Advanced Coastal Navigation

Coast Guard Auxiliary Association Inc. Washington, D. C.

First Edition...... 1987 Second Edition ...... 1990 Third Edition ...... 1999 Fourth Edition...... 2002

ii iii iv v vi Advanced Coastal Navigation

TABLE OF CONTENTS

Introduction...... ix

Chapter 1 INTRODUCTION TO COASTAL NAVIGATION ...... 1-1

Chapter 2 THE MARINE MAGNETIC COMPASS ...... 2-1

Chapter 3 THE NAUTICAL CHART ...... 3-1

Chapter 4 THE NAVIGATOR’S TOOLS & INSTRUMENTS ...... 4-1

Chapter 5 DEAD RECKONING ...... 5-1

Chapter 6 PILOTING ...... 6-1

Chapter 7 CURRENT SAILING ...... 7-1

Chapter 8 TIDES AND TIDAL CURRENTS ...... 8-1

Chapter 9 RADIONAVIGATION ...... 9-1

Chapter 10 NAVIGATION REFERENCE PUBLICATIONS ...... 10-1

Chapter 11 FUEL AND VOYAGE PLANNING ...... 11-1

Chapter 12 REFLECTIONS ...... 12-1

Appendix A GLOSSARY ...... A-1

INDEX ...... Index-1

vii Advanced Coastal Navigation

viii intRodUction

WELCOME ABOARD!

Welcome to the exciting world of completed the course. But it does marine navigation! This is the fourth require a professional atti tude, care- edition of the text Advanced Coastal ful attention to classroom presenta- Navigation (ACN), designed to be tions, and diligence in working out used in con cert with the 1210-Tr sample problems. chart in the Public Education (PE) The ACN course has been course of the same name taught by designed to utilize the 1210-Tr nau - the United States Coast Guard tical chart. It is suggested that this Auxiliary (USCGAUX). Portions of chart be readily at hand so that you this text are also used for the Basic can follow along as you read the Coastal Navigation (BCN) PE text. We recognize that students course. ACN is now used internally from all areas of the United States by USCGAUX as the navigation enroll in the course and that a better text in their member training classes. geographic “balance” to the exam - PE students who successfully com- ples might be attractive. Where nec - plete the ACN course (including essary as, for example, in the discus - necessary examinations) and who sion of polyconic charts of the Great elect to join are credited with com- Lakes in Chapter 3, illustrations pletion of the USCGAUX course. from other areas of the country are This fourth edition builds upon included. But the 1210-Tr chart con - the first three editions of this same tains a wide variety of features of text but has been updated to reflect interest to the navigator and, more - the helpful advice of numerous ded- over, contains a useful summary of icated USCGAUX instructors, stu- chart symbols printed on the reverse dent feedback, and recent technical side for ready reference. In short, developments in the field—particu- this is an ideal training chart. larly in the area of electronic navi - The ACN text has been designed, gation. Graphics and layout have for the most part, to teach you the been substantially improved from “classical” methods of coastal navi- earlier editions. gation applicable to small vessels. The ACN and BCN courses are Shortcuts, such as might be taken for not intrinsically difficult, nor do they routine cruises in very familiar have advanced educational require- waters, are men tioned only briefly. ments. Students from many walks of Experience, sometimes called the life and of widely varying education- “seaman’s eye”, teaches the naviga- al backgrounds have suc cessfully tor which shortcuts can safely be

ix Introduction

taken and under what circum- requires only seconds on a laptop ers who have a technical back - stances. But the basis for shortcuts computer? The answer is very sim - ground and are interested in a more should rest on a firm foundation/ ple. First, not all vessels are complete and detailed exposition. mastery of “the text book solution,” equipped with the latest in marine Students who lack the necessary and not simply arise from laziness, electronics; students come to the background or interest can omit lack of knowledge, or being out of ACN/BCN course owning every - these sections without loss of conti - practice. thing from runabouts equipped with nuity. Material so identified is not Over the past few years there little more than a compass to large included in the final examination for have been revolutionary develop- yachts equipped with virtually the the PE course, but may appear on ments in the field of marine elec- entire contents of one of the numer- the USCGAUX navigation course tronics. Modern Global Positioning ous catalogs of purveyors of marine examination. Additionally, a list of System (GPS) or Loran-C receivers, electronics. The ACN/BCN course ref erences is appended to chapters the fluxgate compass, radar, fuel is designed for skippers of both of for those who wish to learn more flow meters and computers, elec- these types of vessels. Second, and about particular topics. Sidebars are tronic charts, and autopilots serve as perhaps more important, even if a used to set off sum maries, provide examples. These devices, tied vessel is equipped with the latest in highlights, and add items of his - together with onboard navigational navigation systems, the reliability of torical interest that would otherwise computers, have revolutionized seaborne electronics is still far from disturb the flow of the narrative. both the art and science of marine perfect. The classical methods of Perhaps surprisingly, several navigation. Today it is literally true navigation are required to moni tor aspects of navigation (generally that, by merely pressing a few but- the performance of even the best- those relating to the “art” of naviga - tons, you can program an entire equipped vessel. And, should these tion) are controversial. For exam ple, voyage and simply sit back, watch systems fail, classical meth ods can there are those who advocate that all for traffic and be prepared to dock be used to bring you home safe and plotting should be done with respect the vessel at the conclusion of the sound. to magnetic, rather than true north. voyage. Alarms warn of the ACN does include material on Although this text takes a position approach of vessels within a prede- more sophisti cated marine electron- on many of these points of contro- fined exclu sion zone, passage into ics, such as GPS, Loran-C, and versy, books or articles that espouse shallow water, and arrival at way- radar. The coverage is not designed a contrary point of view are includ- points or the final destination. Once, to teach you how to operate any par- ed among the references. Therefore, such an “electronic voyage” would ticular make or model but, rather, to the student should not assume that have come from the dreams of Jules acquaint you with the general prin - either the United States Coast Guard Verne, or been impossibly out of ciples and techniques so that you (USCG) or the USCGAUX specifi- reach for most boaters. But recent can make better use of whatever cally endorse any work listed in the advances in the state of the art of equipment you own or elect to pur- references. These are included to marine electronics have brought sci- chase. add balance, perspective, and inter- ence fiction into the realm of the ACN also con tains a novel chap- est. Likewise, any mention or inclu- possible and affordable. ter on fuel plan ning, a subject usual- sion of pictures of particular makes These modern miracles have led ly given short shrift, or omitted or models of equipment does not some students to question the role of entirely, in the typ ical text on navi- imply that these are recommended classical methods of coastal nav - gation. Unfortunately, fuel starva- or endorsed by USCGAUX or igation, such as the use of visual tion and/or fuel exhaustion cause all USCG. fixes, dead reckoning plots, and the too many preventable Search and Thanks to the many firms like. Why, after all, learn the tedious Rescue (SAR) cases. that supplied pictures and/or proce dures required to calculate tide Several sections of this text are illustra tions for use in this text. height or tidal current, for example, identified as “more technical.” This Acknowledgments are included in when an hour-by-hour calculation material is included for those read - the caption for each illus tration. x Introduction

Although theory is not slighted, material is so valuable, and we apol- Remember that you have the the emphasis in the text is on prac - ogize in advance if the information choice of being the operator of tical, time-tested approaches to nav - referenced is not available. your vessel or merely an igation. The text contains numerous This text contains a glossary of occupant. Knowledge, skill, and discussions of practical methods terms, given in Appendix A. experience are required to trans form and numerical examples to provide Readers encountering an unfamiliar occupants into competent operators. a firm foundation in the art and sci- term or abbreviation may wish to Remember, also, that each voyage ence of coastal navigation. consult this appendix for a brief should be a learning experi ence. In In recent years, the amount of explanation. this sense, all of us should be per- useful material posted on the Each page of the logbook of petual students. Internet has increased substantially. Christopher Columbus was headed May you always have fair skies, Web Site addresses are included as with the title Como Dios Manda (As calm seas, fair currents, and follow- references at various points in the God Ordains), which was testi mony ing winds. But, more important, text. These addresses were valid at both to his religious faith and to the may you learn how to navigate safe - the time that this edition went to primitive state-of-the-art of naviga- ly and efficiently regardless of sea press. Addresses change, as does the tion at the time. This book is dedi- or wind conditions. material posted. We opted to include cated to the notion that you should this information, despite possible also have a hand in the out come of a changes, because the supplementary voyage.

xi Introduction

xii Chapter 1

INTRODUCTION TO COASTAL NAVIGATION

“And after this sort he proceedeth from place to place until he arrive unto his desired porte, which is a conclusion infallible if there be no other impediments (whereof there hath been good consideration had) which may breed errour, for from such negligence may arise many inconveniences.”

–The Seaman’s Secrets by John Davis, 1607, as quoted in Schofield, The Story of HMS Dryad

hat is coastal navigation? In equipment, such as the employment Wsimple terms, marine naviga - of methods to fix the vessel’s posi- tion is “getting your vessel from tion from observation of the sun, where you are to where you want to moon, or stars, coastal navigation go, safely and efficiently.” More often demands a greater degree of formally, it is the “process of direct - accuracy and attention to detail. On ing the movement of a vessel from a long ocean passage, for example, one point to another.” It is derived it may suffice to deter mine the ves- from the two Latin words, navis sel’s position only once or twice a (ship), and agere (to move). Coastal day, and to within a margin of navigation refers to navigation in uncertainty of several square miles. coastal (sometimes termed pilot) A well-found ocean going vessel waters, where the opportunity may afford the naviga tor a dry exists to determine or check the workstation, and numer ous elec- vessel’s position by refer ence to tronic aids, such as the Global navigational aids and obser vations Positioning System (GPS) receiver (by either visual or elec tronic and radar. A passing vessel would means) of the coast and its features. be a curiosity in seldom trav eled Coastal navigation is dis tinguished waters, rather than an object for from “blue water” or ocean naviga- collision-avoidance maneuvers. In tion, terms used to describe naviga- coastal waters, particularly in nar- tion out of sight of land and/or row channels, position fixes might coastal Aids to Navigation be required every 5 to 15 minutes, (ATONs). Although blue-water and required accuracy lim its could navigation may appear to require well be measured in yards. The more sophisticated techniques and navigator’s workspace could be

1-1 1 Introduction to Coastal Navigation

cramped, and the vessel’s naviga - Massachusetts, on Vineyard Sound sequences that are important for tional gear limited to a hand-bear- (at the far right of the chart). recognition and identification pur - ing compass. All this is to be done It is convenient to subdivide poses), while the USCP provides while dodging “heavy iron” (large navigation into two distinct, but useful “local knowledge” in narra - vessels) in busy shipping channels. related phases: voyage planning tive form. For example, following a and underway navigation. The route from Tiverton through

ST G A U O planning phase covers the initial A Buzzards Bay would require a tran - C R

S D What you Will U

A U Y X R ILIA learn in this shoreside paper-and-pencil or sit of the channel between Buzzards (increasingly) computer chores, and Bay and Vineyard Sound. The Chapter ends when the vessel’s anchor is USCP offers the following com - J How the course is weighed or the mooring lines are ments about this area: organized slipped. Underway navigation cov - “The passage through J Principles of voyage ers navigation and decision making Woods Hole, between planning and underway on the water. The overall steps in numerous ledges and shoals, navigation each phase are discussed below. is marked by navigational aids. However, tidal currents J Coordinate systems STEPS IN are so strong that the passage (latitude and longitude) VOYAGE PLANNING is dif ficult and dangerous J Measurement of Figure 1-1 highlights the princi - without some local knowl- direction pal steps in voyage planning. It edge. Buoys in the narrowest starts with the assembly of required part of the channel some- AN OVERVIEW reference materials and trip and times are towed under, and a OF THE COURSE vessel data. Such materials include: stranger should attempt pas- J Up-to-date (and corrected) nau- sage only at slack water.” This section provides an Such information is obviously overview of the Advanced Coastal tical charts at the right scale (discussed in Chapter 3), invaluable for planning purposes. Navigation (ACN) course in the Tide Tables are used to estimate J Tide and Tidal Current Tables context of the navigator’s tasks on a the height of the tides that would be and related materials (discussed typical voyage in coastal waters. To encountered on the voyage to in Chapter 8), make the discussion concrete, sup - ensure that a safe route is chosen. pose that you are the navigator for J Navigation reference materials, Tidal Current Tables provide infor - the 42-ft. trawler, Verloren, on a such as the Light List (LL), U. S. mation on the strength and direction voyage from Tiverton, on the Coast Pilot (USCP), and of the currents, information used to Sakonnet River in the state of J Cruising guides to the area (dis- estimate the vessel’s ground speed Rhode Island, to Woods Hole, cussed in Chapter 10). and the selection of the correct Massachusetts, approximately 40 The nautical charts are used to course to compensate for these cur- miles distant. This area is covered lay out the voyage, measure dis - rents. by the 1210-Tr chart, which is dis- tances and courses, identify land - Other information requirements tributed with the course materi als. marks or ATONs that will be used include operating data for the ves - Reach for this chart now (the first to fix the vessel’s position, ensure sel, such as the relation between of many times that you will be engine revolutions per minute called to do this in the chapters that the course avoids hazards to (RPM) and the speed through the ahead) and locate the place of navigation, and for many other pur - departure on the voyage, Tiverton poses. water (discussed in Chapter 5), and (roughly in the middle of the chart, The Light List is consulted to fuel capacity and consumption data near the top), Rhode Island, and the determine the characteristics of the (presented in Chapter 11). For destination, Woods Hole, relevant ATONs (such as color or example, at 2250 RPM, Verloren light characteristics and horn might make 8 knots (nautical miles

1-2 Introduction to Coastal Navigation 1

ASSEMBLE REQUIRED recreation, or fuel. Even for this distance (TSD) calculations are REFERENCE MATERIALS “simple” trip, there are several reviewed in Chapter 5, fuel con - alternatives that might be consid - sumption calculations in Chapter ll, ered. Time spent developing and the somewhat more com plex FORMULATE VOYAGE thoughtful voyage options is time task of allowing, and compen sating, ALTERNATIVES well spent. A good navigator thinks for currents in Chapter 7. and plans ahead, so that he/she Estimation of the probable currents doesn’t have to exercise extraordi - is discussed at length in Chapter 8, EVALUATE ALTERNATIVES nary seamanship! If, for example, As it happens, the currents in the more northerly route through Buzzards Bay and Vineyard Sound Buzzards Bay is chosen, the trip are often moving in opposite direc - schedule has to be worked out to tions, at speeds ranging from less SELECT BEST VOYAGE PLAN & PREPARE FLOAT PLAN minimize the hazards of transiting than one knot to 2 knots or more. the channel between Buzzards Bay So, depending upon the current pat - and Vineyard Sound. To someone terns prevailing on the day and time unfamiliar with these waters, the of the voyage, the two routes identi - COMPLETE PRE-UNDERWAY VESSEL CHECKS USCP indicates that it would be fied above could have significantly prudent to make this transit during different ETEs. daylight hours at or near slack cur - Moreover, as illustrated in rent. This can and should be figured Chapter 8, it is entirely possible that WEIGH ANCHOR out in advance, rather than “come the longer distance route would also upon” in failing light. be the shorter time route. As noted, L The third step in voyage plan - the length of the Buzzards Bay FIG. 1-1–Steps in Voyage Planning ning is to evaluate systematically route is approximately 37.5 miles, the alternatives identified in step compared to 40 miles for the per hour), and burn 7 gallons per two. Obviously, two important fac - Vineyard Sound route. But if the hour (GPH) of fuel from tanks that tors relevant to the voyage are average current along Vineyard can hold 400 gallons when topped Verloren’s speed and the distance to Sound were, say, 2 knots in the off (filled). be covered along each of the alter- direction of intended travel (a so- The second step in voyage plan - native routes. This distance is deter- called fair current), and that in ning is to consult the materials mined from a rough plot of the Buzzards Bay were 0.6 knots assembled and formulate voyage alternate routes on the nautical against the direction of travel (a so- options for later evaluation. Voyage chart by techniques revealed in called foul current), the time options relevant for this trip would Chapter 3. In this case, the route required for the trip through include the overall route to follow through Buzzards Bay (ap - Vineyard Sound would be approxi - (east through Buzzards Bay, or proximately 37.5 miles for one pos - mately 4 hours, compared to nearly south, then east through Vineyard sible route layout) is slightly short - 5 hours on the “shorter” route. (This Sound are obvious alternatives), er than that through Vineyard calculation must be refined to take departure time (which affects the Sound (approximately 40 miles). account of the fact that the first leg water currents and tide heights that Speed and distance determine the is common to both routes. Even a will be encountered and also how en route time required for the voy - more exact calculation, however, much of the voyage will be con - age (5 hours estimated time en route shows that the longer distance route ducted during daylight hours), the (ETE) to cover 40 miles at 8 knots is the shorter time route.) This ex - speed to be run (which affects the assuming no current), and the fuel ample is not hypothetical—the estimated en route time, fuel con - consumption (35 gallons required, assumed currents are, in fact, the sumption, and arrival time), and assuming 5 hours en route at 7 gal - estimated currents at one point in planned stopovers for amenities, lons per hour). Simple time-speed- the tidal current cycle. Additionally,

1-3 1 Introduction to Coastal Navigation

the Vineyard Sound route avoids the availability of suitable land - ing direction, speed, and, occasion - the trip through the channel next to marks or ATONs to fix the vessel’s ally, distance) and future positions Woods Hole. This benefit may not position or to mark channels all (termed dead reckoning positions) be important to someone with local need to be considered. Discussion at various times in a stylized for - knowledge, but might be a decisive of these important matters can be mat. The DR plot is maintained and factor otherwise. found scattered throughout this text updated throughout the underway For this voyage in Verloren, fuel in the examples used to illustrate portion of the voyage. certainly won’t be a problem, key points. The fifth step in voyage plan - assuming that the tanks are even The fourth step in voyage plan - ning is to complete prevoyage near to being full prior to departure. ning is to select a plan that is “best” checks on the vessel and its equip- But for longer trips, or in vessels in some sense, considering the ves - ment—much as aircraft pilots do in with higher fuel consumption or sel, navigational equipment aboard, the preflight inspection. For exam - lower fuel capacity, fuel planning is skill and local knowledge of the ple, the navigator would verify that often a singularly important activi - navigator and crew, and other rele - all communications and navigation ty. For vessels with what are termed vant factors. Included here is the equipment were functioning prop - “short legs” (limited fuel capacity), important task of making a “float erly and that the correct charts and fuel stopovers would need to be plan” that describes the route and other reference materials were considered, and/or the engine throt- estimated time(s) of arrival so that aboard. Weather information should tle setting altered to stretch fuel the Search and Rescue (SAR) per - be gathered and used as part of the reserves. sonnel can be promptly alerted if “go-no-go” decision. If all goes you become overdue. (The float well in this step, it is time to start plan should also include a descrip - engines, slip Verloren’s dock lines, tion of the vessel, number of per - note the departure time in the navi- sons on board, available safety and gator’s or ship’s log, and get under- radio equipment, and other relevant way. information.) The float plan is left in the care of a responsible person, STEPS WHILE UNDERWAY with instructions to notify the Coast Guard in the event that the vessel Figure 1-2 shows a simplified becomes overdue. The navigator summary of the key underway often prepares a more detailed voy - activities. As noted above, the navi - age plan in this step, identifying gator estimates the future position of the vessel at various times using

PHOTO PHOTO COURTESY OF MAINSHIP checkpoints and turnpoints for each L leg of the trip, courses to steer, time DR (see Chapter 5). But, these esti - A trawler moving serenely through the estimates, and fuel consumption mates are not error free. Neither water. The material in this course will estimates. wind nor current, for example, is enable you to navigate such vessels In this fourth step, the navigator considered in the determination of DR positions—for reasons that are with confidence. also plots the first “legs” (route seg - ments) of the voyage on a tactical apparent on reading Chapter 5. Option evaluation is not limited (underway) dead reckoning plot Therefore, it is very important to to questions of time, speed, or fuel (DR plot). Dead reckoning (DR), check and update the actual consumption. Many other factors explained in Chapter 5, is the name progress of the voyage at frequent need to be considered. For example, given to the process of predicting intervals in coastal waters. This is the difficulty of transiting channels the future position of a vessel from done with a series of “fixes,” points or inlets, availability of “bolt knowledge of its present (or start - in time at which the vessel’s posi - holes” (safe places to anchor or ing) position, the course steered, tion is accurately determined. moor in the event of mechanical and the speed maintained. A tactical The vessel’s position can be problems or adverse weather), and DR plot shows course legs (includ- fixed by three principal methods.

1-4 Introduction to Coastal Navigation 1

J First, visual observation of the START range or bearing of landmarks or ATONs can determine its position. For example, the navi - gator could determine the mag - UPDATE DR netic bearing of the abandoned PLOT lighthouse on Sakonnet Point, and that of the tower on Gooseberry Neck, which could SEARCH FOR fix Verloren’s position by tri - FIX OPPORTUNITIES angulation if the Buzzards Bay route were taken. This method FIX VESSEL’S for position fixing is termed POSITION piloting, and is discussed in Chapter 6. J Second, the vessel’s position can be fixed by use of electron- ELECTRONIC PILOTING CELESTIAL ic navigational systems, such as NAVIGATION GPS, loran, or radar. For exam - USE VISUAL USE SHIPBOARD ple, GPS could be used to read METHODS OBSERVATIONS the vessel’s latitude and longi - ELECTRONICS tude directly. Alternatively, radar could be used to measure the range and bearing to a rec - ognizable landmark. This is COMPARE FIX WITH COMPUTE termed electronic navigation, DR POSITION CURRENT SET & DRIFT and is discussed in Chapters 6 and 9. L J Third, the position of the ship FIG. 1-2–Steps in Underway Navigation can be fixed by observation of the angle (elevation) of heaven - ly bodies (here meant to mean fix with the vessel’s DR position siderably. In practice, the navi - the sun, moon, or stars). This can be used as a “plausibility” or gator’s underway tasks are often process is termed celestial navi - “reality” check on the fix and the more complex and varied. For gation. For various reasons, DR position. Absent blunders, any example, instead of merely estimat- including the limited opportuni - discrepancy between the fix and the ing the set and drift of the current, ties for fixes, and the possible DR position is due to current, so a the navigator would usual ly esti- error of celestial fixes, celestial comparison of these two positions mate a course (by the methods dis- navigation is not extensively can be used to estimate the actual cussed in Chapter 7) to compensate used in coastal waters and is not currents (the term set refers to the for these effects and ensure that the presented in this text. direction toward which the current vessel stays in safe water. On such a Once a fix is determined, this is is flowing, and drift refers to the short voyage, as is illustrated here, plotted on the tactical DR plot (see speed of the current) encountered en route decision making may be Chapter 5) and the plot is updated (the method for estimating set and relatively simple. But on other trips, with this fix. (The data for this fix drift is discussed in Chapter 7). the navigator would be continually are also entered into the navigator’s For introductory purposes, revising fuel consumption estimates or ship’s log.) A comparison of the Figure 1-2 has been simplified con- and the estimated time of arrival

1-5 1 Introduction to Coastal Navigation

in this text. Of course, not all voy- poles and bulges in the middle, as ages are sufficiently long or com- opposed to a prolate spheroid plex to require the formal use of all which resembles a football; but the techniques discussed above. For don’t go calling it prolate spheroid short voyages in familiar and well- ball, or you will wind up being marked waters, and when weather called an oddball!), but the differ- conditions are close to ideal (e.g., ence between the earth’s actual moderate seas, calm winds, and shape and that of a perfect sphere is good visibility), various short cuts not important for this course. The can be taken to simplify the naviga- average diameter of the earth is tor’s duties. This is termed naviga- approximately 6,880 nauti cal miles, tion by seaman’s eye and is and its circumference is approxi- PHOTO COURTESY OF UNITED STATES COAST GUARD COAST PHOTO COURTESY OF UNITED STATES addressed in Chapter 11. L mately 21,614 nautical miles. Since Navigation of high-speed vessels is there are 360 degrees (denoted with Navigation of faster boats and/or navi- both simpler and more difficult. the degree symbol °) of angular gation in rough waters requires greater Currents are less of a factor for measure in a circle, 1 degree of preplanning. Here is a United States high-speed vessels and these com- angular measure along the earth’s Coast Guard 44-ft patrol boat crashes putations are usually omitted. surface is approximately 60 nauti- through a wave. There is little time or However, there is less time to use cal miles. Degrees are fur ther sub- space to plot positions exactly. traditional methods and more pre- divided into minutes (denoted with planning is required. Navigation of (ETA). These revised estimates high-speed vessels is covered an apostrophe, e.g., 30 minutes is could signal the need to change the briefly in this text. written 30’), and sec onds (denoted voy age plan. For example, the In the above discussion, the con- with two apostro phes, e.g., 40 sec- discov ery that fuel consumption tents of two chapters were omitted. onds is written 40”). There are 60 was significantly higher than Chapter 2 covers the minutes in a degree (and 60 seconds planned could mean that the vessel marine magnetic compass, would have to be diverted to an and Chapter 4 provides a alternate destination. summary discussion of the NORTH NORTH MAGNETIC GEOGRAPHIC The navigator should also check navigator’s tools (other POLE POLE the accuracy of the navigation than the vessel’s compass). equipment in use by comparing, Before moving on to whenever possible, fixes deter - some of the interesting mined by various methods. For material in the chapters example, a comparison between an ahead, it is necessary to accurate visual fix and one deter - address two important intro- WEST EAST mined by GPS or Loran-C could be ductory topics: the earth’s EQUATOR used to verify that these systems coordinate system and were functioning properly. measure ment of direction. Likewise, a prudent navigator would make periodic checks on the BACK TO BASICS: accuracy of the vessel’s compass, THE PLANET EARTH perhaps by spot checks of the com- The earth is approxi- pass’ deviation table, as explained mately spherical, as illus- SOUTH SOUTH in Chapter 2. GEOGRAPHIC MAGNETIC trated in Figure 1-3. POLE POLE There you have it—a brief illus- Technically, the earth is L tration of the various navigator’s termed an oblate spheroid FIG. 1-3–The Earth and Its Poles tasks and where these are addressed (a sphere flat tened at the

1-6 Introduction to Coastal Navigation 1

in a minute), so 1 minute of angular GREAT AND measure is approximately equal to 1 SMALL CIRCLES nautical mile. A plane passed The earth rotates about a straight through the cen ter of the line called the axis of rota tion, or earth separates the earth polar axis. The earth com pletes one into two hemispheres, rotation every 24 hours (the solar and inter sects the surface day). The axis of rotation passes of the earth to pro duce a through the center of the earth, geometric figure termed a intersecting the surface at two great circle. On the sur- points, termed the north and south face of a sphere, the geographic poles (denoted Pn and shortest distance between L Ps, respectively). The earth rotates any two points lies along the great FIG. 1-5–A Parallel of Latitude from west to east, i.e., counterclock- circle that connects these two wise when viewed from a point in points. (On the slightly flat tened great circle is termed the equator, as space atop the North Pole. The surface of the earth, the short est dis- shown in Figure 1-4, and the two west-to-east rotation makes the sun tance between two points is techni- hemispheres formed are named the appear to rise in the east and set in cally termed a geodesic, but for the northern and southern hemi spheres. the west. The earth is also a mag- purposes of this course a great cir- A small circle results if a plane net—discussed below—and has cle and a geodesic are one and the is passed through the earth that does North and South Magnetic Poles. same.) not touch the earth’s center. Small These poles (shown also in Figure If the plane is passed so that it is circles parallel to the equator 1-3) are not coincident with the geo- perpendicular to the earth’s axis of (termed parallels) are one of the graphic poles, an important point rotation (i.e., equidistant from the two reference coordinates used to explored below. geographic poles), the resulting

L L FIG. 1-6–The Planes of the Meridians FIG. 1-4–The Equator (Longitude) Meet at the Polar Axis

1-7 1 Introduction to Coastal Navigation

define position on the earth’s sur - through the Naval face. Figure 1-5 shows the equator Observatory in Washing - LONGITUDE OF THREE LOCATIONS and another parallel of latitude. ton, DC, would be identi - Latitude is the angular measure of fied as Lo = 77° 03.9’ W TOKYO EAST the distance north or south of the (77 degrees, 3.9 minutes, equator and is measured in degrees west of the prime meridi- an) or, equivalently as 77 (0 to 90 degrees). PRIME A great circle that passes 03’ 54” (77 degrees, 3 Pn MERIDIAN through the polar axis or axis of minutes, 54 seconds west rotation is termed a meridian. It has of the prime meridian). The degree sign is some - two parts—that on the observer’s WEST side of the earth, which is called the times omitted. In some upper branch, and one on the other writings, E or W is omitted LOS ANGELES WASHINGTON side of the earth, which is called the when it is clear that the L longitude is east or west, lower branch of the meridian. The FIG. 1-7–Longitude for Three Locations on the but this practice should be planes of the meridians meet at the Earth’s Surface polar axis, as shown in Figure 1-6. discouraged. As other examples, the longi -

Meridians are used to define ST G A U O A C R

S D tude of the Griffin Observatory in U historiCal A U Y R the other major coordinate for spec- XI IA L ifying position on the earth’s Los Angeles, CA, is Lo = 118 18.1’ Footnote: surface –longitude. W, and that of the Tokyo Where is the Astronomical Observatory at reference Meridian? LONGITUDE AND LATITUDE Mitka, Japan, is Lo = 139 32.5’ E. As noted in the text, the prime Remember, longitude is always meridian passes through Green - The prime meridian (more specified as east or west of the wich, England. However, unlike specifically, its upper branch) passes prime meridian. Figure 1-7 shows the equator, its location is entirely arbitrary. Ptolemy, for example, through the original site of the Royal the longitudes of these three loca - Observatory in Greenwich, England. chose to place the prime meridian tions on the earth’s surface as through the Canary & Madeira Also called the Greenwich viewed from atop Pn. Islands. Later it was moved to the Meridian, it is used as the origin of It is not sufficient to identify a Azores and the Cape Verde Islands. measurement of longitude (see side- position on the earth’s surface by its Various governments have placed bar). More precisely, longitude it in Copenhagen, Jerusalem, longitude alone, because there are London, Paris, Philadelphia, Pisa, (abbreviated Lo, or sometimes writ - an infinite number of points that lie Rome, and St. Petersburg. At the ten λ, the Greek letter lambda) is the on any meridian. Another coordi - Third International Geophysical angular distance (in degrees, min- nate is necessary to specify position Con gress meeting in Venice in utes, and sec onds, or degrees and uniquely. 1881, several other proposals were deci mal minutes) between a position floated, including use of the Great As noted, this second coordinate Pyramid of Egypt as the prime on the earth and the prime meridian is termed latitude. More formally, meridian. In 1884, representatives mea sured eastward or west ward latitude (abbreviated L or Lat.) is from 26 countries to the through 180 degrees along the arc of the angular distance between a International Meridian Con - the equa tor to the meridian of the position on the earth’s surface and ference in Washington, DC, voted to select Greenwich as the official position. Because longi tude is mea- the equator, measured northward or prime meridian. The French con- sured only through 180 degrees, southward from the equator along a tinued to recognize the Paris rather than 360 degrees, from the meridian and labeled with an “N” Observatory as the “correct” loca- prime meridian, it is necessary to or an “S” to denote whether the tion of the prime meridian until include the word east (E) or west point is located in the northern or 1911. These and other stories are related in Sobel (1995) and (W) to define the longitude uniquely. southern hemispheres, respectively. O’Malley (1990). For example, the meridian passing (Sometimes, when the hemisphere

1-8 Introduction to Coastal Navigation 1

necessary to have some means for GREENWICH Lat 45o30.1’N N specifying direction on the earth’s o Lo 73 20.4’W 60oN surface. 50oN o Direction is not absolute but PARALLELS 40 N o must be keyed to some reference OF LATITUDE 30 N (TYPICAL) 20oN point. Three common reference W 10oN points are true north, magnetic 100o 80o 60o 40o 20o 20o north (discussed below), and the EQUATOR W E ship’s heading (relative bearings are discussed below). If direction is referenced to true MERIDIANS north (geographic north or the OF LONGITUDE North Pole), it is defined relative to (TYPICAL) the local meridian passing through the point of interest (also called the L local geographic meridian). The FIG. 1-8–Grid System of Latitude and Longitude local geographic meridian passes through the north geographic pole, is clearly implicit, the N or S will be along any meridian, is equal to 60 so this direction is relative to the omitted, but this practice is to be nautical miles, and 1 minute is north geographic pole or to north. discouraged.) Latitude ranges from equal to l mile. Note, however, that The direction of true north, or 0 degrees (for a point located at the this does not hold for longitude. northward along the upper branch equator) to 90 degrees N or S (for a Although 1 minute of longitude is of the local geographic meridian, is point at the north or south geo - approximately equal to 1 nautical defined as zero degrees and graphic pole). Lines of constant lat- mile at the equator, as the latitude becomes the reference direction. By itude are called parallels of latitude, increas es, the distance along any convention, the preci sion of angular or simply, parallels. parallel, between two meridians, measurement for courses or bear- Continuing the earlier examples, becomes smaller, reaching 0 miles ings is to the nearest degree. the latitude of the Naval at either pole. (The length of 1 Degrees are reported to three digits; Observatory in Washington, DC, is degree of longitude is approximate- so, for example, north has the direc- L (or Lat.) = 38 55.2’ N, that of the ly equal to 60 times the cosine of tion 000 degrees. Direction is spec- observatory in Tokyo, Japan, L = 35 the latitude. For example, at latitude ified clockwise from true north. 40.4’ N, and that of the Griffith of 41 30’ north, approximately the Thus, east is 090 degrees, south is Observatory, L = 34 06.8’ N. These midpoint of the latitudes given on 180 degrees, west 270 degrees, etc. two coordinates, latitude and longi - the 1210-Tr chart, the length of 1 The direction 360 degrees and 000 tude, are used to define locations on degree of longitude is approximate - degrees are one and the same and, the earth’s surface, as shown in ly 45 nautical miles rather than 60 by convention, this direction is usu- Figure 1-8. By convention, a point’s nautical miles on the equator.) ally written 000 degrees. Therefore, latitude is written first and its lon - Remember that latitude is measured it is said that direction is measured gitude second, so if there are no along a meridian (running north or clockwise from north, and ranges labels, the first number written is south), while longitude is measured from 000 degrees to 359 degrees. latitude, the second longitude (the along a parallel (running east or With a suitable device for mea - notation E or W, versus N or S also west) from the prime meridian. suring angles (see Chapter 4), define the coordinate). directions can be read from a chart One important attribute of lati - DIRECTION off the local meridian. However, for tude (noted implicitly above) is the Latitude and longitude are all reasons that are apparent in later fact that 1 degree of latitude, mea- that are required to specify location chapters, it is useful to have addi- sured up or down (north or south) on the earth’s surface. But, it is also tional sources of directional infor-

1-9 1 Introduction to Coastal Navigation

L FIG. 1-9–True and Compass Rose as Presented on the 1210-Tr Chart

1-10 Introduction to Coastal Navigation 1

of the geographic pole, a line is

ST G A U O A C R

S D

U formed that “spirals” around the

A U Y X R ILIA FaCtoiD globe, continually edging either According to records dating northward (for directions between 271 degrees and 359 degrees or 000 back to the 1700s, the appar- degrees and 089 degrees) or south - ent position of the north ward (for directions between 091 magnetic pole has shifted degrees and 269 degrees). This line, from a position north of termed a rhumb line or loxodrome, Scandinavia across Green - approaches either pole, as shown in land to its present position in Figure 1-10. This line drawn on the the Parry Islands in northern surface of a sphere, such as the earth, is actually curved, not L Canada. Over geo logic time, straight. (However, as noted in FIG. 1-10–A Rhumb Line or the shifts have been even Chapter 3, it will plot as a straight Loxodrome as Given in Bowditch more dra matic; it is believed line on the Mercator chart typically that 200 million years ago used for coastal navigation.) lar, a phenomenon caused by the the magnetic poles were near The earth has a weak magnetic nonuniform distribu tion of magnet- the equator! field, thought to be generated by the ic material through out the earth. flow of the liquid iron alloy core of The angular difference between the planet. This field, termed a dipole mation provided on the nautical the geographic and magnetic merid - field, is similar to the mag netic field chart. One common directional ref- ians at any point on the earth is that would be generated by a large erence is termed a compass rose, called the magnetic variation, or bar magnet located near the center of which provides directional informa - simply variation. (The term mag - the earth. The magnet ic flux lines tion relative to both true and to netic declination is also used.) diagramed in the styl ized and simpli- magnetic north (discussed below). Variation is said to be east if the fied representation of Figure 1-11 magnetic meridian points eastward Figure 1-9 shows a dual compass flow out from the core through the of the north geographic pole, or rose taken from the 1210-Tr chart. auroral zone of the South Pole, west if (as shown in Figures l-12 or Directions relative to true north are around the earth, and return through given on the outer circle of the rose. the auroral zone of the North Pole. True north is generally indicated More important to the mariner, the with a star symbol (presumably a magnetic poles on the earth differ reference to Polaris, the star that is, from the geographic poles. In 1984, to within a degree or so, aligned the North Magnetic Pole, for exam- with true north). The principal ple, was located in Canada’s advantage of printing compass Northwest Territories, at approxi- roses on nautical charts is that it is mately a latitude of 78.9 degrees relatively easy to transfer (i.e., north and longitude 103.8 degrees measure) these direc tions with par- west, several hundred miles removed allel rulers or a para line plotter (see from the geographic North Pole. Chapter 4). At the surface of the earth, lines When a direction other than of magnetic force are termed mag- exactly north, south, east, or west is netic meridians, analogous to geo- specified on the earth, and followed graphic meridians. However, unlike for any distance, such that each sub- geographic meridians, which have a sequent meridian is passed at the simple geometrical interpretation, L same angle relative to the direc tion the magnetic meridi ans are irregu- FIG. 1-11–The Earth is a Magnet

1-11 1 Introduction to Coastal Navigation

1-13) the north magnetic pole is still follows that variation is the able to convert from true to magnet - westward (to the left) of the north angular difference between true ic directions or the reverse. geographic pole as seen by the north and the direction that the ves- Variation data can be found in observer. sel’s compass would point, absent several sources. Perhaps most con - Incidentally, diagrams such as shipboard magnetic influence. Lines venient, variation data are printed Figures 1-12 and 1-13, are very of constant varia tion are termed iso- on the compass rose found on nau - helpful in understanding the con cept gonic lines, and the line where the tical charts, such as is illustrated in of variation but are not, strict ly variation is exact ly zero degrees is Figure 1-9. There the inner circle of speaking, accurate. This is because called the agonic line. These isogo- the compass rose shows magnetic they suggest that a com pass (free of nic lines arc chart ed and are pub- directions while the outer circle shipboard magnetic influences) lished on Chart #42 by the National shows true directions. In this illus - would always point toward the Imagery and Mapping Agency. tration, the magnetic meridian North Magnetic Pole. In fact, a The relevance of all this to the points to the left of the local geo - freely suspended magnetic compass mariner is that the magnets in the graphic meridian—and the varia - needle acted upon by the earth’s vessel’s compass (discussed in tion is approximately 15 degrees magnetic field alone will lie in a Chapter 2) tend to align with the west. (As noted above, the magnet - vertical plane known as the magnet- magnetic meridians, rather than the ic meridians shift around, and have ic meridian. These magnetic meridi- true or geographic meridians. (It is daily (diurnal) and longer term ans, however, do not neces sarily actually slightly more complicated (secular) changes, so, for this rea - point towards the magnetic poles, than that, but Chapter 2 straightens son, the important shifts are identi - because the earth’s magnetic field is out the details.) Therefore, it is nec - fied on the chart. Reference to irregular. This technicality aside, it essary to know the variation to be Figure 1-9, for example, shows that

VARIATION IS THE ANGULAR DIFFERENCE BETWEEN THE GEOGRAPHIC AND MAGNETIC MERIDIANS

GEOGRAPHIC OBSERVER’S NORTH POLE POSITION

MAGNETIC NORTH POLE

L FIG. 1-12–Variation: The Approximate Difference Between the Directions to the North Geographic Pole and the Magnetic Meridian

1-12 Introduction to Coastal Navigation 1

the variation at this location was 15 bearing compass, discussed in ally bear 345 degrees true. This is degrees west in 1985, and that it is Chapter 4, might be used to take a because, at this location, the varia - increasing at the rate of three min - bearing on a shore-based object, tion is 15 degrees west, or to the left utes per year.) and the navigator may wish to con - of true. A glance at the compass Within the continental United vert this to a true bearing for plot - rose shows that all bearings have States, variation ranges from about ting on the nautical chart. Alter - this fixed difference between mag - 20 W in northern Maine, through 0 natively, a mariner may measure a netic and true. Conversion from in portions of Florida, to 21 E in true course on the chart (discussed magnetic to true is, therefore, a northern Washington state. (On the in Chapter 3) and wish to convert simple matter of subtraction of a northern border between Alaska this to a magnetic course. west erly variation, or addition of and Canada, it is approximately 35 Conversion from one reference an easterly variation. Thus, for E as of this writing.) point to another is relatively simple. exam ple, an object bearing 090 Suppose, for example, that the vari- magnet ic, would bear 090 - 015 or CONVERSION FROM ation is 15 degrees west, as is 075 true. (Chapter 2 provides some TRUE TO MAGNETIC AND shown in Figure 1-9, as would handy memory aids to keep the VICE VERSA apply to one portion of the area addition and subtraction straight, It is often necessary to convert cov ered by the 1210-Tr chart. An but a simple one to remember is from direction expressed relative to object located in the direction of “magnetic or compass to true, add magnetic north to direction magnetic north from the perspec- east.”) As discussed in other chap - expressed relative to true north or tive of the observer (said to have ters, magnetic courses or bearings vice versa. For example, a hand- bear ing 000 magnetic) would actu- are identified as such by the use of

VARIATION CHANGES WITH THE OBSERVER’S LOCATION

GEOGRAPHIC NORTH POLE VARIATION WEST AT THIS LOCATION

MAGNETIC NORTH POLE

VARIATION EAST AT THIS LOCATION

L FIG. 1-13–Variation is the Angular Difference between the Geographic (True) Meridian and the Magnetic Meridian. Variation Changes with Locality.

1-13 1 Introduction to Coastal Navigation

RELATIVE BEARINGS

2 POINTS ON DEAD PORT BOW AHEAD 000o

BROAD ON PORT BOW 2 POINTS STARBOARD BOW

o 360 o 2 5 2. o BROAD ON THE 7 . 5 3 E BEAR STARBOARD BOW 2 POINTS FOR 3 TIV ING LA S o RE 0 5 4 FORWARD 1 5 o 3

WAR PORT BEAM

D 2 POINTS

o 0 6 STARBOARD BEAM 5 . 7 2 . 5 9 o 2

0

o

9

BROAD ON 0 BROAD ON

0

7

o

2

PORT BEAM STARBOARD BEAM

1

o

5 1

.

2

7

.

4 5

2 2 POINTS ABAFT

o

STARBOARD BEA

1

o

5

3

2

5

2 POINTS ABAFT 2

o

PORT BEAM

1

M

5

BROAD ON

5

. B 7 o

2

Y . 5

0 DE EES

2 GR

STARBOARD QUARTE 1

o 8 0

o

2 POINTS ON BROAD ON STARBOARD QUARTER

PORT QUARTER DEAD ASTERN R 2 POINTS ON

PORT QUARTER

BY COMPASS POINTS

There are 32 points on a compass each having 11 1/4 degrees. (32 x 11.25 = 360 degrees)

L FIG. 1-14–Relative Bearings by Degrees and Points

1-14 Introduction to Coastal Navigation 1

the word “magnetic,” or the writing dead ahead, directly off the bow, reciprocal bearing is one that differs of an “M” after the number. If no would bear 000 R. Note that rela- from the original by 180 degrees. such prefix or suffix is added, it is tive bearings relate to the fore-and- For example, if a fixed navigational assumed that the course or bearing aft or bow direction of the boat and aid, such as a lighthouse, were to is “true.” change direction as the boat bear 000 degrees true from your changes direction (heading) or posi - vessel (i.e., be directly north of RELATIVE DIRECTIONS tion. If the boat is underway, and your vessel), it could equally be (BEARINGS) the object observed is stationary, said that your vessel is directly It is convenient, from time to the relative bearing will change as south of the lighthouse. That is, time, for mariners to indicate direc - the boat approaches, passes, and your vessel would be on a recipro - tions referenced to objects other continues on. The relative bearing cal bearing from the lighthouse. To than the true and/or magnetic would also change if the boat were calculate a reciprocal bearing, all meridians. In fact, one of the most turned, increasing in a clockwise that is necessary is to add or sub - used direction systems is that refer - manner as the boat turned counter - tract 180 from the given bearing. In enced to the fore-and-aft line paral - clockwise. this example, the reciprocal of 000 lel to, or directly over, the keel of To convert from relative bearing degrees is 180 degrees (obtained by the observer’s vessel. to either a true or magnetic bearing, adding 180). (Helpful hint: in cal- If a direction “rose” were super - all that is necessary is to remember culating a reciprocal by addition or imposed over the vessel (plan view) the equation: ship’s heading + rela- subtraction, it may also be neces - with the 000 line directly forward, tive bearing = bearing to object. sary to add or subtract 360 degrees at the vessel’s bow, 090 on the ves- Thus, for example, if the vessel to the result to ensure that the sel’s starboard beam, 180 directly were heading 070 true, and you answer lies between 000 and 360.) aft, on the vessel’s stern, and 270 on observed an object bearing 135 R, The reciprocal of 270 degrees is the port beam, the relative bearing the object would bear 070 + 135 = 090 degrees, the reciprocal of 315 system is developed as illustrated in 205 true. Of course, because there degrees is 135 degrees, etc. (With a Figure 1-14. Objects relative to the are only 360 degrees of arc measure little practice you can do these in instantaneous direction of the bow in a circle, it may be necessary to your head quickly by first adding of the boat are indicated in degrees subtract 360 degrees from the cal- 200 and taking away 20, or sub - of angular measure, clockwise, just culated bearing of the object. For tracting 200 and adding 20! Thus, as directions are indicated for the example, if the ship’s heading were for example, the reciprocal of 121 true and magnetic direction sys - 315 degrees true and the object’s degrees is 301 degrees, obtained by tems. Shown also in Figure 1-14 are relative bearing were 135 degrees, quickly adding 200 to 121 to get relative bearings in the older “point the true bearing would be 315 + 321, and then subtracting 20 to get system,” in which the 360 degrees 135 = 450; subtracting 360 gives 301). are subdivided into 32 points. (The the correct answer of 090 degrees. If your compass is the top-read- older “point system” is included for The concept of relative bearings ing type (see Chapter 2) you can historical interest only, and is not is fundamental in the practice of read the reciprocal bearing directly. used in this text.) navigation. The concept should be Reciprocals can also be read from a An object 45° off the bow on the thoroughly understood. The three convenient compass rose. starboard side (broad on the star - direction systems will be linked Bearings, whether with respect board bow in the older system) together in the use of the magnetic to true north (true bearings) or mag- would have a relative bearing of (or compass and the practice of pilot- netic north (magnetic bearings), are would bear) 045 R (here the “R” ing. all bearings from the vessel to an denotes “relative”). If the object object. Reciprocal bearings are were 45° off the bow on the port RECIPROCAL BEARINGS bearings from an object to the ves- side, it would have a relative bear - Finally, we conclude with brief sel. ing of: 360 - 045 =315R. A vessel mention of reciprocal bearings. A

1-15 1 Introduction to Coastal Navigation

SELECTED REFERENCES

Bowditch, N., (1995). American Practical Navigator; Naval Training Command, (1972). A Navigation An Epitome of Navigation, Pub No. 9, Defense Compendium, NAVTRA 10494-A, U. S. Mapping Agency, Bethesda, MD. Government Printing Office, Washington, DC.

Brogdon, B., (1995). Boat Navigation for the Rest of Norville, W., (1975). Coastal Navigation. Step by Step, Us: Finding Your Way by Eye and Electronics, International Marine Publishing Co., Camden, ME. International Marine, Camden, ME. O’Malley, M., (1990). Keeping Watch, a History of Budlong, J. P., (1977). Shoreline and Sextant, Practical American Time, Viking Penguin, New York, NY. Coastal Navigation, Van Nostrand Reinhold Co., New York, NY. Queeney, T. E., (1986). “The Wandering Magnetic Pole,” Ocean Navigator, No. 9, pp. 3 et seq. Cline, D.A., (1990). Navigation in the Age of Discovery, Montfleury, Inc., Rogers, AR. Saunders, A. E., (1987). Small Craft Piloting and Coastal Navigation, Alzarc Enterprises, Departments of the Air Force and the Navy, (1983). Air Scarborough, Ontario, Canada. Navigation, AFM 51-40, NAVAIR49, 00—80V— 49, U. S. Government Printing Office, Washington, Schofield, Admiral B.B., (1977). Navigation and DC. Direction, The Story of HMS Dryad, Kenneth Mason, Homewell, UK. Eyges, L., (1989). The Practical Pilot: Coastal Navigation by Eye, Intuition, and Common Sense, Schufeldt, H. H., Dunlop, G.D., and A. Bauer, (1991). International Marine, Camden, ME. Piloting and Dead Reckoning, 3rd edition, Naval Institute Press, Annapolis, MD. Kals, W.S., (1972) Practical Navigation, Doubleday & Company, Inc., Garden City, NY. Sobel, D., (1995) Longitude the True Story of a Lone Genius who Solved the Greatest Scientific Problem Kielhorn, W. V., (1988). A Piloting Primer, privately of His Time. Walker and Company, NY. printed, Naples, FL. Tver, D. F., (1987). The Norton Encyclopedia Maloney, E. S., (1985). Dutton’s Navigation and Dictionary of Navigation, W.W. Norton & Piloting, 14th edition, Naval Institute Press, Company, New York, NY. Annapolis, MD. U.S. Coast Guard Auxiliary, (1983). Coastal Piloting, Markell, J., (1984) Coastal Navigation for the Small U.S. Coast Guard Auxiliary National Board, Inc., Boat Sailor, Tab Books Inc., Blue Ridge Summit, Washington, DC. PA. Wright, F.W., (1980). Coastwise Navigation, Jason Ministry of Defense (Navy), (1987). Admiralty Manual Aronson Inc., New York, NY. of Navigation, BR 45 (1), Volume l, Her Majesty’s Stationary Office, London, UK.

Moody, A. B., (1980). Navigation Afloat, A Manual for the Seaman, Van Nostrand Reinhold Co., New York, NY.

1-16 Chapter 2

THE MARINE MAGNETIC COMPASS

“Truth lies within a little and certain compass, but error is immense.”

–Henry St. John, Viscount Bolingbroke, Reflections upon Exile (1716)

INTRODUCTION determine compass courses from his second chapter of the true courses. TAdvanced Coastal Navigation Compass adjustment refers to (ACN) Course explores the marine the process of adjusting small mag- compass and its use. The compass nets contained in the compass to is one of the simplest and most use- remove as much error as possible. ful navigation instruments to be Many modern textbooks devote considerable space to compass carried aboard a vessel. Arguably, a adjustment, and the reader may competent navigator, well-found wonder why this topic is covered vessel, up-to-date charts, timepiece, only briefly here. The reason is and a good compass are the only simple. Although compass adjust- real requirements for a safe and ment is not impossibly complex, it efficient voyage. Columbus was is not trivial and needs to be done able to do this even without good right! Professional compass charts or timepieces! adjusters are available at reasonable This chapter provides a brief cost to perform this service and, history of the compass, a discussion unless the mariner is willing to of the types and parts of the modern devote a substantial amount of time compass, an exposition of the prin- and intellectual effort, this is a job ciple of operation of the compass, best left to experts. In addition to and finally, a discussion of possible adjusting the compass, a profes- compass errors and their measure- sional can provide good advice on ment so that the mariner can com- the placement of the compass, ship- pensate for these errors and steer board electronics, and other gear correct courses. In particular, this that may affect the compass. For chapter reviews so-called TVMDC those who disagree with this assess- computations, named for the ment and have the time and inclina- sequence True-Variation-Magnetic- tion to master the intricacies of Deviation-Compass that is used to compass adjustment, a brief

2-1 2 The Marine Magnetic Compass

description is included. Moreover, By the time of Columbus, the may revolutionize this field. (Gyro- the references included at the end of compass was well developed and scopes are not discussed in this text, this chapter provide a useful start- there is evidence (from the diaries as these are not presently available ing point for home study. of Columbus) that the phenomenon at reasonable cost to the typical The material in this chapter is of magnetic variation was at least boater.) not difficult. But teaching experi- partially understood. By the early During the mid-1920s, an elec- ence indicates that considerable 1700s, charts showing the locations tronic compass—termed a fluxgate practice is required in order to of lines of equal variation (isogonic compass—was developed for air- rapidly and reliably solve the prob- lines) were available. Likewise, craft to provide better directional lems discussed in this chapter. compass deviation, an important information in turns and during subject discussed below, was under- maneuvers. In recent years, this stood in qualitative terms at about ST G A U O technology has become available at A C R

S D What You Will U

A U Y this same time, although practical X R ILIA learn in this a reasonable cost to the mariner, means for compensating for devia- and for this reason is given passing Chapter tion were not developed until 1801 mention. J The “anatomy” of a by Captain Matthew Flinders (from The “electronics revolution,” a compass which the Flinders bar used in com- phrase used frequently in this text, pass adjustment takes its name). J Compass types also includes directional systems. The modern liquid-filled com- Outputs from a fluxgate compass J Compass deviation and pass, similar to those used on yachts can be “processed” by a wide vari- its measurement today, dates back to the period 1850 ety of computer systems and used J TVMDC calculations to 1860 when it was developed and for automated steering (autopilots), patented by E. S. Ritchie of Boston, J Compass errors and navigational computers (e.g., to Massachusetts. (The company compute current set and drift, as founded by Ritchie is still in busi- discussed in Chapter 7). Yet more ness today.) Since that time, there sophisticated developments are BRIEF HISTORY have been evolutionary rather than likely in the near future. The exact origin of the compass revolutionary developments in the For all these newer develop- is lost in antiquity. Although some magnetic (mechanical) compass. ments, the traditional magnetic accounts claim that the compass For example, new lightweight compass remains one of the most was invented well before the birth materials are used for compass important navigational instruments, of Christ (Hewson, 1983), docu- cards, improved magnets are avail- as evidenced by the fact that even mentary evidence of its use in able, and many other incremental the most sophisticated ship or air- Europe and China dates back only improvements have been made to to approximately 1100 AD (Aczel, increase the accuracy, stability, and craft in service today still has at 2001; Bowditch, 1995; Collinder, utility of the magnetic compass. least one magnetic compass aboard. 1955; Jerchow, 1987). (Incidental- Elmer Sperry, an American, and Its relative simplicity, reliability, ly, by convention the early Chinese Anschutz-Kampfe, a German, dur- and lack of dependence on electrical compasses were said to point south, ing the early part of the 20th centu- power sources will probably ensure as this was considered a more noble ry developed the modern gyrocom- its survival well into the future. aspect.) The modern compass card pass, an instrument capable of indi- (as opposed to needles used on the cating true rather than magnetic PARTS OF THE COMPASS earliest compasses) apparently orig- north. Gyroscopes were widely Over the years, the marine mag- inated with Flavio Gioia of Amalfi used in naval and merchant ships netic compass has evolved into a in southern Italy sometime around since the end of World War I. functional, easy-to-read, conve- 1300 AD (Collinder, 1955), Heretofore, gyroscopes have been nient, and relatively inexpensive although this is questioned by some electromechanical devices, but laser navigational instrument. The damp- (Aczel, 2001). gyros are now in development that ing system of a modern, spherical,

2-2 The Marine Magnetic Compass 2

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PowerDamp Stabilizer Hardened DirectiveForce® Steel Pivot Magnets

Brass Counterbalance Triple Cup Hardened Steel and Sapphire Sapphire Jewel Pivot Jewel

L PHOTO COURTESY OF RITCHIE NAVIGATION FIG. 2-1–Damping System of Modern Compass liquid-filled marine magnetic com- and W) are also indicated on the mariners. Fortunately, mariners pass is shown in Figure 2-1. In this dial. Arrows or other marks are have rediscovered the joy of num- compass, a lightweight dial or com- sometimes used to designate the bers and the older point system is pass rose is graduated in degrees intercardinal points (e.g., NE, SE, now only of historical interest. (If increasing in a clockwise direction SW, and NW). Older compasses you have such a compass, mount it from 000 degrees to 359 degrees to were traditionally graduated in the in your den, not on your boat!) indicate the compass heading. The mariner’s “point” system, men- Attached to the dial are the increments shown on the compass tioned in Chapter 1, in which the “north-seeking” compass magnets. dial can be 1 degree, 2 degrees, or, circle was divided into 32 compass The dial is supported on a jeweled more typically for compasses used points, each of 11.25 degrees. bearing, which turns on a pivot. In on small vessels, 5 degrees. (Stud- These are named, in clockwise turn, the pivot is mounted in a gim- ies conducted just prior to World order from north: north, north by bal system, designed to keep the War II indicated that graduations east, north-northeast, northeast by dial level with the horizon if the every 5 degrees were significantly north, northeast, northeast by east, vessel pitches or rolls. Fastened to easier to read than finer graduations east-northeast, east by north, east, the gimbal is one (or more) lubber’s and, in practical terms, nearly as etc. Naming these points, termed line(s). The lubber’s line (also accurate.) Numbers are typically “boxing the compass,” was an termed lubber line [Moody, 1980]) spaced every 30 degrees, and the unpleasant and confusing task used is the index mark against which the cardinal points (north, south, east, historically in hazing rituals for dial graduations are read to deter- and west, or abbreviated N, S, E, midshipmen and other would-be mine the direction of the vessel rel-

2-3 2 The Marine Magnetic Compass

the magnets, so as dentally, the wires to the compass to reduce wear on light should be twisted to minimize the pivot. The ultra- magnetic effects.) lightweight dials in Many compasses come with a use can be damped hood (adjustable on some models) with fluids that are to reduce glare and improve read- not viscous (thick), ability. Removable protective cov- a combination that ers are also recommended if the provides stability compass is installed in a location and accuracy with- where it is exposed to the elements. out a tendency to “overshoot” and COMPASS DIAL DESIGN oscillate as the ves- There are two principal designs sel is turned to a for the compass rose or dial. These new heading. The are discussed briefly below. compass also con- J The first design is termed a top- tains an expansion reading compass (also a flat diaphragm to allow card compass by some manu- for the expansion facturers). With this design, the and contraction of mariner reads the heading or PHOTO PHOTO COURTESY OF RITCHIE NAVIGATION the damping fluid L bearing “across the card.” The with temperature or pressure lubber’s line is located behind FIG. 2-2–Combination Front and Top changes. A fill plug is used to the card. The numbers that indi- Reading Compass replace or “top off” the damping cate heading or bearing increase fluid. (It is important that there are in a clockwise direction—a cor- no air bubbles in the compass ative to that of the card. The lub- rect geometric representation. A fluid.) ber’s line (or principal lubber’s line heading of 030 degrees is to the if there are more than one) should The bowl is supported by a case be aligned with the fore-and-aft or holder, generally axis of the vessel. called a binnacle. The gimbals, card, and magnets Somewhere near the are enclosed in a bowl with a clear, bowl are found the transparent, hemispherical glass (or compensating mag- plastic) top, within which the card nets, used to adjust and gimbals are free to rotate inde- the compass to pendently of the attitude of the con- compensate for the tainer. The top (dome) may be vessel’s magnetic impregnated with inhibitors to environment. reduce any discoloration of the card Most compasses or fluid from ultraviolet radiation are lighted for night and may also magnify the readings, use. A low intensity so that the apparent card size is red lamp is pre- larger. The bowl is filled with a ferred to avoid or nonfreezing liquid to damp (slow minimize adverse

down) the motion of the dial for effects on the night PHOTO COURTESY OF RITCHIE NAVIGATION increased stability and to support vision of the helms- L much of the weight of the card and man or crew. (Inci- FIG. 2-3–Fluxgate Compass with Digital Readout

2-4 The Marine Magnetic Compass 2

right of a heading of 000 slightly more con- degrees, for example, and the fusing and requires a compass provides the same rep- bit more practice resentation. If the helmsman is before familiarity is asked to turn to 030 degrees assured. An inexpe- from a heading of north, it is rienced helmsman, clear that this must be a turn to asked to come to a the right. It is also relatively heading of 030 easy to read compass bearings degrees from north, over this compass dial. The could glance at the compass dial itself is unob- front-reading dial structed through 360 degrees, and see that this although its placement aboard heading is to the left, the vessel usually limits this and, therefore, begin range. A top-reading compass is a turn in the wrong installed forward of the steering direction before dis- mechanism and beneath the covering the error. From the perspective PHOTO COURTESY OF RITCHIE NAVIGATION helmsman’s eye level. L of ease of interpretation, the top- J The second design is termed a FIG. 2-4–Fluxgate Compass reading compass dial is greatly to front-reading compass (also with Digital Readout be preferred. But, it is also impor- called a direct reading compass tant to consider how the compass and Conventional Displays by some manufacturers). This will be viewed once installed. In the compass dial design is typical of typical powerboat installation (and most aircraft compasses and is in sailboats where the compass bin- graduated in 5-degree increments also used for marine compasses. nacle is integrated with the wheel), and a top-reading dial that omits a With this design, the lubber’s the compass is located on a panel numerical display of degrees. Some line is in front of the dial and immediately in front of the helms- compasses also include inclinome- indicates the direction toward man, and so a top-reading compass ters, to measure the angle of roll of which the vessel is heading. is easy to see. However, in a typical the vessel. However, the dial is graduated in light aircraft the compass is Yet other compass displays, typ- a counterclockwise direction. installed at the top of the cockpit ically those used with fluxgate Thus, for example, the 30-degree (where it is least likely to be affect- compasses, feature a direct digital graduation on the front-reading ed by magnetic interference from readout of the heading. Figure 2-3 dial is located to the left of 000. radios or other electronics), at or shows the display unit of a fluxgate This apparent reversal in direc- above the eye level of the pilot, compass with digital readouts. Dig- tion is made necessary because necessitating a front-reading ital readouts are generally shown to the lubber’s line is located in design. Similarly, in certain sail- the nearest degree, rather than 5 front of the dial. The mounting boats the compass is mounted on degrees as is common on conven- of a front reading compass usu- the outer cabin bulkhead—nearly at tional compasses. Although many ally precludes its use for obtain- eye level for the helmsman seated prefer digital displays, these also ing bearings. This is not a real several feet away—and a front- have limitations or disadvantages. detriment, since a hand-bearing reading compass is necessary. (For example, it is impossible to compass, discussed in Chapter 4, Some compass designs, such as take a bearing on any object that is is a ready substitute. that shown in Figure 2-2, combine not aligned with the vessel’s head- Either design correctly shows both types of compass displays in ing.) Moreover, the digital display the vessel’s actual heading, but the one unit. The model shown in Fig- provides less “situational aware- front-reading compass design is ure 2-2 shows a front reading dial ness” for the helmsman than does

2-5 2 The Marine Magnetic Compass

the top-reading compass. This dis- sels. On average, small vessels are meet, but this is sometimes more of advantage can be overcome if the significantly more “lively” than a problem in smaller craft. Consult fluxgate compass also incorporates a compass adjuster and read the

a conventional dial, as is shown in ST G owner’s manual (or product litera- A U O A C R

S D praCtiCal U

A U Y R Figure 2-4, which provides precise X A ture) for advice on this important ILI tip heading information and situational topic. awareness. The display shown in Carry at least one spare com- There are several types of com- Figure 2-4 cannot be used to take pass mounts, each with advantages bearings, however. pass aboard—if only a hand- and disadvantages. Compass Finally, some compasses— bearing compass. This is mounts include the bracket mount called telltale compasses—should (fast and versatile installation—par- be mounted in an upside down cheap insurance. According ticularly for angled surfaces), flush (overhead mount) position. The to Morrison (1942), early mount, deck or binnacle mount, and telltale compass is usually installed bulkhead/dash mount. in the ceiling of the navigator’s mariners carried plenty of berth, so that the navigator can read spare “needles” (compass PRINCIPLE OF OPERATION: the vessel’s heading when not on DEVIATION duty. Overhead-mount compasses needles) aboard. Ferdinand The modem magnetic compass are a favorite of “single handlers,” Magellan reportedly had is highly sensitive and is able to and can double as a backup com- align itself with weak magnetic pass. (Incidentally, the practice of thirty-five spare needles on fields, such as the earth’s magnetic “single-handing” (voyaging for his ship! field. The magnets underneath the long distances with only one person compass card will align with the aboard) is unsafe, a violation of the magnetic field and indicate direc- navigation rules (a proper lookout larger vessels. Larger and more tion relative to this field. But, the cannot be maintained by a sleeping expensive compasses have better magnetic field aboard a ship is actu- helmsman), and is strongly discour- gimbals and have larger and easier ally a combination (resultant) of aged.) to read dials. The saying “bigger is multiple magnetic fields—that of Whatever display is chosen, it is better” almost always applies to the the earth and those of the vessel and important that the numerals on the selection of a compass. Finally, it is its equipment. dial be large and easy to read. recommended that a vessel be Were the earth’s magnetic field Ornate displays, such as were found equipped with at least two com- acting alone, the compass would on older marine compasses, are less passes as a precaution against com- indicate direction in the magnetic readable than the simple, clean pass failure. A handheld compass direction system—that is, the com- designs of today. (discussed in Chapter 4) can serve pass would point in the direction of as a backup. magnetic north. (Please refer again BRIEF ADVICE ON to Chapter 1.) Determination of the COMPASS SELECTION COMPASS MOUNTING direction with respect to true north The best advice on compass Ideally, the compass should be would involve nothing more than selection is not to be miserly. With mounted where it can easily be adding or subtracting the local mag- compasses, as with other items of read, is protected from the ele- netic variation from the indicated equipment, you generally get what ments, and is free of any magnetic compass direction (more below). you pay for. Because the compass is influences aboard the vessel (see However, the magnetic field such an important navigational below). The lubber’s line should be aboard a vessel is not solely due to instrument, it is essential that it be precisely aligned with the fore-and- the earth’s magnetic field. Ship- of high quality. Incidentally, this aft axis of the vessel. On larger ves- board electronics, windshield wiper comment also applies to small ves- sels, these requirements are easy to motors, compressed-gas horns,

2-6 The Marine Magnetic Compass 2

The difference between these is

ST G A U O WARNING A C R

termed deviation. There are actual- S D U

A useful tip U Y X R Do not plaCe ly three “norths” that the mariner ILIA Metal oBJeCts need be concerned with: true north, It is important to empha size magnetic north, and compass north. near CoMpass Simply put, deviation is the differ- that deviation varies with the Do not use the area near ence between the direction that the vessel’s head ing. When con- compass actually points and the the compass as a resting direction that it would point if there verting a relative bearing to a place for metal objects, were no local magnetic fields true or magnetic bearing, aboard the vessel. Although statis- such as flashlights, cameras, tics on the deviation of uncompen- novices often make the mis- kitchen utensils, sated compasses aboard recreation- take of applying the devia- certain plotting instruments, al boats are not available, these deviations could be quite large, say tion appropriate to the rela- and even metal sunglasses. 10 degrees to 15 degrees, and possi- tive bearing rather than the These can affect the bly even more. It is precisely because of the vessel’s heading. Be careful accuracy of the compass deviation caused by the vessel’s not to make this error! readings and cause serious magnetic field that correcting mag- nets are found in all good compass- way to minimize possible course errors! es. A skilled compass adjuster can sources of deviation. move the adjusting magnets so as to J Second, follow the directions tachometers, electrical motors, tele- remove most of the deviation nor- given below to determine the vision sets, and other equipment mally caused by the vessel’s mag- also generate magnetic fields. netic field. (A good compass compass deviations on various Indeed, flashlights, camera light adjuster can also serve as a consul- headings. If these deviations are meters, tools, and even some tant on compass placement and can “acceptable” (a judgement call), kitchen utensils can also affect the advise the mariner how to stow then use the “For/Steer” table compass. (For skeptics, a simple other gear to minimize deviation in (see below) directly and do not experiment proves this and is high- the first place.) undertake compass adjustment. ly instructive. For example, note the If not, then either call a profes- Compass Adjustment— compass reading, then place a sional compass adjuster or use flashlight near the compass and A Brief Digression the following procedure. Read observe how the reading changes.) As noted above, the use of a the directions for compass The vessel itself—particularly steel professional compass adjuster is adjustment (contained in the vessels—may have magnetic fields recommended. This material is owner’s information supplied oriented in a variety of ways. The added for those interested in a do-it- with the compass) again to vessel’s magnetic field may even yourself project. The material in ensure that you are thoroughly depend upon the direction the ves- this section is adapted loosely from familiar with the procedure and sel was facing when it was con- the former AUXNAV specialty the location of the two adjusting structed or last laid up for the win- course (COMDTPUB P16798.16A) screws. ter (Kielhorn and Klimm, 1978). documentation: J Third, make the following work- These additional fields also affect J First, carefully read the direc- ing tool. Take a sturdy card- the compass, with the result that the tions that come with the vessel’s board and a dowel (a pencil will compass heading of the vessel may compass and ensure that the do). Make a hole for the dowel differ from its magnetic heading. compass is mounted in such a in the center of the cardboard

2-7 2 The Marine Magnetic Compass

and draw a straight line across make another compass devia- the magnetic heading to a true the cardboard through the cen- tion card as described below. heading. However, unlike variation, ter. Select a calm, sunny day which depends solely on the ves- For other perspectives, read (with minimal traffic to avoid) sel’s position, deviation varies with through appropriate sections of for this evolution, in mid-morn- the vessel’s heading. Therefore, it is Brogden (1995), Eyges (1989), ing or afternoon when the sun Denne (1979), and Kaufman (1978) necessary to use the deviation will cast a shadow on the dowel. included in the references at the end appropriate to the vessel’s compass Take the boat out and maintain a of this chapter. heading before it can be used to constant heading of north as It is seldom the case that all the convert to the correct magnetic indicated by the compass. Place effects of this magnetic field can be heading. Although, theoretically, the dowel in the hole and rotate compensated for by the adjusting this deviation could be different for the board until the shadow of the magnets, and usually a small resid- each possible heading, in practice dowel falls on the line. Now ual deviation (say 2 degrees to 4 the deviation is determined for each turn the boat in the opposite degrees, but sometimes more) 15-degree or 30-degree heading direction until the shadow falls remains after adjustment. The increment; then these values are on the other side of the line. You mariner has two options for dealing interpolated to estimate the devia- will have turned 180° (turn and with residual deviation. The first is tion on intermediate headings. This steady on the reciprocal course simply to ignore any residual error process of determining the devia- promptly because the sun moves and effectively compensate for its tion on various headings is termed about 1° in 4 minutes). Read the presence by fixing the vessel’s posi- swinging ship or swinging the com- compass on this heading. Most tion more often. As a rough rule of pass and is discussed below. probably, it will not read exactly thumb, an unrecognized error of 1° 180°. Now, use a stainless steel means that a vessel would be SWINGING SHIP or brass screwdriver on the approximately 1 mile off course Normally, professional compass athwartships (N-S adjustment) (termed cross-track error) if it adjusters will swing ship as part of adjusting screw; remove half of traveled a distance of 60 miles. their services to compensate the the difference between the com- Table 2-1 shows the cross track compass and provide a table of pass reading and 180°. For error as a function of the distance deviations to the mariner. In such example, if the compass were to traveled and the angular error or cases, the mariner will probably read 170°, use the adjusting residual deviation. For short dis- wish to spot-check this table peri- screw to set the compass to tances, small angular errors are odically to verify its continuing 175°. Turn back on the original practically insignificant and can accuracy. However, the procedures course until the shadow falls on sometimes be ignored. However, (discussed below) are the same the other side and take out half for longer distances or in conditions whether the entire deviation table is of the difference between the of poor visibility (which would pre- being prepared or individual values compass reading and 000°. vent detection and identification of are being spot-checked. J Fourth, repeat the process until landmarks, fixed aids to navigation In brief, the procedure for you can’t remove any more (ATONs), or buoys), simply ignor- swinging ship is to steady on a error. ing deviation cannot be recom- known compass course and then J Fifth, do the same thing on east- mended. take bearings on a distant object or west headings. Head 090° by The second, and generally range. The vessel is positioned so compass, align the shadow with preferable, option is to measure the that the magnetic bearing to the the line, turn 180°, read the compass deviation, and use this object to be observed is known. The compass, and take out half the measured value to correct the compass bearing is read directly, or error with the other (E-W) observed compass heading to a converted from a relative bearing adjusting screw. When no fur- magnetic heading in the same man- obtained using a pelorus and com- ther improvements can be made, ner as variation is used to “correct” pared with the object’s known mag-

2-8 The Marine Magnetic Compass 2

DistanCe anGular error (DeGrees) traVeleD Miles 1.00 1.50 2.00 2.50 3.00 4.00 5.00 7.50 10.00 1.00 0.02 0.03 0.03 0.04 0.05 0.07 0.09 0.13 0.18 2.00 0.03 0.05 0.07 0.09 0.10 0.14 0.17 0.26 0.35 3.00 0.05 0.08 0.10 0.13 0.16 0.21 0.26 0.39 0.53 4.00 0.07 0.10 0.14 0.17 0.21 0.28 0.35 0.53 0.71 5.00 0.09 0.13 0.17 0.22 0.26 0.35 0.44 0.66 0.88 6.00 0.10 0.16 0.21 0.26 0.31 0.42 0.52 0.79 1.06 7.00 0.12 0.18 0.24 0.31 0.37 0.49 0.61 0.92 1.23 8.00 0.14 0.21 0.28 0.35 0.42 0.56 0.70 1.05 1.41 9.00 0.16 0.24 0.31 0.39 0.47 0.63 0.79 1.18 1.59 10.00 0.17 0.26 0.35 0.44 0.52 0.70 0.87 1.32 1.76 12.50 0.22 0.33 0.44 0.55 0.66 0.87 1.09 1.65 2.20 15.00 0.26 0.39 0.52 0.65 0.79 1.05 1.31 1.97 2.64 17.50 0.31 0.46 0.61 0.76 0.92 1.22 1.53 2.30 3.09 20.00 0.35 0.52 0.70 0.87 1.05 1.40 1.75 2.63 3.53 22.50 0.39 0.59 0.79 0.98 1.18 1.57 1.97 2.96 3.97 25.00 0.44 0.65 0.87 1.09 1.31 1.75 2.19 3.29 4.41 27.50 0.48 0.72 0.96 1.20 1.44 1.92 2.41 3.62 4.85 30.00 0.52 0.79 1.05 1.31 1.57 2.10 2.62 3.95 5.29 35.00 0.61 0.92 1.22 1.53 1.83 2.45 3.06 4.61 6.17 40.00 0.70 1.05 1.40 1.75 2.10 2.80 3.50 5.27 7.05 45.00 0.79 1.18 1.57 1.96 2.36 3.15 3.94 5.92 7.93 50.00 0.87 1.31 1.75 2.18 2.62 3.50 4.37 6.58 8.82 60.00 1.05 1.57 2.10 2.62 3.14 4.20 5.25 7.90 10.58 70.00 1.22 1.83 2.44 3.06 3.67 4.89 6.12 9.22 12.34 80.00 1.40 2.09 2.79 3.49 4.19 5.59 7.00 10.53 14.11 90.00 1.57 2.36 3.14 3.93 4.72 6.29 7.87 11.85 15.87 100.00 1.75 2.62 3.49 4.37 5.24 6.99 8.75 13.17 17.63 L TABLE 2-1–Cross-track error as a function of distance traveled and residual deviation or other angular helm error. netic bearing, and the deviation is such as a prominent object or range. when conditions are nearly ideal, in calculated. Professional compass The object(s) selected for observa- calm waters and in good visibility. adjusters often use the sun for tion should be a good distance away The need for good visibility is obvi- observation, but most mariners are (e.g., 6 miles) to minimize parallax ous. The reason why calm waters unfamiliar with celestial navigation error in the calibration. It is impor- are preferred is to simplify steady- and elect to use something simpler, tant that swinging ship is done ing the vessel on a compass heading

2-9 310

L FIG. 2-5–The range formed by the spire in New Bedford and the Butler Flats light bears approximately 310 degrees. The Marine Magnetic Compass 2

and reading the compass. Experi- degree increments or bearings are from the south, these two objects ence shows it can take several hours not easily read throughout 360 are exactly in line (one behind the to swing ship, so the exercise degrees, then it is necessary to use a other or in range) on a bearing of should be planned with sufficient pelorus as well (discussed below). 310 degrees true from the vessel to time allowance to be completed the objects. The bearing can be read within the daylight hours. DIRECT OBSERVATION from the chart as discussed in The procedure for swinging ship USING A RANGE Chapter 3, but take the answer as depends upon the compass to be The easiest method for swinging given for the present. The variation examined and the ability to take ship, if circumstances permit, is to in this area, read from the nearest bearings on objects not directly use a range and read the compass compass rose, is approximately 015 aligned with the fore-and-aft axis of directly. A range consists of two degrees west; so the magnetic bear- the vessel. If the compass is gradu- charted objects that can be viewed ing of this range would be 310 + ated to the nearest degree, and and aligned from a distance. For 015 = 325 magnetic. (See below for designed and located so that an example, consider the spire in New a handy rule to remember whether unobstructed view is possible Bedford and the Butler Flats Light to add or subtract variation.) throughout all 360 degrees, then located south and east of New Bed- Suppose that the vessel were only a compass is necessary. (This ford shown in the l210-Tr chart and steadied on a compass course of is likely to be the case for relatively in Figure 2-5. Both of these objects 000 degrees (compass north) while few boats.) If, as is more common, are likely to be prominent and rela- the vessel was somewhere south of the compass is graduated only in 5- tively easy to identify. Approaching the line drawn on the chart. (This

Deviation (Degrees) 8 8 7 7 6 6 5 E 5 E 4 4 3 3 2 2 1 1 0 0 -1 -1 -2 -2 -3 -3 -4 -4 W -5 -5 W -6 -6 -7 -7 -8 -8 0 30 60 90 120 150 180 210 240 270 300 330 360 Compass Heading (Degrees)

Vessel: Auxiliary Facility 273007, All Radios and Electronics on, Data from 09-30-89 for Compass at Lower Helm Station. L FIG. 2-6–A Plot of Compass Deviations

2-11 2 The Marine Magnetic Compass

would, of course, be evident from are any “anom- the vessel because the Butler Flats alous” results Light would be to the right of the that do not fit the spire. At the precise instant that the pattern. Overall, light and the spire appeared to be in the line drawn line, the magnetic bearing of the through the mea- range would be 325 degrees from sured deviations the vessel. At this same instant, sup- should appear as pose that the compass bearing of a smooth curve the range (read over the compass) (actually a mix- was 323 degrees (while the com- ture of trigono- pass heading was 000 degrees). The metric functions deviation on this heading is the dif- for those techni- ference between the magnetic bear- cally inclined) L ing, 325 degrees, and the bearing free of “bumps” or observations FIG. 2-7–Relative Bearing read from the compass, 323 that appear discrepant. Such a curve degrees, or 2 degrees. But, is it 2 is drawn in Figure 2-6 and appears A pelorus consists of a graduated degrees east, or 2 degrees west? It generally to confirm the adequacy compass-like dial (l-degree incre- ments) and sighting vanes that can can, of course, be worked out from of the measurements, although the deviations on some headings, such be rotated around the dial to take first principles (refer to Chapter 1), as 240 degrees, should be bearings. Unlike the “north-seek- but it is easy to remember the sim- rechecked. (In Figure 2-6 easterly ing” compass dial, however, the ple phrase, “compass least, error deviations are shown with a plus position of the dial on the pelorus east.” That is, if the compass bear- sign, and westerly with a minus does not change with the vessel’s ing is less than the magnetic bear- sign.) Additionally, the deviations heading. The pelorus is mounted ing (as it is in this case, 323 degrees on some headings are relatively and the dial fixed so that the 000- is less than 325 degrees), then the large (5 or 6 degrees), so the com- degree mark on the dial is pointed deviation is “east.” (If not, then the pensation is far from perfect. (A so as to be parallel to the vessel’s error would, of course, be “west.”) more technical analysis, omitted bow, precisely aligned with the Thus, in this example, the estimated here, suggests that improved com- fore-and-aft axis of the vessel. compass deviation on a heading of pensation is possible. However, the (Consult the directions supplied 000 degrees is 002 degrees east. To example is continued for illustrative with the pelorus for mounting confirm this result, the process purposes.) instructions and pay particular might be repeated and the average attention to the alignment proce- deviation noted. USE OF A PELORUS dure.) The pelorus should be It is convenient to use a work- As noted, most marine compass mounted so that the navigator can sheet, such as is shown in Table 2-2 installations do not permit direct easily view objects throughout a to record the observations. This reading of compass bearings full 360 degrees. worksheet contains directions as through a full 360 degrees. Addi- The bearings read from the well, which makes it handy to use. tionally, many compasses are grad- pelorus are relative bearings (refer The process is now repeated on a uated only to 2 degrees or 5 to Chapter 1), rather than compass compass heading of 015 degrees, degrees, rather than in 1-degree bearings. For this reason it is neces- 030 degrees, etc., until all observa- increments. If either of the state- sary to calculate the compass bear- tions are recorded. ments is true, it is necessary to ing from the simple equation: modify the procedure given above Ship’s heading + Object’s relative PLOTTING THE RESULTS for swinging ship. The most conve- bearing = Object’s compass bear- The results should be plotted on nient solution is to use a pelorus, ing. Figure 2-7 illustrates this equa- a sheet of graph paper to see if there sometimes called a dumb compass. tion.

2-12 The Marine Magnetic Compass 2 Light notes or other source. This worksheet is designed to be used for determination of deviation where a range is available and can be sighted over the com- pass. The true bearing of the range can be determined from a nautical chart, List, Compass deviation should be determined every 15 degrees of heading, generally starting at 000 degrees. The “notes” section should identify the electronics on board, and operating at the time of the calibration. It is suggested that two tables be prepared, one with all electronics on, and the other with electronics off. Altair 5 2E 3E 5E 6E 5E 5E on this heading Deviation Column 3, estimated Deviation is If Column 4< Columns 3 and 4 Deviation “West” “East,” Otherwise Vessel: 4 per 323 322 320 319 320 320 Range as Compass of range Compass Direction Bearing of Difference Between Read Over LDM Determined from Nautical Chart or Other Source Determined from Nearby Compass Rose -East +West, +/- Variation, True 3 of 325 325 325 325 325 325 Above range Direction Magnetic Calculation Taken From Taken naViGator: 310 015 West 325 Magnetic 2 000 015 030 045 060 075 Directly Read From per per Compass Lubber’s Lubber’s Line Vessel’s head Vessel’s Spire in Spire New and Bedford Butler Flats Light and VHF Radio Loran, ON Radar, 10-15-99 1 entry Where Obtained ranGe: notes: true BearinG ranGe: of Variation: MaGnetiC BearinG ranGe: of Date: L 2-2 –Portion of TABLE Worksheet for Determination of Deviations from Range

2-13 2 The Marine Magnetic Compass

This extra step requires a slight check deviation at least a few times

ST G A U O A C R

S D praCtiCal tip: modification to the worksheet given U during the boating season, and

A U Y X R in Table 2-2 to prepare a compass ILIA use of a whenever a major voyage is deviation table. This modified CoMpass in planned. Deviation can change worksheet is shown in Table 2-3. To repeataBle MoDe whenever new electronic gear is clarify by example, suppose that a brought aboard (or moved), the compass deviation table is being Get in the habit of entering vessel is laid up for the winter, or someone inadvertently leaves prepared using the same range as compass headings in a log on given in Table 2-2. While on a com- a flashlight or camera near the pass heading of 015 degrees, the your trips—particularly compass. navigator sights over the vanes of when in good weather. If the Lightning strikes near the vessel the pelorus as the range is perfectly can also affect deviation, and the aligned. The navigator calls “mark- same trip is repeated in con- compass should be checked after mark-mark” to allow the helmsman ditions of reduced visibility, electrical storms have passed to make slight heading changes to through the area. these same compass head- bring the vessel back to the assigned 015-degree heading, and ings can be used—even if the DEVIATION ON INTERMEDIATE HEADINGS notes the relative bearing, say 307 compass has not been degrees. Alternatively, the helms- The deviation table consists of man sings out the vessel’s heading checked for deviation and entries spaced every 15 degrees or on hearing “mark-mark-mark,” and errors exist. This use of a every 30 degrees. Values for inter- the navigator notes this heading. mediate headings are obtained by navigation instrument is The compass bearing to the range interpolation. For example, if the is, therefore, 015 + 307 = 322 termed its use in repeatable deviation were 3 degrees east on a degrees (360 degrees will have to mode. If you return to the compass heading of 000 degrees be subtracted from this total if the and 0 degrees on a heading of 015 total exceeds 360). The deviation same readings of the com- degrees, it would be approximately on a compass heading of 015 pass (or other navigation 2 degrees on a heading of 005 degrees is, therefore, 3 degrees east degrees. Fancy formulas are not as in the earlier example (remem- instrument), you compensate warranted here; simply prorate the ber, compass least, error east). for its error. For an interest- deviation directly and round the calculation to the nearest degree. SPOT CHECKS ing discussion of this princi- To spot-check a previously pre- ple, refer to Brogden (1995). USE OF THE pared deviation table, all that is nec- DEVIATION TABLE essary is to take advantage of During a typical voyage there The deviation table is used for ranges that may appear near the are many opportunities for such two important purposes. First, it is vessel’s course. In this case, the spot checks, and these should be used to calculate the vessel’s actual helm is put over briefly to align the used to advantage. Throughout the magnetic heading when steering a vessel with the range, the compass known compass heading. Second, it navies of the world, it is common heading noted, and the deviation on is used to calculate the correct com- practice to check the compass at this heading calculated as discussed pass heading to steer to make good above. At a minimum, compass least once daily and to report the a desired magnetic heading. headings should be recorded in a results to the captain. For the aver- The deviation table solves the log (see sidebar) for later analysis age boater, it is not necessary to first objective directly. Refer to when the vessel is anchored or make such frequent or formal Table 2-4, for example, based upon docked. checks, but it is appropriate to the measurements discussed above.

2-14 The Marine Magnetic Compass 2 6 3E on this heading Deviation Column 5, estimated Deviation is If Column 4< Columns 4 and 5 Deviation “West” “East,” Otherwise Altair 5 of 325 Above range Direction Magnetic Calculation Vessel: 4 per 322 of range Compass Direction Bearing (3) Plus Relative (360° may have Vessel’s Head (2)Vessel’s From Taken Difference Between to be substracted) LDM Determined from Nautical Chart or Other Source Determined from Nearby Compass Rose -East) (+West, +/- Variation True 3 307 Read From range Pelorus relative Bearing of naViGator: 310 015 West 325 Magnetic 2 015 Directly Read From per per Compass Lubber’s Lubber’s Line Vessel’s head Vessel’s Spire in Spire New and Bedford Butler Flats Light and VHF Radio Loran, ON Radar, 10-15-99 1 entry Where Obtained ranGe: notes: true BearinG ranGe: of Variation: MaGnetiC BearinG ranGe: of Date: L 2-3 –Portion of TABLE Worksheet for Determination of Deviations Using Relative Bearings

2-15 2 The Marine Magnetic Compass

CoMpass to MaGnetiC MaGnetiC to CoMpass

(a) (B) (C) (D) Compass Magnetic heading Deviation heading Deviation 000 2E 000 2E 015 3E 015 3E 030 5E 030 4E 045 6E 045 6E 060 5E 060 5E 075 5E 075 5E 090 3E 090 3E 105 3E 105 3E 120 2E 120 1E 135 1W 135 1W 150 2W 150 2W 165 3W 165 2W 180 4W 180 4W 195 5W 195 5W 210 6W 210 6W 225 5W 225 5W 240 4W 240 4W 255 5W 255 5W 270 4W 270 4W 285 3W 285 3W 300 2W 300 2W 315 1W 315 1W 330 1E 330 1E 345 2E 345 2E 360 2E 360 2E L TABLE 2-4–Sample Deviation Table [All Values in Degrees(°)]

Suppose that the vessel’s compass heading to a magnetic heading is rected or magnetic heading corre- heading were 045 degrees. From often termed “correcting,” because sponding to a compass heading of the first two columns of this table this process removes (corrects for) 045 would be 045 + 006 = 051 (headed by the phrase “compass to the deviation error. A simple rule to degrees magnetic. Similarly, a com- magnetic”), the deviation corre- remember is “correcting add east,” pass heading of 030 degrees corre- sponding to a compass heading of meaning that in converting from a sponds to a magnetic heading (refer 045 degrees is 6 degrees east. The compass heading to a magnetic to Table 2-4) of 035 degrees. magnetic heading is the compass heading, easterly deviation is added Frequently, however, it is neces- heading plus or minus the devia- (westerly deviation is, therefore, sary to reverse the process. That is, tion. Converting from a compass subtracted). Using this rule, the cor- to find the appropriate compass

2-16 The Marine Magnetic Compass 2

course to steer to make good a par- on a 045 degree heading is approxi- which completes the deviation ticular magnetic heading. For this mately 6 degrees (5 + 10(6-5)/16 = table. Use the left half of the table task it is necessary to reverse the 5.625, rounded to 6). Therefore, the when correcting from compass to logic discussed above. For example, approximate deviation on a magnet- magnetic, and the right half when suppose that the mariner wants to ic heading of 045 degrees is 6 “uncorrecting” from magnetic to make good a course of 045 magnet- degrees east, as shown in Table 2-4. compass. (Unless the deviations are ic. What compass course should be When making interpolations, quite large, the two halves of the steered? From the above discussion, remember that deviations are only table are virtually identical and, in note that on a magnetic heading of measured to the nearest degree. A practice, these differences are often 035 degrees the deviation is 5 calculation is only as accurate as the neglected.) Incidentally, the data degrees east, whereas on a magnetic least accurate number, so round all presented in the right hand side heading of 051 degrees, the devia- interpolated numbers to the nearest (RHS) of Table 2-4 are often used tion is 6 degrees east. A simple degree. to make a simple “For/Steer” cor- interpolation (rounded to the nearest Continuing the process leads to rection card. Entries for this table degree) indicates that the deviation the results shown in Table 2-4, can be computed as follows. Sup-

NAME BRIEF DESCRIPTION REMARKS

Northerly Turning Error Applies principally when vessel is on north- Of principal concern to aircraft, but erly or southerly headings and the compass of relevance to all fast boats. Arises card is tilted with respect to the horizon. from magnetic dip. Phenomenon de - Effect is for compass to lag the turn, or scribed for northern hemisphere only. momentarily show a turn in the opposite direction when turning from north. In turns from south, the compass leads the turn, i.e., shows the vessel turning more rapidly than it actually is. The effect is greatest in a rapid, steeply banked turn.

Acceleration Error Also due to dip, this error is greatest on head- Effect greatest with vessels capable ings of east or west and zero on north or of large accelerations, e.g., speed south. If the vessel is accelerated on either of boats. Also seen with aircraft. Often these headings, the compass will indicate an observed when boat butts into head apparent turn to the north. When decelerating, sea, or planes down a swell while on the compass indicates a turn to the south. an east or west heading. A memory aid to remember the word “ANDS,” for Acceleration – North, Deceleration – South.

Oscillation Error Though listed as a separate error in some texts, this is actually a combination of the above errors. Results from erratic movements of the compass card caused by rough seas or abrupt helm changes. Helmsman has to “av- erage’’ out oscillations mentally for precise steering.

Heeling Error Of particular relevance to sailing vessels, this Adjusted for by heeling magnets on error arises from change in the horizontal some compasses.Adjustment is par- component of the induced or permanent tially a function of the magnetic magnetic fields at the compass due to rolling latitude of the vessel. or pitching of the ship. To a lesser extent heeling errors may be affected by the angle of plane of a powerboat.

note: See article by Kielhorn for details on some of these errors. L TABLE 2-5–Additional Compass Errors Which Arise if Vessel is not Straight and Level, and at Constant Speed

2-17 2 The Marine Magnetic Compass

pose that the mariner wishes to fol- courses to compass courses, and netic course. Since the variation low a magnetic heading of 000. vice versa. Although these calcula- is west, it is added to the true Reference to Table 2-4 shows that tions are quite simple, practice is course. The magnetic course is, on a magnetic heading of 000 the necessary to ensure familiarity. For therefore, 060 + 015 = 075. deviation is 2° east. Therefore, the this reason, the following text and J Second, from Table 2-4, the compass heading in this case would examples are given. deviation corresponding to a be 358. So, for a course of 000 It is often necessary to convert magnetic heading of 075 magnetic, the mariner should steer from true to compass headings or degrees is 5 degrees east. From 358. Under the heading “For” is bearings. As discussed in later the simple rule to add west in placed the magnetic course 000 and chapters, courses are laid out on this sequence, it follows that a under the heading “Steer” is placed nautical charts and, typically, mea- 5-degree easterly deviation 358. The table is completed to cre- sured with respect to true north. But would be subtracted. to undertake the voyage, the navi- ate a For/Steer card that is normally J Third, from the above steps, gator needs to determine how to posted next to the compass. the required compass course is convert this true course to a com- 075 - 005 = 070 degrees. pass course to steer. The overall COMPASS CALCULATIONS The important points to remem- sequence for this conversion is to Navigators need to become ber are the sequence of calculations start with the true course, add or familiar with the calculations nec- and whether to add or subtract vari- subtract variation to calculate a essary for converting from true ation and deviation. If all else fails, magnetic course, and then again reference to the nearest compass add or subtract deviation to calcu- ST G A U O A C R rose on the nautical chart will

S D Variation anD U

A U Y R late a compass course: True, Varia- X ILIA DeViation enable you to figure out whether to tion, Magnetic, Deviation, Com- add or subtract variation (or devia- pass, or as it is sometimes said, Variation is the angular tion). TVMDC. In addition to learning Sometimes it is necessary to difference between true the sequence of calculations, it is reverse the process, and convert useful to have a handy rule to and magnetic north—see from compass to true. For example, remember whether to add or sub- a bearing on a distant object may be Chapter 1. Variation is a tract variation and deviation. taken from the vessel’s compass, Throughout history there have function of the vessel’s loca- and it is necessary to convert this been a series of “salty” mnemonics bearing to true, before plotting on a tion on the earth. It can be used to help remember the TVMDC nautical chart. The sequence of cal- sequence. The current politically found on the nautical chart. culations is just the opposite of that correct mnemonic is TeleVision discussed above, i.e., CDMVT. As Deviation is the difference Makes Dull Children; Avoid Watch- well, the sign to apply to east or ing. Decoded, it reveals the between magnetic north and west variation or deviation is also sequence of calculations TVMDC reversed. That is, east is added, and compass north. It is particu- and the reminder (AW) to add west west is subtracted. Although this is when converting from true to com- lar to each vessel and is simple enough to remember, some pass. For example, what is the com- prefer to use the additional memory a function of the heading of pass heading to steer if the true aid: Can Dead Men Vote Twice? At course is 060, variation is 015 west, the vessel. Although related, Elections! The first letters are the and the deviation table is as given memory aid to the sequence: Com- these are distinct concepts. in Table 2-4? The answer is calcu- pass, Deviation, Magnetic, Varia- lated as follows: Do noT ConFuse tion, True, and Add East. (Some air- J First, start at the true course, craft pilots were taught the phrase: These TWo TerMs. 060, and convert it to the mag- Can Ducks Make Vertical Turns?)

2-18 The Marine Magnetic Compass 2

To illustrate, suppose that the compass heading of the vessel is

ST G A U O A C R

065 degrees and an object is sighted S D U loCal MaGnetiC DisturBanCe A U Y X R bearing directly ahead per the ves- ILIA sel’s compass in an area where the notes indicating magnetic disturbance are variation is 015 degrees west. What is the true bearing? Assuming that printed in magenta on u. s. charts. Where space the deviation table is as given in permits, these notes are printed in the specific area Table 2-4, the deviation on a com- pass heading of 065 degrees is 5 of local magnetic disturbance. here are some degrees east and, therefore, the examples of how these are shown: magnetic heading of the vessel is 065 + 005 = 070 degrees (add east). loCal MaGnetiC DisturBanCe In turn, the true heading is the mag- netic heading plus or minus varia- Differences from normal variation of as much tion. If east is to be added, then as 5° have been observed in Gastineau Channel west should be subtracted for this calculation, so the true heading is in the vicinity of Lat. 58° 15'. 070 – 015 = 055 true. That’s all there is to it. Practice until you are loCal MaGnetiC DisturBanCe proficient. Differences of 12° or more from normal varia- tion may be expected in X Channel in the vicin- ity of Z Point. Where limited by space, the full note is placed else- where on the chart and the following reference note shown (in magenta) in the area of the disturbance:

loCal MaGnetiC DisturBanCe PHOTO PHOTO COURTESY OF INDUSTRIES, KVH INC. L (see note) FIG. 2-8–Fluxgate Digital Compass. Mariners should exercise particular vigilance Both the fluxgate sensor and the Liquid Crystal Display (LCD) are enclosed in when operating in these areas. one watertight unit.

ADDITIONAL POINTERS introduced, as shown in Table 2-5. substantial acceleration (e.g., ON THE COMPASS It is not a good idea to use compass speedboats), and substantial angles It is important to remember that readings obtained while the vessel of heel or bank. (Consult the refer- compass readings are most accurate is heeling, turning, or accelerat- ences at the end of this chapter for only when the vessel is level (as ing/decelerating. The effects of more details.) Directional gyros or opposed to heeling), traveling at a these errors are to make the com- gyrocompasses are less prone to constant speed, and maintaining a pass difficult to read and/or to give these errors and sometimes favored constant course. Otherwise, a series erroneous indications. These errors by mariners for this reason (among of additional compass errors is are largest for vessels capable of others).

2-19 2 The Marine Magnetic Compass

LOCAL MAGNETIC mine if there are areas of local mag- the same, or different, units.) If sep- DISTURBANCES netic disturbance located along arate from the display unit, the sen- In some areas of the world, the your proposed route. Exercise extra sor can be located remotely, in an measured values of magnetic varia- vigilance when transiting these area of the vessel where magnetic tion differ from the expected (chart- areas. Do not rely entirely on the disturbances are at a minimum. The ed) values by several degrees. The compass; steer by reference to land- display unit is small and can be sources of these discrepancies are marks and/or ATONs, if possible, mounted for optimum visibility. termed local magnetic distur- and fix your position frequently. Most modern fluxgate systems are bances, local attractions, or mag- Obviously, you should not attempt integrated with microprocessors, netic anomalies. Magnetic distur- to calibrate a compass in an area of which can perform many useful bance notes identify such areas on known local magnetic disturbance. functions. The model, pictured in U. S. nautical charts where errors (Local magnetic disturbances are Figure 2-8, automatically compen- are greater than or equal to 2° (3° in not a compass error, per se. sates for deviation (to within +/- 1 Alaska). The note will indicate the Nonetheless, this magnetic phe- degree, in most cases) by simply location and magnitude of the dis- nomenon does affect the compass making a 360 degree turn with the turbance. The indicated magnitude and should be noted.) vessel! Other models can also dis- should not be considered as the play headings in either true or mag- largest possible value that may be THE FLUXGATE COMPASS netic (variation data are stored in encountered. Large disturbances Finally, any modern discussion the microchip) and can furnish elec- are more frequently encountered in of compasses would be incomplete tronic inputs to other navigation the shallow water areas near land- without, at least, a passing mention systems such as the Global Posi- masses (particularly mountains) of the fluxgate compass. The flux- tioning System (GPS), Loran-C, or than on the ocean. Fortunately, the gate compass senses the earth’s radar. effect of a local magnetic distur- magnetic field electronically, rather An electronic compass is very bance typically diminishes rapidly than with magnets. (Readers wish- convenient. However, it does not with distance. However, in some ing a more complete discussion eliminate the need for a magnetic locations there are multiple sources should refer to the bibliography.) compass, because electronics are of disturbances and the effects may The fluxgate compass consists of a dependent upon a reliable power be distributed for many miles. Read sensor and a display unit. (The sen- supply and are easily damaged in the nautical chart carefully to deter- sor and the display unit may be in the marine environment.

2-20 The Marine Magnetic Compass 2

SELECTED REFERENCES

Aczel A. D., (2001). The Riddle of the Compass, The Fuson, R. H. (Translator), (1987). The Log of Invention That Changed the World, Harcourt, Inc., Christopher Columbus, International Marine New York, NY. Publishing Company, Camden, ME.

Anon., (1988). The Magnetic Compass, Principles, Ganssle, J. G., (1989). Anatomy of a Fluxgate, Ocean Selection, Navigation, E. S. Ritchie & Sons, Inc., Navigator, September/October. Pembroke, MA. Hempstead, R. L., (1987). Magnetic Compasses in the Bowditch, N. (1995). American Practical Navigator, Small Boat Environment, Ocean Navigator, No. 12, An Epitome of Navigation, Pub. No. 9, Defense March/April, pp. 21 et seq. Mapping Agency Hydrographic/Topographic Center, Bethesda, MD. This publication is avail- Hewson, J. B., (1983). A History of the Practice of able electronically from the National Imagery and Navigation, Second Edition, Brown, Son & Mapping Agency (NIMA) Marine Navigation Ferguson, Ltd. Publishers, Glasgow, Scotland, UK. Department web site (http://www.nima.mil/). Jerchow, E., (1987). From Sextant to Satellite Brogden, B., (1995). Boat Navigation for the Rest of Navigation, 1837-1987, 150 Years, C. Plath, Ham- Us, Finding Your Way by Eye and Electronics, burg, Germany. International Marine, Camden, ME. Kaufman, S., (1978). Compass Adjusting for Small Cohan, L. S., (1988). Compass Deviation Affected Craft, Surfside Harbor Associates, Surfside, FL. by Lightning, Ocean Navigator, No. 17, Kielhorn, W. V., (1988). A Piloting Primer, privately January/February. printed at Naples, FL.

Collinder, P. A., (1955). History of Marine Navigation, Kielhorn, W. V., (1957). The Northerly Turning Error, St. Martin’s Press, Inc., New York, NY. The Rudder, Vol. 73, No. 2, pp. 40-42.

Dahl, N., (1983). The Yacht Navigator’s Handbook, Kielhorn, W. V., and H. W. Klimm, (1978). A Hearst Books, New York, NY. Preliminary Study of the Changes of Magnetic Defense Mapping Agency, Hydrographic/Topographic Structure in a New Steel Trawler, Navigation, Center, (1980). Handbook of Magnetic Compass Journal of the Institute of Navigation, Vol. 6, No. 6, Adjustment, Fourth Edition, Pub. No. 226, pp. 365 et seq. Washington, DC. Moody, A. B., (1980). Navigation Afloat: A Manual for Denne, W., revised by Cockcroft, A. N., (1979). the Seaman, Van Nostrand Reinhold, New York, Magnetic Compass Deviation and Correction, A NY. Manual of the Theory of the Deviations and Morrison, S. E., (1942). Admiral of the Ocean Sea, A Mechanical Correction of Magnetic Compasses in Life of Christopher Columbus, Little, Brown and Ships, Brown, Son & Ferguson, Ltd., Glasgow, Company, Boston, MA. Scotland. Queeney, T. E., (1985). Beyond the Lodestone, Ocean Department of Transportation, Federal Aviation Navigator, No. 3, September/October, pp. 33 et seq. Administration, (1987). Instrument Flying Hand- book, AC61-27C, Washington, DC. Queeney, T. E., (1987). Future Heading Systems, Are Lasers The Next Step, Ocean Navigator, No. 12, Eyges, L., (1989). The Practical Pilot, Coastal March/April, pp. 35 et seq. Navigation by Eye, Intuition and Common Sense, International Marine Publishing Co., Camden, ME.

2-21 2 The Marine Magnetic Compass

SELECTED REFERENCES

Rousmaniere, J., (1989). The Annapolis Book of Seamanship, Simon and Schuster, New York, NY.

Saunders, A. E., (1987). Small Craft Piloting and Coastal Navigation, Alzarc Enterprises, carborough, Ontario, Canada.

Van Heyningen, M. A. K., (1987). The Evolution of the Modern Electronic Compass, NMEA News, January/February.

2-22 Chapter 3

THE NAUTICAL CHART

“It would appear that on some [of the Marshall Islands]…these charts were considered so precious that they might not be taken to sea. This was partly because they might be damaged in the canoes and partly, perhaps, because the people might never come back, in which case the tribe’s precious property would be lost forever”. [Emphasis added]

–Per Collinder, A History of Marine Navigation

INTRODUCTION DIFFERENCES BETWEEN his chapter provides an intro- CHARTS AND MAPS Tduction to the nautical chart, At the outset, get in the habit of how it is constructed, how to deter- calling this a chart—not a map. A mine position, course, and distance map depicts information relevant to on the chart, how to read the chart, terrestrial travel and provides the and, finally, some practical ideas answers to such questions as: about chart interpretation and use. J What are the route numbers? The material in this chapter is not J difficult, but there is a lot to learn. Where do the roads lead? Read this chapter with a nautical J What are the distances between chart, preferably the 1210-Tr, close cities and towns? at hand. Refer to the chart often. J Which roads are improved and Don’t despair, it takes a great deal which are limited access roads? of experience to become fully J Where can you find services familiar with the wealth of relevant information depicted on the nauti- such as fuel, food, and lodging? cal chart! This chapter provides an J Where are the entrances to introduction to the nautical chart. national parks? As you work the homework prob- It is useful to know the answers lems throughout the course, you to these questions if you plan to will gradually become more famil- take a family camping vacation or a iar with the nautical charts and how business trip by automobile. The they are used. map generally emphasizes land-

3-1 PHOTO COURTESY OF UNITED STATES COAST GUARD 3 The Nautical Chart

forms (sometimes including the deal of information, much is omit- used, but also treated with rever- representation of relief) and high- ted—the height of a tower, for ence and care. To this group, a map ways with the shoreline represented example, or the precise locations of is something that can be purchased as an approximate delineation, usu- church spires, water towers, monu- at a gasoline station vending ally at mean sea level. Maps do not ments, and conspicuous dwellings, machine, located next to advertise- usually present detail on water would normally not be shown on a ments for recaps, a cooler full of hot areas. Maps depict roads but, typi- map because this information is of soda, and a dog with an infected cally, do not provide information on limited utility to the map user. ear. If you haven’t guessed already, the condition of any road, although The practice of marine naviga- mariners are snobs. (For that matter, this can sometimes be inferred by tion has unique information so are aircraft pilots, who refer to collateral information (e.g., requirements. Water depths, aids to their maps as “aeronautical whether or not a road is part of the navigation (ATONs) and their rele- charts.”) So, don’t risk being interstate system). vant attributes (radio frequencies, branded a “landlubber”; refer to a Although maps provide a great light colors, sectors, and character- chart by its proper name. istics), landmarks, unmarked haz-

ST G A U O A C R

S D What you Will U ards to navigation, land outlines, A SOURCE OF A U Y X R ILIA learn in this bridge clearances, distance, and VITAL INFORMATION Chapter direction, are all pertinent to marine As noted above, a marine chart J Chart projections navigation. The nautical chart is a depicts information of use for graphic portrayal of the marine marine navigation. This includes a J How to establish a posi- environment. Together with sup- host of data on navigable (and not- tion on a nautical chart plemental navigational aids, it is so-navigable) water and the con- J How to measure direc- used to define courses, fix a vessel’s tiguous land. As you read through tion and distance on a position, ensure the vessel does not the material in this chapter, think nautical chart stray into dangerous waters, and about why cartographers chose to J navigate vessels by a safe and effi- include this information on the Chart scales and their chart and how you could use it to significance cient route. A chart is normally a working advantage. In very broad terms, J Types of nautical document. Courses, lines of posi- information depicted on charts can charts tion (LOPs), fixes, waypoints, and be used to: J How soundings and other relevant data and information J orient the mariner and help bottom characteristics are plotted on the chart. In use, a to establish (fix) the position are depicted nautical chart normally has numer- of the vessel: Landmarks (e.g., ous marks, courses, LOPs, fixes, J spires, buildings, water towers, How heights are and annotations made by the depicted and tanks), geographic features mariner. And, unlike the practice of (e.g., mountains, elevation con- J How to read the early mariners in the Marshall tours), and ATONs (e.g., buoys, General Information Islands (see introductory quota- range markers) are depicted, Block on a chart tion), charts are intended to be car- inter alia (among other things), J Practical pointers on ried on board! In contrast, a map is to help fix the vessel’s position finding your position generally a static document, used as and mark the location of various on a chart a reference guide. channels. Parallels of latitude, Marine navigation, being both meridians of longitude, and J ATONs and dangers to an ancient art and science, has compass roses are included to navigation evolved its own prejudices and jar- enable the user to determine J Chart storage gon. To a mariner, a chart is a veri- geographic coordinates and to table treasure—something to be measure courses and distances.

3-2 The Nautical Chart 3

J highlight and locate possible The above list is by no means

ST G A U O A C R

S D

hazards to navigation: Water complete. It provides instructive U

A U Y X R ILIA Caution notes depths and the location of examples to stimulate your think- obstructions, shoals, rocks, ing. The amount, complexity, and Caution notes are included on sunken wrecks, and fish trap diversity of material presented on nautical charts to alert the areas are included to warn the typical nautical chart is astound- mariner to certain hazards. ing! Nautical cartography is a high- mariners of both marked and Examples of caution notes ly developed art and science. unmarked hazards to navigation. taken from several charts Presenting the required information Some of these data may have include: in easy-to-understand and compact multiple uses. For example, J Extremely heavy tide rips water depths can be used in voy- format requires integrating the efforts of those in numerous disci- and strong currents may be age planning to identify possible encountered in the vicinity “shortcuts” for the trip. Wrecks plines to select the “right” method of chart projection, appropriate of the islands shown on this may be of intrinsic interest to chart. divers and anglers. Nautical choice of scale, chart symbols, J Improved channels shown charts are annotated with a abbreviations, lettering styles, col- series of “caution” notes (see ors, paper, and printing techniques by broken lines are subject to shoaling, particularly at sidebar) that warn the mariner to produce today’s high-quality the edges. about various hazards to naviga- nautical chart. J tion. Mariners are warned that REPRESENTATION OF A J Facilitate voyage planning: numerous uncharted duck SPHERICAL SURFACE UPON blinds and fishing struc- Much information is provided A PLANE OR FLAT SURFACE tures, some submerged, may that facilitates voyage planning exist in the fish trap area. and/or the practice of good sea- It is impossible to represent a Such structures are not manship. For example, symbols spherical surface upon a flat surface charted unless known to be that identify the nature of the without introducing some distor- tion—in distance, direction, shape, permanent. bottom (sand, mud, rocks, grass) J provide information regarding and/or area. Even the school refer- Mariners are warned to stay ence globe made out of paper or the suitability of a possible clear of the protective riprap plastic printed on flat or rotary anchorage area and/or the type surrounding navigational presses is made up of gores of of anchor necessary to optimize light structures. essentially flat surfaces cut to bend holding power. J St. Lucie Inlet: The channel and fit upon a spherical surface. If J is subject to continual identify and locate areas of the globe were disassembled and its regulatory significance: change. Entrance buoys separate pieces laid out, one would and lights are not shown Restricted areas, prohibited have a flat surface where areas areas, and other related informa- because they are frequently would be “correct,” but there would shifted in position. tion is provided to alert the be huge gaps between the gores; mariner to areas that are subject and direction, shape, and distances Where space permits, caution to certain restrictions. would have no meaning in the notes are shown near to the J identify areas/objects of spe- spaces between, nor continuity hazard noted. However, space cific interest to particular user from one gore to another, except at constraints may make this groups: The location of desig- the equator where all gores are impossible. Read the chart nated anchorage areas, pilot joined. carefully to find all applicable operating areas, piers, and oil caution notes. platforms are of interest to spe- CHART PROJECTIONS cialized user groups. Throughout the history of navi-

3-3 3 The Nautical Chart

gation, there have been numerous one line) cones to provide the THE MERCATOR attempts to depict a round surface Mercator and the polyconic projec- PROJECTION with a flat one. Some have been tions, respectively. These are shown This projection has been one of remarkably successful under certain in Figure 3-2 as are the two key the most useful for navigation for conditions, but all such projections nautical chart projections. Each of over 400 years. It was developed by suffer some limitations. The goal of these projections has its particular a brilliant Flemish geographer by the various projections, a sampling utility and each has limitations. A the name of Gerhard Kremer of which is given in Figure 3-1, is to particular projection is chosen (latinized to Gerhardus Mercator) balance and minimize the distor- based upon the intended use. A who published a world chart con- tions to produce a representation Mercator projection, for example, structed by his method in 1569. The that preserves (to the extent possi- distorts areas (particularly near the Mercator projection is a cylindrical ble) direction, distance, shape, area, poles) substantially, and is unsuit- projection, ingeniously modified by and correct angular relationships. A able for depicting the relative size expanding the scale at increasing projection that preserves correct of countries. (Generations of stu- latitudes, to preserve shape, direc- angular relationships is termed con- dents have grown up believing that tion, and angular relationships. formal. This property, or a close Greenland, for example, is larger This projection is conformal, approximation to it, is essential if than the United States, because they the chart is to be used for naviga- have studied Mercator projections but distance and area relationships tion. where this is apparently true. In are distorted. Both the meridians No chart projection is fully ade- actual fact, the United States is and the parallels are expanded at quate over a large area, but several more than 4.3 times larger than the same ratio with increasing lati- are sufficiently so to be useful to the Greenland.) tude. For the technically minded the small craft navigator. For naviga- These projections, their devel- expansion is equal to the secant of tion charts, the spherical surface of opment and use for small craft nav- the latitude (secant H = 1/cos H), the earth has been projected on a igation are discussed in some detail with a small correction to reflect the cylinder and, also, on a series of in this section. fact that the earth is not a perfect coaxial, tangent (touching the sphere. The projection does not earth’s surface at one point or along include the poles and usually not even the uppermost 15° of latitude, because the value of the secant at GENERAL CHARTS Azimuthal-Equidistant these angles is too large, being Gall-Peters infinity at Lat. 90° (N or S). Since Goode Homolosine Lambert Azimuthal Equal Area the expansion is the same for all Lambert Conformal directions and angular relationships Miller Cylindrical are correctly indicated, the projec- Mollweide tion is conformal and compass Orthographic directions (rhumb lines or loxo- Polar Stereographic dromes) are shown as straight Polar Gnomonic lines—properties of value to the Robinson mariner. Simple Conic Sinusoidal Distances can be measured Van der Grinten directly, but not by a single distance scale for the whole chart (unless a CHARTS OF PARTICULAR Mercator large-scale chart is used, see below) INTEREST TO THE MARINER Polyconic or for large areas. Instead, the lati- tude scale is used along any merid- L FIG. 3-1–Illustrative Projections

3-4 The Nautical Chart 3

L FIG. 3-2– Mercator and Conic Projections ian. (Remember, 1° Lat. = 60 M, 1’ COORDINATES FOR THE example, sell Mercator projections Lat. = 1 M.) Great circles appear as MERCATOR PROJECTION in novelty shops with south at the curved lines on the Mercator pro- The format for the Mercator top of the chart! Closer to home, jection except for the meridians and projection is rectangular. Latitude some nautical charts of rivers are the equator, which appear as and longitude are the coordinates typically not aligned north up in the straight lines. Note: The Mercator used for the Mercator projection. interests of saving space.) projection may also be made as an The parallels of latitude usually The meridians appear as oblique projection (at an angle to appear as horizontal, straight lines, straight, parallel lines running from the bottom of the chart to its top. the equator or meridian), using any running from the right to the left (in The scale for the meridians great circle (rather than the equator) the western hemisphere), with the projections oriented such that north (longitude scale) is indicated on the for the base line. Such a projection is at the top of the chart and south at top and bottom margins of the is termed an oblique Mercator pro- the bottom, east at the right margin, chart. The scale for the parallels jection and is used for special navi- and west at the left. (There are some (latitude scale) is provided on the gation applications, which are nautical charts based on Mercator right- and left-hand margins, and is beyond the scope of this course. projections where, for various rea- also used for distance measure- sons, north does not appear at the ment. Compass roses, indicating top of the page. The Australians, for true and magnetic directions, are

3-5 3 The Nautical Chart

and draw a light pencil line from and closed”—with one edge

ST G A U O A C R

S D historiCal U this point across the chart paral- along a parallel of latitude

A U Y X R ILIA Footnote lel to the top, bottom, and other shown on the chart. Holding the Mercator was active in the parallels on the chart. (Use of a base ruler along the parallel, parallel rule or paraline plotter swing the other ruler to the Protestant Reformation. His makes this easy.) Every point on desired latitude value on the left career nearly came to an end this line has that same latitude. or right scale. A light pencil line in 1544 when Mary, Queen J To specify the position uniquely, is drawn only in the vicinity of Dowager of Hungary, had now use the longitude scale at the approximate longitude, him imprisoned as a heretic. the top or the bottom of the chart which is “eyeballed” or estimat- (these scales are along parallels) ed from the longitude scale at She issued an edict that all the top or bottom of the chart. heretics should be put to to locate the desired longitude value and strike another light J After the latitude line has been death. Mercator was one of pencil line up or down, parallel drawn, take the dividers and set forty-three persons con- to the meridians and the sides of their points so that one point demned at Louvain but was the chart. All points on this line falls on the nearest meridian of saved by the intercession of (itself, a meridian) are at the longitude and the other at the value of longitude desired. Now, the parish curate, reportedly same longitude. Where the two light pencil lines intersect, the bring the dividers, carefully so a clever man and shrewd position is uniquely specified. as not to disturb the setting, debater. Fortunately for down along the meridian until modern mariners, Mercator USE OF PARALLEL RULERS the desired latitude line (paral- stuck to making maps after AND DIVIDERS TO PLOT lel) is encountered. If the line crosses the meridian, simply this experience. For more A POSITION ON THE MERCATOR PROJECTION measure along the latitude line information, see Wilford (parallel) with the dividers from (1982) in the references at Parallel rulers and the dividers the meridian. Where the other help to minimize unnecessary the end of this chapter. point of the dividers falls is the marks on the chart while plotting a desired longitude. The two coor- position. (Interrupt your reading of dinates now specify the vessel’s placed at convenient locations on this chapter to take a few minutes to position, uniquely. If the latitude the chart. read about parallel rules and line (parallel) does not cross the dividers in Chapter 4.) meridian, simply swing the par- ESTABLISHING A POSITION A common task in navigation is allel rulers (keeping the base USING THE MERCATOR to plot a position on the chart. For ruler along the parallel) so that PROJECTION example, a navigator may read the the other ruler falls along the Since the Mercator projection vessel’s present position in latitude desired parallel and crosses the results in a rectangular presenta- and longitude from a Global meridian, as well. Then, as tion, a rectangular coordinate sys- Positioning System (GPS) or Loran- above, measure the increment of tem using latitude and longitude C receiver (see Chapter 9) and wish longitude from the meridian makes establishing a specific posi- to plot this position on the chart. along the parallel ruler to the tion on the globe an easy process on The technique is simple and is illus- position. the chart: trated in Figure 3-3: J The process can also be used J Using the latitude scale at the J Place the parallel rulers—which with the meridian first, and the right or left margins (these are two straight edges con- dividers set off of the parallel scales are along meridians), strained to remain parallel as and the latitude scale. In addi- simply take the latitude value they are “walked” or “opened tion, the dividers may be used

3-6 The Nautical Chart 3

to the top or the bottom of the Long. 77˚ 40' W Longitude Scale chart and measure from the meridian along the longitude 78˚ 77˚ 76˚ scale to determine the value of Parallel the longitude increment. Read 34˚ the longitude directly under the point of the dividers. J The dividers can be used alone, without the rulers, keeping them Lat. 33˚ 19' N Lat. 33˚ 19' N parallel by eye, as above. Long. 77˚ 40' W Parallel Practice with the rulers/dividers will develop sufficient confi- dence and familiarity with the 33˚ process, to the point that the Parallel rulers will no longer be

yyyyy Latitude Scale required, and the dividers can

Meridian Meridian then be used alone. MEASURING DIRECTION ON THE MERCATOR PROJECTION A marine chart developed on the Parallel 32˚ Mercator projection usually has several compass roses (true and magnetic) placed at convenient L locations about the chart. Directions are measured from these FIG. 3-3–Plotting a Position on the Mercator Projection Using Latitude and roses using the parallel rulers, a par- Longitude Coordinates aline plotter, or other suitable method. The process is very simple: directly, without the parallel J At the desired position, walk the J Align the parallel rulers along rulers, to locate the position. In parallel rulers from a parallel of this case, the dividers are main- latitude. Measure the latitude the course or bearing line to be tained “parallel” to the meridian increment along a nearby measured. (It is helpful to put or parallel by eye—relatively meridian with the dividers. an arrow in the direction of the course line or the object. This easy to the practiced naviga- J Taking the dividers over to the helps to prevent making an error tor—as the position is struck. latitude scale at the right or left of 180° in reading the compass margin of the chart, determine rose.) READING A POSITION FROM the value of latitude by measur- J THE MERCATOR CHART ing the length set on the dividers Holding the base ruler along the It is often of interest to reverse from the parallel up or down line, walk the rulers, keeping the above process—i.e., to find the along the scale. them parallel to the line, to the center of the rose, which is indi- latitude and longitude of some J Now, take the dividers and mea- cated by a small cross. object on the chart. Taking (read- sure along the parallel rulers ing) such a position from the from the position to the nearest J With the ruler through the center Mercator chart is a very simple meridian. Take the set dividers cross, simply read the direction process, similar to plotting one: off of the true (or magnetic) rose

3-7 3 The Nautical Chart

in the desired direction. The rec- measure distance on the Mercator course in a series of “steps,” men- iprocal (180°) is read in the chart. tally counting in multiples of the opposite direction. Remember, however, that convenient distance. The final Alternatively, if a paraline plot- Mercator’s projection expands the “step” will be shorter, and the ter is used, the plotter is aligned scale as latitude increases. So, it dividers are reset to this length and with the course and rolled to the may be important where on the the increment read on the meridian. nearest compass rose to measure scale the distance is measured. Alternatively, several of the direction, taking care not to disturb (Unless a very small-scale chart is plotters on the market are equipped the alignment. The paraline plotter used, however, this scale expansion with one or more distance scales. is, generally, easier to use if a large is largely of theoretical interest.) For example, the Weems and Plath flat surface is available. Main - Distance is measured using a set of parallel plotter is equipped with taining alignment is quite easy dividers. If the length of the course three distance scales, corresponding under these ideal conditions. If, is shorter than the maximum exten- to chart scales (see below) of however, the navigator’s worksta- sion of the dividers, the dividers are 1:80,000, 1:40,000, and 1:20,000, tion is cramped and uneven, main- extended to the length of the course. respectively. Using this type of taining alignment is more of a trick. The dividers are then moved (with- plotter, the distance can be read In any event, practice is necessary. out changing their setting) so that directly from the appropriate dis- they are aligned with one of the tance scale. Be very careful, how- MEASURING DISTANCE ON meridians at the right or left edge of ever, to note the exact scale of the THE MERCATOR CHART the chart, and the length read out in chart to use the correct scale on the minutes and tenths of minutes or plotter. Otherwise, you have conve- The latitude scale, shown on minutes and seconds. Distance is nience at the expense of error! either side of the Mercator chart, is determined by remembering that Trivial errors like this can easily used for distance measurement. one minute is equal to one nautical arise if charts of more than one Since one degree of latitude is equal mile. A distance of 10.7 minutes, scale are used on the voyage, as for in distance on the earth’s surface to for example, translates to 10.7 nau- example, using both a coastal chart 60 nautical miles, and one minute tical miles. If a very small-scale (see below) for the en route portion of latitude is, therefore, equal to one chart is being used (see below) it is of the voyage and a harbor chart nautical mile, the degrees and min- important to measure this distance (see below) for the final approach. utes and tenths of minutes or sec- at or near the midpoint of the course It is easy to make the mental error onds markers indicating the latitude line. For coastal charts this refine- of continuing to use the coastal along the meridians on the sides of ment is unnecessary. scale on the plotter for the harbor the chart provide excellent scales to If the length chart. (See Chapter 12 for a discus- of the course is sion of other “trivial errors” that longer than the have led navigators to grief!) maximum exten- sion of the THE POLYCONIC dividers, it is PROJECTION measured by set- Nearly all marine navigation ting the dividers charts for the Great Lakes are based to a convenient on a polyconic projection—a series distance on the of cones concentric with the earth’s meridian (e.g., 2, axis and tangent to the sphere at dif- 5, 10 nautical ferent parallels of latitude. When a miles, depending plane portion is taken from the PHOTO COURTESY OF UNITED STATES COAST GUARD COAST COURTESY OF UNITED PHOTO STATES upon the scale of polyconic surface, as shown in L the chart) and Figure 3-4, the meridians appear Use of Paraline Plotter. walked along the almost as straight lines, very slight-

3-8 The Nautical Chart 3

ly curved, converging northward

ST G A U O A C R

S D beyond the top of the chart, toward U

A U Y R true or MaGnetiC north? X ILIA the apexes of the cones. Great circles appear as essen- It is useful to com ment on the appropriate choice of direction on tially straight lines and parallels of nautical charts. In principle, directions can be measured or specified latitude appear as slightly curved, relative to true north or relative to magnetic north. (Compass north, as almost parallel lines intersecting the discussed in Chapter 2, has no place on the chart because compass meridians at 90° angles and diverg- ing as they approach the edges of deviation differs from vessel to vessel.) Traditionally, true north has the chart. Distortion is least along been used for directional reference by the merchant marine and all the central meridian and increases navies of the world. More re cently, some small boat navigators have toward the sides of the chart, as the distance between parallels of lati- (some times passionately) advocated the use of mag netic north as a ref- tude increases. Although the poly- erence. conic chart is not conformal, great Traditionalists point out that most celestial computations are car- circles do appear as essentially straight lines, and radio signals (fol- ried out in terms of true north, marine gyrocompasses “point” to true lowing great circles) can be plotted north, the meridians on either Mercator or poly conic projections are as straight lines on this projection. oriented with respect to true north, tidal current data (see Chapter 8) LATITUDE AND are given with respect to true north, and, finally, that most ATON LONGITUDE SCALES ON information (e.g., the bearings of ranges) is provided with respect to THE POLYCONIC CHART true north. On the polyconic chart the par- Advocates of the use of magnetic north argue that true directions allels of latitude appear as slightly curved lines diverging toward the must generally be converted to magnetic (and ultimately to compass sides of the chart, and the meridians north) before use, introducing the possibility of error in the TVMDC converge to an imaginary spot off computations (see Chapters 2 and 12). Moreover, they add, nearly all the top of the chart. These “distor- tions” are readily apparent on a commercial continental aircraft navigation is carried out without so chart of a large area, but are virtual- much as a mention of true north. They concede that true north is ly undetectable on the charts gener- appropriate for celes tial, bluewater, or polar navigation, but argue that ally used by operators of small ves- sels. Figure 3-5, for example, repro- it has little relevance for the coastal small boat skipper. duces a portion of the 14881 poly- Each side of this “controversy” has points in its favor. In truth, conic chart (produced in the origi- either system can be made to work for coastal navigation (at least in nal at a scale of 1:80,000). As a the lower latitudes) provided that it is used with intelli gence. However, practical matter, latitude and longi- tude can be determined in the same one system should be chosen, to avoid the confusion associated with a manner as on a Mercator chart. For mixed system of units. Therefore, for purposes of this course, true extremely accurate plotting, an north will be taken as the appropri ate reference point. The choice is interpolator is typically reproduced somewhere on the polyconic chart, made largely because plotters that read off parallels or me ridians mea- as illustrated in Figure 3-6. sure relative to true north, and also because the ATON and tidal cur- (Because its use is so specialized, rent informa tion (see Chapter 8) is generally keyed to true north. the interpolator is not discussed in this text.) Incidentally, bar charts

3-9 L FIG. 3-5–Illustration of Polyconic Chart (14881, Scale 1:80,000) The Nautical Chart 3

Central Meridian

68˚ N

67˚ N Tangent Parallel

66˚ N

65˚ N Tangent Parallel

64˚ N

Poly (Several) Conic L FIG. 3-4–The Polyconic Projection for measurement of distance (in scales used by the small craft oper- polyconic projections. feet, yards, meters, and statute ator, any lack of conformality is miles) are also shown in Figure 3-6. practically unmeasurable. Compass CHART SCALES roses are provided on charts of the Since a chart is a representation DISTANCE ON POLYCONIC Great Lakes, and the nearest rose of the physical and geographical CHARTS FOR THE GREAT should be used when measuring nautical features of the earth’s sur- LAKES REGION true and magnetic directions. Also, face on a plain piece of paper, it is Distance on Great Lakes charts variation changes about one degree important to be able to relate dis- are indicated in statute miles (mi.) for every change of about a degree tance on the earth’s surface to dis- and not in nautical miles (M). in longitude. Thus, for long east- tance shown on the chart. The term Large-scale Great Lakes charts may west trips, corrections of magnetic for this earth-to-chart distance rela- also indicate distances in meters and compass courses should be tionship is scale. Scale is nothing (m), yards (yd.), and feet (ft.). Bar made about every 40 statute miles. more than the number of distance graphs are provided for measuring When the distance over which the units on the earth’s surface repre- distances as shown in Figure 3-6. direction is to be measured is very sented by the same distance unit on long, and is in a predominately east- the chart. The unit may be inches, DIRECTION ON POLYCONIC west direction, measure the course using the English (or American) CHARTS FOR THE GREAT line angle at the charted meridian system of units, or meters, if using LAKES REGION nearest the halfway point of the line the metric system, or any other between the two points of interest. units for that matter. Although the polyconic projec- Figure 3-7 summarizes the key dif- The scale on the nautical chart is tion is not conformal, for the chart ferences between the Mercator and expressed as a ratio of the units,

3-11 L FIG. 3-6–Plotting Interpolator included in Polyconic Chart. such as 1 inch on the chart is equiv- the value of the fraction, the larger chart. A large-scale chart covers a alent to 2,500 inches on the earth’s the scale. A scale of 1:2,500 small area. A small-scale chart cov- surface, written 1:2,500. There are (expressed as a fraction, 1/2,500) has ers a large area. Put another way, a several scales in use, depending on a much larger value than the scale of large-scale chart shows a large the area being charted and the detail 1:5,000,000 where the fraction has a amount of detail, a small-scale chart to be included. value of 1/5,000,000. Thus, a chart The relative size of the scale with a scale of 1:2,500 would be contains a small amount of detail. (large- or small-scale) is determined considered a large-scale chart, and a Always use the largest scale chart by the relative size of the ratio chart with a scale of 1:5,000,000 available for navigation to show the expressed as a fraction. The larger would be considered a small-scale maximum detail.

3-12 The Nautical Chart 3

ATTRIBUTE MERCATOR POLYCONIC (Great Lakes) Conformal Yes No Distance scale Variable, given in nautical miles Nearly constant, given in statute miles (measure at mid-latitude) Angle between parallels and meridians 90° 90° Appearance of parallels Parallel straight lines, unequally spaced Arcs of nearly concentric circles nearly equally spaced* Appearance of meridians Parallel straight lines, equally spaced Straight lines converging at pole Straight line crosses meridians Constant angle (rhumb line) Variable angle (approximate straight line) Great circle Curved line (except at equator meridians) Approximated by straight line Rhumb line Straight line Curved line Distortion of shapes and areas Increases away from equator Very little Illustrative uses Large-scale, coastal, and pilot charts Great Lakes How direction measured Reference to any meridian, parallel, or Direction at any point should be measured compass rose by reference to the meridian passing through that point or compass rose.

*Parallels are equally spaced only along the central meridian of the chart and are not concentric, but become farther apart toward the edges of the chart. However, these differ- ences are not noticeable on large-scale charts. L FIG. 3-7–Mercator and Polyconic Charts Contrasted

TYPES OF MARINE CHARTS J General Charts. Scales These charts show considerable There are several types of between 1:150,000 and detail, even to individual piers marine charts. These are differenti- 1:600,000. Used for offshore, and slips. ated by their scale and their intend- but within coastal zones of nav- J sMall CraFt Charts. ed use. These types and their prin- igation outside of outlying reefs Scales of 1:40,000 and larger cipal uses (refer to Figure 3-8) are: and shoals when the vessel is (although some are at smaller generally within sight of land or J sailinG Charts. Scales of scales). These are special com- aids to navigation and its course 1:600,000 and smaller. Used in posite type charts of inland can be directed by coastal pilot- navigation offshore, outside of waters, including the intra- ing techniques. coastal areas, or for sailing coastal waterways. The J between distant coastal ports. Coast Charts. Scales Mercator projection is used on The shoreline and topography between 1:40,000 and these charts, but it may be are generalized. Offshore 1:150,000. Used for inshore nav- skewed—north does not neces- soundings, principal lights, igation of bays and harbors of sarily appear at the top—to fit outer buoys, and landmarks vis- considerable width and for large the expanse of water on to the ible at considerable distances inland waterways and coastal chart. A river, for example, are shown. Detail needed for passages. The 1210-Tr chart, for would run lengthwise on the close-in navigation is lacking. example, is a coast chart. chart, and north could be toward Charts of this series are useful J harBor Charts. Scales the side. Small craft charts are for plotting the track of major larger than 1:40,000. Used in printed on lighter weight paper tropical storms, and/ or long navigating harbors, anchorage and folded rather than rolled. voyages. areas, and small waterways. These charts contain additional

3-13 3 The Nautical Chart

ST G A U O A C R

S D U iMportanCe A U Y X R ILIA oF up-to-Date Charts

The date of a chart is of vital importance to the navigator. When information becomes obso- lete, further use of the chart for navigation may be dangerous. Natural and artificial changes, many of them critical, are occur- ring constantly, and it is impor- tant that navigators obtain up-to- date charts at regular intervals and hand correct their copies for changes published in the Notice

L PHOTO COURTESY OF NATIONAL OCEANOGRAPHIC to Mariners. AND ATMOSPHERIC ADMINISTRATION Nautical Chart Catalogs Charts are revised at regular information of interest to small provides only limited detail. Even intervals. Users should consult craft operators, such as data on the entrances to the harbor of refuge the pamphlet, Dates of Latest marinas, tide predictions, and are difficult to see clearly on the weather broadcast information. coast chart. Editions, for the dates of current They are designed for use in chart editions. This pamphlet is open boats and runabouts. WHAT CHARTS It is noted above that you should ARE AVAILABLE? issued quarterly and available use the largest scale chart available Nautical charts of U. S. waters free from the Distribution for navigation. Figure 3-9 illus- are available from authorized Division (N/ACC3), National trates this point by comparing Point National Oceanic and Atmospheric Ocean Ser vice, Riverdale, MD, Judith harbor of refuge as shown on Administration (NOAA), National the 1210-Tr chart (scale 1:80,000) Ocean Service (NOS), and Coast 20737-1199. with a harbor chart (scale 1:15,000) Survey sales agents. The names and Other relevant information is of the same area. Which of these addresses of these agents are given would you wish to have aboard if in a Nautical Chart Catalog issued provided for those with Internet headed for Point Judith? The harbor in several short volumes (charts access at NOAA’s web site chart shows much more detail in the themselves, really) that cover the (www.noaa.gov). Notice to harbor of refuge area and in Point entire United States. Volume 1, for Judith Pond as well. example, is titled “Atlantic and Mariners information can be Soundings, discussed below, Gulf Coasts, including Puerto Rico found on the National Imagery land details, and even individual and the Virgin Islands.” Nautical and Mapping Agency (NIMA’s) houses can be seen on the harbor Chart Catalogs are available free of chart. In contrast, the coast chart charge from the Coast Survey. web site (www.nima.gov).

3-14 L FIG. 3-9–The 1210-Tr (inset) and a 1:15,000 Harbor Chart Compared. (These have been reduced from actual size for this illustration.) Which would you want aboard if headed for Point Judith? 3 The Nautical Chart

These same chart catalogs list duce nautical charts—actually

ST G A U O A C R

S D eQuiValents to all the available charts and provide reproduce Coast Survey charts. U

A U Y X R information on the chart scale, Some products are sold in chart ILIA Chart nuMBer 1 chart number, and price. For books, containing portions of many example, the area covered by the charts. These may be convenient Most nations that produce 1210-Tr chart (a special-purpose for the mariner. However, these nautical charts issue a chart training chart) is actually covered charts are not supported by the by a coastal Chart Number 13218, Coast Survey and are not easy to that corresponds to Chart at a scale of 1:80,000. (Portions update with information from the Number 1. For example, the of this area are also covered by Notice to Mariners or the Local Canadian Hydrographic sailing and general charts, as well Notice to Mariners. as harbor charts.) Many marinas Service issues Chart 1, have copies of the chart catalog SOUNDINGS AND BOTTOM Symbols, Abbreviations, and available, and it is a good idea to CHARACTERISTICS Terms and in the United pick one of these up on your Two of the most important types Kingdom, the Hydrographic next visit. The NOAA web site pro- of information presented on a nauti- vides information available from cal chart are the depth of water and Office issues Chart 5011, the Nautical Chart Catalog. This the bottom characteristics. This Symbols and Abbreviations system (http://anchor.ncd.noaa.gov/ information is depicted by the use noaa/noaa.html) is particularly easy of a combination of numbers, used on Admiralty Charts. to use. A small-scale map is dis- underwater contour lines, color Although chart symbols have played and, by clicking on the codes, and a system of standardized been standardized to a large desired portion of the map, the symbols and abbreviations. image is “zoomed” until the rectan- The soundings (depth of water) degree, mariners who use gular outlines of available charts on the chart represent the water charts published by other are shown. When an “identify” but- depth as measured or estimated governments should carry a ton is pressed, the desired chart from a specified vertical datum. name, number, scale, and latest edi- The vertical datum (not to be con- copy of the analog to Chart tion and print date are displayed. fused with horizontal datum) refers Number 1. Several commercial firms pro- to the base line or plane from which

1 INCH EQUALS APPROXIMATELY

SCALE OR NAUTICAL STATUTE CHART REPRESENTATIVE FRACTION MILES MILES CLASSIFICATION 1:2,500 0.03 0.04 Harbor 1:10,000 0.14 0.16 1:20,000 0.27 0.32 Small Craft 1:40,000 0.55 0.63 Coast 1:80,000 1.10 1.26 1:150,000 2.06 2.37 General 1:300,000 4.11 4.73 1:600,000 8.23 9.47 Sailing 1:1,500,000 20.57 23.67 1:14,000,000 192.01 220.96

L FIG. 3-8–Chart Scales and Classification

3-16 The Nautical Chart 3

a chart’s depth measurements are equal depth and profile the bottom Cod channel and the accompanying made. Historically, different datum shape. These lines are either num- data block that provides informa- levels were used for charts of the bered or coded according to depth, tion on channel depths. East and West Coast. On the East using particular combinations of The nautical chart also provides Coast, the tidal datum was mean dots and dashes. Figure 3-10, for information on the nature and low water (MLW)—the average example, reproduces an illustration “quality” of the bottom. Figure 3- low tide over a long period. The from Chart Number 1 (Nautical 10 also shows a selection of some tidal cycle on the East Coast gener- Chart Symbols and Abbreviations, of the many abbreviations (e.g., ally produces approximately two Tenth Edition, November 1997, see “rky,” “S,” “S/M,” “M”) that are low tides daily. But, these two low the attached references), that shows used to describe the quality of the tides are of approximately equal some of the standardized symbols bottom. This information is useful height—and the chart datum, there- and conventions for soundings and in deciding where to anchor and fore, was MLW. On the West Coast, depth information. Every mariner which anchor to use in an unfamil- and at some other locations, the two should have a copy of Chart iar harbor. For example, a “rky” low tides are generally of unequal Number 1 for ready reference. The (rocky) bottom generally presents depth, and the average of the lower panels of this chart are also repro- difficulties in anchoring or remov- of the two, designated mean lower duced on the back of the 1210-Tr ing the anchor, and a sandy bottom low water (MLLW), was used for chart for instructional purposes. may present poor holding condi- datum. However, this convention is However, Chart Number 1 is more tions, whereas clay (“Cy” or “Cl”) being changed so that datum in convenient to have on your boat. is typically the best holding bottom. either case will be MLLW in future Charted depths of water may be Finally, the style of the lettering editions of U. S. charts. (This datum given in feet, fathoms (1 fathom provides some additional depth- change is only a matter of definition equals 6 feet), or, increasingly, in related information. Vertical letter- and will not, in and of itself, affect meters (1 meter equals approxi- ing is used to represent features that any of the charted depths on the mately 3.3 feet). New charts will are dry at mean high water, where- East Coast. In any event, the specif- typically contain metric units. The as leaning or slanting letters are ic datum selected will be noted on chart legend, discussed below, used for water and underwater fea- the nautical chart.) informs the user of the depth units. tures that cover and uncover with The purpose of using a “low In some cases, more than one set of tidal action. water datum” is to produce reason- units may be found on the same able, yet “conservative,” depth chart, so close inspection is neces- HEIGHTS information for the chart. That is, sary to interpret correctly the depth Clearances of bridges and the actual depth of water will typi- information given. Different colors heights of landmarks are given in cally be greater than the charted or tints are used to convey depth height above mean high water depth. But it is important to remem- information; refer to Chart Number (MHW). This convention is analo- ber that average values are used in 1 for details. gous to using mean lower low water determining the chart datum. At any Dredged channels are shown on for soundings—the intent is to pro- given time (see Chapter 8) in any the nautical chart by two dashed duce a “conservative” number in location, the actual water depth lines to represent the approximate that the actual clearance is likely to may be lower or higher than the horizontal limits of dredging. The be greater than the charted clear- chart datum. So, although this depth controlling depth of the channel and ance. For example, a bridge height convention is arguably conserva- date of measurement are shown on might be shown as 30 ft., but the tive, the mariner is not relieved of the chart near the lines enclosing mean tidal range might be 8 ft. (see the responsibility of considering the the channel or in a separate data Chapter 8), so the bridge height actual height of tide at the time of block on the chart. Figure 3-11, for could range (on average) from 30 ft. interest. example, shows a portion of the to 38 ft. from the surface of the Contour lines connect points of 1210-Tr chart depicting the Cape water. This could be of substantial

3-17 3 The Nautical Chart

I Depths

Depth Contours

Feet Fm/Meters Low water line –——–——–———0———––——–—— 0 0 –——–——–———2———––——–—— 6 1 –—––——3–—–—— 12 2 –——–——–———5———––——–—— 18 3 –——––—6–——— 24 4 —–———————10———————— 30 5 ––––——15–——— 36 6 One or two lighter —–———————20———————— 60 10 blue tints may be used instead of the ——––—25–——— 120 20 ‘ribbons’ of tint at 10 —–———————30———————— 180 30 or 20 m 240 40 ——––—40–——— 300 50 —–———————50———————— 600 100 ——––—75–——— 1,200 200 –––––––––––——100–—––––––––––— 1,800 300 –––––––––––——200–—––––––––––— 2,400 400 –––––––––––——300–—––––––––––— 3,000 500 –––––––––––——400–—––––––––––— 6,000 1,000 –––––––––––——500–—––––––––––— –———0———– ——––—600–——— 30 –———1–——— ——––—700–——— –———2–——— –———3–——— ——––—800–——— –———4–——— ——––—900–——— –———5–——— ––———————1,000–—–————— –———6–——— ––———————2,000–—–————— —–——10——— —–——20——— ––———————3,000–—–————— —–——30——— ––———————4,000–—–————— —–——40——— ––———————5,000–—–————— —–——50——— ––——100–—–— ––———————6,000–—–————— ––——200–—–— ––———————7,000–—–————— ––——300–—–— ––———————8,000–—–————— ––——400–—–— ––———————9,000–—–————— ––——500–—–— ––——1,000–—–— –––——————10,000–—––————

Approximate depth 20 contour Approximate depth 31 (blue or contours Continuous lines, 50 black)————–100————– with values

Note: The extent of the blue tint varies with the scale and purpose of the chart, or its sources. On some charts, contours and figures are printed in blue.

L FIG. 3-10–Soundings and Bottom Characteristics interest to the skipper of a sailboat BASIC CHART INFORMATION J The chart title, which is usually with a 34-ft. mast. Once again, the name of the prominent navi- however, averages are used. On Essential Information gable body of water within the occasion, the tidal height may be Essential chart information is area covered in the chart. higher than mean high water, so this contained in a number of places on J A statement of the type of pro- convention does not excuse the a nautical chart. These are jection and the scale. mariner from consulting the appro- described below. J The unit of depth measurement priate references (see Chapter 8) to General information Block (illus- (feet, fathoms, or meters) and determine the actual clearance. trated in Figure 3-12) contains the datum plane. following items: notes: All notes should be read

3-18 The Nautical Chart 3

J Nature of the Seabed

Types of Seabed

Rocks ¨ K Supplementary national abbreviations: a – ag

1 S Sand S

2 M Mud M

3 Cy; Cl Clay Cy

4 Si Silt Si

5 St Stones St

6 G Gravel G

7 P Pebbles P

8 Cb Cobbles Cb

9 Rk; rky Rock; Rocky R

10 Co Coral and Coralline algae Co

11 Sh Shells Sh

12 S/M Two layers, eg. Sand over mud S/M

13.1 Wd Weed (including Kelp) Wd

13.2 Kelp Kelp, Seaweed

14 Sandwaves Mobile bottom (sand waves)

15 Spring Freshwater springs in seabed L FIG. 3-10 (cont’d.)–Soundings and Bottom Characteristics carefully because they contain J Tidal information. J In addition, handwritten nota- information that cannot be present- J Reference to anchorage areas. tions may be placed on a chart ed graphically such as: incorporating corrections occur- edition number. The edition J The meaning of special abbrevi- ring after the date of issue, which and/or revision numbers of the ations used on the chart. were published in the Notice to chart provide information on how J Special notes of caution regard- Mariners (NM) or Local Notice recently the chart was prepared. ing danger areas, prohibited to Mariners (LNM). Corrections J areas, dumping areas, safety The edition number and date of occurring after the date of issue areas, firing areas, vessel traffic the chart are located in the mar- and published in this manner zones, etc. gin of the lower left-hand corner. must be entered by hand on the

3-19 3 The Nautical Chart

L FIG. 3-11–The Cape Cod Channel as Shown on the 1210-Tr Chart

3-20 The Nautical Chart 3

Charts of the world’s

ST G A U O A C R

S D

waters prepared by the U Why CharteD

A U Y X R National Imagery and ILIA oBJeCts May Mapping Agency (NIMA) Be MisseD are being converted to the World Geodetic System There are several reasons 1984 (WGS 84) datum, which is equivalent to why objects shown on the NAD 83. chart may not be seen by the CHART TERMS HAVE navigator. These include: VERY SPECIFIC J The object may no longer MEANINGS be there. This can occur if Just a glance over a buildings or other struc- nautical chart will show tures are demolished or that a very specialized moved and the chart has vocabulary is used in con- not yet been corrected. nection with the chart sym- J bols. For example, several The navigator may be terms are used to describe looking in the wrong L various landmarks, such as towers place. FIG. 3-12–General Information Block or similar structures on the island of J Terrain or foliage may Martha’s Vineyard (refer to the of 1210-Tr Chart mask the object from a 1210-Tr chart). There are radar tow- particular viewpoint. ers, lookout towers, and spires. Just J chart. This is the chart owner’s north, on the Elizabeth Islands and The prevailing meteoro- responsibility. Note: no matter nearby mainland Massachusetts, logical visibility may be how recent a chart is, between can be found monuments, water too poor to see the object. the time of the survey and the towers, houses, cupolas, standpipes, This is often a problem time of printing and distribution, stacks, tanks, and more lookout with radio towers, power it is likely that some changes or towers. Each term has a unique and lines, or other objects that updating needs to be made to specialized meaning. Although the do not have a large visual differences among many of these ensure its accuracy. cross section. terms are obvious, there are sub- Title notes also contain the hor- J izontal datum used for the chart— tleties and specialized “terms of The navigator may not this defines the horizontal coordi- art” that are used. Figure 3-13, for understand what to look nate system used for the chart. This example, shows a sampling of for—his/her “mental tem- information is very important to terms and associated definitions for plate” may be wrong. users of radionavigation systems man-made landmarks as used in the J The navigator may not construction of nautical charts. (A such as GPS. The navigator should have read the chart cor- note the horizontal datum used for more complete list can be found in rectly. For example, a the chart (e.g., North American Bowditch and other references Datum of 1983 [NAD83]) and given at the end of this chapter.) stack may be charted ensure that the GPS receiver is Compare, for example, the differ- (e.g., “tallest of 5”) and adjusted so that it presents positions ences among the terms spire, cupo- the navigator is looking based on this same datum—other- la, and dome. All are found atop for a single isolated stack. wise, material errors may occur. buildings, but each is recognizably

3-21 3 The Nautical Chart

Building or House—One of these terms, as appropriate, is used when the entire structure is a landmark, rather than an indi- vidual feature of it. Spire—A slender pointed structure extending above a building. It is seldom less than two-thirds of the entire height of the structure, and its lines are rarely broken by stages or other features. The term is not applied to a short pyramid-shaped struc- ture rising from a tower or belfry. Cupola—A small dome-shaped tower or turret rising from a building. Dome—A large, rounded, hemispherical structure rising above a building or a roof of the same shape. Chimney—A relatively small, upright structure projecting above a building for the conveyance of smoke. Stack—A tall smokestack or chimney. The term is used when the stack is more prominent as a landmark than accompany- ing buildings. Flagpole—A single staff from which flags are displayed. The term is used when the pole is not attached to a building. Flagstaff—A flagpole rising from a building. Flag tower—A scaffold-like tower from which flags are displayed. Radio tower—A tall pole or structure for elevating radio antennas. Radio mast—A relatively short pole or slender structure for elevating radio antennas, usually found in groups. Tower—A structure with its base on the ground and high in proportion to its base, or that part of a structure higher than the rest, but having essentially vertical sides for the greater part of its height. Lookout station (watchtower)—A tower surmounted by a small house from which watch is kept regularly. Water tower—A structure enclosing a tank or standpipe so that the presence of the tank or standpipe may not be apparent. Standpipe—A tall cylindrical structure, in a waterworks system, the height of which is several times the diameter. Tank—A water tank elevated high above the ground by a tall skeletal framework. The expression gas tank or oil tank is used for the distinctive structures described by these words.

SOURCE: Bowditch, N. American Practical Navigator, An Epitome of Navigation, Defense Mapping Agency, Bethesda, MD, 1995. L FIG. 3-13–Illustrated Definitions for Charted Landmarks different. It is worth the time the landmark, e.g., MONUMENT, accurately located can be used for required to study the meaning of CUP (or CUPOLA), DOME. recognition purposes and to form such specialized terms and to com- However, a small circle, without a the visual context for identifying pare what is shown on the chart to dot, is used for landmarks not accu- other accurately known landmarks what can be seen from a vessel. rately located. These are denoted by for LOPs. For example, a MONU- Experienced mariners have a men- capital and lowercase letters, e.g., MENT might be located between a tal picture of each of the items Mon, Cup, Dome. (In some cases, Cupola and a Dome. Observation of the letters PA, denoting position defined in Figure 3-13, which these latter objects could serve to enables them to locate landmarks approximate, is also used to mini- identify the MONUMENT, which quickly from the chart descriptions. mize ambiguity.) In other cases, would be used for determination of There are numerous specialized only one object of a group is chart- an LOP. conventions employed in the con- ed. This is denoted by a descriptive struction of charts to convey precise legend in parentheses, including the These specialized definitions meanings. Continuing with land- number of objects in the group, e.g., and conventions are used to save marks as an example, a large circle (TALLEST OF FOUR). The pru- space and convey a wealth of infor- with a dot at its center is the symbol dent mariner soon learns to use only mation on the nautical chart. But, used to denote selected landmarks accurately located landmarks to all this efficiency is to little avail if that have been accurately located. determine LOPs or fixes (see the mariner doesn’t take the time to Capital letters are used to identify Chapter 6). Landmarks that are not learn these definitions.

3-22 The Nautical Chart 3

PRACTICAL POINTERS identify (recognize) these objects. ON FINDING YOUR Object detectability is a complex POSITION BY REFERENCE function of size, shape, color, light- TO THE NAUTICAL CHART ing, obstructions (masking), and The detail presented on the other factors. A complete discus- modern nautical chart is designed to sion of this topic is well beyond the facilitate piloting by reference to scope of this course. However, charted objects. Examination of the some practical aspects are notewor- 1210-Tr chart shows a sample of thy. the myriad of charted objects. But it An object generally needs to be requires substantial practice before visible to be detected (see below for mariners can rapidly and reliably some important exceptions.) orient themselves from a compari- Although this point may seem obvi- son of visual observations with ous, the implications are more sub- what is depicted on the nautical tle. The mere fact that an object is chart. In principle, the process is charted does not imply that it is vis- easy. The mariner should have at ible from all distances, at all least an approximate idea of the aspects, and at all times. For exam- vessel’s position. Plotting a fix (dis- ple, objects can be masked (blocked) by other objects, terrain, cussed in Chapter 6) is simply a GUARD COAST COURTESY OF UNITED PHOTO STATES matter of selecting two or more foliage, or simply the curvature of L prominent objects shown on the the earth. If the height of the object The light is partially obscured by terrain chart, locating these visually, deter- is given (as it is for most lights and and vegetation masking when viewed mining their bearings using a com- selected other objects), calculation from this angle. pass (either the vessel’s main com- of the maximum distance at which pass or a handheld compass), and it can be seen is relatively straight- distance, and/or from some aspects, plotting the bearings on the chart. forward, as discussed in Chapter 6 but it does not mean that the object The vessel’s position is fixed at the (in general) and Chapter 10 (for can be seen from all aspects and point (or in the area) where these lights in particular). If the observer from all distances. Refer to the two (or more) bearings intersect. is beyond this distance, the object 1210-Tr chart, for example, and However, things are not always as cannot be seen. Terrain, foliage, look at Nashawena Island. Two simple as they seem. Visual detec- meteorological visibility, and other objects are charted on this island, a tion and identification of charted factors also affect target detectabili- monument and a house. No height objects often presents a challenge to ty. Not all of these factors can be is given for either object and no ele- the mariner. Some practical tips on determined by inspection of the vation contours are given for the this topic are offered here. chart. In some cases, elevation con- island, so it is not possible to deter- Additional material can be found tours are shown for terrain, and the mine from the chart alone whether scattered throughout this text. The mariner can make an approximate or not (and from what angle) either reader may wish to consult the determination whether or not an of these objects would be visible. interesting book by Eyges (1989) object is likely to be visible. But, As a matter of fact, the house on the listed among the references for often these contours are not shown island cannot be seen from a posi- additional insights. and/or the height of the object is not tion to the southwest, and the mon- given. In these cases, the mariner ument is quite difficult to see at all, OBJECT DETECTION cannot determine whether or not an except at close range. To use charted landmarks for object is visible. The fact that an Objects may be masked (hidden position determination it is general- object is charted generally means from view) by trees or foliage as ly necessary to detect (see) and that it can be seen from some useful well as terrain or the earth’s curva-

3-23 3 The Nautical Chart

ture (see Chapter 6). The maximum some time period when the chart range that a 5-ft. buoy could be seen does not exactly match reality. For if the observer’s height of eye were example, look at Cuttyhunk Island 10 ft. (based upon horizon distance on the 1210-Tr chart. A lookout calculations shown in Table 6-2) is tower is charted very near the “k” in 6.3 nautical miles. Beyond this dis- “Cuttyhunk.” This structure was tance, the buoy will be masked by there when the 1210-Tr chart was the horizon. And, indeed, at night reissued. However, on the current (depending upon the visibility and chart of the 1210-Tr waters (Chart intensity of the light) the light on 13218) this structure is no longer the buoy might be seen at this dis- shown. Instead, a tank is shown at tance. During the daylight hours, approximately the same location. however, the maximum distance Students often ask why roads that the buoy could be seen (at least are shown on the nautical chart. with the unaided eye) is probably They reason that a road is most much less, say 1/2 to 1 nautical unlikely to be seen and, therefore, is mile, give or take. So, the second a useless addition to the chart. In reason why object height is impor- fact, roads can be useful to depict tant relates to the resolving power because, although the road itself of the eye. may well be invisible, vehicles can PHOTO COURTESY OF UNITED STATES COAST GUARD COAST COURTESY OF UNITED PHOTO STATES L Object height, incidentally, is often be seen. This is particularly “Home at last.” Both the breakwater the chief reason why a harbor true at night, when headlights or and the light will be shown on the breakwater is often a poor reference taillights are conspicuous. chart. However, breakwaters are often mark in coastal navigation despite not seen at great distances due to the its relatively massive appearance on IDENTIFICATION/ curvature of the earth and the limited the nautical chart. Particularly at RECOGNITION height of most breakwaters. high tide, a breakwater may only be As noted, an object needs to be seen when quite close to the harbor identified (recognized) as well as entrance. Trainees in the United detected. Identification is almost as ture. Here again, the unwary navi- States Coast Guard Auxiliary complex a subject as detection, but gator may have difficulty reconcil- (USCGAUX) boat crew program it is possible to provide some prac- ing the observed scene with the often waste time trying to spot har- tical pointers. chart. In the case of tree masking, bor entrances by looking for break- J First, take the time to study the many trees lose their leaves in the waters, and overlook otherwise various object definitions and winter, and an object may be readi- conspicuous targets that could be abbreviations to be found in ly visible, but not so in the summer used for position fixing. Chart Number 1 and related when the leaves shield the object. Another important reason why a publications. As noted else- The color of the object and the charted object may not be able to be where in this chapter, the terms lighting conditions also affects visi- observed is that it is no longer used often have very specific bility. Objects that are backlighted there! Charted structures may be meanings, which are useful for or in shadow have a different visual demolished and/or removed. In due recognition purposes. One day it appearance than when under other course, these changes are reported may be very important for you lighting conditions. This effect can and chart corrections listed in the to know the difference between be quite dramatic in some cases. NM/LNM (see Chapter 10) and a standpipe and a gas tank. The size of the object is impor- made in subsequent editions of the Nonetheless, as precise as the tant for two reasons. First, objects chart. However, either process is cartographers attempt to be, can be masked by the earth’s curva- not instantaneous and there will be some of the terms admit to vari-

3-24 The Nautical Chart 3

ous interpretations, and even object identification is generally visible. Although the control diligent study of the definitions easier if the object is considered tower or the hangers may not be may be insufficient to solve all as part of an entire scene, rather able to be seen from the bridge recognition problems. For than in isolation. For example, a of a sport fisherman, aircraft example, consider the term tank located next to other descending to land can be “abandoned light house.” This prominent objects is often more observed. Just remember that term is used in the technical easily recognized and identified the prevailing approach paths of sense to denote a lighthouse that than a lone tank. Nonetheless, aircraft may depend upon the is no longer functioning in that the alert observer should antici- wind direction, so some caution capacity. But, what does it look pate the possibility that some of needs to be exercised. like? Aside from the fact that the surrounding objects shown J Fourth, it is often easier to rec- working lighthouses often differ on the chart may be hidden or ognize objects and determine considerably in appearance, difficult to recognize. The your position if you are reason- abandoned lighthouses vary observer’s “mental image tem- ably sure of it to begin with. even more depending upon con- plate” may not match exactly This may sound foolish or, at dition. In the Delaware Bay, for the chart appearance. best, obvious, but this is not a example, there is an abandoned Incidentally, visibility plays a trivial statement. If you keep a lighthouse that is little more part in object identification as good dead reckoning plot than a foundation. (The word well as detection. If the visibili- (Chapter 5) and take frequent “ruins” shown on the Delaware ty is poor, perhaps only one or position fixes (Chapter 6), you Bay chart might alert us to this, two prominent objects may be have a good (though approxi- but also admits to various inter- visible and it may be difficult to mate) idea of your position at all pretations. The “ruins” of the locate oneself. Alternatively, if times. When you decide to fix temples of the sun and moon the visibility is greater, many the vessel’s position again, it is near Mexico City are massive objects may be visible and ori- not necessary to go hunting all structures that can be seen entation much simpler. over the chart to find the charted for miles!) Not all light struc- J Third, notwithstanding the fact location of the two towers visi- tures are in the same state of col- that objects generally need to be ble off the port beam! lapse. The abandoned light- visible in order to be detected Sometimes, knowledge of your house off Sakonnet Point, and identified, there are a few approximate position enables shown as “Tower, Abandoned noteworthy counterexamples to you to make a plausible identifi- Lighthouse” on the 1210-Tr this “obvious” truth. For exam- cation of an object that cannot chart, is fully restored and (dur- ple, in many locations of the be identified directly. For exam- ing daylight hours) looks just world, large powerplants (typi- ple, many shoreside communi- like an operating lighthouse. It cally shown on nautical charts if ties have conspicuous water takes time, experience, and local located near the shore) can be towers. Unless the tower has the knowledge to become expert in found. The steam exhaust from town name written on it (as identifying charted features. the cooling towers of these many do), or presents some Descriptions of selected features plants is often visible at a much unique “scene,” the identifica- (sometimes including pho- greater range than the cooling tion of these towers may be dif- tographs) can be found in the tower itself. Although the steam ficult. If you have kept track of United States Coast Pilot cloud may not enable precise your position well, the tower (USCP) and many commercial- position fixing, it can assist in might be plausibly identified ly published cruising guides. orientation. Airports serve as because it is the only tower that The USCP is discussed in more another example of an object is consistent with the vessel’s detail in Chapter 10. generally shown on nautical approximate position. Con - J Second, it is useful to note that charts that may not be directly versely, if you are much less cer-

3-25 3 The Nautical Chart

tain of your position, it’s a much nals. In the case of lighted ATONs, that precluded great accuracy of tougher job to match the chart for example, the nautical chart pro- detail. A chart based on such a with what can be seen. The vides information on the location, survey should be regarded with same tower might conceivably markings or numbers, color caution. Except in well-fre- be that belonging to any of sev- (including sectors of various col- quented waters, few surveys eral towns along the coastline. ors), light characteristics, nominal have been so thorough as to J Fifth, beginning navigators range, height of light, whether or make certain that all dangers to often focus on man-made fea- not sound signals or a radio beacon navigation have been found and tures shown on the chart for ori- are part of the same facility, and charted. entation and position determina- other pertinent information. Refer J The scope of sounding data is tion. Landmasses can often to Chart Number 1 for details. another clue to estimating the assist in the identification of completeness of a survey. Most charted objects and/or be useful DANGERS TO NAVIGATION charts seldom show all sound- directly. Gibraltar (“The Rock” The nautical chart provides a ings that were obtained. to many of WW II vintage) great deal of information on dan- However, if soundings are serves as a dramatic example of gers to navigation, such as sub- sparse or unevenly distributed in a land feature that is easily visi- merged rocks, reefs, pilings, snags, charted coastal waters, the pru- ble (in good weather) and readi- wrecks, and obstructions. Figure 3- dent navigator exercises care. ly identifiable. Closer to the 14 contains a sample of the many J Large or irregular blank spaces 1210-Tr waters, it should be separate symbols used to depict among soundings may mean clear that Gay Head and the these dangers to navigation on the that no soundings were obtained associated light should also be modern nautical chart. It is a good in those areas. Where the nearby easily recognizable. Small hash idea to highlight dangers along an soundings are “deep,” it may marks shown on the chart near intended route as a reminder. logically be assumed that in the Gay Head suggest some sort of blanks the water is also deep. bluffs, and the elevation of the OTHER CHART FEATURES However, when surrounding light (170-ft.) is unlikely to be The nautical chart also contains water is “shallow,” or if it can be the result of a very tall tower. a wealth of additional data on such seen from the rest of the chart This conjecture is correct; these items as the characteristics of the that reefs are present in the area, 150-ft. palisades are very char- coastline, land, ports and harbors, such blanks should be regarded acteristic and easy to identify. topography, buildings and struc- with great caution. This is espe- Make sure that you familiarize yourself with the land features tures (as noted above and shown in cially true in areas with coral shown on the nautical chart. Figure 3-15), and even tides and reefs and off rocky coasts. Give Object recognition is as much a currents. Again, reference should be such areas a wide berth. mental as a physical activity, made to Chart Number 1 for details. J Everyone responsible for the rather like putting together the safe navigation of a vessel must pieces of a navigational puzzle. ACCURACY OF CHARTS have a thorough working This point is made at several A chart is only as accurate as the knowledge of the nautical chart. places in the chapters ahead. survey on which it is based. The Select a chart you commonly prudent navigator must consider: use in navigating and (with this AIDS TO NAVIGATION J The source and date of the chart in hand) reread this chapter. (ATONs) are generally given in the title J The NIMA (NOS has a similar The nautical chart provides along with the changes that have specification) specified accura- information on ATONs, such as taken place since the date of the cy for harbor, approach, and lights, buoys, daybeacons, beacons, survey. Earlier surveys often coastal charts is that features radio/radar stations, and fog sig- were made under circumstances plotted on the chart will be with-

3-26 The Nautical Chart 3

K Rocks, Wrecks, Obstructions

35 35 Rk 35 Non-dangerous rock, depth known 21 35 15 Rk R R R.

Co Co Reef line Co 16 3l Coral reef which covers

19 5 17 Breakers Breakers 8 Br Br 18

Wrecks

Plane of Reference for Depths ¨ H

Wreck, hull always dry, on 20 Hk large-scale charts Wk

Hk Wreck, covers and uncovers, on Wk Wk 21 large-scale charts Wk Wk Wk

9 Wk Submerged wreck, depth known, 52 22 on large-scale charts Wk

Submerged wreck, depth unknown, Hk Wk Wk 23 on large-scale charts Wk

Wreck showing any portion of hull or Wk 24 superstructure at level of chart datum. Wk Wk

Masts Mast (10) ft Masts Wreck showing mast or masts Mast 25 Funnel above chart datum only

1 Wreck, least depth known by 4 25 5/2 Wk 52 Wk 6 Wk Wk (9) 26 sounding only

Wk Wreck, least depth known, swept 21 5 4 4 25 Rk 27 Wk 6 Wk by wire drag or diver 6 Wk Wk 21

28 Dangerous wreck, depth unknown

Sunken wreck, not dangerous 29 to surface navigation

Wreck, least depth unknown, but considered to have a safe clearance 30 8 Wk 25 Wk 25 Wk 15 to the depth shown L FIG. 3-14–A Sampling of Symbols Used to Denote Dangers on the Nautical Chart

3-27 3 The Nautical Chart

point navigation, and approach charts, used for the final approach to an airport. A similar categorization is appropriate to marine navigation. From the point of view of the coastal mariner, the nautical equivalent of an en route chart would be the coastal charts, and the equiva- lent to the approach charts would be the harbor and/or small craft charts. However, no competent instrument pilot would be content to fly with only en route charts and the approach chart for the intended destination. Pilots recognize that

PHOTO COURTESY OF UNITED STATES COAST GUARD COAST COURTESY OF UNITED PHOTO STATES unanticipated difficulties (e.g., L weather beneath landing mini- United States Coast Guard buoy tender have on board the latest charts mums at destination, mechani- servicing lighted buoy. Buoys and other of the area to be sailed. The cal malfunction, unforecast ATONs are shown on nautical charts. point is made at several places headwinds, etc.) may require in the text that changes in land- them to divert to alternate desti- in 1 millimeter (1 mm) at chart marks, ATONs, and other chart- nations. Indeed, under certain scale with respect to the pre- ed features occur almost con- flight conditions, the specifica- ferred datum, at a 90 percent stantly. Charts are revised peri- tion of an alternate is legally confidence interval. For a large- odically to reflect these changes, required when an instrument so the latest charts need to be scale chart of 1:15,000 scale, a flight plan is filed. The situation used. Additionally, changes are 1-mm error equates to +/- 15 facing mariners is almost exact- recorded in a publication, Notice meters (16.2 yards), which is the ly analogous to that of aircraft to Mariners, available in both same order of magnitude as the pilots. Weather at the intended hard copy (see Chapter 10) and GPS error. For a smaller scale destination may be unaccept- on the Internet. The prudent chart of 1:80,000 scale, the chart able; larger than anticipated cur- navigator notes these changes error is +/- 80 meters (86.4 rents (see Chapter 11) or other on the latest charts available. yards), which will become the factors (e.g., standing by to J limiting factor in position plot- Second, ensure that you have assist a disabled vessel, towing a ting accuracy. charts at the proper level of disabled vessel) may cause fuel detail or scale for the intended to be expended at a greater rate PRUDENT ADVICE voyage. As a practical matter, than anticipated, mechanical REGARDING CHARTS this means that you should gen- difficulties (e.g., a rough run- erally use the largest scale chart ning engine), and other factors This is a good point in the narra- available for the waters cruised. may make a diversion to an tive to inject some prudent advice Aircraft pilots (at least, those alternate harbor attractive and on the use and storage of nautical that are instrument rated) prudent. However, the safety charts from the point of view of the observe a useful distinction margin afforded by alternates navigator. between what are termed en could be seriously eroded if the J First, ensure that you always route charts, used for point-to- right approach charts are not

3-28 The Nautical Chart 3

E Landmarks

Plane of reference for Heights ¨ H Lighthouses ¨ P Beacons ¨ Q

General

TANK Tk 1 Examples of landmarks Building Hotel

CAPITOL DOME FACTORY WATER TR 2 Examples of conspicuous landmarks WORLD TRADE CENTER HOTEL WATER TOWER

3.1 Pictorial symbols (in true position)

3.2 Sketches, Views (out of position)

Height of top of a structure above plan of (30) 4 reference for height

Height of structure above ground level 5 (30)

Landmarks

10.1 Ch Church Ch.

Tr. 10.2 Church tower Tr.

Church spire Sp. 10.3 SPIRE Spire Sp.

10.4 CUPOLA Cup Church cupola Cup. Cup.

11 Ch Chapel

12 Cross, Calvary

13 Temple

14 Pagoda

15 Shinto shrine, Josshouse

L FIG. 3-15–Building and Structure Symbols Shown on Chart No. 1

3-29 3 The Nautical Chart

repair facilities. But, a glance at landmarks and ATONs to be

ST G A U O A C R

S D

U sourCe the 1210-Tr chart shows this used for fixes can be evaluated

A U Y X R ILIA DiaGraMs area in a uniform shade of blue, before the vessel slips a moor- with no sounding detail whatso- ing line. To simplify the logis- All U. S. nautical charts of ever. (In ancient days, this area tics of planning, some mariners would have been labeled Terra actually have two copies of the scale 1:500,000 or larger Incognita and festooned with relevant charts and other publi- produced after 1992 are images of sea serpents and other cations, one set for their home unsavory characters!) The inset or office, and another set for the required to include a source in the 1210-Tr chart indicates vessel. diagram. The source dia- that these waters are covered J Fourth, any chart discrepancies in Chart Number 13221 gram provides details of the noted during the voyage should (Narragansett Bay). As a pru- be reported to the appropriate hydrographic surveys from dent navigator, you should have agency. Mariners’ reports are an foreseen this contingency and which the chart has been important source of chart cor- taken along the 13221 (and rections and these are strongly compiled. It serves as a guide other) chart(s). If this chart were encouraged. not on board, an otherwise rou- to navigators, is useful in In short, simply remember the tine diversion could have the four R’s: charts should be Recent, at the planning of routes, pro- makings of a real problem. All the Right scale, Readily accessible, may end well, but the level of vides information on data and Reviewed beforehand. cockpit tension is sure to go up a quality, and allows users to few notches. This brief scenario CHART STORAGE also illustrates another point: make their own judgements Most textbooks on large ship charts should be stowed in a navigation include at least some of the data’s fitness for a par- known and easily accessible passing remarks on chart storage. location (see below). You should ticular use. Source diagrams The advice usually reads that charts be able to find the 13221 chart should be stored flat, to avoid creas- are also provided on certain in a hurry if necessary—now is es, or “rolled-up.” This idea is just not the time to have to rummage foreign charts. fine if you own the Queen Elizabeth around the mayonnaise jars and (either edition) or other large ves- cleanser (just behind the pickle sel. However, most of us do not available. Suppose, for example, relish) to find it. have the luxury of a draftsman’s that you are voyaging in a twin J Third, take the time to study the storage cabinet and have to make do engine sport fisherman on the charts (and related publications) with much less space. It is virtually waters covered by the 1210-Tr when you plan a voyage—in the impossible to store charts flat on the chart. You depart from some safety and comfort of your den typical recreational vessel. There - port on the Sakonnet River or living room, rather than in a fore, it boils down to a choice of intending to head into the harbor possibly cramped and wet cock- whether to roll or fold your charts. at Point Judith (please refer to pit. Although not every aspect of Either scheme has passionate advo- the 1210-Tr chart). Just off a voyage can be planned in cates, odd for such a mundane Brenton Reef (near Newport advance, most can. Overall voy- issue. In truth, either scheme can Neck), the port engine starts to age legs, time-speed-distance work and the choice depends in part run rough, and you begin to con- (TSD) calculations (at least, for on the available storage and work- sider options available. The har- power vessels), fuel consump- ing space in the vessel. If the navi- bor at Newport is close by and tion, tides and currents, destina- gator’s workstation will permit a likely to have marinas with tion alternates, and even the chart to be laid out without folding,

3-30 The Nautical Chart 3

rolling might be a better idea. Plastic tubes can be purchased to store charts, often easily attached to The Four “Rs” for Charts the overheads of the cabin. If, how- ever, the chart table is better suited Recent to serving tea than navigation (as is true for many vessels produced Right Scale today), charts will have to be fold- ed, in any event. Folding is a better Readily Accessible option for storage in this case. (If Reviewed Beforehand charts are folded, try to avoid putting sharp creases in the chart, this increases the difficulty of plot- ting and introduces weak points where tears can originate.) More important than the “fold wish to write on your chart: ticular needs and interests. Don’t or roll” controversy, charts should J Names of marinas or yacht be afraid to write on the chart. It is be stored in a dry (well, relatively clubs that you might visit intended for this purpose. dry) location and arranged for easy J access. Use gummed labels or some Areas of productive fishing EPILOGUE: IF CHARTS other method to identify the chart as J Good anchorage areas COULD SPEAK, OH, WHAT filed; otherwise, it can be a chore to J Approximate locations of TALES THEY COULD TELL find the right chart. Also, take the objects not depicted on the chart time to lay out the charts necessary As the foregoing material J Drawbridge operating times and for each voyage beforehand. shows, the modern nautical chart is Always check to ensure that you radio frequencies for communi- a marvel. The compact symbols and have the correct charts for the cation format enable a chart to convey on planned voyage (including alternate J Timing characteristics of one sheet of paper what would destinations) as part of the pre-voy- foghorns (not shown on charts) require volumes of written material to cover. Yet, the chart leaves fasci- age checklist. It is much easier to J Additional information on nating stories untold, concealed by replace a chart at the marina than to ATONs not shown on the chart such compact symbols and text as: discover it missing when en route. that is relevant, such a descrip- wreck, PA, or “Danger, unexploded tion of particular lights, taken depth charges.” Part of the fascina- CHART AS A from other navigational refer- tion of these charts is the fun of WORKING DOCUMENT ence publications or personal imagining or researching the stories observation It is noted at the beginning of behind these cryptic notations. J this chapter that a nautical chart is Probable arcs of visibility of Such chart research can be done intended to be a working document, certain lights during the long winter nights, por- rather than a passive reference J Navigational hazards of particu- ing over dusty volumes from the guide. In subsequent chapters of lar relevance library or used bookstore. To intro- this book, dead reckoning and pilot- J Written courses and distances duce you to this sport, the following ing are covered. Intended and actu- historical anecdote, taken from The J Key items from the NM or LNM al voyage tracks are actually plotted Navigator (official publication of affecting your voyage (see on the chart. However, these are USCGAUX), is offered with Chapter 10) not the only annotations that can be respect to the waters covered by the With these additions, you cus- made on a nautical chart. Here are 1210-Tr chart. tomize the chart to reflect your par- some things that you might also On 4 May 1945, Grossadmiral

3-31 3 The Nautical Chart

Donitz, commander of the German vessel to arrive on the scene was the circle centered on coordinates 41° submarine fleet, gave the order to Coast Guard Frigate Moberly, com- 14’N and 71° 25’W, marked cease hostilities. At this same time, manded by LCDR L. B. Tollaksen, “DANGER”—unexploded depth the U-853, under the command of USCG. He was later joined by the charge—May 1945.” This was the Oberleutenant Helmut Froemsdorf, Destroyer Ericson and the general area of the attack. was making a deep penetration of Destroyer Escorts Amick and Approximately three miles to the Eastern Sea Command. So far, Atherton. the SE of this area is a wreck in fact, that the boat was in Rhode By 7:20 P.M. the U-853 had marked “PA.” This is the final rest- Island Sound. The U-853 was a been located and was depth- ing place of the U-853, the last Type IX-C, 750 tons, and charged. The boat slipped away but German submarine to be sunk in snorkel-equipped, which meant that was again detected at 11:37 P.M. A WWII. As an exercise, take the time the submarine could remain sub- hedgehog attack by the Atherton to plot the various positions and merged for extended periods. settled the matter and the U-853 locate these on the 1210-Tr chart. Unfortunately, it also meant that sank. Despite this, the attack con- Readers interested in learning Oberleutenant Froemsdorf did not tinued until 1:10 A.M. on 6 May. more about the wrecks shown on learn that the war was over. Numerous vessels participated in their charts can consult the In any event, on 5 May at 5:40 this action. Later in the morning of Automated Wrecks and P.M., the U-853 torpedoed the 6 May, two blimps joined the sur- Obstructions Information System Black Point, a British steamer of face vessels and the attack resumed. (AWOIS). NOAA has a searchable some 5,300 tons, breaking it in half. Finally one of the blimps reported a AWOIS database available on their This was off Point Judith on a true large oil slick and the action was web site (http://www.noaa.gov/). bearing of 129 degrees and a dis- broken off. One of the surface ves- tance of 5,910 yards. On the 1210- sels recovered Oberleutenant Tr chart a wreck symbol is located Froemsdorf’s cap and the chart at coordinates 41° 19.7’N and 71° table in the floating debris. 25.8’W. This marks the final rest- Over 200 depth charges were ing place of the Black Point. dropped in the area during this The passing Yugoslav freighter engagement. On the chart you will Kamen sent out an SOS and the first notice a two-mile diameter

3-32 The Nautical Chart 3

SELECTED REFERENCES

Anon., (1986). The Secret of the Chart. The Navigator, Ministry of Defense (Navy) BR 45(1), (1987). USCG Auxiliary National Board Inc. Admiralty Manual of Navigation, Volume 1, Her Majesty’s Stationary Office (HMSO), London, UK. Bowditch, N. (1995). American Practical Navigator, An Epitome of Navigation, Pub. No. 9, Defense Mixter, G. W. (edited by D. McClench), (1967). Mapping Agency Hydrographic/Topographic Primer of Navigation, Fifth Edition, Van Nostrand Center, Bethesda, MD. This publication is avail- Reinhold Company, New York, NY. able electronically from the National Imagery and Mapping Agency (NIMA), Marine Navigation Naval Training Command, (1972). A Navigation Department web site (http://www.nima.mil/). Compendium, NAVTRA 10494-A, U.S. Government Printing Office, Washington, DC. Brogdon, W., (1995). Boat Navigation for the Rest of Us—Finding Your Way by Eye and Electronics, Schofield, B. B., (1977). Navigation and Direction, International Marine, Camden, ME. The Story of HMS Dryad, Kenneth Mason, Homewell, Hampshire, UK. Brown, L. A., (1977). The Story of Maps, Dover Publications Inc., New York, NY. Wilford, J. N., (1988). The Impossible Quest for the Perfect Map, The New York Times, Tuesday, 25 Committee on Map Projections of the American October, New York, NY. Cartographic Association, (1986). Which Map is Best? American Congress on Surveying and Wilford, J. N., (1982). The Mapmakers, The Story of Mapping, Falls Church, VA. the Great Pioneers in Cartography from Antiquity to the Space Age, Vintage Books, a Division of Chriss, M., and Hayes, G. R., revised by Squair, W. H., Random House, New York, NY. (1994). An Introduction to Charts and Their Use, Fifth Edition, Brown, Son & Ferguson, Ltd. Williams, J. E. D., (1992). From Sails to Satellites, The Publishers, Glasgow, Scotland. Origin and Development of Navigational Science, Oxford University Press, Oxford, UK. Collinder, P. A., (1955). History of Marine Navigation, St. Martin’s Press, New York, NY. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Eyges, L, (1989). The Practical Pilot, International Service, and Department of Defense, National Marine Publishing Co., Camden, ME. Imagery and Mapping Agency, (1997). Chart No. 1, United States of America Nautical Chart Hewson, J.B., (1983). A History of the Practice of Symbols and Abbreviations, Tenth Edition, Navigation, Second Edition, Brown, Son & Washington, DC. Ferguson, Ltd. Publishers, Glasgow, Scotland, UK. U.S. Department of Commerce, National Oceanic and Hinz, E., (1986). The Complete Book of Anchoring and Atmospheric Administration, National Ocean Mooring, Cornell Maritime Press, Centreville, MD. Service, (1997). Nautical Chart User’s Manual, Lamb, J.C., (1981). Chart Language—A Self-Teaching First Edition. Washington, DC. This publication is Course in Reading Marine Charts, AquaTech, available electronically at the NOAA web site Chapel Hill, NC. (http://www.noaa.gov/).

Maloney, E. S., (1983). Chapman, Piloting, U.S. Departments of the Air Force and Navy, (1983). Seamanship and Small Boat Handling, Hearst Air Navigation, AFM 51-40, NAVAIR 000-80V-49, Marine Books, New York, NY. U. S. Government Printing Office, Washington, DC.

3-33 3 The Nautical Chart

SELECTED REFERENCES

3-34 Chapter 4

THE NAVIGATOR’S TOOLS & INSTRUMENTS

“Man is a tool-using animal…Without tools he is nothing, with tools he is all.” –Thomas Carlyle

“Out of the ruck we have now instruments of beautiful precision, and much greater capabilities than heretofore. It would, however, be wrong to infer that thereby the duties of the navigator had been rendered less arduous…” –Squire Thornton Stratford Lecky

INTRODUCTION listed in this chapter can fit into one n addition to the magnetic com- carry-on bag. Many navigators Ipass, charts, parallel rules, and routinely carry a kit or “flight dividers mentioned in the foregoing case,” whether aboard their own chapters, navigators of small craft vessels or as guests. may use several other tools or Most of the tools described in instruments. This chapter provides this chapter are relatively inexpen- brief descriptions of several tools sive, particularly in comparison to used by the navigator, including the obvious benefits of their use. plotters, drafting compass, pencils, Accuracy depends on good quality, the nautical slide rule, electronic but also upon familiarity and good calculator, personal computer, technique in the tool’s use. A navi- hand-bearing compass, binoculars gator is encouraged to obtain the (conventional and stabilized), best tools for the money and pro- night-vision equipment, the marine jected use. The most expensive sextant, charts and reference publi- tools, however, are not necessarily cations, tools for measuring water the best, nor are the least expensive depth (electronic and manual), a bargain. VHF-FM homer, radio, and elec- Just as any artisan knows, good tronic navigation systems. With the tools cost money—but quickly exception of the VHF-FM homer prove their worth. For example, and radar, this equipment is made novices rapidly find, to their dis- in small and lightweight versions. may, that the inexpensive student- Indeed, nearly all the tools/devices drafting compass and dividers

4-1 4 The Navigator’s Tools & Instruments

(made of plated, stamped steel) grated wristwatch-GPS unit! The Augmentation System (WAAS) sig- quickly fall apart or rust in the appeal of miniaturized units is nals. Even hikers carry handheld marine environment. But, the same great. However, it is important to GPS units and some rental car com- tools made of marine brass prove consider ease of use when buying panies now offer vehicles equipped their worth by long and dependable these units. Very small electronic with GPS units. service. calculators, for example, have dis- Technology useful for marine Remember that the marine envi- plays that are difficult to read and navigation has advanced rapidly. ronment (salt or fresh water) is the keys are placed so close togeth- Chapters in navigation texts written harsh, and only well-designed and er that it is difficult to avoid data- only 20 years ago typically covered constructed devices survive its test. entry errors when used aboard a the parallel rule, divider and draft- A selection of tools, which the nav- pitching or rolling vessel. ing compass, pencils, a simple igator may find useful, is described handheld compass, a binocular, a

ST G below. Not all are appropriate for A U O A C R timepiece, and a sextant. Night-

S D

U What YOU

A U Y X R each and every small vessel. Loran- ILIA WILL LearN IN vision apparatus, handheld radios C, for example, would not be thIS Chapter and navigation receivers, and per- appropriate for use in an area with sonal computers were the exclusive no loran coverage, and radar too J Which tools are province of the military or the costly and inconvenient to mount particularly useful to the dreams of inventors. for use on very small vessels oper- navigator? ating in fog-free areas during day- J What are the uses of PLOTTERS light hours. Nonetheless, these are these tools? A plotter is one of the most com- discussed briefly in this chapter mon and important of the tools used (and in more detail in Chapter 9) for J What criteria should be by the navigator. The basic purpose the sake of completeness. used in selecting these of a plotter is to be able to draw Marine electronics have been tools? straight lines on the nautical chart miniaturized substantially in recent and to measure the angle of these years. For example, numerous The first two editions of this text lines with respect to either the par- handheld Global Positioning did not include GPS because the allels of latitude or the meridians of System (GPS) and Loran-C units system was not fully operational longitude. For example, a common are on the market. Most are not and receivers cost thousands of dol- much larger than a pocket calcula- lars. Now multi-channel handheld task is to draw a course line (or tor and weigh only a few ounces. units sell for little more than intended track line) on the chart and One company even makes an inte- $100 (discounted)—less than the to measure the true or magnetic price of a good ship’s direction of this course. Plotters are compass. Formerly, also used to draw lines of position GPS and Loran-C (LOPs) to represent visual or elec- receivers were carried tronic bearings. Finally, plotters are only on naval and com- used, in lieu of the mileage scales mercial ships. Now the on the nautical chart, to estimate the well-equipped recre- distances involved over the course ational vessel may carry legs drawn. (These uses are dis- several GPS units- cussed in Chapters 5 and 6 follow- including the ultra- ing.) accurate Differential The device traditionally used by

PHOTO COURTESY OF USCG PHOTO AUX GPS (DGPS) and navigators for plotting is a set of L receivers equipped to parallel rules (also written parallel Now, the well-equipped recreational vessel carries process Wide Area rulers), and many mariners still pre- several GPS units. fer this tool. But, there are several

4-2 The Navigator’s Tools & Instruments 4

other plotting devices in common

ST G A U O A C R

use by small craft operators. One S D U

A U Y KeepING aND MarKING tIMe, X R such tool that is particularly easy to ILIA use is the “paraglide” or Paraline® aN hIStOrICaL aSIDe plotter. This is a graduated rule, with protractors and parallel lines, Measurement of time has always been essential to accurate nav- attached to a roller, constrained to roll such that the rule always (well, igation. An accurate estimate of the ship’s speed and the elapsed nearly always) remains parallel as it time were the essential requirements to estimate distance trav- is moved on a chart. With this tool, the user has the option of either: eled. In the era of the Spanish Armada, the lilting of ship’s boys J rolling the sliding rule from the rather than the sounding of bells marked the passage of time. course line over to a convenient compass rose and taking the According to Felipe Fernandez-Armesto (The Spanish Armada, reading from the rose directly The Experience of War in 1588, Oxford University Press, along the edge of the rule, or J rolling the rule from the course Oxford, New York, NY, 1989), one boy’s job was to mind the sand line over to a meridian or paral- clock, which measured time in glasses each of one half hour lel and reading the direction using one of the protractors duration. (Early mariners believed in redundancy—each ship inscribed on the face of the rule. carried several glasses, including spares!) Each turn of the sand Both methods are suitable and easy to use on many small craft, clock “was announced by a traditional lilt, probably similar to or provided that there is a large enough flat surface available to identical to those recorded by Eugenio de Salazar on a voyage to spread the chart. There are other Santo Domingo is 1574. After the first half hour of every watch, plotting devices, termed “coursers,” which are similar to the “paraline- for instance, the cry rang out: type” plotter, without the roller, but “One glass has gone. with many parallel lines scored lengthwise. These are used in the another’s a-filling. same manner as the rolling type only rather than roll; the parallelism More sand shall run. is maintained by eye relative to the multiple parallel lines printed on If God is willing. the plotter. to my God let us pray. Another tool used for basic plotting consists of two identical to make safe our way. drafting triangles (30° 60° 90°) arranged, hypotenuse-to-hypo - and his Mother, Our Lady, who prays for us all. tenuse, to form two parallel lines which can be expanded by sliding to save us from tempest and threatening squall.” the triangles together until the com- pass rose center is touched (see During the night, the boys had the additional duty of exchanging Figure 4-1). Although drafting tri- calls with members of the watch to ensure that each was awake. angles have their supporters, most navigators find these inconvenient

4-3 4 The Navigator’s Tools & Instruments

circles on charts or displaying time in the 24-hour for- B maneuvering boards. mat, as this eliminates the need to The many uses of the convert from one time system to drafting compass are another. detailed in Chapters Several firms manufacture small 6, 7, 9, and 10 and but easy-to-read electronic timers. include drawing cir- These can be used in the “count-up” A cular LOPs, solving (for measuring elapsed time) or current triangles, and “count-down” (to give time to go) THE PARALINE PLOTTER B drawing arcs of visi- modes. Some portable GPS or bility of lights. Loran-C receivers also have this Dividers should function. also be carried for TWO 30˚ -60˚ -90˚ measuring distances

ST G A U O A C R

DRAFTSMAN’S TRIANGLES S D WrItING ON on the nautical chart. U A U Y XI AR A Dividers and drafting LI ChartS compasses are often sold as sets in a con- As another historical aside, venient carrying the modern practice of draw- case. So-called “one- ing course lines and LOPs on L handed” dividers are FIG. 4-1–The “Paraglide” or “Paraline” made. These are particularly easy to charts is of relatively recent Type Plotters and the Draftsman’s use when underway. origin and reflects the wide- Triangles Used to Determine Direction WRIST WATCH (OR OTHER spread availability and rela- to use on small vessels. These are ACCURATE TIMEPIECE) tively low cost of the modern better suited for voyage planning in Time is one of the basic dimen- nautical chart. Charts used your office or den. sions of piloting. Throughout histo- There are many other novel ry, numerous devices have been by early mariners were devices for plotting. (Descriptions used for this purpose, from hour- extremely valuable and diffi- glasses (see sidebar) through and advertisements for these can be cult to acquire, and most of found in many of the popular boat- chronometers. A reliable timepiece ing magazines.) Some have the is essential. Without a means of what is now termed chart- capability of solving current sailing telling time, dead reckoning navi- work—such as the simple problems (see Chapter 7), relative gation, running a search pattern, or bearing problems, and other navi- identifying the proper characteristic task of maintaining a DR plot gational tasks. As these are typical- of an aid to navigation are impossi- discussed in Chapter 5—was ble. Every crew member should get ly relatively inexpensive, you can done by laborious calculation try several before settling on the in the habit of wearing a wristwatch plotter of choice. when under way. The wristwatch rather than simple drafting. should be waterproof and be able to Cutting up charts to produce DRAFTING COMPASS indicate hours, minutes, and sec- onds. Digital electronic watches the many illustrations in this The drafting compass is an with stopwatch features are fully instrument similar to a pair of text is a procedure that would satisfactory for piloting and naviga- dividers, except that one leg has a tion work. It is also convenient to have left Columbus and his pencil lead attached. This tool is use a watch that has the option of peers aghast! used for swinging arcs and drawing

4-4 The Navigator’s Tools & Instruments 4

PENCILS DISTANCE An ample supply of pencils (and 2 GRN-NAUT. MILES 2.5 2 BLACK-YARDS sharpener) or a mechanical pencil 3 3.5 1.0

(with spare lead) is required for 4 4000

4500 3500 0.90 5000

3000 5500

4.5 6000 chart work. Pencils are used to draw 2500 0.80

5 7000 8000 2000

courses, LOPs, fixes, and for chart 0.70 9000 1800

1600

labels. It is important to use a cor- 6 10000 0.60

1400 rect type of pencil for plotting. The 7 12000 15 20 12 25

30 1200 0.50 pencil should leave a line which is 14000 8

.08 1000 0.45

easy to see, does not smudge, and is 16000

9 .07 900

0.40 18000 SPEED easy to erase completely without 4

.06 800 20000 3.5

10

0.35 leaving a permanent mark on the 3

TIME 50 700 11

25000 2.5

chart. (Erasures, incidentally, do MINS SEC. HRS 12

40 700 0.30 13 2

not arise solely because of errors in 35 600 30000

14 30 550 plotting. Many mariners reuse 0.25

charts and erase the previous voy- 15 500 40000 NAUTICAL SLIDE RULE 450

age plots.) A medium (No. 2) pencil 0.20

400

50000 20

is best. Keep your pencils sharp; a 350 60000

300

0.15

dull pencil can cause considerable 70000

25 250 80000

200 90000

error in plotting a course and will 180

100 160 30 140

also smudge up the chart. A 0.5- 120 35

100

40 90

45 mm mechanical pencil is excellent 80

50 70 for chart work. 60

NAUTICAL SLIDE RULE L The purpose of the nautical slide for details of operation. Note: with FIG. 4-2–The Nautical Slide Rule rule is to solve time-speed-distance the advent of inexpensive electronic (TSD) problems quickly and accu- calculators, the nautical slide rule is you appear eccentric but can pre- rately (see Chapter 5). Although now obsolescent. However, a nau- vent water damage. there are many varieties of nautical tical slide rule does not require bat- slide rules available, they all func- teries and is a useful backup tool. Personal computers (PCs) have tion in the same manner. For any a variety of possible uses on board two values of a TSD problem, the CALCULATORS a vessel. These include: third value can be readily deter- AND COMPUTERS J PCs are used to perform numer- mined. Figure 4-2 illustrates a nau- The electronic digital handheld ous navigation calculations, tical slide rule. The nautical slide calculator is also quite satisfactory from simple TSD problems rule is constructed of waterproof for navigation purposes, and use of through celestial navigation cal- and durable plastic. It has three such devices is encouraged. culations. (PCs are particularly clearly labeled scales: speed, time, Calculators with engineering or sci- useful for computing tide and distance. By adjusting the entific (e.g., ability to do square heights and tidal currents, dis- appropriate dials, values can be roots and trigonometric functions) cussed in Chapter 8.) independently set into the indexes. capability are particularly useful. It J PCs are used to display and With values set into any two scales, is also a good idea to place the cal- the third, or unknown value, will culator in a small plastic bag that manipulate electronic charts. automatically appear at the appro- can be sealed. With practice, the J PCs are used to download way- priate index. Directions supplied calculator can even be used while point information from GPS and with the rule should be consulted inside the plastic bag, which makes Loran-C receivers and manipu-

4-5 4 The Navigator’s Tools & Instruments

late these data by use of data- ing compass unless the boat is first inating parallax error. It does take a base management programs. pointed at the object or the boat is few seconds for a liquid damped J PCs can interface with ship- equipped with a pelorus and/or has compass to reach equilibrium, how- board electronics, such as a GPS been carefully calibrated for beam ever, and requires some practice receiver, and display real-time bearings. This is particularly true if before accurate bearings can be read position information. an electronic compass is used as the from a rolling or pitching vessel. boat’s standard compass. A newer type of hand-bearing J PCs (equipped with appropriate (Depending upon the boat’s design, compass is the fluxgate electronic software) can perform most of it may or may not be convenient to compass. As discussed in Chapter the voyage planning tasks, mount a pelorus.) A hand-bearing 2, the fluxgate compass is a revolu- including determination of compass is a valuable backup unit tionary development of the 1920s courses, distances and bearings and should be carried in every nav- and extensively used in aircraft dur- between waypoints, estimated igator’s carry-on case. ing World War II. Unlike the tradi- time en route, and fuel planning The hand-bearing compass may tional compass, it does not use mag- calculations. be moved anywhere on the vessel nets but, rather, accomplishes the J PCs are used for vessel mainte- by the navigator, independent of the same function by electronically nance logs and other “clerical” steering compass. Care should be sensing the magnetic field. Rapid functions. exercised, however, to ensure that innovation has now made this tech- J PCs with access to the Internet the handheld compass is not influ- nology accessible in a device that enable the navigator to access enced by nearby magnetic fields set can fit in the palm of your hand! valuable information. up by masses of ferromagnetic Several different handheld Laptop PCs are particularly con- (iron, steel) metals, such as the ves- fluxgate compasses are sold. One venient, although it may be difficult sel’s standing rigging, rails, model features digital readout of to read screens in bright sunlight. anchors, engines, and electrical bearings (to within plus or minus 2 Moreover, these units do not equipment. It is often assumed that degrees accuracy when used by an respond well to being dropped, jos- the hand-bearing compass is free of experienced mariner) that are stored tled, or splashed with water. deviation. However, this assump- in memory whenever a bearing is Special-purpose, rugged, and tion should be verified on your boat taken. Several memory locations water -resistant PCs are manufac- (see below). are available, so that an entire round tured. These reportedly work well, Hand-bearing compasses are of bearings can be taken rapidly but are costly. Although laptop PCs produced in a wide variety of sizes, while on deck and later recalled for can use battery power, running time shapes, and prices. The traditional plotting at the navigator’s station. on batteries is limited. For this rea- liquid-filled magnetic compass with Many models also include a stop- son it is more convenient to use vane sighting has been joined by watch for leg timing, estimation of these on vessels with a reliable sup- other, more novel, compass types. a running fix (see Chapter 6), or ply of alternating current (AC). One popular, though still traditional, other purposes. The handheld flux- type is termed a “hockey puck,” gate compass has many advantages, HAND-BEARING COMPASS because of the similarity in appear- but there are two disadvantages that A hand-bearing compass is con- ance to its namesake. The hockey should be noted. First, this and venient for taking bearings to land- puck type is liquid filled and con- other fluxgate compasses require a marks and Aids to Navigation tains a tritium gas capsule to pro- power source. Therefore, batteries (ATONs). This is not a required vide light for reading under condi- are required and spares should be tool if the steering compass may be tions of restricted visibility. carried. Second, unless the fluxgate readily used for this purpose. In Additionally, this compass projects compass is gimbaled, take care to most cases, however, bearings the image of the compass card to hold it as level as possible; other- accurate enough for a precise fix infinity, so that the compass can be wise, bearing errors up to several cannot be determined with a steer- held close to the eye and read, elim- degrees could result. The navigator

4-6 The Navigator’s Tools & Instruments 4

should practice taking bearings, the location(s) on the vessel where first under ideal conditions (e.g., in the handheld compass is relatively calm or protected waters) and later free of deviation. Particularly on a under more challenging circum- power vessel, the mass of metal in stances, before routine use of this the engine(s) may cause apprecia- hand-bearing compass. ble compass deviation if bearings A more sophisticated variant of are taken while standing over or the handheld fluxgate compass is near the engine(s). shown in Figure 4-3. This particular device combines a 5 x 30 mm BINOCULAR monocular, fluxgate compass, elec- A good 7 x 50mm binocular (see tronic range finder, and timer into Figure 4-5) is very useful for locat- one convenient unit. The range ing and identifying visual land- finder enables the vessel’s “distance marks and ATONs. For example,

off” to be determined by using the the identifying numbers or letters COURTESY OF KVH PHOTO INC. INDUSTRIES, known height of an object sighted on buoys can often be read using a L (e.g., a lighthouse) and its relative binocular at a much greater distance FIG. 4-3–Handheld monocular (KVH size (as seen in the monocular) to than with the unaided eye. A binoc- Datascope®) combines a fluxgate com- determine the object’s range. Figure ular is also useful for search and pass, timer, and range finder. 4-4 shows how this is done. First, rescue (SAR) work to identify dis- the height of the object is entered abled vessels. (Incidentally, by hand alone. Also avoid smaller into the instrument. Next, the although the term “a pair of binocu- objective lenses, as these have too observer looks through the monoc- lars” is commonly used, this is not limited a field of view and gather ular and adjusts the height of the technically correct. A binocular too little light to be useful in the horizontal bars to match that of the denotes an optical device with two operational environment—particu- object. The distance off is read in lenses. The plural form “binocu- larly at night. The 7 x 50 mm is the the digital display in the same units lars” refers to more than one standard binocular in use by most as used to enter the object height. device.) of the world’s navies. Rubber-coat- (Circular LOPs resulting from this The meaning of the numbers 7 x ed binoculars are recommended. type of observation are discussed in 50 used above, is as follows: 7 rep- Some mariners prefer a model with Chapter 6.) This particular model is resents the “power” or magnifica- a self-contained illuminated bearing gimbaled, which minimizes any tion of the binocular and 50 mm is compass. A good binocular is error arising from failure to hold the the diameter of the objective (front) waterproof or, at least, water-resis- compass perfectly level. lens in millimeters. Magnification tant. The best binocular is filled Additionally, the device enables is the ratio of the size as seen with with dry nitrogen or some other average bearings to be read directly, the unaided eye compared to the inert gas to prevent condensation increasing the precision of the bear- apparent size when viewed through and fogging. ings taken. the binocular. If a binocular has 7x Some binoculars have a built-in As noted above, it is often magnification, for example, an hand-bearing compass and/or a assumed that the handheld compass object will be enlarged 7 times. A range finder. Though convenient, is free of deviation. The prudent buoy 700 yards away will look only the hand-bearing compass comes navigator should take bearings from 700/7 = 100 yards distant. with a weight penalty. The simplest several locations on the vessel and Unless these are image- or gyro- range finder on a compass is a compare these to the vessel’s stabilized (see below), avoid binoc- series of calibrated reticles. If an known position (e.g., on a pub- ulars with greater magnification object of known width or height lished range) to test the validity of than 7 power, as these are nearly fills one 10-mil unit marking on the this assumption and to determine impossible to hold steady enough horizontal reticle and is known to

4-7 4 The Navigator’s Tools & Instruments

weight is comfortable for you. Novices are sometimes disappoint- Form your judgement on comfort ed that they cannot “see more” only after wearing the binocular for through binoculars. But it is impor- several hours’ use—not from a few tant to note that you see “with your minutes in a marine supply store. mind” as well as through your eyes. Remember, also, that it takes Although there are different patience and experience to learn to points of view on the subject, expe- use the binocular effectively. For rience shows that it is tiring to use a example, reading the numbers or binocular to scan the horizon con- letters for identification of a buoy is tinually. Instead, it is preferable to sometimes more difficult than you use the binocular to examine and PHOTO COURTESY OF KVH PHOTO INC. INDUSTRIES, L might think. In conditions of poor identify an object already detected visibility or haze, it may be very by the unaided eye. FIG. 4-4–Use of KVH Datascope® to difficult to establish a positive iden- Measure Range from Object of Known tification using the binocular—even STABILIZED BINOCULAR Height at short range. Even in good visibil- Image-stabilization (IS) tech- be 20-meters wide, the object is ity, it may still be difficult to do, nology similar to that used in video 1,000 meters away. The formula to particularly if the buoy is backlight- cameras is now employed by one calculate the range is 1,000 times ed in hazy sunlight. Use of a binoc- manufacturer (Canon) of marine the known size divided by the num- ular will only make the silhouette of binoculars. The big advantage of IS ber of mils. the buoy larger, rather than making technology is that binoculars of Quality binoculars allow users the writing legible. In this visibility substantially greater magnification to focus the left and right eyes sep- condition, it may be necessary to (e.g., powers of 10-15 rather than 7) arately, a real benefit to users who use other observable cues and con- can be used on the vessel without wear eyeglasses. textual information to assist in iden- objectionable blurring. Because of Weight is an important consider- tification of an object. Suppose, for greater magnification, these binoc- ation in the selection of a binocular. example, that reference to the chart ulars can detect and/or identify Various models weigh as little as indicates that a buoy is isolated, objects at substantially greater 3 - 7 ounces for a compact set, to 48 without prominent landmarks or ranges than is possible using con- ounces or more for larger binocu- other buoys nearby. A sweep of the ventional binoculars, depending lars. Borrow someone else’s binoc- area with a binocular could verify upon prevailing visibility. ular to develop a sense of what that no other buoys are in the vicin- Despite the use of advanced ity. This evidence, in concert with technology, models now on the estimates of the vessel’s probable market weigh between 21 and 36 position or an electronic fix, could ounces, roughly comparable to be adequate to confirm a provision- weights of conventional binoculars. al identification. Now suppose that Figure 4-6 illustrates an IS (Canon) the buoy were one of a pair marking binocular. the lateral limits of the channel. IS binoculars are more expen- The binocular could be used to sive than conventional models (but, locate the matching buoy. The point not greatly so) and require batteries. of these examples is to illustrate Nonetheless, the additional magni- that several types of information fication is a substantial benefit. can sometimes be integrated to Another manufacturer (Fujinon) establish the identification of an produces a stabilized binocular by PHOTO COURTESY OF STEINER-OPTIK PHOTO GMBH L unknown object, even if it cannot using technology similar to that FIG. 4-5–High Quality 7 x 50 Binoculars be verified using direct means. used on big-ship systems. This

4-8 The Navigator’s Tools & Instruments 4

binocular has a high-speed internal mariners to observe gyroscope to damp image motion. channel markers, As of this writing, this particular nearby boating activi- model has comparable perfor- ty, and floating mance, but is heavier and more obstructions (e.g., costly than those based on IS tech- logs, crab trap mark- nology. ers, half-submerged Mechanical IS technology (a so- containers) under low called Cardanic Suspension system) light conditions. The has also been developed. latest devices are not Technology is likely to advance toys, but precision rapidly, however, which may equipment. If these change the relative costs and bene- prevent one ground- fits of these and other technologies. ing, allision, or colli- sion they will have

paid for themselves COURTESY OF CANON PHOTO USA NIGHT-VISION EQUIPMENT L many times over. Night-vision equipment was FIG. 4-6–IS binoculars reduce once the sole province of the mili- OTHER PARAPHERNALIA motion and enable greater tary. In recent years, however, the magnification to be used. cost (and weight) of this equipment Other tools that might be con- has decreased dramatically. New sidered by the coastal navigator izontal angles. These, in turn, can units are available that weigh in the include an optical range finder and be employed to fix the vessel’s range of 16 - 26 ounces—lighter a sextant. position. In principle, extremely than many binoculars! Figure 4-7 As noted, an optical range find- accurate two-angle fixes can be shows a lightweight handheld er is used to determine the distance determined using a sextant. The model. Night-vision devices are between the vessel and an object of United States Coast Guard battery powered. known height. In this way, a circu- (USCG), for example, used hori- Night-vision equipment uses a lar LOP can be determined, as dis- zontal sextant angles to place its lens to focus available light on a cussed in Chapter 6. Range finders buoys in this manner. (More recent sensitive plate (photocathode). The differ in features, cost, and ease of practice is to position buoys using photocathode plate turns the energy use. It is recommended that experi- DGPS.) In practice, it requires both ence be gained in using from photons into electrons that are these devices before any amplified by a microchannel plate purchase decision is made. and impact upon a phosphor screen Many mariners purchase visible to the eye. Civilian users these from glossy have access (as of this writing) to Christmas catalogs, try three generations of night-vision these once or twice, and devices. Generation III units are then, frustrated, consign the best, but also the most expen- these to a garage sale. sive. A marine sextant is typ- A night-vision scope amplifies ically used for measuring minute amounts of ambient light vertical elevations of celes-

(even starlight) to give the appear- COURTESY OF ITT PHOTO NIGHT MARINER tial bodies for celestial L ance of daylight—albeit a greenish LOPs and fixes—a subject beyond FIG. 4-7–This third generation daylight—rather like living in a the scope of this text. But, sextants futuristic smog-shrouded city. can also be used for measuring hor- night vision device is Night-vision devices enable lightweight and powerful.

4-9 4 The Navigator’s Tools & Instruments

because the use of obsolete charts can be dangerous. Natural and man- made changes, many of these criti- NARROW RETURN: Hard Bottom cal, are occurring constantly, and it is essential to have access to the lat- est information. Additionally, the charts should be updated by refer- ence to Notice to Mariners (see Chapter 10) to insert changes to the charts before a new edition is BROAD, FUZZY: Soft, Silty Bottom issued. These updates are important because it may be years before a new edition of a chart is issued. As of this writing, for example, there are numerous charts that are as many as ten years old. Although the number of changes in these older charts may be insufficient to justify MULTIPLE, SHARP: Rocky Bottom publication of a new edition, the likelihood that there are no material changes over a ten-year period is small indeed. Chart Number 1 (see Chapter 3) fits easily into a navigation case and should be carried. MULTIPLE, BROAD: Fish, or Other Other publications of use by the Objects not on Bottom navigator include the Light List, Tide and Tidal Current Tables, and the U.S. Coast Pilot (USCP). These, too, are discussed in Chapter 10 and should be studied carefully L this purpose. Space and scope con- by the navigator as part of the pre- FIG. 4-8–Interpretation of Depth straints preclude a more complete voyage planning process and, of Sounder Flash Returns discussion here. course, carried aboard the vessel. CHARTS AND REFERENCE training and experience before reli- FLASHLIGHT/MAGNIFIER PUBLICATIONS able positions can be determined A small flashlight (with red from sextant measurement of hori- Although not, strictly speaking, lens) is helpful to read charts on a zontal angles. Even the selection of tools of navigation, appropriate darkened bridge. As well, a magni- the proper objects to sight, so as to nautical charts and navigation refer- fier is sometimes helpful to read the maximize the accuracy of the ence publications are essential. small print on charts or navigation- resulting fix, is a sophisticated sub- With respect to charts, ensure that al reference publications. ject. Readers interested in this topic all necessary charts are carried and Finally, it is appropriate to are directed to the sources listed in that the largest scale charts are include some electronic devices the bibliography at the end of this available, as noted in Chapter 3. among the navigator’s tools. These chapter. Suffice it to say that few Ensure, also, that you have the lat- include a depth sounder, portable coastal navigators use a sextant for est editions of the nautical charts, radio, and radio navigation appara-

4-10 The Navigator’s Tools & Instruments 4

tus such as GPS, Loran-C, and pulses of high frequency sound radar units. These latter aids are waves that reflect off the bottom described in more detail in Chapter and return to the receiver. The 9, but mentioned briefly below. In reflected waves or echoes are con- particular, there are several relative- verted to electrical signals and read ly inexpensive, light weight, and from a visual scale or indicating easily portable GPS and Loran-C device. A device called a transducer receivers on the market. These transmits the sound waves. The make great backup units for the transducer is usually mounted vessel’s main receivers and fit easi- above the low point of the hull ly into the navigator’s kit. (often on the stern). Therefore, this distance must be subtracted from all DEPTH SOUNDER readings to determine the actual The depth of water is one of the water depth below the boat’s hull or key variables that a navigator con- propeller. (Some models of depth siders in safety of navigation. (It sounders can be adjusted to com-

can also be used to provide a “real- pensate for differences in mounting CORP. COMMUNIATIONS COURTESY OF STANDARD PHOTO ity check” on positions determined location to provide an accurate L by other means.) Groundings have depth under the keel or FIG. 4-10–Submersible Handheld accounted for approximately 10% propeller(s).) VHF-FM Radio of recreational boat incidents in Water depth is indicated on a recent years, according to USCG screen by a flashing spot on a circu- bers often jump around, which can SAR statistics. Although a depth lar dial, on a recorder by a trace or, sounder, per se, does not prevent a cause confusion. with modern, digital devices, Some of the more elaborate grounding incident, intelligent use directly as numbers on an electron- of depth information can help to models have the capability to inter- ic indicator. With practice and expe- face with other electronic equip- lower the frequency of these inci- rience, you can also tell what the dents. ment on board, such as a GPS, bottom characteristics and condi- DGPS, or Loran-C receiver and The depth sounder is an elec- tions are, which is useful in select- tronic instrument designed to pro- chart link system, to enable other ing an anchorage. Illustrative read- specialized navigational tasks to be vide information on a continuous ings from an older model are shown basis through the transmission of accomplished. Figure 4-9, for in Figure 4-8. example, illustrates an integrated More modern depth sounders four-in-one navigation system. use Light Emitting Diodes (LED), This unit can display information Liquid Crystal Displays (LCD), or from radar, GPS, a chart link sys- video screens and may combine a tem, and a depth sounder/fish find- variety of other functions in addi- er. The split screen, evident in tion to providing depth information. Figure 4-9, shows electronic chart For example, these depth sounders and depth information. The ability may include a temperature probe to to display a depth history (even determine seawater temperature, a over only a short period) is useful. speedometer to give the vessel’s This provides information on the speed through the water (STW), a rate of change of depth, which is log to indicate the total distance potentially important. Many mod- PHOTO COURTESY OF FURUNO, USA COURTESY OF FURUNO, PHOTO L covered with respect to the water, els have the capability of displaying FIG. 4-9–Multi-functional Navigation and a “fish-finder.” Digital dis- depth in various units, such as feet, Display plays are easy to read, but the num- meters, or fathoms. This is a con-

4-11 4 The Navigator’s Tools & Instruments

venient feature that can be used to bar). Now most lead lines have

ST G A U O match the units with those on the plastic inserts with numbers dis- A C R

S D MarKINGS ON U

A U Y X R ILIA the LeaD LINe nautical chart being used. Constant played. vigilance is necessary, however, to A lead line is difficult to use Historically the “leadsman” avoid misinterpreting depth infor- except in relatively shallow, calm mation. Several groundings have water, at slow speed. Nonetheless, had to learn to recognize resulted when the navigator keep a lead line neatly stowed and marks in the lead line by feel assumed that the display was read- ready for use at all times in the ing in one set of units (e.g., fath- event the depth sounder becomes as it was brought aboard. oms), when another (e.g., feet) was inoperative or electrical power is These markings were stan- programmed. lost and soundings are required. At least one manufacturer Historically, the lead line was dardized. For example, 2 makes a battery-powered, hand- considered one of the most impor- fathoms (1 fathom equals 6 held depth sounder, roughly the size tant navigator’s tools. Waters of a conventional flashlight. (1958) noted that text descriptions feet) was marked by two This text does not explain how of the lead line appeared as early as strips of leather, 3 by three to operate electronic equipment, 1584. Aczel (2001) reported that because the information would be the lead line was regarded as so strips of leather, 5 by a white model-specific, in most cases, and critical to navigation that ships cotton rag, 7 by red woolen quickly become out of date due to detained in port for nonpayment of the rapid technology advances in duty or other infractions had their bunting, 10 by a piece of the marine electronics field. Refer sounding lines confiscated by the leather with a hole in it, etc. to the owners’ manuals for these authorities—a practice that contin- details. ued even after ships carried more Markings on the line were modern instruments. For those called “marks” and the esti- HANDHELD LEAD LINE intent on escape without payment The handheld lead line may be of fees, this is an early example of mated intermediate fathoms used to measure depths of water the benefits of carrying spares! between the marks called when the depth sounder is not avail- able or usable, such as around a VHF-FM DIRECTION FINDER “deeps.” The leadsman used grounded vessel, perhaps from a A very high frequency (VHF)- a quaint phraseology to sing dinghy. The lead line should be frequency modulated (FM) homer used periodically to check the accu- (direction finder or homing device), out the sounds. “Mark racy of the electronic depth although not generally considered a Twain” (two fathoms), “By sounder. piloting tool, per se, allows zeroing The lead line consists of a line in on the source of any VHF-FM the deep six” (six fathoms), marked in feet or fathoms and a radio signal being received. Each “Less a quarter 5” (4 3/4 lead weight, hollowed at one end in time a signal is received, the rela- which tallow can be inserted (called tive bearing of the transmitting sta- fathoms), are examples. The “arming the lead”) to gather sam- tion can be measured and read on a American author Samuel ples of the bottom. As noted in digital display. Errors in relative Chapter 3, bottom characteristics bearing are least when the transmit- Langhorne Clemens (1835- are charted and this information can ting station is on the homer- 1910) adopted “Mark sometimes be used to assist in fix- equipped vessel’s bow, and within ing the vessel’s position. At one plus or minus 5 degrees to 20 Twain” as a pseudonym. time, there were a stylized set of degrees for other orientations. markings on the lead line (see side- (These accuracy figures are specific

4-12 The Navigator’s Tools & Instruments 4

to each model. However, accuracy both conventional and distress com- is typically greatest at zero relative munications. Figure 4-10 shows a bearing.) Direction finding gear is typical marine-band handheld especially useful for SAR work to radio. The manufacturer claims locate the position of a vessel in that this model can withstand sub- distress, but can be a useful tool for mersion up to one meter for thirty navigation as well. Fishing vessels minutes—a handy product feature. use this gear to determine the bear- As with all VHF radios, communi- ing of other vessels that are report- cations are limited by line-of-sight edly catching fish. constraints. Operation of marine- band VHF-FM band radios (base HANDHELD RADIO station or handheld) is regulated by Lightweight and compact VHF- the Federal Communications FM handheld radios are manufac- Commission (FCC). FCC rules tured that are excellent backups for must be observed. While not unrea- the vessel’s principal radio equip- sonable, these rules do place certain

ment. Most have weather channels limits on within-ship, ship-to-ship, COURTESY OF GARMIN PHOTO INTERNATIONAL that provide weather broad casts and ship-to-shore use. L from the National Oceanic The FCC has established a new FIG. 4-12–Handheld GPS unit is and Atmospheric Administration ultrahigh frequency (UHF) family versatile navigation system. (NOAA). Weather information is radio service (FRS) to satisfy the essential to the safety of navigation. need for FM clear-quality, short- in the areas of loran coverage that Handheld radios can be used for range communications between any include (among others) virtually all individual or organization without of the coastal waters off the conti- any FCC licensing requirements. nental United States. The modern The FRS is free from many of the loran receiver is typically integrated marine-band rules. Handheld FRS with a self-contained navigation receivers, such as that shown in computer so that, in addition to the Figure 4-11, enable communica- vessel’s position, other relevant tions over water at distances in voyage parameters can be dis- excess of three miles. These are played. For example, the vessel’s also very handy for crew communi- speed and course over the ground cations on larger recreational ves- can be calculated. Additionally, the sels. For example, the skipper on user is able to establish electronic the flybridge can communicate “waypoints” (which could be clearly with crew hauling in the buoys, other ATONs, turnpoints, or anchor or releasing lines. arbitrary locations) to mark the var- ious legs of the voyage. The loran LORAN-C receiver can display distance to the Loran is an acronym that stands waypoint, miles off a direct course for long-range navigation and, to to the waypoint (so-called cross civilian users, is one of the technol- track error [XTE]), average speed ogy “spin-offs” of World War II. over the ground (SOG), velocity to Details of this system are provided the next waypoint, estimated time PHOTO COURTESY OF ICOM PHOTO INC. AMERICA, L in Chapter 9. But, briefly, a loran until the waypoint is reached FIG. 4-11–Family Radio Service receiver enables a user to determine (sometimes termed time-to-go [TTG]), current set and drift (see Handheld Receiver a highly accurate position fix with-

4-13 4 The Navigator’s Tools & Instruments

tems. The handheld extent that this device is affordable model shown in Figure by many small boat operators. The 4-13 has cartographic lowest priced radar sets are in the capabilities. It $1,000 to $2,000 price range as of includes a base map this writing. and has “up-loadable” detailed map data SPARE BATTERIES available from com- The portable electronic tools pact disks (CDs) or described in this chapter typically “down-loadable” from use batteries as a source of power, Internet sites. although some are equipped to use

PHOTO COURTESY OF GARMIN PHOTO INTERNATIONAL A portable GPS L the boat’s power supply. To ensure receiver makes a great backup for FIG. 4-13–Another Handheld GPS Unit the availability of these portable the vessel’s principal navigation electronics, it is necessary to carry a with Navigation Database receiver(s) and can easily be carried plentiful supply of spare batteries of in the navigator’s bag. appropriate sizes. Some equipment requires special-purpose batteries Chapter 7), and a host of other rele- RADAR vant navigational information. that are not typically available at Finally, a series of adjustable Radar, an acronym for radio marinas or local stores. alarms can alert the navigator to detection and ranging, is also an arrival at a waypoint, penetration of outgrowth of World War II. Radar is ANCHOR a preselected track, an XTE of more also discussed at length in Chapter Except in unusual circum- than a predefined amount, or even 9, but briefly described here. It con- stances (e.g., when running an the fact that the anchor is dragging! sists of a transmitter and a highly inlet), it is possible to stop the boat sensitive receiver. It transmits and to give the navigator time to think, GPS receives in rapid succession and, by consult publications, and make measuring the time until the reflect- GPS operates on a different deliberate decisions. Running a ed signal is received, converts this boat is fundamentally different principle than loran (see Chapter 9), time to a distance, and shows an but the receiver has the same dis- from piloting an aircraft in this electronic image of the radar target regard. When faced with continu- play capabilities. Unlike Loran-C, on a display screen termed a plan GPS is a worldwide system and is ing uncertainty over the vessel’s position indicator (PPI). Radar is position, proximity to navigational presently scheduled to replace used both for navigation, to provide Loran-C. hazards, or the proper course of an all-weather electronic image of action, the wisest choice is to stop Small, portable, and inexpen- the vicinity of the vessel, and for sive GPS receivers are produced— and think. In this sense, it is appro- collision avoidance. Radar can also priate to consider the vessel’s most have the capability of display- be used to detect and warn of areas ing chart or map information and anchor as one of the navigator’s of precipitation (e.g., rain, hail, or most important tools! serving as full-function navigation snow). Radar can be coupled with instruments. Figure 4-12 provides GPS, Loran-C, and other onboard CONCLUDING COMMENTS a photograph of a modern handheld electronics. Even as recently as a GPS unit. This small unit is also a few years ago, the mere thought of The tools and instruments listed full-function navigation receiver. including radar in a course designed above span a great range of uses, For example, the user can enter 500 for small boat navigation would sophistication, and prices. waypoints and 20 routes of up to 30 have been foolish. But rapid tech- (Compare a ten cent No. 2 pencil waypoints each. An adapter nological progress has brought with modern radar!) Navigators enables this unit to be connected to down the price of radar, to the must choose which tools are appro- a PC and electronic charting sys- priate to their needs and budgets.

4-14 The Navigator’s Tools & Instruments 4

Relevant factors include the size skeptical. A commonly heard opposite is true. The most accurate and configuration of the vessel, remark is to the effect, “I don’t need navigation is necessary in coastal waters cruised, prevailing weather, all of this gear or to use such formal waters, a point made in Chapter 1. and the availability of ATONs. At a techniques; I either remain in inland Of course, mere possession of a minimum, a watch, charts, relevant waters or venture only a few miles sophisticated navigational tool or publications, pencils, plotter, offshore.” The most cogent instrument does not guarantee its dividers, a hand-bearing compass, a response is to note that, according proper use or confer immunity from slide rule or calculator, portable to USCG SAR statistics, fewer than trouble. Nonetheless, the proper navigation receiver, and some 10% of all SAR cases occur more tools and instruments used with device for measuring water depth than ten miles offshore. More than skill and judgment certainly lower should be on board. 90% of SAR cases occur on inland the likelihood of incident. this Student reaction to the extensive waterways or within ten miles of said, the most important naviga- list of equipment presented in this shore. Staying close to shore does tor’s tool is an alert and inquiring chapter and, indeed, the formal nav- not eliminate incidents or the need mind, and the most important igation techniques presented else- to navigate with precision. Indeed, quality is constant vigilance. where in this text is sometimes bluewater sailors know that just the

4-15 4 The Navigator’s Tools & Instruments

SELECTED REFERENCES

Aczel, A. D., (2001). The Riddle of the Compass, the Lecky, S.T.S., revised and enlarged by Bowen, G.P., Invention That Changed the World, Harcourt, New (1942). Wrinkles in Practical Navigation, Twenty- York, NY. Second Edition, D. Van Nostrand Company Inc., New York, NY. Anonymous, (1996). Battle of the Level Binoculars, Canon is our choice for self-stabilizing binoculars Maloney, E. S., (1983). Chapman, Piloting, over the heavier and more costly Fujinon. Seamanship and Small Boat Handling, Hearst Powerboat Reports, Volume 9, Number 3. Marine Books, New York, NY.

Bowditch, N. (1995). American Practical Navigator, Markell, J., (1984). Coastal Navigation for the Small An Epitome of Navigation, Pub. No. 9, Defense Boat Sailor, Tab Books, Blue Ridge Summit, PA. Mapping Agency Hydrographic/Topographic Center, Bethesda, MD. This publication is avail- Melton, L., (1987). Piloting with Electronics, able electronically from the National Imagery and International Marine Publishing Co., Camden, ME. Mapping Agency (NIMA) Marine Navigation Norville, W., (1975). Coastal Navigation Step by Step, Department web site (http://www.nima.mil/). International Marine Publishing Co., Camden, ME.

Brogdon, W., (1995). Boat Navigation for the Rest of Waters, D. W. (1958). The Art of Navigation in England Us—Finding Your Way by Eye and Electronics, in Elizabethan and Early Stuart Times, Yale International Marine, Camden, ME. University Press, New Haven, CT.

Budlong, J. P., (1977). Shoreline and Sextant, Practical Wright, F. W., (1980). Coastwise Navigation, Jason Coastal Navigation, Van Nostrand Reinhold Co., Aronson Inc., New York, NY. New York, NY. Van Heyningen, M. K., (1987). The Evolution of the Coote, Capt. T. O., (1988). Yacht Navigation—My Way, Modern Electronic Compass, NMEA News, W. W. Norton & Co., New York, NY. Jan/Feb. Eyges, L, (1989). The Practical Pilot, International Marine Publishing Co., Camden, ME.

4-16 Chapter 5

DEAD RECKONING

“The greatest hazard to navigation is a bored navigator.”

–Anonymous, quoted in Schlereth (1982)

Pettingill said he never plots anything on a chart and rarely refers to them. “I don’t have to plot; I just know it all by heart”

(Capt. Joseph Pettingill, skipper of the Little Gull, who ran the tug aground in four- to five-foot surf off Brigantine, New Jersey, in November 1992, as quoted in Professional Mariner, Vol.1, No. 1, 1993.)

INTRODUCTION vessel’s stern to pass the log (which ead Reckoning (DR) is a rather was, by hypothesis, “dead” in the Dforbidding name given to one water) to form a crude estimate of of the simplest and most basic tech- the vessel’s speed through the niques of navigation. The origin of water (STW). Still others with a this term is lost in antiquity. Some more macabre sense of humor believe that the word “dead” should believe that “dead” is used to be written “ded,” a contraction for describe your future condition if “deduced,” because the method you don’t follow this procedure deduces the future position of a carefully! vessel from a present (or past) Whatever the origin of the term, known position and knowledge of professional navigators believe that the course steered and speed main- DR is one of the most important tained (see Hewson, 1983). The navigational techniques. Piloting Dutch equivalent, “ruwe bereken- and electronic or celestial naviga- ing” (rough estimate) and the tion are not in themselves complete French “route estimée” are clearer systems of navigation—just names in this regard (Hewson, 1983). given to describe ways of checking Others believe that the term the accuracy of DR positions. It is originated with the activity of important to remember that sophis- throwing a piece of wood (a so- ticated electronics can (and do) fail, called Dutchman’s log) in the water cloud cover can prevent observation from the vessel’s bow, and measur- of celestial bodies, and poor visibil- ing the length of time taken for the ity can prevent aids to navigation

5-1 PHOTO COURTESY OF UNITED STATES COAST GUARD 5 Dead Reckoning

plot is considered one of the essen- wise, important that common plot-

ST G A U O A C R

S D

U What YOU tial skills of the navigator. ting conventions are used. After all,

A U Y X R ILIA WILL LearN IN This chapter provides the basics when you hand over the watch to thIS Chapter of DR, nomenclature, symbols, another navigator, it is essential that conventions (rules) for preparing he/she be able to understand chart J Definition of dead DR plots, and a discussion of how a notations, the log, and other perti- reckoning vessel’s speed curve is prepared and nent voyage information. Take the used. The material in this chapter is time to be disciplined, to prepare J How to prepare DR plots closely related to that presented in neat plots, and to record all perti- J Definition of various Chapter 6. Therefore, it is a good nent information when obtained. terms related to course, idea to read through both chapters The midnight to 4 A.M. watch is no heading, bearing, and briefly before studying this chapter time to have to reconstruct earlier in detail. plots or to sort out whether a partic- speeds ular chart notation represented a J Rules of construction of NOMENCLATURE AND DR position, estimated position, or DR plots PLOTTING CONVENTIONS fix! As you gain experience (or Much of the material in this and develop a “seaman’s eye”, as dis- J Time-speed-distance the next chapters is concerned with cussed in Chapter 12), you will be calculations nomenclature and conventions for better able to judge just how much constructing DR plots. There is, information to record and the J How to prepare and unfortunately, no universal agree- appropriateness of various short- interpret a speed curve ment on these definitions and con- cuts. For the present, take the time ventions. Plotting symbols to study this material carefully until employed in Great Britain, for the preparation of DR plots is sec- (ATONs) or other visual references example, differ substantially from ond nature. from being seen and identified. In those used in the United States or such circumstances, it is particular- Canada. (Once again, we appear to UNITS, SIGNIFICANT ly important to be able to estimate a be divided by a common language.) FIGURES, AND PRECISION vessel’s position. An estimate Even within the United States, there Before discussing DR and plot- derived from DR may be all that is are differences in practice and con- ting conventions, it is useful to available. vention—as any careful compari- address the question of accuracy, DR is a relatively simple, some son of the excellent texts given in precision, and significant figures would even say crude, method of the references for this chapter will used in small boat navigation. navigation. Crude though it may be, reveal. The United States Coast These are related but distinct con- history shows that it is possible to Guard Auxiliary (USCGAUX) and cepts. Accuracy refers to the close- accomplish great feats of naviga- the United States Power Squadrons ness of a measured or estimated tion solely by means of DR. The (USPS) have agreed to a common value to the true (but often great Genoese navigator, set of plotting conventions that are unknown) value. Precision correct- Christopher Columbus, for exam- presented in this text. ly refers to the repeatability of a ple, was by all accounts a superb Definitions and conventions are measurement, but is used here to DR navigator, and his historic voy- sometimes tedious reading, and denote the “fineness” of the degree ages were completed almost solely many a student is tempted to try of measurement, or number of dec- by the use of DR. Even today, with expedient shortcuts, or to become imal places, if you will. Table 5-1 all the advances in the art and sci- impatient with an instructor who presents the generally accepted lim- ence of navigation, from computers “nitpicks.” Resist this temptation! its of precision used for small boat to use of navigational satellites, the It is important that a common set of navigation. A vessel’s speed (and ability to maintain a competent DR definitions is employed and, like- related speed terms defined in this

5-2 Dead Reckoning 5

GENERIC CONCEPT ILLUSTRATIVE QUANTITIES MEASURED OR REPORTED TO

Position Latitude Nearest 0.1’ Longitude

Direction Course/Heading Three digits to nearest whole degree Bearing Deviation Variation Current Set

Speed Speed Nearest 0.1 knot or Speed Through Water statute mile per hour Speed of Advance Drift

Distance Distance Nearest 0.1 nautical or statute mile Cross-Track Error unless otherwise specified

Time Time to Go Four digits to nearest minute Fix Time unless otherwise specified Estimated Time of Arrival

L TABLE 5-1–Standards of Precision Used in Small Craft Navigation chapter), for example, is generally shown to enable the reader to fol- position determined by use of taken to the nearest 0.1 knot (or low the calculations or to avoid course and speed alone is termed a statute mile per hour where this unit round-off error. In the end, howev- dead reckoning position (DR posi- is employed). Thus, the value 4.0 er, all values should be rounded to tion). The term dead reckoning is knots is written to distinguish it the limits given in Table 5-1.) sometimes erroneously used to from, say, 3.9 knots or 4.1 knots. refer to the determination of a pre- Technically, 4 knots as written DEAD RECKONING DEFINED dicted position by use of the course means any number between The practice of estimating posi- and speed expected to be made approximately 3.5 and 4.5 knots tion by advancing a known position good over the ground taking into and, therefore, is different from 4.0 for courses and distances run is consideration the effects of wind knots. Those who insist on being called dead reckoning (DR). It is and current. Although positions formally correct label charts with generally accepted that the course determined by consideration of the precision of the measurements (C) known to have been steered and these other factors might be superi- shown in Table 5-1. This text is less the speed (S) or speed through the or in some respects, they are prop- formal and, for example, would use water (STW) are used in the prepa- erly termed estimated positions 4 knots and 4.0 knots interchange- ration of a DR plot. The DR plot is (EP), and not DR positions. DR plots are used for two basic ably (it being understood that, always started from a known posi- purposes: unless otherwise specified, speed is tion or fix and always restarted at measured to the nearest 0.1 knot). the time another fix is determined. J First, a DR plot is used for over- (In some of the numerical examples The effects of current and wind all voyage planning. Voyage presented in this text, a greater drift are not considered in determin- planning includes the determi- number of decimal places may be ing a position by dead reckoning. A nation of the overall course(s)

5-3 5 Dead Reckoning

and speed(s) to be run, identifi- sured relative to true north. If the point.” Elsewhere in this same cation of checkpoints or way- course is specified relative to mag- book, he notes that in the era of sail points where courses or speeds netic north, an “M” is written after mariners termed a position deter- are to be changed or the vessel’s the course. mined solely by dead reckoning the position determined, fuel or Beneath the course line is writ- “point of fantasy.” Whatever its amenity stops, or points of inter- ten the speed being run by the ves- limitations, the DR position pro- est. In short, the voyage plan sel. As noted, the vessel’s speed is vides a useful indication of the ves- describes the intended trip in normally written in nautical miles sel’s positon. general terms sufficient for per hour, knots using an “S” fol- planning purposes. These plans lowed by one, two, or three digits. BEGINNINGS: are developed in sufficient detail Speed can generally be measured to ILLUSTRATIVE DR PLOTS to ensure adequate fuel reserves the nearest tenth of a knot and, so, Figure 5-1 shows an illustrative (see Chapter 11) and safe pas- one decimal place is used. The DR plot that might be suitable for a sage. The plan is best made in speed may also be indicated in powerboat. This plot shows a por- the comfort of your home or statute miles per hour if distances tion of a voyage in Buzzards Bay office or on the vessel while still are customarily measured in statute on the 1210-Tr chart. At 1:00 P.M. at the dock. miles on the chart being used. The (1300), the vessel fixes its position J Second, a DR plot is maintained abbreviation kn., kt., or mph is not using a Global Positioning System while underway to reflect the necessary and is omitted. The dis- (GPS) receiver (see Chapter 9) in vessel’s actual progress and tance to the next objective (e.g., the the vicinity of buoy “BR” (Qk Fl G) position. This tactical DR plot end of a course leg, turn point, or north of Quicks Hole. A circle with reflects all the factors that may other waypoint) may be written a dot in the center is used as the alter plans—a late start, next to the speed with a “D” fol- symbol for a fix, with the figure unplanned diversions, winds or lowed by the distance (to the near- 1300 (to denote the time) written currents different than anticipat- est 0.1 mile). parallel to one of the chart axes, and ed. This on-the-water tactical The position where the vessel the letters GPS to denote the basis DR plot may not be as elaborate would be expected to be, after run- of the fix. The vessel proceeds on or neat as the voyage plan (or ning a specified course at a speci- course 000 true at a speed of 6.1 planning plot) but makes up in fied speed for a particular length of knots as shown by the labels above accuracy for what it loses in time (expressed in hours and min- and beneath the course line respec- appearance. Both types of plots utes), is calculated, the resulting tively. (As discussed below, the are included in this discussion. distance is marked off on the course direction is taken to be with respect line, and the DR position is plotted. to true north unless otherwise spec- ELEMENTS OF The DR position is labeled by a dot ified; so it is not necessary to use a DEAD RECKONING surrounded by a semicircle (or par- special chart label to denote true. In the process of dead reckon- tial circle) and the time, using the Likewise, as noted above, ing, courses are drawn on the chart 24-hour system, is written on the “degrees” are understood to be the as solid lines from a known position chart at an angle to the horizontal. unit of angular measurement unless (usually a point of departure) or a As a historical aside, Pérez- otherwise specified; so, no degree fix. Course lines are identified by Reverte (2000) quotes Gabriel de symbol is employed.) A DR posi- their true direction, which is written Ciscar in Curso de Estudios tion is plotted at 1335, written at an preceded by a “C” on top of, and Elementales de Marina as follows, angle to the course line, to show the parallel to, the course line in the “The location at which the ship time when the vessel changes usual three digits (e.g., 000 for finds itself as a result of prudent course (or will change course) to North); the degree sign (°) is judgement only, or of data about 057. The DR position is shown by implicit and is omitted. Unless oth- which there is considerable uncer- the semicircle. For route segments erwise specified, courses are mea- tainty, is called the reckoning that involve course changes, as is

5-4 1426

C 152 S 6.1

1447

C 057 S 6.1

1335

C 228 S 6.1

1518

S 6.1

C 000

C 228 S 4.0

1606

1300 GPS

L FIG. 5-1–An Illustrative DR Plot 1700

C 024 S 4.0

1615 227 1630

1615

1615 1615 321

1615 349 1600

1530

C 024 S 4.0

1500

L FIG. 5-2–DR Plot Restarted after Fix 1430 Dead Reckoning 5

Date: VeSSeL: NaVIGatOr:

position Course Speed time L: Lo: Compass Magnetic true rpM StW remarks

L TABLE 5-2–An Illustration of a Ship’s Log Format the case in Figure 5-1, a partial cir- ficult to verify in practice (without leg beginning at 1606, as this is cle is used. At 1426, another DR a GPS or Loran-C receiver aboard). only an excerpt from the DR plot. position is plotted where the vessel But that is getting ahead of the Figure 5-2 shows another DR changes course to 152 (speed main- story. The purpose of Figure 5-1 is plot. The voyage begins at Block tained at 6.1 knots). At 1447, anoth- simply to illustrate DR plotting Island. Here there are no planned er DR position is plotted at the conventions. course or speed changes, and DR course change to 228 for the return Figure 5-1 illustrates many of positions are shown on the hour and trip. At 1518 the vessel’s speed is the symbols and conventions half hour as would be more typical slowed to 4 knots (written 4.0 on employed for DR plots. Normal for small craft navigation in pilot the DR plot), shown by the semicir- practice is to start the plot at a point waters. At 4:15 P.M. (1615), the cle, etc. This simple DR plot shows of departure, denoted a circle with a vessel is able to fix its position by the DR positions, course legs, dot in the center. Courses and taking three bearings on land-based times, and speeds for each leg of the speeds are appropriately labeled, objects. (Fix labels and plotting voyage. and DR positions (and times) are techniques are discussed in Chapter For clarity, the turn points in the plotted at each course or speed 6). Accordingly, the position repre- DR plot shown in Figure 5-1 were change. Although not shown on sented by the fix is plotted as a cir- drawn in the open water. In well- Figure 5-1 to avoid a cluttered cle with a dot in the center. For rea- marked waters, it is convenient to appearance, DR positions would sons that will be apparent in plan course “legs” so as to begin or also normally be plotted at regular Chapter 7 on current sailing, a DR end near buoys, along the lines intervals (typically every half hour position is typically plotted at the defined by ranges, or at other loca- or hour) and whenever a fix or line time of the fix. Finally, the tactical tions where the vessel’s position is of position (see below) is deter- DR plot is renewed (started again) easily determined. For example, the mined. Finally, Figure 5-1 omits from the fix determined at 1615 and 1426 turn point shown might be dif- writing a course and speed on the not from the 1615 DR position. The

5-7 5 Dead Reckoning

practice of restarting a tactical DR

ST G A U O A C R

S D

U plot at a fix increases the accuracy

A U Y X R IS aLL thIS reaLLY NeCeSSarY? ILIA of subsequent DR positions, to min- Students frequently ask this question. They note from their imize cumulative errors arising from factors neglected in the prepa- experience that most recreational boaters do not go to the trou- ration of the DR plot. ble of making a DR plot and updating it with fixes. The short answer is “not always.” Cruising in familiar and easy-to-navi- SHIP’S LOGS—A COMPLEMENT TO THE gate waters may not require formal plotting. However, in more TACTICAL DR PLOT challenging waters or those less familiar to the navigator, some The above examples show that a planning and plotting is essential. “Navigating by Seaman’s properly prepared DR plot can be Eye”, as it is called, can have disastrous consequences. The both self-contained and informa- tive. Even so, a ship’s log is a use- magazine, Professional Mariner, has a semiregular feature ful complement to the DR plot. “maritime casualties” which chronicles such misadventures. There are probably as many pro- Failure to maintain a DR plot is a common explanation for acci- posed formats for pages in a ship’s log as there are textbooks on navi- dents. Here are a few samples: gation. The essentials of the log are J The mate of a Reinhauer tugboat drove a loaded barge up onto the summarized in Table 5-2. In addi- rocks outside of New London, Connecticut, in December 1992. He tion to the “header” entries, was navigating by eyeball and became confused over the identity of columns are provided for time, buoys and reportedly did not fix his position on a chart nor plot a position (latitude, longitude), course into the harbor. course (compass, magnetic, true), J The Norwegian cruise ship Starward ran aground on the island of St. engine speed (in revolutions per John (U.S. Virgin Islands) in February 1994. Among the causes of minute [RPM]), vessel speed, and a this grounding, according to United States Coast Guard (USCG) space for remarks. (Remarks could sources, was the failure of the crew to plot DR positions or to deter- include weather information, a brief mine set and drift of current. discussion of the impertinence of Mr. Christian, and plans for subse- J The mate of the 989-foot freighter Indiana Harbor drove directly quent disciplinary action.) The into the Lansing Shoals Light in September 1994. According to time-honored ship’s log is the offi- USCG sources, the mate was navigating by “eyeball” without plot- cial record of the voyage. Later in ting his position and failed “somehow” to notice the lighthouse ahead Chapter 7 on current sailing, a of the ship. somewhat more elaborate work- J The first mate of the 207-foot cruise ship Camden grounded his ship sheet is presented, but the essentials on rocks in Penobscot, Maine, in September 1992. The mate lowered are shown in Table 5-2. the vessel’s speed, sounded fog signals, and monitored progress by The log provides space for not- radar. Nonetheless, he did not plot the ship’s position nor maintain a ing relevant calculations (in the DR track. “remarks” section), and other infor- mation that is not shown on the DR Each of these incidents was preventable. Each resulted in regu- plot. Normally, the log book entries latory action to suspend the license of the mariners involved. and the tactical DR plot are pre- Presumably, none of these officers would now ask the question, pared at the same time. In some cir- cumstances, for example, on long “Is all this really necessary?” distance cruises, the helmsman will make entries into the ship’s log, and

5-8 Dead Reckoning 5

can choose courses and speeds For purposes of this chapter, it is

ST G A U O A C R

S D INfOrMaL U almost at will, sailing vessels have sufficient to note that the various A U Y X R ILIA LOGS some unique operating restrictions. tacks can be diagramed on a DR In particular, sailing vessels cannot plot, which presents a “hem- Many recreational boaters sail directly against the wind nor, stitched” (zigzag) appearance as do not prepare a formal log for that matter, usually any closer shown in Figure 5-3. This figure than within approximately plus or assumes that the desired track is as discussed in this chap- minus 35° to 45° of the direction directly upwind (e.g., overall ter—or for that matter a DR from which the wind is blowing. course is 090 and the wind is from The sailing vessel’s speed through 090) and that the vessel cannot sail plot. Nonetheless, even if a the water is a complex function of any closer to the wind than 45°. In formal log or DR plot is not the boat length, design, sail trim, this illustration each separate tack is sea state, wind speed, and direction identified with course, speed, and justified, it is important to of the wind relative to the course. DR positions at the turn points. In write down, at least, the (For a fixed wind direction, a so- this case, the DR positions on the called polar diagram that varies hour and half hour coincide with courses, times, and fixes. with the wind speed and is unique the turn points. Additionally, DR The entries in this abbreviat- to each vessel design describes the positions should be plotted whenev- relation between boat speed er a fix, running fix, or LOP is ed log will be sufficient to get through the water and course.) determined. If the tacks are short, as Therefore, when proceeding in an would be the case, for example, in a you home. Professional upwind direction, the sailor travers- narrow river, this detail is generally skippers always have an es the overall course in a series of omitted and factored instead into a “tacks”—short legs off the wind an reduced speed through the water. approximate idea of their appropriate number of degrees. The But, if the tacks are of appreciable vessels’ positions. Skippers determination of the proper angle of length, diagrams such as shown in these tacks relative to the desired Figure 5-3 should be used. In blue- who don’t note courses and overall track line is a matter for water or ocean sailing, depending times are likely to become optimization. Small angles act to upon the circumstances, individual minimize the extra distance cov- tacks could be quite long, a matter lost unless in very familiar ered, but boat speed is adversely of several hours or even days. In waters. affected if the vessel sails too close such cases it would be particularly to the wind. Large angles increase important to plot these tacks as the navigator or captain will update the average speed through the water actually sailed. On courses that are the DR plot at a later time. But nor- but also increase the total distance not directly upwind, the sailing ves- mally, the entries in the ship’s log that needs to be covered relative to sel would be functionally equiva- and the updating of the tactical DR the rhumb line course. For any lent to a power vessel, and so, too, plot are contemporaneous. given vessel, prevailing wind, and would be the resulting DR plots. current, there is an optimal tack When a sailboat has to tack DR PLOTS FOR sequence that minimizes the time upwind, the actual distance sailed SAILING VESSELS taken to travel between any two between any two points can be sub- (This section is somewhat more points. The determination of this stantially greater than the rhumb technical and may be omitted with- optimal sequence is a sophisticated line (straight-line) distance. If a out loss of continuity.) The prepara- topic well beyond the scope of this sailboat were forced to tack at an tion of DR plots for sailing vessels course in navigation, but is angle, theta, to the desired track presents some novel aspects. This is addressed (at least implicitly) in line, the actual distance covered because, unlike power vessels that some of the texts referenced at the compared to rhumb line distance end of this chapter. will be increased by a factor

5-9

1800

1730

S 3.0 S C 045 C

1700

1630

1600

1530

C 135

1500 S 3.0

1430

S 3.0 S C 045 C L FIG. 5-3–DR Plot for a Sailing Vessel Showing Individual Tacks 1400 Dead Reckoning 5

1/cos[theta]. Thus, for example, if little alternative but to await more ation on that heading. Deviation theta were 45°—as it is in the illus- favorable winds or turn on the “Iron can change with time as dis- tration in Figure 5-3, the actual Genoa” (engine). cussed in Chapter 2. The result length of the “hemstitched” course Finally, winds are often quite is that course lines left on charts through the water would be greater variable and, therefore, the tacks for long periods of time may than the rhumb line distance by may have to be adjusted from those bear no relation to the correct some 41%. Consequently, the speed originally planned. Consequently, value if they have been labeled made good (absent current or lee- the navigator aboard a sailing ves- as compass courses and the way) along the rhumb line would be sel may have to make frequent revi- deviation has changed. sions to the DR plot to “stay with only approximately 71% of the J Course of Advance (COA): the vessel.” Navigators aboard speed through the water. In fact, the This indicates the direction of power vessels also have to adjust actual situation is somewhat worse the intended path to be made DR plots, but with less frequency. than the idealized diagram in Figure good over the ground. 5-3 would indicate. This is because Little wonder, then, that sailboat the wind tends to blow the boat far- navigators get to stay in practice! J Course over Ground (COG): ther downwind of the course This indicates the direction of steered. The angle between the DEAD RECKONING TERMS the path actually followed by course steered and the course over DEFINED AND ILLUSTRATED the vessel over the ground, usu- the ground due solely to wind There are several important ally an irregular line. effects is termed leeway and can terms used in the practice of dead J Course Made Good (CMG): amount to 5 degrees or more. reckoning. These terms are defined This indicates the single resul- (Leeway or current would not be and explained below: tant direction from a point of shown on a DR plot but would J Course (C): Course is the aver- departure to a point of arrival or affect the overall ground speed age heading and the horizontal from waypoint to waypoint. attained. Power vessels are also direction in which a vessel is (Synonym: Track Made Good) affected by leeway, which can be intended to be steered, J particularly important for slow- Current: The horizontal veloci- expressed as the angular dis- ty (speed and direction) of water moving vessels with a large surface tance relative to north, usually area above the waterline. Consult over ground. (See Chapter 7 for from 000 at north, clockwise a more complete definition.) the references at the end of this through 359 from the point of chapter for details.) Sailboats take departure or start of the course J Dead Reckoning (DR) position: longer to complete a voyage than to the point of arrival or other A position determined by dead power craft, not only because of point of intended location. The reckoning. their hull design and generally reference direction is true and if J Dead Reckoning Plot: A DR slower speeds through the water, so used, need not be labeled. plot is the charted movement of but also because of the additional However, some navigators find a vessel as determined by dead track length required for tacking it more convenient to also use reckoning. upwind. Because the extra distance magnetic courses. These mag- covered can be material in certain netic courses are labeled after J Drift: The speed in knots at circumstances, it is necessary to go the three-digit direction with the which the current is moving. to the trouble of factoring tacks into letter “M.” Others have advo- Drift may also be indicated in the DR plot. Laying out the tacks is cated going one step further and statute miles per hour in some also important to ensure that the indicating compass courses, areas—the Great Lakes, for sailing vessel remains in safe labeling them with the letter example, or other areas for waters. If there is not sufficient safe “C.” This practice is discour- which nautical charts are based water (sea room) to accommodate aged, since a compass course on units of statute miles. This these tacks, the sailing vessel has depends upon the value for devi- term is also commonly used to

5-11 5 Dead Reckoning

mean the speed at which a ves- servative navigators would not taken at different times and sel deviates from the course regard these as fixes at all), adjusted to a common time. This steered due to the combined because floating aids may be off practice involves advancing or effects of external forces such as station. (See Chapter 6 for more retiring LOPs as discussed in wind and current. details.) The angle between the Chapter 6. intersecting LOPs is important. J Estimated Position (EP): An J Set: The direction in (towards) For two bearings, the ideal angle improved position based upon which the current is flowing. is 90°. If the angle is small, the DR position and which may This term is also commonly however, a slight error in mea- include, inter alia, factoring in used to mean the direction in suring or plotting will result in a the effects of wind and current, which a vessel is being deviated relatively large error in the plot- or a single line of position. from an intended course by the ted position. LOPs from visual combined effects of external J Line of Position (LOP): A line bearings to nearby objects are force such as wind and current. of bearing to a known origin or preferable to those obtained reference, along which a vessel from objects at a distance. The J Speed (S): The rate at which a is assumed to be located (see most accurate visual fix is vessel advances relative to the Chapter 6, Line of Position). An obtained from three objects water over a horizontal distance. LOP is determined by observa- above the horizon 50° to 70° (or When expressed in units of nau- tion (visual bearing) or mea- 110° to 130°) apart in azimuth. tical miles per hour, it is referred surement (e.g., radar). An LOP A dead reckoning plot is always to as knots (kn. or kt.). One knot is assumed to be a straight line renewed at a fix or running fix. equals approximately 1.15 for visual bearings, or an arc of statute miles per hour. J Heading (HDG): The instanta- a circle (radar range), or part of neous direction of a vessel’s J Speed Made Good (SMG): some other curve such as hyper- bow. It is expressed as the angu- Indicates the average speed bola (loran). LOPs resulting lar distance relative to north, actually accomplished relative from visual observations (mag- usually 000 at north, clockwise to the ground. netic bearings) should be con- through 359. Heading should verted to true bearings prior to J Speed of Advance (SOA): not be confused with course. plotting on a chart. Indicates the speed intended to Heading is a constantly chang- be made relative to the ground J Fix: A known position deter- ing value as a vessel yaws back along the track line. mined by passing close aboard and forth across the course due an object of known position or to the effects of sea, wind, and J Speed Through the Water determined by the intersection steering error. Heading is (STW): The apparent speed indi- of two or more LOPs adjusted to expressed in degrees of true, cated by log-type instruments or a common time, determined magnetic, or compass direction. determined by use of tachometer from terrestrial, electronic, and speed curve or table, at a J Position: On the earth, this refers and/or celestial data (see particular point in time. to the actual geographic location Chapter 6). The accuracy, or of a vessel defined by two pa - J Speed over Ground (SOG): The quality of a fix, is of great rameters called coordinates. actual speed made good at any importance, especially in coastal Those customarily used are lati- instant in time with respect to waters, and is dependent upon a tude and longitude. Position may the ground. number of factors. LOPs must also be expressed as a bearing be based on known and/or iden- J Track (TR): The intended hori- and distance from an object, the tified sources. Fixes determined zontal direction of travel with position of which is known. based on the charted positions respect to the ground. of buoys need to be regarded J Running Fix (RFIX): A fix (Synonym: Intended Track, with particular suspicion (con- obtained by means of LOPs Track Line)

5-12 Dead Reckoning 5

SUBTLETIES IN THE

ST G A U O A C R

S D DEFINITIONS LISTED ABOVE U A U Y X R ILIA What IS a pOSItION? On quick reading, many of the terms listed above may appear quite In this chapter, a position is defined in terms of coordinates similar—and, indeed, some are (latitude and longitude). Some experienced mariners would nearly identical. But, there are some challenge this definition as being excessively narrow. To them, subtle differences that are drawn. This section explores some of the a position only has relevance when plotted on a chart and cor- similarities and differences among rectly interpreted. For example, does the position described by the above terms. the fix indicate that the vessel is along the intended track at the The basis for distinction among “proper” time? If so, there are no currents or these currents some of these terms relates to the have been correctly compensated for in the selection of the difference between average and instantaneous values. For example, course. Does the position indicate that the vessel is standing course relates to the average direc- into danger? If so, adjustments must be made to avoid the dan- tion of travel. A helmsman may ger and return the vessel to the intended track. Does the posi- attempt to maintain a course of 090 tion indicate that an intended turn point is near? If so, prepa- degrees but, for a variety of reasons (e.g., motion of the sea, collision rations are made to change the vessel’s course. avoidance), the heading of the ves- Professional navigators know the approximate position of sel might vary from 080 to 100 their vessel at all times—regardless of whether or not they know degrees. The course steered may the specific coordinates of this position. Moreover, profession- average 090, but at a given instant al navigators have situational awareness—they are aware of the in time the heading might be 097. The average of the heading values position of the vessel with respect to their intended track and will equal the course steered, how- possible hazards to navigation. If hazards are close by, the pro- ever. The reader may be puzzled by fessional navigator pays much more attention to the vessel’s the subtlety: is this a distinction exact position. without difference? Emphatically not! The course (090 in this illustra- tion) is the appropriate variable to The distinction between speed Likewise, speed is defined relative be plotted on the DR plot. But, sup- (average speed) and speed through to the water, whereas SMG is rela- pose a relative bearing of an ATON the water (instantaneous speed) is tive to the ground. You steer a were observed to be 045 while the less important for most applications course and maintain a speed, but vessel was heading 097. You need and, therefore, these terms are used these are elements of relative to add the vessel’s magnetic head- interchangeably in this text. Strictly motion. The CMG and SMG refer ing (not the course) to the relative speaking, it is speed, rather than to the vessel’s actual progress rela- bearing to calculate a magnetic STW, that should be shown on the tive to the earth and, hence, refer to bearing (142 in this example, DR plots, but this distinction is aca- what is termed true motion. assuming no deviation). When tak- demic. (Current accounts for the difference ing such a bearing and calling Another basis for distinction between these two concepts—the “mark” to the helm, the helmsman between the concepts defined above horizontal flow of water relative to should answer 097 (the heading), is the plane of reference. Course, the earth as is discussed in Chapter not 090 (the course)—hence, the for example, is defined relative to 7 on current sailing.) As with CMG need to distinguish between these the average heading. But, CMG and SMG, COG and SOG are concepts. Not all such distinctions refers to the average progress rela- defined relative to the ground, but are equally meaningful, however. tive to the ground or earth. COG and SOG relate to the actual

5-13 5 Dead Reckoning

(often irregular) path of the vessel’s fast rules for determining the accu- minutes than if these are taken progress with respect to the earth, racy of a DR position. Brogdon hourly. Fixes should not be viewed while CMG and SMG relate to the (1995) notes that the USCG esti- as “happenstance”, or chance average course and speed traveled mates that the probable error in a occurrences, but rather should be with respect to the earth. As a prac- boat’s DR position is approximate- deliberately planned for in any voy- tical matter, COG and SOG are rel- ly 15% of the distance traveled— age considering the location of atively unimportant—CMG and but this rule of thumb is very ATONs, other suitable charted fea- SMG are the key terms here. approximate. tures, and available navigation The basis for distinction The factors contributing to error equipment on board the vessel (e.g., between yet other terms defined in DR positions include all the ele- GPS, Loran-C, radar). When you above refers to the distinction ments of current (see Chapter 7) examine a chart and lay out a between anticipated or planned such as leeway and helmsman’s course to your destination, get in events and actual events. The terms errors. These factors vary with the the habit of preplanning fix oppor- track and COA are synonymous and circumstances of the voyage. Tidal tunities. Indeed, the availability of refer to the planned route for the and other currents often affect river suitable fix opportunities should be vessel with respect to the ground. voyages. These are typically ori- one of the important factors in route So, too, does CMG, but the distinc- ented along the intended track, so selection. This subject is developed tion between these terms is that that lateral errors in position are not more thoroughly in Chapter 6. COA is an intended path, whereas large—even if the vessel is ahead of DR position accuracy is also CMG is what has actually occurred. or behind schedule. Experience related to the care taken by the nav- Thus, on the basis of forecast or shows that it is difficult to maintain igator in the preparation of the DR estimated currents, a course may be a constant course. The necessity to plot. In the age of electronics, set in order to achieve a certain avoid traffic, to avoid taking seas at where a fix can be taken with no track or COA, but the net result an uncomfortable angle, and helms- more effort than reading the display with the actual currents is the CMG. man’s inattention/distraction all of a navigation receiver, there is a If the forecast or estimate was accu- contribute to the variability of the temptation to pay less attention to rate, then the COA and CMG will course steered. Hewson (1983) more traditional methods of naviga- be virtually identical. Otherwise, cites an excerpt from early text by tion. Overreliance on automated these will be different, and the dif- John Davis (Seaman’s Secrets, systems should be avoided, howev- ference is meaningful. Similarly, 1594), which said that one of the er. It deprives the navigator of the the SOA is the intended or estimat- reasons why the results found in opportunity to maintain necessary ed speed along the track with navigation so often differed from skills and (as boaters soon learn) respect to the ground, but the SMG those expected was “the stredge automated systems can fail. is the actual ground speed deter- (steerage) may be so disorderly Columbus could certainly have mined after the fact. handled so that thereby the Pylote benefited from GPS, Loran-C, and In subsequent chapters, the may be abused.” radar. But, odds are he would have meaning and use of these terms will The effects of these errors (cur- been a better navigator than most of become more familiar. For purposes rent and helmsman’s error) over the us equipped with the same tools of this chapter, the key terms need- time period since the vessel’s last because he knew well how to live ed are speed, course, DR position, known position are relevant to the without these advances. (See LOP, and fix or departure point. accuracy of DR. Therefore, to a Morrison, 1942, for a discussion of large degree, the navigator can con- DR in the age of Columbus.) ACCURACY OF DR PLOTS trol the accuracy of the DR posi- Students frequently ask about tions by taking more frequent fixes. SUMMARY OF THE RULES the accuracy of DR plots, so it is Other factors being equal, the accu- FOR CONSTRUCTION OF appropriate to address this topic. racy of DR positions will be twice DR PLOTS Unfortunately, there are no hard and as good if fixes are taken every 30 At this point it is appropriate to

5-14 Dead Reckoning 5

summarize some of the key conven- Third, and more generally:

ST G A U O A C R J S D hIStOrICaL tions in the preparation of DR plots. U Make every effort to produce a

A U Y X R First, with respect to symbols: ILIA aSIDe— neat, uncluttered, and easy-to- J The circle with a dot inside is Dr pLOtS IN understand plot. Measure cours- used to denote a fix, or running the aGe Of es and distances carefully. fix. Each DR plot must begin COLUMBUS J Keep the plot up to date by with a circle. The time (4 digits recording and annotating fixes and 24-hour time) is written Christopher Columbus used DR and LOPs, etc., as these are next to the known position, almost exclusively. Charts were taken. Note relevant data and preferably parallel to one of the appropriate remarks in a log as chart axes. made of sheepskin. Courses well as maintaining the plot on J Courses are drawn with a solid and times were noted. However, the nautical chart. line. The course steered is writ- ten above the line beginning charts were not drawn upon. REPRISE: VOYAGE PLANS with a capital “C” and followed Rather, a divider was used to AND TACTICAL DR PLOTS by the true course (in degrees to As noted above, DR plots are place a pinprick at the calculat- three digits without the degree prepared both for planning purpos- symbol). If a magnetic course is ed DR position. The process es (the voyage plan) and to keep written, the letter “M” should track of the vessel’s actual progress was called by the Spanish fazer follow. Refrain from writing (the tactical DR plot). Now that the compass courses on the chart, or echar punto (to make, or details of DR plots have been although these should be noted covered, it is appropriate to apply the point), or cartear (do in the ship’s log. The speed discuss these plots and their should be written beneath the the chart). According to purposes again. course line beginning with a The voyage plan is prepared Morrison (1942), this was called capital “S” and the speed to one prior to leaving the dock. It is a decimal place, if known. “pricking the chart” or “prick- complete plot, covering the entire length of the planned voyage, pre- J DR positions are denoted with a ing it off” by English mariners pared in sufficient detail to ensure semicircle (or partial circle) in the same period. that enough fuel is carried, en route with a dot inside, and the time (4 and arrival time estimates are digits and 24-hour time) noted approximately correct, a safe route J DR positions should also be at an angle to the course line is chosen, and other planning objec- plotted whenever a fix, running written nearby. tives are satisfied. But, because no fix, or LOP is determined. Second, with respect to plotting fixes or LOPs are available at the J conventions: DR positions should also be time that this DR voyage plan is J DR positions are to be based plotted whenever a course or prepared, this plot contains only solely on the course steered and speed change takes place. In the DR positions (not fixes or lines of speed using TSD calculations event that numerous course or position), courses, and other anno- (see below), and not upon any speed alterations, fixes, or LOPs tations deemed suitable by the nav- igator. Both times and positions allowance for current or leeway. are performed, the hourly or half-hourly DR positions may shown on this DR plot may later J DR positions should be plotted be omitted for clarity. turn out to be in error. In particular, at appropriate time intervals because the starting time is often (e.g., every hour or every half J A new DR plot should be started uncertain, the navigator may find it hour on the hour and half hour, whenever a fix or running fix is more convenient to represent time or more frequently if necessary). obtained. as elapsed time (e.g., hours and

5-15 5 Dead Reckoning

minutes after trip start), rather than

ST G A U O A C clock time. R

S D U

A U Y X R The tactical DR plot is prepared ILIA NaVIGatION at SpeeD “as you go” and reflects the actual starting time, actual fixes when Navigation in fast boats (say those capable of 15 knots or more) determined, and the vessel’s actual offers additional challenges and opportunities. These boats (as opposed to planned) progress. The tactical DR plot is updated and often lack the navigational “amenities” such as a chart table, are started again after each fix and shows DR positions whenever an typically more “lively” (making underway plotting more LOP or fix is determined. (See side- difficult), but are less affected by current (which increases the bar on fast boat navigation for alter- natives to the tactical DR plot.) On accuracy of the planning chart). Operators of fast boats a voyage with many legs where borrow tricks used by aircraft pilots and compensate for the numerous course or speed changes are planned, it is not convenient to difficulty of maintaining a tactical DR plot by: (1) taking more replot the entire subsequent voyage on the tactical plot whenever a fix is care in the preparation of the planning chart (greater use of determined, because the chart could annotations), (2) “precomputing” fix opportunities, and (3) rely- become impossibly cluttered. (Moreover, much of this work may ing on navigational electronics to a greater extent. Thus, for be wasted, because additional fixes require renewing the tactical example, mariners will program routes into their navigation DR plot.) Therefore, the navigator receivers made up of a sequence of waypoints (see Chapter 9) usually plots only the next (few) DR position(s) in sequence on the and choose courses to minimize the indicated cross-track error tactical plot. In short, the navigator uses the tactical DR plot simply to (XTE) displayed on the GPS or Loran-C receiver. Preplanned monitor progress and “stay ahead of fixes are checked to ensure that bearings, depths, or other read- the vessel.” The original DR voyage plan serves as a reference ings are “where they are supposed to be.” If not, the mariner plot, if needed. Of course, if the always has the opportunity to slow down or stop and reassess the tactical DR plot shows the vessel to be “impossibly” behind schedule situation. Because speeds can be chosen at will, these are select- or out of position, it may be necessary to rethink and revise the ed to facilitate mental calculations. For example, at 30 knots the entire plan. For example, the vessel covers a mile in 2 minutes, at 20 knots, 3 minutes, at 15 fuel calculations in the voyage plan may be predicated on the assump- knots, 4 minutes. Running at one of these speeds makes it easy tion of catching an incoming tide (which will stretch the to estimate times en route or times to the next waypoint without available fuel as discussed in formal calculation. For useful discussions of fast boat naviga- Chapter 11). A delay in getting started could mean that adverse tion, see Brogdon (1995), Bartlett (1992), Pike (1990), and currents would be encountered. In this situation, it may be wise to Kettlewell (1993). redo the fuel budget to ensure

5-16 Dead Reckoning 5

that sufficient fuel remains and, D = ST/60, known, the formula to use is: if necessary, divert to alternates where: D=ST/60 to take on additional fuel. D= Distance (nautical miles), Insert the values for S Alternatively, courses may have to S=Speed (knots), and (Speed = 10 knots) and T be recalculated (see Chapter 7) if T=Time (minutes). (Time = 20 minutes): currents turn out different than D = (10) (20) / 60 anticipated. J To calculate speed from distance It is a matter of judgment and and time, use the formula, Carry out the arithmetic: individual choice how these plots S = 60 D/T. D = 200 / 60 = 3.3 nautical miles are prepared and used. Some navi- J To calculate time from distance example #2. You can make it from gators lay out the original voyage and speed, use the formula, your present position to the ship- plan on the chart and maintain the T = 60 D/S. ping channel in 3 hours and 45 min- tactical plot on a transparent over- It is important to note that the utes if you maintain a speed of 10 lay. Others may use separate charts time used in the above equations is knots. What is the distance to the for this purpose (a more expensive the time of the run, or the elapsed shipping channel? option). Yet others use plotting time between two DR positions. You must use minutes to solve sheets available in blank pads with Since DR positions are to be the TSD formula. Convert the 3 only latitude and longitude shown. labeled with their appropriate time, hours and 45 minutes to min- (This practice is not recommended the interval is easy to determine. As utes. To do this, multiply 3 for coastal navigation, however, noted above, time is indicated in the hours by 60 (60 minutes in an because a blank pad does not pro- twenty-four hour system, where vide information on hazards to nav- hour), which is 180 minutes, 1:00 P.M., for example, is 1300. To igation.) Finally, some navigators and then add 45 minutes. Time determine the interval between two “rough out” the voyage plan on a (T) is 225 minutes. times, simply subtract the smaller series of worksheets and draw the time from the larger, by hours and Write down the appropriate tactical DR plot only on the nautical minutes. Remember, however, that formula: D = ST/60 chart (this practice requires that there are 60 (and not 100) minutes there is sufficient space on the ves- Compute using information you in each hour. If you do not have sel for plotting). With experience, a have for the appropriate vari- enough minutes from which to sub- satisfactory method can be chosen. able: tract, “borrow” 1 hour or 60 min- D = (10 knots) (225 minutes)/60 TIME, SPEED, utes. If you cross into another day, D = 2250 / 60 = 37.5 miles AND DISTANCE and pass 2359, reverting back to (nearest tenth) 0000, then borrow 24 hours from Dead reckoning is accomplished that day, and add them to the old example #3. Solving for speed by calculating distance (D) run, or day to make the calculation. when time and distance are known. to be run, speed (S) of the vessel, Examples of TSD calculations are Assume that it took you 40 minutes and time (T) of the run. Distance is provided below to illustrate the to travel 12 miles, what is your measured to the nearest tenth of a process of computation and deter- speed? nautical mile, speed to the nearest mination of the proper time inter- Since distance and time are known, tenth of a knot, and time to the near- val. Distance = 12 miles, Time = 40 est minute. If any two of these three minutes, use the equa tion S = 60 quantities are known, the third can example #l. Solving for distance D/T = (60) (12) / 40 easily be computed using the three when speed and time are known: forms of the basic TSD formula Suppose you are running at 10 Carry out the arithmetic: shown below. knots, how far will you travel in 20 S = 720 / 40 = 18.0 knots minutes? J To calculate distance from speed example #4. A second problem and time, use the formula, Since speed and time are solving for speed when distance is

5-17 5 Dead Reckoning

known and time can be calculated: Since distance and speed are age out the effects of any current to Your departure time is 2030. The known, use this formula: produce an estimate of the vessel’s distance to your destination is 30 speed through the water. A plot is T = 60D/S where D = 12 miles miles. You want to arrive at 0000 made of RPM, usually in incre- and S = 15 knots. (2400). Obtain the speed you must ments of 250 RPM, 500 RPM (or maintain. Substitute the appropriate val- other convenient interval), over the ues for D (Distance = 12 miles) range of available engine speeds, Find the time interval between and S (Speed = 15 knots): against the average measured 2030 and 2400 (2400 will not be speed. The resulting curve can be displayed on a 24-hour clock. T = (60) (12) / 15 = 48 minutes. used to predict the resulting speed Nonetheless, 0000 is logically equivalent to 2400). To do this, SPEED CURVE corresponding to any RPM, or the appropriate RPM for any desired subtract 2030 from 2400. In order to be able to do TSD speed. As a by-product for boats Remember you are subtracting calculations, it is necessary to be capable of getting on plane, this “hours” and “minutes.” able to have an accurate value for point is clearly evident as a discon- Determine the time interval as speed. Even if the small craft is tinuity in the curve, and the most follows (note that 23 hours and equipped with a log of some kind to economical speeds are quickly indi- 60 minutes is equivalent to 24 provide speed indication directly, cated. This subject is explored in hours and 00 minutes): this device must be calibrated Chapter 11. hr min hr min against known speeds to determine It is important to realize, howev- 24 00 23 60 any deviation of the indicated speed er, that such a speed curve is valid ______-20 30 -20 ______30 from actual speed. Small paddle- only for the conditions as tested. If 3 hr 30 min wheel sensors can become fouled, the hull is degraded by additional which introduces errors in the indi- Remember, you use minutes to drag from marine encrustation, for cated speed. For boats equipped solve the TSD equation. example, the speed curve applying only with engine speed indicators Convert the 3 hours and 30 min- to the clean hull is no longer applic- (tachometers), some means must be utes to minutes. To do this, mul- able. The hull must be restored to provided to convert engine revolu- tiply 3 hours by 60 (60 minutes its initial condition or a new curve tions per minute (RPM) into speed in an hour), which is 180 min- must be developed. For smaller through water. The mechanism used utes. Add the 30 minutes craft, which are sensitive to load, it to accomplish calibration of speed remaining from step 1. Time (T) may be necessary to develop sepa- logs or tachometers in terms of = 210 minutes. rate speed curves for different load actual speed through the water is conditions (e.g., fuel carried and Write down the appropriate for- the development of the speed curve, crew aboard). mula, S = 60D/T, and compute a graphic plot of observed speed The speed curve is developed as using the information you have versus RPM. follows: for appropriate variables. The procedure is simple. A S = (60) (30 miles) / 210 minutes measured distance (often, but not J A measured mile or other mea- necessarily, one nautical mile) is sured course is located on the S = 1800/210 = 8.6 knots (as run at various engine speeds chart or laid out on the shore so rounded to the nearest tenth) (RPM), in both directions over a set that it is visible in an area safe example #5. Solving for time course, and the speeds are measured enough for conducting speed tri- when speed and distance are (calculated from the TSD formula). als. Usually, each end of the known: You are cruising at 15 knots Ideally, the speed curve is devel- speed course is denoted by visi- and have 12 miles to cover before oped on a day when the wind is ble range markers, for example, arriving at the next waypoint. What calm to eliminate wind effects. diamond shapes at one end and is your estimated time en route The purpose of running the squares or circles at the other. (ETE)? course in both directions is to aver- These ranges form perpendicu-

5-18 Dead Reckoning 5

COURSE DESCRIPTION: ______DATE: VESSEL: COURSE LENGTH: 1.00 NAUTICAL MILES HULL CONDITION: FUEL QTY: WATER QTY: NUMBER CREW: “OUt” LeG “BaCK” LeG

CaLCULateD CaLCULateD aVeraGe CaLCULateD SpeeD SpeeD SpeeD CUrreNt rpM MIN SeC (KNOtS) MIN SeC (KNOtS) (KNOtS) (KNOtS) 250 22 24 2.7 19 35 3.1 2.9 0.2 500 20 30 2.9 17 55 3.3 3.1 0.2 750 18 6 3.3 15 48 3.8 3.6 0.2 1000 15 15 3.9 13 15 4.5 4.2 03 1250 12 15 4.9 10 35 5.7 5.3 0.4 1500 9 45 6.2 8 20 7.2 6.7 0.5 1750 7 49 7.7 6 45 8.9 8.3 0.6 2000 6 23 9.4 5 35 10.7 10.1 0.7 2250 5 14 11.5 4 44 12.7 12.1 0.6 2500 4114.9 3 45 16.0 15.5 0.5 2750 3 23 l7.7 3 l3 l8.7 18.2 0.5 3000 3 23 17.7 3 12 18.8 18.2 0.5

NOTE THAT SPEEDS, AND NOT TIMES, ARE AVERAGED. SPEEDS ARE AVERAGED TO THE NEAREST 0.1 KNOT. CURRENTS ARE CALCULATED AS WELL, NOT FOR SPEED CALCULATION PURPOSES BUT RATHER TO ENSURE THAT THE COMPUTATIONS ARE PLAUSIBLE. NOTE HERE THAT ESTIMAT- ED CURRENTS FIRST INCREASE, THEN DECREASE SLIGHTLY, AS MIGHT BE EXPECTED WITH A TIDAL CURRENT. L TABLE 5-3–Illustrative Speed Table

lars to the measured distance tion, and the average of the two cal course. (Note that each course to be run. Alternatively, speeds. For this discussion, rows course is developed individual- natural ranges, ATONs, etc., can are indicated for increments of ly. Remember that the deviation be used, but the distance 250-RPM engine speed from depends on the compass or mag- between these must be mea- 250 RPM up to the maximum netic heading, and reciprocal sured on the chart. permissible engine RPM. The headings do not necessarily rest of the columns will be filled have the same deviation!) J A table is made up prior to con- in during the speed trial. ducting any speed runs. The When the boat is approaching table consists of several J The speed trial is conducted. the speed course, it is brought columns: RPM, time of run in The boat is taken out to the up to the RPM desired and initial direction, calculated speed course, which is determined turned on to the course. A stop- in initial direction, time of run in from the chart, and the direction watch is started when the vessel reciprocal direction (in minutes, converted from true to compass at speed crosses the first range or minutes and seconds), calcu- for the initial course and from and continued until the vessel lated speed in reciprocal direc- true to compass for the recipro- crosses the second range, when

5-19 5 Dead Reckoning

it is stopped. The time is noted current. Thus, you end up with a A graph is drawn in an X-Y on the table, the resulting speed so-called “out” and a “back” leg coordinate system (see Figure 5- calculated, and the reciprocal (terms such as “upwind” or 4). The X, or horizontal axis, is course is then run. (If seconds “downwind” are sometimes calibrated for RPM, and the Y, are recorded, these must be con- used). The speeds for each leg or vertical axis, is calibrated for verted to decimal minutes by are calculated and these speeds speed in knots. The values of dividing by 60 and adding this averaged. Do not average times RPM and Speed are plotted for fraction to the minutes.) Again, and then calculate a single each pair of RPM/Speed values the watch is started upon cross- speed. As a plausibility check on in the table. Any symbol can be ing the range and stopped when the speed curve, compare the used for the plot. A “smooth” crossing the next range. The calculated current with the esti- curve may be developed (possi- time is again noted and the mated current using the methods bly by eye) through the data resulting speed calculated and of Chapter 8. points, to refine the estimates, entered into the table. The two but this is omitted here. Either J The process is continued for speeds are averaged and the the graph or the table may be each value of RPM desired, resulting value entered into the used, subsequently, to determine until the table is completely last column in the table. Speed the speed, which would result filled out (see Table 5-3). runs are done in each direction for a specific RPM or the RPM, to account for the effects of any J The speed curve is then plotted. which must be made to accom- plish a specific speed. Note that the speed indicated from the curve is speed through the water (STW), not speed over the ground, because the ground speeds have been averaged. Speed read from the speed curve can be used in the TSD calculations to develop your DR plots. For boats with two engines, it is useful to plot a speed curve for single- as well as twin-engine operation. In the event of engine failure, the curve can be used for estimation of

STW (knots) speed. Consult your owner’s man- ual before making single-engine speed runs. There may be limits to allowable RPMs for single-engine operation. If your vessel is equipped with a navigation receiver (GPS or Loran- C), many of the above steps can be simplified. All (or nearly all) Engine RPM (Hundreds) receivers in production today have Note: Actual data plotted. Most navigators would an option of reading speed over the “smooth” the curve for actual use. ground (SOG) directly. All that is necessary is to run the out and back L legs as discussed above and read the FIG. 5-4–Illustrative Speed Curve Based on Data Taken from Table 5-3 SOG or SMG directly from the dis-

5-20 Dead Reckoning 5

play. The out and back leg speeds SOG, SMG, velocity towards desti- for calculation of SOG, the receiv- are averaged as discussed in step 3, nation, and velocity along route. er’s estimate may be less precise above. However, care must be used SOG comes the closest to that than if the “timed speed run” to ensure that the correct speed is required for development of a speed method discussed above is taken from the unit. (Most receivers curve.) Additionally, depending on employed. have the capability to display sever- the receiver’s quality and technical al speeds including, for example, features, such as the averaging time

5-21 5 Dead Reckoning

SELECTED REFERENCES

Anderson, J., (1987). Exercises in Pilotage, David & Marchaj, C. A., (1988). Aero-Hydrodynamics of Charles, London, UK. Sailing, Revised and Expanded Edition, International Marine Publishing Co., Camden, ME. Bartlett, T., (1992). Navigation at Speed, Fernhurst Books, Briton, UK. Marden, Luis., (1986). The First Landfall of Columbus, National Geographic, Vol. 170, No. 5, Bowditch, N. (1995). American Practical Navigator, November, pp. 572, et seq. An Epitome of Navigation, Pub. No. 9, Defense Mapping Agency Hydrographic/Topographic Mixter, G. W. (edited by D. McClench), (1967). Center, Bethesda, MD. This publication is avail- Primer of Navigation, Fifth edition, Van Nostrand able electronically from the National Imagery and Reinhold Company, New York, NY. Mapping Agency (NIMA), Marine Navigation Department web site (http://www.nima.mil/). Morrison, S.E., (1942). Admiral of the Ocean Sea, A Life of Christopher Columbus, Little, Brown and Brogdon, W., (1995). Boat Navigation for the Rest of Company, Boston, MA. Us—Finding Your Way by Eye and Electronics, International Marine, Camden, ME. Pérez-Reverte, A., (2000). The Nautical Chart, (trans- lated from the Spanish), Harcourt, Inc., New York, Collinder, Per., (1995). A History of Marine NY. Navigation, St. Martin’s Press Inc., New York, NY. Pike, D., (1990). Fast Boat Navigation, Adlard Coles Gotelle, P., (1979). The Design of Sailing Yachts, Nautical, London, England. International Marine Publishing Co., Camden, ME. Pocock, M., (1986). Inshore-Offshore: Racing, Henderson, R., (1970). The Racing Cruiser, Reilly and Cruising, and Design, Nautical Books, London, Lee Books, Chicago, IL. UK.

Hewson, J.B., (1983). A History of the Practice of Saunders, A. E., (1987). Small Craft Piloting and Navigation, Second Edition, Brown, Son & Coastal Navigation, Alzarc Enterprises, Ferguson, Ltd. Publishers, Glasgow, Scotland, UK. Scarborough, Ontario, Canada.

Hobbs, R. R., (1974). Marine Navigation 1, Piloting, Schlereth, H., (1982). Commonsense Coastal Second edition, Naval Institute Press, Annapolis, Navigation, W.W. Norton & Company, New York, MD. NY.

Johnson, P., (1972). Ocean Racing and Offshore Yachts, Shufeldt, H. H., and Dunlap, G.D. as revised by Bauer, Second edition, Dodd Mead & Co., New York, NY. B.A., (1991). Piloting and Dead Reckoning, Third Edition, Naval Institute Press, Annapolis, MD. Kane, G. R., (1984). Instant Navigation, Second edi- tion, Associated Marine, San Mateo, CA. Toghill, J., (1975). The Yachtsman’s Navigation Manual, John de Graft, Clinton Corners, NY. Kettlewell, J. J., Ed., (1993). Reed’s Nautical Companion, Thomas Reed Publications, Boston, United States Power Squadrons, (1998). Plotting & MA. Labeling Standards, The United States Power Squadrons®, Raleigh, NC. Maloney, E. S., (1985). Dutton’s Navigation and Piloting, 14th edition, Naval Institute Press, Waters, D. W., (1958). The Art of Navigation in Annapolis, MD. England in Elizabethan and Early Stuart Times, Yale University Press, New Haven, CT.

5-22 Chapter 6

PILOTING

“Piloting doesn’t just imply plotting; it insists upon it. Those who mark their positions only in the cranial furrows of their memory, instead of recording them with pencil marks and log notations, may one day find themselves completely lost, pounding their bottom out on an unsuspected shallow place that “warn’t supposed to be thar!”

–Frederick Graves

“It is established for a custom of the sea that if a ship is lost by default of the lodesman, the mariners may, if they please, bring the lodesman to the windlass and cut off his head without the mariners being bound to answer before any judge, because the lodesman had committed high treason against the undertaking of the pilotage, and this is the judgement.”

–The Laws of Oleron, 1190 as quoted in Schofield

INTRODUCTION DR positions need to be checked his chapter follows directly the for accuracy and updated as neces- Tdiscussion of dead reckoning sary. In principle, these accuracy (DR) and amplifies and expands checks can be made by several upon the material presented in means, such as visual reference to Chapter 5. As noted, DR is used to charted objects (termed piloting), project the position of the vessel at electronic measurements (termed some future time using information electronic navigation), and mea- on the course steered and speed surement of the elevation of celes- maintained. If the helmsman steers tial bodies (termed celestial naviga- a straight and accurate course and tion). In coastal (sometimes termed maintains constant speed, and water pilot) waters, visual reference to current and/or wind effects are min- charted features (both natural and imal, the DR position will be quite man-made) is convenient and com- accurate. However, these ideal con- monly used. The principal tasks of ditions do not always prevail and piloting are: (1) to check, refine,

6-1 PHOTO COURTESY OF FURUNO 6 Piloting

and update DR positions, (2) to J Third, in certain parts of the is essential for every mariner and, determine a sequence of fixes, (3) country, the shoreline may be in a very real sense, constitutes the to calculate such revisions to the nearly featureless and aids to core of both the Basic and vessel’s course or speed as are nec- navigation (ATONs) few and far Advanced Coastal Navigation essary to maintain the intended between. Unless the vessel has (BCN/ACN) courses. Take the time track, and (4) to monitor progress electronic navigation receivers, to read and reread these chapters and select alternate tracks and/or DR positions may be all that are thoroughly and to work out the ports as necessary to ensure a safe available when operating in problems in the Study Guide (SG). and efficient passage. these areas. The objective of this chapter is Therefore, even though DR

ST G A U O A C R

to show how positions can be deter- D positions may be inaccurate and S U What YOU

A U Y X R mined by visual reference to chart- should be checked at suitable inter- ILIA WILL LearN IN ed objects. However, material on vals, the preparation of DR plots is thIS Chapter position fixing by electronic means essential to prudent navigation. is also introduced in this chapter to For the most part, the material J Definitions of piloting, familiarize the reader with the rele- in this chapter is not difficult, line of position, fix, run- vant nomenclature, notation, and either in principle or in practice. ning fix, estimated posi- procedures. Radionavigation is dis- Nonetheless, statistics on search tion and danger bearing cussed in Chapter 9. and rescue (SAR) cases from the Because fixes are presumed to United States Coast Guard (USCG) J How to determine and be more accurate than DR posi- indicate that many distressed plot a line of position tions, students often ask why mariners do not know their posi- mariners take the trouble to main- tions with any accuracy. Inaccurate J Utility of a line of posi- tain DR plots. This question is or incomplete position information tion addressed implicitly in Chapter 5, not only contributes to accidents J but it is worthwhile to restate the (e.g., groundings and fuel exhaus- How to determine and answer here. There are at least three tion) but also delays rescue, plot a visual fix important reasons why a DR plot is because search and rescue units J How to determine and necessary: (SRUs) waste time searching for J First, a fix, no matter how accu- the distressed vessel. Therefore, it plot an electronic fix rate, is simply a determination is particularly important that this J How to determine and of a vessel’s position at an material be mastered. plot a running fix instant in time. By itself, a fix Aside from the obvious safety cannot be used to determine or element, there are many other bene- J Danger bearings and estimate a vessel’s future posi- fits to being able to determine posi- their uses tion—as is done by DR. tion rapidly and accurately. In gen- J Second, although it is recom- eral, these relate to the comfort and mended that a vessel’s position efficiency of the voyage. A captain PILOTING be fixed frequently—particular- who knows the vessel’s position (at Piloting (also called pilotage) is ly in pilot waters where it is nec- least approximately) at all times is a navigation involving frequent refer- essary to ensure that the vessel more confident captain, and so are ence to charted landmarks, ATONs, does not stray into danger— any guests aboard. The determina- and depths. Piloting entails frequent opportunities for fixes may be tion that the vessel is not on the pre- comparison of the physical features limited. Fog or other weather planned track means that corrective of the earth’s surface (including phenomena that impair visibility action can be initiated promptly. man-made features) and their rela- may prevent frequent fixes, for For all these reasons, the infor- tionships to those same features as example. mation covered in Chapters 5 and 6 depicted on the chart to reconstruct

6-2 Piloting 6

the same relationships of direction, maintaining buoys and their The Admiralty Manual of angular differences, and distances to sinkers in precise geographi- Navigation states: establish the position of the vessel. cal locations. Buoy positions “The position of buoys and The charted features include are normally verified during small-size floating structures those of natural origin, such as periodic maintenance visits. must always be treated with promontories, hill and mountain Between visits, atmospheric caution even in narrow tops, and fixed, man-made (some- and sea conditions, seabed channels... Remember in times termed cultural) objects such slope and composition, and particular that buoys can as towers of various types, buildings, collisions or other accidents quite easily drag or break smoke stacks, tanks, cooling towers, may cause buoys to be adrift; that they are frequent- lighthouses, and other structures. sunk or capsized…Prudent ly moved as a shoal extends; Also included in these fixed features mariners will use bearings or and that they may not always would be “invisible” ATONs such as angles from fixed aids to nav- display the correct character- radio beacons (RBn) and passive igation and shore objects, istics…Buoys should not be radar reflectors (Ra Ref). (The soundings and various meth- treated as infallible aids USCG has phased out radio bea- ods of electronic navigation to navigation, particularly cons, although some aeronautical to positively fix their posi- when in an exposed position. beacons are still in service.) tion.” [Emphasis added.] Whenever possible, navigate According to Bowditch, piloting by fixing from charted shore “is practiced in the vicinity of land, On many nautical charts, particular- ly small craft charts, the following objects; use the echo dangers, etc., and requires good sounder; check the DR/EP judgment and almost constant warning is printed: “The prudent mariner will against the position; use but attention and alertness on the part do not rely implicitly on of the navigator.” not rely solely on any single aid to navigation, particular- buoys.” (Emphasis in origi- Buoys in Piloting ly on floating aids. See U.S. nal.) Buoys are also used in piloting, Coast Guard List, and U.S. It also notes: but the navigator should regard Coast Pilot for details.” “When shore marks are diffi- fixes determined solely from float- The General Information chapter of cult to distinguish because of ing aids with caution because buoys the U.S. Coast Pilot (USCP) con- distance…or thick weather, are susceptible to accidental dam- tains the following caution: buoys and light-vessels must age (sinking) or relocation (drag- “...A prudent mariner must often be used instead.” ging) and cannot be guaranteed to not rely completely upon the be on station at all times (see side- Many excellent texts on navigation charted position or operation bar). On this point, all prudent nav- provide cautionary statements of floating aids to navigation, igators agree. What is more contro- regarding use of buoys for position- but will also utilize bearings versial is whether or not buoys fixing, but don’t rule this practice from fixed objects and aids to should be used for position fixing. out completely. Brogden (1995), a navigation on shore.” This section highlights alternative former Captain in the USCG, offers viewpoints. In the introduction to Bowditch offers the following guid- the standard caution, but adds: “I the Light List (see Chapter 10), pub- ance: don’t hesitate to use buoys for bear- lished by USCG, the following “For these reasons, a mariner ings, but the bearings to them aren’t statements can be found: must not rely completely as accurate or as reliable as bear- “Buoy positions represented upon the position or opera- ings to lights.” on nautical charts are tion of buoys, but should nav- The record of the USCG in approximate positions only, igate using bearings of chart- maintaining buoys in U.S. waters is due to the practical limita- ed features, structures, and excellent. For example, the perfor- tions of positioning and aids to navigation on shore.” mance goal set by USCG (1999) is

6-3 6 Piloting

to maintain navigation availability preferred for position fixing. LOP BY BEARING FROM on 99.7% of all days. The actual Mariners should treat any fix deter- CHARTED OBJECT availability of short range ATONs mined solely from bearings on One of the simplest and most over the years from 1994-1998 has buoys (or passing close to a buoy) common LOPs is developed from a been consistently over 98%— with caution unless this position is single bearing on a charted object. impressive performance, indeed. checked and confirmed with other The bearing can be taken using a Readers should note, however, that information. To use one term of art, pelorus (see Chapter 2), or a hand- this availability is measured against mariners take a “position reference” bearing compass, or by orienting a standard of where the buoy is sup- from one or more buoys, not neces- the boat so that the resulting rela- posed to be, not necessarily where sarily a fix. The prudent mariner tive bearing is dead ahead (000R), it is shown on a particular edition uses several means of navigation in abeam (090R or 270R), or astern of a chart. Unless the mariner has concert (see Chapter 12), rather (180R). Bearings so determined updated the chart (see Chapter 10), than relying on any individual aid could be compass bearings if the the buoy (even if correctly posi- or system. As written, this is not ship’s compass is used, magnetic tioned) may not be where it is intended to be an absolute prohibi- bearings if a hand-bearing compass shown on the chart. tion on position fixing using buoys. is used in a location where it is free Some texts (e.g., Pike, 1990) This said, with the availability of of deviation, or relative bearings if that argue that: low-cost electronic navigation a pelorus is used. If a relative bear- “Passing a buoy gives an receivers (such as Global ing is taken, it must be converted to accurate fix in eyeball navi- Positioning System [GPS] or Loran- a true bearing by first determining gation and it can be worth C) it is a simple matter to verify that the compass heading at the instant diverting from the straight a buoy is in its proper location. of the relative bearing observation, line course to obtain such a correcting this compass heading to fix.” THE LINE OF POSITION a magnetic heading by applying Markell (1984) is likewise sanguine A line of position (LOP) is a line deviation, and then to a true head- about the use of buoys for position established by observation or mea- ing by allowing for variation as dis- fixing. He notes: surement on which a vessel can be cussed in Chapter 2. The relative “The easy way to get an expected to be located. A vessel can bearing is then added to the vessel’s accurate fix is to come be at an infinite number of positions true heading to determine the true alongside a charted buoy and along any single LOP. In piloting, bearing to the object. The true bear- record the time.” an LOP may be established by a ing is plotted on the chart to the measured bearing to a known and object as a solid line. The United Elsewhere he is more cautious and charted object, or by an alignment States Coast Guard Auxiliary states: of two visible and charted objects, (USCGAUX) encourages the use of “The most reliable line of or by a measured distance from a standard plotting and labeling tech- position is from a bearing charted object. In the case of bear- niques adopted by the USCG, the taken on a range. Next in ings, LOPs are straight lines. In the U.S. Navy, as well as other authori- order of reliability is LOP case of distances, LOPs are circles, tative sources such as the United from a bearing on a fixed and sometimes referred to as circles States Power Squadrons® (USPS). object ashore, and third is a of position (COP). LOPs are used in These standards specify that LOPs floating aid to navigation, this text to refer to both. These should be drawn with solid lines such as a buoy. For our pur- methods for determination of LOPs over the range of possible positions poses...buoys are plenty are discussed below. LOPs can also of the vessel. “Impossible loca- accurate enough.” be determined by electronic means, tions” along an LOP (e.g., on land, On balance, this ACN text takes the such as radar. in the area between two range position that fixed ATONs and other markers used to define the LOP) shore-based objects are strongly should be drawn as dashed lines.

6-4 Piloting 6

Figure 6-1 shows the LOP

ST G A U O A C R

S D

U Great BUOY VOYaGeS, construction and labeling. The

A U Y X R ILIA aN hIStOrICaL aSIDe vessel is somewhere on the solid portion of the LOP. This LOP The admonition not to rely solely on floating aids to fix your is labeled on “top” with the time boat’s position reflects the fact that buoys are not always where of the observation (using four digits in the 24-hour system) and they are supposed to be. Wind, flooding, ice, and other environ- on the “bottom” with the true bear- mental factors may cause a buoy to break its mooring and float ing from the vessel to the object. Top and bottom are placed in away. Perhaps the most famous buoy “voyage” is reported in quotation marks because it is Amy Marshall’s A History of Buoys and Tenders published as an not always obvious. However, there won’t be any confusion insert in the Commandant’s Bulletin of November 1995. The if time is always written as four story reads in part: The whistle buoy, which marked Nantucket digits and bearing as three digits. Thus, for example, 1:20A.M. is Shoals Light Vessel Station parted its mooring and headed written 0120. southwest on Jan. 20, 1915. “Two months later, mariners sight- An Example ed it 15 miles off Cape Hatteras, NC. On April 7, the crew of the The sailboat Perdida is steering Italian steamship Andrea intercepted the wandering buoy 400 a course of 264 true, speed 4.0 knots. The Point Judith Light is vis- miles east of Cape Hatteras, and towed it for more than 200 ible off to starboard. It is clearly miles. For unknown reasons, the crew abandoned the buoy on visible from Perdida and is easily identified on the chart by its char- April 16. On Aug. 7, reports of the errant buoy came in with a acteristics (magenta overlaying a sighting 220 nautical miles NNE of San Salvador, Bahamas. By dot within a circle, and the legend: Gp (0cc (1+2) 15 sec 65 ft 16 M). November, it was well on its way northward with confirmed At 1015, the vessel is on a compass sightings SSE of Cape Fear, NC (Nov. 9), ESE of Cape Hatteras heading of 274C when the light- house is observed with a pelorus (Dec. 12), 290 miles east of Bodie Island (Dec. 30), and finally (see Chapter 2) bearing 073R (73° 165 miles WSW of Bermuda (March 25, 1916). to starboard of the bow). The head- ing, relative bearing, and the time In the meantime, the Lighthouse Service dispatched tenders are recorded in the vessel’s log: to search for the buoy. Although its distinct shape, red paint, reg- time, 1015; heading, 274C; relative bearing, 073R. The navigator cor- ulation markings, and blowing whistle made this buoy conspic- rects the compass heading to a true uous, the tenders had no success in locating it. heading (TH) using the deviation On Aug. 16, 1916, after 19 months and 3,300 nautical miles, for that compass heading observed from the deviation table or devia- the buoy’s voyage came to an end. The private vessel that finally tion curve (assume that the com- recovered the buoy noted that its whistle still sounded and bits of pass deviation is 5° E on a heading of 274C) and applies the local vari- its mooring chain were still attached.” ation (assume that the variation is 15 W) using the method discussed in Chapter 2:

6-5 0930 1000 C 264 S 4.0

1030

337 1015

L FIG. 6-1–A Single LOP from a Visual Bearing Piloting 6

DIreCtION heaDING MeMOrY aID maximum possible error in an LOP

ST G Compass 274C Can A U O A C R

S D

U FaCtOID: U.S. as plotted might easily be 3 or 4

Deviation +005E Dead A U Y X R Magnetic 279M Men ILIA atON SYSteM degrees. Many of these errors are Variation -015W Vote random, rather than systematic, and True 264T Twice (Add East) At Elections The United States has one of tend to average or “cancel out.” Although the probable error is less the best ATON systems in the The true bearing (TB) to the object than the maximum possible error, it from the boat is then determined by world—in terms of both the is a mistake to believe that the over- adding the relative bearing (RB) quality and quantity of all error is negligible. (073R) to the true heading (TH) For these and other reasons, (264): ATONs. From a purely quan- visual LOPs are only approximate, trUe BearING OF OBJeCt FrOM VeSSeL: titative perspective, the num- perhaps no more accurate than to TH + RB = TB within plus or minus 3 degrees on a 264 + 073R = 337 ber of Federal ATONs small vessel. Electronic LOPs The LOP is plotted to the light- (including, for example, developed by radar are also subject house on a bearing of 337, and to bearing error. To avoid chart clut- labeled with the time of observation buoys, beacons, sound sig- ter, it is generally appropriate to use (1015) on top and the true bearing nals) is nearly 50,000. the best estimate of the LOP and from the vessel to the object (337) plot this alone, rather than attempt underneath the line. The vessel is bearing (after conversion of the to calculate possible error limits assumed to be located somewhere heading from compass to true). The (confidence intervals) and plot on that line. The recommended ship’s heading is not known with these on the chart. Nonetheless, it is practice is to plot a DR position certainty, and errors of 2 degrees or important to keep these errors in whenever an LOP is determined. so would not be regarded as uncom- mind and to take steps to minimize (The purpose of plotting a DR posi- mon. (All the more so if the naviga- them by careful measurement and tion is to determine the current set tor and the helmsman are the same judicious selection of objects for and drift as discussed in Chapter 7.) person!) The conversion process observation. Additionally, it is Therefore, the 1015-DR position from relative to true bearings, even important to recognize and allow should also be plotted in Figure 6-1. though it is only a simple matter of for these errors in voyage planning. addition and/or subtraction, intro- For example, courses should never ACCURACY OF LOPs duces the possibility of additional be laid out so as to pass close to It is customary to regard visual errors. Leaving aside outright blun- hazards to navigation unless other- LOPs as being highly accurate— ders, such as applying the deviation wise unavoidable. Rather, an ample that is, to assume that the vessel’s appropriate to the relative bearing safety factor should be incorporated position must be located exactly rather than the ship’s heading or to ensure that the vessel remains in along an LOP. This convention is errors in arithmetic, the vessel’s safe water. followed here. Keep in mind, how- compass is not perfectly accurate ever, that the possibility of error (particularly if the sea is other than TYPES OF LOP always exists in the determination flat calm) and the deviation table is Bearing from Charted Object of an LOP. An LOP determined by only approximate. Thus, bearing —Visual Observation conversion of a relative bearing, for errors of 2 or 3 degrees would not —Radar Bearing example, is subject to several be thought of as unusual—even —RDF Bearing errors. The error in the measure- more if the seas are rough. LOP from Directional Range ment of the relative bearing could Finally, the LOP needs to LOP by Distance from Object —Vertical Angle easily be of the order of 1 or 2 be plotted on a chart for —Distance from Radar Obs. degrees, even if a pelorus is used. position determination. The relative bearing is added to the Plotting errors of 1 degree ship’s heading to determine a true or more can occur; so the

6-7 1700

L FIG. 6-2–A range LOP as determined by a charted tank and tower. Only the time is noted. Piloting 6

must be charted. LOP BY DISTANCE Range markers place the taller FROM CHARTED OBJECT of the markers behind the shorter Distance to an object can be marker viewed along the course determined by measurement using a line. The top object appears to be to radar observation, by measurement the left of the lower object when the of vertical angle with a sextant, observer is left of the line deter- stadimeter, or range finder. In this mined by the range, and the top case, however, the LOP is a circle object appears to be to the right of rather than a straight line. The the lower object when the observer resulting COP (or arc of position) is is to the right of the range. drawn as the circumference of a cir- An LOP by range is the easiest cle with origin at the observed LOP to determine and among the object and radius equal to the mea- most accurate. Because no calcula- sured distance. The arc is drawn as tion is required, only the time needs a solid line and labeled with the to be noted and indicated on the time and the distance. The time is chart. A range LOP is illustrated in placed on the “top” of the arc (four Figure 6-2. Here, the navigator digits) and the distance “below” notes that a charted tank and a (nautical miles and tenths preceded PHOTO COURTESY OF UNITED STATES COAST GUARD COAST COURTESY OF UNITED PHOTO STATES L tower are in range at 1700. The by the capital letter D). The entire Buoy being serviced by buoy tender. range line is drawn as a solid line COP is not shown, only the portion Prudent mariners should treat fixes on the chart (except for the portion denoting the locus of possible posi- between the objects used to define determined by floating aids with tions of the vessel. Unless there is the range and/or any portion over great caution. ambiguity, there is no need to draw land) and labeled on top with the a radius from the point of origin. LOP BY time. No direction label is required DIRECTIONAL RANGE on the chart. A directional range consists of two objects in line, one behind the other, and defines a unique direc- tion and a range line (the line drawn through the two objects). (British nomenclature for a range is a tran- sit.) Ranges can provide highly accurate visual LOPs and can be used in compass calibration as dis- cussed in Chapter 2. They are also

helpful in steering difficult channel LEFT OF RIGHT OF approaches. The objects may be RANGE LINE ON RANGE LINE RANGE LINE man-made, such as range markers for a channel, a steeple and a tank, or natural, such as two tangents of land (taken carefully to account for slope of land and curvature of earth’s surface), a large rock and a waterfall, or a combination of both. L In order to be used, objects in range Range lights provide accurate directional information.

6-9 1800 D 2.5

L FIG. 6-3–A Circular (ARC) LOP from a Range Determined from Radar Piloting 6

DISTANCE BY DISTANCE BY RADAR 1210-Tr chart, there is an abun- VERTICAL ANGLE OBSERVATIONS dance of charted objects suitable for (This section is more technical Using radar, the object is identi- LOP determination or position fix- and can be omitted without loss of fied on the plan position indicator ing, and some thought should be continuity.) A sextant can be used to (PPI), the radarscope or screen. The given to selecting those that are measure the vertical angle of a fixed range markers (FRMs) or the “best.” As the mariner becomes charted object of known height. The variable range marker (VRM) can familiar with the area, the identifi- distance in nautical miles to the be used to determine the range of cation of objects suitable for deter- object can be estimated by the the object from the vessel. This is mination of LOPs or fixes becomes height of the object (from the water illustrated in Figure 6-3, where the second nature. But, for unfamiliar level to the height of the light light on the NW tip of Gay Head waters, it is important to study the source—not the top of the struc- provides a “recognizable” radar tar- applicable nautical chart(s) and ture—for a lighthouse) divided by get at range 2.5 M. (See Chapter 9 other relevant publications to plan the tangent (tan) of the angle mea- for a discussion of radar detection for LOP opportunities. sured (H), and divided by 6076 (to and identification of targets. The J First, the object(s) selected for convert ft. to nautical miles), or: light structure, per se, may not be the LOP must be charted and detected on radar, but the western readily identifiable. Land fea- d = h/[6076* tan H] end of the island at Gay Head is tures, such as isolated and This calculation can be performed likely to have a characteristic radar uncovered rocks or rocky bluffs with an electronic calculator or by signature.) A circular (or arc in this can sometimes be used to using special-purpose tables found case) LOP is scribed around the advantage; but beware of gently in Bowditch. To illustrate the above object on the chart using a drafting sloping shorelines, because equation, suppose a light of height compass set to the distance (here, these are not distinct (particular- 85 ft were observed to have a verti- 2.5 M) using the latitude or bar ly if radar is used) and may cal angle of 0.53° (31.8 minutes, scale. The circle is labeled with the change apparent position with remember to convert to decimal time (1800) above the line and the the height of the tide. Man-made degrees by dividing minutes by 60). distance (D 2.5) below the line. The structures, such as standpipes, For this angle, tan H = tan (0.53 arc is drawn to cover the possible water towers, radio towers, degrees) = 0.00925, h = 85 ft positions of the vessel. The vessel is cupolas, or spires on buildings, (neglecting the difference between somewhere on this circle (or arc). as well as ATONs, can be readi- the chart datum for heights and the Although not specifically recom- ly used. Cartographers decide height of the tide) and d = mended by labeling conventions, which objects to chart based on 85/(6076) (0.00925) = 1.5 M. the word RADAR might be added to their possible navigational utili- After the range to the object is the COP to denote the method used. ty. However, examine the chart determined, a COP is drawn on the closely to ensure that the object chart around the object (or an arc is PRACTICAL POINTERS FOR selected cannot be mistaken for swung from the object if it is clear- DETERMINATION OF LOPs another. In this connection, iso- ly apparent as to which direction Before discussing how fixes are lated objects can sometimes be the vessel lies) using the drafting determined, it is useful to provide easier to see and identify than compass set on that range from the some brief practical pointers on the those in close proximity can. An latitude scale on the chart. This cir- selection of objects for determina- object has to be both detected cular LOP is labeled with the time tion of LOPs. In some cases, identi- and identified in order to pro- and with the distance (D 1.5—the fiable objects are few and far vide a valid LOP. In some cases, distance units are implicit and are between and the navigator has little for example, with municipal not normally labeled). The vessel is choice but to use any possible water towers the object can be assumed to be somewhere on the opportunity. In others, however, readily identified by writing (the circle or the arc. such as in the waters covered by the town’s name) or other distin-

6-11 6 Piloting

guishing marks on the side. In the object observed is 1/8th of a ST G A U O A C R

S D

U tIp: USe

A other cases, a unique color or mile distant, but more than U Y X R ILIA the USCp tO color pattern may make some 2,000 yards if the object is 20 heLp SeLeCt objects easy to recognize. nautical miles away. Nearby prOMINaNt (Local knowledge can be a great objects are also easier to see and OBJeCtS help.) The nautical chart or identify, particularly under con- USCP often contains informa- ditions of restricted visibility. the USCp has a section tion that can determine the suit- J Third, choose objects that are titled “prominent Features” ability of objects for position taller in preference to those that that identifies features that determination (see sidebar). For are shorter. The ease with which example, the 1210-Tr chart might be used for position an object can be detected and notes that the Gay Head Light identified is a complex function fixing. here is an excerpt shown in Figure 6-3 cannot be of the prevailing visibility, shape from the 30th edition of the seen (and, therefore, would not of the object, color contrast USCp that describes promi- be usable) in the area south of compared to the surroundings, nent features on the Nomans Land. Identification of and other factors. But it is also a objects using radar (discussed in approaches to Newport function of height of the object, Chapter 9), rather than visual a subject explored in more detail harbor: means, requires some experi- in Chapter 9. The distance to the “The following objects are ence in interpretation of radar visible horizon (arising from prominent when approaching images. Remember, also, that atmospheric refraction and the different visual cues are impor- Newport Harbor either from curvature of the earth) is a func- tant for recognition of objects at tion of the observer’s height of the southward or northward: night compared to daylight eye and the height of the object, a hotel on Goat Island; a viewing conditions. Lighted as shown in Table 6-2. Thus, for white building of the yacht ATONs are often preferable example, an observer on a vessel because the color and character- club near Ida Lewis Rock in with a height of eye of 7.5 feet istics of the light provide more could, under ideal conditions, the southerly part of the har- positive identification during the see an object of height 10 feet as bor; church spires in the hours of darkness. many as 6.9 nautical miles town; and the buildings of the J Second, choose objects that are away. If the object were 100 feet Naval Education and as close as possible to the ves- high, it might be seen at a dis- sel’s position in preference to tance of 14.9 nautical miles. Training Center and Naval those that are located farther (This is yet another reason why War College on Coasters away. Remember a 1-degree small buoys are difficult to see.) Harbor Island in the north error in determining an object’s Actual distances of visibility part of the harbor. To the bearing translates approximate- could be smaller than shown in ly to an error of 1 mile off in 60 the table (which is based solely westward on Conanicut (lateral error). Table 6-1 illus- on average atmospheric refrac- Island are several large trates this point. It shows the lat- tion and the earth’s curvature) hotels and a standpipe.” eral error in yards as a function but in no event would be much these objects facilitate of the distance off (in nautical larger than shown in this table. miles or yards) and the error in In any event, Table 6-2 shows orientation and should be determining an object’s bearing. the potential benefits of looking considered for determina- Thus, for example, a bearing for taller objects. It is sometimes tion of LOps. error of 3 degrees translates into useful to draw an arc or circle of a lateral error of only 13 yards if the distance to the visible hori-

6-12 Piloting 6

DIStaNCe tO OBJeCt BearING errOr IN DeGreeS NaUtICaL MILeS YarDS 123457.5 10 0.125 250 4913 17 22 33 44 0.250 500 9 17 26 35 44 66 88 0.375 750 13 26 39 52 66 99 132 0.500 1,000 17 35 52 70 87 132 176 0.625 1,250 22 44 66 87 109 165 220 0.750 1,500 26 52 79 105 131 197 264 0.875 1,750 31 61 92 122 153 230 309 1.000 2,000 35 70 105 140 175 263 353 1.125 2,250 39 79 118 157 197 296 397 1.250 2,500 44 87 131 175 219 329 441 1.500 3,000 52 105 157 210 262 395 529 1.750 3,500 61 122 183 245 306 461 617 2.000 4,000 70 140 210 280 350 527 705 2.500 5,000 87 175 262 350 437 658 882 3.000 6,000 105 210 314 420 525 790 1,058 3.500 7,000 122 244 367 489 612 922 1,234 4.000 8,000 140 279 419 559 700 1,053 1,411 5.000 10,000 175 349 524 699 875 1,317 1,763 7.500 15,000 262 524 786 1,049 1,312 1,975 2,645 10.000 20,000 349 698 1,048 1,399 1,750 2,633 3,527 12.500 25,000 436 873 1,310 1,748 2,187 3,291 4,408 15.000 30,000 524 1,048 1,572 2,098 2,625 3,950 5,290 17.500 35,000 611 1,222 1,834 2,447 3,062 4,608 6,171 20.000 40,000 698 1,397 2,096 2,797 3,500 5,266 7,053 L TABLE 6-1–Lateral errors as a function of bearing error and the distance to fix show importance of selecting nearby objects for fixes. Table entries show lateral error in yards. zon on the chart to determine the means to confirm (verify) any failure to apply properly the cor- maximum range at which key LOP or fix so determined. rections for deviation or varia- objects can be seen. J Fifth, ranges make for the most tion are eliminated. Look for J Fourth, use fixed rather than accurate LOPs, so these should “natural” as well as man-made floating structures for determi- be used whenever possible. This ranges. (Remember, though, that nation of LOPs. As noted above, is partially a consequence of the many other vessels could also be this guidance is not intended to precision with which small devi- using marked ranges, and keep a be an absolute prohibition ations can be seen on a range. sharp lookout for traffic.) against use of buoys for deter- But, it is also important to recall J Sixth, it takes some practice mining an LOP or position fix- that no conversion of bearings before accurate compass bear- ing. In some cases, buoys may from compass or magnetic are ings can be reliably obtained be the only objects available, necessary before LOPs can be from the deck of a wave-tossed and their distinctive markings plotted from ranges. Thus, any boat. If in doubt, take the aver- aid in identification. However, errors due to compass deviation, age of several bearings rather as soon as possible use other local magnetic anomalies, or than a single bearing. Learn to

6-13 6 Piloting

heIGht heIGht OF eYe IN Feet OF OBJeCt (Ft.) 0.0 2.5 5.0 7.5 10.0 15.0 20.0 30.0 0.0 0.0 1.8 2.6 3.2 3.7 4.5 5.2 6.4 2.5 1.8 3.7 4.5 5.1 5.5 6.4 7.1 8.3 5.0 2.6 4.5 5.2 5.8 6.3 7.1 7.8 9.0 7.5 3.2 5.1 5.8 6.4 6.9 7.7 8.4 9.6 10.0 3.7 5.5 6.3 6.9 7.4 8.2 8.9 10.1 12.5 4.1 6.0 6.8 7.3 7.8 8.7 9.4 10.5 15.0 4.5 6 4 7.1 7.7 8.2 9.1 9.8 10.9 17.5 4.9 6.7 7.5 8.1 8.6 9.4 10.1 11.3 20 0 52 7.1 7 8 &4 8 9 9 8 10.5 11.6 22.5 5.5 7.4 8.2 8.8 9.2 10.1 10.8 12.0 25.0 5.8 7.7 8 5 9.1 9.5 10.4 11.1 12.3 27.5 6.1 8.0 8.8 9.3 9.8 10.7 11.4 12.5 30.0 6.4 8.3 9.0 9.6 10.1 10.9 11.6 12.8 35.0 6.9 8.8 9.5 10.1 10.6 11.5 12.2 13.3 40.0 7.4 9.2 10.0 10.6 11.1 11.9 12.6 13.8 45.0 7.8 9.7 10.5 11.1 11.5 12.4 13.1 14.3 50.0 8.3 10.1 10.9 11.5 12.0 12.8 13.5 14.7 55.0 8.7 10.5 11.3 11.9 12.4 13.2 13.9 15.1 60.0 9.1 10.9 11.7 12.3 12.8 13.6 14.3 15.5 65.0 9.4 11.3 12.0 12.6 13.1 14.0 14.7 15.8 70.0 9.8 11.6 12.4 13.0 13.5 14.3 15.0 16.2 75.0 10.1 12.0 12.7 13.3 13.8 14.7 15.4 16.5 80.0 10.5 12.3 13.1 13.7 14.2 15.0 15.7 16.9 85.0 10.8 12.6 13.4 14.0 14.5 15.3 16.0 17.2 90.0 11.1 12.9 13.7 14.3 14.8 15.6 16.3 17.5 95.0 11.4 13.3 14.0 14.6 15.1 15.9 16.6 17.8 100.0 11.7 13.5 14.3 14.9 15.4 16.2 16.9 18.1 110.0 12.3 14.1 14.9 15.5 16.0 16.8 17.5 18.7 120.0 12.8 14.7 15.4 16.0 16.5 17.3 18.0 19.2 130.0 13.3 15.2 16.0 16.5 17.0 17.9 18.6 19.7 140.0 13.8 15.7 16.5 17.0 17.5 18.4 19.1 20.3 150.0 14.3 16.2 16.9 17.5 18.0 18.9 19.6 20.7 175.0 15.5 17.3 18.1 18.7 19.2 20.0 20.7 21.9 200.0 16.5 18.4 19.2 19.8 20.2 21.1 21.8 23.0 L TABLE 6-2–The distance to the visible horizon in nautical miles as a function of the observer’s height of eye in feet and object height in feet shows the importance of selecting tall objects for LOP determination.

hold a hand-bearing compass as gator’s desk aboard a vessel. accepted that two LOPs determine a level as possible to minimize Check plotted bearings (includ- fix and that three or more LOPs can observation errors. ing the conversion from com- determine the possible accuracy of J Finally, take the time to plot pass to true) at least twice. a fix. It is also important to note that LOPs carefully. Parallel rules or even a single LOP can be useful. paraline plotters can slip when WHAT IS THE WORTH One LOP may not enable you to transferring lines–particularly OF A SINGLE LOP? determine exactly “where you are,” on a crowded and pitching navi- It is often (but not universally) but it can be useful to determine

6-14 1100

1030 1030 280

C 338 S 4.0

1000

L FIG. 6-4–Construction of an Estimated Position 6 Piloting

“where you are not.” In other plot for a voyage in the waters cov-

ST G A U O A C R

S D words, a single LOP can enable the U hINt: aVOID ered by the 1210-Tr chart. At 1000,

A U Y X R mariner to rule out possible loca- ILIA thIS pLOttING sailing vessel Vagaroso takes depar- tions. And, as shown below, a single errOr ture from buoy N “8” and heads LOP has many other uses. towards Mattapoisett Harbor at 4.0 In certain situations, objects can One of the common plotting knots. At 1030, an LOP from the be selected for a visual (or electron- tower on West Island is determined, er rors made by students is to ic) LOP that are more or less direct- and (following the instructions ly in line with the vessel’s course, draw the EP at the intersec- given in Chapter 5) a 1030 DR posi- either ahead or astern. A single LOP tion of the DR course and the tion is immediately plotted. Now from such an object can be used to consider the problem posed by this LOP. Except for the fortu- determine whether or not the vessel information. By hypothesis, the is making good the intended course, itous case where the course vessel is somewhere along the 1030 even if a fix is not obtained. Such an line is exactly perpendicular LOP (subject, of course, to the LOP is sometimes said to be a above provisos on the accuracy of to the LOP to begin with, this course or track line LOP. Such an LOP). But, absent the LOP, the opportunities occasionally happen incorrect procedure will best estimate of the Vagaroso’s posi- by chance, but should be a deliber- result in a position that is tion is the 1030 DR position. The ate part of the voyage plan itself. In recommended solution to this other words, a course can be laid out farther away from the DR dilemma (see some of the references in such a way that several objects position than if a perpendic- for an alternative procedure) is to can be used for trackline LOPs ular had been drawn. locate the vessel’s EP along the along the intended course. Tracking LOP, but as close to the DR position toward a specific object (albeit with vessel. In this case, the LOP can be as possible. It is easy to show from reference to the compass) can sim- used to compare the vessel’s speed the principles of plane geometry plify the job of the helmsman. made good (SMG) with the vessel’s that the shortest distance between a Course turn points can often be speed through the water (STW) and point and a line is formed by a per- conveniently identified and speci- compute the fair or foul current (see pendicular drawn from the point to fied by means of a single LOP. That Chapter 7). These LOPs are termed that line. In other words, the EP is is, the vessel proceeds along one speed LOPs. Speed LOPs are par- that point along the LOP where a course until the bearing of a pre- ticularly useful if the vessel is oper- line drawn from the DR position identified (preferably fixed rather ating in a narrow channel or river intersects the LOP at a right angle. than floating) object reaches a pre- where the margin for lateral error is Figure 6-4 illustrates this construc- determined bearing. For example, a small. In fact, many river mariners tion. The EP is denoted with a portion of the voyage plan might be do not go to the trouble of deter- square around the intersection of the to proceed on a course of 090 mining fixes in the conventional single LOP and the dashed line per- degrees until a particular lighthouse sense at all; they simply use buoys pendicular to the LOP drawn from bears 045 degrees, then come left to to stay in the channel and speed the comparable time DR position. If 045 and track inbound. Because LOPs to monitor progress and the time of the EP is clear from the large ships do not respond instantly, revise estimates. construction, it may be omitted to it is necessary to locate the “wheel Single LOPs can be used to reduce chart clutter. over” point before reaching the turn determine an estimated position Although some argue otherwise point. For most recreational boats (EP) when used in conjunction with (see some of the references at the the radius of turn can be disregarded. a DR position. (EPs allowing for end of this chapter), conventional Objects can also be selected for current are considered in Chapter 7.) practice is not to renew the DR plot determination of LOPs that are Figure 6-4 illustrates the idea. This at an EP—only at a fix, or running (more or less) directly abeam of the figure shows a conventional DR fix (see below). This is because the

6-16 Piloting 6

EP so determined lacks the preci- single LOP can later be advanced

ST G A U O A C R

S D sion of a fix. The only circumstance U atONs FOr (or retired) to determine a running

A U Y X R where it is recommended that a new ILIA pOSItION fix, as discussed below. DR track should be started from an FIXING Navigation Without a Fix EP (rather than continued from the In many cases it may be possible DR position) is if the EP were clos- ATONs are designed er to hazards to navigation and to navigate efficiently and safely prospects for a prompt fix are poor. precisely for the purpose of without an explicit fix. For exam- ple, the navigator can maintain a The reason for this suggestion is helping the mariner deter- that, if the vessel’s position is constant bearing towards an identi- mine the position of the ves- uncertain, it would be prudent to fiable charted object without explicit determination of a fix, pro- assume that the vessel is located at sel. These are able to be iden- vided that the direct course to the the more hazardous of the two loca- tified by appearance, sound, object passes through safe water tions (DR and EP) and a course be (consult a chart to make this deter- set to remain well clear of the haz- markings, light patterns (if mination). Perhaps even more con- ards. This situation arises more lighted) etc. Take special venient, a range (either a specific commonly in areas that do not have navigational range or a simple the abundance of ATONs shown on pains to positively identify alignment of charted objects) can the 1210-Tr chart. In the exercises ATONs before plotting posi- be used for this purpose. Many given in the SG, therefore, assume tions. Remember Alton ranges can only be used over a fixed that a new DR track is started only Moody’s famous statement, area and it is important that the nav- from a fix or running fix. igator does not continue to follow To distinguish the construction “An incorrectly identified the range into hazardous waters. To line used to determine the EP from mark is a hazard, not an aid, ensure safe passage, use appropri- the ship’s course, it is recommended ate measures (e.g., reference to a that the perpendicular construction to navigation.” depth sounder or channel marker). line be dashed rather than solid. The Depending upon the bottom true direction of the construction line does not determine a fix but is very contour and presence of hazards to is easily seen to be the true bearing useful nonetheless. If reference navigation, following a depth con- of the LOP plus or minus 90 degrees, objects are selected so as to be tour may provide safe passage with- or 010 degrees in the example along the course line, a single LOP out having to determine an explicit shown in Figure 6-4. (As with all enables the navigator to determine fix. In order to use this method, directions, 360 degrees may have to whether or not the vessel is making there must be a reasonable depth be added to, or subtracted from, the good the intended course. If refer- gradient in the vicinity of the result.) It is preferable to calculate intended route. Therefore, this ence objects are selected so as to the direction and plot this exactly method cannot be used in an area of pass abeam, the single LOP can be with parallel rules or a paraline plot- uniform depth (no position refer- used to check the vessel’s SMG. ter rather than attempt to estimate ence) or where there is abrupt the perpendicular by eye alone. (It is Additionally, the single LOP can be shoaling (may not be able to take not necessary to label the bearing of used to determine an EP, which evasive action in time). To use this the dashed line.) Any discrepancy incorporates both the DR position method, the navigator monitors the between the EP and the DR position and the LOP. Absent an allowance depth gauge; if the depth appears to is a result of the factors discussed in for current, the EP is determined by increase, the vessel’s heading is Chapter 7 on currents and whatever drawing a dashed perpendicular shifted in a direction towards shal- lack of precision is entailed in the line from the DR position to the lower water, and the converse. At determination of the LOP. LOP. The EP is denoted by a square some point in the voyage, it may be To summarize, a single LOP surrounding the point. Finally, a necessary to abandon the track

6-17 1015 248

1015

1015 L FIG. 6-5–Fix Determined by Intersection of Range with Visual Bearing Piloting 6

defined by the depth contour. For mation would prevail. Remember

ST G A U O A C R

S D FIX OptIONS example, a depth contour might be U that a dead reckoning plot is always A U Y X R followed until an inlet is identified ILIA eXpLaINeD IN started over at a fix. at which point the vessel would thIS Chapter The coordinates of a fix (e.g.. shift direction to enter the inlet. The following fix options are latitude and longitude, time differ- Although this topic is referred to ences [TDs]) can also be read from explained in this chapter: as “navigating without a fix,” it a navigation receiver. Electronic J Cross bearings, does not mean navigating without navigation is covered in Chapter 9, J position information. In the exam- range and bearing, but the plotting conventions for ples given above, the navigator J Intersection of two electronic fixes are noted below. must have knowledge of the ranges, approximate position of the vessel J two distances, CROSS BEARINGS and appropriate charts to ensure J Distance and bearing to Crossing two or more LOPs that following a range or depth con- object, based on bearings is a common tour can be done safely. J Bearing and line of means of establishing a fix. Figure 6-5 illustrates crossing two bearing THE FIX soundings, J passing close to a fixed LOPs, one from a range consisting The fix is an accurate position of the two towers on Beavertail Pt charted object, established by the simultaneous, or and the other a visual LOP on the J nearly simultaneous, intersection electronic fix, Point Judith Light sighted on a true (crossing) of two or more LOPs, J running fix, and bearing of 248 degrees from the ves- passing close aboard to a fixed J two bearings on the bow. sel. The resulting fix is plotted with ATON, or read directly from a nav- a circle around the point of intersec- igation receiver (GPS or Loran-C). J Distance and bearing: Inter - tion and the time (1015) written hor- Some cautious navigators would section of a distance COP with a izontally. Should the navigator be argue that at least three LOPs are bearing to one or more objects. running a DR plot at this time (not required to determine a fix. The shown in Figure 6-5), as is recom- J Known reference: Passing intersection of two LOPs would be mended, a new plot would be initi- close to a fixed ATON or other treated as an estimated position. ated using the fix as its beginning. In addition to direct readout of charted object. position from a navigation receiver, To assure maximum accuracy of RANGE AND BEARING fix possibilities include: the fix, it is important that the inter- section angle of any two LOPs be Establishing a fix by the intersec- J Cross bearing: The intersection as close to 90° as possible. If three tion of a range and a bearing LOP is of two or more LOPs deter- LOPs are used, these should inter- a convenient way to determine a mined from visual or electronic sect at angles as close to 60°-120° turning point while following the bearings. as possible. Following these criteria range down a channel at night. J While the two objects (lights) are in range and bearing: The inter- minimizes fix errors, but reasonable range, the boat is known to be on this section of a range LOP with a deviations of a few degrees (10° - bearing on another object. LOP. By taking frequent bearings on 20°) from the optimum will provide another object (light) to one side, a J two ranges: The intersection of an acceptable fix (and as such, series of fixes are determined and the LOPs formed by two visual should not be discarded), but the the progress of the vessel can be ranges. accuracy will be less than optimal. tracked. When the bearing reaches J two distances: The intersection Intersection angles of less than 40° that marking the turning point, the of two (or more) COPs deter- should be avoided for fixes unless navigator notes the location and rec- mined from visual or electronic they are all that are available. Then, ommends a change to the new means. the rule of using all available infor- course. It is essential that some other

6-19 6 Piloting

object (or method of fixing position) ings taken first on the light on the point of intersection of the ranges is used in conjunction with the point at 2000 (036), and 2015 and the time. Each range is an LOP, range, else the vessel runs the risk of (342), and then shifted to the flash- and the intersection constitutes a fix. running into dangerous waters. ing green light at 2030 (037), and Well-marked rivers where range finally, to the turning bearing (000) TWO DISTANCES lights are used, such as the at 2042, at which time the boat The intersection of two circular Delaware, employ a sequence of changes to its new course of 158 to LOPs also establishes a fix, ranges to aid navigation. One range stay in the next channel. Fixes although there is a possibility of is followed until the next in obtained at 2000, 2015, 2030, and ambiguity since circles can inter- sequence is aligned (ranges are dis- 2042 are labeled accordingly with a sect at two points. Unless the cor- tinguished by the colors and flashing circle around the point of intersec- rect intersection is known, say by characteristics of the lights), where- tion and the time horizontally with an approximate bearing on some upon the vessel turns and comes to the base of the chart. the new course. Of course, it is other reference, two “fixes” are important to maintain a sharp look- TWO RANGES possible, but only one is correct. out for other vessels using the same Circular LOPs are often devel- range! Perhaps the easiest fix to plot is oped by radar observations. This fix This process is illustrated in that obtained by the intersection of is indicated in Figure 6-7 with radar Figure 6-6, where the range of 090 two ranges. The only label required range determinations of 3.0 M from is followed, with successive bear- for this fix is a circle around the Buzzards Bay Light and 2.5 M from

Bearings shifted to this light to keep angles of intersection near 60-90°

Bearings started Gp Occ 6 sec Note TURNING BEARING on this light to 55ft 13M OF 000 keep angle of intersection near 60-90° 2015 342 000 2042 2030 037 RANGE 2000 036 LIGHTS FR C 090 Range 090° 2042 2000 S 5 2015 2030 RR C 158 S 5

L FIG. 6-6–Fixes determined by intersection of bearings with range and illustration of turning bearing of 000 for course change to C 158. All directions indicated are true.

6-20 NEW DR PLOT

C 313 S 10.0

0930 RADAR 0930 D 2.5

0930

ORIGINAL 0930 C 313 S 10.0 DR PLOT D 3.0

L FIG. 6-7–Circular LOP’s determine radar fix; DR track is restarted. the SW tip on Cuttyhunk Island. useful opportunity for a brief probably no reason to look further, The point of intersection is labeled digression to cover some practical except to note that there appears to with a circle (dot in center). The aspects of the navigator’s job. be a current (or wind/current com- word RADAR may also be added Perhaps most important, the navi- bination) setting the vessel north after the time if the LOP was deter- gator should not be content with and slightly west of the DR posi- mined by radar. As with any other simply following rote procedures, tion. File this information away fix, a new DR track is initiated after such as immediately starting a new and see if this trend is continued at the fix as shown in Figure 6-7. DR plot from the fix, without the next fix. Perhaps this current (Note also in Figure 6-7 that the attempting to interpret the available should be compensated for in plan- time is written above, and distance data. For example, the navigator ning the next course using the written below, the LOP regardless should determine whether the methods discussed in Chapter 7 on of whether these are within or out- apparent discrepancy between the current sailing. side the arc. This is easier to read.) DR position and the fix is plausible. Suppose, however, that the DR Figure 6-7 shows both the DR In this illustration, the two posi- position and the fix were further position and the fix. This provides a tions are quite close, so there is apart. Rather than simply accepting

6-21 6 Piloting

this conclusion as fact, the navigator activities done according to rote. side, which removes some of the en should use the opportunity to check Instead, position fixes provide the route work, another benefit of this the accuracy of the DR plot, the fix, basis for considered action. procedure. or both. The fix might be checked, for example, by taking another DISTANCE AND BEARING BEARING AND LINE range, bearing, or by measuring the TO OBJECT OF SOUNDINGS depth of water and consulting the This fix can be determined A vessel coasting (running off chart to see if this information is visually by a range finder and shore, essentially parallel with the consistent with the apparent fix. handheld compass. This method coast) may use this method by tak- This example illustrates another can also be used if radar is ing soundings and a bearing when important point. Information should available. The object is first identi- the opportunity presents itself. As be plotted and analyzed as soon as fied on the PPI. Next, the VRM the course is run, the soundings are possible after it is gathered. Merely is set to measure the distance and an plotted at intervals, according to the recording data for later analysis fore- electronic bearing line (EBL) is speed and course of the vessel, on a closes the opportunity to check the used to measure the bearing to the piece of drafting parchment or vel- fix by additional measurements. A object. (Caution should be used, lum (a smooth heavy tracing paper) good navigator should never “get however, with radar bearings, the using the same scale as on the chart. behind” the vessel; new information precision of which are about 1° – 2° When the bearing is taken, the dis- should be analyzed when developed. in azimuth at best (see Chapter 9) tance from the object is determined If this cannot be done, the navigator and could be worse, depending by aligning the soundings along the may wish to slow or stop the vessel upon the amount of yaw, pitch, and course and the bearing, and moving (see Chapter 12). the sounding trace toward or away Navigators should always be roll of the vessel. The precision of from the object (on the bearing line) alert to the possibility that their ves- the distance measurement is usually until close agreement with the sels are “standing into danger.” In greater. In this case, the intersec- charted soundings is obtained. This this example, the vessel is likely to tion of the circular distance LOP determines a fix. However, this fix pass closer to the Sow and Pigs and the straight-line bearing LOP is not very precise unless the sound- Reef than originally planned. A determines the fix. ings change abruptly or offer some glance at this portion of the chart The same type of fix results if a other unique feature which distin- (see Figure 6-7) indicates that the navigation receiver is being used. guishes the specific area from oth- buoy R “2 S & P” bell marks the For any defined waypoint (specific western edge of this reef. Perhaps location stored in the receiver’s the lookouts should be alerted to memory), the receiver can display watch for this buoy, or the radar the bearing and distance to the way- should be adjusted to a smaller point. The bearing and distance to range setting to help ensure safe the waypoint can be plotted to passage. Alternatively, perhaps the establish the fix—although the lati- vessel’s course should be tude and longitude can be read altered more towards the directly. Many mariners believe Buzzards Bay entrance light to t h a t ensure that the reef is left a safe plotting distances and bearings distance to starboard. Which is quicker and less prone to error choice is best depends upon numer- than plotting latitude and longitude ous factors, such as the prevailing readings on the chart. Waypoints should be chosen (see Chapter 9) visibility, draft of the vessel, and GUARD COAST COURTESY OF UNITED PHOTO STATES the navigator’s familiarity with the to define the various “legs” of L local waters. Plotting and fix the planned voyage. Waypoints Fixed markers are preferable for taking should not be “clerical” are entered (and checked!) dock- position fixing.

6-22 Piloting 6

ers. A navigator may be more com- the time, and the means used to running fix (abbreviated R Fix). fortable terming this an estimated determine the fix. Examples J A new dead reckoning plot is position and labeling this position include 1225 GPS, 0745 DGPS, started at the position of the run- with a square to denote an EP. 1130 LORAN, etc. ning fix. In circumstances of reduced vis- J Avoid advancing (or retiring) an ibility (e.g., in fog), it is sometimes RUNNING FIX (R FIX) LOP for more than 30 minutes if convenient to navigate by following It is not always possible to you are in near-shore waters a depth contour, rather than laying obtain two LOPs at nearly the same where currents are variable, out straight-line courses—particu- time. For example, poor visibility unpredictable, and/or uncertain. larly if the vessel is not equipped may limit the number of objects that However, in open waters (free with GPS or Loran-C. Although the can be seen. In this case, a running of navigational hazards) it is computation of DR positions along fix may be used. A running fix uses common practice to advance (or a curved course are more tedious, an LOP on one object, with a previ- retire) LOPs for longer periods, the actual course can be approxi- ous (or subsequent) LOP on the providing you have good course mated by a series of straight lines same or another object, which is and speed records. and, with practice, quite accurate “time corrected” (advanced or DR plots can be made. If the sea retired) by dead reckoning calcula- Example of a Running Fix bottom is well shaped, it is surpris- tion of the vessel’s direction and dis- Vessel Perdida is southwest of ingly easy to follow a depth contour. tance traveled during the interval Point Judith on a course of C065 between observations of the LOPs. maintaining 5.0 knots in intermit- PASSING CLOSE TO This section considers the running tent showers and 4-ft. seas. A por- A FIXED CHARTED fix without any allowance for cur- tion of the DR plot for this segment AID TO NAVIGATION rent. The running fix with current is (labeled “original DR plot”) is Every time the vessel passes a discussed in several of the refer- shown in Figure 6-8, along with DR fixed charted ATON (such as a ences. To plot a running fix, follow positions at 0930, 1000, and 1030. light), or other charted object, a fix the steps below, which are also illus- At 1000, a visual bearing (convert- is obtained. As noted above, it is a trated in Figure 6-8: ed to 025 degrees true) is taken on matter of judgment whether or not J Allow for the time lapse the light at the tip of Point Judith, to label it a fix if a floating aid is between the first and second and the LOP is plotted and correct- passed close aboard. In Figure 6-4, bearing. This is done by advanc- ly labeled with the time and the for example, passing close to buoy ing (or retiring) the first LOP bearing (it is denoted “AB” in N 8 was judged equivalent to a fix along the vessel’s dead reckon- Figure 6-8). The 1000 LOP crosses and so denoted with a circle. The ing course as if it were advanc- ahead of the 1000 DR position, sug- factors contributing to this judgment ing (or retiring) at the same gesting that the SMG is greater than included depth soundings indicating speed and in the same direction 5 knots and/or that Perdida is far- a shoal and the close proximity to as the vessel’s course and speed. ther south than the DR position identifiable objects on the shore. J The first LOP is advanced would indicate. Perdida’s navigator (retired) by moving it parallel to monitors the depth sounder and ELECTRONIC FIX itself, forward (backward) along alerts the lookout to watch for buoy Navigation receivers for GPS, the course line a distance equal R “2” as a check on the vessel’s Differential GPS (DGPS), or to the distance covered by the position. But, R “2” cannot be seen Loran-C can provide fix data in vessel in the interval between (possibly because of the seas) or terms of latitude and longitude or the two LOPs. The intersection heard. In any event, at 1030 Point (in the case of Loran-C) time differ- of the advanced (or retired) LOP Judith Light is again seen, bearing ences (see Chapter 9). These coor- at the time of taking the second 342 degrees true. This second LOP dinates define a fix. The fix is bearing represents the best esti- is drawn and the navigator decides denoted with a circle (dot in center), mate of position and is called a to plot a running fix.

6-23 C A 1100 1030

C 065 RESTARTED S 5.0 DR PLOT 1000 1030 RFIX

C 065 S 5.0 0930

ORIGINAL 1000 1030 DR PLOT 025 342

B 025

1000 – 1030 D

L FIG. 6-8–Construction of a Running Fix Piloting 6

The steps are detailed below: explore other means to determine

ST G A U O A C R

S D

J Perdida’s position. The crossing U aVOID theSe Obtain the time interval and dis- A U Y X R ILIA tance that the vessel traveled angle of the advanced LOP and the COMMON since the 1000 LOP. 1030 LOP (43° in this illustration) errOrS IN 10 hr 30 min is much smaller than the ideal 90 pILOtING ______-10 hr 00 min degrees and the failure to observe 0 hr 30 min time interval or hear buoy R “2” are factors that According to Bowditch, the J Calculate the distance that the would be of concern. Depth infor- more common errors in vessel would move in the inter- mation could rule out the possibili- piloting include: val between the LOPs. ty that Perdida is too close to Point D = ST/60 Judith but is not of much help in J Failure to obtain or D = 5 x 30/60 D = 150/60 = 2.5 nautical miles deciding between the 1030 DR evaluate soundings J Using a pair of dividers, mea- position and the running fix (both J Misidentification of lie nearly along the 60-ft. depth sure the distance (2.5 M) off the atONs latitude or nautical mile scale contour). This might be a good J along the 065-course line in the time to stop imitating Christopher Failure to use naviga- direction traveled from the point Columbus and look at the GPS tional aids effectively where the 1000 LOP crosses the receiver! In the illustration in Figure 6-8, J Failure to adjust a mag- course line. the running fix was determined netic compass or keep a J Advance the first LOP, ensuring using nonsimultaneous observa- table of corrections it is moved parallel to itself for- tions of the same object. A running J ward along the true course line fix can also be determined from two Failure to apply devia- an amount equal to the distance non-simultaneous observations of tion or variation traveled (2.5 M). Draw the two different objects. In this case, J Failures to check com- advanced LOP. Label the the procedure is identical to that pass readings regularly advanced LOP with the time described above, except that the interval (1000-1030) written second LOP is plotted from another J Failure to maintain a above the line to indicate that it object. Dr plot is an advanced LOP over the The running fix assumes that the J Failure to plot new period from 1000 to 1030. The vessel’s SMG and CMG are equal to information bearing (025) is written beneath the STW and course, respectively. the line. This advanced LOP is In other words, the procedure J Failure to properly eval- denoted “CD” in Figure 6-8. assumes that the DR plot over the uate new information J Plot and label the LOP resulting time interval between the two obser- J from the second bearing. The vations is without error. To the poor judgement running fix is located at the extent that this assumption is valid, J Failure to use informa- point of intersection with the the running fix is likewise error free. tion in charts and navi- advanced LOP. Label this point However, current and wind conspire gation publications with a circle, and horizontally to undermine the validity of this with 1030 R Fix, clear of the assumption. Other things being J Failure to “keep ahead course line. A new DR plot is equal, therefore, a conventional fix of the vessel” started at this running fix. is superior to a running fix. J Failure to have backup The above example focuses on In the illustration in Figure 6-8, navigation methods the mechanics of determining and the vessel is assumed to maintain a plotting a running fix. In practice, constant course and speed. A run- in place the navigator should actively ning fix can also be determined if

6-25 6 Piloting

the vessel changes course and/or vessel maintains course and speed Chapter 9.) An electronic calculator speed. All that is necessary is to until a second bearing is taken. The equipped with trigonometric func- find the resultant course and speed second bearing is likewise converted tions is most convenient for calcu- over the interval. The resultant to an angle on the bow, and is shown lating the distances y and h. course and speed can be determined as angle H in Figure 6-9. The dis- Alternatively, special purpose by drawing a single line between tance run between the bearings, tables can be used to find the multi- the DR positions at the time that the denoted x, can be calculated from the plying coefficients in the above original bearing is taken and the usual TSD formula discussed in equations. To illustrate, let F1 be time that the second LOP is taken Chapter 5 as x = ST/60. Now if equal to sin Y/sin (H - Y). Table 6- and treating this as the overall angle H is exactly twice angle Y–for 3 gives values for F1 as a function course and speed line. Consult the example, if Y = 20 and H = 40, or Y of the angles Y and H. Simply enter references given in the bibliography = 45 and H = 90–then it follows that the table at the row corresponding for details. the triangle formed by the distance to the angle on the bow at the first run and the two bearings is an isosce- bearing and the column correspond- ANGLE ON THE BOW les triangle, with the “distance off” at ing to the angle on the bow at the (This section is more technical the second bearing, denoted y in second bearing. The value read and can be omitted without any loss Figure 6-9, exactly equal to the dis- from the table, Fl, is then multiplied of continuity.) Running fixes on the tance run between the fixes. Because by the distance traveled, ST/60, to same object can also be treated no mathematics is involved in figur- calculate y. mathematically, without reference ing this out, it is a common method Likewise, let F2 be defined as to the charts. Figure 6-9 provides an of figuring distance off. Choosing (sinY) (cos [90-H])/sin (H-Y). illustration of the running fix and such a pair of angles and calculating Table 6-4 shows F2 as a function of symbols used in this discussion. distance off is termed “doubling the the angles Y and H. F2 is read from Here the vessel takes a first bearing angle on the bow.” the table and multiplied by the dis- on an object (e.g., a lighthouse) and Although the computations are tance between bearings, ST/60, to converts this to an angle on the particularly simple if the angle on determine the distance off when bow. (Also referred to as an angle the bow is doubled, the method can abeam, or the CPA. (A more com- off the bow.) This angle, shown as be employed for any pair of angles prehensive set of tables can be angle Y in Figure 6-9, is measured (ideally, not too closely spaced for found in Bowditch.) from 0 through 180 degrees. (For maximum accuracy) on the bow, Y The calculations go faster if a this purpose, it is not necessary to and H. In particular, it can be shown worksheet, such as that shown in distinguish right from left, as it is (using the law of sine’s from ele- Table 6-5, is used to organize the assumed that the object remains on mentary trigonometry) that the dis- calculations. As a specific numeri- one or the other side of the vessel.) tance off at the second bearing, y, is cal illustration, suppose that the After taking the first bearing, the given by the equation, y = (ST/60) sportfish Esplendido is maintaining sin Y/sin (H-Y), and a true course of 090, and speed of 6 that the distance off knots, in an area where the magnet- when directly abeam, ic variation is 015W. At 1200, a denoted by the symbol magnetic bearing of 090 is taken on h in Figure 6-9, is h = a lighthouse using a hand-bearing (ST/60) (sin Y) (cos compass, assumed here to be free of [90-H]) divided by sin deviation. The true bearing is, (H-Y). (This distance, therefore, 090 - 015W = 075. The h, is sometimes called angle on the bow is the absolute dif- the distance at the clos- ference (difference without regard est point of approach to sign) between the true bearing L (CPA) and is discussed 075 and the vessel’s course (090), FIG. 6-9–Two-bearing problem illustrated at greater length in or 015 degrees in this example.

6-26 Piloting 6

F1 SeCOND aNGLe, h, (DeGreeS) FIrSt aNGLe, Y (DeGreeS) 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 5 1.000 0.502 0.337 0.255 0.206 0.174 0.152 0.136 0.123 0.114 0.106 0.101 0.096 0.093 0.090 0.089 0.087 10 1.992 1.000 0.671 0.508 0.411 0.347 0.303 0.270 0.246 0.227 0.212 0.201 0.192 0.185 0.180 0.176 15 2.970 1.490 1.000 0.757 0.612 0.518 0.451 0.403 0.366 0.338 0.316 0.299 0.286 0.275 0.268 20 3.924 1.970 1.321 1.000 0.809 0.684 0.596 0.532 0.484 0.446 0.418 0.395 0.377 0.364 25 4.849 2.434 1.633 1.236 1.000 0.845 0.737 0.657 0.598 0.552 0.516 0.488 0.466 30 5.737 2.879 1.932 1.462 1.183 1.000 0.872 0.778 0.707 0.653 0.610 0.577 35 6.581 3.303 2.216 1.677 1.357 1.147 1.000 0.892 0.811 0.749 0.700 40 7.375 3.702 2.484 1.879 1.521 1.286 1.121 1.000 0.909 0.839 45 8.113 4.072 2.732 2.067 1.673 1.414 1.233 1.100 1.000 50 8.789 4.411 2.960 2.240 1.813 1.532 1.336 1.192 55 9.399 4.717 3.165 2.395 1.938 1.638 1.428 60 9.937 4.987 3.346 2.532 2.049 1.732 65 10.399 5.219 3.502 2.650 2.145 70 10.782 5.411 3.631 2.747 75 11.083 5.563 3.732 80 11.299 5.671 85 11.430 Note: A more complete table can be found in Bowditch. L TABLE 6-3–Abbreviated Table of Values of Factor F1 for “Angle on the Bow” Computations

FIrStF2 SeCOND aNGLe, h, (DeGreeS) aNGLe, Y (DeGreeS) 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 5 1.174 0.130 0.115 0.108 0.103 0.100 0.098 0.096 0.094 0.093 0.092 0.091 0.090 0.090 0.089 0.088 0.087 10 0.516 0.342 0.284 0.254 0.236 0.223 0.214 0.207 0.201 0.196 0.192 0.188 0.185 0.182 0.179 0.176 15 1.016 0.630 0.500 0.434 0.394 0.366 0.346 0.330 0.317 0.306 0.297 0.289 0.281 0.274 0.268 20 1.658 0.985 0.758 0.643 0.572 0.524 0.488 0.461 0.438 0.420 0.403 0.389 0.376 0.364 25 2.425 1.396 1.050 0.874 0.766 0.692 0.638 0.596 0.562 0.533 0.508 0.486 0.466 30 3.291 1.851 1.366 1.120 0.969 0.866 0.790 0.731 0.683 0.643 0.608 0.577 35 4.230 2.336 1.698 1.374 1.175 1.040 0.940 0.862 0.799 0.746 0.700 40 5.215 2.836 2.034 1.628 1.378 1.208 1.082 0.985 0.906 0.839 45 6.215 3.336 2.366 1.874 1.572 1.366 1.214 1.096 1.000 50 7.200 3.820 2.682 2.105 1.751 1509 1.330 l.192 55 8.140 4.275 2.974 2.313 1.909 1.632 1.428 60 9.006 4.686 3.232 2.494 2.04 11.732 65 9.772 5.041 3.449 2.640 2.145 70 10.414 5.329 3.617 2.747 75 10.914 5.541 3.732 80 11.256 5.671 85 11.430 Note: A more complete table can be found in Bowditch. L TABLE 6-4–Abbreviated Table of Values of Factor F2 for “Angle on the Bow” Computations

Now suppose that at 1230, moved a distance, x, of (6) (30)/60, the distance of the CPA is 0.366 Esplendido again takes a bearing on or 3.0 nautical miles. From Table 6- (3.0) = 1.1 nautical miles. Table 6-6 the light, 060M or 045 true. The 3, factor Fl is 0.518, and, from shows the completed worksheet for second angle on the bow is, using Table 6-4, F2 is 0.366. The vessel’s this example. the absolute value, equal to 045 distance off at the second bearing is This procedure is exactly equiv- degrees. In the time interval 0.518 (3.0) = 1.6 nautical miles alent to plotting the running fix. As between the bearings, the vessel (rounded to one decimal place), and discussed here, no allowance for

6-27 6 Piloting

current is included. Although the navigation were on the computations may appear tedious, other side, NMT (denoting with practice these can be done Not More Than) would be rapidly and reliably, particularly if a used. Conventional prac- worksheet is employed. Bear in tice would be to draw mind, also, that no calculations are “whiskers” or hash marks, required in the event that the angle not shown in Figure 6-10, on the bow is doubled. on the side of the danger bearing facing the hazard. DANGER BEARINGS In order to use danger Before summarizing the material bearings to advantage it is L presented in this chapter, one last necessary that there be a charted FIG. 6-10–Danger Bearing Illustrated topic, danger bearings (called clear- object (here the lighthouse) on ing bearings in British texts), is which bearings can be taken. chart relative to the earth. However, included. To make the discussion Obviously, danger bearings only when the information is adequate, concrete, refer to Figure 6-10. This make sense if the hazard is not oth- such as three reliable LOPs cross- figure provides an extract from a erwise well marked. However, even ing at a point (or, more typically, vessel’s DR plot. The vessel is steer- if a buoy were shown on the chart, within a very small triangle), little it might be worthwhile to lay off a ing a course generally east, and judgment is necessary to establish a danger bearing as a precaution in intends to round a point of land with satisfactory estimate of the vessel’s the event that the buoy were off sta- a lighthouse before turning south- position. tion. (This advice is particularly east en route to its destination. But, there are also circum- helpful in areas of the world and Suppose, however, that there is an stances when the navigator has seasons where buoys are less fre- unmarked danger area (e.g., fish uncertain or conflicting information quently on station.) Readers inter- traps, shoals, or coral) that is locat- regarding the vessel’s position. ested in additional detail on danger ed just west of the light, denoted by Three LOPs, for example, may not bearings should consult the refer- the shaded area in Figure 6-10. A cross at a single point or be located ences provided at the end of this prudent mariner would ensure that a within a small triangle, or the chapter. wide berth was given to this area. apparent fix may be located an One simple procedure is to lay off a SUMMARY OF SYMBOLS implausibly large distance away line (shown in Figure 6-10) well from the vessel’s DR position. Figure 6-11 presents a summary north of the area, and to note the Situations such as these require a and brief description of the drafting bearing of this line (here 090 true). careful weighing of the information symbols used for DR and piloting. Provided that the actual true bearing at hand and intelligent and cautious These plotting conventions differ to the light was never less than 090 rejection of suspicious information. from those employed in other parts true, the mariner could be sure that The resulting position, adjusted for of the world (Europe, principally). the vessel would not enter the dan- such discrepancies, is termed the Aside from some additional notation ger area. The bearing, 090 in this most probable position (MPP). introduced in Chapter 7, this com- example, is termed a danger bear- Many mariners equate an MPP prises the entire set of symbols used. ing and labeled NLT (denoting Not with an EP. Although an MPP Less Than) above the danger bear- THE MOST PROBABLE could be an EP in certain circum- ing. As a practical matter, this is one POSITION (MPP) stances, the MPP is a more general of the areas where it would also be concept than an EP. In principle, an prudent to write the magnetic bear- Because there are always errors MPP could be a fix, running fix, EP, ing on the chart as well, since a associated with all measurements, it DR position, or some combination hand-bearing compass is likely to be is almost impossible to establish the of these. In the example given in used as a check. If the hazard to exact position of the vessel on the Figure 6-8, many navigators might

6-28 Piloting 6

TABLE 6-5 (TOP) & TABLE 6-6 (BELOW)–Worksheets for Angle on the Bow Computations

090 6 015 W 1200 090 075 015 1230 060 045 045 30 3.0 0.518 0.366 1.6 1.1

6-29 6 Piloting

IteM DIaGraM DeSCrIptION

DR plot C 090 Course (090 true) written above S 10.5 line, speed (10.5 knots) written below line.

DR position 0930 1000 Time (24 hour) written at angle to semicircle denoting DR position.

LOP Lightly drawn line with time (24 1030 hour) above LOP and true bearing 045 beneath.

Estimated Position 1030 1030 Square located where dashed per- 270 pendicular line from DR position touches LOP. 1030

Visual Fix 1100 Circle where two or more LOPs 1100 cross. Time written parallel to 1100 130 chart axis. 045 Electronic Fix 1500 RADAR GPS LORAN Time and method (if relevant).

Running Fix

1030 R FIX Circle containing the intersection of a given LOP and another LOP advanced (or retired) in time, with RFIX and the time specified.

L FIG. 6-11–A Concise Summary of Navigation Drafting Symbols decide that the running fix deter- denote an MPP; however, the letters navigators on whether or not to start mined was an MPP or EP rather MPP written next to the position a new DR plot at an MPP. The more than a fix. There is no generally would be clear enough. Likewise, “conservative” navigators would agreed upon symbol or label to there is no clear consensus among certainly not renew a DR plot at an

6-30 Piloting 6

MPP, waiting instead until a “prop- selected so as to minimize the position. A series of light dashed er” fix was determined. Other necessary precision of naviga- lines can be drawn from each of equally competent practitioners tional fixes. For example, the the objects with various possible would have a more flexible attitude course can be positioned on the bearings noted. When under- on this question. chart “one or two depth contours way, simply measure and record Many of the “judgment” issues, greater than the vessel’s draft,” the bearings, then observe the such as whether or not to use float- where less precision is required. preplotted position where these ing aids for fixes, how much faith to Fix opportunities can be identi- measured bearings cross on the put in a running fix, whether or not fied and preplanned. Danger chart. to start a new DR plot from an MPP bearings can be determined and J After plotting the intended track or EP, serve to remind us that navi- plotted on the chart. and annotating the nautical gation is both an art and a science. J Annotate the nautical chart chart, cut and/or fold the chart extensively to highlight dangers into a strip that covers the area PERSPECTIVES AND and fix opportunities. For exam- of this track line and necessary PRACTICAL IDEAS ple, if there are buoys and fixed surrounding detail. Underway, FOR SMALL BOATS ATONs near the planned route, the strip can be unrolled to The techniques described in circle these with a pencil and match the vessel’s progress Chapters 4 and 5 are ideally suited write the identifying letters, without taking up a great deal of to vessels that have sufficient space numbers, and light characteris- room—a poor man’s electronic aboard to plot courses and fixes. tics in large characters. chart display. Ships, yachts, larger sportfishing J For many trips, it is not neces- J If your boat is equipped with vessels, trawlers, and many sail- sary that every fix be of high electronic navigation receivers boats offer sufficient space and sta- accuracy. An approximate posi- (given the low cost, size, and bility to permit use of most or all of tion will do. The preplanned weight of these receivers, there these techniques. Aboard a center- route can be followed and prin- is little excuse not to have at console boat or a runabout, it is dif- ciples described in this chapter least one aboard), waypoints ficult to use the piloting methods can be used to fix the vessel’s marking the checkpoints to be discussed in this chapter. Having position periodically. For exam- used along the track can be pre- said this, the extent to which these ple, you can use a hand-bearing programmed. The cross-track techniques can be used when the compass to take a bearing on a error (XTE) displayed by the navigator is convinced that accura- suitable object. Rather than plot receiver can be monitored and cy is important is amazing. (As this LOP exactly, the navigator’s used to correct courses from observed by William Thomas hand can be used as a surrogate those determined from the chart. Cummings (1903-1944) in a field for the parallel rulers. With If more electronic equipment is sermon on Bataan in 1942, “There practice, you can estimate LOPs carried, additional methods can are no atheists in the foxholes.”) to within 5 degrees or so. be used. For example, if radar is Here are some additional ideas and Certain fixes, such as those in carried, the vessel can navigate perspectives for you to consider: the vicinity of hazards to navi- a fixed distance off the shoreline J To a large degree, preplanning gation, may have to be quite by maintaining one of the FRMs can substitute for on-the-water accurate and conventional plot- or a preset VRM tangent to the plotting. Even operators of ting used for these critical fixes. apparent shoreline. In areas with small (and/or fast) boats can lay J You can preplot (RYA 1990) a regular depth gradient, the out and plot the track of the possible fixes by drawing a depth sounder can be used for intended voyage, including bearing lattice. Suppose, for the same purpose. courses and DR positions. In example, that two objects are to J If you plan to cruise in a well- many cases, courses can be be used for fixing the vessel’s defined inshore channel, such as

6-31 6 Piloting

portions of the Intracoastal of) various ATONs, and keep a tion is accomplished by a judicious Waterway where buoys and sharp eye on the depth sounder. combination of DR plots, seaman’s other markers are closely However, when planning trips eye, and electronic navigation. spaced, it is not really necessary out of sight of land, or where fix to maintain a tactical DR plot. opportunities are less frequent, Just lay out the track line (with ensure that you follow the meth- courses), steer these courses, ods presented in this chapter. look for (and note the passage Nowadays, most coastal naviga-

6-32 Piloting 6

SELECTED REFERENCES

Anon, “Celestial Navigation, the Basics (Part VII) This Moody, A. B., (1980). Navigation Afloat, A Manual for Segment Offshore Plotting,” Ocean Navigator, No. the Seaman, Van Nostrand Reinhold Co., New 13. May/June 1987, pp. 49 et seq. York, NY.

Bowditch, N. (1995). American Practical Navigator, Pike, D., (1990). Fast Boat Navigation, Adlard Coles An Epitome of Navigation, Pub. No. 9, Defense Nautical, London, UK. Mapping Agency Hydrographic/Topographic Center, Bethesda, MD. This publication is avail- Royal Yachting Association, (1990). Navigation, An able electronically from the National Imagery and RYA Manual, David & Charles, Newton Abbot, Mapping Agency (NIMA), Marine Navigation UK. Department web site (http://www.nima.mil/). Sexton, J., (1987). Interpreting the Single LOP, What Bright, C., (1989). Celestial Navigation, The Basics: to Do When Your DR Position is Several Miles Celestial Draftsmanship, Ocean Navigator, From a New LOP?, Ocean Navigator, No. 11, Jan/Feb, pp. 53 et seq. Jan/Feb, pp. 48 et seq.

Brogdon, W., (1995). Boat Navigation for the Rest of Schofield, B.B. (1977). Navigation and Direction, The Us—Finding Your Way by Eye and Electronics, Story of HMS Dryad, Kenneth Mason, Hampshire, International Marine, Camden, ME. UK.

® Campbell, S., (1985). The Yachting Book of Practical United States Power Squadrons (USPS), (1998). Navigation, Dodd, Mead & Company, NY, espe- Plotting & Labeling Standards, The United States cially pp. 81 - 101. Power Squadrons, Raleigh, NC.

Eyges, L, (1989). The Practical Pilot: Coastal Commandant USCG, (1999). United States Coast Navigation by Eye, Intuition, and Common Sense, Guard, Fiscal Year 2000 Budget in Brief, International Marine Publishing Co., Camden, ME. Washington, DC.

Fraser, B., (1981). Weekend Navigator, John de Graff, Vesey, T., (1989). Dead Reckoning in Fog, Cruising Inc., Clinton Corners, NY. World, Vol. 15, No. 10, October, pp. 33 et seq.

Graves, F., (1981). Piloting, International Marine Walsh, G., (1988). Coastal Navigation, The Basics, Publishing Co., Camden, ME. Building on DR: The Line of Position, Ocean Navigator, No. 19, May/June, pp. 55 et seq. Groth, L., (1988). Sticking With the DR Position, Ocean Navigator, Nov/Dec, pp. 15 et seq. Williams, J. R., (1988). Starting a Fresh DR Track, A Navigator Makes the Case for Using EPs and Kielhorn, W. V., (1988). A Piloting Primer, privately Running Fixes, Ocean Navigator, No. 20, published, Naples, FL. July/August, pp. 68 et seq.

Ministry of Defense (Navy), (1987). Admiralty Manual of Navigation, BR 45 (1), Volume 1, Her Majesty’s Stationary Office, London, UK.

Markell, J., (1984). Coastal Navigation for the Small Boat Sailor, Tab Books Inc., Blue Ridge Summit, PA.

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6-34 Chapter 7

CURRENT SAILING

So, we beat on, boats against the current, borne back ceaselessly into the past. –Francis Scott Fitzgerald, The Great Gatsby

We must take the current when it serves, or lose our ventures. –William Shakespeare, Julius Caesar

INTRODUCTION trast, often regard wind and current his chapter covers the impor- as minor annoyances except when Ttant topic of current sailing either assumes extreme propor- for the mariner. It builds upon tions. Currents are thought to pre- the material presented in Chapters sent more of a challenge for power- 5 and 6 to include allowance for boat seamanship than powerboat the effects of current in voyage navigation. This view is overly sim- planning and en route navigation. plistic and misleading. As shown in Although sailing vessels are affect- this chapter and later in Chapter 11, ed to a greater degree than most the effects of current can be very powerboats, this chapter is impor- important for powerboats—particu- tant reading for sailors and power- larly in terms of fuel efficiency and boat skippers alike. range. Operators of commercial Sailors are long accustomed to vessels routinely select advanta- dealing with wind and current in geous times to enter a harbor or voyage planning. Winds affect the navigate a river so as to minimize vessel’s speed through the water fuel consumption. Powerboats are (STW) and create the necessity to also affected by current in less sub- tack rather than run direct courses tle ways; ask any navigator why to a destination. Current affects the DR positions are not exact! speed made good (SMG) and the This chapter covers three princi- required course to achieve a desired pal topics: track. These truths are obvious to J First, it illustrates the additional sailors. notation necessary for plotting Powerboat operators, in con- current problems.

7-1 PHOTO COURTESY OF RAYTHEON MARINE 7 Current Sailing

J Second, it explains the calcula- This chapter is slightly shorter CURRENT SAILING TERMS tions necessary to determine the than Chapters 5 or 6. But, teaching The following terms are used in actual current set and drift. experience suggests that the materi- current sailing: J Third, it shows how to deter- al in this chapter requires fully as J Estimated Current: This is the mine the course necessary to much time to master as the earlier current developed from evalua- compensate for estimated cur- chapters on dead reckoning (DR) tion of known or predicted rents and to calculate the result- and piloting. Reread both of these forces using calculations, cur- ing SMG/SOA. chapters, particularly the defini- rent tables, diagrams, and/or To master these topics requires tions contained therein, before charts. These methods are pre- some elementary knowledge of studying this chapter. sented in Chapter 8. vectors. These are introduced early J CURRENT Actual Current: This is the cur- in this chapter from a mechanical, rent measured as the difference rather than a mathematical, per- The process of allowing for cur- between the vessel’s actual posi- spective. Steps to calculate a solu- rent in determining the predicted tion (fix) and that predicted with- tion are clearly described and illus- course made good, or in determin- out taking into account the effects trated with numerical examples. ing the effect of a current on the of current (i.e., the DR position). Additionally, the Maneuvering direction of motion of a vessel, is The actual current incorporates Board (M Board) is introduced in termed current sailing. For the pur- all of the effects of current this chapter as a tool for rapid and poses of this chapter, the term cur- described above. (Purists may accurate analysis of current sailing rent as used in current sailing challenge the use of the word problems. Use of M boards for includes: actual, arguing that even a fix has other applications (e.g., radar plot- J The horizontal motion of water uncertainty so that even this actu- ting) is deferred until Chapter 9. over the ground, including al current is an estimate. To avoid Finally, a more complete voyage ocean current, tidal, and river confusion with the estimated cur- planning form is introduced to sim- currents, rent as determined above this plify and organize current sailing objection is put aside and the J The effect of wind and seas, and calculations. term remains actual current.) J The effects of steering errors J Set: Set is the direction toward made by the helmsman, com- which a current flows, or the

ST G A U O A C R

S D pass error, speed curve error, U What YOU direction toward which the ves- A U Y X R ILIA WILL LearN IN tachometer or other engine sel has been moved as a result of thIS Chapter error, log or speedometer error, the current. Set is expressed in and fouled bottom or unusual degrees, true. (This convention J Definitions of terms used trim. is exactly opposite to that for in current sailing. wind. The wind direction is the APPLICATIONS J How to determine current direction from which the wind is set and drift. Current sailing calculations may blowing.) also be used to correct course and J Drift: Drift is the magnitude or J How to determine the speed for the effects of known speed of the current. Drift can vessel’s estimated posi- (measured or calculated) or sus- be expressed in knots, statute tion. pected current in order to arrive at miles per hour, or kilometers per the intended destination at the hour. The choice of units J How to figure the course intended time. Current sailing cal- depends upon the cruising area. to steer to compensate for culations may be applied directly to It is convenient to use speed set and drift. a DR plot to convert a DR position units to match the distance units into an estimated position (EP). given on the chart of the area.

7-2 Current Sailing 7

THE CURRENT TRIANGLE statute, or nautical miles) or speed “ec.” It is drawn outward from the Graphical calculation of the (e.g., in knots or statute miles per origin (e) a distance of 7.0 units effects of current involves the use hour). The M board also contains along a bearing of 090 degrees. In of the current triangle, which is a 36 equally spaced bearing lines, vector terminology, the “tail” of the simple vector diagram. The compo- one for every 10 degrees in azimuth vector is at the origin, and the nents of the current triangle have (written on the outermost ring), “head” of the vector is located at the both magnitude and direction. The originating at the center and radiat- point (090 degrees, 7.0 knots). For current triangle can be constructed ing towards the outer rings. Along this example, a distance scale of 1:1 on a separate piece of blank paper, the outermost ring are smaller is used, so the “range rings” can be or directly on the chart, using the degree gradations, enabling read in knots directly. If a different compass rose as a convenient azimuths to be drawn every 1 scale were used, it would be neces- means of measuring direction and degree from 0 through 359 degrees. sary to set the dividers using the the latitude scale as a means of Just beneath the outer bearing scale auxiliary scales to the right or left determining magnitude in units of is an inner bearing scale, shown in and mark off the vector length. (It is speed (knots or mph). Additionally, smaller print, that presents the reci- good practice to record the scale current problems can be solved on procals of the principal bearings used somewhere on the diagram for an M board, the approach recom- (e.g., the reciprocal of 10 degrees is later reference.) To draw the ec vec- mended here. 190 degrees, and this is shown tor, simply use a paraline plotter or beneath the 10-degree mark). At the parallel rules, align the origin and MANEUVERING BOARD bottom (not shown in Figure 7-1 the 090 mark on the outer bearing (M Board) because of space limitations) is a ring, and draw a solid line of length The M board is a special-pur- self-explanatory nomograph that 7.0 units. (If a bearing were inter- pose plotting aid that is published can be used to solve time-speed-dis- mediate between the outer gradua- by the National Imagery & tance (TSD) problems. On each tions, it would be convenient to use Mapping Agency (NIMA), formerly side of the sheet are two auxiliary a divider preset to a 7-unit length to the Defense Mapping Agency scales, four in total (representing locate the head of the vector.) Label (DMA). It is sold in pads of 50 alternative scales of 2:1, 3:1, 4:1, this first vector with the course and sheets and comes in two sizes, a 10- and 5:1), that can be used in lieu of speed in the same manner as for a inch diameter (DMA number the principal distance scale. There DR plot, course above the line and 5090), and a 20-inch diameter are also inset tables for recording speed beneath the line. (DMA number 5091). The smaller time, bearing, and range, a feature Now suppose that an estimated diameter pad (5090) is more conve- that is useful for radar plotting (see current with a set of 180 degrees nient aboard small vessels and can Chapter 9) but is not relevant here. and drift of 4 knots affects the be purchased at authorized agents The use of the M board is explained progress of the vessel. Obviously, for NIMA publications and many below. the overall effect of this current will dealers in navigation supplies. It is be to shift (set) the vessel to the reproduced in Figure 7-1. The M VECTORS TO DETERMINE right (south) of its course. In other board is a so-called polar diagram, THE EFFECTS OF A KNOWN words, the estimated course of consisting of ten equally spaced OR ESTIMATED CURRENT advance (COA) or course made concentric rings, termed distance The effects of current on a ves- good (CMG) will be greater than circles, around an origin (denoted sel are easily determined with an M 090. Likewise, the SOA or speed by the symbol “e,” drawn by hand board. As a first example, assume made good (SMG) may differ from in Figure 7-1) graduated with the that a vessel is steering a course of the speed through the water (STW, numbers 2 through 10 (1 is not 090 degrees and maintaining a or more simply, speed, S). (SOA shown but is implicit). These dis- speed of 7.0 knots. The course or and SMG are virtually—but not tance circles can be used to repre- velocity vector for the ship is dia- exactly—interchangeable terms. sent either distance (e.g., in yards, grammed in Figure 7-1 and denoted The distinction is discussed later.)

7-3 7 Current Sailing

MANEUVERING BOARD

e C 090 c S 7.0

TR (CMG) 120 4.0 DRIFT 180 SET SOA (SMG) 8.1

d

SCALE 1:1

L FIG. 7-1–Maneuvering Board with Example Problem

The operative question is, by how In this case, the current vector is DRIFT is sometimes written Dft or much? drawn from the head of the course simply “D” to save space. (Since To answer this question, it is vector, c, in the direction of the set navigators also use the symbol “D” first necessary to draw another vec- of 180 degrees, a length of 4.0 for distance, it is preferable not to tor, the current or drift vector, to units, and labeled SET 180, DRIFT use “D” for drift to avoid ambigui- represent the effect of the current. 4.0, as is also shown in Figure 7-1. ty.) To draw this vector, the parallel

7-4 Current Sailing 7

rule, or paraline plotter, is first and when the drift is large com- respect to the ground, so the student aligned with the origin along a pared to the vessel’s speed, as may wonder again why the DR plot bearing of 180 degrees and then shown in Table 7-1, for a “beam” is not abandoned in favor of the EP walked or rolled until aligned with current. Inspection of Table 7-1 plot. Although there is some merit the head of the course vector. A pair (produced by computer) shows why to this idea, remember that the drift of dividers preset to length 4.0 (the fast powerboats are less likely to be vector is merely an estimate and drift in this example) is used to affected by the current than slower may be proven wrong with subse- determine the length of the current sailboats. But, it also shows that quent fixes. All that is known with vector. The tail or the drift vector is CDAs are significant, nonetheless, (near) certainty is that the vessel at point c, and the head is labeled d. and should not be disregarded. steered 090 and maintained 7.0 Finally, the resultant vector, ed, Assuming a 4-knot drift, for exam- knots. Thus, it is recommended in which denotes either the track ple, the CDA would be nearly 6 situations where there is a pre- (TR)/SOA or CMG/SMG, can be degrees, even if the vessel’s STW dictable current that both DR posi- determined. It is drawn from the were 40 knots, hardly a negligible tions and EPs be drawn. origin (e), to the head of the current effect. The above method for estimat- vector (d). The TR or CMG can be The effect of current on the ves- ing the effects of a known current read from the outer bearing ring and sel’s progress in the original exam- on the vessel’s position is also used is approximately equal to 120 ple can also be shown in a more in correcting a running fix for cur- degrees, some 30 degrees to the familiar manner, by use of the DR rent. In the conventional running right of the course steered in this plot, shown in Figure 7-2. The DR fix calculation, the first LOP is example. The SOA or SMG can be course and DR positions are now advanced along the course steered a measured from the length of the shown beginning with a fix or distance equal to the DR distance vector just drawn in, approximately departure assumed to take place at over the time period that the first 8.1 knots in this example, 16% 1000. Remember that the DR posi- LOP is advanced. In the presence of a known current, the above proce- more than the STW. (Any current tions do not include the effect of dure is used to determine the resul- for which the SMG is greater than current, so the DR course line is tant CMG/SMG, and the line of the STW is termed a fair current. If plotted in a conventional manner. position (LOP) is advanced along the SOA/SMG is less than the Instead, the effects of the estimated the CMG (rather than the course) a STW, the current is termed foul.) current are shown in a series of esti- distance calculated using the SMG This vector is labeled with the TR mated positions (EPs), or DR posi- (rather than the speed) over the time or CMG above the line and SOA or tions, corrected for drift. The vector interval that the first LOP is being SMG beneath the line. This com- diagram on the M board described a advanced. The position so deter- pletes the analysis of the problem. time period of one hour, so the drift mined might be termed a running The effect of the current is easily vector, cd, is drawn from the 1100 seen and determined quantitatively DR position to determine the 1100 by use of the M board. EP. The drift vector is drawn with a In this example, the difference dashed line and appropriately between the course steered and esti- labeled as shown. EPs for times mated track, which might be termed other than 1100 are easily drawn the current drift angle (CDA), is from the corresponding DR posi- quite substantial. In general, the tions parallel to the drift vector a CDA is a function of the relative length sufficient to touch the line orientation of the course and drift drawn from departure to the 1100 vectors and of the vessel’s STW EP. compared to the drift. CDAs are Arguably, the line connecting L largest when the current is abeam the EPs is a more realistic estimate FIG. 7-2–Current problem in Figure 7-1 (90 degrees to the vessel’s course) of the vessel’s actual path with as it might appear on a nautical chart.

7-5 7 Current Sailing

VeSSeL DrIFt OF BeaM CUrreNt (KNOtS) StW (KNOtS) 1.00 1.50 2.00 2.50 3.00 4.00 4.50 5.00 1.0 45.0 56.3 63.4 68.2 71.6 76.0 77.5 78.7 2.0 26.6 36.9 45.0 51.3 56.3 63.4 66.0 68.2 3.0 18.4 26.6 33.7 39.8 45.0 53.1 56.3 59.0 4.0 14.0 20.6 26.6 32.0 36.9 45.0 48.4 51.3 5.0 11.3 16.7 21.8 26.6 31.0 38.7 42.0 45.0 6.0 9.5 14.0 18.4 22.6 26.6 33.7 36.9 39.8 7.0 8.1 12.1 15.9 19.7 23.2 29.7 32.7 35.5 8.0 7.1 10.6 14.0 17.4 20.6 26.6 29.4 32.0 9.0 6.3 9.5 12.5 15.5 18.4 24.0 26.6 29.1 10.0 5.7 8.5 11.3 14.0 16.7 21.8 24.2 26.6 14.0 4.1 6.1 8.1 10.1 12.1 15.9 17.8 19.7 16.0 3.6 5.4 7.1 8.9 10.6 14.0 15.7 17.4 18.0 3.2 4.8 6.3 7.9 9.5 12.5 14.0 15.5 20.0 2.9 4.3 5.7 7.1 8.5 11.3 12.7 14.0 25.0 2.3 3.4 4.6 5.7 6.8 9.1 10.2 11.3 30.0 1.9 2.9 3.8 4.8 5.7 7.6 8.5 9.5 35.0 1.6 2.5 3.3 4.1 4.9 6.5 7.3 8.1 40.0 1.4 2.1 2.9 3.6 4.3 5.7 6.4 7.1 L TABLE 7-1–Current Drift Angles (Degrees off Course) as a Function of Vessel STW and Current Drift for Beam Current fix corrected for drift, or an estimat- other authoritative sources such as or estimated current but no LOP. ed position, depending upon the the United States Power Squadrons® Here a dashed construction line likely accuracy of the drift estimate. (USPS). This section reviews these is drawn from the DR position at In following this procedure, make plotting conventions for EPs, with an angle equal to the current set. sure that you properly take into and without current or LOPs, when The length of this construction account the time between LOPs and plotting on a nautical chart. line is equal to the distance that do not, for example, arbitrarily the vessel would move in the J Figure 7-3 illustrates the plotting assume that the current is affecting time interval since the last fix. convention for an EP assuming the progress of the vessel for 1 hour, For example, if the time since that only an LOP is available (no when the interval between LOPs is the last fix were 2 hours and the current information). The proce- only 20 minutes. current drift 1.1 knots, the length dure is to determine and plot a of the drift vector would be 2.2 DR position at the time corre- CHART PLOTTING nautical miles. It is unnecessary sponding to the LOP and to draw CONVENTIONS to draw a head on this arrow a dashed construction line from As noted in Chapters 5 and 6, the because lines are always drawn the DR perpendicular to the United States Coast Guard from a DR position. Auxiliary (USCGAUX) encourages LOP. (This is a review of mater- J Figure 7-5 illustrates the case the use of standard plotting and ial covered in Chapter 6.) where both current information labeling techniques adopted by the J Figure 7-4 illustrates the plotting and an LOP are available. First, USCG and the U.S. Navy, as well as convention if there is an actual a DR position is plotted. Next, a

7-6 Current Sailing 7

dashed construction line is drawn from the DR position (in the direction of the set) a length equal to the distance that the vessel would drift since the last fix. Finally, another dashed con- struction line is drawn from the end of the drift vector perpen-

dicular to the LOP. COURTESY OF USPS ILLUSTRATION L Some of this formalism is omitted if expected average ground speed, FIG. 7-4–Plotting convention for whereas speed made good (SMG) current problems are being solved estimated position with current but refers to an actual average ground on a maneuvering board. no LOP. speed. The next current problem, dis- cussed below, refers to the determi- SOA are relevant. Put somewhat nation of the actual current, from differently, TR and SOA refer to knowledge of the course steered before-the-fact estimates, whereas and speed maintained, as compared CMG and SMG refer to actual, to the actual average ground speed rather than planned, courses and and direction. In other words, for speeds. determination of the actual current, These differences are subtle and the vessel’s course and speed vector are often ignored by mariners.

ILLUSTRATION COURTESY OF USPS ILLUSTRATION are compared to the CMG/SMG. Indeed, if the before-the-fact esti- L (Whatever the skipper’s intentions mates of current are accurate, then FIG. 7-3–Plotting convention for or expectations were is beside the the TR is almost exactly equal to estimated position without current but point.) Alternatively, for planning the CMG, and the SOA is, likewise, with LOP. purposes, it is necessary to decide equal to the SMG. what course and speed to run in order to maintain an intended track DETERMINATION OF DISTINCTIONS BETWEEN (TR) and, possibly, intended aver- SET AND DRIFT TR/CMG AND SOA/SMG age ground speed as well. Solutions Set and drift are usually deter- It is noted above that the terms to this problem are also discussed in mined from an analysis of the DR TR and CMG (likewise, SOA and this chapter. But here the TR and plot when a fix is obtained, by sim- SMG) are virtually, but not exactly, interchangeable. The distinctions are not central to the first example but become more important for dis- cussion of the other current sailing problems. As used in current sail- ing, the track (TR) refers to an intended or expected direction of travel with respect to the ground (course of advance (COA)), where- as course made good (CMG) refers to the actual direction of travel with respect to the ground. Likewise, L COURTESY OF USPS ILLUSTRATION SOA refers to an intended or FIG. 7-5–Plotting convention for estimated position with LOP and current.

7-7 7 Current Sailing

ply reversing the procedure and 301 degrees true to the The drift vector just determined described above. Briefly stated, cur- Buzzards Bay entrance light) at is termed the actual current, rent set and drift account for the dif- 1450. (If desired, a CMG/SMG because it is based on actual data ference between a fix and a DR vector can be drawn in by con- rather than an estimate derived position. If there is no current, then necting the initial fix and the from current tables, charts, or the fix and the DR position should newly acquired fix with a diagrams (see Chapter 8). coincide. straight line, measuring the

The procedure for determination CMG, and calculating the SMG ST G A U O A C R

S D

U COMMON

A U Y of current set and drift is carried out by the TSD formula.) XI AR LI in the following four steps and is Immediately plot the DR posi- errOrS IN illustrated with a numerical exam- tion corresponding to the fix CUrreNt prOBLeMS ple from a portion of a cruise in the time (1450), as is illustrated in experience (both in the waters covered by the 1210-Tr Figure 7-6. classroom and on the water) chart. Although an M board could J Third, draw a dashed line, the shows that students often be used for estimation of the current drift vector, from the DR posi- make the following errors. vector, it is more common to solve tion just plotted to the fix taken avoid these errors! this problem directly on a chart, so at the same time. Measure the this technique will be illustrated. bearing of this drift vector (from J Drawing the drift vector J First, prepare a DR plot in the the DR position to the fix) using from the fix to the DR conventional manner, noting a paraline plotter or parallel position rather than from either departure or the vessel’s rule. This bearing (320 degrees the DR position to the fix earlier fix, together with true in the example) is the set of and, therefore, calculat- course(s) steered and speed(s) the current. Label the drift vec- ing the reciprocal of the maintained. (Note that it is not tor appropriately with the set drift vector necessary that the vessel steer written above the dashed line only one course and/or maintain representing the drift vector. J Failing to take in to only one speed for the proce- J Finally, measure the length of account the actual time dure to produce correct the drift vector—1.6 nautical between fixes (which may answers! This point is made miles in this example. This is the differ from 30 minutes or clear in a later example.) An distance that the vessel drifted 1 hour) and, therefore, illustrative DR plot is provided over the time period between the figuring the drift incor- in Figure 7-6. This plot shows two fixes. Remember that drift is rectly that the vessel takes departure the distance covered in one hour J from the green lighted buoy (it is measured in knots or statute Failing to plot a DR posi- near Quick’s Hole on the east miles per hour), and, therefore, it tion corresponding to the side of Nashawena Island at is necessary to divide by the time exact time a fix is taken time 1330 and maintains a period (either in decimal hours J Failing to renew the DR course (213 degrees true) and or, using S = 60D/T, in minutes) speed (5.0 knots). Figure 7-6 to calculate the rate. In this plot at every fix also shows the appropriate DR example, the time period positions plotted every 30 min- between fixes was 1 hour and 20 As noted above, there is no utes on the hour and half hour. minutes (1450 minus 1330 = 80 requirement that the vessel main- J Second, plot a fix whenever it is minutes), so the drift = 60 tain a constant course or speed for obtained. In this example, a (1.6)/80, or 1.2 knots. (A com- these calculations to be accurate. cross-bearing fix is obtained mon error is to forget this step.) Any combination of course and (355 degrees true to the light on Add the label to the drift vector speed changes can be accommodat- the SW tip of Cuttyhunk Island directly beneath the set notation. ed, provided that these are recorded

7-8

1330

1400

S 5.0 S

C 213 C

1430 1450

SET 320

DRIFT 1.2

355

1450 1450

1450 301

L FIG. 7-6–Determination of Actual Current by Comparison of a Fix with a DR Position 1030

SET 226 1030

1030 DRIFT 0.6 272

C 016 S 5.0

0900 1000

C 127 S 7.0 C 037 S 5.0

0930

L FIG. 7-7–Determination of Set and Drift with Multiple Course and/or Speed Changes Current Sailing 7

and reflected in the DR plot. Figure follow. That said, common errors vector, ed, is drawn based on the 7-7 illustrates this slightly more include drawing the drift vector vessel’s actual positions at the times complex case. Here the vessel takes from the fix to the DR position of the two fixes. Finally, the current departure from the Harbor of rather than from the DR to the fix vector, cd, is drawn from the Refuge near Point Judith at 0900 (thus obtaining the reciprocal of the course/speed vector to the and makes several course and/or set), failing to take into account the CMG/SMG vector. The orientation speed changes, as shown on the DR elapsed time between fixes, and and length of this vector give the set plot. At 1030, a fix is obtained from failing to plot a DR position corre- and drift, respectively. (Remember a range line consisting of the two sponding to the time that the fix to draw the vector from c to d, and towers located on Beavertail Point was taken. Remember, also, to not from d to c; else the set will be and the tank (bearing 272 degrees renew the DR plot at the fix. off by 180 degrees!) true) located on Point Judith Neck. As noted above, an M board can (Note from Figure 7-7 that the bear- be used as well as the chart. For ADDITIONAL REMARKS ing on the range is omitted, in cases where a constant course and ON DETERMINATION accordance with the plotting con- speed have been maintained, the OF CURRENT ventions discussed in Chapters 5 problem is particularly simple and The above examples show how and 6.) This 1030 fix is plotted, amounts to reversing the logic used to estimate the actual current by together with the 1030 DR position. in constructing Figure 7-1. That is, comparing the DR position with a The drift vector is drawn in and the the course and speed vector, ec, is fix. It is also possible to estimate bearing (set = 226 degrees) and first drawn. Next, the CMG/SMG the current by comparing a DR length (0.9 nautical miles) deter- position with an EP derived from a

mined. The time between departure ST G A U O A single LOP neglecting any C R

S D

U USe OF the

A U Y X R and the first fix was 1.5 hours (1030 ILIA M BOarD allowance for current. Just substi- – 0900), so the estimated drift is tute the word EP for the word fix in 0.9/1.5 or 0.6 knots. The set and Current problems can be the above discussion and follow the drift are labeled appropriately. As solved (plotted) on either an same procedure. However, because can be seen, the procedure required M board or a chart. the EP derived from a single LOP is to incorporate course and/or speed likely to be less accurate (perhaps changes is only slightly more com- Experience shows that the M substantially less accurate) than a plicated (because the DR plot is board usually provides more fix, the “actual” current so deter- more complex) but is otherwise mined is, likewise, less accurate. precise answers and is easier identical to the simpler case. Even if a fix, rather than an EP, Incidentally, this example is a nice to grasp for many students. is used for estimation of actual cur- illustration of the fact that the CMG Solving the current problem rent, it is important to realize the and SMG vector is, to some degree, limitations of this estimate. First, off the chart results in less an abstraction. The CMG/SMG the “actual” current includes all of vector is determined by connecting chart clutter. However, every the factors listed under the defini- the two fixes with a straight line. In marina or marine retailer tion of current, e.g., helmsman’s this case, the straight line would errors, errors in the speed curve, carry the vessel over a portion of does not sell M boards. wind effects, and other factors Point Judith! Do not think of this as Moreover, doing the work besides the horizontal movement of an error; it follows directly from the “off the chart” may make it water. Even without this considera- definition of the CMG/SMG vector tion, the actual current should be as the resultant vector. more difficult for your relief termed an “historical estimate” rel- These two examples show that to check (if you have the lux- evant to a particular time and the procedure for calculation of the ury of multiple navigators). expanse of water. Conventional actual current is quick and easy to practice is to apply this historical

7-11 7 Current Sailing

estimate to future legs. However, make good an intended track (TR),

ST G A U O A C R

S D CUrreNt what is needed is a forecast of assuming that the current set and U

A U Y X R future current in the area where the drift are known (either because ILIA SaILING IN aN vessel will be cruising, rather than these are estimated in advance eLeCtrONIC estimates of past currents for areas using the methods of Chapter 8 or WOrLD already traveled. Judgment is determined by a comparison of the required to decide whether or not to vessel’s DR position with a fix as The availability of modern use the actual current determined discussed above), as well as the navigation systems, such as the on one leg of a trip as a surrogate vessel’s speed (S). This problem is Global Positioning System for those applicable to subsequent easily solved on an M board in the (GPS) or Loran-C, can simplify voyage legs. Current diagrams and following five steps. current sailing. As noted in related material, discussed in To make the discussion con- Chapter 9, the navigator can Chapter 8, provide an indication of crete, assume that the skipper wish- specify the intended voyage the spatial distribution of current es to make good a track of 050 track as a series of waypoints and, hence, the possible differences degrees, and that the current set and that define the ends of route between the current in one area and drift are 130 degrees and 2.0 knots, segments. that in another. If these differences respectively. The vessel’s speed is Once started along this are small, the mariner may use the 8.0 knots. The M board solution is route, the helmsman steers the historical estimate with more confi- shown on Figure 7-8 and is dis- vessel in accordance with the dence. Local knowledge is also use- cussed below. navigation receiver’s display to ful in forming sound judgments. J First, lightly draw a line of arbi- keep the cross-track error To return to the topic of the (XTE) at or close to zero. Most accuracy of the actual current, it is trary length in the direction of navigation receivers have a dis- important to note that the overall the intended track (050 play with a heading arrow accuracy is a function of the fix degrees). When completed, this directing the helmsman to steer accuracy and the time period over line will represent the vessel’s right or to steer left. The aver- which the estimate was developed. TR and SOA. However, the age heading necessary to keep If the time period is “short,” the fix length (SOA) cannot yet be the XTE at or close to zero is will be “close” to the DR position, determined. The vector begins at the correct course to compen- and any errors in the fix or the DR the origin, e, of the M board. plot will have a substantial effect on J Second, draw the current vector sate for the current. the error in the resulting current from the origin, e, and label the The navigation receiver can estimate. However, if too long a set and drift. The head of the also display the SMG or veloci- time period is used, the calculated current vector, located in this ty along route (VAR) from actual current is less proximate and, example at the point (130 which an estimated time of therefore, less relevant. As a practi- degrees, 2.0 knots), is designat- arrival can be calculated. cal matter, a time period of at least ed with the letter “g” in Figure Navigating in this way, there is 30 minutes, and as much as 1 hour 7-8. As in the earlier M board no need for explicit current cal- or more, is often used for current example, a scale of 1:1 is conve- culations of the type presented determination. nient and is also noted. in this chapter. However, not all vessels are equipped with navi- J Third, set the drafting compass DETERMINATION OF gation receivers, any piece of to a length equal to the speed, S, equipment can fail (especially A COURSE TO STEER of the vessel using the distance on a boat), and “hand calcula- (CURRENT GIVEN) or auxiliary scales as appropri- tions” can serve as a useful A common problem in current ate. Place the metal point of the check on navigation receivers sailing is to determine the appropri- compass on the head of the cur- (see Chapter 9). ate course to steer (C) in order to rent vector (at g), and “swing”

7-12 Current Sailing 7

MANEUVERING BOARD

f

COMPASS ARC

TR 050 SOA 8.1

e SET 130 DRIFT .2 C 036 S 8.0

g

SCALE 1:1

L FIG. 7-8–Determination of Course to Make Intended Track with Current Given

an arc of length S (8.0 units in J Fourth, draw a solid line from degrees in this example, is the this example) until it intersects point g to point f. This line is the course that needs to be steered at the track line lightly drawn in course/speed vector for the ves- 8.0 knots to make good the the first step. Note and darken sel. Using a parallel rule or par- intended track. Label the course this intersection point, labeled aline plotter, measure the bear- (C 036) and speed (S 8.0) as on “f” in Figure 7-8. ing of this line. This angle, 036 a DR plot.

7-13 7 Current Sailing

J Fifth, darken and measure the of the set compared to the vessel’s

ST G A U O A C R

S D

length of the TR/SOA vector ef. course and the ratio of the drift to U USe OF

A U Y X R This length (approximately 8.1 the vessel’s speed. Current correc- ILIA eLeCtrONIC units in the example) is the esti- tion angles are largest when the cur- CaLCULatOrS mated SOA along the track. rent is abeam the intended track and Label the track (TR 050) and when the drift is large in relation to Graphical methods are devel- SOA (SOA 8.1) on the vector ef the vessel’s speed. This point is as shown. illustrated in Table 7-2, which oped in this chapter. These This completes the procedure. shows computer calculated values calculations can also be The answer is interpreted as fol- of the current correction angle in lows. To make good an intended degrees as a function of the relative made mathematically, using track of 050 degrees in a current bearing of the set in relation to the with drift 2.0 knots and set 130 track (the rows in the table) and the trigonometric formulas (see ratio of the drift to speed (the degrees, it is necessary to steer a Rogoff, 1979; Shufeldt and course of 036 degrees if the vessel’s columns). In this example, the rela- speed is 8.0 knots. The estimated tive bearing of the set is 80 degrees, Dunlap, 1991). Electronic SOA is approximately 8.1 knots. It and the ratio of drift to speed (2/8) is good practice to check the plausi- is 0.25. Reading along the row cor- calculators are convenient bility of the results before setting responding to the relative bearing for these calculations. off on the calculated course. In this (080) and down the column corre- example, the current set is to the sponding to the speed ratio (0.25), Additionally, several manu- right of the intended track. That is, the table entry, -14 degrees, is the current is tending to push the shown. Thus, to maintain a track of facturers make special-pur- vessel to the right of the intended 050 the vessel would need to steer pose calculators (typically track. It makes sense, therefore, that 050-014, or 036 degrees—exactly the course to compensate for this the value determined by use of the for aviators) that are current should be to the left of M board. designed for these calcula- track. And, indeed, 036 degrees (the course) is to the left of 050 degrees CURRENT COMPENSATION tions. Mariners using one of (the intended track). The difference WHEN THE SOA IS between the course steered and PRESPECIFIED these special-purpose calcu- track (036-050 = 014 degrees) is The technique described above lators made for aircraft pilots termed the current correction angle is applicable to determination of the (CCA). This result is plausible. course to steer to compensate for should remember that the Next, consider the SOA esti- known or estimated currents in the mate. Because the current set is for- conventional case where the ves- convention for wind direc- ward of the beam, albeit only slight- sel’s speed is specified. This is the tion differs by 180° from that ly, the SOA should be (slightly) usual situation in current sailing. greater than the vessel’s speed. In The speed is either estimated (in the used for current set. other words, the current in this case of sailing vessels) or “known” Therefore, it is necessary to example is (slightly) fair. This, too, as in the case of powerboats main- checks, as the calculated SOA (8.1 taining a specified engine revolu- add (or subtract) 180° to the knots) is slightly greater than S (8.0 tions per minute (RPM). In either knots). case, the vessel’s speed is assumed set in order to make these The current correction angle can to be fixed in advance rather than a calculations correctly. be determined mathematically and decision variable to be determined. is a function of the relative bearing It is also of interest to solve cur-

7-14 Current Sailing 7

reLatIVe ratIO OF CUrreNt DrIFt tO VeSSeL’S SpeeD thrOUGh the Water (StW) BearING OF Set (DeGreeS) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.60 0.70 0.80 0.90 1.00 00000000000000000 10 00-1 -1 -2 -2 -3 -3 -4 -4 -5 -6 -7 -8 -8 -10 20 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -12 -14 -16 -18 -20 30 0 -1 -3 -4 -6 -7 -9 -10 -12 -13 -14 -17 -20 -24 -27 -30 40 0 -2 -4 -6 -7 -9 -11 -13 -15 -17 -19 -23 -27 -31 -35 -40 50 0 -2 -4 -7 -9 -11 -13 -16 -18 -20 -23 -27 -32 -38 -44 -50 60 0 -2 -5 -7 -10 -13 -15 -18 -20 -23 -26 -31 -37 -44 -51 -60 70 0 -3 -5 -8 -11 -14 -19 -19 -22 -25 -28 -34 -41 -49 -58 -70 80 0 -3 -6 -8 -11 -14 -17 -20 -23 -26 -29 -36 -44 -52 -62 -80 90 0 -3 -6 -9 -12 -14 -17 -20 -24 -27 -30 -37 -44 -53 -64 -90 100 0 -3 -6 -8 -11 -14 -17 -20 -23 -26 -29 -36 -44 -52 -62 -80 110 0 -3 -5 -8 -11 -14 -16 -19 -22 -25 -28 -34 -41 -49 -58 -70 120 0 -2 -5 -7 -10 -13 -15 -18 -20 -23 -26 -31 -37 -44 -51 -60 130 0 -2 -4 -7 -9 -11 -13 -16 -18 -20 -23 -27 -32 -38 -44 -50 140 0 -2 -4 -6 -7 -9 -11 -13 -15 -17 -19 -23 -27 -31 -35 -40 150 0 -1 -3 -4 -6 -7 -9 -10 -12 -13 -14 -17 -20 -24 -27 -30 160 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -12 -14 -16 -18 -20 170 00-1 -1 -2 -2 -3 -3 -4 -4 -5 -6 -7 -8 -8 -10 180 00 0 0 0 0 0 0 0 0 0 00 0 00 190 00 1 1 2 2 3 3-4 45678810 200 01 2 3 4 5 6 7 8 910 12 14 16 18 20 210 01 3 4 6 7 910 12 13 14 17 20 24 27 30 220 02 4 6 7 911 13 15 17 19 23 27 31 35 40 230 02 4 7 911 13 16 18 20 23 27 32 38 44 50 240 02 5 710 13 15 18 20 23 26 31 37 44 51 60 250 03 5 811 14 19 19 22 25 28 34 41 49 58 70 260 03 6 811 14 17 20 23 26 29 36 44 52 62 80 270 03 6 912 14 17 20 24 27 30 37 44 53 64 90 280 03 6 811 14 17 20 23 26 29 36 44 52 62 80 290 03 5 811 14 16 19 22 25 28 34 41 49 58 70 300 02 5 710 13 15 18 20 23 26 31 37 44 51 60 310 02 4 7 911 13 16 18 20 23 27 32 38 44 50 320 02 4 6 7 911 13 15 17 19 23 27 31 35 40 330 01 3 4 6 7 910 12 13 14 17 20 24 27 30 340 01 2 3 4 5 6 7 8 910 12 14 16 18 20 350 00 1 1 2 2 3 3 4 4 5 67 8 810 L TABLE 7-2–Current Correction Angle as a Function of Relative Bearing of Set and Ratio of Drift to STW rent problems where the SOA, low tide, etc. For these problems, Solution of this problem is a rather than the vessel’s speed, is the vessel’s speed is assumed to be minor variant—a simplification, in specified. For example, you may a decision variable, rather than fact—of the technique discussed wish to arrive at a drawbridge at a fixed in advance. It is also assumed above. It, too, is solved on an M time of scheduled opening or arrive that the current is known (or able to board, using the sequence of three at an inlet at a time of slack water. be estimated) as are the intended steps explained below. As an exam- Other examples include attempts to track and SOA. The problem is to ple, assume that the estimated time of rendezvous at a specified place and determine the course to steer and departure (ETD) is 1015 and an esti- time, cross a shoal at or near high speed to maintain in order to make mated time of arrival (ETA) of 1045 tide, pass under a fixed bridge at good the intended track and SOA. at a location bearing 280 degrees true

7-15 7 Current Sailing

MANEUVERING BOARD

f TR 280 SOA 9.0 e C 292 DRIFT 3.0SET 150 S 11.2

g

SCALE 1:1

L FIG. 7-9–Determination of the Course and Speed to Achieve a Specified Track and SOA, with Known Current and 4.5 miles distant is desired. The degrees. The M board solution is knots) to the point f. (Unlike the estimated current sets 150 degrees at shown in Figure 7-9. case where the speed, rather 3.0 knots. What is the course to steer Here are the steps to follow to than the SOA is specified, this and speed to maintain? A TSD cal- solve this type of problem: vector can be drawn completely culation readily shows that the SOA J First, draw the TR/SOA vector in the first step.) A scale of 1:1 is must be 9.0 knots to attain the ETA, from the origin, e, out along the used here. so the problem givens are: SOA 9.0 intended track (280 degrees) a J Second, draw the current vector knots, set 150 degrees, drift 3.0 length equal to the SOA (9.0 from the origin, e, to point g, as knots, and intended track 280

7-16 Current Sailing 7

in the earlier case, and label the board. Pay particular attention to

ST G A U O A C R

S D WheN tO set (150 degrees) and drift (3.0 what is being plotted as well as to U

A U Y X R knots). the steps. For example, suppose you ILIA USe theSe J Third, draw the course/speed estimate a current and use this esti- teChNIQUeS vector from the head of the cur- mate to calculate a course to com- pensate so as to make good an The competent navigator rent vector (point g) to the head should be able to solve cur- of the TR/SOA vector (point f). intended track. Later, you determine rent sailing problems quick- Measure the length of this vec- a fix and wish to calculate the actu- tor (11.2 units); this is the al current. Remember to compare ly, accurately, and with con- required speed (11.2 knots) to the DR track with the CMG/SMG— fidence. To maintain this (or make good the SOA. Next, mea- neither the TR nor the SOA enter any other skill), it is neces- sure the bearing (using a parallel into this calculation. Practice alone sary to stay in practice. rule or paraline plotter), 292 prevents these blunders. Therefore, it is necessary (at degrees in this example. This is a minimum) to make these the course required to compen- VOYAGE PLANNING: calculations at least occa- sate for the current. PUTTING IT ALL TOGETHER sionally. The answer is interpreted as fol- As discussed above, current For many voyages, cur- lows. To make good an intended sailing techniques are useful for rents can be substantial, and track of 280 degrees and SOA of two purposes. First, these are used formal use of these tech- 9.0 knots in a current with set of for prevoyage planning to estimate niques is necessary. For 150 degrees and drift 3.0 knots, it is the requisite courses and SOAs example, in crossing the necessary to steer 292 degrees and based on forecasted currents (see maintain 11.2 knots. The answer is Chapter 8 for details), and second, Gulf Stream from Florida to plausible; the current is setting the in a more or less “real time” mode, the Bahamas, it is essential ship to the left of track, so the CCA where a comparison of the vessel’s to correct for currents (these should be to the right. Additionally, DR plot with positions enables cal- have a northward set). Even the current is foul, so the speed will culation of actual currents and nec- on voyages where the current have to exceed the intended SOA. essary course adjustments. correction angles are less, All current problems are subdi- Normally, current problems are say in navigating a tidal vided into one of four categories: (i) solved sequentially. For example, a river, current can have a sub- determination of the EP from course preliminary estimate of set and drift stantial effect on SMG and steered and speed maintained, (ii) is used to calculate a course and fuel requirements. As noted, determination of the actual current estimate an SOA. This course is electronic navigation systems steered until a subsequent fix is from knowledge of the course can do much of the work. steered and maintained compared to obtained. The fix is compared to the But, valuable as these sys- the CMG/SMG, (iii) determination DR position, and a revised estimate of the course to steer to compensate of current set and drift is made. This tems are, these do not substi- for known currents in order to make revised set and drift is compared to tute for the human brain. good an intended track given the known information (see Chapter 8) There are also voyages speed, and (iv) determination of the and adjusted (if necessary) based that do not require formal course to steer and speed to main- upon the vessel’s future intended use of these techniques. tain, given actual current, in order to track. The revised set and drift are Trips in local waters, areas make good an intended track and used to determine a new course and where currents are modest, attain a specified SOA. Review the SOA, and so forth in an iterative and areas with well-marked steps for solution of each of these manner. channels may not require problem types and practice plotting Prevoyage planning is an impor- use of these techniques. these on both a chart and an M tant activity in itself. From the point

7-17 7 Current Sailing

lems,” the aeronautical equivalent NORMALLY, CURRENT PROBLEMS ARE SOLVED SEQUENTIALLY of current problems. If you elect to use one of these, remember that wind directions are always given as FIX VESSEL’S the direction from which the wind POSITION is blowing, not towards which the wind is blowing. When entering the inputs to the calculator, it is nec- essary to add (or subtract) 180° to the current in order to use this as a ESTIMATE DETERMINE “wind.” Drawing even an approxi- SET AND DRIFT COURSE AND SOA mate vector diagram provides a rough “reality check” on plausibili- ty of the answers determined by calculator. DEALING WITH CURRENTS of view of the ACN course, it pro- which they would normally be cal- USING ELECTRONIC vides an opportunity to integrate the culated for planning purposes. NAVIGATION RECEIVERS various topics covered thus far, e.g., The first (handwritten) line Full function navigation charts, the ship’s compass, DR, describes one leg of a voyage from receivers (whether Loran-C or piloting, and current sailing. The Block Island to Wood’s Hole on the GPS) simplify dealing with currents essence of prevoyage planning is to 1210-Tr Chart. Follow through the when underway. These receivers select the appropriate route and tim- calculations and ensure that you are enable the navigator to set up a ing of the voyage (from inspection able to understand how these are series of course legs (a route) by of relevant charts and other refer- developed. defining the waypoints (points of ence materials), lay out the voyage specified latitude and longitude) legs (including the identification of USE OF ELECTRONIC that mark the ends of each of the suitable objects for fixes, turn- CALCULATORS legs. The receiver displays the points, etc.), precalculate courses, bearing and distance to the next Electronic calculators can be SOAs, fuel consumption, and other waypoint in the route sequence as used for many of the problems tasks. well as the vessel’s ground speed. There is no single “best” solved by graphical methods in this Navigation receivers also display approach to this planning process, chapter. Programmable calculators the cross-track error (XTE)—the but it is helpful to have a worksheet, or laptop computers with spread- amount by which the vessel is off or checklist, to structure the process. sheet programs can also be used. course and the direction to turn to One trip planning worksheet is The necessary equations are given put the vessel back on course. shown in Figure 7-10. It contains in many texts (e.g., Schufeldt and XTEs are typically shown in frac- checklist information and entries for Newcomer, 1980). A calculator or tions of a mile. Initially, the helms- such items as track, leg distance, computer provides rapid and accu- man steers a course to match the engine RPM, STW, current set and rate answers to current problems intended track—or the track +/- the drift, course to steer (true), the and is particularly handy when CCA if an estimate of current is TVMDC calculations, estimated evaluating several voyage plan available. If there is any apprecia- SOA, estimated time en route options (route and timing). ble “sideways” component to the (ETE), ETA, and fuel planning data Several manufacturers make current, the vessel will gradually (discussed in Chapter 11). These electronic calculators used by air- develop an XTE error. You must entries are arranged in the order in craft pilots for solving “wind prob- turn back towards the intended

7-18 Current Sailing 7 (GAL) EST. FUELEST. FUEL EST. 1030 ESTIMATED TIME START:______FOB (GAL): ______COMPASS SOA ETE ETA CONSUMPTION REMAINING   MAGNETIC     CHECK ITEMS     TIDE CHARTS:______CHARTS: PILOT ______TRACK DISTANCE SETTING STW CURRENT TRUE INTENDED LEG POWER ESTIMATED LDM PERDIDA 6-9-99 BLOCK ISLAND TO BLOCK WOOD’S ISLAND HOLE TO 1 “1A” WOR “A” 050 9.2 1250 5.3 130 2 028 15W 043 3E 040 5.2 1:46 1216 LEG FROM TO (TRUE) (N.M.) (RPM) (KNOTS) SET DRIFT COURSE VARIATION COURSE DEVIATION COURSE (KNOTS) HH:MM HH:MM (GAL) L FIG. 7-10 –Illustrative Voyage Planning Worksheet with First Leg of Voyage entered. REMARKS ON VOYAGE PLAN: VOYAGE REMARKS ON GENERAL INFORMATION: LEG PLAN: DETAILED DATE:______REQD:______CHARTS NAVIGATOR: ______TABLES: CURRENT ______COAST PILOTVESSEL: GEAR: ______NAV. ______TABLE:______DEVIATION LIGHT LIST: TIMEPIECE: ______TABLE: FUEL/RPM ______

7-19 7 Current Sailing

track to compensate for the effect of tities provided by the receiver. slower than the STW—ensure that the current. By trial and error, a The simple expedient of turning you will have adequate fuel course can readily be determined towards the intended track to cor- reserves to complete the voyage. that keeps the vessel close to the rect for cross-track error works well Otherwise, use of the navigation intended track. if the cross-track error is modest. receiver will ensure that you stay on These navigation receivers sim- However, if a large XTE is allowed track to the point of fuel exhaus- plify navigation and reduce under- to develop, it is necessary to verify tion! Unanticipated fair or foul cur- way workload. However, the navi- by reference to the nautical chart rents may also mean that you have gator should not rely exclusively on that turning back to the intended any one source of data. Waypoints track, or resetting the origin way- to adjust power settings in order to can be entered incorrectly, the point, will not place the vessel in arrive on schedule (e.g., to reach receiver may lose signal, and other dangerous waters. Additional drawbridges before scheduled errors can occur. Therefore, it is details are provided in Chapter 9. openings or to run inlets at slack prudent to check the accuracy of In dealing with a foul current— current). bearings, distances, and other quan- which results in a SMG that is

7-20 Current Sailing 7

SELECTED REFERENCES

For additional material on current sailing, please refer Schufeldt, H. H., Dunlop, G.D., and A. Bauer, (1991). to the same references as given at the end of Chapters 5 Piloting and Dead Reckoning, 3rd Edition, Naval and 6. Note that books published in Great Britain use a Institute Press, Annapolis, MD. different set of plotting symbols than are employed here. Schufeldt, H.H., and K. E. Newcomer, (1980). The Calculator Afloat: A Mariner’s Guide to the Coolen, E., (1987). Nicholls’s Concise Guide to Electronic Calculator, Naval Institute Press, Navigation, Volume 1, Brown, Son & Ferguson Annapolis, MD. Ltd, Glasgow, Scotland, pp 314 et seq. United States Power Squadrons, (1998). Plotting & Ministry of Defense, (Navy) BR45 (1) (1987). Labeling Standards, The United States Power Admiralty Manual of Navigation, Volume 1, Squadrons®, Raleigh, NC. D/DNW/ 102/3/14/2, Her Majesty’s Stationary Office (HMSO), London, UK, pp 204 et seq.

Rogoff, M., (1979). Calculator Navigation, W.W. Norton & Company, New York, NY.

7-21 7 Current Sailing

7-22 Chapter 8

TIDES AND TIDAL CURRENTS

“As they say on my own Cape Cod, a rising tide lifts all the boats.”

(President John F. Kennedy, address in the Assembly Hall at the Paulskirche, Frankfurt, Germany, June 25, 1963.)

“The tides are the heartbeat of the ocean, a pulse that can be felt all over the world.”

(Albert Defant, Ebb and Flow: The Tides of Earth, Air, and Water)

INTRODUCTION concepts, then again more carefully This chapter provides a discus- to study the details and work sion of two important phenomenas through the problems. to the mariner: tides and tidal cur- Estimating the height of tides is rents. Tides and tidal currents are of necessary in order to identify safe great interest to the oceanographer water, know how much anchor line as well, but the focus of this chapter to deploy when anchoring, and esti- is on items of interest to the mate the vertical clearance under mariner. Experience in teaching the bridges. Estimating tidal currents is Advanced Coastal Navigation equally important, but for different (ACN) course indicates that stu- reasons. Tidal current information dents typically find the material in is essential for voyage planning. It this chapter straightforward. The is used, for example, to select a exposition is clear and the calcula- time to enter inlets, rivers, or bays tions are not difficult. However, the to maximize safety and/or mini- computations are tedious and mize travel time, and to estimate numerical errors are easily made. the speed made good (SMG) and To minimize errors and simplify fuel consumption for voyages in computations, several worksheets areas affected by tidal currents. (containing directions) are includ- Prediction and estimation of ed. This said, some perseverance is tides or tidal currents is facilitated required. Read the chapter through by use of the Tide Tables, or Tidal once to get an overview of the key Current Tables. Formerly published

8-1 8 Tides and Tidal Currents

and sold to the general public annu- As noted, tides result from the ally by the National Ocean Service gravitational effects of the moon (NOS), these are now reproduced by and sun, and experience (and theo- commercial publishers based upon ry) indicates that tides are periodic. NOS data. The bulk of this chapter There are predictable variations in is devoted to a discussion of how to tidal heights, hour-by-hour, day-by- use and interpret these tables. day, season-by-season, and year- by-year. Tide and tidal current pre-

ST G A U O A C R

S D U What YOU dictions can be made years in A U Y X R ILIA WILL LearN IN advance—though periodic recali- thIS Chapter bration of the mathematical models based on actual observations enable J The relevance of tide and more accurate predictions to be tidal current information made. J How to read and interpret All tidal heights are measured Tide Tables from a vertical datum or reference J How to use Tide Tables plane. In the United States, and cer- to make tide height tain other parts of the world, this predictions reference plane is what is termed L J How to read and interpret mean lower low water (MLLW). FIG. 8-2–Typical Tide Curves for United Tidal Current Tables The term high water denotes the States West Coast Ports J How to use Tidal Current Tables to make current predictions maximum height reached by each J Uses of tidal current infor- rising tide, and low water the mini- mation in voyage planning mum height reached by each falling tide. The tidal range is the height J Calculation of the time of difference between consecutive sunrise and sunset high and low waters. Figure 8-1 and Figure 8-2 show TIDES AND the hourly and daily patterns of tide TIDAL PATTERNS heights (measured relative to At the outset, it is important to datum) for several locations on the distinguish between two related but East, Gulf, and West Coasts of the distinct terms. Tide refers to the ver- United States. This small sample is tical motion of the water, tidal cur- sufficient to indicate the most rent (called tidal stream by the important patterns (discussed British) to the horizontal motion of below) and some typical tidal the water. Both tides and tidal cur- heights and ranges. rents have the same origin and vary As can be seen from the patterns with the same factors, including the in Figure 8-1 and Figure 8-2, there gravitational effects of the moon are often two high and two low and sun, and runoff from rain or waters in a day, but other patterns snowfall. Although tides and tidal L occur (see below). Tides follow the currents are related, these are logi- FIG. 8-1–Typical Tide Curves for United moon more closely than the sun. cally distinct concepts. States East Coast Ports Although the sun is much larger, the

8-2 Tides and Tidal Currents 8

moon is very much closer to the perigee (closest to the earth). Buzzards Bay) where the tidal range earth, more than compensating for When the moon and the sun are is greater (4 to 5 feet). Mariners the mass difference in the gravita- in conjunction and “pulling togeth- using inland lakes and many rivers tion equation. Thus, the period evi- er” (i.e., at a new or full moon a phe- are not affected by tides. dent in Figures 8-1 and 8-2 tends to nomenon termed syzygy and pro- This brief sample of tidal data match that of the lunar day— nounced sizz-a-gee), larger tidal indicates that the practical impor- approximately 24 hours and 50 ranges, termed spring tides, occur. tance of tidal phenomena varies minutes—rather than the solar day. (The term spring has nothing to do from region-to-region and even Because the lunar day is slightly with the season.) Alternatively, when location-to-location within regions. longer than the solar day, the times the gravitational forces of the sun Mariners from some locations in for high and low tides occur later on and the moon are in quadrature (i.e., New Jersey, for example, may each succeeding calendar day. at right angles), tides with smaller worry little about tides, while tides There are three principal daily range, termed neap tides, occur. are a central fact of life for boaters patterns observed for tides: diurnal, Tidal range varies with location in Maine, Alaska, or other areas semidiurnal, and mixed. on the earth, as well as with the with large tidal range. J Diurnal: A diurnal tide has a positions of the sun and moon. period or cycle of approximate- Examination of the patterns shown PRACTICAL REASONS WHY ly one tidal (lunar) day—that is, in Figure 8-1 and Figure 8-2, shows TIDES ARE IMPORTANT a diurnal tide will have one high that there are substantial differences Before discussing how tidal and one low tide each day. in the tidal ranges among U.S. ports. heights are estimated, it is useful to At Anchorage, Alaska, for example, J Semidiurnal: A semidiurnal review briefly why knowledge of the tidal range is 30 feet or more, tides is important to the mariner. tide has a period of approxi- whereas at Key West or Pensacola, mately one-half of a tidal day Anyone who has run aground, had Florida, the tidal range is consider- to replace or straighten a bent prop (see, for example, the patterns ably smaller (only 2 or 3 feet). Other for Boston, Massachusetts, and or shaft, had their anchor drag, or locations with large tidal ranges trimmed the top few feet off the New York, New York, in Figure include the Bay of Fundy (adjoining 8-1); that is, there are typically mast of their sailboat by crossing the provinces of Nova Scotia and under a bridge with insufficient two times of high water and two New Brunswick in Canada) where times of low water in each day. clearance can skip this section— mean tidal ranges of 40 feet or more such experience provides more than J Mixed: A mixed pattern (see, for are observed, some locations on the ample motivation for careful study example, Seattle, Washington, or coast of Maine (20 feet or more), of this chapter! San Francisco, California, as and near the southern tip shown in Figure 8-2) is a type of of South America. tide with a large inequality in the Locations with relative- high and/or low water heights, ly small tidal ranges with two high waters and two include Barnegat Bay, low waters usually occurring New Jersey, (less than 1 each tidal day. foot), and portions of In addition to the daily patterns the Chesapeake Bay. evident in the illustrations in Figure The area covered by the 8-1 and Figure 8-2, there are more 1210-Tr chart includes subtle patterns with a longer period. some locations with Tides have a smaller range when the small ranges (e.g., Little

moon is in apogee (farthest from the Harbor near Woods MARINE INSURANCE COURTESY OF BOAT/US PHOTO earth, denoted with the symbol “A” Hole, with a range of 1.4 L in Figure 8-1 and Figure 8-2), and a feet) and others (e.g., Tide height calculations are not difficult, but care larger range when the moon is in several locations in must be taken to avoid numerical errors.

8-3 8 Tides and Tidal Currents

that you have a choice of alternate ble to deploy sufficient anchor line

ST G A U O A C R

S D

U hIStOrICaL routes to your final destination—a to ensure a safe scope at high tide.

A U Y X R ILIA FOOtNOte deep but indirect channel or a short- The only alternative is to maintain a er, more direct route that takes you constant anchor watch. Mariners in days gone by over the shallows. Knowledge of Anchoring at low tide risks were taught to enter an unfa- the height of the tide may enable deploying an insufficient scope if miliar harbor cautiously at you to take a shortcut—in perfect the tidal range is not considered. safety—that will save substantial But, anchoring at high tide also has “half tide and rising.” Half distance and (therefore) time. Plan risks if tidal heights are ignored. tide ensured that there would the voyage at a time when the tides This is because the radius of the are relatively high. swing circle (the distance from the be water under the vessel, Knowledge of the height of the anchor to the stern of the vessel) and a rising tide meant that tide is also very important when increases for a fixed anchor line anchoring or mooring. Suppose, for length as the tide height decreases. the vessel could be promptly example, that you anchor in 3 feet If this effect is not considered, you refloated—assuming a soft of water and the distance from the run the risk of being on intimate bottom—in the event of mis- bow to the waterline for your vessel terms with other vessels in a crowd- is 2 feet. Following recommended ed anchorage! calculation. This advice is procedures, you elect to deploy 40 Finally, knowledge of tidal still sound. Modern predic- feet of anchor line, figuring that this heights is important to determine will give you a “safe” scope of 8 to whether or not there is sufficient tion methods, however, are 1 (40/(3 + 2) = 8). But suppose it clearance between the vessel’s more accurate. was low tide when you anchored, mast(s) or outriggers and overhead and that the tidal range is 5 feet. At obstructions, such as bridges or In Chapter 3, it is noted that the high tide the water depth would be power lines. In some critical cases, charted depth of water is based on a 8 feet (5 + 3) and the effective it may be necessary to pass under a vertical datum of mean lower low scope would be reduced to only 4 to bridge only at low tide to ensure water (MLLW, the average of the 1 (40/(3 + 5 + 2) = 4), not nearly that sufficient clearance exists. lower of the low tides each day). enough. (Table 8-1 shows how the For these and other reasons it is The actual water depth varies with actual scope of anchor line at high important for the mariner to know the height of the tide, so in order to tide varies with the scope at low how to estimate tide heights. calculate this depth—and, thus, tide and the tidal range, assuming whether or not safe water exists—it that the distance from the bow to SOURCES OF is necessary to calculate tidal the waterline is 2 feet) The point of TIDAL INFORMATION heights. Knowledge of water depth these simple calculations is that you There are several sources of is obviously important to the safety should plan to put out enough line tidal information. Many newspa- of the voyage—after all, who wants to ensure a sufficient scope at high pers, for example, routinely publish to run aground or have to wait days tide. This means that you need to information on the time and height until high tides are sufficient to know the state of the tide at anchor- of tides at several locations. clear a harbor? Vessels imprisoned ing and the depth of water at high Television weather broadcasts, par- in harbors with shallow entrances tide. These calculations can be real- ticularly those oriented towards awaiting favorable tides to depart ly dramatic for areas such as mariners, often give tidal informa- were often said to be “neaped.” Anchorage (a misnomer, to be sure) tion, as do scheduled government Knowledge of water depth can or the Bay of Fundy, where (as dis- weather broadcasts. Several private also be important to the efficiency cussed above) the tidal range can be companies publish almanacs or of a voyage as well as its safety. It 30 or 40 feet! At some of these other guides that contain tidal infor- sometimes happens, for example, locations it may not even be possi- mation. Increasingly, computer

8-4 Tides and Tidal Currents 8

Water Depth Intended When Scope anchoring tidal range(Feet) (Ft.) 12345710

63 5.0 4.3 3.8 3.3 3.0 2.5 2.0 64 5.1 4.5 4.0 3.6 3.3 2.8 2.3 65 5.3 4.7 4.2 3.8 3.5 3.0 2.5 66 5.3 4.8 4.4 4.0 3.7 3.2 2.7 67 5.4 4.9 4.5 4.2 3.9 3.4 2.8 68 5.5 5.0 4.6 4.3 4.0 3.5 3.0 69 5.5 5.1 4.7 4.4 4.1 3.7 3.1 6 10 5.5 5.1 4.8 4.5 4.2 3.8 3.3 6 12 5.6 5.3 4.9 4.7 4.4 4.0 3.5

73 5.8 5.0 4.4 3.9 3.5 2.9 2.3 74 6.0 5.3 4.7 4.2 3.8 3.2 2.6 75 6.1 5.4 4.9 4.5 4.1 3.5 2.9 7 6 6.2 5.6 5.1 4.7 4.3 3.1 3.1 77 6.3 5.7 5.3 4.8 4.5 3.9 3.3 78 6.4 5.8 5.4 5.0 4.7 4.1 3.5 79 6.4 5.9 5.5 5.1 4.8 4.3 3.7 7 10 6.5 6.0 5.6 5.3 4.9 4.4 3.8 7 12 6.5 6.1 5.8 5.4 5.2 4.7 4.1

83 6.7 5.7 5.0 4.4 4.0 3.3 2.7 84 6.9 6.0 5.3 4.8 4.4 3.7 3.0 85 7.0 6.2 5.6 5.1 4.7 4.0 3.3 86 7.1 6.4 5.8 5.3 4.9 4.3 3.6 87 7.2 6.5 6.0 5.5 5.1 4.5 3.8 88 7.3 6.7 6.2 5.7 5.3 4.7 4.0 89 7.3 6.8 6.3 5.9 5.5 4.9 4.2 8 10 7.4 6.9 6.4 6.0 5.6 5.1 4.4 8 12 7.5 7.0 6.6 6.2 5.9 5.3 4.7 L TABLE 8-1–Actual Scope of Anchor Line at High Tide as a Function of Low Water Depth and Intended Scope for Various Values of Tidal Range, Assuming that Vessel is Anchored at Low Tide software is coming on the market formerly published annually by the Tables presented in this chapter are that automates the calculation of U.S. Department of Commerce, taken from the NIMA publication tidal heights. Tide height calcula- National Oceanic and Atmospheric based on NOS camera-ready mas- tions can be done in advance as part Administration (NOAA), National ters.) Tide Tables and directions for of the voyage planning process or Ocean Service (NOS). Now, the their use are available in electronic when underway—particularly if a National Imagery and Mapping form at the NOAA web site laptop computer is available. Agency (NIMA) publishes these for (http://co-ops.nos.noaa.gov/). The most authoritative source of internal government use only. NOS Tide Tables are published annu- tidal information (and, indeed, the still provides the same information ally in four volumes, as follows: source of the reference data repub- to commercial publishers. Some Europe and West Coast of Africa lished by other firms) is found in elect to reproduce these in the exact (including the Mediterranean Sea); the Tide Tables. These tables were NOS format. (Excerpts from Tide East Coast of North and South

8-5 8 Tides and Tidal Currents

America (including Greenland); are locations from which a However, Reed’s (1998) and West Coast of North and South shorter series of observations is Eldridge (1998) correct for daylight America (including the Hawaiian summarized by comparison savings time. Islands); Central and Western with simultaneous observations It occasionally happens (even in Pacific Ocean and Indian Ocean. at reference stations, as dis- areas with semidiurnal tides) that Together, these four volumes con- cussed below. there will only be two highs and a tain daily predictions for more than Tide prediction for reference low, or two lows and a high in a 200 reference stations or ports (dis- stations is made using Table 1 given day. This occurs because the cussed below), and differences and (“Daily Tide Predictions”) of the tides follow the slightly longer other constants necessary for tide Tide Tables, which presents daily lunar day. This can be seen in Table height calculation for about 6,500 tide predictions for high and low 8-2 for 28 February 1999, for exam- other ports/locations. Several other water. Table 8-2 of this text, for ple, when there are two high tides at countries (e.g., Great Britain, example, contains an excerpt from 0610 and 1831, and a single low Canada, Brazil, and Argentina) also Table 1 of the Tide Tables for the tide at 1234. publish accurate and useful tables, reference station Newport, Rhode Examination of the daily predic- but these are not discussed in this Island, over the months of January, tions for Newport shown in Table 8- chapter. February, and March 1999. This 2 indicates that, although the major- table contains the predicted time ity of the tidal heights are positive USE OF THE TIDE TABLES and height (in feet and centimeters) (i.e., above datum), low tides are The publication Tide Tables of the high and low tides for each occasionally beneath datum (a neg- includes seven separate sets of day in the year. Reference to Table ative entry). At these times, the tables. Of these, Tables 1, 2, and 3 8-2, for example, shows that on 1 actual water depths will be less than are required for estimation of tidal January, high tides are expected at the charted values. heights and are discussed in this 0642 and 1906 (tidal heights of 4.8 Tide predictions are now made chapter. The remaining four tables and 3.9 ft., respectively), and low by computers, using algorithms relate to estimation of the time for tides are expected at 0001 and 1255 based upon the predictable motions sunrise/sunset, moonrise/moonset, (tide heights of -0.8 and -0.7 ft., of the sun and moon, measurements and some units conversions (feet to respectively). The significance of a of past tide heights, and an meters and reduction of local mean negative number is that the estimat- allowance for seasonal variation time to standard time). Sunrise and ed tide height is below the vertical representing average freshet (heavy sunset calculations are covered at (charted) datum at these times. rains or snow) and drought (dry) the end of this chapter. All times given in the Tide conditions. Unusual conditions Locations for which tidal pre- Tables prepared by NOS are in stan- (higher or lower than average snow dictions can be made with the aid of dard time (in the 24-hour system). or rainfall, etc.), however, will the Tide Tables are subdivided into Therefore, during the period when cause the tides to be higher or lower two classes: reference stations and daylight savings time is in effect, it than predicted from the tables. subordinate stations (substations): is necessary to convert either to or Likewise, changes in winds or baro- J reference stations (called stan- from standard time. Mariners who metric conditions cause variations dard ports by the British) gener- use tide tables produced by other in sea level. Typically, an onshore ally include commercially sources should consult the notes wind or a low barometer will important ports and locations accompanying these tables to see if increase the heights of both the high for which a long series of tidal the same convention is used. and low waters compared to predic- observations are available. Tidal Unfortunately, time conventions are tions based on average values. predictions are generally more not uniform. For example, Likewise, offshore winds or a high accurate for reference stations. Lighthouse Press (1998) reprints barometer will decrease high and J Subordinate stations (called the NOS tables directly, so that low waters relative to predictions. secondary ports by the British) standard time is used throughout. (Consult Bowditch for equations

8-6 Tides and Tidal Currents 8

Newport, Rhode Island, 1999 Times and Heights of High and Low Waters

L TABLE 8-2–Excerpt from Table 1 (Daily Tide Predictions) of Tide Tables

8-7 8 Tides and Tidal Currents

that can be used to correct for these J place name: This entry con- the reference station to estimate the meteorological conditions.) There - tains a place name correspond- times of the corresponding high and fore, the tide height predictions ing to each location for which low water at the subordinate sta- should not be regarded as error- predictions are available. In this tion. For example, the time differ- free. It is a matter of judgment cou- example, Penikese Island is the ences shown for Penikese Island are pled with local knowledge how name given to substation num- -0 hrs 17 minutes for high water much of a safety margin to apply to ber 1103. and -0 hrs 16 minutes for low water. these predictions. NOAA’s Center J Latitude and longitude: For The significance of the minus signs for Operational Oceanographic accurate work, take the time and in these table entries is that both Products and Services (CO-OPS) trouble to plot this position on high and low water occur earlier at provides useful information on the the nautical chart, rather than Penikese Island, than at Newport. accuracy of tide height predictions simply rely on the place name. Had the signs been positive, the and answers to frequently asked Occasionally, these plots will times would have been later. It is questions (FAQs) at the web site put you on dry land, but the noted above, (Table 8-2) for exam- (http://co-ops.nos.noaa.gov/). exercise is instructive in any ple, that high water would occur at Estimating tide heights at refer- event. Newport on 1 January 1999 at ence stations is simply a matter of J time and height difference 0642, and later at 1906. The tide time differences for Penikese Island reading Table 1. In principle, an data: Constants used for adjust- applied to these times mean that identical table could be prepared for ing the times and heights of high high water will occur 17 minutes each subordinate station as well. and low tides at a reference sta- earlier, or at 0625, and 1849. In However, the Tide Tables would be tion to those for the subordinate almost impossibly bulky if daily pre- station. making this subtraction or addition, dictions were provided for all 6,500 To illustrate how these data are it is necessary to keep track of the subordinate stations. Therefore, it is used, please refer to substation date, as well as the time. For exam- necessary to devise alternative meth- 1103, Penikese Island, in Buzzards ple, the time difference for low tide ods to handle the large number of Bay in Table 8-3. The first two is -16 minutes. Therefore, the 0001 subordinate stations. numbers shown in the “differences” low tide at Newport 1 January 1999 columns are the time differences (in occurs at Penikese Island at 2345 SUBORDINATE STATIONS hours and minutes) for high water one day earlier, on 31 December and low water at this substation as 1998. The next set of differences Table 2 (“Tidal Differences and compared to a designated reference shown in Table 8-3 are height dif- Other Constants”) of the Tide station. A glance over the “differ- ferences at high and low water. Tables, excerpted in Table 8-3 of ences” columns indicates that all Height difference information in this text, shows what are termed substations between 1085 and 1193 the Tide Tables can be expressed in “tidal differences and other con- in numerical sequence shown on any of three ways: stants.” This table enables data this page are keyed to the same ref- J First, the table entry may be a from the reference stations to be erence station, Newport. (Reference number with either a plus or used, together with port-specific stations are selected in an attempt to minus sign in front. This is to be information, to estimate the heights match the tidal characteristics of the interpreted as the quantity to be of tides at subordinate stations. This substation. Often these are quite added or subtracted from the section explains the numerical pro- close to the substation, but occa- height of the corresponding tide cedures required. sionally these are far removed. A height at the reference station to The entries of this table include: number of substations in calculate the height at the subor- J Station or substation identifi- Antarctica, for example, are keyed dinate station. The significance cation number: For example, to Galveston, Texas.) These time of a table entry of -0.2 at high Penikese Island is substation differences are to be added to the water, for example, is that the number 1103. times (algebraic sign important) at height of the high tide at the

8-8 Tides and Tidal Currents 8

L TABLE 8-3–Excerpt from Table 2 (Tidal Differences and Other Constants) of Tide Tables

8-9 8 Tides and Tidal Currents

subordinate station is 0.2 feet Tide Tables, reproduced in Table 8- the height of the tide at any desired lower than the high tide at the 3 in this text. Two ranges are given: time. corresponding reference station. mean range and spring range. The The computations of tide J Second, the table entries may be mean range is the difference in heights are conveniently done using prefixed with an asterisk, as is height between mean high water a tide table worksheet, one version the case for Penikese Island. and mean low water. At Penikese of which is illustrated in Table 8-4. The asterisk is used to denote a Island, the mean range is 3.4 ft., as It provides space to write down the multiplicative (rather than addi- shown in Table 8-3. The spring relevant entries from the Tide tive) correction factor. For range is the average semidiurnal Tables and directions for making example, the factors given in range occurring semimonthly as the the required computations. You Table 8-3 for the high tide and result of the moon being new or full may wish to make copies of the the low tide are both 0.97 (this is (syzygy again). Numerically, it is blank forms for later use. a numerical coincidence) for equal to 4.2 ft. in this example. In Table 8-5 contains a worked-out Penikese Island. Thus, the general, the spring range is greater example of a tide height calculation height of the 0625 high tide at than the mean range where the type for 0900 on 1 January 1999, at Penikese is the height of the cor- of tide is either semidiurnal or Penikese Island. This example is responding tide at Newport, 4.8 mixed, and is of no practical signif- used to illustrate the calculations. ft., multiplied by 0.97, or icance where the type of tide is Follow through these calculations (rounded to one decimal place) diurnal. For locations where the to ensure familiarity with the proce- 4.7 ft. Likewise, the height of tide type is diurnal, the table gives dure. The worksheet given in Table the 1849 high tide at Penikese the diurnal range, which is the dif- 8-4 has three sections. The top sec- Island, is the height of the corre- ference in height between mean tion is used to record the relevant sponding tide at Newport, 3.9 higher high water and mean lower inputs to the problem, the middle ft., multiplied by 0.97, or low water. section to calculate the times and approximately 3.8 ft. Finally, Table 2 of the Tide heights of high and low tides at the Tables (reproduced in Table 8-3 of J Third, table entries may consist reference station (and, if necessary, this text) shows the mean tide of both an asterisk and an alge- the substation), and the bottom sec- level—1.8 ft. in the case of braic sign, such as (*0.4 + 1.5). tion to calculate the height of the Penikese Island. The mean tide In this case, the height of the tide at any time. level (also termed half tide level) is tide at the reference station is to Completion of the top and mid- a plane midway between mean low be multiplied by the number dle sections is simple (although water and mean high water, mea- denoted with an asterisk, and tedious) and is illustrated in Table sured from chart datum. To estimate then added to the next number. 8-5. One or two additional com- the level of mean high water For example, a reference height ments are appropriate, however. (MHW), add one-half of the mean of 5 ft. would then translate to First, take the time to copy correct- tidal range to the mean tide level. 0.4 (5.0) + 1.5 = 3.5 ft. ly the high water and low water Thus, for example, MHW at The reason for these different time differences and height differ- Penikese Island would be 1.8 + 0.5 formats in the tide height difference ences. A common error, made by (3.4) = 3.5 ft. column is that several relationships students and experienced mariners are considered for correlation of USE OF A TIDE WORKSHEET alike, is to apply the low water time reference stations and substations; difference, for example, to the high the best (most accurate) relation- The above discussion shows water time at the reference station. ship is chosen for inclusion in the how the Tide Tables can be used to Second, ensure that you use the cor- Tide Tables, but this “best predic- estimate the time and height of high rect formula for calculating tide tor” varies from station to station. and low water at either a reference heights at subordinate stations from Tide ranges are shown in the or subordinate station. These same reference stations. For example, an next two columns of Table 2 of the tables can also be used to estimate asterisk in the tide difference for

8-10 Tides and Tidal Currents 8

Substation:______Date: ______Look up these values from Table 2, “Tidal Differences and Other Constants.” This section can be omitted if the Ref. Station: ______Substation #: ______desired location can be found in Table 1, “Daily Tide Predictions”, of the Tide Tables. Height differences HW Time Diff: ______Diff of Hgt. At HW: ______denoted with an asterisk are to be multiplied rather than LW Time Diff:______Diff of Hgt. at LW: ______added to reference station height.

Calculations: Look up heights and times for reference stations in Table Ref. Station: ______Substation:______1, Daily Tide Predictions. Add or subtract time differ- ences for substations to Table 1 times for reference sta- Condition Time Height Condition Time Height tions. Calculate the height at the substation from the LW ______LW______height of the tide at the reference station plus or minus the height difference tabulated above, unless denoted HW ______HW ______with an asterisk — in which case, the tabulated factors should be multiplied by the heights of the corresponding LW ______LW______tides at the reference station. Keep in mind that the time differences may place the required reference tide on the HW ______HW ______day before or after the date in question for the substa- LW ______LW______tion. Remember, times given in tables are standard zone time, not daylight savings time. Subtract 1 hour from HW ______HW ______daylight savings time to calculate zone time.

Height of Tide at Any Time: Location: ______Time: ______Date:______Duration of Rise or Fall: ______Length of time between high and low tides that bracket desired time Time from Nearest Tide: ______Use the lesser of the times from the last tide, or time until the next tide Range of Tide: ______Difference in height between tides on either side of desired time: re that subtracting a negative number is logically equivalent to addition Height of Nearest Tide: ______Height of tide closest to desired time Tabled Correction: ______From Table 3 Height of Tide at Time: ______Add above correction if nearest tide is low water, subtract otherwise Charted Depth: ______Determined from chart Depth of Water at Time: ______Add tide height to charted depth to calculate depth at required time L TABLE 8-4–Tide Table Worksheet this station means that the tide lated in the middle section of the from the nearest high or low tide to heights are to be multiplied, rather worksheet. To simplify these calcu- estimate the height of the tide at any than added. lations, the Tide Tables contain time. Its use is illustrated in the fol- The bottom section of the work- Table 3 (“Heights of Tide at Any lowing steps: sheet is used to calculate the height Time”), reproduced in this text as J Step 1: Enter the desired time of tide at any time, from knowledge Table 8-6. It consists of a series of and date of tide height predic- of the daily highs and lows calcu- factors, to be added or subtracted tion in the bottom of the work-

8-11 8 Tides and Tidal Currents

Substation:______Penikese Date: ______1 January 1999 Look up these values from Table 2, “Tidal Differences and Other Constants.” This section can be omitted if the Ref. Station: ______Newport Substation #: ______1103 desired location can be found in Table 1, “Daily Tide Predictions”, of the Tide Tables. Height differences HW Time Diff:-0:17 ______Diff of Hgt. At HW: *0.97 ______denoted with an asterisk are to be multiplied rather than added to reference station height. LW Time Diff:______-0:16 Diff of Hgt. at LW: ______*0.97

Calculations: Look up heights and times for reference stations in Table 1, Daily Tide Predictions. Add or subtract time differ- Ref. Station: ______Newport Substation:______Penikese ences for substations to Table 1 times for reference sta- Condition Time Height Condition Time Height tions. Calculate the height at the substation from the height of the tide at the reference station plus or minus LW ______LW______0001 -0.8 2345 -0.8 the height difference tabulated above, unless denoted HW ______0642 4.8 HW ______0625 4.7 with an asterisk — in which case, the tabulated factors should be multiplied by the heights of the corresponding LW ______1255 -0.7 LW______1239 -0.7 tides at the reference station. Keep in mind that the time differences may place the required reference tide on the HW ______1906 3.9 HW ______1849 3.8 day before or after the date in question for the substa- tion. Remember, times given in tables are standard zone LW ______LW______time, not daylight savings time. Subtract 1 hour from HW ______HW ______daylight savings time to calculate zone time.

Height of Tide at Any Time: Location: ______Penikese Time: ______0900 Date:______1 January 1999 Duration of Rise or Fall: ______6:14 Length of time between high and low tides that bracket desired time Time from Nearest Tide: ______2:35 Use the lesser of the times from the last tide, or time until the next tide Range of Tide: ______5.4 Difference in height between tides on either side of desired time: re that subtracting a negative number is logically equivalent to addition Height of Nearest Tide: ______4.7 Height of tide closest to desired time Tabled Correction: ______1.9 From Table 3 Height of Tide at Time: ______2.8 Add above correction if nearest tide is low water, subtract otherwise Charted Depth: ______Determined from chart Depth of Water at Time: ______Add tide height to charted depth to calculate depth at required time L TABLE 8-5–Complete Tide Table Worksheet

sheet. This time is 0900, 1 sheet are expressed in daylight between the high water at 0625 January 1999. (In the event that savings time.) and the low water at 1239. The daylight savings time is in use J Step 2: Compute the duration of difference between these times on the date, ensure that either rise or fall. Referring to the sub- (remember that there are 60, not this time is converted to stan- ordinate station calculations for 100 minutes in an hour) is 6 dard time, or that the calcula- Penikese Island in Table 8-5, for hours and 14 minutes. tions in the middle of the work- example, note that 0900 falls J Step 3: Compute the time from

8-12 Tides and Tidal Currents 8

L TABLE 8-6–Exerpt from Table 3 of Tide Tables–Height of Tide at Anytime

8-13 8 Tides and Tidal Currents

the nearest tide. In this case, lower table and stop at the table as a large ship bearing down on there are two adjacent tides to be entry in the row that is closest to your vessel), and having to recon- compared: the high water, the range of the tide (5.4 ft. in struct calculations from these paper which occurs at 0625, and the this example, so 5.5 ft. is the scraps (which have now fallen off low water, which occurs at closest row). This entry, termed the chart table from the wake). 1239. The time in question, the correction to heights, is 1.9 Finally, tide height calculations 0900, is closer to the high water in this example, and is entered are useful to estimate the vertical at 0625. The time difference in the worksheet in Table 8-5. clearance under obstructions. The (0900 - 0625) is 2 hours 35 min- J Step 7: Compute the height of worksheet given in Table 8-7 struc- utes. (Remember that 0900 is the tide at the desired time as the tures the calculations. The direc- equivalent to 0860 for calcula- height of the nearest tide plus or

tion.) ST G A U O A C minus the table entry. If the R

S D

U COMMON

A U Y J X AR Step 4: Compute the range of nearest tide is high water (as in ILI errOrS IN the tide. The range is the differ- this example), the table entry is tIDe heIGht ence between the high water subtracted, and therefore, the (4.7 ft.), and the adjacent low height of the tide at 0900 is 4.7 - CaLCULatIONS water (-0.7 ft.), or 5.4 ft. in this 1.9 = 2.8 ft. If the nearest tide Listed below are several com- example. (Subtracting a nega- were low tide, the correction mon errors in tide height cal- tive quantity is equivalent to determined in step 6 would be culations. Check your work addition.) added, rather than subtracted. carefully to avoid these errors: J Step 5: Enter the height of the J Step 8: Compute water depth: If J Neglecting to convert from nearest tide in the worksheet. it is desired to estimate the depth daylight to standard times. Since the nearest tide is the high of water at a particular location, All tide data are given in water at 0625, the number, 4.7 the height of the tide calculated standard times in the gov- ft., corresponding to this tide in step 7 is added to the charted ernment Tide Tables. Check height is entered. depth to determine the depth of explanatory notes carefully in other publications. J Step 6: Read Table 3 of the Tide water at the desired time. J Tables (reproduced as Table 8-6 These above steps are required Not being alert to date of this text) to determine the fac- to estimate tide heights at any time changes when calculating tor to be added or subtracted and, also, water depths at any time. tides at subordinate stations. from the height of the nearest Although it takes some time to read J Applying the high water tide. Enter the top section of through and complete a calculation time difference to low water Table 8-3 at the row closest to for the first time, with practice or the converse. the duration of rise or fall just comes familiarity and speed. Be J Failing to understand that determined in step 2. (No inter- careful to avoid the common errors factors denoted with an polation is required.) In this discussed in the sidebar. These asterisk should be multiplied example, the nearest entry is 6 same errors occur in making tidal by rather than added or sub- hours and 20 minutes. Read current calculations. (Practice soon tracted from the reference across this row to find the num- convinces you of the desirability of station heights. ber closest to the time from the using a computer for the calcula- J Failing to select the correct nearest high water or low water, tions!) The worksheet helps to reference station for the which is 2 hours 35 minutes in ensure accuracy in the event that desired subordinate station. this example. The closest time manual calculations are made. J Making arithmetic errors. to 2:35 listed in this row is 2 There is nothing more irritating J (Less common) Failing to hours 32 minutes. Now, read than writing these entries on a piece use the tables for the correct down the column that has the of scratch paper, losing your place year. entry 2 hours 32 minutes to the (from some minor distraction such

8-14 Tides and Tidal Currents 8

preliminary Information: Vessel:______Date: ______Location: ______Navigator: ______Time: ______Object to be Cleared:______

Clearance Computations: Item Value Source/Remarks

1. Published Clearance: ______Read from applicable chart or other source. 2. Minimum Clearance: ______Masthead height. 3. Safety Margin: ______Judgment input (recommended as at least 3 ft.). 4. Required Clearance: ______Line 2 plus Line 3. 5. Height of Tide at Specified Time: ______From completed Tide Worksheet. 6. Mean Tide Level: ______From Table 2 (last column) of Tide Tables for appropriate station. 7. Mean Range: ______From Table 2 of Tide Tables for appropriate station. 8. Mean High Water: ______One-half of Line 7 plus Line 6. 9. Clearance Increment: ______Line 8 minus Line 5 (may be negative quantity). 10. Predicted Clearance: ______Line 1 plus Line 9 (take note of sign). 11. Sufficient Clearance: ______Is predicted clearance (Line 10) greater than required clearance (Line 4)? L TABLE 8-7–Vertical Clearance Worksheet tions for use are reproduced on the time consuming and it is all too Island on 1 January 1999. This out- worksheet and are clear enough not easy to make errors. Moreover, it is put includes predictions for each half to require an example. The only impractical to make hand calcula- hour throughout the day. The predic- judgmental input to the calculations tions for multiple times at multiple tion for 0900 (2.3 ft.) is in reasonable detailed in Table 8-7 is the required locations. Fortunately, there are agreement with the 2.8 ft. estimated safety margin. A safety margin is several computer packages avail- in Table 8-5. More to the point, gen- appropriate to allow for the inaccu- able that are easy to use and inter- eration of this output required only a racies in the tide height estimation pret. These programs vary consider- few mouse clicks. It took much longer to print this table than to per- and the possibility that waves ably in numerical complexity, accu- form the calculations. These pro- caused by wind or wakes will racy, and cost. For example, some are revised annually; others are grams make it easy to generate simi- reduce clearance. There are no “perpetual.” The perpetual pro- lar predictions for several destina- hard-and-fast rules for selection of grams are slightly less accurate (not tions and waypoints along the an appropriate margin, but many updated with the latest NOS data), planned route and use this informa- mariners would want at least a 3- but substantially less expensive. tion intelligently. foot margin. Some programs imbed tide height calculations in voyage planning TIDAL CURRENT COMPUTER PROGRAMS software; others are “stand alone.” Accompanying the periodic rise FOR TIDE PREDICTIONS Output from one such program and fall of the tide is a horizontal As noted above, these calcula- (Nobeltec/Nautical Software (1996)) flow of the water, known as the tidal tions are not difficult. But they are is shown in Figure 8-3 for Penikese current. The relevance of current

8-15 8 Tides and Tidal Currents ft. 0 2 4 6:25a 4.6 12:39p -0.7 6:49p 3.8

L FIG. 8-3–Illustrative Computer Output (Reproduced with Permission from Nobeltec/Nautical Software, Inc.)

8-16 Tides and Tidal Currents 8

information to voyage planning is go, using the methods dis- explored in some detail in Chapters cussed in Chapter 7 for esti- 7 and 11, and therefore, not repeat- mation. That is, current set ed here. and drift can be estimated The set (direction towards from a comparison of the DR which the current is flowing) and position with a fix. Third, cur- drift (speed) of a current can be rents can often be estimated or estimated in several ways. First, calculated from various pub- currents can often be observed lished aids, including the directly from the movement of the Tidal Current Tables or Tidal water around stationary objects, Current Diagrams, formerly such as bridge pilings, lobster traps, published by NOS and now or buoys. This phenomenon is com- available from commercial monly termed the “current wake.” sources. Finally, publications With practice, the mariner can learn such as the U.S. Coast Pilot to judge the speed of the current (USCP), discussed in Chapter from the size and other characteris- 10, provide useful data or tics of the current wake. Rules of information on currents. thumb developed from experience The focus of this discus- indicate that a 1-knot current causes sion is on the use of Tidal a definite ripple, a 3-knot current Current Tables, although causes swirls and eddies for several passing mention is made of yards, and a 5-knot current causes a the other sources. At the out- boiling wake to stretch out “down set, it is important to make current” for perhaps 50 yards. The one point quite clear. The time of L angle of lean of the harbor buoys is high and low water (discussed FIG. 8-4–Typical Current Curves for another indication of the direction above) calculated for tides is not, in Reference Stations and strength of the current, but here general, a reliable indication of the it is more difficult to provide useful times of turning of the current or rules of thumb. Some buoys are del- slack (slow) water. For some loca- follows a very similar procedure to icately balanced and lean heavily to tions (e.g., those on the outer coast), that employed for tide heights, so the side in light currents. Others the difference between the times of learning how to read and use the require more substantial currents high or low water and the beginning Tidal Current Tables is not such a before a definite visual lean is evi- of the ebb (flowing out of the river chore. dent. Take the time to study the or bay) or flood (flowing into the As with tides, tidal currents are buoys in your area and expand your river or bay) current may be small. periodic, and vary with both time store of local knowledge on your But, for other locations, such as and location. Figure 8-4, taken from next voyage. An alternative proce- narrow channels, landlocked har- the Tidal Current Tables, shows a dure is to hold the vessel stationary bors, and certain tidal rivers, the time plot of tidal current data for in the current, relative to, say, a time of slack may differ from the several locations in the United buoy or fixed structure, and drop a time of high or low water by as States. As with tides, there is sub- piece of wood over the side, timing much as a matter of hours. stantial variability in the strength how long it takes to move from the Therefore, do not use the tide calcu- (maximum drift) and temporal pat- bow to the stern. If the piece of lations as a surrogate for necessary tern of these currents. Average val- wood requires t seconds to travel L current calculations. That, so to ues for maximum currents at loca- feet, the current drift (in knots) is speak, is the bad news. The good tions listed in the Tidal Current approximately 0.59L/t. Second, news is that the estimation of tidal Tables vary from less than 1 knot to currents can be calculated as you currents from the published tables 5 knots or more. At some other

8-17 8 Tides and Tidal Currents

mariner risks run- tant to note the exact location (lati- ning into danger- tude and longitude) for which a pre- ous waters and/or diction is made in such publications becoming lost as the Tidal Current Tables. unless currents Second, a careful mariner can (with are considered due regard for shallows or other and/or frequent dangers to navigation) often select a fixes are taken in course that reduces the strength of tidal waters (see the foul currents encountered, or Chapters 5 and 6). increases the strength of the fair It is somewhat current. surprising, but PHOTO COURTESY OF UNITED STATES COAST GUARD COAST COURTESY OF UNITED PHOTO STATES L nonetheless true, TIDAL CURRENT TABLES that there is little Commercial vessels often plan trips to take advantage of These tables are based on pre- relation between favorable currents. dictions made by NOS in the the range of the United States. Many other countries locations, such as Seymour tide and the strength of the maxi- of the world also produce equiva- Narrows in British Columbia, mum current, if comparisons are lent tables. Two volumes formerly Canada, currents can be as strong as made among locations. Thus, for published by NOS are now pub- 14 knots. Although these maxi- example, the maximum currents are lished commercially; one titled the mum values may not seem large on relatively weak in areas off the Atlantic Coast of North America, initial reflection, particularly to coast of Maine, even though the and the other the Pacific Coast of power boaters, a drift as small as 1 tidal range is typically large in these North America and Asia. The Tidal or 2 knots can have profound areas. Conversely, the currents are effects on fuel economy (see relatively large in such areas as Current Tables are organized in a Chapter 11) and create challenges Nantucket Island and Nantucket very similar format to the Tide to seamanship. Anyone who has Sound (near the area covered by the Tables. For example, daily predic- transited a tough inlet with an 1210-Tr chart), even though the tions are available for reference sta- ebbing current matched against a tidal range is relatively small in tions, and currents at subordinate stiff onshore wind has a healthy these areas. However, for a fixed stations are calculated from pub- respect for the effects of even a location, the speed of the maximum lished tidal current differences. “modest” current and can appreci- current (ebb or flood) is correlated However, the number of reference ate the utility of tidal current pre- with the tidal range. Thus, for stations and subordinate stations are dictions for selecting the best time example, during periods of the far fewer than those given for the of transit. On long passages, small month when the tides at a station Tide Tables. Additionally, the loca- gains in efficiency by selecting the are largest (new or full moon, etc.), tions themselves may differ—even best routing, with due consideration the maximum currents are also if the station name is the same. This for the effects of currents, translate largest. The speed of the current is is another reason to plot the latitude into large cumulative gains. generally not constant across a and longitude of the station as part Commercial vessels often plan trips body of water, but varies with loca- of the planning process. Finally, the so as to take advantage of a favor- tion. It is generally greatest at mid- reference station for a particular able current. channel, and drops off near the substation may differ between the Uncompensated currents can set shore. But, in winding rivers, the Tide Tables and the Tidal Current the vessel off course (see Chapter currents are generally largest Tables. For example, there are actu- 7), as well as affect the SMG. towards the concave shore (along ally two locations near Penikese Currents degrade the accuracy of the “outside” curve). These state- Island (unusual, since fewer sta- the running fix unless these are con- ments have two important practical tions are given in the Tidal Current sidered in its construction. The implications. First, it is very impor- Tables) for which current predic-

8-18 Tides and Tidal Currents 8

Pollock Rip Channel, Massachusetts, 1999 F–Flood, Dir. 035° True E–Ebb, Dir. 225° True

L TABLE 8-8–Excerpt from Table 1 (Daily Current Predictions) of Tidal Current Tables

8-19 8 Tides and Tidal Currents Tidal Tidal Current Tables L 8-9 –Excerpt 2 of (Current TABLE Table Differences and Other Constants) of

8-20 Tides and Tidal Currents 8

tions are available, neither of which erence stations is made using Table next to the current velocity), 0954 corresponds in latitude or longitude 1 (“Daily Current Predictions”) of (when the current is ebbing, denot- to that used for Penikese Island in the Tidal Current Tables, which ed by the letter E, at a speed of 1.7 the Tide Tables. In this case, the dif- presents daily predictions of the knots), 1639 (2.2 knot flood), and ference in location is not great, but times of slack water, and of maxi- 2236 (1.6 knot ebb). it is at other locations (e.g., near mum current (flood and ebb). Table At some locations on the West San Francisco, California). 8-8 of this text, for example, con- Coast of the United States, the pat- Moreover, Penikese is referenced to tains an excerpt from Table 1 of the tern of floods, slacks, and ebbs is the daily predictions for Newport, Tidal Current Tables for the refer- more complex. Slacks, for example, Rhode Island, in the Tide Tables, ence station, Pollack Rip channel, do not always follow a flood or ebb. but to Pollack Rip Channel near the over the months of January, These special current patterns are entrance to Nantucket Sound in the February, and March of 1999. explained in the Tidal Current Tidal Current Tables, which is not At the top of this table, the direc- Tables. even shown on the 1210-Tr chart! tion of the flood (035 degrees true) The accuracy of these predic- and the ebb (225 degrees true) is tions is generally taken to be within USE OF THE TIDAL given for the reference station. Note one-half hour, although deviations CURRENT TABLES that the flood and ebb directions are of as much as one hour or more can not necessarily reciprocal (180 occur. Therefore, a prudent mariner The publication, Tidal Current degrees apart). The set of the current plans to arrive at a particular Tables, consists of five separate sets is given because the directions for entrance or strait at least one-half of tables. Of these, Tables 1, 2, and flood and ebb are not always obvi- hour before the time estimates 3 are required for estimation of tidal ous, and, moreover, the accurate given in the tables, to ensure pas- currents at any time. Table 4 of the direction is necessary to current sail- sage at a slack or favorable current. Tidal Current Tables is used for ing computations. Directions of estimation of the duration of slack flood and ebb at any subordinate sta- SUBORDINATE STATIONS water (particularly important for tion keyed to this reference station divers), and Table 5 (Atlantic Coast Table 2 (“Current Differences are not necessarily the same as those and Other Constants”) of the Tidal only) presents data for rotary tidal for the reference station and must be currents. These latter two tables are Current Tables, excerpted in Table read from a separate table. For each 8-9 of this text, provides the current discussed only briefly here, but are day, three columns of data are pro- relatively easy to use from the differences and other constraints vided: the times of slack water and necessary for predictions at subor- directions included in the Tidal the time and velocity of maximum Current Tables. dinate stations. The entries of this current. Refer, for example, to the table include: The Tidal Current Tables also table entry for 27 February 1999. J contain information on how cur- Slack water times are estimated to Station or substation identifi- rents are combined (vector addi- occur at 0036, 0656, 1258, and cation number: For example, tion) and provide Current 1939. As with the Tide Tables, stan- substation 2051 is assigned to Diagrams, which graphically por- dard times in the 24-hour system are the station located 0.8 miles tray currents in several locations. employed in those publications that northwest of Penikese Island. Only a brief discussion of these reproduce the NOS Tidal Current There is no correspondence tables and charts is included in this Tables exactly. Other conventions between the number assigned to text, because of size and scope con- are used in some cases. Read the a particular location in Tide straints, but clear directions are pro- directions that accompany the tables Tables and that assigned in the vided in the Tidal Current Tables. that you are using. Tidal Current Tables. (It almost goes without saying, that Times and strengths of maxi- J place name: As noted, the actu- this publication is very important mum currents on this day are 0408 al position should be plotted, and should be aboard any vessel.) (when the current is 1.8 knots and rather than relying on the pub- Tidal current prediction for ref- flooding, denoted by the letter F lished place name.

8-21 8 Tides and Tidal Currents

Substation:______Ref. Station: ______Date: ______Look up these values from Table 2, “Current Time Differences: ___ Speed Ratios: ______Directions: ______Differences and Other Constants.” This sec- Min. Bef. Flood: _____ Flood: ______Flood: ______tion can be omitted if the desired location Flood: ______Ebb:______Ebb: ______can be found in Table 1, “Daily Current Min. Bef. Ebb:______Predictions.” Pay careful attention to any Ebb: ______footnotes applicable to the station.

CALCULATIONS: Look up times and speeds for reference sta- Ref. Station: ______Substation:______tion in Table 1. Add or subtract time differ- Condition Time Speed Condition Time Speed ences for substations to Table 1 times for Slack ______Slack ______reference station (pay attention to date). Ebb ______Ebb ______Estimate the drift at the substation by multi- Slack ______Slack ______plying the appropriate speed ratio by the Flood ______Flood ______drift at the reference station. Remember, Slack ______Slack ______times given in these tables are standard time Ebb ______Ebb ______in the 24-hour system. Slack ______Slack ______Flood ______Flood ______Slack ______Slack ______Ebb ______Ebb ______

VELOCITY OF CURRENT AT ANY TIME: Location: ______Time:______Date: ______Interval Between Slack and Time difference between desired time Desired Time: ______and nearest slack. Interval Between Slack and Max Time difference between slack and Current: ______maximum current that bracket desired time. Drift of maximum current (ebb or Max Current: ______flood) closest to desired time. From Table 3—be careful to use correct Tabled Correction: ______table if more than 1. Calculated Velocity: ______Multiply correction by max current.

Direction: ______Take direction from top data block. L TABLE 8-10–Current Table Worksheet

J Location: The latitude and lon- ebb. For Penikese Island, station worksheet (reproduced in Table gitude of the station or substa- 2051, these time differences are 8-10) for tidal current computa- tion are given. As with the Tide keyed to Pollack Rip Channel tions. A worked out example is Tables, these coordinates are (look for this station in the Time given in Table 8-11. In tran- approximate, and occasionally Differences column) and are scribing information from this plot on dry land. numerically equal to - 1 hour, 37 table to the worksheet, pay par- J time differences: Four time minutes, - 0 hours 25 minutes, - ticular attention to ensure that differences are given, that corre- 0 hours 55 minutes, and -0 hours the correct numbers are entered. spond to minimum current (gen- 57 minutes, respectively. As As with the tidal data, these time erally slack water) before flood, with tidal predictions, it is con- differences are to be added flood, minimum before ebb, and venient to use a structured (algebraic sign important) to the

8-22 Tides and Tidal Currents 8

Substation:______Penikese Ref. Station: ______Pollack Date:______27 February 1999 Look up these values from Table 2, “Current Time Differences: ___ Speed Ratios: ______Directions: ______Differences and Other Constants.” This sec- Min. Bef. Flood: _____-1:37 Flood: ______0.6 Flood:______050 tion can be omitted if the desired location Flood: ______- :25 Ebb:______0.6 Ebb: ______254 can be found in Table 1, “Daily Current Min. Bef. Ebb:______- :55 Predictions.” Pay careful attention to any Ebb: ______- :57 footnotes applicable to the station.

CALCULATIONS: Look up times and speeds for reference sta- Ref. Station: ______Pollack Rip Substation:______Penikese tion in Table 1. Add or subtract time differ- Condition Time Speed Condition Time Speed ences for substations to Table 1 times for Slack ______Slack ______reference station (pay attention to date). Ebb ______Ebb ______Estimate the drift at the substation by multi- Slack ______0036 0 Slack ______2259 0 plying the appropriate speed ratio by the Flood ______0408 1.8 Flood ______0343 1.1 drift at the reference station. Remember, Slack ______0656 0 Slack ______0601 0 times given in these tables are standard time Ebb ______0954 1.7 Ebb ______0857 1.0 in the 24-hour system. Slack ______1258 0 Slack ______1 1 2 1 0 Flood ______1639 2.2 Flood ______1614 1.3 Slack ______1939 0 Slack ______1844 0 Ebb ______2236 1.6 Ebb ______2139 1.0

VELOCITY OF CURRENT AT ANY TIME: Location: ______Penikese Time:______0900 Date:______27 February 1999 Interval Between Slack and Time difference between desired time Desired Time: ______0221 and nearest slack. Interval Between Slack and Max Time difference between slack and Current: ______0224 maximum current that bracket desired time. Drift of maximum current (ebb or Max Current: ______1.0 flood) closest to desired time. From Table 3—be careful to use correct Tabled Correction: ______1.0 table if more than 1. Calculated Velocity: ______1.0 Multiply correction by max current.

Direction: ______254 Take direction from top data block. L TABLE 8-11–Current Table Worksheet

corresponding times at the refer- ous day at Penikese Island. To multiplied by the drift of the ence station. For example, the see this, note that 0036 on 27 corresponding current at the ref- first current at Pollack Rip February is equal to 2396 on 26 erence station. Numerically, the Channel on 27 February 1999 is February; 2396 - 0137 = 2259. speed ratios are 0.6 for the flood a slack before flood at 0036. The next current is a flood cur- and ebb current alike at station The time difference correspond- rent which occurs at Pollack Rip 2501, but they differ at many ing to this current at Penikese Channel at 0408—equivalent to other locations. These are Island is - 1 hour 37 minutes 0343 at Penikese Island, from entered into the top data block earlier. Therefore, the corre- the - 25 minute time difference. of the worksheet in Table 8-11. sponding current occurs at J Speed ratios for flood and J average speeds and directions (0036 - 0137) or 2259 the previ- ebb: These factors are to be of the various currents:

8-23 8 Tides and Tidal Currents

Directions are given with February 1999 is 1.0 knot (ebb) and Use of such programs is highly rec- respect to true north, and aver- has a set of 254°. As with similar ommended. age speeds are given to the near- tide height computations, it is nec- est 0.1 knot. These average essary to take care in the computa- DURATION OF SLACK speeds are of general interest, tions to avoid numerical errors. It is sometimes of interest to but are not used for individual estimate the length of time at which predictions. LATE BREAKING NEWS a current will be slack, or nearly so. Note that the table of current NOAA has recently completed a Divers, for example, are generally differences and other constants uses major statistical study of the accu- constrained to operate in circum- the phrases “Min. before Flood” racy of tidal current predictions. stances where the current is nearly and “Min. before Ebb,” rather than Results of this study are being eval- slack. As a second example, local “slack.” This is because in some uated as this edition of ACN goes to knowledge may indicate that cer- locations the current does not press. A major policy decision to tain inlets or bridges are best tran- diminish to a true slack water or revise these tables may be made in sited during periods of near slack. zero speed stage. Indeed, reference the near future. This decision could The time predictions for slack to Table 8-9 shows that the mini- result in the removal of 50 percent current, discussed above, attempt to mum current before flood at Rose or more of the tidal current subordi- give the exact instant of zero (or Island, northwest of (station #2241) nate stations now listed in the Tidal minimum) speed. However, there is is 0.1 knots, not zero. Current Tables. a period on each side of slack water when the current is quite weak. In THE TIDAL COMPUTER PROGRAMS principle, the estimated current at CURRENT WORKSHEET As with tide height computa- various times could be computed The top two blocks of the work- tions, software for tidal current using the above procedure. A plot of sheet can be completed, and show computations is commercially the estimated current versus time the times of the slack and maximum available. Figure 8-5 shows illustra- could then be used to estimate the currents at the subordinate station tive output from one program duration of interval within which as calculated above. To estimate the (Nobeltec/Nautical Software) for the estimated current is arbitrarily speed of the current at any time, it is this station on 27 February 1999. small. But, such computations necessary to use Table 3 (“Speed of The estimate for 0900 agrees exact- would be tedious, indeed, and sub- Current at Any Time”) from the ly with that computed from the ject to numerical error unless a Tidal Current Tables, reproduced Tidal Current Tables. computer program is used. here as Table 8-12. Note that two There are two major advantages Fortunately, the architects of the separate tables are given for this to using computer programs for Tidal Current Tables have devised a purpose, each referenced to differ- tidal current computations: simpler procedure. This involves ent stations. (The same is true for the use of Table 4 (“Duration of J the Pacific Coast stations.) Pay First, computers take the Slack”) of the Tidal Current Tables, careful attention to ensure that the drudgery out of these computa- which is reproduced as Table 8-13 correct table is selected. The entries tions. It is possible to make indi- of this text. Actually, two separate for this table are the factor to multi- vidual sheets of predictions tables are used, as is the case for ply the estimated maximum current (such as that illustrated in Table 3 of the Tidal Current Tables, at the substation to calculate the Figure 8-5) for numerous possi- each applicable to different stations. current at any time. Directions are ble ports and waypoints. This The duration of slack (in min- given in the worksheet, and not contributes greatly to the effi- utes) is correlated with the strength repeated here. ciency of voyage planning. of the maximum current. Thus, the Table 8-11 provides a worked J Second, computers greatly tables provide the duration (in min- out numerical example. The esti- reduce the possibility of numeri- utes) over which the estimated cur- mated current at 0900 on 27 cal error. rent will be less than a prespecified

8-24 Tides and Tidal Currents 8

L TABLE 8-12–Table 3 (Speed of Current at Any Time) from Tidal Current Tables, 1999

8-25 8 Tides and Tidal Currents E F kt. -1 0 1 2 3:42a 1.1 6:00a 8:57a -1.0 11:20a 4:14p 1.3 6:44p 9:39p -1.0 11:58p

L FIG. 8-5–Illustrative Output of Tidal Current Program (Reproduced with Permission from Nobeltec/Nautical Software, Inc.)

8-26 Tides and Tidal Currents 8

value of from 0.1 knots to 0.5 knots, 1.3 knots, closer to the entry 1.5 Locations where rotary currents in increments of 0.1 knots, for vari- knots in Table 8-13. The dura- prevail are designated by the phrase ous values of the drift of the maxi- tion of weak current from this “see Table 5” in the table of current mum current. table is 31 minutes, of which differences and other constants. To illustrate, suppose that it is of half, say 16 minutes, would Directions are provided in Table 5 interest to predict the approximate occur on either side of slack. of the Tidal Current Diagrams and duration of the slack at 1121 at Thus, the period after slack at are not reproduced here. Penikese Island (substation #2051) which the current would be 0.2 on 27 February 1999—an example knots or less is from 1121 to TIDAL CURRENT DIAGRAMS discussed above. Further, for pur- 1137. Tidal Current Diagrams are poses of this example, take “slack” J Combining these two results, published for several well-traveled to mean a current of less than 0.2 the estimated period at which areas of the United States in the knots. Note that the estimated max- the current is predicted to be less Tidal Current Tables. As with other imum ebb (at 0857) before this than 0.2 knots is from 1058 to tables, these diagrams are no longer slack is 1.0 knot (see the completed 1137. distributed to the general public by current table worksheet in Table 8- It is easier if the currents on NOS, but are available from some 11; notice the entries for the substa- either side of the slack are identical. commercial sources. Figure 8-6 tion Penikese), and the estimated (It is yet easier to use a computer illustrates one such diagram for flood after this slack (at 1614) is 1.3 program for calculation!) Vineyard and Nantucket sounds, knots. Because these maximum covering a part of the area on the currents are not identical, it is nec- ROTARY CURRENTS 1210-Tr chart. Other locations for essary to solve the problem in two (ATLANTIC COAST ONLY) which Tidal Current Diagrams are parts: The preceding discussion is published include the East River, J Consider, first, the time period applicable to what are termed New York, New York Harbor via after the ebb at 0857 but before reversing currents (termed rectilin- Ambrose Channel, the Delaware the slack at 1121. Reference to ear by the British). According to Bay and River, and the Chesapeake the directions indicates that NOS, a reversing current is defined Bay. These current diagrams repre- Table 8-13 A (top) is the correct as “a tidal current, which flows sent average conditions of the sur- table for this station. Enter this alternately in approximately oppo- face currents in the middle of the table at the column headed 0.2 site directions with a slack water at channel. knots. The correct row corre- each reversal of direction.” For the illustration in Figure 8- sponds to the maximum ebb However, some currents are better 6, easterly currents are designated current of 1.0 knots. The entry, described as rotary. A rotary current “flood,” and westerly currents 46 minutes, is the estimated is “a tidal current that flows contin- “ebb.” The small numbers in the duration of weak current near ually with the direction of flow diagram are the average speeds of the time of slack water. Half of changing through all points of the the current in knots and tenths at this period, 23 minutes, occurs compass during the tidal period.” various locations and times. All before slack. Thus, before the Rotary tidal currents are generally times, shown at the top and bottom slack current, the estimated found offshore where the direction axes of the diagram, are keyed to length of time that the current of flow is not (or is less) restricted slack waters at Pollock Rip will be beneath 0.2 knots is 23 by shoreline or other barriers. The Channel. These times can be esti- minutes, or from approximately rotary currents tend to rotate clock- mated from the daily predictions 1058 to 1121. wise in the Northern Hemisphere, given in the Tidal Current Tables. J Next, consider the period after and counterclockwise in the Speed lines are shown to the the slack but before the flood at Southern Hemisphere. The speed of right of the diagram and are used 1614. Here the maximum cur- a rotary current generally varies directly with the diagram. By trans- rent (row to enter the table) is throughout the tidal current cycle. ferring to the diagram the direction

8-27 8 Tides and Tidal Currents

L TABLE 8-13–Table 4 (Duration of Slack) from Tidal Current Tables of the speed line equal to the ves- waters. This diagram also depicts removed from the tables and placed sel’s speed (with a paraline plotter the most favorable time for depart- on a flat surface.) To determine the or parallel rules), the diagram will ing any place shown on the left set and drift of the current, use the show the general set and drift of the margin. (Experience suggests that it parallel rulers to transfer from the current that would be encountered is easier to use a parallel ruler or speed lines to the diagram at the by any vessel passing through these paraline plotter if this diagram is left, the direction of the speed line

8-28 Tides and Tidal Currents 8

L FIG. 8-6–Illustrative Tidal Current Diagram

8-29 8 Tides and Tidal Currents

L FIG. 8-7–World Time Zone Chart–Source: Bowditch

8-30 Tides and Tidal Currents 8

corresponding to the vessel’s speed Whether or not a “free ride” is pos- TIDAL CURRENT CHARTS through the water (STW), moving sible depends upon the speed of the The last source of tidal current the edge of the ruler to the point vessel (too fast, and it may outrun information discussed in this chap- where the horizontal line represent- the following current, too slow, and ter is the Tidal Current Charts. ing the place of departure intersects the current may reverse) and the These were published for locations the vertical line representing the tidal current patterns at the location in the United States by NOS for time of day. If the ruler’s edge lies in question. For example, it is not several areas, including Boston within the shaded portion of the possible to ensure a fair current Harbor, Narragansett Bay, diagram, a flood current will be throughout an uninterrupted voyage Narragansett Bay to Nantucket encountered; otherwise, an ebb cur- for southbound vessels transiting Sound, Block Island and East Long rent will be encountered. If the the length of the Delaware River. Island Sounds, New York Harbor, ruler’s edge lies along the bound- As noted above, these diagrams Delaware Bay and River, Upper ary, slack water will be found. represent average conditions. On Chesapeake Bay, Charleston To illustrate, suppose that sport- any day, the currents calculated Harbor, Tampa Bay, San Francisco fish Esplendido, cruising at 12 from the worksheet for Pollack Rip Bay, Puget Sound (north), and knots, enters Pollock Rip Channel Channel may be larger or smaller Puget Sound (south). Now these are at 0800 on 27 February 1999. than those given in the Tidal no longer published by NOS, Reference to the daily predictions Current Diagram. The predicted although several commercial firms (Table 8-8) indicates that the ebb current drift for the day in question publish smaller versions of these. begins at 0656 on this date and can be used to determine approxi- Figure 8-7 provides an example, flood begins at 1258. The time, mate correction factors to adjust the taken from the Narragansett Bay to 0800, therefore, will be about 1 averages in the Tidal Current Nantucket Sound charts. These hour after ebb begins. With parallel Diagrams to the circumstances pre- rulers, transfer to the diagram the charts (each a set of charts, really) vailing on the day when a predic- 12-knot speed line “westbound,” include a series of 12 copies of a tion is required—in which case, a placing the edge of the ruler on the chart of the area covered, superim- simple ratio can be used. point where the vertical line “1 hour posed with arrows and numbers to Differences in the vessel’s esti- after ebb begins at Pollack Rip indicate the typical set and drift of mated speed of advance (SOA) as a Channel” intersects with the hori- the current (at spring tides) for a zontal 47-mile line (the starting result of making appropriate use of specific hour in the tidal current point). It will be found that the ruler these diagrams and selecting favor- cycle for a major reference station. passes through the unshaded (ebb able times of departure can be sub- In Figure 8-7, for example, the current) portion of the diagram, the stantial. For example, ebb currents times are taken with respect to drift averaging approximately 1 of as much as 2.4 knots can be Pollock Rip Channel. Factors are knot. Esplendido will, therefore, encountered in the stretch from East also provided (in the text accompa- have a favorable ebb current aver- Chop to just west of Tarpaulin nying these charts) for adjusting the aging about 1 knot all the way to Cove. A 12-knot vessel, therefore, drift estimates from average spring Gay Head, a welcome consolation could make nearly 14.4 knots if the tidal currents to those on the day in after the night’s fishing activities. voyage were perfectly timed, ver- question. Therefore, for accurate The most important use of these sus only 9.6 knots if an inauspicious results, it is necessary to use these diagrams is to determine the most time were chosen, a difference of charts in conjunction with the Tidal advantageous time of departure to 50 percent in SOA (and fuel econo- Current Tables. (The Narragansett maximize fair currents over the my, as well). These differences Bay chart, keyed to Newport, intended voyage route. Incidentally, could be even more important in the Rhode Island, is unique in this it may not always be possible to case of slower sailing vessels. A 6- series as it is based upon times of ensure fair currents throughout the knot vessel could either average 8.4 tides, rather than tidal currents, and voyage, even if there is complete knots or 4.6 knots, a difference of is designed to be used with the Tide flexibility as to starting time. over 80 percent! Tables.)

8-31 8 Tides and Tidal Currents

These charts are most useful for for the current on each leg of the later, south to Woods Hole, than providing a synoptic view of the voyage. to take the shorter route east temporal and spatial distribution of J Second, it is useful to make through Vineyard Sound, currents—and eliminate an incredi- rough computations of the depending, of course, on the ble amount of drudgery in making expected currents at various voyage timing. Reference to the station-by-station computations of times of day. For this purpose, it Tidal Current Chart also indi- current set and drift. Many of the is not necessary to make hourly cates that currents are generally current patterns depicted on these computations, but rather to iden- stronger on the north side of charts are complex—at a given tify the times and strengths of Vineyard Sound when these cur- time, currents may be going in one slack water and maximum tidal rents are flowing to the south- direction in one area, and another current. These approximate cal- west. So if Vineyard Sound is just a few miles away. Refer, for culations are sufficiently accu- selected for the route at the time example, to the currents in rate for identifying the major depicted in the chart, steering a Buzzards Bay and Vineyard South timing options of the voyage. course closer to Martha’s shown in Figure 8-7. As noted For example, it may be apparent Vineyard will result in reduced below, these diagrams can be used that a delay of only a few hours foul currents on average. Local to advantage for determining the in the start of a voyage offers the knowledge gained from experi- best overall routing to follow, the advantage of more favorable ence, or a careful reading of best timing of a voyage, or even, if currents, or that one route option such publications as the USCP both the overall route and time are is superior to another in terms of (see Chapter 10), helps to estab- fixed, the specific route that is like- the likely currents at voyage lish the importance of currents ly to maximize fair currents or min- time. Remember that it is the at various locations and to iden- imize foul currents. Your only com- speed with respect to the earth tify potentially attractive voyage plaint after a careful examination of (SMG, SOA) over the course options. these charts is that one may not be that is important, not the speed The choice between a longer available for the waters where you through the water (STW). A distance route with fair currents and cruise! longer route (in distance) may a shorter distance route with foul

actually be shorter (in time) than currents is easy to evaluate. Let D1 STRATEGIES FOR USING another if the currents are right. be the distance (M) on the longer TIDAL CURRENT DATA In the area of the 1210-Tr chart, route, X1 the drift of the fair current

This section contains some brief a little time spent with the Tidal along this route, D2 the shorter dis- remarks about the use of tidal cur- Current Tables or a Tidal tance, X2 the drift of the foul cur- rent information in voyage planning. Current Chart (see Figure 8-7) rent, and S the vessel’s STW. J First, it is useful to annotate the available for the area will con- Given these definitions, it can be voyage-planning chart with the vince you of the proposition that shown that the longer distance route locations for which tidal current the currents are often flowing in will be the shorter time route pro- predictions are available. opposite directions in Vineyard vided that: Browse through the index found Sound, and Buzzards Bay. That D1/D2 < (1 +X1/S) / (1- X2/S). at the back of the tables to iden- is, the currents are generally tify the potentially relevant northeasterly in Buzzards Bay Table 8-14 facilitates these com- locations. In this way, you can when they are southwesterly in putations. To illustrate the use of identify the available stations Vineyard Sound and the con- this formula, approximately one that can be used for planning verse. Starting from a point hour after flood begins at Pollock each of the voyage legs. That is, southwest of Cuttyhunk Island Rip Channel the currents are south- for planning purposes, a differ- on a voyage to Woods Hole, it westerly in Vineyard Sound at ent reference station or substa- might be much faster to go approximately 1.5 knots and north- tion can be used as a surrogate through Buzzards Bay and, easterly in Buzzards Bay at

8-32 Tides and Tidal Currents 8

RATIO OF RATIO OF FAIR CURRENT TO VESSEL’S SPEED THROUGH THE WATER FOUL CUR RENT TO VESSEL’S STW 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.00 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38 0.02 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 1.18 1.20 1.22 1.24 1.27 1.29 1.31 1.33 1.35 1.37 1.39 1.41 0.04 1.04 1.06 1.08 1.10 1.13 1.15 1.17 1.19 1.21 1.23 1.25 1.27 1.29 1.31 1.33 1.35 1.38 1.40 1.42 1.44 0.06 1.06 1.09 1.11 1.13 1.15 1.17 1.19 1.21 1.23 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1.40 1.43 1.45 1.47 0.08 1.09 1.11 1.13 1.15 1.17 1.20 1.22 1.24 1.26 1.28 1.30 1.33 1.35 1.37 1.39 1.41 1.43 1.46 1.48 1.50 0.10 1.11 1.13 1.16 1.18 1.20 1.22 1.24 1.27 1.29 1.31 1.33 1.36 1.38 1.40 1.42 1.44 1.47 1.49 1.51 1.53 0.12 1.14 1.16 1.18 1.20 1.23 1.25 1.27 1.30 1.32 1.34 1.36 1.39 1.41 1.43 1.45 1.48 1.50 1.52 1.55 1.57 0.14 1.16 1.19 1.21 1.23 1.26 1.28 1.30 1.33 1.35 1.37 1.40 1.42 1.44 1.47 1.49 1.51 1.53 1.56 1.58 1.60 0.16 1.19 1.21 1.24 1.26 1.29 1.31 1.33 1.36 1.38 1.40 1.43 1.45 1.48 1.50 1.52 1.55 1.57 1.60 1.62 1.64 0.18 1.22 1.24 1.27 1.29 1.32 1.34 1.37 1.39 1.41 1.44 1.46 1.49 1.51 1.54 1.56 1.59 1.61 1.63 1.66 1.68 0.20 1.25 1.28 1.30 1.33 1.35 1.38 1.40 1.43 1.45 1.48 4.50 1.53 1.55 1.58 1.60 1.62 1.65 1.68 1.70 1.73 0.22 1.28 1.31 1.33 1.36 1.38 1.41 1.44 1.46 1.49 1.51 1.54 1.56 1.59 1.62 1.64 1.67 1.69 1.72 1.74 1.77 0.24 1.32 1.34 1.37 1.39 1.42 1.45 1.47 1.50 1.53 1.55 1.58 1.61 1.63 1.66 1.68 1.71 1.74 1.76 1.79 1.82 0.26 1.35 1.38 1.41 1.43 1.46 1.49 1.51 1.54 1.57 1.59 1.62 1.65 1.68 1.70 1.73 1.76 1.78 1.81 1.84 1.86 L TABLE 8-14–When to choose a longer route with fair currents compared to a shorter route with foul currents. Table entries are the ratio of the distances such that the voyage time is the same for both routes. If the actual ratio of distances is less than or equal to the tabulated value, the longer distance route is the shorter time route. between 0.5 and 0.6 knots. If sailing tively. Interpolating, the ratio are tedious and always subject to yacht Despacio can make 6.0 knots D1/D2 would be 1.47, as deter- numerical error. Just how many (STW), under what circumstances mined from the above equation. computations are made depend would a longer route through The phenomenon of currents upon whether or not a computer is Buzzards Bay to Woods Hole going in opposite directions or at available to automate the computa- require less time? Here, X1 = 0.6 different speeds in contiguous tions. Use of the Tidal Current

(approximately), X2 = 1.5, and S = waters is not unique to Buzzards Charts or Tidal Current Diagrams

6.0, and, therefore, X1/S = 0.10, Bay/Vineyard Sound. For example, reduces the computational drudgery

X2/S = 0.25. Substitution of these the currents on opposite sides of if no computer is at hand.) In haz- values into the above equation Whidbey Island in Puget Sound ard-strewn waters and in conditions shows that the longer distance route (North) or Vashon Island in Puget of poor visibility when fixes are not is the shorter time route provided Sound (South) exhibit the same pat- likely to be frequent, hourly com- that D1 < 1.47 D2. In this example, terns. putation of currents, estimated posi- distance D1 could be nearly 50 per- The computations (referred to tions, and courses to steer (see cent longer than D2 and still be the above) identify and narrow down Chapter 7) might be in order. shorter time route. Referring to the precise times (more accurately, Alternatively, in open water or if Table 8-14, read down the column time intervals or “windows”) for all-weather navigation aids, such as corresponding to X1/S = 0.1 to the which more exact current predic- radar or Global Positioning System row where X2/S is approximately tions are required or useful. In this (GPS), are aboard, the use of aver- 0.25. Rows are shown in Table 8- more detailed planning phase, you age currents over a period of sever-

14 for X2/S equal to 0.24 and may need or want to make hourly al hours might be appropriate. 0.26—the corresponding ratios of estimates of current. (As a practical The navigator should consider

D1 to D2 are 1.45 and 1.49, respec- matter, these detailed computations the purchase of computer programs

8-33 8 Tides and Tidal Currents

L TABLE 8-15–Sunrise, Sunset and Other Information for New Bedford, Massachusetts on 6 June 2001, as Downloaded from the USNO Web Site to automate the calculation of tide predictions should be checked fre- attempt to start their engines at the heights or tidal currents. Software quently against the vessel’s actual end of the day. that is both powerful and easy to progress for voyage decision mak- There are numerous sources that use is now on the market. Use of a ing. provide the times of sunrise and computer program speeds up the sunset. Newspapers often provide planning process greatly, and SUNRISE/SUNSET this information in their daily means that many more voyage It is sometimes of interest to weather page(s), as do television options can be considered and eval- know the times of sunrise and sun- stations. Some web sites uated. Computers help ensure accu- set. For example, members of the provide shareware or other racy as well. Coast Guard Auxiliary conduct sun- calculators for this purpose. For Remember that tidal current set patrols designed to identify pos- example, the U.S. Naval predictions are not error free. These sible distress cases as boaters Observatory (USNO) web site

8-34 Tides and Tidal Currents 8

Times of sunrise and sunset can also be calculated (albeit, from a more laborious process) from infor- mation provided in Tables 4 (“Local Mean Times of Sunrise and Sunset”) and 5 (“Reduction of Local Mean Time to Standard Time”) supplied in the Tide Tables. Table 8-16 (in this text) provides an excerpt from Table 4, and Table 8- 17 (in this text) reproduces Table 5 of the Tide Tables. As shown in Table 8-16, the Tide Tables provide the local mean times of sunrise and sunset in 5-day increments for various latitudes, North or South. For most purposes, it is not necessary to interpolate either dates or latitudes. Rather, enter these tables to the nearest date or latitude. However, the times given in the Tide Tables are local mean times, not standard times, and it is necessary to convert from one to the other. Local mean times are referenced to a particular meridian of longi- tude. At any instant, the local mean times differ for each distinct merid- ian of longitude. However, it would L be inconvenient to have to reset TABLE 8-16–Excerpt from Table 4 (“Local Mean Times of Sunrise and Sunset”) of watches whenever moving from Tide Tables one meridian of longitude to anoth- er. To simplify matters, 24 stan- (http://aa.usno.navy.mil/AA/data) ly. This calculator keeps track of dard time zones have been estab- offers an easy-to-use calculator daylight savings time as well. lished throughout the world, corre- that requires entering only the Other web sites that offer sunrise or sponding to the length of the solar desired city, state, and date. sunset calculations include day. As a historical footnote, it was This calculator provides the (http://www.suncreations.com/sun. the introduction of railroads that times of sunrise, sunset, moonrise html/) and that maintained by created a need for common time and moonset among other informa- Australia’s National Mapping units—prior to the 1840s, many tion. Table 8-15 provides Agency(http://www.auslig.gov.au/g communities kept “local time”— illustrative output from the USNO eodesy/astro/sunrise. htm). which was inconvenient for creat- calculator for 6 June 2001 at Many computer programs for ing train schedules. Greenwich New Bedford, Massachusetts. The estimation of tides or tidal currents Mean Time (GMT)—one of the 24 times for sunrise and sunset are also calculate the times of sunrise time zones—was adopted in the 5:10 a.m. and 8:18 p.m., respective- and sunset as an added feature. United States at noon on 18

8-35 8 Tides and Tidal Currents

L TABLE 8-17–Reproduction of Table 5 (“Reduction of Local Mean Time to Standard Time”)

8-36 Tides and Tidal Currents 8

L TABLE 8-18–Worksheet for Estimating the Time of Sunrise or Sunset

8-37 8 Tides and Tidal Currents

06/09/01 New Bedford, MA

41° 38' N 70° 55' W

06/10/01 42° N

0424

75° 00' W 70° 55' W 4° 05'

0016

0408

0508

L TABLE 8-19–Numerical Example of Use of Time Worksheet

8-38 Tides and Tidal Currents 8

November 1883 when the telegraph As a second example, “R” (Romeo, find the local mean time of sunrise lines transmitted time signals to all or Eastern Standard Time [EST]) or sunset for the closest date and major cities. Prior to that, there applies between 67.5° West and latitude (42° N on 10 June), 0424. were over 300 local times in the 82.5° West and is the standard time Write down the relevant informa- United States! zone for the Eastern United States. tion (date, location, latitude, nearest It may seem surprising today, The numbers on the bottom of date, nearest latitude, and time of but the introduction of standard Figure 8-7 show the number of sunrise/sunset at nearest date and time in the United States was very hours that each time zone differs latitude) on the worksheet for esti- controversial, as related in an inter- from GMT. Consider the Romeo mating the time of sunrise or sunset esting book by O’Malley, (1990). time zone, for example, the number (Table 8-19). O’Malley relates an apocryphal given “+5” is the number of hours Referring to station 1133 in story told by a union leader about to be added to Romeo time to cal- Table 2 of the Tide Tables indicates an Irishman who asked a train con- culate GMT. Thus, if the Romeo that the longitude of this station is ductor at what time the train depart- time is 1400, the GMT is 1900. 70° 55’ West and that the standard ed. When told by the conductor “8 Time zones on land do not time meridian for this location is o’clock, standard time”, the match these zones exactly but, 75° West (the Romeo time zone). Irishman complained about use of rather, tend to follow political The longitude of New Bedford “standard time” (a reference in his boundaries between nations or sub- (70° 55’ W) is farther east than the mind to Standard Oil Co.) and divisions of nations. Land time longitude of the standard time noted “They’ll be gittin’ the wind zones are also depicted in Figure 8- meridian (75° 00’ W). Therefore, next, they’ve got the time now.” 7. For most countries, these are in local time at New Bedford is later GMT was adopted universally even hours, but there are several than that of the standard meridian. in 1884 when the International exceptions. The adjustment for the difference in Meridian Conference met in In order to convert local mean longitudes (75° 00’ – 70° 55’ = 4° Washington, DC. After this confer- time (the time given in the Tide 05’) must correct by subtracting the ence, the International Date Line Tables) to standard time, it is neces- time adjustment. How much time was established and 24 time zones sary to adjust for the time (hence, should be subtracted? Table 5 created. For additional details, see longitude) difference between the (“Reduction of Local Mean Time to the Greenwich Observatory’s web local meridian (i.e., the meridian Standard Time”), which is repro- site (http://greenwichmeantime. corresponding to the location duced in Table 8-17 provides this com/info/gmt.htm) or that main- desired) and the appropriate stan- answer. Enter this table with the tained by NASA (http://liftoff.msfc. dard time meridian. Standard time difference of longitude between nasa.gov/Academy/Rocket_Sci/clo meridians for each of the stations local and standard meridian, 4° 05’ cks/time-gmt.html/). Coordinated listed in the Tide Tables can be in this example, to find the time Universal Time (UTC) replaced found in Table 2 of these tables. correction in minutes or hours. The GMT as the World standard in For example, consider New time adjustment read from Table 8- 1986. It is based on atomic mea- Bedford, Massachusetts. Here is 17 is 16 minutes. Standard time is surements rather than the earth’s how to calculate the estimated time 16 minutes earlier than local time. rotation. of sunrise on 9 June 2001. First, Completing the example, the stan- Figure 8-7 shows these 24 refer to the index to stations in the dard time of sunrise in New zones—one for each 360/24 = 15 back of the Tide Tables to determine Bedford is the local time of sunrise, degrees of longitude. Each is given the station (1133) corresponding to 0424, less 16 minutes, or 0408. All a separate alphabetical designator. New Bedford, Massachusetts. From times are given as standard times “Z” (Zulu or GMT) is the standard Table 2 of the Tide Tables, the lati- and must be converted to daylight time zone that applies (on the water, tude of this station is 41° 38’ N. times for the period when daylight at least) to the area of the globe Refer now to Table 4 of the Tide time is in effect. Therefore, sunrise between 7.5° East and 7.5° West. Tables (Table 8-16 of this text) to is estimated to occur one hour later,

8-39 8 Tides and Tidal Currents

at 0508. This estimate is quite close a worksheet suitable for reproduc- or sunset from the Tide Tables is not to that determined from the USNO tion. Each entry and computation is difficult. However, it is very much calculator. explained in Table 8-18. Table 8-19 more convenient to read the news- To facilitate use of the Tide shows this worksheet filled in for paper or access the web sites listed Tables for estimating times of sun- the example presented above. above. rise and sunset, Table 8-18 provides Estimation of the times of sunrise

8-40 Tides and Tidal Currents 8

SELECTED REFERENCES

Bowditch, N. (1995). American Practical Navigator, Maloney, E. S., (1985). Chapman Piloting, Seamanship An Epitome of Navigation, Pub. No. 9, Defense and Small Boat Handling, Hearst Marine Books, Mapping Agency Hydrographic/Topographic New York, NY. Center, Bethesda, MD. This publication is avail- able electronically from the National Imagery and Markell, J., (1984). Coastal Navigation for the Small Mapping Agency (NIMA) Marine Navigation Boat Sailor, TAB Books Inc., Blue Ridge Summit, Department web site (http://www.nima.mil/). PA.

Defant, A., (1958). Ebb and Flow, The Tides of Earth, Mays, J. (1987). Thorough and Versatile Tide Air, and Water, University of Michigan Press, Ann Prediction by Computer, Sea Technology, May Arbor, MI. 1987.

Eldridge Tide and Pilot Book, (1999). Marion Jewett Nobeltec/Nautical Software, (1996). Tides & Currents ® White, Robert Eldridge White, Jr., Linda Foster for Windows , Version 2.0 Users Guide, 3rd White, Publishers, Boston, MA. Edition, Nautical Software, Beaverton, OR.

Graves, F., (1981). Piloting, International Marine O’Malley, M. (1990). Keeping Watch, a History of Publishing Co., Camden, ME. American Time, Viking Penguin, New York, NY.

Hinz, E., (1986). The Complete Book of Anchoring and Reed’s Nautical Almanac, North American East Coast Mooring, Cornell Maritime Press, Centreville, MD. 1999, Thomas Reed Publications Inc., Boston, MA. 1998. Lighthouse Press, (1998). Tide Tables 1999 High and Low Water Predictions East Coast of North and Wylie, F. E., (1979). Tides and the Pull of the Moon, South America, Including Greenland, Lighthouse The Stephen Greene Press, Brattleboro, VT. Press, Marina Del Rey, CA.

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8-42 Chapter 9

RADIONAVIGATION

“The winds and waves are always on the side of the ablest navigators.”

–Edward Gibbon (1737-1794)

“None so blind as those that will not see.”

–Matthew Henry (1662-1714)

INTRODUCTION decreased in price to levels well t several points in this text, within reach of the average boater A“revolutionary developments” and have become very much more in marine navigation systems are sophisticated and user friendly. mentioned. Nowhere is evidence of Additionally, advances in com- this revolution more dramatic than puters have resulted in navigation in the area of radionavigation sys- systems that do many of the tasks tems. In years past, small boat skip- formerly done exclusively by the pers were lucky to own a crystal navigator. A modern navigation controlled marine radio, capable of receiver can do much more than transmitting on only four or five provide a continuous display of lat- channels, and a radio direction itude and longitude. It can calculate finding (RDF) set. Dead reckoning speed made good (SMG), course (DR) positions were typically made good (CMG), current set and checked using visual piloting tech- drift, cross-track error (XTE), niques for coastal voyages and course to steer to reach an intended celestial fixes for blue water trips. destination, time-to-go (TTG) to Both radar and loran were bulky, reach the destination, and can expensive, and limited, for the most sound an alarm to warn of arrival. part, to merchant vessels and naval Navigation receivers can share data ships. with radar, autopilot, and computer Now, advances in the state-of- charting systems for a variety of the-art of electronics and computers purposes that would amaze even a have enabled the manufacture of merchant marine skipper of 20 miniaturized and much less expen- years ago. sive components. Navigation This chapter provides an intro- receivers, for example, have duction to these systems.

9-1

PHOTO COURTESY OF FURUNO 9 Radionavigation

Constraints on length and scope It may require reading and reread- J Choose equipment that has limit the discussion to only the most ing to ensure that the concepts are easy-to-read (large numerals) important systems: Loran-C, the understood. Don’t give up. This is and intuitive displays. The need Global Positioning System (GPS), important material. Even if you to maintain a proper lookout and Differential GPS (DGPS), Wide don’t own any of this equipment at complete other necessary tasks Area Augmentation System present, you probably will in the means that you may have only a (WAAS), and radar. Readers inter- near future. few seconds to read and inter- ested in more details about these pret navigational displays. systems, and others not covered in J Make sure that it is easy to use ST G A U O A C R

S D this chapter, should consult the U What YOU A the keyboard to enter data, U Y X R ILIA extensive references provided at the WILL LearN IN change displays, or perform end of this chapter. thIS Chapter other functions. Keys that are Radionavigation sets made by J well separated and offer tactile different manufacturers share many Principles of operation of feedback help to reduce data features, but also differ consider- Loran-C, GPS, and errors. Remember that you may ably in other features and details of DGPS have to use the keyboard in var- operation. For this reason, it is J How to use a navigation ious circumstances (e.g., heavy impossible to write a single chapter seas). that explains how to use specific receiver for coastal pilot- J Reliability and ruggedness models. Even if size constraints on ing should be prized. Unfortunately, this chapter did not prevent a J Principles of operation of it is difficult to learn whether a model-by-model discussion, the radar particular model is reliable. rapid pace of innovation would There are several magazines that quickly render this text obsolete. J Use of radar for collision offer relatively objective product Rather, this chapter describes the avoidance and navigation reviews for radionavigation general principles of operation of equipment. Lack of reliability the radionavigation system and J How to plot radar targets during the product test (if common features of sets now in observed) should be an impor- general use. In short, this chapter is Equipment Selection—A tant consideration in the ratings. not an owner’s manual, but serves Brief Digression J as a complement or introduction to Consider the quality of the that document. Students often ask ACN owner’s manual as one of the Care has been taken in this instructors for specific recommen- most important product features. chapter to avoid excessively techni- dations on makes and models of Many manufacturers attach a cal descriptions. Most mariners, for radionavigation equipment. The high priority to novel technical example, are uninterested in the United States Coast Guard features yet fail to produce particular frequency of operation, Auxiliary’s (USCGAUX) general understandable manuals. Model- pulse length, beam width, etc., policy is to discuss desirable fea- specific instructional videotapes except insofar as this information is tures of equipment, but not to are produced and sold for many required to understand the use and endorse specific products. Here are products. If clear and easy to fol- limitations of the equipment. Again, a few general observations that low, the instructional videotape look to the references at the end of might prove useful: can substitute for a weak owner’s this chapter for these details. J Unless you use this equipment manual. Despite simplification, this is very frequently, simplicity and J There are many benefits to inter- one of the longest and most techni- ease of installation and use are connection of electronic equip- cal chapters in the Advanced likely to be much more impor- ment. Most GPS receivers, for Coastal Navigation (ACN) course. tant than advanced features. example, have the ability to pass

9-2 Radionavigation 9

CHAPTER ORGANIZATION The chapter presents informa- tion on Loran-C and GPS/DGPS/ WAAS, followed by material on radar. Because navigation receivers for Loran-C and GPS are very sim- ilar from a user perspective M • (although different internally), the principles of operation of Loran-C and GPS/DGPS/ WAAS are dis- cussed first. Following this discus- W • sion, the use of these systems for coastal navigation is explained. Finally, use of radar for navigation L and collision avoidance is covered. FIG. 9-1–Master and Secondary should fit conveniently in the space available on the bridge. LORAN-C Equipment should be laid out in Loran, an acronym for Long- information to a radar unit, so a logical way to facilitate use. Range Navigation, is a highly reli- that bearings and distances to Take care that equipment bulk able and accurate long-range navi- waypoints, speed, and distance and placement do not create gation system. It was an outgrowth can be displayed on the radar blind spots that impair your abil- of military research during the screen. Likewise, navigation ity to maintain a proper lookout. 1930s in England and, later, in the receivers can provide informa- J Although not a criterion for United States. The first practical tion to an electronic charting equipment selection, it is appro- system for civilian maritime navi- system, so that the intended priate to add another point here: gation (later termed Loran-A) was track and vessel’s actual posi- make sure that when you add, replaced by Loran-C during the tion can be displayed on a chart change, or remove electronics 1970s. The Loran-C system will facsimile. from your bridge, you swing ultimately be replaced by GPS. The Standard protocols (connec- ship and build a new compass U.S. Government has indicated that tions) have been devised to deviation table. Changes to adequate notice will be provided in enable various units to inter- e q u i p m e n t change information. In theory, and wiring these work. However, it is gen- may affect 13370.0 erally easier to interconnect units the deviation (of the same vintage) made by table. Also the same manufacturer. There - c o n s i d e r fore, it may be worthwhile to try making two 13370 to standardize on one manufac- d e v i a t i o n turer in purchasing an electronic tabels, one M • system—even if this means that w h i c h you make trade-offs with respect applies when to individual units. In any event, the electron- W • check with the manufacturers to ic equipment ensure that the components will is on and be fully compatible. a n o t h e r L when off. J The size and shape of units FIG. 9-2–Loran LOP

9-3 9 Radionavigation

Figure 9-1 (taken from the Loran-C User Handbook published by the USCG) illustrates two sta- tions from a hypothetical chain, the master (M) and the whiskey (W) secondary. The TD be tween arrival of the signals from the master and any of the secondary stations is typ- ically very small, and measured in millionths of a second, designated micro seconds, and abbreviated µsec. (Radio waves propagate at essentially the speed of light, 161,829 nautical miles per second. At this speed, it requires approxi- mately 6.179 µsec for the signal to PHOTO COURTESY OF RAYTHEON MARINE CO. COURTESY OF RAYTHEON PHOTO L travel one nautical mile.) Modern Integrated GPS/Loran-C Navigation Receiver. Note CDI at top of display. loran receivers can measure TDs to within 0.1 µsec. In the illustration in Figure 9-2 advance of closure of the Loran-C Guard (USCG)), generally separat- the measured TD is 13,370 µsec system. ed by several hundred miles. The (shown in the upper right hand cor- Loran-C coverage includes vir- onboard receiver can be “tuned” ner) and, therefore, the vessel lies tually the entire United States to a (placed in quotation marks because somewhere along the 13,370 LOP. distance offshore of approximately the tuning principle differs from This LOP is curved rather than a 1,000 nautical miles. There are, selection of a frequency) to receive straight line, because (mathemati- however, offshore areas without the transmissions from all stations cally) the locus of points located a adequate Loran-C coverage at dis- in the chain. constant difference in distance tances considerably less than 1,000 One station in the chain is between two points is a hyperbola. miles from the U.S. coast. designated the master (denoted by For this reason loran is sometimes Coverage Diagrams are published the symbol M), and the remaining called a hyperbolic system. showing the areas where Loran-C two to four stations are designated However, on larger scale charts the can be used effectively. Other coun- secondaries (denoted by the letters apparent curvature is very slight. tries also operate loran systems. W, X, Y, and Z). The The absolute accuracy (see below) master transmits a of Loran-C is between 0.1 and 0.25 signal (actually a nautical miles. Loran-C repeatable complex pulse), fol- X • 32200.0 accuracy is much greater. lowed at predeter- Loran-C Principle of mined intervals by a transmission from Operation I each of the secon- M • In simplified form, Loran-C daries. The onboard 32200 operates as follows. It consists of receiver measures two components, an onboard navi- the slight time dif- W • gation receiver and a chain of three ference (TD) to five land-based transmitting sta- required for these tions (maintained in the United separate signals to L States by the United States Coast reach the vessel. FIG. 9-3–Another Loran LOP

9-4 Radionavigation 9

of continuity.) Mapping Agency [NIMA]), and 13370.0 Unlike many other sources. For example, Figure X • 32200.0 other navigation 9-5 shows the location and cover- systems that age of the Northeast U.S. chain. For use discrete fre- this chain, the master is located in 13370 quencies for Seneca, NY, and the secondary sta- M different stations, tions are located in Caribou, ME • all loran chains (Whiskey), Nan tucket, MA (Xray), 32200 transmit on the and Carolina Beach, NC (oddly same frequency enough, Yankee). Two criteria, a W • in the low fre- minimum signal-to-noise (S/N) quency band, ratio and an accuracy specification, 100-kilohertz define the coverage area enclosed (kHz). At this by the dotted lines in Figure 9-5. L frequency, radio is not limited to The accuracy criterion used for this FIG. 9-4–Loran fix determined by TDs line of sight, and reception ranges coverage diagram requires that 95 of hundreds of miles are possible. percent of the fixes determined Now, suppose another similar To prevent interference between should be within 0.25 nautical mile. TD measurement is taken between stations, each chain employs a dif- The transmission sequence the arrival times for the signals ferent time sequence from the master and the Xray sec- of trans missions, ondary, as illustrated in Figure 9-3. termed the Group LORAN-C The measured TD, 32,200 µsec, is Repetition Interval displayed in the receiver, indicating (GRI). In the Loran-C 90° W 80° W 70° W 60° W that the vessel lies somewhere chains, these intervals along the 32,200-µsec LOP. The are between 40,000 µ intersection of these two LOPs and 99,990 sec. The chain designations are fixes the vessel’s position as shown 50° N four-digit numbers in Figure 9-4. In a similar manner, W the TDs for the other master-sec- derived from the repe- M tition interval by ondary combinations also deter- X mine LOPs and can be used to dividing by ten, e.g., Z 40° N determine a fix, as well. (Signal the group repetition strength, crossing angles, and other interval of the Y factors determine the best station Northeast Chain is 99,600 µsec, so this pair for position determination.) 30° N According to the chart labeling con- chain is designated ventions discussed in Chapters 5 GRI 9960. and 6, a Loran-C fix is symbolized The Loran-C with a circle enclosing a dot with chains, their designa- the word “LORAN” and the time tions, and coverage (four-digit, 24-hour time) written data can be found in next to the fix symbol. the Loran-C User Handbook, Publication Loran-C Principle of 117, of the Defense Operation II Mapping Agency (This section is more technical (DMA) (now the L and can be skipped without loss National Imagery and FIG. 9-5–GRI 9960, The Northeast U. S. Loran-C chain

9-5 9 Radionavigation

of the signal from the master factors, the onboard loran should and that from one of the sec- record this TD for the Whiskey sec- ondaries is illustrated as fol- ondary if located at the assumed lows. Suppose that your vessel position. is located at latitude 41° 20’ N Examination of the 1210-Tr and longitude 71° 20’ W, chart shows that the assumed posi- approximately seven miles tion is located almost exactly on the south of Newport Neck on the 14,420 µsec TD contour, shown in 1210-Tr chart, and that the light blue, so this approximate cal- receiver is set to receive the culation is remarkably accurate. 9960 chain. The master station Loran Overprinted Charts at Seneca, NY, is located approximately 258 miles from Many nautical charts (e.g., the this assumed position (great 1210-Tr) of coastal scale or smaller circle computation), so the are overprinted with the loran TD signal from the master will contours shown as faint gray, light L require approximately 6.18 µsec/M blue, or magenta lines with the FIG. 9-6–Use of LORAN Interpolator to x 258M = 1,594 µsec to reach the GRI, station, and µsec printed Plot Position onboard receiver. (Strictly speak- somewhere along the contour. ing, radio waves propagation differ- (Harbor charts usually do not come operates as follows. First, the mas- ences over land and seawater with loran overprinting.) The TD ter station transmits a signal (actu- should be accounted for, but these contours are typically printed at ally a series of pulses). Next, after are omitted in this illustration.) convenient intervals. On the 1210- receiving the master signal, one of The distance between the master Tr chart, for example, these are the secondary stations transmits. and Whiskey station in Caribou, printed at 5- or 10-µsec intervals. But, to allow time for the signal ME, is approximately 451 miles, Obtaining a position from the loran from the master to propagate equivalent to a time delay of 2,787 receiver entails reading the TD dis- through the coverage area, the sec- µsec (i.e., 6.18 µsec/M x 451M = play from the navigation receiver, ondary station delays its transmis- 2,787 µsec). The secondary station noting the TDs, and locating their sion by a prespecified length of transmits after an 11,000-µsec SCD position on the chart. Options dif- time termed the secondary coding (given in the Loran-C User fer, but most loran receivers auto- delay (SCD). The SCD for each sta- Handbook). The secondary station matically select the best GRI and tion in each chain can be read from in Caribou, ME, is approximately the best two TDs for use. Users a loran coverage diagram, pub- 360 nautical miles from the wishing to select a different GRI or lished in the Loran-C User assumed position, and, therefore, it master-secondary pair can override Handbook. For the Northeast U.S. will take an additional 6.18 µsec/M the automatic feature and select chain, the SCD is 11,000 µsec, for x 360 M = 2,225 µsec for the sec- these manually. The specific TDs example. In turn, after another cod- ondary signal to reach the assumed being displayed by the loran are ing delay, the third station in position. generally obvious from inspection sequence transmits, etc., until the The loran receiver measures the of the magnitude of the TDs. transmission cycle is completed. TD between the arrival of the mas- However, most sets have the capa- The process is initialized again with ter signal and the secondary signal, bility of displaying these directly, the transmission of the master. The or 2,225 + 11,000 + 2,787 - 1,594 = typically on a different page of the GRI is selected so as to avoid over- 14,418 µsec in this example. Aside electronic display. (Because naviga- lap with other chains using the from rounding errors and slight cor- tion receivers can provide so much same frequency. rections to allow for variations in data, the display would be impossi- Computation of the expected signal propagation speed as a result bly crowded if all were displayed time difference between the arrival of variations in terrain and other simultaneously. Therefore, the data

9-6 Radionavigation 9

are segmented into several possible However, in some cases survey

ST G A U O A C R

S D displays, each on one or more data are available, and more sophis- U a USeFUL

A U Y X R pages. A switch on the receiver ticated adjustments are possible. ILIA WOrLD WIDe enables any page to be selected.) For charts that have these addition- WeB aDDreSS Interpolation of TDs (i.e., posi- al adjustments included, the dis- tion location for TDs between those claimer is reworded: “the lines of USCG operates a very useful printed on the contour lines) can be position shown here have been website for users of electron- done by eye or, for more precision, adjusted based on survey data.” TD ic navigation systems. The by using the interpolator printed on contours are located with more the chart or available as a separate accuracy if adjusted based upon site address is http://www. plastic or cardboard overlay. Figure survey data, and so, generally navcen.uscg.gov. The site 9-6 shows the use of a Loran-C speaking, these charts are more provides a wealth of infor- plotter. In this example, it is desired accurate. to plot the vessel’s position as mea- It is noted above that harbor mation about navigation sys- sured by the Xray TD 25,744. charts are not overprinted with TD tems (including Loran-C, Contours are overprinted on the contours. One important reason GPS, and DGPS) and their chart for 25,740 and 25,750 (shown why this is so is that the corrections as 9960-X-25750), respectively. for signal propagation are more status. This site also contains The plotter is aligned so that both complex and variable for nearshore all the Notice Advisories to ends of the 10-unit scale are placed areas than for outlying areas. NAVSTAR Users (NANUs) on the neighboring TD contours. A Additionally, the requirements for pencil marks the point 4/10ths navigational accuracy when operat- –rather like the Notice to along this line. This procedure is ing in restricted waterways are Mariners but focused on the repeated at another location and the often greater than the 0.1 to 0.25 needs of GPS users. dots connected to produce a 25,744 nautical mile accuracy advertised contour. for loran. As noted above, Loran-C TD Unless the user has a loran with harbors. The advice offered here is contours are printed on many nauti- the capability to automatically con- to use loran (if possible), but as a cal charts. The accuracy of location vert from TDs to latitude and longi- secondary or supplemental naviga- of these contours is generally quite tude (most do) or has visited the tion tool. That is, navigate using high. All nautical charts are not harbor before and noted the TDs GPS or traditional methods (e.g., identically accurate, however. With corresponding to key features of DR plots, visual fixes if visibility older charts, the location of the TD navigational interest (e.g., buoys, permits, soundings, etc.) and sup- contours is determined by a slight channels, anchorages, etc.), the lack plement the available information variation of the simple calculation of loran TD overprinting prevents with GPS or Loran-C readings. illustrated above to account for dif- the use of loran for navigation with- Particularly, if the navigator has ferences in the speed of the propa- in harbors. Even if the onboard local knowledge and has calibrated gated signal over land versus water. receiver has the capability to dis- the loran by comparison of loran In such cases, a disclaimer is writ- play latitude and longitude directly, positions with “ground truth” ten on the chart: “the lines of posi- there are many experienced naviga- (knowledge of the actual positions), tion shown have been adjusted tors who would argue that this fea- loran can be a valuable supplemen- based on theoretically determined ture should not be used because of tal aid for harbor navigation. overland signal propagation delays. the possible inaccuracy in the TD to Inspection of the Loran-C TD They have not been verified by lat./lon. conversion in harbor areas. lines as printed on the nautical chart comparison with survey data.” This Although this cautious advice is provides two additional important is exactly the wording on the 1210- certainly conservative, it overlooks pieces of information to the naviga- Tr chart, for example. the possible benefits of loran use in tor. First, the navigator can see the

9-7 9 Radionavigation

USCG publishes a useful document, Specifications of the Transmitted Loran-C Signal, that contains charts depicting the possible error of posi- tion estimates arising from the geo- metric component of signals from Loran-C chains (actually, in terms of triads of master-secondary pairs). Inspection of these diagrams (together with the range limits of loran coverage) can enable the user to find the best station pairs for use. Most modern Loran-C receivers do this automatically. Automatic Coordinate PHOTO COURTESY OF NORTHSTAR TECHNOLOGIES COURTESY OF NORTHSTAR PHOTO L Conversion Sensor Units for GPS/DGPS (microseconds) between these Early Loran-C receivers dis- lines. For example, in the general played only TD information which crossing angles of the various loran area of Brenton Light on the 1210- required the user to make the con- TD lines. Recall from the discus- Tr chart, the Xray TD gradient is versions manually from TDs to lat- µ sion on the accuracy of fixes in approximately 0.82 M per 10 sec itude and longitude using the pub- µ Chapter 6 that crossing angles of 90 interval, or 0.082 M/ sec. lished loran chart. This procedure is degrees are optimal. The loran nav- Generally speaking, the smaller the not difficult, but time consuming igation receiver generally selects gradient, the better in terms of fix and decidedly inconvenient if sin- the TDs automatically. Criteria used accuracy. Suppose, for example, gle-handing. Indeed, a long section by the various manufacturers vary, that the onboard receiver could of the earlier edition of this text was µ but may or may not include cross- resolve TDs to 0.1 sec. With a devoted to a detailed exposition of µ ing angles. The alert navigator can gradient of 0.082 M/ sec, a 0.1 the tricks of TD interpolation. Now, determine the Loran-C LOPs in use µsec resolution would translate into however, all loran sets being manu- and alter these, if necessary, to a possible position error of 0.1µsec factured have automatic coordinate ensure that a good crossing angle x 0.082 M/µsec = 0.0082 nautical conversion routines built in, so all results. Referring to the 1210-Tr miles, or approximately 50 ft. In that is necessary is to enable this chart, for example, it can be seen this same area the gradients for the function and read out latitude and that in this area the crossing angles Yankee and Whiskey secondaries longitude directly. Some of the ear- for the magenta (Xray) and blue are 0.138 and 0.118 nautical miles liest sets had crude conversion rou- (Whiskey) LOPs are generally per microsecond, respectively. tines that did not take into account much less than 90 degrees. Were Considering both gradient and certain propagation corrections, these selected by the receiver, a crossing angle, the best TDs in this termed additional secondary phase manual override to select another small area would be Xray and factors (ASF). With these earlier TD pair would be in order. Yankee. (Gradient alone would sug- sets, use of TDs was clearly prefer- Second, it is useful to introduce gest Xray and Whiskey, but, as able for maximum accuracy. the concept of gradient. The gradi- noted, these TDs have a poor cross- However, all the new sets now have ent as used in this discussion is the ing angle.) highly accurate conversion routines ratio of the spacing between adja- The above calculations are sim- programmed in, so there are now cent Loran-C TDs, as measured in ple, but tedious if a large number of only small differences in accuracy nautical miles, and the TD computations are required. The obtainable by using TDs in place of

9-8 Radionavigation 9

latitude and longitude readouts. operated by the U.S Department of J User segment: This segment Even so, as of this writing, there is Defense (DOD). DOD developed consists of the receivers, proces- no industry standard for lat./lon. GPS originally to meet its needs for sors, and antennas that allow conversion routines and, for the worldwide coverage (not available users to receive GPS signals and most demanding applications, TDs with Loran-C). Initial research in compute position, velocity, and are to be preferred over lat./lon. the early 1970s was successful and time. readouts. full-scale development was com- GPS Principle of Operation Loran Accuracy pleted by 1984. In late 1992, the system was operable. Since then, GPS is based on satellite rang- The absolute accuracy of the additional satellites have been ing. NAVSTAR is the name of the Loran-C system ranges from 0.1 deployed and other enhancements satellite used in the GPS. Each GPS nautical mile to 0.25 nautical mile. have been made. GPS reached Full satellite transmits an accurate posi- Absolute accuracy is calculated by Operational Capability (FOC) on tion and time signal. The user’s comparing the loran’s estimate of July 17, 1995. navigation receiver measures the the vessel’s position with a known GPS permits land, sea, and air- time delay for the signal to reach position (ground truth). Thus, for borne users to determine their the receiver, which (using the example, if you were to go to the three-dimensional position, veloci- velocity of propagation) can be Buzzards Light on the 1210-Tr ty, and time 24 hours per day, in all converted to the apparent range chart and read the loran display, the weather, anywhere in the world, (pseudorange) from the satellite to latitude and longitude shown would with accuracy greater than any the receiver. The word pseudorange be within less than 0.1 to 0.25 nau- other system now available. is used because inaccuracies in the tical miles of the charted position. GPS consists of three segments: clock in the navigation receiver will Accuracy is a function of gradient, space, control, and user: result in corresponding error in the crossing angle, and other factors. J estimated range. If the receiver’s However, another accuracy con- Space segment: This segment clock is fast (slow), the apparent cept is also relevant here, repeat- includes 24 operational satel- time of travel from the satellite to able accuracy. In the above exam- lites in six circular orbits 20,200 the receiver—hence, apparent ple, if you were to note the apparent kilometers (km) (10,900 M) range—is too small (great). More TDs (or latitude and longitude) of above the earth with a 12-hour on this topic is given below. the Buzzards Light on the loran and period. The satellites are spaced Measurement of a range from either write these down or save in orbit so that at any time a one satellite enables the user’s posi- them in the loran’s memory and minimum of six satellites will tion to be specified somewhere on a later return to the same indicated be in view to users anywhere in sphere with radius equal to this position, the vessel would be much the world. Satellites continuous- range. Measurement of the range closer to Buzzards Light. The ly broadcast position and time from another satellite determines repeatable accuracy is of the order data to users throughout the another sphere of position. The of 75 feet or even less. Indeed, world. intersection of these two spheres is some mariners claim to have nearly J Control segment: This segment a circle. Measurement of the range collided with buoys and fixed aids consists of a master control sta- from a third satellite determines to navigation (ATONs) once the tion in Colorado Springs, with another sphere of position, which apparent coordinates were entered five monitor stations and three (absent measurement errors) will into the memory! ground station antennas located intersect with the circle determined throughout the world. The mon- from the first two measurements at GLOBAL itor stations track all GPS satel- only two points. Measurement of POSITIONING SYSTEM lites in view and collect range the range from a fourth satellite GPS is a satellite-based radio- information from the satellite reduces the possible positions to navigation system developed and broadcasts. only one. (Actually, because the

9-9 9 Radionavigation

error, all lines of posi-

ST G A U O A C R

S D

U USe COrreCt tion (LOPs) determined

A U Y X R ILIA Chart from TD measurements DatUM FOr will intersect at a com- GpS reCeIVerS mon point—the user’s GPS receivers must be set location in space. But, what if the receiver’s to the same chart datum as clock is in error? In used in the production of the this event, the LOPs nautical chart. Otherwise, will not intersect at a common point. substantial position errors However, because (see may result. As shipped, most above) all range mea- L TECHNOLOGIES COURTESY OF NORTHSTAR PHOTO receivers are set to the chart surements must be in error by the same Steering screen on modern GPS receiver shows datum commonly used in the amount, it is possible graphical display of vessel and provides relevant United States. However, for the receiver to use information on bearing, distance, and XTE. an iterative approach to users should check to ensure estimating the correct position. In J Standard positioning Service that the proper chart datum principle, this problem can be (SpS): This is intended for gen- is used. Charts for foreign solved by assuming a series of pos- eral public use. SPS signal accu- sible clock errors (either fast or racy was intentionally degraded waters use a variety of refer- slow). For each assumed clock to protect U.S. national security ence points. The horizontal error, a series of pseudoranges can interests. Selective Availability chart datum is listed on the be determined and checked to see if (SA) is the name given to the the resulting spheres of position process to control (degrade) sys- chart. Consult the owner’s meet at a common point. If so, the tem accuracy. The accuracy for manual for the GPS to clock error is determined. If not, SPS (95 percent probability) is learn how to adjust the additional assumptions can be eval- that the user’s position can be uated. Although tedious to describe, determined within 100 meters chart datum. the problem can be solved very horizontally. In accordance with quickly with computer chips a Presidential Directive, SA was marine user is located somewhere imbedded in the navigation receiv- turned off a few minutes past on the earth, three measurements er. Students sometimes ask why not midnight EDT after the end of 1 use more accurate clocks in naviga- are satisfactory.) May 2000. According to infor- tion receivers to eliminate this extra It is noted above that errors in mation posted in the Interagency work. The answer is that highly the clock aboard the navigation GPS Executive Board’s (IGEB) accurate clocks are very expensive, receiver can lead to errors in range web site (http://www.igeb.gov), whereas powerful computer chips determinations from each of the the United States has no intent to are comparatively cheap. Thus, it satellites. If the clock is in error, pays to couple a cheap clock with a use SA again. To ensure that then the radius of each sphere is computer, rather than use an expen- potential adversaries of the either too small or too large. This is sive clock. United States do not use GPS, true because each of these errors the military intends to develop results from the clock error in the System Accuracy and deploy systems and methods receiver. Therefore, each is off (plus GPS was designed to provide for regional denial in lieu of or minus) by the same amount. two levels of service: global degradation of signal If there is no receiver clock accuracy.

9-10 Radionavigation 9

J precise positioning Service However (Dubay and Johns, 2001),

ST G A U O A C R

S D

(ppS): This in intended for use DGPS is reported to provide 1- U hINt: aLert

A U Y X R by DOD and other authorized meter horizontal positioning accura- ILIA NaVIGatOrS users. The accuracy for PPS is 16 cy (95 percent) when the user is less meters (95 percent probability). than 100 nautical miles from the Alert navigators realize that The accuracy listed above is DGPS transmitting site. Accuracy it is easy to make mistakes in absolute accuracy. Because of the then degrades at a rate of approxi- continually shifting relation of the mately 1-meter per hundred nautical entering data into navigation satellites and the user, the repeat- miles as the user moves away from receivers–particularly when able accuracy of GPS is the same as the transmitting site. underway. They preplan and the absolute accuracy. In theory, DGPS receivers gather naviga- civil GPS receivers should now tional signals from all GPS satel- do as many of the data entry match the accuracy of PPS (16 lites in view, together with differen- chores as possible when meters) under normal circum- tial corrections from a nearby stances without any modification to DGPS site. Coverage diagrams for dockside. Entries are the receiver. the DGPS system are available on checked and double- the USCG’s Navigation Center checked. The course and dis- DIFFERENTIAL GPS (Navcen) website (http:///www.nav tance between all waypoints Because the (original) accuracy cen.uscg.gov). of SPS was not deemed adequate GPS or DGPS fixes are plotted is measured on the nautical for harbor entrance and approach using a circle with a dot inside and chart. When the waypoint is navigation, the USCG developed may have GPS or DGPS noted activated, the receiver should another system called Differential along with the time. GPS (DGPS). This system reached read approximately the same Initial Operational Capability GDOP/HDOP as measured on the chart. (IOC) on January 30, 1996, and As is the case with conventional If not, there is an error FOC in 1999. LOPs, there are optimal angles of DGPS is the regular GPS with an intersection of GPS LOPs. The somewhere. additional correction added. The optimum case would be to have one correction signal improves the accu- satellite directly overhead and the view, which displays the azimuth of racy of the GPS signal, whether or other three spaced 120° around the the various satellites. These also not SA is enabled. The USCG receiver on the horizon. The worst display HDOP and an estimate of Maritime Differential GPS Service case would occur if the satellites the user’s probable position error. includes two control centers and were spaced closely together or in a over 50 remote broadcast sites line overhead. WIDE AREA AUGMENTATION (located in former RDF sites). This The phenomenon by which fix SYSTEM (WAAS) service broadcasts correction signals accuracy is degraded is referred to Another DGPS-type system, on marine radiobeacon frequencies as geometric dilution of position to improve the accuracy of GPS- (GDOP). The GDOP depends upon intended primarily for use by air- derived positions. DGPS was the geometry of the satellites in craft, has been developed/funded by designed to provide 10-meter accu- relation to the user’s receiver. There the Federal Aviation Administration racy in the designated coverage area. are several related ideas. For (FAA). Development of this sys- This coverage area includes the mariners, the horizontal dilution of tem, termed the Wide Area coastal continental United States, the precision (HDOP) refers to the Augmentation System (WAAS), Great Lakes, Puerto Rico, portions effects of the satellite geometry on along with DGPS, was recommend- of Alaska and Hawaii, and portions position (latitude and longitude) ed by a 1994 Telecommunications of the Mississippi River Basin. errors. Many receivers have a sky and Information Administration

9-11 9 Radionavigation

(NTIA) study. WAAS became GPS/DGPS/ translates to ±15 meters (16.2 available for use in August 2000. WAAS ACCURACY yards), which is approximately the The WAAS (Queeney, 2001) The accuracy of GPS—particu- same as that for the GPS PPS. For consists of 25 ground-based monitor larly if augmented with DGPS or a smaller-scale chart of 1:80,000, stations, termed wide area ground WAAS—raises additional issues rel- the chart error is 80 meters (86.4 reference stations (GRS). Position ative to chart accuracy. Before GPS, yards), which becomes the limiting errors determined at these GRS are when the vessel’s position was esti- factor in position plotting accuracy. transmitted to a wide area master mated by traditional methods, it was Currently, NOAA surveys are being station (WMS) for analysis and pro- understood that vessels’ geographi- conducted to DGPS accuracy. cessing. The corrections are relayed cal positions could be as much or However, for surveys conducted to a ground earth station (GES) and more than 1 M in error (particularly prior to the mid-1990s, the accura- transmitted to two geosynchronous on the open ocean). Given this cy requirement was only 1.5 mm at communication satellites. The cor- potential uncertainty, mariners gave the scale of the chart. rections are broadcast on the same a wide berth to hazards, such as Navigation receivers have the frequency as one of the GPS signals. shoals and obstructions, depicted on capability of displaying information A WAAS-capable GPS receiver is charts. There was general accep- according to several horizontal able to use this information to cor- tance that the available navigational chart data. The horizontal datum of rect estimated positions in the same information and cartographic each chart is listed on the chart. way as a DGPS receiver. The processes used by the chartmaker to The user should check that the hor- WAAS system was designed to pro- position the hazards were more izontal datum in use by the receiver vide uniform 7-meter (95 percent) accurate than the typical position matches that of the chart; other- accuracy, regardless of the location uncertainty of a vessel. wise, substantial position errors of the receiver within the WAAS Now the accuracy of GPS- (with respect to features depicted service area (Dubay and Johns, derived positions (particularly those on the chart) may result. 2001). Actual accuracy (Queeney, determined with DGPS/WAAS) 2001) of WAAS is reported to be often exceeds that of charts used for NAVIGATION RECEIVERS better than 3 meters. navigation. All mariners should A navigation receiver takes continue to give wide berths to position information from the NEED FOR DGPS/ charted hazards until charting Loran-C, GPS, DGPS, or WAAS. GPS RECEIVERS worldwide can be brought to the In some cases, these are distinct Now that SA has been turned required new accuracy. Check to units. More expensive GPS units off, it is reasonable to ask if it is ensure that the GPS receiver is set also have built-in DGPS sensors. In necessary to purchase a DGPS or to the correct horizontal chart other cases GPS receivers can WAAS capable GPS receiver. The datum for the chart in use and refer accept information from DGPS answer depends upon your specific to the source diagram to find the position sensors (i.e., sensors only needs. Users who wish to have hydrographic survey on which var- without the display unit). Some navigational accuracy of ten meters ious parts of the charts are based. navigation receivers can accept or better will need to have DGPS or The specified NIMA chart accu- information from multiple systems. WAAS. Moreover, as new features racy for harbor, approach, and Use of these receivers is described are developed, these are typically coastal charts is that features below. introduced into the “high-end” plotted on a chart will be within The relatively low cost of most receivers. A user desiring other 1 millimeter (mm) at chart scale receivers makes it possible for receiver features may find that with respect to the identified chart boaters to afford to carry backup these are available only on WAAS datum, at a 90 percent confidence units. Portable battery-powered capable or DGPS receivers. interval. NOAA has similar accura- units are especially suitable. cy specifications. For a large-scale The modern navigation receiver chart of 1:15,000, a 1-mm error provides the capability of display-

9-12 Radionavigation 9

N a v i g a t i o n can be “memorized” by the naviga- receivers differ in tion receiver while on a leisurely terms of the number cruise. Then, on a later day when of waypoints that the ceiling and visibility are can be stored. zero/zero, these waypoints prove Waypoints might their worth. correspond to the Most receivers have built-in bat- mariner’s dock, teries that are used to power the ATONs such as memory, so waypoints are not lost lights or buoys, whenever the external power is cut wrecks, areas of or the unit is switched off. Still, it is productive fishing, a good idea to write down the coor- or simply imaginary dinates in a waypoint log for future L reference points that reference. Some mariners list way- FIG. 9-8–Cross-Track Error Illustrated are used as turn points or leg mark- points in a computer program. With ers for an intended voyage. certain makes and models of navi- ing the vessel’s position continu- Navigation receivers differ consid- gation receiver, it is possible to ously. The receiver also contains a erably in the ease (and number of either upload or download way- clock and small computer. Because keystrokes) with which waypoints points to a personal computer (PC). time and position data are available, can be entered and in the allowable This option can be handy and saves it is possible to calculate the ves- number/type of characters and icons considerable effort if the receiver’s sel’s speed over the ground (SOG) that can be stored with the way- memory is lost. (with various averaging times), point. Some receivers provide only Navigation receivers have some for numeric identification of way- speed made good (SMG) since form of “GO TO” key, which points. Others allow the user to store passing the latest waypoint (see enables the user to define a specific an icon (e.g., a replica of an anchor, below), course made good (CMG), waypoint as a destination. Once the a buoy, or other symbol) and an and related quantities. waypoint is defined as the destina- alphanumeric descriptor. This is an Many receivers have the capa- tion, the navigation receiver com- important feature of the receiver. bility of displaying the vessel’s putes and displays the bearing and Waypoints can be entered in track. Most are capable of being distance to the waypoint. (At the advance of a voyage, using the lati- integrated with chart plotters, option of the user, these bearings tude and longitude or TDs of the which overlay the position informa- can be shown in reference to either position identified on the chart. tion on an electronic chart. true or magnetic north.) A track is Alternatively, waypoints can be also defined, from the boat or air- ELECTRONIC NAVIGATION entered by actually voyaging to the waypoint and entering the coordi- craft’s position (when the waypoint One of the very useful features nates automatically in the receiver’s was initially activated directly) to of all modern navigation receivers is memory. The advantage of this lat- the waypoint. If the boat or aircraft the ability to define waypoints (WP ter method for Loran-C receivers is drifts off course, most receivers or WPT). Waypoints are pre-speci- that, because repeatable accuracy have a course deviation indicator fied locations that are stored in the rather than absolute accuracy is (CDI)—an icon to display the loca- receiver’s memory. Options differ, involved, the waypoints are more tion of the course from the vessel’s but most navigation receivers accurate. The trick is to enter these present position. enable the user to enter waypoints in waypoints on a day with excellent Figure 9-8 shows an aircraft terms of latitude/longitude, TDs (for visibility when these can be readily traveling between two waypoints. Loran-C receivers), and/or range identified. For example, the The intended track is the line join- and bearing from the present posi- entrance buoys to an inlet or harbor ing the two waypoints. In this tion or another waypoint. example, the aircraft has strayed to

9-13 9 Radionavigation

point in the route sequence Waypoints, routes, and CDIs are becomes the destination. When so convenient to use that the navi- this destination is reached, the gator may become sloppy. As with receiver sounds an alarm (see any navigational tool, it is possible below) and changes the desti- to make unrecognized and cata- nation to the next waypoint in strophic blunders using a naviga- sequence. tion receiver. For example, the Mariners are cautioned to receiver will not “complain” if establish routes with due con- adjacent waypoints in a route are sideration to navigational haz- located on opposite sides of an ards and the accuracy of the island! Unless you are operating a navigation system. For exam- dredge this will have unpleasant ple, in establishing a route consequences. based on Loran-C (assuming Still on the subject of a need that waypoints are entered for care in using electronics, it is based on lat./lon. coordinates, important to check electronic L rather than on a previous visit), position fixes by as many addition- FIG. 9-9–A route is a defined the route should be laid out to give al means as possible, such as prox- sequence of waypoints. shoals 0.1 to 0.25M clearance, if imity to DR position, visual fixes, possible. Nearly all navigation the right of course and the actual receivers have the capability of set- and water depth. It is all too easy to track (shown by the dashed line) is ting an XTE alarm. This alarm transpose digits when entering to the right of the intended track. sounds whenever the XTE exceeds waypoints or to make other The cross-track error (XTE) is a user-defined limit (e.g., 0.05 M). trivial errors. Whenever possible, it defined as the closest distance of Figure 9-10 illustrates a route laid is appropriate to perform the aircraft or vessel from the out to be well clear of the rocks and “reality checks” on the information intended track. It is determined by shoals shown in the illus- presented. dropping a perpendicular line from tration. The XTE alarm set- the user’s position to the intended ting ensures that the vessel track. Navigation receivers have the will remain in safe water capability of displaying the XTE as whenever within the XTE well as the bearing to the destina- boundaries. tion waypoint. Most navigation Most navigation receivers have receivers have the capabili- some route capability. As shown in ty to bypass one or more Figure 9-9, a route is described by a waypoints in a route defined sequence of waypoints. In sequence, as shown in this illustration, the route includes Figure 9-11. (Receivers dif- waypoints 02, 03, 04, etc. The route fer in the ease with which is first defined in terms of the this can be done.) This fea- numerical sequence of waypoints. ture can be handy. Waypoints can correspond to However, mariners should checkpoints, turn points (as shown remember that it is neces- in Figure 9-9), or to suit some other sary to check carefully to relevant purpose. Routes can also ensure that the revised rout- be given alphanumeric designators. ing keeps the vessel in safe In use, the navigator activates the water. L route function and the next way- FIG. 9-10–Routes should be well clear of hazards.

9-14 Radionavigation 9

able means (e.g., Alarms depth finder) to avoid Most navigation receivers are encounters with the equipped with alarms or indications marked hazard. that warn the mariner if a signal is lost or no longer usable for naviga- OTHER tion. You should spend time study- FEATURES ing the owner’s manual to ensure Modern naviga- that you are fully familiar with tion receivers have a these indications. Many receivers variety of other fea- continue to display position coordi- tures that facilitate nates even in the event of a lost or navigation. For insufficient signal, which can fool example, the naviga- the unwary mariner (see material in tion receiver can be Chapter 12). coupled to an autopi- Most navigation receivers are lot. With this setup, also equipped with a variety of the autopilot will adjustable alarms. Examples of var- steer towards the ious alarms include: waypoints in the J arrival alarm: This sounds L route, obtaining when the vessel passes within a FIG. 9-11–Bypassing a waypoint on course corrections user-adjustable distance from a a route. automatically from destination waypoint. As way- CDI in the navigation points are often used to mark receiver. Use of ATONs as Waypoints turn points, it alerts the mariner Many receivers are integrated to put the wheel over. Many mariners find it conve- with electronic charts—a particu- J nient to use ATONs (e.g., entrance larly handy feature. The receiver passing alarm: (sometimes buoys, channel markers) as superimposes the vessel’s position termed arrival off-course alarm): waypoints. The Canadian Coast and the intended track on an elec- This alarm warns the mariner Guard recommends that mariners tronic chart of the area. Deviations that a waypoint has been passed do not use ATONs as waypoints from track are apparent and easily (technically that the vessel has because of the risk of collision with corrected before these passed a line perpendicular to the the aid or grounding on the danger become appreciable. The that the aid is marking (see electronic charts are http:///www.ccg-gcc.gc.ca/ available on a variety of dgps/guide_7_e.htm). A practical storage media. However, idea is to offset the waypoint from as with paper charts, elec- the ATON 1/4 to 1/2 mile (if there is tronic charts become out- sufficient sea room) so that the dan- dated and must be ger of a close encounter with the replaced with current edi- ATON or the hazard that it marks is tions. Also, remember relatively small, yet the ATON will that most electronic still be visible. If ATONs are used charts do not have any facility to add changes as waypoints, mariners are cau- that have been noted in tioned to monitor the vessel’s L the Local Notice to progress carefully in the vicinity of Mariners. FIG. 9-12–Illustration of the difference between an the waypoint and to use all avail- arrival and a passing alarm.

9-15 9 Radionavigation

heIGht OF heIGht OF aNteNNa IN Feet tarGet (Ft.) 0.0 2.5 5.0 7.5 10.0 15.0 20.0 30.0 0.0 0.0 1.9 2.7 3.3 3.9 4.7 5.5 6.7 2.5 1.9 3.9 4.7 5.3 3.9 4.7 5.5 6.7 5.0 2.7 4.7 5.5 6.1 6.6 7.5 8.2 9.4 7.5 3.3 5.3 6.1 6.7 7.2 8.1 8.87 10.0 10.0 3.9 5.8 6.6 7.2 7.7 8.6 9.3 10.5 12.5 4.3 6.2 7.0 7.7 8.2 9.0 9.8 11.0 15.0 4.7 6.7 7.5 8.1 8.6 9.5 10.2 11.4 17.5 5.1 7.0 7.8 8.4 9.0 9.8 10.6 11.8 20.0 5.5 7.4 8.2 8.8 9.3 10.2 10.9 12.1 22.5 5.8 7.7 8.5 9.1 9.6 10.5 11.2 12.5 25.0 6.1 8.0 8.8 9.4 10.0 10.8 11.6 12.8 27.5 6.4 8.3 9.1 9.7 10.3 11.1 11.9 13.1 30.0 6.7 8.6 9.4 10.0 10.5 11.4 12.1 13.4 35.0 7.2 9.1 9.9 10.6 11.1 11.9 12.7 13.9 40.0 7.7 9.6 10.4 11.1 11.6 12.4 13.2 14.4 45.0 8.2 10.1 10.9 11.5 12.0 12.9 13.6 14.9 50.0 8.6 10.6 11.4 12.0 12.5 13.4 14.1 15.3 55.0 9.0 11.0 11.8 12.4 12.9 13.8 14.5 15.7 60.0 9.5 11.4 12.2 12.8 13.3 14.2 14.9 16.1 65.0 9.8 11.8 12.6 13.2 13.7 14.6 15.3 16.5 70.0 10.2 12.1 12.9 13.5 14.1 14.9 15.7 16.9 75.0 10.6 12.5 13.3 13.9 14.4 15.3 16.0 17.2 80.0 10.9 12.8 13.6 14.3 14.8 15.6 16.4 17.6 85.0 11.2 13.2 14.0 14.6 15.1 16.0 16.7 17.9 90.0 11.6 13.5 14.3 14.9 15.4 16.3 17.0 18.3 95.0 11.9 13.8 14.6 15.2 15.7 16.6 17.3 18.6 100.0 12.2 14.1 14.9 15.5 16.1 16.9 17.7 18.9 110.0 12.8 14.7 15.5 16.1 16.7 17.5 18.3 19.5 120.0 13.4 15.3 16.1 16.7 17.2 18.1 18.8 20.0 130.0 13.9 15.8 16.6 17.3 17.8 18.6 19.4 20.6 140.0 14.4 16.4 17.2 17.8 18.3 19.2 19.9 21.1 150.0 14.9 16.9 17.7 18.3 18.8 19.7 20.4 21.6 175.0 16.1 18.1 18.9 19.5 20.0 20.9 21.6 22.8 200.0 17.3 19.2 20.0 20.6 21.1 22.0 22.7 23.9 L TABLE 9-1–The distance to the radar horizon in nautical miles as a function of the antenna height in feet and the target height shows why high antennas are desirable. Note that these ranges are, typically, less than the “advertised” range of the radar.

intended track at the waypoint) sel’s XTE has exceeded an tion lane. Alternatively, a com- without triggering the arrival adjustable XTE maximum. mercial fisherman, to avoid fish- alarm. Figure 9-12 provides an J Boundary (border) alarm: ing in an illegal fishing area of illustration of the difference This may be thought of as the defined dimensions, might use between the arrival and passing mirror image of the XTE alarm, this feature. Penalties for fishing alarms. The arrival alarm zone is warning the mariner that the within illegal zones can be quite the circular area in this illustra- vessel has penetrated a “lane” of substantial, so many commer- tion, the passing alarm zone the defined width between two cial fishing vessels find this fea- darker shaded area. waypoints. This could be used ture useful. J Cross-track error alarm: This to warn the mariner that the ves- J anchor watch: This might be warns the mariner that the ves- sel has entered a traffic separa- thought of as the mirror image

9-16 Radionavigation 9

such as navigation tronic navigation and seaman’s eye receiver, depth for navigation. sounder (shallow- or deep-water alarms), RADAR: GENERAL radar (guard zone Radar is the last of the electron- penetration), and ic systems to be described in this engine alarms. If text. Introduced in Chapter 4, it is overused, these can described in detail here. Radar is be very confusing. another development that originat- Also, unless hooked ed in World War II. The name is an to amplifiers and acronym for RAdio Detection And speakers, these Ranging coined in the U.S. Navy in alarms may not be 1940 and adopted for general use loud enough to be heard over the by the Allied Powers in 1943. The engine noise. Even invention of radar, probably more PHOTO COURTESY OF RAYTHEON MARINE CO. COURTESY OF RAYTHEON PHOTO with a quiet bridge, than any other development, an anchor watch changed the necessary skills of the L alarm that sounds in the night may professional mariner from the so- Modern Radar Display Unit not be heard in the sleeping com- called “three Ls” (Lookout, Log, partments unless a remote speaker and Leadline) to those necessary for of an arrival alarm. The mariner is rigged. the modern environment. defines a waypoint where the This completes the discussion of Unlike GPS or Loran-C, which anchor is dropped and an alarm the Loran-C and GPS/DGPS/ require satellite or ground-based circle sufficient to accommodate WAAS systems. Navigation is transmitters for operation, radar is the swing circle of the vessel. much easier as a result of the intro- completely self-contained. A radar The alarm sounds whenever the duction of these new tools/instru- set consists of a transmitter, receiv- actual distance from the way- ments on the bridge. er, antenna (either open array or point exceeds the preset value. However, lack of familiarity radome), and display unit. Figure 9- Alarms are potentially valuable, with these navigation instruments 13 shows pictures of both the open but should not be used to excess. can actually make navigation more array and radome antennas. The modern electronic bridge is confusing. Most recreational The radar set alternately trans- typically fitted with many alarms, boaters use a combination of elec- mits, “listens” for the reflected sig- PHOTO COURTESY OF RAYTHEON MARINE CO. COURTESY OF RAYTHEON PHOTO L FIG. 9-13–Open Array (left) and Radome (right) contrasted.

9-17 9 Radionavigation

nal, and measures the time for the given by these formulas. Table 9-1 Determining a radar fix from obser- reflected signal to return. This time shows the distance to the radar hori- vation of the range and bearing of a is translated into a distance (radar zon in nautical miles as a function charted, isolated, and well-defined waves travel at the speed of light) of the height of the antenna in feet radar-reflective object is relatively and the reflected signal or radar echo and the height of the target in feet. simple. It becomes much more is displayed on a circular screen, Compare this with the distance to complex, however, if the shoreline termed a plan position indicator the visible horizon given in Chapter is not well defined and/or promi- (PPI), that shows the azimuth (bear- 6.) Table 9-1 shows only the dis- nent and radar-reflective targets are ing) and range of the target or pip. tance at which it is normally possi- few and far between. The reaction ble that a target can be “seen” by of a first-time radar user to the the radar. As discussed below, image on the radar screen is often

ST G A U O A C R S D one of disappointment. Novices are U praCtICaL detection and identification are A U Y X R ILIA tIp: USe a complex phenomena and detection often preconditioned to expect a lit- raDar ranges may be appreciably shorter eral image (e.g., as seen by eye or than given in Table 9-1. as in a photograph) rather than the reFLeCtOr Radar also has a minimum initially confusing mass of electron- Use of a radar reflector range at which targets may be ic “blobs” on the screen. A promi- detected. This minimum range is nent inlet (see below) may be virtu- increases the likelihood that related to the radar’s pulse length. ally invisible to the radar (at a given range), as may fiberglass or wooden you will be observed on Radar waves travel at the speed of light; so if, for example, the pulse vessels, while a small radar reflec- radar. These devices are rel- length were 1 µsec, the wave could tor held aloft from a small boat may “bloom.” Hilly ground surrounded atively inexpensive and easy travel approximately 984 feet, or 492 feet out and back. With this by low-lying areas may appear as a to install. pulse length, any target closer than series of islands. A visually promi- 492 feet cannot be detected, nent tower on land may present only a small echo because its because the radar will still be emit- The maximum range of the rounded shape scatters the return ting a signal when the return wave radar is determined by several fac- signal, whereas a much smaller bounces back and the receiver will tors, including the radar’s power structure of different shape and still be off. If p is the pulse length in and the antenna height. Radar sig- materials of construction may pre- microseconds, then the minimum nals are broadcast on frequencies sent a larger target. With time, radar range is at least 492 p (in that are essentially line-of-sight, so patience, and a clear understanding feet). Most radar units have a vari- antenna height is often critical to of the characteristics and limita- able pulse length. The minimum range. The actual equations for dis- tions of radar, navigators can learn range for most small boat radar tance in nautical miles to the visible to interpret radar imagery with units is about 30 yards or so, and is and radar horizons are D = 1.17* almost the same ease as photo inter- He0.5 and D = 1.22* Ha0.5. usually given in the manufacturer’s pretation. Textbook discussions are Respectively, where D is the dis- specifications. valuable, but no substitute for on- tance to the visible horizon (first the-water experience with radar. equation) or radar horizon (second RADARSCOPE The PPI provides a chart-like equation), He is the height of eye INTERPRETATION presentation (plan view). But, the above the surface of the water in For navigational applications— image painted on the screen as the feet and Ha is the distance from the determination of position from antenna rotates is not a literal repre- water to the center of the radar range and bearing of known objects sentation of the shore. The width of antenna in feet. Under certain cir- as discussed in Chapter 6—radar the radar beam and the length of the cumstances, the distance could be can be very useful if its characteris- transmitted pulse are two factors substantially greater or lesser than tics and limitations are understood. that distort the image as it appears

9-18 Radionavigation 9

on the scope. More specifically, the on the radar screen. On relative leading when a radar position is width of the radar beam acts to dis- motion displays (discussed below), being determined relative to the tort the shoreline features in bearing landmasses apparently move in shoreline. and the pulse length may cause off- directions and at rates opposite and J Mud flats and marshes normally shore features to appear as though equal to the actual motion of the reflect radar pulses only a little part of the landmass. Inlets, for observer’s ship. Actually, most better than a sandpit. The weak example, cannot generally be seen landmasses are readily recogniz- echoes received at low tide dis- on radar until the vessel is suffi- able; the real problem is to identify appear at high tide. Mangroves ciently close that the size of the specific features so that these fea- and other thick growth may pro- inlet is wider than the antenna’s tures can be used for determining a duce a strong echo. Areas that beam width. (Generally, open array fix. Identification of specific fea- are indicated as swamps on a antennas have a narrower beam tures can be quite difficult because chart, therefore, may return width than radomes, a significant of various factors, including distor- either strong or weak echoes, advantage.) Knowledge of the beam tion resulting from beam width and depending on the density and width, from the manufacturer’s pulse length and uncertainty as to size of the vegetation growing specifications, enables an approxi- just which charted features are in the area. mate computation of the maximum causing the echoes. Often the iden- J When sand dunes are covered distance at which an inlet will be tification of a specific feature (e.g., able to be identified. For this and a tower or light) is based on a con- with vegetation and are well other reasons, the actual radar textual association. For example, a back from a low, smooth beach, image of a scene depends upon the light on the end of a recognizable the apparent shoreline deter- range at which it is observed. landmass may be identified based mined by radar appears as the It is not always easy to deter- upon the fact that the landmass has line of the dunes rather than the mine which features in the vicinity a characteristic shape and the light true shoreline. Under some con- of the shoreline are actually reflect- is the only radar prominent object ditions, sand dunes may return ing the echoes painted on the scope. on the tip of the landmass. strong echo signals because the Any uncertainty or ambiguity, of The following clues or interpre- combination of the vertical sur- course, undermines the accuracy of tation keys are suggested in face of the vegetation and the the resulting fix. Additionally, cer- Defense Mapping Agency (DMA) horizontal beach may form a tain features on the shore will not Publication 1310 (DMA’s name has sort of corner reflector. be visible on the scope, even if they been changed to the National J Lagoons and inland lakes usual- have good reflecting properties, if Imagery and Mapping Agency ly appear as blank areas on a PPI they are hidden from the radar [NIMA]): because the smooth water sur- beam by other physical features or J Sandpits and smooth, clear face returns no energy to the obstructions. This phenomenon is radar antenna. In some called masking or radar shadow. beaches normally do not appear on the PPI at ranges beyond one instances, the sandbar or reef Land Targets or two miles because these tar- surrounding the lagoon may not Landmasses are readily recog- gets have almost no area that appear on the PPI because it lies nizable because of the generally can reflect energy back to the too low in the water. steady brilliance of the relatively radar antenna. Ranges deter- J Coral atolls and long chains of large areas painted on the PPI. mined from these targets are not islands may produce long lines of Knowledge of the vessel’s position reliable. If waves are breaking echoes when the radar beam is relative to nearby land is an impor- over a sandbar, echoes may be directed perpendicular to the line tant clue to interpretation. This is returned from the surf. Waves of the islands. This indication is one of the reasons why it is desir- may, however, break well out especially true when the islands able to integrate navigation infor- from the actual shoreline, so that are closely spaced. The reason is mation from a navigation receiver ranging on the surf may be mis- that the spreading resulting from

9-19 9 Radionavigation

the width of the radar beam caus- J Low islands ordinarily produce brighten at times and then slowly es the echoes to blend into con- small echoes. When thick palm decrease in brightness. Normally, tinuous lines. When the chain of trees or other foliage grow on the the pip of a ship target fades from islands is viewed lengthwise, or island, strong echoes often are the PPI only when the range obliquely, however, each island produced because the horizontal becomes too great. In heavy seas, may produce a separate pip. Surf surface of the water around the however, small vessels can be lost breaking on a reef around an atoll island forms a sort of corner in radar shadow and appear only as produces a ragged, variable line reflector with the vertical surfaces intermittent targets. of echoes. of the trees. As a result, wooded Radar Shadow J Submerged objects do not pro- islands given good echoes and can duce radar echoes. One or two be detected at a much greater While PPI displays appear to rocks projecting above the sur- range than barren islands. offer a chart-like presentation if landmasses are being scanned by face of the water, or waves Atmospheric Targets breaking over a reef, may the radar beam, there may be siz- appear on the PPI. Obviously, Radar has the capability of detect- able areas missing from the display when an object is submerged ing moisture in the form of rain and because of certain features being entirely and the sea is smooth snow. Commercial aircraft use radar blocked from the radar beam by over it, no indication is seen on for weather avoidance. Mariners can other features. A shoreline, which is the PPI. use radar in the same way. continuous when the ship is at one J If the land rises in a gradual, Vessels as Targets position, may not be continuous when the ship is at another position regular manner from the shore- Vessels, unlike landmasses pre- and scanning the same shoreline. line, no part of the terrain pro- sent only a small pip on the PPI. The radar beam may be blocked duces an echo that is stronger But other objects, such as small from a segment of this shoreline by than the echo from any other islands, rocks, and aids to naviga- an obstruction such as a promontory. part. As a result, a general tion (ATONs) also present small An indentation in the shoreline, such “haze” of echoes appears on the targets. Other clues, in addition to as a cove or bay, appearing on the PPI, and it is difficult to ascer- target size, are required to classify PPI when the ship is at one position tain the range to any particular the target as a vessel. For example, may not appear when the ship is at part of the land. Land can be a check of the vessel’s position can another position nearby. Thus, radar recognized by plotting the target indicate that no land is within radar (see below). Care must be exer- range. The size of the pip can also shadow alone can cause consider- cised when plotting because, as be used to exclude the possibility of able differences between the PPI dis- a vessel approaches or retreats land or precipitation, both usually play and the literal chart presenta- from a shore behind which the having a massive appearance on the tion. This effect in conjunction with land rises gradually, a plot of the PPI. The rate of movement of the beam width and pulse length distor- ranges and bearings to the land pip on the PPI can eliminate the tion of the PPI display can cause may show an apparent course possibility of aircraft. even greater differences. For this and speed. Having eliminated the foregoing and other reasons, users need to gain J Blotchy signals are returned from possibilities, the appearance of the experience in radar interpretation. hilly ground because the crest of pip at a medium range as a bright, each hill returns a good echo, steady, and clearly defined image on RADAR USES although the valley beyond is in a the PPI indicates a high probability From the above it should be shadow. If high receiver gain that the target is a steel ship. Unless clear that marine radar is useful for (one of the adjustable controls on equipped with special radar reflec- two broad applications, navigation some radar sets) is used, the pat- tors, detection of wooden or fiber- and collision avoidance. (As noted tern may become solid except for glass vessels is more problematic. above, radar can also be used for the very deep shadows. The pip of a ship target may weather avoidance.)

9-20 Radionavigation 9

TYPE OF TYPE OF KEY MOTION DISPLAY CHARACTERISTICS ADVANTAGES DISADVANTAGES

Relative motion dis- Relatively low cost Off-screen plotting play with own ship at (no gyrocompass on maneuvering UNSTABILIZED center and instanta- input required). board required to (SHIP’S HEAD neous relative bear- Relative bearings of determine target UP SHU) ing of targets dis- targets may be easier speed and direction. played. Plan view to see than with stabi- Bearing accuracy rotates in direction lized (NU) displays. adversely affected RELATIVE opposite to turn. by yaw. MOTION Relative motion dis- Direct readout of Unless “base course play with own ship at actual target bearing up” mode available, center. Linked to and on screen plot- can cause interpre- gyrocompass to dis- ting is possible. tation difficulties ALL STABILIZED play continuous Target smearing due when ship’s course (NORTH north-up picture on to ship’s yaw is is far from north. UP NU) PPI. Absolute bear- reduced. Targets Units are more ing of targets dis- retain same position expensive than SHU played. Own ship’s on-screen when own radar. heading displayed by ship turns, lowering heading flash. the likelihood that tar- gets will be lost.

TRUE Own ship and other Simplest to under- Cost, complexity of MOTION moving objects dis- stand and visualize. operating controls, played in true True motion radars need to reset display motion. Stationary can generally be as own ship moves North up or ship’s objects such as land, operated as relative to edge of display. head up alternatives buoys, etc., remain motion radars, so has exist, but North up stationary. Display all advantages of rela- more common. most resembles tive motion display, if “chart view” on PPI. necessary. L TABLE 9-2–Types of Radar Display

With respect to navigation, radar radar rather than the eye. The trick at least, systematic observation of can be used to “see” land objects in using radar for this purpose targets—as discussed below. and ATONs under conditions of involves learning the appearance of darkness or otherwise restricted vis- targets on a radar screen, rather than RADAR PLOTTING: ibility. In this sense, radar naviga- as seen by eye. As noted above, this REGULATORY tion is analogous to electronic is not a simple process and facility IMPLICATIONS piloting. That is, the techniques of comes only with experience. The following sections have navigation with radar are nearly The second major application of been written to simplify radar plot- identical to those used for visual radar is for collision avoidance. ting as much as possible. But, it piloting (e.g., determination of That is, radar is used to detect and may be necessary to study this range or bearing to identified identify other vessels and the radar material carefully in order to master objects, determination of LOPs, data are used to assess the likeli- the topic. Nonetheless, it is impor- fixes etc.) except that the objects are hood of collision. This requires tant that the operator of any radar- detected and identified with the familiarity with radar plotting—or, equipped vessel fully understand

9-21 9 Radionavigation

radar interpretation and plotting. RADAR DISPLAYS though the perspective of the PPI Mastery of any onboard naviga- Teaching experience with the would encourage the naive observ- tional tool is important. Why, after ACN course shows that the differ- er to believe that he/she can “see” all, own something you can’t use? ent types of radar displays frequent- what’s on the other side of a target. But, knowledge of the use of radar ly confuse students—particularly as Radar observers should be particu- is important for an additional rea- many of the texts on this subject larly alert to the possibility that pre- son. The navigation rules assume (without explicit mention) viously unseen targets can emerge (NAVRULES) or collision regula- that a so-called stabilized north up suddenly from behind larger targets tions (COLREGS) explicitly or display is used, whereas most in congested waterways. implicitly mention radar in several displays on small boats are unstabi- To simplify interpretation of sections. For example (in Rule 7, lized with ship’s head up. This Figure 9-14, extraneous features, “Risk of Collision”) the rules section has been added for such as land echoes, side lobe require that: clarification. echoes, sea or rain clutter, etc., are “Proper use shall be made of Three principal types of display suppressed in this view of the PPI. radar equipment if fitted and are used on marine radar: (i) a rela- On the outer edge of this PPI is a operational, including long- tive motion display (ship’s head ring displaying bearing informa- range scanning to obtain up), (ii) a relative motion display tion. A series of concentric rings early warning of risk of col- (north up), and (iii) a true motion (six in this figure) termed Fixed lision and radar plotting or display (which can be either ship’s Range Markers (FRM) is also equivalent systematic obser- head up or north up). Key charac- shown. The scale of these FRMs is vation of detected objects.” teristics, advantages, and disadvan- generally adjustable from a switch [Emphasis added.] tages of each of these displays are on the display unit. The number In other words, if you have work- summarized in Table 9-2. Some and spacing of the FRMs vary ing radar aboard, it must be used and radar units can be switched from among sets of different manufac- used properly. Because this com- one display mode to another; other ture. Normally the FRMs can be mon sense dictum is made so explic- radar units have only one fixed type switched on or off. Additionally, it, the mariner may risk a legal lia- of display. These three principal many radar units have an adjustable bility in the event of a collision for types are discussed below. intrusion alarm, which alerts the any failure to use the radar properly. navigator if any target penetrates a Radar ownership, thus, involves cer- Unstabilized Ship’s Head Up user-adjustable zone. In this exam- tain responsibilities. A complete dis- (SHU) Display ple (see top of diagram), the scale is cussion of this topic is beyond the Figure 9-14 shows a stylized set so that a target or pip located at scope of this section and this brief replica of a radar display or PPI, so the outermost ring is 6 M distant mention does not purport to give named because it presents a plan and, therefore, each of the concen- legal advice. Navigators interested view (top view) of the scene to the tric rings is 1 M apart. The range of in following up on this should con- observer. Although this display is the target can be estimated from its tact an appropriate professional. particularly convenient, it is impor- position relative to the FRMs. To tant to remember that it was not cre- increase the accuracy of range esti- OUTLINE OF DISCUSSION ated by observation from above. mates, most radar units are also The remainder of this chapter is Rather, it is “painted” from a point equipped with one or more Variable structured as follows. First, a brief near the ground. Radar shadow Range Markers (VRM) (not shown discussion on the types of radar dis- (discussed earlier), though not nec- in Figure 9-14), which insert addi- plays is presented. Next, radar-plot- essarily obvious from the display, tional ring(s) in the display at an ting techniques are explained is, nonetheless, an important phe- adjustable distance from the center. through the use of several examples. nomenon. Smaller vessels, for The range of the VRM is generally Finally, other relevant details and example, can be masked or hidden shown on a separate digital readout situations of interest are covered. from view behind larger ships, even located on the screen.

9-22 Radionavigation 9



6 M 1 M

L FIG. 9-14–Replica of SHU Radar Display

If accurate range information is each of the FRMs was misinterpret- versus ATON, fishing vessel versus required, the VRM (if available) ed. Use of the VRM display pre- merchant ship, etc.), as discussed should be used rather than the vents this error from being made. later in this chapter. But, until this FRMs. This recommendation is It is not possible to tell the ori- classification process is complete, offered for two reasons. First, the entation or even the size of the tar- all detected objects are referred to VRM generally enables a more pre- get from the size or the intensity of as simply targets or pips. cise estimate of range to be made. the radar image alone. As noted In the display shown in Figure Second, it occasionally happens above, target detection is a function 9-14, the observer’s vessel (termed that the operator inadvertently of many variables, including the own ship, or reference ship or ves- switches the range setting on the target’s height, size, shape, aspect, sel) is located at the center, and the radar. In this event, the distance texture, composition, and range. target’s location is, therefore, a rel- corresponding to each range ring is Local knowledge, contextual clues, ative position. Some radar units altered. Chapter 12 provides an and radar plotting can often be used have a feature that enables the posi- example where a vessel grounded to classify the target (e.g., land tar- tion of the observer to be offset because the range corresponding to gets versus targets at sea, vessel from the center of the screen. This

9-23 9 Radionavigation

 6 M 1 M

L FIG. 9-15–Replica of NU Radar Display might be done, for example, to pro- counterclockwise direction. Only were heading 030 compass, the vide a greater display area forward relative bearings (to the ship’s deviation on this heading were 2 of the observer. head) of targets can be read from degrees east, and the variation were The radar display type shown in this display. The target shown in 10 degrees west, the true bearing of Figure 9-14 is termed unstabilized Figure 9-14 has a relative bearing the target would be 030 + 002 - 010 with ship’s head up (SHU). (The of 050 degrees. To calculate a true + 050 = 072 degrees. With the notation SHU is checked in the top bearing from this relative informa- unstabilized SHU display, true (or right corner.) This is the most com- tion it is necessary to note the refer- magnetic) bearings must be calcu- mon display on small boat radar ence ship’s compass heading at the lated, rather than simply read off units. The word unstabilized is time of observation, convert this the screen. Moreover, it is neces- used because the image in the PPI heading to true heading from the sary to note the reference vessel’s will change with the reference ves- CDMVT sequence, and finally, to heading at the instant the relative sel’s heading. A turn to starboard, add the target’s relative bearing to bearing is measured; otherwise, for example, will cause the target(s) the reference ship’s heading. Thus, yaw errors are introduced. in the image to appear to rotate in a for example, if the reference vessel Some unstabilized SHU dis-

9-24 Radionavigation 9



12 M 2 M

L FIG. 9-16–Replica of Radar Display–Target as Seen at Successive Times plays are capable of displaying the most modern radar units have one Stabilized North Up (NU) vessel’s heading (e.g., from a flux- or more electronic bearing markers Display (EBM) or electronic bearing lines gate compass) or COG (e.g., from a In contrast to the unstabilized (EBL). These appear as an GPS or Loran-C input). These dis- display, some radar units (typically adjustable spoke on the screen with plays are termed north referenced more expensive models that would an off-screen (or on-screen) digital unstabilized displays. Care must be be found on larger vessels) feature a bearing readout. The EBL can be used, however, because the COG stabilized display with north up rotated until it is aligned with a tar- may be very different from the ves- (NU) presentation. The stabilized get, and the relative bearing read sel’s heading at any instant. display accepts heading information directly. In Figure 9-14, a dotted Heading, not COG is required for from the vessel’s gyrocompass or, EBL is shown (actual lines may be bearing calculations. alternatively, from a fluxgate or solid on the screen but are shown as To increase the accuracy with electronic compass. This informa- dotted in this illustration to avoid which relative bearings can be read tion is shown schematically in confusion with other markers). (particularly for targets that are not Figure 9-15, which presents a NU located near the edge of the screen), display for the same case as shown

9-25 9 Radionavigation

in Figure 9-14. In this display, a the target, for example, are relative image displayed on a conventional heading flash (shown by the dashed to the reference vessel and not to the screen decays (becomes less bright) line) shows own ship’s instanta- ocean or the bottom. For example, if over time until it is refreshed with neous heading. The heading flash one vessel were steaming north at the next revolution of the antenna. shows the true or magnetic heading, 10 knots and meeting another vessel Conventional displays require the depending on whether a gyrocom- on a southbound course moving at use of a viewing hood during day- pass or fluxgate compass is used. In the same speed, the relative closure time hours. the example above, the true heading rate is 20 knots. This would be the Within the past few years, digi- would be 030 + 002 - 010 = 022 apparent speed of the target as cal- tal or digitized displays, also called degrees, as shown in Figure 9-15. If culated from successive observa- raster scan displays (also termed the reference vessel were to change tions of the target from a relative cathode ray tube (CRT) displays) course, the radar image would not motion radar. Similarly, land and have become available. Raster scan change (as it would with an unstabi- other fixed features will have an displays are an outgrowth of mod- lized display)—only the heading apparent velocity when observed ern television technology. The prin- flash would change position, hence from a relative motion radar. (As cipal advantage of these new dis- the name stabilized display. noted above, these objects will have plays is that the image does not As most vessels yaw somewhat, apparent motion opposite, but at a decay, but rather remains bright as even if the helmsman attempts to speed equal to that of the reference the antenna revolves. Viewing maintain a steady course, target vessel.) hoods are not generally required smear is sometimes a problem with Some radar units are so-called with this display. unstabilized displays. Bearing true motion units. These are able to Some radar units are equipped accuracy, in consequence, is gener- display the actual movements of the with color, rather than monochrome ally higher with a stabilized display. reference vessel as well as that of displays. On some models, different Additionally, true (or magnetic) tar- the other targets. (This requires colors are used only to differentiate get bearings can simply be read additional computers and some fixed or variable range markers (see directly from the NU display, with- means of determining the vessel’s below), but on others, different out any additional calculations. true course and speed.) Land and hues are used to denote the intensi- Finally, because the target loca- other fixed targets remain fixed on ty of the echo. tion does not change when the ref- the display and the vessels move More recently, liquid crystal erence ship changes course, targets about in very much the same fash- displays (LCDs) have become pop- are less likely to be “lost” (e.g., ion as would be seen by an over- ular. LCDs draw less power (an confused with other targets in the head observer. This advantage attractive feature for sailboats), are display) by the observer in the comes about at the expense of com- thinner and lighter, and more easily event that the reference ship plexity in operation and cost. visible in sunlight. CRTs typically maneuvers. In the example shown Because true motion radar units are have greater resolution and are easy in Figure 9-14, there is only one tar- (at present) priced out of the reach get in view, so the target is unlikely to read and interpret in low-light of all but a small proportion of to be lost. However, in a “target- conditions. LCD displays are gen- recreational boaters, these are not rich environment” (i.e., if many tar- erally the display of choice for sail- discussed further in this chapter. gets are present) it can be difficult boats and small open powerboats. Many radar units are able to dis- to keep track of the individual tar- DISPLAY SCREENS gets on the SHU display. play waypoints (from a Loran-C or AND OPTIONS GPS unit) on the radar screen. True Motion Display The display types discussed These are typically shown as a Both the stabilized and unstabi- above should not be confused with small circle with a dashed line from lized displays discussed above pre- the display screen. Conventional the center of the display—a so- sent a picture of relative, rather than screens, also called analog displays, called “lollipop.” This is a conve- true, motion. Rates of movement of were standard at one time. The nient feature.

9-26 Radionavigation 9

RADAR PLOTTING— degrees. The navigator records the legal implications. At a minimum, A FIRST EXAMPLE observation (target bearing, Altair’s all CBDR targets should be plotted Radar observations of targets heading, time, and target range) and or otherwise observed systematical- are generally plotted to determine attends to other duties. Altair main- ly, particularly where visibility is the answers to key questions tains course and speed. At 1006, the reduced. As noted in Rule 7, sys- regarding the likelihood of colli- process is repeated and a second tematic observation may be used in sion, speed, and course of the tar- observation recorded. The target lieu of plotting. The easiest way to get, etc. Plotting may be done man- vessel bears 321 degrees relative at accomplish this is to place an EBL ually, as discussed in this chapter, range 6.3 M while Altair is on a on the target. If the target appears to or automatically if the radar has this heading of 110 degrees. (Although “walk down the EBL” towards your feature. range and bearing information can vessel, a CBDR situation exists and Radar plotting is better be taken at any interval of time, it is some corrective action needs to be described by example than abstract customary to take observations at taken. discussion of theory. So, let’s sup- equally spaced instants in time. To return to the example: at pose that you are the navigator Moreover, to simplify later calcula- 1012, when Altair’s heading is 111 aboard the yacht, Altair, maintain- tions, observations are often taken degrees, the target bears 316 ing a course of 112 degrees at 5 at intervals of 3, 6, or 12 minutes. degrees, at range 5.3 M. Figure 9-16 knots, outbound for Bermuda just For example, if observations are shows the position of the target as it south of the shipping channel off taken every 6 minutes (0.1 hour), would be seen at successive times of the mouth of the Delaware Bay. the speed of the relative motion is observation—denoted /XX, where Altair is equipped with radar with simply 10 times the distance trav- XX refers to the number of minutes unstabilized SHU display. At 1000, eled during the interval.) past the hour when the observation a radar target can be identified bear- Because the target’s relative was taken. As can be seen from the ing 322 degrees relative (using the bearing appears nearly constant and successive PPI images, the relative EBL) at a range of 7.3 M (using the the target’s range is decreasing, a motion (SHU) of the target is gener- VRM). Means for identification of so-called constant bearing decreas- ally southeastward (moving “down- targets are discussed later in this ing range (CBDR) situation, the scope” and closing from left to chapter. Factors considered in target navigator decides to monitor the tar- right) on the SHU display, and some identification include the vessel’s get more closely and to prepare a danger of collision may exist. An M location relative to charted ATONs, radar plot on a maneuvering board board plot is judged to be appropri- known fishing areas, the water (M board). Interpretation of radar ate. To facilitate the preparation of depth at the target’s location, the plot is considerably simplified if the the M board plot, the worksheet range at which the target is detect- reference ship maintains course and shown in Table 9-3 has been devel- ed, and the apparent course and speed, so the navigator ensures that oped. Directions for filling in the speed of the target. In this example, Altair keeps on course and speed worksheet and the preparation of the suppose that there are no ATONs or until all observations are completed. plot are given below. fixed obstructions at the target’s Whether or not to plot a target Worksheet Preparation location. Altair’s navigator tenta- requires some judgment. It is gener- tively identifies that target as anoth- ally impractical and unnecessary to and Plotting er vessel and decides to monitor the plot all targets that appear on the The worksheet shown in Table target’s progress. screen. This is particularly true in 9-3 has three sections: a data sec- Because Altair has an unstabi- congested areas, such as harbor tion, a space for remarks on the lized display, the navigator calls entrances or fishing grounds, where plot, and, finally, a guide to the “mark” when using the EBL to the sheer number of targets would computations. Experience has determine the relative bearing, and preclude plotting of all targets. shown that the use of the worksheet records the helmsman’s response. However, if a collision results, the simplifies plotting and interpreta- At 1000, Altair’s heading is 112 failure to maintain a plot may have tion of the results.

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L TABLE 9-3–Basic Worksheet for Solving Radar Problems on Maneuvering Board

The first section of the work- first example, assume that the tion, reference ship’s head + rela- sheet to be filled in is the data sec- ship’s compass has no deviation. tive bearing = bearing to target. For tion. Accordingly, the data on the Additionally, this problem is solved example, in the first observation at time of observation, target range, in magnetic, rather than true, direc- /00, the target’s magnetic bearing is own ship’s heading, and target rela- tions to eliminate the need to con- 112 + 322 = 434 degrees. Since this tive bearing are entered, along with sider variation. With these conven- exceeds 360, 360 is subtracted and own ship’s course and speed. To tions, the target’s magnetic bearing the calculated magnetic bearing is simplify the calculations for the can be calculated from the equa- 074 degrees. The other times, dis-

9-28 Radionavigation 9

Altair 112° 5 KTS

1000 7.3 112 322 074 1006 6.3 110 321 071 1012 5.3 111 316 067

1 1 M 1 1 KT

Straight Line & Constant Interval Between Points on RMP indicate Both Vessels Maintaining Course & Speed

271 2.1 12 Min. 10.5 2.2 001 249 4.9 28 Min. 1040 254 6.1

L TABLE 9-4–Completed Worksheet for First Example tances, relative bearings, and calcu- pared either with grease pencil on a the radar screen is small, and the lated magnetic bearings are similar- transparent plastic overlay on the navigator may wish to change range ly recorded and entered into the radar screen itself, on an M board, scales as the target vessel draws worksheet, as shown in Table 9-4. or on another special purpose chart closer, a M board is used for plot- Next, it is necessary to plot such as a radar transfer plotting ting rather than plotting on the radar these data. In principle, the relative sheet (RTPS). But, because an screen. Two separate plots are pre- motion plot (RMP) could be pre- unstabilized display is being used, pared: an RMP and a speed or vec-

9-29 9 Radionavigation

Scales MANEUVERING BOARD Distance: 1SD = 1M Speed: 1SD = 1 Knot

RMP CPA DRM = 271°

M3/12 M2/06 M1/00 60(2.1) e SRM =______= 10.5 12 112° 254° 5 Knots 6.1 Knots 271° r m 10.5 Knots Vector Plot

L FIG. 9-17–Radar Plot for “Altair” Example tor diagram (VD). It is essential The first plot required is the closest point of approach (TCPA), that these two plots not be con- RMP. The RMP is used to deter- and other quantities if required. The fused. For convenience, these two mine whether or not a danger of second of these plots, the VD, is separate graphs are generally plot- collision exists, the closest point of used to determine the target’s ted on the same M board, but the approach (CPA), the direction of course and speed. plots are logically distinct. relative motion (DRM), the time to

9-30 Radionavigation 9

Relative Motion Plot (RMP) include failing to use the scale fac- be the same when the reference To prepare the RMP it is neces- tors (if other than 1:1) and misread- ship is heading north.) The sary to plot successive observations ing the bearing scale (e.g., plotting DRM is the first entry on the of the target’s distance and either 074 degrees as 066 degrees, etc.). third (bottom) section of the true or magnetic bearing. Although Figure 9-17 shows the M board worksheet. a useful plot can be made from as RMP. The three observations are J Second, it is useful to calculate few as two observations, generally plotted. For each observation the the speed of relative motion three or more are plotted if time and coordinates are range (M) and mag- (SRM) from the RMP. If the circumstances permit. Your vessel netic bearing. Passing a straight edge points are equally spaced and lie (Altair) is termed the reference ship or paraline plotter through these in a straight line, the speed is or vessel and is labeled “R” if points indicates that all lie approxi- constant. The SRM can be cal- required. (Symbols for the RMP are, mately on the same straight line and culated by measuring the dis- by convention, all uppercase.) In a are equally spaced. This situation tance between the first and last RMP, own ship is located at the ori- will occur whenever the reference observations on the RMP with a gin (center of the plot) at all times. ship and the maneuvering ship both pair of dividers, and calculating The target is termed the maneu- maintain course and speed. Altair the speed using the formula S = vering ship and is labeled “M.” has maintained speed and course (by 60D/T. In this example, the Successive data points for the design), so this plot enables the con- SRM works out to approximate- maneuvering ship are denoted with clusion to be drawn that the target ly 10.5 knots. Alternatively, the the symbol Mi /XX to denote the ith vessel has also maintained course nomogram on the bottom of the observation and the time when it and speed. This conclusion is noted M board (not shown in this illus- was made. In this example, the first in the worksheet in Table 9-4. tration) can be used to estimate observation would be denoted From the RMP it is possible to the SRM. These entries are also M1/00, the second observation calculate several useful quantities. given on the completed work- M2/06, and the third observation These include: sheet in Table 9-4. M3/12. The target range and true or J First, the direction of relative J Third, the RMP can be used to magnetic (magnetic in this exam- motion (DRM) can be estimat- calculate the CPA. As the name ple) bearings are plotted on the ed. This is done by connecting implies, the CPA is the smallest polar diagram (M board). M boards the points on the line and sliding distance that the maneuvering are discussed in Chapter 7. the paraline plotter to a line par- ship (the target) will approach For ease in plotting, it is some- allel to the DRM but through the the reference ship, to use the times appropriate to plot the target origin. The DRM (271 degrees common terminology. It is at a different scale than 1 scale divi- in this example) can be read on determined by either extrapolat- sion (SD) equals 1 nautical mile the outer (bearing) scale of the ing or interpolating the RMP. (M). However, a scale of 1 SD M board. The DRM is noted on The CPA is found by drawing a equals 1.0 M is convenient in this the M board and the worksheet. line from the origin perpendicu- example. There is provision on the It is important to avoid reading lar to the RML. The point where worksheet to note the scale, but, the reciprocal direction on the the perpendicular and the RML additionally, it is useful to write the M board. In this instance, suc- intersect is the CPA. Although scale on the M board. If scales other cessive points lie generally to this can be done graphically, it is than 1:1 are used, it is necessary to the left of each other, and this easier to calculate the bearing factor the scale into the calculations. determines the direction. (Note from the DRM plus or minus 90 Care should be taken in prepar- that the DRM, nearly westward degrees. The distance, approxi- ing the plot. Small plotting errors in this case, differs from the mately 2.2 nautical miles in this are carried through the calculations apparent southeastward motion example, can be transferred to and can influence the final results of the target on the SHU display. one of the distance scales with a substantially. Common errors These two directions will only pair of dividers. By extending

9-31 9 Radionavigation

the line from the origin through attempting to contact the target ves- course and speed vector is plot- the CPA to the bearing scale, a sel on VHF/FM (Channel 13) to ted. This vector, labeled er (low- true (magnetic) bearing of the arrange details of passage, or alter- ercase letters are used on the CPA can be obtained. In this ing course or speed to increase the vector plot to avoid confusion example, the magnetic bearing separation at CPA. The effect of with the uppercase symbols on of the target at the CPA is course or speed adjustments can be the RMP), is plotted on the M approximately 001 degrees. evaluated on an M board, but this is board. The vector is drawn out- These entries are also shown on beyond the scope of this chapter. ward from the origin in the the worksheet in Table 9-4. The direction of the course of the relative bearing of the CPA can THE VECTOR DIAGRAM reference vessel. The length of be calculated from the true The second plot generally vector er is proportional to the (magnetic) bearing minus the required for complete solution of speed of the reference vessel. In ship’s head. In this example, the the radar problem is the speed trian- this example, Altair’s speed vec- relative bearing is 001 degrees gle or vector diagram. It is used to tor has a length of 5 SDs (5 SDs magnetic minus 112 degrees estimate the target’s course and correspond to 5 knots at a 1:1 (360 must, therefore, be added), speed. There are two principal scale) and is drawn outward on a or 249 degrees. In other words, methods for plotting the vector dia- heading of 112 degrees. at the point of closest approach gram. The method discussed here J Second, the relative motion vec- the target vessel will be approx- can be found in the Maneuvering tor is laid out on the M board. imately 21 degrees abaft Altair’s Board Manual or the Radar The “tail” of the relative motion port beam. Navigation Manual published by vector is the DRM (271 degrees J Fourth, the TCPA can be deter- the DMA (now NIMA), and cited in in this example), and is conve- mined, by measuring the dis- the attached references. Another niently laid out by moving the tance between the last observa- method, called the Real Time paraline plotter parallel to the tion, M3, and the CPA, approxi- Method (RTM), is gaining favor DRM on the RMP. The length of mately 4.9 M, and calculating among radar users. This is also the relative motion (rm) vector how long it would take to tra- described in some of the references is the SRM (10.5 knots, or 10.5 verse this direction at the SRM. but is not discussed here. SDs at the 1:1 scale used in this In this example, the TCPA is Unlike the RMP, which shows example), determined from the estimated to be 1040. distances and bearings, the vector RMP. To summarize the results for this diagram shows speeds and courses J Third, the maneuvering vessel’s example, the RMP has enabled the of the maneuvering and reference vector, denoted em, is drawn following estimates to be made. ships. Therefore, it is possible that a from the origin to the head of The target vessel will pass as close different scale would be appropriate the relative motion vector. The as 2.2 M to Altair at 1040 with rel- for plotting the vector diagram. In orientation of this vector (254 ative bearing of closest approach this example, however, a scale of 1 degrees in this example) is the 249 degrees. Actions to be taken by knot equals 1 SD is appropriate and course (magnetic in this exam- Altair’s crew in this case depend is so noted on the M board and ple) of the maneuvering or tar- upon additional factors, such as worksheet alike. get vessel. The length of this Altair’s ability to maneuver, visibil- The vector diagram is a graphi- vector (measured as 6.1 SDs, or ity, etc. In good weather, Altair’s cal method for vector addition and 6.1 knots at a 1:1 scale) is equal skipper would probably elect to is used to determine the course and to the speed of the target. The maintain course and speed, alert the speed of the maneuvering ship. It is fact that the target’s estimated lookout(s) to the target’s position, simple to use and interpret, and is speed is positive confirms the and continue to monitor the situa- described in the three steps shown navigator’s initial classification tion until the target vessel was well below: of the target as another vessel. If clear. Other options include J First, the reference vessel’s the target were stationary, or

9-32 Radionavigation 9

nearly so, it could have been a RADAR PLOTTING— course, and speed of the target. vessel at anchor (the plausibility A SECOND EXAMPLE What actions, if any, should you of this hypothesis could be As a second example, range and take as the master of the Nowy Sacz checked from the depth of bearing data are presented from one on the basis of these data? water), dead in the water, or, reconstruction of a collision that Solution to Second Example possibly, a fixed ATON. occurred in clear weather between Table 9-5 and Figure 9-18 show This completes the radar plot for the Polish M/V Nowy Sacz and the the completed worksheet and M the first example. Inspection of Cypriot ship Olympian under con- board plot, respectively, for this both plots shows that Altair and the ditions of darkness early one morn- example. From the RMP, it is deter- target vessel are almost on opposite ing on 14 February 1972 approxi- mined that the DRM is 285 degrees courses, and should pass port-to- mately 20 M south of Cape St. and the SRM is 3 knots. From the port at a distance of approximately Vincent, off the Iberian Peninsula, M board plot, it is clear that the 2.2 M if both vessels maintain along the coast of Portugal. CPA is 0 M, i.e., a collision situa- course and speed. In practice, the Although both ships were using tion exists. The TCPA (time of esti- RMP would probably be continued radar for navigation, radar observa- mated collision) in this case is tions were not taken nor were M or otherwise systematically 0359, and the target’s course and board plots actually made in this observed to monitor the progress of speed are estimated to be 331 case. Nonetheless, sufficient data the maneuvering vessel and ensure degrees and 14.6 knots respectively. exist to reconstruct the sequence of that no subsequent course or speed Incidentally, the Cypriot vessel radar observations that would have claimed to be on a course of 290 adjustments were required to avoid resulted had these actually been collision. degrees making only 13 knots, a taken. claim that was rejected by the court To make the problem concrete, AUTOMATIC RADAR on the basis of the evidence pre- suppose that you are the master of sented. PLOTTING AID the Nowy Sacz, enroute from From the M board plot it is clear Even though there are faster Casablanca in North Africa to that the Olympian is overtaking the plotting methods than illustrated Gdynia, Poland. At 0245, Nowy Nowy Sacz. So under the above, it should be clear that radar Sacz is making 12.5 knots on a true NAVRULES (assuming that the plotting could be a chore. To facili- course of 341 degrees. A radar tar- vessels were in sight of one anoth- tate this process, large ship radar get is identified with a relative bear- er) the Nowy Sacz was the stand-on units are equipped with an ing of 124 degrees, approximately vessel and the Olympian should Automatic Radar Plotting Aid 3.7 miles distant. (For purposes of have altered course/speed to avoid (ARPA). ARPA automates prepara- this example, assume that Nowy collision. The actual situation was Sacz has an unstabilized SHU dis- tion of a plot. A target can be desig- somewhat more complex and the play.) By 0300 the target has closed nated and a computer in the radar Nowy Sacz was initially held to approximately 2.9 M while still responsible for the collision that unit makes the necessary computa- bearing 124 relative. A third obser- resulted. However, on appeal, this tions. ARPA can display all the rel- vation is taken at 0315, when the verdict was reversed and the evant quantities computed labori- target bears 124 degrees relative at Olympian was held to be partially at ously above. a range of approximately 2.2 M. fault. The actual collision occurred Several manufacturers have Assume that, because an autopilot at 0358, in agreement with the pre- introduced radar units with “Mini- was in use, Nowy Sacz’s heading dicted TCPA. ARPAs” for use on recreational and speed remained constant at 341 This example is instructive in boats. Use of such radar units can degrees and 12.5 knots, respective- that it shows the value of an M reduce cockpit workload consider- ly, while the bearings were taken. board plot, rather than simple ably—but only if the mariner is Prepare an M board plot for these examination of the radarscope fully familiar with their operation. observations and estimate the CPA, images alone. However, even the

9-33 9 Radionavigation

NOWY SACZ 341° 12.5 KTS

0245 3.7 341 124 105 0300 2.9 341 124 105 0315 2.2 341 124 105

2 1 M 1 2 KTS

CBDR Situation—Straight Line Plot

285° 1.5 M 30 Min. 3 0! NA NA 2.2 44 Min. 0359 331° 14.6

L TABLE 9-5–Nowy Sacz/Olympian Collision radar observations alone should water rather than on the course and ence and maneuvering ships that have alerted both vessels that a col- at the speed indicated, the identical could have produced the same lision was likely. Note that the series of radar images could have RMP. It is only the vector plot that same succession of radar images resulted if the Olympian were enables the actual course and speed could have resulted from many approaching on a course of 285 of the maneuvering ship to be deter- combinations of reference and tar- degrees at 3 knots. In fact, there are mined. get ship movements. For example, an infinite number of combinations if the Nowy Sacz were dead in the of speed and course for the refer-

9-34 Radionavigation 9

MANEUVERING BOARD Distance: 2SD = 1M Speed: 1SD = 2 Knot

rm 285° m 3 KTS r

em er 341° 331° 12.5 14.6

e DRM = 285° M3/15 M2/00 SRM = 3 KTS M1/45

L FIG. 9-18–Nowy Sacz/Olympian

THIRD EXAMPLE above, the M board plot was linear course and speed. Indeed, it is The third example presented and the distances were constant important to note that the reference here is of interest because the plot is between evenly spaced observa- vessel should maintain course and somewhat complex and presents tions. As noted, this situation results speed throughout the plotting inter- more of a challenge to interpreta- only when both the reference and val if valid data are to be obtained. tion. In all the problems given maneuvering vessels maintain However, even if the reference ves-

9-35 9 Radionavigation

sel maintains course and speed, the motion vector should be inserted in J Suppose first that the prevailing maneuvering vessel(s) may not do the speed diagram. But, in this visibility is such that Pollux and so. Thus, it is important to consider example there are two relative the target vessel are in sight of some cases involving a maneuver- motion vectors—one valid for the one another throughout the ing target. first period, denoted rm1, and encounter. In this case, Pollux is In this example, the M/V Pollux another valid after xx38, denoted the give-way vessel, because the is proceeding due north at a speed by rm2. From the plot of the first target is in Pollux’s danger zone of 9 knots inbound to Valdez, three points it is determined that the –that is, between 0 degrees and Alaska. At XX29 a target is identi- DRM is 291 degrees and the SRM 112.5 degrees relative bearing. fied bearing 111 degrees at a range is approximately 11.3 knots. The target, therefore, is a cross- of 2.3 M. To avoid tedious compu- Connecting rm1 to er enables deter- ing vessel within the meaning of tations, it is assumed that a NU dis- mination of the target vessel’s ini- the NAVRULES. (Although the play is available and true bearing tial course and speed to be 321 situation is close, the target is information is given directly as degrees and 17 knots, respectively. within 2 degrees of being an shown below. To determine the target’s course overtaking and, thus, the give- time range (M) Bearing (Degrees) and speed after xx38 it is necessary way vessel.) Close or not, Pollux xx29 2.30 111 to plot another vector diagram. But, should have given way, by either xx35 1.20 111 Pollux’s course and speed were alteration of course or speed to xx38 0.60 111 unchanged, so the same vector, er, ensure that the vessels passed at xx41 0.35 140 is used for the second vector dia- a safe distance (see Rule 15). xx47 0.60 225 J xx53 1.20 243 gram. The RML for the second time Now suppose that this encounter period shows that the DRM and took place in restricted visibili- Solution SRM are approximately 259 ty. In this situation, unlike that The M board plot shown in degrees and 6.6 knots, respectively. of vessels in sight of one anoth- Figure 9-19 presents a somewhat These computations define the sec- er, there are no stand-on or give- more complex picture than the ear- ond rm vector, rm2, which is plot- way vessels. However (Rule lier examples. In this case, the ted as shown. 19), “a vessel which detects by points on the RMP do not plot as a Examination of the second vec- radar alone the presence of single straight line but, rather, tor diagram indicates that the tar- another vessel shall determine if appear to fall along two straight get’s speed was reduced to approxi- a close-quarters situation is lines joined at xx38, indicating that mately 10 knots, but that the course developing or risk of collision either the reference or the target was unchanged, because rm2 lies exists.” The rule continues, “If vessels changed course and/or along the same radial (approximate- so, she shall take avoiding speed. Since Pollux did not change ly) as rm1. This example is particu- action in ample time.” Because either course or speed, the target larly interesting because it clearly Pollux is equipped with operat- vessel must have—possibly to illustrates the value of plotting. It ing radar and able to determine avoid collision. Note that the first would be clear from inspection of that a risk of collision existed, three points at 29, 35, and 38 past the radar screen alone that “some- Pollux should have taken appro- the hour indicate the classic CBDR thing” has happened, but only the priate action to minimize the pattern. M board plot enables the navigator risk of collision. The object of this exercise is to to determine exactly what action Therefore, regardless of the pre- determine what happened. It is was taken by the target vessel. vailing visibility, Pollux should have solved by making two RMPs and It is useful to consider this initiated action to avert collision, two vector diagrams. Note the example from another perspective, albeit for different reasons. A com- speed scale of 2 SD = 1 knot is that of the NAVRULES. In this con- plete discussion of the NAVRULES, used. After plotting the RMP and nection, it is instructive to consider radar, and collision-avoidance is Pollux’s speed vector, the relative the circumstances of visibility: beyond the scope of this text.

9-36 Radionavigation 9

MANEUVERING BOARD Distance: 4SD = 1M Speed: 1SD = 2 KTS

29 111 2.3 35 111 1.2 38 111 0.6 42 240 .35 47 225 0.6 53 243 1.1

m2

r

m2

e

DRM = 285 DRM2 M3/38 1 = 291 M 60(1.65) 2/35 M6/53 SRM2 =______= 6.6 15 SRM 1 = ______60(1.7) M1/29 9 = 11.3

L FIG. 9-19–Example Involving M/V Pollux

However, the reader is advised to to an M board for radar plotting. RTPS. Before discussing the plot in review the references given in the The RTPS is also available from detail, a brief review of the RTPS is bibliography for more details. NIMA (No. 5089, DMA Stock No. in order. Basically, the RTPS offers WOXZP 5089), but is produced in a similar presentation to the M USE OF THE RTPS only one size (10-inch). board, except that 6 FRMs are given A Radar Transfer Plotting Sheet Figure 9-20 shows the RML for and scales are adjusted to be multi- (RTPS) can be used as an alternative the first example as plotted on the ples of 6 (i.e., 3 miles, 6 miles, 12

9-37 9 Radionavigation

L FIG. 9-20–Altair Example Plotted on RTPS miles, and 24 miles). Suppression ple, observation Ml/00 is located transferred to the radar scope fac- of a radiating distance (or speed) 7.3 miles at a bearing 074 magnet- simile in the center. scale found on the M board presents ic. If a 12-mile scale is assumed Use of an RTPS is particularly a less cluttered plot, but auxiliary (i.e., FRMs located 2 miles apart), convenient if the range scales on scales, given on the right- or left- as denoted by placing a circle your radar set match those on the hand side, need to be used and dis- around the 12 in the auxiliary scale RTPS. The auxiliary scales elimi- tances (or speed) transferred with a on the right-hand side, the distance nate the need to convert from SDs to pair of dividers from the auxiliary (7.3M) is first measured with a set either distance or speed. However, scales to the plot. Thus, for exam- of dividers on this scale and then as most small boat radar units have

9-38 Radionavigation 9

range scales that differ from the points that should be made with wise, self-evident. Frequently, choices presented on the RTPS, use respect to the interpretation of radar buoys can be identified from the of an M board is recommended. for collision avoidance. range at which detection first J First, it is important to note that occurs. Detection ranges for RADAR PLOTTING: THE if the reference ship is dead in buoys vary from perhaps 1/4 M STEPS SUMMARIZED the water (DIW), aside from for a conical buoy to 6 M or As illustrated in the above whatever drift and/or yaw may more for a buoy with a radar examples, the overall steps in radar be present, the relative motion reflector. This also shows the plotting are as follows: display behaves almost as value of a continuous radar J Observation and detection: A though it were a true motion dis- watch. Sporadic observation continuous radar watch is main- play. Thus, if presented with an will not enable the target’s tained and the appearance of tar- ambiguous situation and condi- detection range to be estimated gets noted. On the basis of early tions otherwise permit, you can with any accuracy. A target observation of the range and simply halt the vessel to enable detected at 10 or more miles dis- bearing of the target(s) and other the course and speed of other tant, for example, is unlikely to factors, a decision is made targets to be observed directly. be a buoy. If two targets are pre- whether or not to plot or other- J Second, it should be evident that sent, reducing the gain may wise monitor the target(s). If a target which is apparently sta- enable differentiation to be practical, the reference ship tionary when viewed from the made; the buoy will disappear should maintain course and radar screen of a moving vessel first, while the echo of a nearby speed. is not DIW, but rather maintain- ship may remain visible. As well, M board plots can J plotting and analysis: If the ing the same course and speed as be used to differentiate between target(s) is plotted, the plot is the reference ship. Such targets DIW targets, such as buoys and used to determine the risk of are often observed by vessels moving vessels. A target that is collision (CPA, TCPA) and transiting inlets or enroute to DIW will have a DRM opposite nature of the encounter (true and popular destinations as they that of the reference vessel and a relative bearings). Alternatively, keep station relative to other ves- speed the same as the reference this can be done automatically if sels. vessel. This can be determined the radar has this feature. J Third, it is often of interest to solely from a RMP, although a J identify stationary targets such Selection of evasive action: vector diagram will, of course, Although not discussed in this as buoys, or other ATONs. There are several ways to do confirm the stationary position chapter, M boards can be used to this. Often the location of the of the target. In practice, a vec- evaluate the effects of course target and absence of other tar- tor diagram for a DIW target and speed changes to ensure that gets will render identification of will result in a small apparent the vessels pass at a safe dis- buoys self-evident. This also speed due to observation errors, tance. highlights the importance of but values close to zero are suf- J Monitoring results: Radar knowing the reference vessel’s ficient confirmation of the tar- plots are maintained to monitor position accurately and is one of get’s status. the results of any collision the many reasons why it is con- From the above, it should avoidance maneuvers to verify venient to have a loran or GPS be clear that targets on identical their success and/or to select that “talks” to the radar and dis- (or nearly) courses from the ref- alternative maneuvers. plays the vessel’s latitude and erence ship that have the same longitude on the radar screen. speed as the reference vessel OTHER SITUATIONS In some cases, for example, appear stationary. Targets mov- OF INTEREST with a transponder-equipped ing downscope on the SHU dis- There are several other salient buoy, the identification is, like- play at greater speed (SRM)

9-39 9 Radionavigation

than the reference vessel are CONCLUDING COMMENTS sive on-the-water practice (in good other vessels on (nearly) recip- Proficiency in radar interpreta- visibility) will enable you to rocal courses. And “upscope” tion and plotting comes only with become fully familiar with radar. targets apparently moving practice. Plotting can (and should) Finally, the reader should downscope at slower speeds be practiced in the living room or remember that manufacturers are (SRM) than the reference ship study. There are simulation pro- continually seeking ways of making are vessels being overtaken by grams suitable for use on a PC that radar more user-friendly. For exam- the reference ship. These simple are helpful for home study. ple, new displays are now available rules of thumb can easily be ver- However, there are important that integrate (either via split screen ified with the use of an M board. differences between the shipboard or integrated one-screen displays) J Fourth, it is important to empha- environment (e.g., the time avail- radar and electronic charts. In the size that the material presented able for plotting, the size of the single-screen option, the electronic in this chapter gives only a sam- plotting table, if any, the need to chart overlays the radar display, pling of the possible uses of M divide your attention between plot- which may simplify radar interpre- boards. Examination of one of ting and other duties) and actual on- tation and facilitate target identifi- the many texts given in the list the-water conditions. The problems cation. Keep abreast of these devel- of references will show addi- in this text provide useful opportu- opments by reading the navigation tional applications. nities to test your mastery of the literature. subject. But nothing short of exten-

9-40 Radionavigation 9

SELECTED REFERENCES

Anderson, J., (1987). Exercises in Pilotage, David and Coolen, E., (1984). Nicholls’s Concise Guide to Charles, London, UK. Navigation Examinations, Vol. 2, Brown, Son Ferguson, Ltd., Glasgow, Scotland. Block, R. A., Editor, (1983). Radar Observer Manual (Fifth Revision), Marine Navigation Textbooks, Dahl, B., (1986). The Loran-C Users Guide, Houma, LA. Richardsons Marine Publishing Inc., Evanston, IL.

Bole, A. G. and K D. Jones, (1981). Automatic Radar Dahl, B., (1993). The Users Guide to GPS, Plotting Aids Manual, Cornell Maritime Press, Richardsons Marine Publishing Inc., Evanston, IL. Centreville, MD. Defense Mapping Agency, Hydrographic/Topographic Bowditch, N. (1995). American Practical Navigator, Center, (1983). Loran-C Correction Table, An Epitome of Navigation, Pub. No. 9, Defense Northeast, USA, 9960, DMA Stock No. LCTUB Mapping Agency Hydrographic/Topographic 2211 200 C, Washington, DC. Center, Bethesda, MD. This publication is avail- able electronically from the National Imagery and Defense Mapping Agency, Hydrographic/Topographic Mapping Agency (NIMA) Marine Navigation Center, (1984). Maneuvering Board Manual, Pub. Department web site (http://www.nima.mil/). No. 217, 4th edition, Bethesda, MD.

Brogdon, W., (1995). Boat Navigation for the Rest of Defense Mapping Agency, Hydrographic/Topographic Us—Finding Your Way by Eye and Electronics, Center, (1985). Radar Navigation Manual, Pub. International Marine, Camden, ME. No. 1310, 4th edition, Bethesda, MD.

Brooke, G. A. G. and S. Dobell, (1986). Radar Mate, Defense Mapping Agency, Hydrographic/Topographic Adlard Coles Ltd., London, UK. Center, (1988). Radar Navigational Aids, Pub. No.117, DMA Stock No. RAPUB 117, Bethesda, Burger, W., (1978). Radar Observer’s Handbook for MD. Merchant Navy Officers, 6th edition, Brown, Son & Ferguson, Ltd., Glasgow, Scotland. Departments of Transportation and Defense, (1999). Federal Radionavigation Plan 1999, NTIS Cahill, R. A., (1983). Collisions and Their Causes, Springfield, VA. Fairplay Publications, London, UK. Dixon, C., (1999). Using GPS, Second Edition, Canadian Power and Sail Squadrons, (1999). Sheridan House, Dobbs Ferry, NY. Navigating With GPS, Ontario, Canada. Dubay, C. and T. Johns, (2001). WAAS, DGPS and the Carpenter, M. and W. Waldo, (1975). Real Time Mariner’s Toolkit, available electronically at Method of Radar Plotting, Cornell Maritime Press, (http://www.navcen.uscg.gov/news/archive/2001/ Centreville, MD. may/WAAS-DGPS.htm/).

Chinery, M., (1994). Simple GPS Navigation, French, J., (1977). Small Craft Radar, Van Nostrand Fernhurst Books, West Sussex, UK. Reinhold Company, New York, NY.

Clissold, P., (1970). Radar in Small Craft, Thomas Hoffer, W., (1979). Saved! The Story of the Andrea Reed Publications Limited, London, UK. Doria—The Greatest Sea Rescue in History, Summit Books, New York, NY. Coolen, E., (1987). Nicholls’s Concise Guide to Navigation, Vol. 1, Brown, Son & Ferguson, Ltd., Glasgow, Scotland.

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SELECTED REFERENCES

Holdert, H. M. C. and F. J. Bozek, (1984). Collision Queeney, T., (2001). A New Way to Increase GPS Cases, Judgements, and Diagrams, Lloyd’s of Accuracy, Professional Mariner, Issue 56, p 49 et London Press Ltd., Legal Publishing and seq. Conferences Divisions, London, UK. Safford, E. L., (1971). Modern Radar: Theory, Hurn, J., (1989). GPS: A Guide to the Next Utility, Operation and Maintenance, Tab Books, Blue Trimble Navigation, Sunnyvale, CA. Ridge Summit. PA.

Hurn, J., (1989). Differential GPS Explained, Trimble Sonnenberg, G. J., (1988). Radar and Electronic Navigation, Sunnyvale, CA. Navigation, 6th Edition, Butterworths, London, UK. McClench, D., Ed. (1967). Primer of Navigation by George W. Mixter 5th Edition, Van Nostrand United States Coast Guard Academy, (1990). Loran-C Reinhold Co., New York, NY. Engineering Course Notes, New London, CT.

Melton, L., (1986). The Complete Loran-C Handbook, United States Coast Guard, (1992). Loran-C User International Marine Publishing Co., Camden, ME. Handbook, COMDTPUB P16562.6, Washington, DC. Melton, L., (1987). Piloting with Electronics, International Marine Publishing Co., Camden, ME. United States Coast Guard, (1984). Radionavigation Systems, G-NRN, Washington, DC. Ministry of Defense (Navy), (1987). Admiralty Manual of Navigation, BR 45 (1), Volume 1, Her United States Coast Guard, Specifications of the Majesty’s Stationary Office, London, UK. Transmitted Loran-C Signal, COMDINST 16562.4, Washington, DC. Monahan, K. and D. Douglass, (2000). GPS Instant Navigation, A Practical Guide from Basics to United States Power Squadrons®, (1997). GPS Global Advanced Techniques, 2nd Edition, Fine Edge Positioning System, An Introduction to the Global Productions, Bishop, CA. Positioning System, Raleigh, NC.

Moss, W. D., (1965). Radar Watchkeeping, The Van Wyck, S. M. and M. H. Carpenter, (1984). The Maritime Press Ltd., London, UK. Radar Book, Cornell Maritime Press, Centreville, MD. Motte, G. A. and T. M. Stout, (1984). Chartwork and Marine Navigation for Fishermen and Boat Walsh, G., (1988). Coastal Navigation, the Basics: Operators, Second Edition, Cornell Maritime Press Radar Tactics and the Rules of the Road, Ocean Centreville, MD. Navigator, September/October p.55 et seq.

Oudet, L., (1960). Radar and Collision, D. Van West, J., (1988). Boat Owner’s Guide to Radar, Nostrand Company, Princeton, NJ. International Marine Publishing Company, Camden, ME. Pierce, J. A. (1989). Memoirs of John Alvin Pierce: Development of Loran. Journal of the Institute of West, J., (1978). Radar for Marine Navigation and Navigation, Vol. 36, No. 1, pp. 1-8. Safety, Van Nostrand Reinhold Company, New York, NY. Pike, D., (1977). Electronic Navigation For Small Craft, Granada Publishing, London, UK. Wylie, F. J., Editor, (1982). The Use of Radar at Sea, Royal Institute of Navigation, Naval Institute Press, Annapolis, MD.

9-42 United States Coast 2 Pilot Atlantic Coast: Cape Cod to Sandy Hook 30th Edition CHAPTER 10

NAVIGATION

U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Ocean Service U.S. Department REFERENCE PUBLICATIONS of Transportation United States Coast Guard “Many receive advice, few profit by it.” LIGHT LIST –Publilius Syrus, First Century BC Volume 1 ATLANTIC COAST “Any ship can be a survey ship…once.” St. Croix River, Maine to Shrewsbury River, New Jersey –Capt. T.W. Richards, NOAA This publication contains a list of lights, sound signals, buoys, daybeacons, and other aids to navigation.

THIS PUBLICATIONIMPORTANT SHOULD BE CORRECTED INTRODUCTION information and are worthwhile EACH WEEK FROM THE LOCAL NOTICES TO MARINERS OR NOTICES TO MARINERS AS APPROPRIATE. y this point in the course/text, additions to the ship’s library. you should be quite familiar Finally, in this information age, United 1998 B there is a great deal of useful infor- StatesCOMDTPUB P16502.1 with the 1210-Tr chart and, more generally, the amount, variety, and mation posted on the Internet. Coast 2 utility of material depicted in the Important Internet addresses are modern nautical chart. Nonetheless, provided. Pilot space and format constraints limit Atlantic Coast: the amount of information that can ST G A U O A C R S D WHAT YOU

Cape Cod to Sandy Hook U

be placed on the chart. Therefore, A U Y X R 30th Edition the mariner needs additional mater- ILIA WILL LEARN IN ial to help provide for the safe and THIS CHAPTER efficient navigation of the vessel. A J variety of additional publications Additional sources of chart- related information are available, some published by commercial enterprises, and others J Overview of contents of the U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric AdministrationU.S. Department of government origin. This chapter National Ocean Service of Transportation United States Coast Pilot United States provides a brief overview of rele- and Light List Coast Guard vant government publications, including the United States Coast J Purpose and utility of LIGHT LIST Pilot (USCP), Light List, Notice to Notice to Mariners and Local Notice to Mariners Volume 1 Mariners (NTM or NM), and the Local Notice to Mariners (LNM). ATLANTIC COAST J Important web site These three publications supple- St. Croix River, Maine to addresses Shrewsbury River, New Jersey ment the nautical chart. This publication contains a list of lights, sound signals, buoys, daybeacons, and other aids to navigation. Additionally, commercially pro- J How to estimate the visible duced cruising guides and boating range of a light THIS PUBLICATIONIMPORTANT SHOULD BE CORRECTED EACH WEEK FROM THE LOCAL NOTICES TO MARINERS almanacs often contain valuable OR NOTICES TO MARINERS AS APPROPRIATE. 10-1 1998 COMDTPUB P16502.1 10 Navigation Reference Publications

COAST PILOT mation furnished by other federal J Wharf descriptions, The United States Coast agencies and state and local govern- J Dangers, ments, maritime and pilotage asso- Pilot, to use its formal title, is J Routes, traffic separation ciations. Mariners are also encour- published periodically by the schemes, National Ocean Service (NOS), aged to recommend corrections and J Small craft facilities, and Charting and Geodetic Services additions to the USCP. These J (C&GS), National Oceanic changes can be sent to NOAA on Federal regulations applicable and Atmospheric Administration Form 77-6 (Coast Pilot Report), to navigation. (NOAA), United States Department copies of which can be found at the In short, the USCP provides a virtual encyclopedia of local knowl- of Commerce. It is published back of each USCP. These correc- edge. Some topics in the above list, in nine volumes covering tions can also be filed electronically. such as the availability of pilotage, the Atlantic and Pacific coasts Topics covered in the USCP towage, and wharf descriptions, are (including Alaska and Hawaii), include: of particular interest to commercial and the Great Lakes and their J Coast and channel descriptions, shipping. However, the USCP pro- connecting waterways. (The area J Designated anchorages, vides useful information to the covered by the 1210-Tr chart, for J Communication frequencies, operators of recreational vessels example, is contained in the Coast (see below). As of this writing, J Drawbridge schedules and sig- Pilot, Vol. 2, Atlantic Coast: Cape NOAA is searching for ways to nals (which differ from bridge to Cod to Sandy Hook.) Coverage make the USCP more useful to bridge), diagrams and dates of latest operators of small craft. editions of the USCP are avail able J Bridge and cable clearances, Corrections to the USCP since from the NOAA web site J Currents, the date of publication are pub- (http://chartmaker.ncd.noaa.gov/). lished in the NTM (see below). J Tide and water levels, The USCP supplements the nav- Critical corrections (both important J igational information depicted on Prominent features, and time sensitive) to the USCP are the nautical chart. This information J Availability of pilotage, published on the following web site is developed in several ways, J Availability of towage, (http://critcorr.ncd.noaa.gov/crit including field inspections conduct- J corr2.htm). Critical corrections ed by NOAA, excerpts from the Weather, provide advance notice of critical NTM (discussed below), and infor- J Ice conditions, chart and USCP identified and orig-

L FIG. 10-1–Excerpt from the U. S. Coast Pilot, 30th Ed. (Emphasis Added)

10-2 Navigation Reference Publications 10

inated corrections by NOS cartog- you go through Quicks Hole (which

ST G A U O A C R

S D raphers. These corrections include appears fairly broad and deep on U CHANGES TO

A U Y X R information on hazards to naviga- the chart) the distance is only about ILIA COAST PILOT tion or other information consid- 19 miles (3.8 hours at 5 knots). Mariners who enjoy the ben- ered essential for safe navigation, What should you do? such as channel conditions, bridge A glance at the USCP (30th edi- efits of the USCP have a and cable clearances, and regulato- tion) descriptions for either responsibility to identify any ry changes. Information provided Canapitsit Channel or Robinsons errors and recommend on the critical corrections web site Hole (refer to Figure 10-1) suggests changes so that others can do not include all corrections pro- that either of these options would benefit from your experi- vided in the NM or LNM. These be a “white knuckle special,” best ence. NOAA Form 77-6 pro- publications (see below) must also left either to those with local vides a convenient way to be consulted. knowledge or the foolhardy. Quicks submit these changes. On the Hole (see Figure 10-2 for the USCP Use of the Coast Pilot: back of the form, space is excerpt) looks to be a better provided for the mariner to An Example option—in particular, the remark Examples, rather than lengthy about avoiding possibly heavy seas request additional surveys or discourse, best indicate the value of in the entrance to Vineyard Sound is chart changes and/or to pro- the USCP. Suppose, therefore, that tantalizing. However, the advice vide additional information. you are in a 32-ft. sailboat with a that sailing vessels should not The “request for surveys or 3.5-ft. draft located in the basin at attempt to pass through Quicks chart change” instructs Menemsha, on Martha’s Vineyard, Hole unless there are favorable mariners to list the area(s) and wish to travel to Fairhaven, on winds and a favorable current is for which surveys and/or the east side of New Bedford potentially unsettling because, with changes in chart format, Harbor. The auxiliary engine has an eastward current in Vineyard scale, or layout are needed. been acting up, but you have Sound, foul currents in Quicks Hole Additional information replaced the plugs and feel that the are to be expected. (Perhaps the problem is solved. Winds are out of engine cannot be relied upon.) requested by NOAA includes the south at 10 to 15 knots. The cur- Hmmm, looks like the long way information about unusually rent is setting eastward through around is not such a bad idea after strong currents; prominent Vineyard Sound. all. If the seas are ugly at the landmarks; objects which As can be seen by referring to entrance to Vineyard Sound, you provide particularly good the 1210-Tr chart, there are two can turn around and try to use the radar returns; sheltered broad options for this voyage: try to engine, or run east to some location anchorages (including direc- make passage through one of three like Tarpaulin Cove. The USCP tion of weather and type of channels (termed holes in that part would be a good place to look to bottom observed); draw- of the country) between the identify possible alternatives. Of bridge operation changes Elizabeth Islands (Canapitsit course, you could always remain Channel, Quicks Hole, or where you are and await more (e.g., drawbridge remains Robinsons Hole) or detour west and favorable conditions. permanently in open posi- pass clear to the west of Cuttyhunk The point of this example is not tion); changes in pilot pick- Island. The distance “the long way to recommend a particular course of up points; and changes in around” is at least 28 miles, equiva- action or to offer a discourse on radio frequencies monitored lent to 5.6 hours at 5 knots, but it voyage decision making, but, by pilots, marine exchanges, could be longer considering the rather, to illustrate the value of the harbor masters, or draw- eastward set of the current in USCP. It puts a wealth of informa- bridges. Vineyard Sound. Alternatively, if tion and local knowledge in your

10-3 10 Navigation Reference Publications

hands that cannot be provided by ning of each volume is a chart that ing maritime services, distress sig- the chart alone. United States Coast depicts the areas covered by this nals and communications, distress Guard (USCG) and United States and other volumes. Following this assistance and coordination proce- Coast Guard Auxiliary chart is a graphic chapter index that dures, radio navigation warnings (USCGAUX) vessels (in some identifies the geographic areas cov- and weather, information about Districts) are required to have the ered in each chapter of the specific nautical charts and aids to naviga- latest editions of these aboard, and volume consulted. The first two tion, and related information. The the above example shows you why. chapters of the USCP provide (1) chapter on navigation regulations general information and (2) naviga- contains some general information, Organization of the Coast Pilot tion regulations. As the name but focuses on regulations applica- All volumes of the USCP share implies, general information is not ble to the areas covered in this vol- a common organization. Familiarity region-specific. It includes defini- ume. Navigation regulations with this organization makes it tions of terms, an identification of include the location of COLREGS much easier to use. At the begin- U.S. government agencies provid- demarcation lines (the boundaries

L FIG. 10-2–Excerpt from the U. S. Coast Pilot, 30th Ed. (Emphasis Added)

10-4 Navigation Reference Publications 10

L FIG. 10-3–Excerpt from the Main Tables of Light List

10-5 10 Navigation Reference Publications

separating areas subject to Differential GPS (DGPS)). Iso - Isophase (Equal interval) International and Inland Rules), The “heart” of the Light List is a kHz - Kilohertz anchorage regulations, drawbridge massive series of tables that LFl - Long Flash operation regulations, ports and describe nearly every aid to naviga- Lt - Lighted waterways regulations, vessel traf- tion in the area covered by the spe- LNB - Large Navigational Buoy fic management regulations, inland cific volume. Entries in the table MHz - Megahertz waterways regulations, regulated include the name and location of the Mo - Morse Code navigation areas and limited access ATON, position (latitude and longi- Oc - Occulting areas, shipping safety fairways, tude); light color and characteristic, ODAS - Anchored Oceanographic danger zones and restricted area height and nominal range (see dis- Data Buoy regulations, and other applicable cussion below), nature of structure, Q-Quick (Flashing) regulations. Following the chapter and a catchall category titled Ra ref - Radar reflector on navigation regulations is a chap- “remarks.” Figure 10-3, for exam- R-Red ter that provides an overview of the ple, provides an excerpt from the RBN - Radiobeacon areas covered. In turn, this chapter 1998 Light List relevant to the 1210- s-seconds is followed by others that provide Tr chart. The last entry shown in this si - silent information on specific areas. The figure (No. 630) provides informa- SPM - Single Point Mooring USCP volume concludes with a tion on the Buzzards Bay Entrance Buoy series of useful tables and an index. Light. Although some of this infor- W-White Read the USCP carefully as you mation can be read from the nautical Y-Yellow prepare for your trip. Take the time chart, the Light List provides addi- Visibility of Lights to annotate your charts with partic- tional details, for example, on the The Light List provides a useful ularly relevant information and/or location, description of the struc- graph for estimating the distance of put adhesive tabs on relevant pages ture, existence of an emergency visibility of lights at night. This is in the text. light, and the pattern of sound sig- reproduced in the logarithmic chart nals. Other useful information could LIGHT LIST of Figure 10-4. The abscissa, or x include, for example, the note that axis, shows the nominal range of First published in 1838, the the ATON is seasonal, or that it the light in nautical miles. The nom- Light List is now published annual- might be removed if threatened by inal range of the light is the maxi- ly by the USCG. Seven volumes ice, the true bearings of range lights, mum distance a light can be seen in cover the United States. The area and limits to visibility. clear weather (meteorological visi- covered by the 1210-Tr chart, for The Light List employs a num- bility of 10 nautical miles). This example, is contained in Volume 1, ber of abbreviations that may be value is listed for all lighted aids to Atlantic Coast, St. Croix River, unfamiliar to the mariner. These navigation except range lights, Maine to Shrewsbury River, New abbreviations include: directional lights, and private aids Jersey. Briefly stated, the purpose A1 - Alternating to navigation. Referring to Figure of the Light List is to provide more bl - blast 10-3, the nominal range of the complete information concerning C-Canadian Buzzards Bay Entrance Light is 17 aids to navigation than can be ec - eclipse nautical miles. shown on nautical charts. In addi- ev - every The ordinate, or y-axis of Figure tion to specific information, the F-Fixed 10-4, shows the luminous range of Light List contains useful general fl - flash the light in nautical miles. The lumi- information on such topics as F1 - Flashing nous range of the light is the great- buoyage, bridge markings, and FS - Fog Signal est distance a light can be seen electronic aids to navigation Fl(2) - Group flashing given its nominal range and the (including radar, Loran-C, Global G-Green prevailing meteorological visibility. Positioning System (GPS), and I-Interrupted Figure 10-4 shows the relationship

10-6 Navigation Reference Publications 10

L FIG. 10-4–Luminous Range Diagram from Light List

10-7 10 Navigation Reference Publications

between the nominal and luminous approximately 5 nautical miles. the Light List, calculate the geo- range of the light. To convert from Neither the nominal range nor graphic range, gr. In this exam- the nominal to the luminous range, the luminous range makes any ple, the geographic range is enter the figure with the nominal allowance for the curvature of the approximately 13.5 nautical range and read upwards to the line earth, distance to the horizon, or miles. that best describes the prevailing geographic range. In other words, J Finally, calculate the actual range visibility. For example, in looking the luminous range captures only the for the Buzzards Bay Entrance effects of the power of the light and as the smaller of the luminous Light (nominal range 17 nautical the attenuation of the atmosphere. range or the geographic range. miles, read from the Light List) The geographic range is the In the above example, the lumi- when the prevailing visibility could greatest distance the curvature of nous range (5 miles) is smaller than be classified as “haze” (category 5 the earth permits an object of a the geographic range (13.5 miles), on the chart, visibility 1 to 2 nauti- given height to be seen from a par- so the luminous range would be the cal miles), the luminous range ticular height of eye without regard best estimate of the distance at to luminous intensity or visibility would be between 3 and 5 nautical which the Buzzards Bay Entrance conditions (synonym: distance to miles (the smaller figure corre- Light could be seen. the optical horizon). This concept is sponding to a visibility of 1 nautical discussed in Chapters 6 and 9. In As an aside, David Burch writ- mile, and the larger to a visibility of particular, Table 6-2 provides a con- ing in the book Emergency 2 nautical miles). To be more pre- venient basis for estimating geo- Navigation offers the following cise, if the visibility were 2 nautical graphic ranges. For example, if the simple approximation to the lumi- miles, the luminous range would be height of eye were 10 ft. on a ves- nous range, sel, and the height of the object lr = 1 + 0.1 v N,

ST G A U O A C R S D were 70 ft. (note from Figure 10-3 where lr = luminous range U CHART

A U Y X R ILIA ANNOTATIONS that the height of the Buzzards Bay (nautical miles), Entrance Light is 67 ft.), then the v = visibility (nautical As noted in other chapters, it geographic range would be approx- miles), and is a good idea to make imately 13.5 nautical miles. N = nominal range (nautical miles). numerous annotations on The procedure for estimation of the distance at which the light can the nautical chart to avoid Using this formula, the approximate be seen is as follows: luminous range in the above example having to refer to other pub- J First, read the nominal range of would be 1 + 0.1 (2)(17) = 4.4 nauti- lications. Many mariners the light from the Light List and cal miles, approximately the same as make it a habit to draw cir- note the prevailing visibility. that found from Figure 10-4. This The nominal range from the cles on charts around major equation is believed to be correct to Light List is 17 miles and the lights to indicate the approx- prevailing visibility is 2 miles in within approximately +/- 20 percent. imate distance at which these this example. Other useful information con- tained in the Light List applicable to should be visible at night. J Second, use Figure 10-4 to con- radio navigation is discussed in Other relevant information vert the nominal range to a lumi- nous range, lr, considering the Chapter 9. Light List corrections might include drawbridge prevailing visibility. In this can be found on the National operating schedules, promi- example, the luminous range is Imagery & Mapping Agency nent landmarks, sound char- approximately 5 nautical miles. (NIMA) website(http://www.nima. J acteristics of aids to naviga- Third, using estimates of the mil/) and, also, the USCG’s height of eye aboard the vessel Navigation Center (navcen) web tion, and related data. and the height of the object from site (http://www.navcen.uscg.gov/).

10-8 Navigation Reference Publications 10

NOTICE TO MARINERS Mariners are posted on the web site very latest navigational safety infor- Nautical charts and publications (http://www.notmar.com/). Other mation pertinent to all craft within are kept up-to-date through use of countries that provide Internet the area of the district. Notices of the NTM, prepared jointly by the access to the NTM include marine regattas and other important National Ocean Service and the Australia, Hong Kong, Malaysia, marine events are also announced U.S. Coast Guard. The NTM is New Zealand, Singapore, and the through the LNM. The basic differ- published weekly, and is available United Kingdom. ence between the NTM and the from NIMA in Washington, DC. LNM is that the NTM focuses on This pamphlet contains the latest ANMS information relevant to areas fre- information on navigational safety, ANMS is an acronym that quented by oceangoing vessels, changes in aids to navigation, chan- stands for Automated Notices to whereas the LNM includes infor- nels, and chart information over a Mariners System. Anyone having a mation relevant to all (including broad area useful to the oceangoing personal computer with a modem smaller) vessels. The LNM is more and coastal vessel alike. The can dial up this system (contact comprehensive than the NTM. USCPs are kept current by updating NIMA for details and the latest Information contained in the them from information contained in access numbers) and, after entering LNM includes special notices, the weekly NTMs. a user identification number (avail- discrepancies in Aids to Navigation The NTM can also be down- able on request from NIMA), (ATONs), temporary changes to loaded from the Internet. Start with access the system. The system is ATONs, chart corrections, advance the web site (http://www.nima.mil), very easy to use and can print out notice of pending changes, click on the “About NIMA” button chart corrections, Light List revi- Light List corrections, military and then the “Marine Navigation sions, Broadcast Warnings, and operations (e.g., helicopter airborne Home Page.” Follow directions to other important corrections. mine-countermeasure operations, the NTM. Once the NTM database Ultimately, ANMS is likely to be sonobuoy operations, firing exercis- is located, it can be searched by superceded by the NIMA web site. es), shoaling, construction/dredg- chart number, NTM (week and One very nice feature of ANMS ing, bridge information (e.g., year), or by a minimum bounding is that it can provide all of the cor- closures, painting, and repairs), rectangle (MBR). The MBR is rections to each chart from the date and other relevant information. specified by the northwest and of issue. This eliminates the possi- Perhaps the easiest way to obtain a southeast corners of the rectangle bility of missing an important cor- copy of the LNM is to access the using latitude and longitude. All rection because you neglected to navcen web site (http://www.navce changes that fall within this rectan- enter a given mailing. Additionally, n.uscg.gov/). Follow the directions gle are displayed and can be down- changes/corrections are first input to access the LNM for the applica- loaded. to ANMS before the NTM is print- ble USCG District. Other agencies of the U.S. gov- ed; so, with ANMS you can get Key to Abbreviaitons Used ernment provide NTM for specific information sooner. areas in electronic form. For in LNM example, the National Park LOCAL NOTICE One of the most important pur- Service (NPS) provides NTM for TO MARINERS poses of the LNM is the dissemina- Lake Powell as part of the Glen The Commander of each local tion of information on changes to Canyon National Recreational Area USCG District issues the Local aids to navigation. In this process, (http://www.nps.gov/glca/notice. Notice to Mariners (LNM)—also most Districts use abbreviations to htm). on a weekly basis. There is no save space. Although the meanings Several other countries of the charge for this publication, which of many of these are self-evident world have established Internet web may be obtained upon request from (e.g., LT EXT means light extin- sites containing NTM information. the local USCG District guished, HAZ NAV means haz- For example, Canadian Notices to Commander. The LNM contains the ardous to navigation, DEST means

10-9 10 Navigation Reference Publications

destroyed, DISC means discontin- OFF STA Off station sad truth that many mariners are ued, W/P means watching properly), PT Point either unaware of these publications the following list may be helpful: or believe that the information pro- PVT Private aid ANCH Anchorage vided in these valuable publications RACON Radar beacon isn’t worth the effort required to APP Approach RBN Radiobeacon read and update charts and other ART Articulated REDINT Operating at publications. BB Bell buoy reduced intensity Failure to pay attention to these important changes is akin to playing BKHD Bulkhead RF Range front BKW Breakwater the marine equivalent of Russian RIV River Roulette. You may be lucky; the post BNM Broadcast Notice RR Range rear on which the daymark was secured to Mariners RPTD Reported that was knocked down by a tow BY Buoy S South/southerly (and duly reported by the USCG in a CH Channel Broadcast Warning) may not pene- SHL Shoal DMGD Damaged trate your hull. And the entrance TRDBN Temporary day- buoy that you depend on for naviga- DAYBD Dayboard beacon tion may not have been blown off DBN Daybeacon TRLB Temporary lighted station by last week’s storm. (In fact, E East/easterly buoy the Brenton Reef Light on which ENT Entrance TRUB Temporary you have taken bearings throughout EMERGY LT Emergency light unlighted buoy the course is now no longer in exis- tence.) But to assume that everything F/S Fog signal TEMP Temporary is as represented in any of these GB Gong buoy TERM Terminal charts or publications, particularly HBR Harbor UIB Unlighted ice buoy when operating in unfamiliar waters IMPCHA Improper charac- W West/westerly is to take an uncalculated risk. The imprudent mariner is always sur- teristic WRK Wreck IN Inlet prised by events, while the prudent WW Waterway mariner is always prepared. INOP Inoperative IS Island BROADCAST WARNINGS NONGOVERNMENT JCT Junction Finally, the Coast Guard trans- SOURCES LB Lighted buoy mits broadcast warnings (monitor In addition to the publications channel 16) or broadcast notice to LBB Lighted bell buoy discussed above, a number of com- mariners (BNM) that provide infor- mercial firms publish cruising LGB Lighted gong buoy mation otherwise contained in the guides and related materials that are LHB Lighted horn buoy LNM that is too urgent to be left to useful to take along. The commer- LIB Lighted ice buoy a weekly publication cial guides often emphasize ser- vices available at various marinas LLNR Light List Number ADVICE YOU SHOULD HEED LWB Lighted whistle and provide interesting historical buoy Although most commercial ves- information about the areas cov- sels regularly receive NTMs (and ered. For example, commercial LNB Large navigational LNMs), the number of subscrip- guides provide information on buoy tions to these publications is very marinas such as brands of gaso- MSLD SIG Misleading signal much smaller than the number of line/diesel fuel available, types of N North/northerly registered vessels. This proves the credit cards accepted, available ser-

10-10 Navigation Reference Publications 10

vices, and amenities. Some guides nery and bombing range for mili- Additionally, other govern- contain small charts that identify tary aircraft, and a type of reform ments, such as Canada or Great particular marinas. school for troubled youths. Britain, publish guides that are use- Many cruising areas are inter- Although this information is not ful. The English publication, Ocean esting from the perspective of the essential for navigation, it does pro- Passages of the World, is viewed as tourist, and it is handy to have a vide interesting reading. a compact classic. A trip to your guide that includes this informa- Some of these guides also con- nearest source of nautical publica- tion. For example, Penikese Island tain information on tidal and other tions is definitely in order. (located in Buzzards Bay and currents, tide tables, a Nautical Finally, there are several web referred to elsewhere in this text) Almanac, or other material avail- sites that provide useful informa- was at one time a leper colony, gun- able from the government. tion, as shown on Table 10-1.

Nautical Internet Addresses of Interest as of April, 2002

USCP – Coverage diagrams and edition dates: http://chartmaker.ncd.noaa.gov

USCP – Critical and/or time sensitive corrections: http://critcorr.ncd.noaa.gov/critcorr2.htm

NIMA List of Lights: http://pollux.nss.nima.mil/pubs/NIMALOL/pubs_j_nimalol_list.html

NIMA Light List Corrections: http://pollux.nss.nima.mil/pubs/NIMALOL/pubs_j_nimalol_list.html Click the link at the bottom of the page for the current Light List Corrections

US Notice to Mariners information: http://www.nima.mil Click on “About NIMA” button and then the “Marine Navigation Home Page”

USCG Local Notice to Mariners information: http://www.navcen.uscg.gov/lnm/default.htm

USCG Navigation Center: http://www.navcen.uscg.gov

Canadian Notices to Mariners: http://www.notmar.com/eng/index.asp

Nautical Chart User’s Manual: http://chartmaker.ncd.noaa.gov/staff/NCUM/ncum.htm

Online Nautical Charts: http://anchor.ncd.noaa.gov/noaa/noaa.html

Dates of latest Nautical Charts: http://chartmaker.ncd.noaa.gov/mcd/dole.htm

These addresses are subject to change at the need and desire of the originating agency.

L TABLE 10-1–Useful Web Sites

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SELECTED REFERENCES

Borton, M. C. et al., (1989). The Complete Boating Holland, F. R., (1988). America’s Lighthouses, An Guide to Rhode Island & Massachusetts, Embassy Illustrated History, Dover Publications Inc., New Marine Publishing, Essex, CT. York, NY.

Burch, D., (1986). Emergency Navigation, International Marine Publishing Co., Camden, ME, p 205.

10-12 Chapter 11

FUEL AND VOYAGE PLANNING

“Excuses for failure attributed to shortness of coal will be closely scrutinized; and justly.”

–Mahan: Naval Strategy, 1911

INTRODUCTION tow. Running fuel tanks dry may ew things are as frustrating as create additional problems. Water Fhearing the familiar rhythmic (which has a greater density than hum of the vessel’s engine(s) inter- fuel) and dirt typically settle to the rupted by a deafening silence and bottom of the fuel tank. Drawing a realizing that the vessel has run out water-dirt mixture into the engine of fuel. Add some other elements to risks damage to injectors and other the scenario, such as deteriorating components. weather and an adverse current set- Accurate statistics on the fre- ting the vessel towards shoal waters quency of fuel exhaustion for plea- or a busy shipping channel, and sure craft are not available—in part, inconvenience quickly turns to because out-of-fuel vessels are potential disaster. Perhaps most dis- often towed in by friends or Good turbing is the fact that, in most Samaritans and not reported to the cases of fuel exhaustion, the assign- United States Coast Guard (USCG) able cause is operator error rather or state law enforcement officials. than mechanical malfunction. But, in the opinion of most experts, Neglecting to verify the amount of incidents of fuel exhaustion are sur- fuel in the tanks before setting out prisingly common. for a demonstration ride around the Aircraft also run out of fuel, harbor, a decision not to buy fuel at despite explicit regulations on the the last marina visited because of amount of fuel to be carried. high prices or lack of the proper According to available statistics, in credit card, a failure to allow for an average year more than 200 air- foul currents or poor weather, craft are destroyed and 30 people neglecting to fill the tanks in order are lost in fuel exhaustion acci- to avoid a long waiting line at the dents. In 1977, one of the worst fuel dock, etc., all sound like feeble years for this type of accident, 55 excuses while drifting under a hot persons were killed, and 60 serious- sun and waiting for an expensive ly injured. These accidents are not

11-1 PHOTO COURTESY OF POST YACHT COMPANY 11 Fuel and Voyage Planning

limited to recreational pilots. are equally useful to the mariner. J Second, the fuel consumption Commercial aircraft and airliners Several of these techniques are dis- chart includes data on the rela- also are involved in fuel exhaustion cussed in this chapter and illustrat- tionship between engine RPM accidents, albeit less frequently. On ed with actual fuel consumption and fuel consumption rate, mea- 25 January 1990, Avianca Flight 52 data for recreational boats. sured in gallons per hour inbound to New York from Bogota, The material in this chapter pre- (GPH). Table 11-2 shows these Colombia, crashed, killing 72 per- sents a systematic view of fuel data for the same vessel. sons. The National Transportation planning and management for ves- Possible sources of these data Safety Board (NTSB) concluded sel operators. Practical tips on this are discussed below. that fuel exhaustion was the proba- important subject complement J Third, data from these two ble cause of this accident. Earlier in development of fuel consumption sources are combined into a fuel 1978, a United Airlines DC-8 curves and related material. planning worksheet, shown in crashed near Portland, Oregon, Table 11-3. (Sources and meth- after running out of fuel, killing 10 BEGINNINGS: A FUEL ods for obtaining these data are of 189 persons on board. CONSUMPTION CHART One of the reasons for dis- discussed later in this chapter.) Careful fuel planning and man- cussing aircraft-related fuel inci- The fuel-planning worksheet agement should be an integral part dents in a text on marine navigation presents basic data on the throt- is that aircraft manufacturers, air- of powerboat navigation. A basic tle setting (RPM), speed through line operators, and pilots have input to the fuel planning process is the water (STW), and the fuel developed systematic approaches to the vessel’s fuel consumption chart consumption rate (GPH). In fuel planning and management that or curve. This curve ties together Table 11-3, no current (either the vessel’s speed curve with fair or foul) is assumed; so the engine fuel consumption informa- speed of advance (SOA) is

ST G A U O A C R

S D U What YOU tion. numerically equal to the STW at A U Y X R ILIA WILL LearN IN J First, the fuel consumption chart any throttle setting. The other thIS Chapter includes the relationship columns are discussed below. between the engine throttle set- To facilitate following the calcu- J How to construct and ting in revolutions per minute lations in this chapter, several sig- interpret a fuel (RPM) and the vessel’s speed nificant figures are retained for consumption chart through the water (STW) in each entry in the tables. However, the reader should bear in mind that J How to make efficiency, knots, the familiar speed curve range and endurance for the vessel discussed in fuel consumption data are seldom calculations Chapter 5. An illustrative speed exact—error margins of ±10 per- curve for a semi-displacement cent, or even more, should be J How fuel efficiency hull vessel, Aventura, is shown attached to these figures. This varies with speed for dis- in Table 11-1 (Laudeman, 1986). injunction is particularly applicable placement and planing RPMs to be used in the develop- to manufacturer’s test data—not hull vessels ment of this curve vary from the because of any deliberate attempt to inflate performance statistics, but J Sources of fuel consump- lowest attainable (clutch speed) tion information to the maximum recommended rather because these data are often continuous power setting. developed under ideal test condi- J How to make time-fuel According to the data given in tions. Therefore, it is appropriate to efficiency tradeoffs Table 11-1, the vessel’s clutch round off actual fuel–planning cal- culations to within this margin. J How to make a speed is only 1 knot. For many “howgozit” chart boats, clutch speeds are signifi- cantly higher.

11-2 Fuel and Voyage Planning 11

ST G A U O A C R

S D

U SINGLe-

A U Y X R ILIA eNGINe OperatION WIth tWIN eNGINe BOatS For twin engine vessels, it is useful to develop speed curves and fuel consumption L estimates for both single- TABLE 11-1–Illustrative Speed Curve for Vessel Aventura and twin-engine operations. In this way, you can be pre- pared with relevant data in The Fuel Consumption Chart Combines the case of engine failure. Check Vessel’s Speed Curve with Engine Fuel Consumption Data with the manufacturer before attempting to develop a single-engine speed curve, VESSEL’S ENGINE’S FUEL CON- SPEED FUEL BURN SUMPTION however. Typically, manu- CURVE RATE CHART L TABLE 11-2–Relationship facturers impose limits on Between RPM and the maximum RPM to be Fuel Consumption used for single-engine opera-

FUEL EFFICIENCY, RANGE, Therefore, the fuel efficiency is 7 allow for unusable and reserve fuel AND ENDURANCE nautical miles per hour/5 gallons as discussed below. Thus, if a 10 Before discussing the sources of per hour = 1.4 MPG. Fuel efficien- percent fuel reserve is assumed, the these data, it is instructive to exam- cy figures at this and other throttle fuel capacity is reduced by this ine the uses of this information. settings are given in the fifth col- amount [0.1 (200) or 20 gallons in From these basic data, it is possible umn of Table 11-3. this example], leaving 180 gallons to calculate other quantities that are Another important fuel planning “voyage” or “en route” fuel. The important in fuel planning. One factor is the range of the vessel. The estimated range is calculated by important measure is the fuel effi- range is the distance in nautical multiplying the en route fuel (gal- ciency, defined as the distance the miles that a vessel can travel with lons) by the fuel efficiency (MPG). vessel can travel on each gallon of the fuel available. The range To continue the above example, the fuel. It is measured in nautical miles depends, inter alia (among other range of this vessel at a throttle set- per gallon (MPG) and is calculated things), upon the throttle setting. In ting of 2000 RPM is the en route as the speed of advance (knots) Table 11-3, the fuel capacity of the fuel (180 gallons) times the fuel divided by the fuel consumption vessel is given as 200 gallons, and efficiency (1.4 MPG) or 252 nauti- rate in GPH. Thus, for example, this capacity could be used for cal miles, assuming a 10 percent referring to Table 11-3, at 2000 range estimates. More typically, a fuel reserve. RPM the SOA is 7 knots and the contingency factor of 10 percent, The vessel’s fuel endurance is the fuel consumption is 5 GPH. 20 percent, or more is assumed to length of time in hours that the ves-

11-3 11 Fuel and Voyage Planning

VESSEL: AVENTURA FUEL CAPACITY: 200 GALLONS FOUL CURRENT: 0 KNOTS

Speed thrU Speed Of fUeL eStImated raNGe eStImated eNdUraNCe thrOttLe the Water advaNCe CONSUmptION fUeL IN NaUtICaL mILeS IN hOUrS SettING StW SOa rate effICIeNCY WIth fUeL reServe WIth fUeL reServe (rpm) (KNOtS) (KNOtS) (Gph) (mpG) 10% 20% 10% 20% 1000 1.0 1.00 0.33 3.03 545 485 545 485 1250 2.0 2.00 0.80 2.50 450 400 225 200 1500 3.5 3.50 1.80 1.94 350 311 100 89 1750 5.0 5.00 3.30 1.52 273 242 55 48 2000 7.0 7.00 5.00 1.40 252 224 36 32 2250 8.0 8.00 6.70 1.19 215 191 27 24 2500 9.0 9.00 9.00 1.00 180 160 20 18 2750 9.5 9.50 11.90 0.80 144 128 15 13 3000 10.0 10.00 16.00 0.63 113 100 11 10 L TABLE 11-3–Illustrative Fuel Planning Worksheet sel can be operated at a given throt- rates than assumed in the fuel con- Part 91 of the Federal Aviation tle setting until the en route fuel is sumption curve, the necessity to Regulations (FARs) under instru- exhausted. The endurance is given divert to an alternate destination ment flight rules (IFR) are required by the en route fuel (total fuel less due to adverse weather, an unantic- under FAR 91.23 to carry sufficient reserve) divided by the fuel con- ipated request to tow another ves- fuel (considering weather reports sumption rate. For example, the sel, unanticipated adverse currents, and forecasts, and actual weather endurance of this vessel at a throttle and the need to stand off a poten- conditions) to complete the flight to setting of 2,000 RPM is the en route tially dangerous inlet until condi- the airport of intended landing, fly fuel (180 gallons) divided by the fuel tions improve. There are no hard- from that airport to an alternate air- consumption rate (5 GPH), which is and-fast rules for deciding on the port, and fly after that for 45 min- equal to 36 hours (10 percent fuel appropriate fuel reserve. For a voy- utes at normal cruising speed. reserve). Table 11-3 displays these age in familiar waters where cur- The above rules or guidelines calculations for both a 10 percent rents are generally predictable, en are offered for illustration only; the and 20 percent fuel reserve at vari- route fueling opportunities are navigator is responsible for select- ous power settings. The reader abundant, seas are calm, and the ing the most appropriate fuel should repeat the calculations shown weather outlook is good, a reserve reserve, considering the circum- in Table 11-3 to ensure familiarity of only 10 percent might be ade- stances of the voyage. with the various quantities and how quate. In less ideal conditions, a they are calculated. larger reserve would be prudent. FUEL EFFICIENCY Reserve margins of 20, 30, or even VERSUS SPEED WHY FUEL RESERVES? 50 percent might be justified in The simple calculations dis- In the above numerical exam- more adverse circumstances. played in Table 11-3 provide useful ples, the term fuel reserve is used. Many mariners are taught a sim- information for fuel planning deci- The fuel reserve is just that—a ple “one-third” rule for fuel plan- sions. In particular, these calcula- reserve or set-aside to allow for ning: plan to use one-third of the tions show that (for this vessel) contingencies or unforeseen cir- fuel on the outbound voyage, one- increases in the throttle setting cumstances that could arise in a third for the return, and keep one- (and, hence, speed) are associated voyage. Possible contingencies third in reserve. with decreases in fuel efficiency could include greater fuel-burn Aircraft pilots operating under and range. In this example, depend-

11-4 Fuel and Voyage Planning 11

(the theoretical number is 1.34) the square root of the water line length in feet. For example, the hull speed of a vessel with a 25-foot length at the water line would be approxi- mately 1.35 times 5, or 6.8 knots. Examples of displacement hull ves- sels include sailboats, motorsailers, many trawlers, and most merchant vessels. The constant, 1.35, is only approximate and depends upon the hull design. Values between 1.25 and 1.5 are reported for displace- ment hull vessels. As can be seen in Figure 11-1, in each case, extra speed is attained only at the L expense of decreased fuel efficien- FIG. 11-1–For displacement or semi-displacement hulls, fuel efficiency is highest cy (and range) for displacement or at low speeds. semi-displacement vessels. ing upon the throttle setting and, article in Boating magazine Hull speed serves as a physical therefore, how fast the vessel is [Berrien, 1987].) Recall that a ves- limit in the case of a true displace- operated, the range (assuming a 10 sel with a displacement hull ment-hull vessel. Installation of a percent reserve) can vary by nearly achieves its buoyancy by displacing more powerful engine, or adding a a factor of five from only 113 nau- a volume of water equal to the ves- second engine, to vessels with this tical miles at a speed of 10 knots to sel weight (as loaded). The maxi- hull design will not result in speeds 545 nautical miles at a speed of 1 mum speed, or hull speed, for a true appreciably greater than the hull knot! For the example given in displacement vessel in knots is speed. The additional power simply Table 11-3, a throttle setting of approximately equal to 1.35 times creates a larger bow wave, causes 1,000 RPM maximizes both range and endurance. That is, there is no other throttle setting for which Aventura’s range or endurance is higher. Typically, however, the throttle settings for maximum range and maximum endurance are not the same. The relationship between fuel efficiency and throttle setting shown in Table 11-3 is broadly characteristic of displacement or semi-displacement hull vessels. Figure 11-1 shows fuel efficiency versus speed curves for a small, but representative, sample of displace- ment hull or semi-displacement hull L vessels including trawlers or trawler-yachts. (The data for the FIG. 11-2–Vessels with planing hulls may increase fuel efficiency when “on plane”. Grand Banks 46 are taken from an Boat-specific differences are important.

11-5 11 Fuel and Voyage Planning

and fuel efficiency falls off. The top speed of a planing ves- sel is determined by the hull design, propulsion, and planing angle. Installation of larger engines in a planing vessel will increase the top speed (for a fixed hull design), but not in direct proportion to the increase in horsepower. Several the- oretical and empirical equations have been proposed for the speed- horsepower relationship (see the references at the end of the chap- ter). Although these differ some- what, most indicate that the top speed varies with horsepower raised to an exponent between approxi- mately 0.33 to 0.55—that is, if the engine horsepower were to double, L the top speed would increase by FIG. 11-3–Fuel burn rate and efficiency for the same boat with two power plants. only 25 to 50 percent. The attain- ment of significantly faster speeds for planing hulls depends upon hull the stern to sink deeper in its wake, fuel efficiency first decreases as design and powerplant selection and sharply increases fuel con- speeds are increased within the dis- jointly. Nonetheless, unlike the situ- sumption. Typically, the normal placement range and then increases ation with displacement-hull ves- cruising speed for a displacement- at speeds above hull speed when the sels, adding power to a planing-hull hull vessel is slightly lower (say, 10 vessel comes “unstuck” and water vessel can increase the top speed. percent or so) than the theoretical drag is reduced. In these cases there At the other end of the speed hull speed. is more than one speed with range, planing-hull vessels can be Adding a second engine to a dis- increased range. Figure 11-2 shows operated at displacement speeds. placement-hull vessel confers many fuel efficiency curves for a sample Operation at these lower speeds benefits (chiefly, increased reliabil- of vessels with planing hulls as often (but not always, see Figure ity and maneuverability), but nei- taken from various issues of 11-2) results in increased fuel effi- ther speed nor fuel efficiency is Boating magazine given in the ref- ciency. Advocates of planing hull improved. Typically, the twin- erences at the end of this chapter. In cruisers for long distance voyages engine version of a displacement- each case, there is a secondary are quick to point this out. hull vessel will burn almost twice power setting for increased fuel However, for vessels of comparable the fuel of the single-engine version economy that is above hull speed. length and overall size, a vessel and be only slightly faster. Range, But substantial boat-to-boat differ- with a planing hull will generally be therefore, is lower compared to the ences are evident—even in this less fuel-efficient than a “similar” single-screw version. Trawlers small sample—so that it is difficult one with a displacement hull. The designed for long distance voyages to provide useful rules of thumb. difference in fuel efficiency are typically equipped with a sin- Nonetheless, it is common for fuel between the two vessels, which gle-screw diesel engine. efficiency to increase above planing could range from 20 to 40 percent For vessels with hulls that can speeds. As speed is increased still or more, is the penalty paid for the get on plane, it is often found that further, drag forces again increase flexibility of being able to get on

11-6 Fuel and Voyage Planning 11

plane and attain higher speeds. What is clear and relevant, how- include published test results and There is little doubt that the full dis- ever, is that the fuel efficiency of a measurements on the actual vessel. placement vessel is optimal for long diesel engine is typically greater Published Test Results distance cruising. than that for a comparable gasoline Incidentally, skippers who engine. (The word “typically” is There are numerous sources of habitually run turbo-charged engine used here. Not all diesels are more fuel consumption information of equipped vessels at displacement fuel-efficient. Some turbocharged the kind displayed in Table 11-3. speeds should check with the vessel diesels have comparable fuel effi- Vessel owner’s manuals sometimes or engine manufacturer to deter- ciency to gasoline engines.) Figure provide these data. Additionally, mine the proper procedures to 11-3 shows relevant comparisons several periodicals, such as Boating ensure long engine life at these based on data presented by Kreisler and Power & Motoryacht maga- speeds. One source recommends (1995). The two solid curves show zines, offer compilations of test that turbo-charged engines should the relation between STW and fuel data for newer vessels. Such boat be run up to 1800 RPM periodical- burn rate for a Bertram 33 powered tests are often an excellent source ly during a voyage to avoid engine by twin gasoline inboards (markers of data. However, the reader should damage. are the open circles) or twin diesels note the conditions of the test, par- References given at the end of (markers are the open squares). The ticularly the loading and reported this chapter provide a theoretical dis- two dashed curves show the corre- sea conditions. As noted below, cussion of fuel consumption for dis- sponding fuel efficiencies. The these can have an important effect placement- and planing-hull vessels. dashed line marked by the closed on fuel efficiency. These data offer circles represents the gasoline-pow- guidance, but should not be used GAS VERSUS DIESEL ered version, that with the closed unless validated to estimate fuel squares, the diesel-powered version. consumption for your vessel, even The speed curve and the fuel if similarly equipped. consumption curve for a vessel As can be seen, the fuel efficiency depend upon the type of engine of the diesels is greater—and dra- Measured Data installed. Typically, diesel engines matically so for low speeds. Measurement of fuel consump- are more fuel-efficient than gaso- Powered with these diesels, this ves- tion can be accomplished in a vari- line engines and diesel fuel costs sel has very “long legs” (can travel ety of ways. Commercial fuel flow less than gasoline. Modern turbo- much farther). meters, sometimes integrated into charged diesels are often faster, as Figure 11-4 shows fuel efficien- microprocessor-based fuel manage- well. Diesels are typically heavier cy estimates (Gorant, 1995) for a ment systems, are probably best for and substantially more expensive Luhrs T-320 open sportfish avail- this purpose. Another advantage of than gasoline engines. Whether able with three twin-engine inboard a fuel flow meter is that instanta- diesels are a cost-effective choice powerplants. The solid black line neous measurements are available, (i.e., whether the annual savings in (filled square markers) shows the which means that the most efficient fuel cost is greater than the annual- fuel efficiency with a gasoline power setting can be determined ized capital cost increment for the inboard and the dashed lines the while underway and adjusted as cir- diesel) depends upon the estimated fuel efficiency of the same vessel as cumstances dictate. These systems usage. Several references (e.g., powered with two different diesels. (particularly for diesel-powered Moss, 1981; Kreisler, 1995; and Again, the fuel efficiency of the twin-engine boats) are expensive, West, 1975) provide relevant data diesels is greater at all speeds, but however. and economic analyses applicable dramatically so at low speeds. A simpler, although less accu- to initial purchase or repowering. rate, method is to fill the tank, run Discussion of the merits of alterna- SOURCES OF the vessel at a constant RPM for, tive powerplant selection and/or FUEL CONSUMPTION say, half an hour, and then measure repowering is beyond the scope of a INFORMATION the amount of fuel it takes to refill text on navigation. Sources of fuel consumption data the tank to its original level. This

11-7 11 Fuel and Voyage Planning

of thumb recommended in older texts is to reduce the speed corre- sponding to any RPM by approxi- mately 1 percent for each month since the vessel was out of dry dock. Rather than use this approxi- mation, it is preferable to recali- brate the speed curve and update the fuel consumption table periodi- cally. The weight of the vessel (and how this weight is distributed) can also affect the speed curve. Devotees of so-called predicted log races (where even seconds matter) make corrections to the speed curve to account for the change in the ves- sel’s weight as fuel is burned. Adjustments to the speed curve to L reflect fuel burn are generally not FIG. 11-4–Fuel burn rate and efficiency for the same boat with three different significant for most fuel planning power plants. purposes, however. The installation and positioning process can be repeated at other cruise RPM, and the maximum of trim tabs also affects the vessel’s throttle settings to produce data continuous-duty RPM. speed curve and fuel economy. Data such as are shown in Table 11-2. on the effectiveness of trim tabs can Even if data are available from FACTORS THAT AFFECT be found in Moss (1981). Ensure manufacturer’s literature or period- FUEL EFFICIENCY that the trim tabs (if installed) are icals, it makes sense to check Hull design, powerplants, and set to the correct position when the (audit) some of the data points to throttle setting are important deter- speed curve is determined. ensure that these are appropriate for minants of fuel efficiency and Engine ventilation and engine- the vessel. The accuracy of this range. Table 11-4 shows several compartment temperature (West audit is one of the factors that other factors that are also impor- (1975)) are also important determi- should be used in determining a tant. Some of these are discussed nants of fuel efficiency. safe reserve. If, for example, the below. Many of these factors are Some of the factors listed in fuel consumption for a vessel was self-evident, such as vessel hull Table 11-4 are subtle, such as vessel 10 percent higher than published design, condition of hull, throttle trim and the ability of the person at data indicate, the curve should be setting (RPM), engine tuning, and the helm to maintain a constant adjusted by this amount and/or an engine horsepower. course. Environmental conditions adequate reserve included. The diameter and pitch of the such as wind and sea conditions or Unless the vessel is equipped propellers can affect both speed and currents are also important. The with a fuel flow meter, it is tedious fuel consumption, as can the bal- effect of currents is discussed and time consuming to check fuel ance of the propellers. below. Winds affect powerboats as consumption data for each possible The condition of the hull can well as sailing craft—in part, by throttle setting. A practical compro- affect the relationship between direct action (e.g., increased drag or mise would be to check only key RPM and speed (and, hence, fuel leeway) and, also, because winds points in the chart, such as the max- efficiency) quite significantly. can induce fair or foul currents. imum range RPM, the normal A rough and time-honored rule Finally, sea conditions are impor-

11-8 Fuel and Voyage Planning 11

tant. It is found for large and small craft alike that SOAs decrease as  Vessel Hull Design wave heights increase–even if the  same throttle setting is maintained. Condition of Hull Additionally, it is often necessary to  Engine Type, Horsepower, and Condition reduce engine RPMs or alter course  Engine Throttle Setting (RPM) when wave heights increase in order to ensure a more comfortable  Pitch and diameter of propellors ride. Some of these effects—such  Weight (Load) and Weight Distribution (Balance) as RPM, hull design, and engine  Trim Settings variables—are considered explicit- ly in the fuel-consumption chart.  Fair or Foul Currents But other factors, such as trim, sea  Wind and Sea Conditions state, and prevailing winds, are not.  Ability of Helmsman to Maintain Course For this and other reasons noted above, a prudent planner makes allowance for these contingencies L and maintains an adequate fuel TABLE 11-4–Factors that Affect Fuel Efficiency reserve.

EN ROUTE TIME-THROTTLE Table 11-3 to show the time lons) at the start of the trip, this SETTING TRADE-OFFS required to reach the destination would leave 200 - 40 = 160 gallons Returning to the example given and the estimated fuel, time, and remaining in the tanks, as shown in in Table 11-3, this section shows cruising distance remaining when column 7 of Table 11-5. This how these data can be used in a the destination is reached. This fuel remaining fuel (160 gallons) would practical manner for improved fuel remaining must include fuel for allow the vessel to cruise at 1000 management and voyage planning. entry into the harbor, additional RPM for 160/0.33 or approximately The fact that, with displacement- maneuvering, and a contingency for 486 hours, and to travel an addi- hull vessels, high cruising ranges foul currents, adverse weather or tional 486 hrs x 1 MPH = 486 nau- can only be achieved at low speeds sea conditions, waiting outside an tical miles as shown in columns 8 does not mean that low speeds are inlet for favorable passage condi- and 9 of Table 11-5. always desirable. Not all voyages tions, diversion en route, etc. Note, But the speed (1 knot) of are long enough to require great first, that at a throttle setting of advance at this throttle setting is so range, and sometimes fuel stops can 1000 RPM the fuel remaining at the slow that 120 hours would be be found even on long voyages. The destination is maximized—only required for the trip! Alternatively, fuel consumption chart provides approximately 40 gallons en route at a throttle setting of 3000 RPM essential information for speed- fuel is consumed. To see this, note corresponding to a STW of 10 range tradeoff analysis. that the SOA (assuming no current) knots, the 120 M trip will take only One way to make use of the is 1 knot, so it would take 120 hours 12 hours; but fuel consumption is information given in the fuel con- (120 M/1 MPH) for the trip as so high (192 gallons) that the tanks sumption chart is to reformat it as shown in column 5 of Table 11-5. would be virtually dry upon arrival, shown in Table 11-5. In this exam- The fuel required for this trip equals leaving no margin for error. ple, a trip of 120 nautical miles is the trip time (120 hours) multiplied Compared to this plan, a compro- being planned. For purposes of this by the fuel consumption rate (0.33 mise throttle setting of, say, 2250 illustration, the possibility of inter- GPH) or approximately 40 gallons RPM would add only 3 hours to the mediate fuel stops is not consid- as shown in column 6 of Table 11- trip but would increase the estimat- ered. Table 11-5 repeats the data in 5. Assuming full tanks (200 gal- ed fuel on board at the destination

11-9 11 Fuel and Voyage Planning

VESSEL: AVENTURA FUEL: 200 GALLONS ON BOARD DISTANCE: 120 NAUTICAL MILES CURRENT 0 FOUL CURRENTS ARE NEGATIVE

fUeL tIme tO fUeL fUeL reqUIred trIp remaINING exhaUStION reServe thrOttLe CONSUmptION trIp fUeL at at raNGe at SettINGS StW SOa rate dUratION reqUIred deStINatION deStINatION deStINatION (rpm) (KNOtS) (KNOtS) (Gph) (hOUrS) (GaLLONS) (GaLLONS) (hOUrS) (mILeS) 1000 1.0 1.00 0.33 120.0 40 160 486.1 486 1250 2.0 2.00 0.80 60.0 48 152 190.0 380 1500 3.5 3.50 1.80 34.3 62 138 76.8 269 1750 5.0 5.00 3.30 24.0 79 121 36.6 183 2000 7.0 7.00 5.00 17.1 86 114 22.9 160 2250 8.0 8.00 6.70 15.0 101 100 14.9 119 2500 9.0 9.00 9.00 13.3 120 80 8.9 80 2750 9.5 9.50 11.90 12.6 150 50 4.2 40 3000 10.0 10.00 16.00 12.0 192 8 0.5 5

Note: Effects of current not considered in this run L TABLE 11-5–Time-Throttle Setting Tradeoffs Illustrated to nearly 100 gallons—enough to The appropriate choice of RPM current can either increase or travel nearly 120 additional miles at depends upon many factors (e.g., decrease the estimated SOA relative this throttle setting. how much reserve is deemed pru- to the STW. The choice between throttle set- dent, time schedules, etc.), but the Currents are important to fuel tings of 2250 RPM and 3000 RPM value of a systematic analysis of planning for two principal reasons: is, in reality, no choice at all. It time/throttle setting trade-offs J Currents can increase or would be sheer folly to attempt this should be clear. All that is required decrease the range attainable at voyage at 3000 RPM without a fuel for this analysis is the vessel’s basic any throttle setting (RPM). Fair stop. And, even if a fuel stop were fuel consumption chart and a few (i.e., following) currents feasible along the route of this voy- additional computations. age, the additional time required for the fuel diversion (queuing at the EFFECT OF fuel dock, fueling, and returning to CURRENT the original track) would have to be Currents can be very added to the voyage time at a throt- important to fuel plan- tle setting at 3000 RPM for a valid ning, and information portal-to-portal time comparison. If on current set and drift the delay occasioned by the fuel stop should be considered, if available. As is noted in were, for example, 1-1/2 hours, the Chapters 5 and 7, cur- total time difference would shrink to rents may alter the 1-1/2 hours (13.5 hours compared to required course to

15 hours). As this example clearly GUARD COAST COURTESY OF UNITED PHOTO STATES maintain a desired shows, there is generally little pur- L track, and the fair or The only time there is too much fuel on board is pose in running displacement-hull foul component of the vessels at maximum speed. when the vessel is on fire.

1Data used in this illustration are taken from the original source. Generally, however, the STW at the lowest feasible throttle setting (the so-called “clutch speed”) is greater than 1 knot—often 5 or 6 knots.

11-10 Fuel and Voyage Planning 11

VESSEL: AVENTURA FUEL CAPACITY: 200 GALLONS FOUL CURRENT: 2 KNOTS

Speed thrU Speed Of fUeL eStImated raNGe eStImated eNdUraNCe thrOttLe the Water advaNCe CONSUmptION fUeL IN NaUtICaL mILeS IN hOUrS SettING StW SOa rate effICIeNCY WIth fUeL reServe WIth fUeL reServe (rpm) (KNOtS) (KNOtS) (Gph) (mpG) 10% 20% 10% 20%

1000 1.0 -1.00 0.33 UNDEFINED INFEASIBLE INFEASIBLE 545 485 1250 2.0 0.00 0.80 0.00 INFEASIBLE INFEASIBLE 225 200 1500 3.5 1.50 1.80 0.83 150 133 100 89 1750 5.0 3.00 3.30 0.91 164 145 55 48 2000 7.0 5.00 5.00 1.00 180 160 36 32 2250 8.0 6.00 6.70 0.90 161 143 27 24 2500 9.0 7.00 9.00 0.78 140 124 20 18 2750 9.5 7.50 11.90 0.63 113 101 15 13 3000 10.0 8.00 16.00 0.50 90 80 11 10

L TABLE 11-6–Illustrative Fuel Planning Worksheet

increase the range, foul (head) current rules out operation at throt- the range is 2000 RPM. In general, currents decrease the range. tle settings less than 1500 RPM (2- it can be shown in fuel consumption J Currents can alter the optimal knot STW) if a positive SOA is curves typical of displacement ves- throttle setting for maximum desired. But, in this instance, unlike sels, foul currents increase the RPM range. Foul currents increase the that for the zero current calcula- for maximum range. Table 11-7 and tions, the maximum range does not Figure 11-5 show how the optimal optimal power setting. occur at the lowest feasible RPM, throttle setting and resulting fuel These points are illustrated in nor are the throttle settings identical efficiency vary with the strength Table 11-6, which builds upon the for maximum range and maximum (drift) of the foul current. data given in Table 11-3 except that endurance. As shown in Table 11-6, Table 11-7 shows the results of a 2-knot foul current is assumed. the throttle setting that maximizes varying the current from 5 knots The SOA is 2 knots less than the STW in this instance, but the calcu- lations are otherwise identical to those in Table 11-3. For example, consider a throttle setting of 1750 RPM. The STW at this RPM is 5 knots but, because a 2-knot foul current is assumed, the SOA is only 3 knots. Fuel consump- tion at this RPM is 3.3 GPH; so the fuel efficiency at this throttle setting is 3/3.3 = 0.91 MPG (shown in Table 11-6) rather than the 5/3.3 = 1.52 MPG shown in Table 11-3. The range with 10 percent fuel reserve is 180 times 0.91 or 164 miles, rather L than 273 miles, nearly a 40 percent FIG. 11-5–How optimal throttle setting and resulting fuel efficiency vary with foul reduction. Obviously, a 2-knot foul currents for vessels with displacement hulls.

11-11 11 Fuel and Voyage Planning

OptImaL OptImaL OptImaL raNGe thrOttLe OptImaL OptImaL fUeL WIth 10% CUrreNt drIft SettING StW SOa effICIeNCY reServe deSCrIptION (KNOtS) (rpm) (KNOtS) (KNOtS) (mpG) (mILeS) Fair 5.00 1000 1.0 6.00 18.18 3273 Fair 4.00 1000 1.0 5.00 15.15 2727 Fair 3.00 1000 1.0 4.00 12.12 2182 Fair 2.00 1000 1.0 3.00 9.09 1636 Fair 1.00 1000 1.0 2.00 6.06 1091 None 0.00 1000 1.0 1.00 3.03 545 Foul -0.25 1000 1.0 0.75 2.27 409 Foul -0.50 1250 2.0 1.50 1.88 338 Foul -0.75 1250 2.0 1.25 1.56 281 Foul -1.00 1500 3.5 2.50 1.39 250 Foul -1.25 1500 3.5 2.25 1.25 225 Foul -1.50 1500 3.5 2.00 1.11 200 Foul -1.75 2000 7.0 5.25 1.05 189 Foul -2.00 2000 7.0 5.00 1.00 180 Foul -2.25 2000 7.0 4.75 0.95 171 Foul -2.50 2000 7.0 4.50 0.90 162 Foul -2.75 2000 7.0 4.25 0.85 153 Foul -3.00 2000 7.0 4.00 0.80 144 Foul -3.25 2000 7.0 3.75 0.75 135 Foul -3.50 2000 7.0 3.50 0.70 126 Foul -3.75 2000 7.0 3.25 0.65 117 Foul -4.00 2000 7.0 3.00 0.60 108 Foul -4.25 2250 8.0 3.75 0.56 101 Foul -4.50 2250 8.0 3.50 0.52 94 Foul -4.75 2250 8.0 3.25 0.49 87 Foul -5.00 2250 8.0 3.00 0.45 81 L TABLE 11-7–Optimal Power settings and Associated Information, as a Function of the Drift of the Assumed Current fair to 5 knots foul. For each case, calculations for each vessel to be near Philadelphia, Pennsylvania. the optimal RPM is determined as operated because the results will Fortunately, a United States Coast well as the resulting fuel efficiency differ from vessel-to-vessel. This Guard Auxiliary (USCGAUX) and range. Although these calcula- example however, shows that cur- patrol vessel was in the immediate tions are tedious, they are not com- rent affects can be substantial—par- vicinity and managed to get a line plex and are readily programmed ticularly for displacement vessels. on and towed the disabled cabin into scientific calculators or person- Many mariners have a qualitative cruiser out of the path of an oncom- al computers (PCs). Once complet- idea of the affect of an adverse cur- ing tanker. When asked how it came ed, the results of these calculations rent on the vessel’s range, but have to be that the cabin cruiser had run are easy to interpret and use. not made the types of calculations out of fuel, the distraught skipper Note that, in this example, even shown above and, so, lack a responded, “I don’t understand it. I a small adverse current can substan- quantitative understanding. In one filled the tanks yesterday morning tially decrease the maximum range; Search and Rescue (SAR) case, for and traveled down to the Delaware a 1-knot adverse current reduces the example, a small cabin cruiser ran Bay and back—a trip I’ve made maximum range by more than 50 out of fuel in the middle of the chan- before without needing fuel en percent! It is useful to make these nel of the Delaware River, route.” Reconstruction of the details

11-12 Fuel and Voyage Planning 11

Before defining these terms, it is useful to imagine an “out and back” voyage from a port of origin with- out the possibility of refueling en route—for example, on an offshore fishing trip from the coast of New Jersey to the Hudson Canyon. The radius of action is the great- est distance (out) that may be trav- eled and still leave sufficient fuel aboard to return to the port of ori- gin without touching the fuel reserve. The point of no return is the point beyond which there is insufficient fuel to return using the L entire fuel reserve. Mathematically, FIG. 11-6–Assumed currents can substantially affect speed–fuel consumption the radius of action, RA, is given by trade-offs for displacement or semi-displacement hull vessels. the equation, RA = F(1-R)/G([1/SOAo] + [1/SOAr]) of the voyage showed that, on this 11-5, except that a foul current of 2 where, occasion, the run down and back on knots is assumed. The range of the river was timed unfortunately— throttle-setting options is more RA = radius of action (nautical miles), F = fuel capacity (gallons), the cruiser had foul currents on both restricted—1000 RPM and 1250 R = fuel reserve (fraction), legs. (See Chapter 8 for a discussion RPM are not feasible, and a 3000- G = fuel consumption rate (gallons per hour), of how to select starting times to RPM throttle setting guarantees SOAo = speed of advance out (knots), and avoid this situation.) After-the-fact fuel exhaustion before the destina- SOAr = speed of advance on return leg (knots). range calculations for this vessel tion is reached. The attractiveness The distance to the point of no indicated that the effective range of operation at 2250 RPM (a possi- return, Dponr, is calculated using given a 2-knot foul current was ble compromise) is reduced; en the same equation given above, reduced by 50 percent in compari- route time has been increased from except that the reserve, R, is set son to the no-current condition. 15 hours to 20 hours and the cruis- equal to zero for this computation. Additionally, because the trip was ing reserve decreased from nearly In the special case where there is no longer, the skipper ran at a higher 120 miles to 60 miles—a substan- current and the SOAo and SOAr are than customary throttle setting to tial effect from what is arguably a each equal to the STW, the radius of complete the voyage during the day- modest current. Figure 11-6 shows action is equal to the range divided light hours. As it was, the skipper this graphically. by two. nearly made it, the cruiser was only To illustrate, suppose that the a few miles from homeport when RADIUS OF ACTION vessel Aventura (for which fuel the engine quit. This skipper now AND RELATED CONCEPTS consumption and capacity data are has fuel consumption charts (of the (This section is more technical given in Table 11-3) is taken on a type shown in Tables 11-3 and 11-6) and can be omitted without any loss trip and a throttle setting of 2250 pasted into the vessel’s log! of continuity. This material is not RPM is selected. The STW corre- Current considerations are also covered in the final examination.) sponding to this throttle setting is important to the kinds of trade-offs Aircraft pilots employ two other 8.0 knots (see Table 11-3). Suppose considered in Table 11-6. Table 11- fuel-planning concepts that are also also that current sailing calculations 8, for example, shows similar calcu- relevant to the mariner, the radius of (discussed in Chapter 7) employing lations to those displayed in Table action and the point of no return. estimated set and drift (discussed in

11-13 11 Fuel and Voyage Planning

VESSEL: AVENTURA FUEL: 200 GALLONS ON BOARD DISTANCE: 120 NAUTICAL MILES CURRENT 2.00

fUeL tIme tO fUeL fUeL reqUIred trIp remaINING exhaUStION reServe thrOttLe CONSUmptION trIp fUeL at at raNGe at SettINGS StW SOa rate dUratION reqUIred deStINatION deStINatION deStINatION (rpm) (KNOtS) (KNOtS) (Gph) (hOUrS) (GaLLONS) (GaLLONS) (hOUrS) (mILeS) 1000 1.0 -1.00 0.33 N/A N/A N/A N/A N/A 1250 2.0 0.00 0.80 N/A N/A N/A N/A N/A 1500 3.5 1.50 1.80 80.0 144 56 31.1 47 1750 5.0 3.00 3.30 40.0 132 68 20.6 62 2000 7.0 5.00 5.00 24.0 120 80 16.0 80 2250 8.0 6.00 6.70 20.0 134 66 9.9 59 2500 9.0 7.00 9.00 17.1 154 46 5.1 36 2750 9.5 7.50 11.90 16.0 190 10 0.8 6 3000 10.0 8.00 16.00 15.0 240 -40 -2.5 -20 L TABLE 11-8–Time-Throttle Setting Trade-offs Illustrated

Chapter 8) indicate that the best example, on a trip from Florida to INTEGRATION OF estimate of the SOA on the “out” the Bahamas. For any distance out FUEL PLANNING INTO leg is 7.1 knots, and on the “back” less than the point of no return, the VOYAGE PLANNING leg is 7.3 knots. (Both would be vessel has sufficient fuel to return The above concepts are readily beneath the STW in the case of a to Florida if a diversion is required. integrated into general voyage plan- beam current.) The fuel consump- Beyond the point of no return, the ning. Table 11-9, for example, tion rate at this throttle setting, G, is vessel is committed (at least in shows a voyage planning worksheet 6.7 GPH (see Table 11-3). Assume terms of available fuel) to continue that contains entries relevant to fuel that a 20 percent fuel reserve (R = to the Bahamas. This voyage to the planning (e.g., RPM, STW, SOA, 0.2) is desired. The radius of action Bahamas also serves as an example fuel consumption rate, and fuel at this throttle setting calculated where the SOAo and SOAr would required and remaining for each leg) from the above equation is, both be less than the STW, because as well as the TVMDC course RA = 200(1-0.2)/6.7([1/7.1] + [1/7.3]), the Gulf Stream would be a beam entries and current sailing entries. equal to approximately 86 nautical current on both the out and return Checkpoints (the ends of the trip miles. The point of no return in this legs. Normally, the point of no legs) can be defined, not only for numerical example would be 20 return would be calculated in navigational convenience (e.g., percent larger, or equal to approxi- advance as part of the prevoyage where courses change), but also as mately 119 nautical miles. planning. It serves as a convenient “option points” where diversions for Both the radius of action and the memory aid to enter the point of no additional fuel stops can be consid- point of no return vary with the return into the Loran-C or Global ered. Remember the old adage that throttle setting. This is because G Positioning System (GPS) receiver the only time there is too much fuel and STW (hence, SOAo and SOAr) as one of the en route waypoints. aboard is when the vessel is on fire! vary with the throttle setting. Obviously, if the estimated set and Computations of the radius of drift differ appreciably from those IS ALL THIS REALLY action and point of no return are assumed in the plan, it is appropri- NECESSARY? also useful for trips where the ori- ate to recompute the point of no The above fuel planning exam- gin and destination differ—for return. ples may seem unnecessarily elabo-

11-14 Fuel and Voyage Planning 11 EST. FUELEST. FUEL EST. ESTIMATED TIME START:______FOB (GAL.):______COMPASS SOA ETE ETA CONSUMPTION REMAINING MAGNETIC VOYAGE PLANNING WORKSHEET VOYAGE CHECK ITEMS TIDE TABLES: ______PILOT CHARTS: ______TRACK DISTANCE SETTING STW CURRENT TRUE INTENDED LEG POWER ESTIMATED LEG FROM TO (TRUE) (M) (RPM) (KNOTS) SET DRIFT COURSE VARIATION COURSE DEVIATION COURSE (KNOTS) HH:MM HH:MM (GAL.) (GAL.) L 11-9 –Voyage Planning TABLE Worksheet REMARKS ON VOYAGE PLAN: REMARKS ON VOYAGE INFORMATION: GENERAL DATE:______REqD:______CHARTS NAVIGATOR: ______TABLES:______CURRENT GEAR:______COAST PILOT:______VESSEL: NAV. ______TABLE: DEVIATION ______LIGHT LIST:______TIMEPIECE:______FUEL/RPM TABLE: ______LEG PLAN: DETAILED

11-15 11 Fuel and Voyage Planning

CateGOrY deSCrIptION

General Planning – Develop a fuel use versus throttle settings (RPM) curve for your vessel. – Consider voyage routing or timing options to minimize exposure to foul currents or adverse weather conditions. – Study publications to learn the locations along a proposed route where fuel is available. – Use formal fuel planning and allow adequate reserve margins. – Consider speed versus fuel consumption trade-offs.

While Underway – Do not trust fuel guages–verify fuel levels before and during voyage. – Maintain a “Howgozit” chart, or – Maintain a fuel log and be prepared to divert to alternate destinations.

Maintenance and Related – Keep engines tuned. – Clean bottom periodically. – Fill up tanks after each trip to minimize condensation problems.

Vessel Equipment – Consider installation of cross-feeds in multiple tank/engine setups. – Consider installation of fuel management system. – Consider installation of vessel trim tabs.

L TABLE 11-10–Tips for Improved Fuel Management

11-16 Fuel and Voyage Planning 11

new as well as used boats. There have been instances where the man- ufacturer inadvertently installed the wrong fuel tank. Moreover, manu- facturers sometimes offer different capacity tanks as options. A simple means for keeping track of the vessel’s fuel status is to maintain a “Howgozit” (a corrup- tion of “How goes it”) chart. Figure 11-7 illustrates such a chart. It is constructed as follows. J First, do conventional fuel plan- ning for the intended voyage to L ascertain the appropriate throttle FIG. 11-7–“Howgozit” Chart for Example Voyage setting, etc. Suppose, for exam- ple, that a trip of 150 miles is planned and a fuel load of 200 rate. It is important to note that for the material presented in this chap- gallons is available at the start of short voyages with frequent oppor- ter and are not discussed further. the voyage. In this example, it is tunities to refuel, some computa- While underway assumed that the vessel tional shortcuts can be taken. Just as Aventura is used, for which the detail of DR plots, frequency of The “while underway” category fixes, etc., can be adjusted to cir- includes the suggestion to verify basic fuel consumption data are cumstances, so, too, can the detail fuel gauges before and during a given in Table 11-3. of the fuel planning calculations. At voyage. Gauges (and other compo- J Second, construct a graph with a minimum, however, the vessel nents of the system) can fail and, total gallons remaining plotted operator should determine that the moreover, the complex (rounded) on the y or vertical axis, and fuel on board is sufficient for the shape of some fuel tanks may make miles to go on the x or horizon- voyage and provide a reasonable the gauge calibration inaccurate, tal axis, as shown in Figure 11- reserve. particularly at low fuel levels. 7. Using a straight edge or plot- Many tanks can be checked by ter, draw a line starting in the ADDITIONAL TIPS inserting a stick in the fuel-filler upper left hand corner (200 gal- FOR IMPROVED hose. Calibrate the reading on this lons on board, 150 miles to go) FUEL MANAGEMENT dipstick by adding measured quan- of the graph to the lower right tities of fuel and marking the stick Table 11-10 lists 14 experience- hand corner of the graph (0 gal- accordingly. These measurements proven suggestions for improved lons on board, 0 miles to go). are most valid while the vessel is fuel management that should be Identify this as the “dry tanks” stationary; so it is necessary to stop considered by the prudent mariner. line. the vessel briefly during a voyage These are grouped under four broad J to ensure maximum accuracy. Keep Third, specify the desired fuel headings: general planning, while an accurate log showing RPM and reserve, say 20 percent, and cal- underway, maintenance and related, length of time the vessel is operated culate the remaining fuel on and vessel equipment. at each RPM as an additional check board at the end of the trip if this General planning on fuel burn. reserve is maintained. In this example, the reserve fuel is (0.2) Those suggestions in the “gen- Incidentally, the capacity of the (200) or 40 gallons. Draw eral planning” category summarize fuel tanks should be verified—on another straight line on the

11-17 11 Fuel and Voyage Planning

“Howgozit” graph from the increase fuel efficiency. (Generally, Therefore, the vessel will consume upper left-hand corner (200 gal- with a displacement-hull vessel, approximately 3.3/5, or 0.66 gal- lons on board, 150 miles to go) this amounts to slowing down.) In lons per mile. To draw this “5-knot to the reserve fuel (40 gallons) the example shown, the fuel line,” pick an uncluttered part of the at the destination (0 miles to remaining is 100 gallons with 90 chart and draw a line with this go). Label this the “20% miles to go. If it is desired to regain slope. Suppose that you start, as reserve” line. the 20 percent fuel reserve (40 gal- shown on Figure 11-7, at the point Once the “Howgozit” chart is lons remaining at the destination), 200 gallons, 80 miles to go. Pick a prepared, keep track of the remain- then no more than 60 gallons of fuel distance, say 50 miles, sufficient to ing fuel and distance to go while can be consumed during the provide a clear indication of the underway. For example, you might remaining 90 miles, a fuel efficien- slope. If a vessel traversed 50 miles note the fuel remaining from inspec- cy of 90/60 or 1.5 MPG. Reference at this throttle setting, it would con- tion of the gauges or from a fuel to Table 11-3 shows that, theoreti- sume 0.66 gallons per mile times 50 totalizer (an instrument that is cou- cally at least, operating at a throttle miles, or 33 gallons. Starting with pled to the fuel flow meter and notes setting of 1750 RPM (equivalent to 200 gallons, the fuel remaining the total fuel consumed on the voy- a STW of 5 knots) or lower should would be 200 - 33, or 167 gallons. age) at selected DR positions, fixes leave you sitting at the dock with a Draw the fuel consumption line (even better), and/or at the end of 20 percent fuel reserve. If this solu- from the original point (200 gal- the various legs of the journey. Plot tion were chosen, you would con- lons, 80 miles to go) to the point these data as you go. In order to tinue to maintain the “Howgozit” (167 gallons, 30 miles to go), and ensure that you have sufficient fuel chart and monitor the vessel’s fuel label this with the speed (5 knots). to complete the voyage, it is clear status. If the plotted points did not Repeat this procedure for each of that the plotted points must always rise closer to the “dry tanks” line or, the other throttle settings to produce lie above the “dry tanks” line; and to even better, the “20% reserve” line, the fuel consumption lines shown in ensure the desired fuel reserve at the it’s time to rethink options. (The Figure 11-7. Although this might destination, these points must stay calculations, for example, might be sound like a tedious effort, in prac- above the “20 percent reserve” line. in error because of the presence of tice it is quick to do. Moreover, If all points plotted remain above an unanticipated foul current.) once done, it radically simplifies en both lines, the fuel will be adequate As an alternative to having to route decision making. Whether or to reach the destination. calculate a feasible throttle setting not you go to the trouble of drawing However, if the plotted points as you go, it is relatively easy to in the fuel consumption lines, it is dip beneath these lines (as shown in superimpose the vessel’s fuel con- important to use the “Howgozit” Figure 11-7), you have the makings sumption data on the “Howgozit” chart en route. of a problem because, at the fuel chart directly. This idea can be Once drawn, the fuel consump- consumption rates experienced thus implemented if a series of fuel con- tion lines are very simple to use. far in the journey, the vessel will sumption lines are drawn, as shown Just take a paraline plotter and align run out of fuel before reaching the in the upper right hand corner of one end with the last solid dot in destination. Now, early in the voy- Figure 11-7. Separate lines are Figure 11-7, corresponding to the age when options remain, some- drawn for each throttle setting, with vessel’s actual fuel state and dis- thing needs to be done. One solu- the slope of the line equal to the tance to go. Next, take the other end tion might be to divert to an alter- fuel consumed per mile (reciprocal of the paraline plotter and align it nate marina where fuel is available. of the fuel efficiency) at that throttle with the desired reserve at the desti- (Identification of suitable alternates setting or STW. nation. Now roll the paraline plotter where fuel is available is an impor- For example, at a throttle setting up to the speed lines above, taking tant part of the preplanning of 1750 RPM, the vessel’s STW care not to change the alignment, process.) Another solution is to (Table 11-3) is 5 knots, and the fuel and read directly the speed neces- change the throttle setting so as to consumption rate is 3.3 GPH. sary to reach the final destination.

11-18 Fuel and Voyage Planning 11

Maintenance and related careful mariners will not take on an fog bank or other period of restrict- The suggestions listed under the entire load of fuel from one source ed visibility, for example) but not “maintenance and related” heading to minimize the possibility of cont- carry on a DR plot or take frequent fixes. are self-evident for the most part. aminated fuel in all tanks. This con- Informal navigation is appropri- Filling the tanks after each trip min- sideration is particularly important ate under these circumstances and imizes the chance that moisture in at some foreign ports of call. is termed navigation by seaman’s the airspace above the fuel in the eye. Its proper and safe use comes tank condenses and (since water is ADDITIONAL PERSPECTIVE: THE SEAMAN’S EYE only after the mariner has learned heavier than fuel) settles at the bot- the navigation skills and techniques When the mariner has learned tom of the tank where it could be sufficiently and has gained suffi- navigation skills thoroughly, he or drawn into the engine. Avoid top- cient experience to make sound she will know when use of the for- ping off the tanks, however, as this judgments regarding the use of the mal techniques (such as plotting, leads to spills. various techniques available for making detailed float plans such as Vessel equipment navigation of the vessel. are shown in Table 11-9, or detailed Thus, seaman’s eye navigation The suggestions listed under the fuel projections) is necessary and is used after sufficient knowledge “vessel equipment” heading are when departures from these formal of the area is obtained through also self-explanatory, but some processes are appropriate and can study of the available charts, Tide remarks are appropriate for vessels be made safely. In familiar areas, Tables, Tidal Current Tables, Coast equipped with more than one fuel such as on a particularly frequented Pilots, and other navigation publi- tank. Installation of cross-feeds embayment or sound, river or cations and combined with local (i.e., interconnections among fuel coastal area, where the charted haz- knowledge of the area. A thorough tanks and the engines) enable the ards, aids to navigation, currents, mastery of the navigational skills vessel to be kept in better trim. tides, and other essential naviga- and techniques offered in the mate- They also enable any tank with bad tional information are well known, rial in this textbook should provide fuel or a clogged line to be it may be sufficient only to keep the a firm foundation for the navigator bypassed to maintain fuel flow to chart handy for marking positions to know when to apply seaman’s the engine(s). Some especially when desired (at the approach of a eye.

11-19 11 Fuel and Voyage Planning

SELECTED REFERENCES

Beebe, R., (1984). Voyaging Under Power, Seven Seas Pike, D., (1976). Motor Sailors, David McKay Press Inc., Newport, RI. Company, Inc., New York, NY.

Berrien, A. D., (1987). Boat Test No. 437: Grand Rains, J., and P. Miller, (1989). Passagemaking Banks Motor Yacht, Boating, February, pp. 93 et Handbook: A Guide for Delivery Skippers and Boat seq. Owners, Seven Seas Press, Camden, ME.

Collins, R. L. (1985). Out of Gas; Accidents Caused Shufeldt, H. H. and K. E. Newcomer (1980). The By Running the Tanks Dry are Among the Most Calculator Afloat, Naval Institute Press, Annapolis, Avoidable of Mishaps, Flying, Vol. 112, No. 9, MD, pp 188 et seq. September, pp. 25, et seq. Stapleton, S., (1989). Stapleton’s Powerboat Bible, Gorant, J., (1995). Three for One; We not only tested Hearst Marine Books, New York, NY. the Luhrs T-320, we tested her with three different engine packages, Power & Motoryacht, January. Stearns, B., (1986). Boat Test No. 423: Rampage 28 Sportsman, Boating, September, p 94. Kinney, F. S., (1981). Skene’s Elements of Yacht Design, Eighth Edition, Dodd, Mead & Company, Stearns, B., (1987). Boat Test No. 443: Grady-White New York, NY. Trophy Pro 25, Boating, February, p 132.

Kreisler, K., (1995). To Repower Or Not To Repower? Stearns, B., (1987). Boat Test No. 458: Fountain 8.8 Seaplane, a 1988 Bertram 33, confronts the eternal Meter Center Console, Boating, May, p 110. question and offers some answers, Power & Teale, J., (1977). Designing Small Craft, David McKay Motoryacht, January, pp 70 et seq. Company, Inc., New York, NY.

Laudeman, W. J., (1986). The Speed/RPM chart revis- Weems, P. V. H., (1940). Marine Navigation, D. Van ited, The Navigator, Fall, p 16. Nostrand Company, Inc., New York, NY.

Lord, L., (1963). Naval Architecture of Planing Hulls, West, J., (1975). Modern Powerboats, Second Edition, Third Edition, Cornell Maritime Press Inc., International Marine Publishing Co., Camden, ME. Cambridge, MD.

Lydecker, R. (undated). Reducing Fuel Costs: A Guide for Recreational Boating, University of Maryland Sea Grant Extension Program.

Lyon, T. C., (1972). Practical Air Navigation, Eleventh Edition, Jeppesen, Denver, CO., pp. 136 et seq.

McFadden, R. D., (1990). Toll at 72 in L. I. Air Crash; 89 Survive, Investigators Say the Colombian Plane May Have Run Out of Fuel, The New York Times, Vol CXXXIX, No. 48,128, 27 January, p 1, contin- ued on p 30.

Moss, F.T., (1981). Modern Sportfishing Boats, International Marine Publishing Co., Camden, ME.

11-20 Chapter 12

REFLECTIONS

“Vision is the art of seeing things invisible” –Jonathan Swift, Thoughts on Various Subjects; from Miscellanies

“The sea is at its best in London, near midnight, when you are within the arms of a capacious chair, before a glow- ing fire, selecting phases of the voyages you will never make” –Henry Tomlinson, The Sea and The Jungle [1912]

INTRODUCTION errors, some with disastrous conse- aving reached this point in the quences, most with happy endings. Htext, you should have a work- These examples are not intended to ing knowledge of the essential frighten, but rather to instruct. tools, techniques, and theory of George Santayana’s famous remark coastal navigation. This chapter is that “Those who cannot remember written to go beyond technique to the past are condemned to repeat it” share insights on the practice and/or is equally applicable to the practice art of navigation. It presents ten of navigation. experience-proven principles of It has been said that the sea is a navigation practice for your consid- cruel mistress, totally unforgiving eration. This chapter has no home- of error. However, careful study of work questions or problems to slave marine mishaps indicates that iso- over, simply points to ponder. lated navigation errors are seldom Nonetheless, this is one of the most the sole probable cause of an acci- important chapters in the Advanced dent. Typically, many adverse fac- Coastal Navigation (ACN) text. tors are required before error turns Each of the principles discussed from mere embarrassment to disas- below is illustrated with exam- ter. It may be prudent to believe ples—many taken from the vast lit- otherwise, but an isolated naviga- erature on accidents with commer- tional error is often recoverable. In cial ships, some from personal the open ocean, far removed from experience of the writer and other navigational hazards, an error may members of the United States Coast be virtually inconsequential. Even Guard Auxiliary (USCGAUX). The in more restricted waters, an error examples all involve navigational may not be critical if promptly

12-1 12 Reflections

detected and corrected. An error in mates published in Boating example, according to recent issues taking or plotting a fix, for exam- Statistics, operator error was a con- of Boating Statistics, navigation ple, can be revealed by a compari- tributing factor in nearly three-quar- error was judged to be the principal son of a fix with the dead reckoning ters of the boating fatalities where cause for reported accidents involv- (DR) position, or from consistency fault could be determined. This ing only about 2 percent of recre- checks with collateral information statement is not intended as an ational vessels. Again, this statistic (e.g., depth soundings, additional indictment—rather as a challenge to is furnished to lend useful perspec- bearings). Even if the fix error is increase professionalism. tive, and not to suggest that such a not immediately discovered and Navigation errors, per se, gener- low rate is acceptable. causes the navigator to change ally account for only a small per- This chapter is organized into a series of maxims or principles that course towards potentially danger- centage of the recreational boating are useful to study and explore in ous waters, such as reefs, there is accidents in the United States. For the possibility that the reefs can be search of wisdom. Space con- detected and course altered to safer straints limit the detail that can be

ST G A U O A C R included in each of the examples.

S D

waters, unless night or storms also U What YOU

A U Y X R conspire to reduce visibility or ILIA WILL LearN IN References are included for those seeking more detail. mask the noise of breaking waves. thIS Chapter In the famous case of the Titanic, PRINCIPLE 1: for example, it has been argued that Ten key principles of only an unfortunate conspiracy of professional navigation, PAY ATTENTION TO DETAIL timing was responsible for the dis- including: Professional navigation is as aster. Had the iceberg been seen much an attitude of mind as an art J just a few minutes earlier, the vessel Pay attention to detail or science. In this regard, it is could have turned to avoid colli- J Practice is essential and important to heed the advice of the sion. Had the iceberg been seen a can also be fun late Frances W. Wright to practice few minutes later, the Titanic would J Do not rely on any one “constant vigilance.” This is partic- ularly true with respect to the have hit head on, and emerged with technique for determin- a bashed-in bow—seriously dam- everyday details or computations ing the vessel’s position aged—but with most of the water- that many mariners take for grant- tight compartments intact and the J Be alert to anomalies ed. A casual attitude can lead other- majority of the passengers and crew J Emphasize routine in wise competent navigators to com- would have survived. times of stress mit the most amazing blunders. So it is that boating is a general- Consider the following examples. J Slow down or stop the ly safe activity. Moreover, available J vessel if necessary and A navigator tuned in the statistics indicate that fatality rates circumstances permit Brenton Reef (RBn 295) (per 100,000 recreational vessels) radiodirection finding (RDF) J have declined in recent years. To Preplan as much as station but dialed in 291 kilo- note these trends lends valuable per- possible hertz (the frequency for Nobska spective, but it is also important to J Be open to data or infor- Point) by mistake (an error in stress that any accidents or fatalities mation at variance with the last digit only) and failed to are, in some sense, intolerable. your understanding of verify the Morse code identifier Statistics show that, for most boating the situation or to plot the bearing and dis- and aircraft accidents as a group, cover the error. Misled by the J human error rather than equipment Know and operate within false bearing, the navigator failure is the probable cause. For your limits altered course and nearly example, according to recent United J Maintain a DR plot grounded the vessel on the States Coast Guard (USCG) esti- rocks.

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J A navigator using a Global 1990) the third mate on the bulk Factor (ASF) corrections to a Positioning System (GPS) freighter Mavro Vetranick sight- Loran-C, as published in the receiver with provision for way- ed Pulaski Light in the Dry Canadian Coast Guard publica- points and a course deviation Tortugas too close on the star- tion, Radio Aids to Marine indicator (CDI) allowed the ves- board beam. The mate adjusted Navigation (Hall, 1989). The sel to drift off course to the left. the course on the autopilot to instructions indicated that the On realizing that the CDI indi- compensate but, unfortunately, tabulated correction was to be cated a course correction to the turned the autopilot dial in the subtracted. For example, if the right, the navigator put the helm wrong direction before immedi- published entry were -0.9 µsec over to return the ship to the ately retiring to the chart table to and the navigator incorrectly intended track to reach the way- check the navigation receiver. entered -0.9 µsec, rather than -(- point without plotting the ves- The mate did not stay on deck to 0.9)= +0.9 µsec, in the Loran-C sel’s position or realizing that a verify that the correct course with the result that the corrected direct return to track put the ves- change resulted and, instead of loran position was less accurate sel in shoal waters. turning away from the reef, the than the original position. J J Captain John M. Waters, Jr., in vessel turned towards the reef Richard Cahill, in his excellent his book, Rescue at Sea and subsequently grounded! book, Strandings and Their (Walters, 1989), writes of a B- J The late Sir Francis Chichester Causes (Cahill, 1985), describes a 1979 accident to the tanker 29 that was forced to ditch 200 (a superb navigator of both air- Messiniaki Frontis near the miles away from its intended craft and vessels) was most can- island of Crete. En route to a island destination because the did in his book, Gypsy Moth planned anchorage, the mate navigator, in adjusting the true Circles the World (Chichester, took a visual bearing on course for magnetic variation 1967), to reveal that, on occa- Megalonisi Light and completed had subtracted, rather than sion, he consulted the wrong the fix with a radar range on this added. This simple arithmetic page of the Nautical Almanac, same light. The variable range mistake cost the lives of several and that he had left behind the marker (VRM) was not used for people and an aircraft. required tables for celestial nav- this purpose. Instead, the range J igation when he originally set out Cameron Bright, writing in the was interpolated from the fixed on a round-the-world voyage! magazine Ocean Navigator range markers (FRMs). (Bright, 1987), describes a rou- J John Milligan, in his book, The Unfortunately, the range setting tine delivery of the cutter Amateur Pilot (Milligan, 1974), on the radar had inadvertently Acadia from West Palm Beach recounts the story of a partici- been switched to a 3-mile range to New York. One storm-tossed pant in the Transpac race who from a 6-mile range, with the night on this voyage near Cape made an error in misidentifying result that the estimated range Hatteras, the navigator finally the Molokai Light as the was in error by a factor of two realized that the reason for the Makapuu Light, a navigation and, therefore, the fix was in increasing discrepancy between error that caused the yacht to error. No attempt was made to DR and Loran-C positions was lose the race. The navigator had obtain another visual bearing on accounted for by essentially the carefully timed the 10-sec light other lights in the area, and so same arithmetic error noted but failed to note that the the position error went undetect- above in adjusting for variation. Molokai Light is flashing while ed. Shortly thereafter, the vessel In this episode, fate took a the Makapuu Light is occulting! went aground. kinder hand, and the only dam- J While cruising near the J Leonard (1998) relates the story age was a bruised ego. Newfoundland port of Grand of a yacht, Silk, which nearly J In another story from the pages Bank, a navigator prepared to went aground off New of Ocean Navigator (Anon, enter Additional Secondary Caledonia because the wrong

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GPS datum was set into the PRINCIPLE 2: figure out where to go without receiver. As noted in Chapter 9, PRACTICE IS ESSENTIAL charts” and “I don’t have to plot; I the datum used in the GPS AND CAN BE FUN just know it all by heart.” In short, receiver must match that used Upon completion of this course, experience in a benign environment for the chart. the average student is well equipped misleads beginner and expert alike. J Hoffer and Hoffer (1989) relate to begin to learn the art of coastal Moreover, without practice, skills the story of Air Canada flight navigation. However, experience, are lost. 143 from Montreal to Ottawa on though essential, is not necessarily Studies by the Federal Aviation 23 July 1989. The Boeing 767 the best teacher. Twenty years of Administration (FAA) indicate that ran out of fuel and had to make subsequent experience may simply many aircraft pilots go through the an emergency landing. amount to one year, repeated twen- same process. Knowledge (at least, Fortunately, all were saved as a ty times. Many students first eager- that knowledge required to pass the result of the remarkable skills of ly begin to apply the tools and tech- pilot’s written examination) is actu- the pilot. There were several niques taught in the course. In ally lost at a fairly rapid rate. Unless reasons for this incident. One of familiar waters and during periods the aviator works to gain additional the major reasons was a failure of good visibility and calm seas ratings, there is a real need to stay to correctly convert between (which the beginning navigator in practice to maintain/regain English and metric units in cal- wisely insists on), the techniques knowledge. For this reason, among culating fuel quantities. are diligently practiced, at first. others, the FAA insists on periodic None of these errors are pro- Soon, it becomes apparent that for- retraining for all licensed pilots. found—all appear quite foolish in mal techniques are not always nec- In a benign environment (e.g., hindsight. All were preventable by essary—at least, under the circum- no equipment failure, operations in the simple expedient of rechecking stances of these relatively simple good visibility and in familiar calculations, seeking to validate voyages. Fuel reserves, first labori- waters), this loss of skills and/or estimates by other means, or taking ously calculated, are found to be knowledge may not be critical. a little more time to think. The key more than adequate in home waters. However, lack of practice and dili- quality necessary to prevent, or at DR plots, originally laid out in gence can take its toll; isolated least reduce, the likelihood of these meticulous fashion, are also found thunderstorms may break out dur- errors is to realize that small details to be “unnecessary.” Lights are first ing the “dog days” of late summer, are important and deserve attention. carefully timed for positive identifi- shutting down visibility and kicking The USCG uses the phrase sweat cation, but then the local lights up substantial waves. Or a pleasant chart to describe a conning chart become familiar and the stopwatch fishing trip off the coast of Maine, used for harbor approaches. This is left ashore. Other potentially unsafe or portions of California, turns into a particularly apt term because it habits evolve, justified first as expe- a real challenge as fog sets in, along reinforces the concept of constant dient shortcuts, then later dignified with the realization that the bottom vigilance. with the appellation, seaman’s eye. is not the familiar soft mud of the Another relevant aspect of the The delivery captain of the 137- Chesapeake Bay, but rather sharp constant vigilance theme is the need foot offshore clamboat Little Gull rocks, interrupted with pinnacles. to maintain a proper lookout. Every (Anon, 1993a) ran the vessel Grounding in these waters does not year, “improper lookout” is found aground on what should have been simply call for volunteers to get wet to be a major cause of recreational a routine trip from Atlantic City, pushing the vessel off a muddy bot- boating accidents, according to New Jersey, to New York City. tom, but may lead to a nasty gash in Boating Statistics. A proper lookout Among other problems, the vessel the hull and thoughts of “Mayday.” is required by the Navigation Rules had no fixed compass and no charts. The gradual attrition of skills is and good common sense besides. The captain reportedly said (Anon not limited to amateur navigators 1993a) that “My brain is so impreg- and other casual boaters. Full-time nated with loran bearings that I can paid navigators also need to work to

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ensure that skills do not go rusty. mary navigator. Captain John O. ed time of arrival (ETA) at the An article in the pages of Ocean Coote (Coote, 1988) recounts the checkpoint that marks the end of his Navigator (Walsh, 1987) tells of the following anecdote in his interest- or her leg. The navigator closest to 672-foot Venezuelan tanker, ing and well-written book, Yacht the actual time past the checkpoint Lagoven Caripe, that went aground Navigation–My Way: wins dinner, or is excused from 60 miles outside the vessel’s proper “Sailing Roundabout back cleaning the dishes or washing shipping channel on Cultivator from Copenhagen after the down the vessel. Shoal while en route to Boston, One-Ton Cup in 1961, I left Another game (in vessels appro- Massachusetts. This vessel depart- a guest artist on the tiller priately equipped and configured) ed Venezuela in early February of after clearing the roads off is for the navigator to con the vessel 1987 with a full load of 14.5 million Copenhagen. Around 0500 I from the navigator’s station only, gallons of No. 6 fuel oil. After some awoke to realize that we without being able to come on deck early difficulties, including the loss were hard aground and in for visual observations. Fog or of a navigation receiver off thick fog. There had not been other limitation to visibility is sim- Savannah and the loss of one of two a single entry in the log or on ulated in excellent weather by hav- radar sets, the problems began in the chart for four hours, so it ing the lookouts report sightings earnest. Bad weather and an inabil- was anyone’s guess where only within a fixed distance of the ity to obtain RDF fixes forced the we had fetched up. We vessel, e.g., one mile or one-half vessel to rely on DR throughout backed off and sailed a reci- mile. The navigator’s task is to most of the voyage. However procal course until we reach an identifiable object, such as (Walsh, 1987), the ship should have picked up an identifiable a buoy, from these limited data. The passed a range of the Highland navigational landmark, but it measure of success is the actual dis- Light RDF beacon and the was a creepy feeling.” tance away from the buoy when the Nantucket LNB beacon. Moreover, navigator indicates that the buoy prior to grounding, there was an THREE KEY WORDS should be alongside. (Assign a interval of clear weather when lookout to ensure that the vessel celestial sights should have been PRACTICEN does not pass too close to the buoy, taken to fix the ship’s position. however.) Some celestial sights were attempt- PRACTICEN Variations on these ideas are lit- ed, but the celestial worksheets erally endless. For example, certain were later described by the ship’s PRACTICE electronic aids can be presumed agent as a “joke.” The captain was inoperative and the display covered relieved of duty upon entry to by cardboard or rubber inset. Boston Harbor. Although a defini- The principle here is summa- (Aircraft pilots, at least those with tive investigation was not conduct- rized in three keywords: practice, instrument ratings, are well familiar ed, the provisional explanation for practice, and practice. Practice with this idea, called partial panel this near-disaster (the vessel was these techniques even if the voyage work.) Or, perhaps all the electron- able to be refloated without a spill) does not require the extensive use ics can be presumed disabled and is that the captain and officers were of formal navigation techniques. navigation conducted only by DR simply out of practice with the This is not an exhortation to and visual fixes. appropriate navigational techniques unremitting drudgery, donning the In the USCGAUX boat crew (here, celestial as a backup) neces- nautical equivalent of a hair shirt. program, vessel coxswains-in-train- sary for the voyage. “Use it or lose Try making a game of it. For exam- ing must pass a task called “night it,” is an apt expression. ple, divide the voyage into legs and ops.” Here, one participating vessel The requirement to stay current have various crewmembers act as (the notional victim) ventures out at with navigational techniques navigators for each of the legs. night and simulates a distress trans- applies to all aboard who will han- Each navigator uses all available mission. (The word “Mayday” is dle the vessel, not simply the pri- information to calculate an estimat- never used in this simulation and

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the phrase “this is a drill” begins ference. Calculate the apparent cur- throughout the text. Simply stated, and ends each transmission.) The rent set and drift (using the methods this principle urges the navigator to search and rescue unit (SRU), of Chapter 7) and test the plausibil- use all available information in fix- conned by the apprentice coxswain, ity of this estimate from the data in ing the vessel’s position, rather than tries to find the “lost” vessel. The the Tidal Current Tables or Charts to rely on any one measurement, operator of the SRU asks a series of (Chapter 8). Perhaps the DR was in method, or technique. For example, questions of the lost vessel by radio error, or the navigator made the radar, GPS, or visual bearings can to narrow down the area to be mistake of drawing the current vec- be used to check a Loran-C fix. searched. “Can you see any bridge tor from the fix to the DR (rather Even depth information can be use- over the river?” “What color is the than the reverse). Perhaps the navi- ful in assessing the plausibility of light on the buoy?” “When did you gator failed to calculate the DR the fix. leave your home port, and in what position corresponding to the exact The essential idea here is to use direction?” are illustrative ques- time of the fix, or made an error in the various methods as cross- tions. (USCGAUX Qualification the 60D = ST formula. Tracking checks on each other to increase the Examiners (QEs), those entrusted down the error is itself a learning confidence of the navigator in the with examining and certifying the opportunity. vessel’s position. Excluding power skills of the prospective coxswain, Navigators should stay within failures, which can prevent any can tell a great deal about the appli- the proven limits of their skills and electrical system from functioning cant’s navigational skills merely the capabilities and equipment of properly, the possible errors among from the questions asked.) This their vessels—a point addressed the principal methods of fixing training exercise provides invalu- below. But voyages could also be position are largely independent. able experience in night navigation planned with the idea of pushing Therefore, having several pieces of and decision making, and is regard- and stretching these limits. An evidence all pointing to the same ed as the highlight of the on-the- excellent way to do this is to crew conclusion increases its reliability. water training program by many with more experienced navigators Professor R. V. Jones, one of the Auxiliarists. to learn their tricks of the trade. key figures in British scientific The obvious point of this prac- Even crewing with less experienced intelligence during World War II, tice in ideal conditions is to prepare navigators can be instructive. writing on the use of deception in the navigator to cope with less than It’s appropriate to close this sec- hoaxes, explains the principle as ideal conditions when these arise. tion with the apocryphal story of a follows (Jones, 1967): Good habits are formed and, equal- famous navigator once asked the “The ease of detecting coun- ly important, the navigator becomes secret of his success: “Not making terfeits is much greater when more alert to the possibility of error. mistakes” was the tongue-in-cheek different channels of exami- Boating is supposed to be fun, reply. This led to the inevitable fol- nation are used simultane- and it is not suggested here that low-up question: “How did you ously. This is why telephonic every portion of every voyage be learn to avoid making mistakes?” hoaxes are so easy—there is the topic of a training exercise. The truthful answer was: “By mak- no accompanying visual Nonetheless, every voyage offers ing mistakes!” appearance to be counterfeit- some opportunity to learn some- ed. Metal strips (chaff or thing worthwhile, and the navigator PRINCIPLE 3: window) were most success- should strive to perfect skills for at DO NOT RELY ON ANY ONE ful when only radar, and that least a part of the voyage and, in TECHNIQUE FOR DETERMINING of one frequency, was particular, attempt to learn from his THE VESSEL’S POSITION employed. Conversely, the or her inevitable mistakes. For Many navigators would consid- most successful naval mines example, when a fix and DR posi- er this the one cardinal rule of suc- were those which would tion do not coincide, the navigator cessful navigation. Indeed, there are only detonate when several can attempt to rationalize this dif- scattered reminders of this point different kinds of signal,

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magnetic, hydrodynamic, homed in on the lightship’s autopilot, detected the problem and acoustic, were received radio signal, without regard for and attempted to return the ship simultaneously. A decoy, the distance to the lightship, to safe water. However, the ship which simulates all these with disastrous results went aground before this correc- signals, is getting very like a (McClench and Millar, 1979). tive action took effect. ship. From these considera- Nor is this an isolated Passengers reported that the tions, incidentally, we can example. A relief lightship to the captain claimed earlier that draw a rather important con- Ambrose Lightship (on the same evening that his ship clusion about the detection approaches to New York “could never run aground of targets in defense and Harbor) was rammed in 1960 by because of space-age navigation attack: that as many different the freighter Green Bay. Closer equipment installed on the physical means of detection to the waters covered by the bridge.” as possible should be used in 1210-Tr chart, the Texaco tanker J The Cypriot-registered grain parallel. It may, therefore, be Lightburne ran aground on ship Katya V ran aground in fog better in some circumstances rocks just beneath the Block the Chesapeake Bay (Anon, to develop two or three inde- Island Southeast Lighthouse on 1993b) after the pilot had pendent means of detection, 10 February 1939. The tanker misidentified a fishing vessel as instead of putting the same had been using the RDF station, the Morse A “CR” fairway buoy total effort into the develop- then located at this light, for off Tilghman Island. The ship ment of one alone.” navigation and misjudged the was reportedly equipped with Though obviously ventured in a distance off. The wreck remains three operating radar sets, some different context than the practice today, another memorial to the with ARPA, as well as GPS, but of navigation, the point is equally dangers of homing without posi- the pilot did not properly fix the valid here. The navigator is not nec- tion fixing. The wreck of the vessel’s position using all avail- essarily the victim of intentional Lightburne is not shown on the able means. deception by another, but rather of 1210-Tr chart, but can be found Relying on one method of navi- self-deception. on chart number 13218 at L: 41° gation can actually create two types The advice to use as many sys- 04.3’ N, Lo: 71° 32.3’ W. of problems. Perhaps the worst out- tems of position fixing as possible Similar stories of sinkings and come is if the method chosen is is particularly apt in the electronic near misses abound, all prompt- flawed in some respect and the nav- era. The nautical literature abounds ed by the same mental error. igator remains unaware of the prob- with examples of problems caused J The 568-foot cruise ship Royal lem. Again, the pages of Ocean by over-reliance on one method of Majesty ran aground near Rose Navigator, Professional Mariner, navigation. Here are a few exam- & Crown shoal, 10 miles east of and other nautical magazines are ples: Nantucket Island in 1995. filled with examples of this type of J RDF (now largely obsolete in According to one account error. One correspondent (Howell, U.S. waters) provides bearing (Anon, 1995a), a loose antenna 1985) writes of undetected loran information only. If used for connection on the GPS receiver problems that led to the grounding homing, it is essential to include caused it to fail and default to a of the 48-foot sloop Victory, near some additional means (perhaps dead reckoning mode; but the Conception Island in Exhuma an RDF cross bearing from a ship’s officers did not detect this Sound, in the 1985 race from different station) to determine and failed to make comparisons Miami to Montego Bay, Jamaica. the distance of the vessel from of the GPS position with those Although lights in this area are few the RDF station. The case of the determined by Loran-C, radar, and far between, visual LOPs collision of the liner, Olympic, or depth sounder. The ship’s apparently were not attempted and with the Nantucket Lightship in captain, called to the bridge to the crew sailed on unaware of the 1934 is relevant. The Olympic handle a problem with the problem until striking the reef.

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There is also another difficulty sounder cannot be used directly to sea conditions, or an inattentive caused by overreliance on one fix the vessel’s position, depth helmsman who allows the vessel to method of navigation. This happens information can sometimes be quite slowly drift off course. when the one method chosen fails useful in confirming a fix deter- The clues to changed conditions catastrophically, e.g., a Loran-C or mined by other methods. Mention or problems may be quite subtle. GPS receiver outage caused by a of the text, Strandings and Their The famous Sherlock Holmes story power failure or surge as an engine Causes, has already been made. of the dog that did not bark in the is started. Losing the one tool in use This invaluable compilation of case night reminds us that negative creates chaos, as neglected DR studies of large ship groundings information can also be important. tracks are hastily reconstructed. lists numerous examples that were A marine VHF radio that is silent, It is particularly important to either caused by negligence in the when normally abuzz with clatter, seek confirmatory evidence in con- use of, or failure to use, the depth may indicate that someone has nection with the location of floating sounder. Specific accidents cited in altered the squelch setting, the ves- aids to navigation. Buoys in U.S. this text involve the tanker Argo sel has departed normally crowded waters are, in fact, on station the Merchant in December 1976, the sea lanes, or a power failure has vast majority of the time. But colli- tanker Messiniaki Frontis in occurred. A light that is not seen sions, storms, and, particularly, ice February 1979, the liberty ship may be the first indication that the can lead to buoys being sunk, dam- Valiant Effort in January 1959, the vessel is not where it is supposed to aged, or off station. Major as well tanker Hindsia in January 1965, the be or that the visibility is deteriorat- as minor buoys can be off station. liberty ship Bobara in January ing. In early October of 1986, for exam- 1955, the steamship Cornwood in In addition to being concerned ple, the Nantucket Large November 1957, the tanker Cristos about the failure to observe some- Navigation Buoy (LNB), which is Bitas in October 1978, the tanker thing expected, the sight or sound an important aid located south of Transhuron in September 1974, and of something unexpected could be the Nantucket Shoals, broke free of the freighter Bel Hudson in May cause for alarm. For example, John its mooring chain and drifted out to 1970. A careful study of these cases Rousmaniere quotes Hewitt sea still flashing its light, transmit- shows conclusively the value of the Schlereth as noting that the naked ting its RDF broadcast, and return- depth sounder. eye has sufficient resolving power ing a racon signal. Thus, RDF and A professional navigator should to enable the mariner to count indi- radar navigation, as well as visual use all available means—radar, vidual trees on a shoreline from a observations, could have been in Loran-C, GPS, depth information, distance off of 1 M., count windows error over the time period from the and maintain a DR plot to fix and in waterfront houses at 2 M., and buoy’s escape until it was located navigate the vessel. In truth, these see the junction line between land and towed back by the USCG buoy means are complementary and and water at 3 M. Using these rules tender Evergreen. Fortunately in mutually reinforcing, rather than of thumb, if a vessel’s intended this case, alert navigators of com- competitive. track were to pass no closer than 4 mercial vessels noted the discrepan- M to shore but windows could be cy and reported it to the USCG. PRINCIPLE 4: counted on waterfront houses, the In earlier chapters, mention has BE ALERT TO ANOMALIES alert navigator should investigate been made of the utility of the depth Many professional navigators further. sounder in checking positions seem to develop a sixth sense about In the days before inertial navi- determined by other methods. the well-being of the vessel. An gation systems became common, an Although there are circumstances experienced navigator asleep in a aviator broke out “on top” of the (e.g., areas where the depth is near- comfortable bunk may awaken if clouds after flying some distance ly constant over a broad area, or the vessel’s motion changes. over the ocean by reference to where dangerous reefs rise abruptly Changes in the motion could come instruments alone to find that the off the ocean floor) where a depth about from wind shifts, changes in sun was on the right, rather than on

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the left where it should have been if fee to the helmsman. A quick glance slowing down may be important for the aircraft were on course. This showed the lighthouse, apparently other reasons as well.) was the first indication of a com- larger now over the bow. To the pass failure. Because the failure casual navigator all was well, an PRINCIPLE 5: was gradual in this case, the pilot occasion to swap lies with the EMPHASIZE ROUTINE had kept resetting the directional helmsman over coffee. A more alert IN TIMES OF STRESS gyro to the compass, and the direc- navigator would check the vessel’s Now and again, the tapes or tional gyro was similarly useless. compass heading upon returning transcripts of communications To the uninitiated, such a sixth topside. A heading significantly between commercial aircraft pilots sense may seem almost mystical. In more (less) than 270 degrees would and air traffic controllers are made truth, this sixth sense has more indicate a current is setting the ves- public after an accident or near- pedestrian origins and, with a little sel to the south (north) of the origi- accident. The startling thing to most care and planning, can be devel- nal track. In effect, this vessel is nonpilots is the high degree of pro- oped by any navigator. The “unseen homing (using visual means) rather fessionalism and cool decision- light” provided a clue only because than tracking. Perhaps this distinc- making revealed in these communi- the navigator took the trouble to tion is of no great consequence, but cations. To most of us this appears estimate the range at which the light it could be if there was unsafe water almost heroic, and well it may be. should be seen (using methods dis- to the south (north). In any event, cussed in Chapter 10) and either an alert navigator gains potentially But it is also the product of exten- drew a circle of visibility on the valuable information from a simple sive conditioning. Pilots are sys- chart or made a note to look for the glance at the compass. tematically trained (in actual air- light when the vessel was suppos- Sound and kinesthesia (motion craft and sophisticated simulators) edly within this arc of visibility. sense) can also furnish important to cope with all manner of emer- The pilot who identified the com- clues to the mariner. After many gencies. This training alone lends pass error simply exploited the hours operating the same vessel, the an element of the routine to emer- common knowledge that the sun navigator develops a clear, although gencies. Equally important, howev- rises in the east and sets in the west. intuitive, sense of the relation er, is the programmed method of The navigator who counted win- between speed, power setting, and instruction for dealing with emer- dows and wisely fixed the vessel’s trim that goes beyond the mere abil- gencies. Pilots are taught early and position and altered course also ity to identify a rough-running often in their careers to use check- exploited elementary knowledge. engine. Slight changes in trim lists. There are checklists used in Good weather and sea condi- and/or bottom condition affect the the preflight inspection, checklists tions and abundant Aids to speed curve (Chapter 5) and (absent used in starting engines, pre-takeoff Navigation (ATONs) lower the a speedometer) may be a clue to the checklists, cruise checklists, etc. level of cockpit tension and lull the reason for a discrepancy between There are also various emergency navigator into a state of complacen- the DR position and a fix that can- checklists, often neatly categorized cy. For example, after completing a not be accounted for by current as fire, engine out, landing gear difficult landfall a navigator left the alone. In operating powerboats in inoperative, etc. Reading out the helmsman at the wheel of a trawler conditions of restricted visibility, it items in this checklist, and follow- with instructions to maintain course is often useful to slow down or stop ing the concise instructions, is not so as to keep the bow pointed at a the vessel periodically and shut off only an orderly way of problem distant lighthouse visible on the the engine. In the silence that fol- solving that lowers the likelihood of bow with the vessel’s heading 270 lows it is sometimes possible to omission of critical items, but also degrees. After going below for cof- hear the sound of diaphones or serves to lower the likelihood of fee, a well-thumbed copy of the other sound clues (let’s hope not the panic among the aircrew. In short, appropriate Light List, and a few sound of breaking surf!) that would these checklists act to make emer- minutes of conversation, the navi- otherwise be masked by the noise gencies routine, to place these in the gator returned topside to bring cof- of the engine. (As noted below, context of the familiar.

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Mariners can benefit from this precisely in time of emergency that same approach. For example, important items are most likely to Marvin Creamer (an accomplished be overlooked. PRE-UNDERWAY CHECKS “bare bones” blue-water navigator), En route and Approach Charts writing in Ocean Navigator PRINCIPLE 6: LNM Corrections to Above (Creamer, 1986), makes the point SLOW DOWN OR STOP that DR is both possible and neces- THE VESSEL IF NECESSARY Coast Pilot and Light Lists sary during storms. He cites several AND CIRCUMSTANCES PERMIT Tide Tables positive aspects to attempts at storm Sometimes a crisis is self- navigation: it assists in prompt Tidal Current Tables induced. It may be a crisis to be post-storm location, helps focus unsure of your position when Tidal Current Charts minds of personnel and combats approaching a reef-strewn harbor at lethargy, increases the confidence Cruising Guide 20 knots. At this speed the vessel of crew in navigator/skipper, pro- will travel over 333 yards in 30 sec- vides practice in heavy-weather onds, roughly the length of time navigation, and helps to pass the that it has taken you to read two or time. All too frequently, the drama get you to the scene of the accident three paragraphs of this text. But of a crisis event distracts the navi- faster! part of this crisis is self-induced and gator from necessary tasks. In Slowing down may be prudent can be removed by the simple expe- storms, particularly severe storms, even if you know approximately dient of slowing down or stopping. seamanship, rather than navigation, where you are. You can use extra This is why the anchor is listed may be the most immediate priority. time to think, identify buoys or among the navigator’s tools in However, assuming that the vessel lights, and/or take more frequent Chapter 4. (Obviously, there are cir- survives the storm, it is essential to position fixes. In a harbor with a cumstances where stopping is ill- know the vessel’s position and complex and/or unfamiliar advised, such as in the middle of proximity to dangerous waters. approach, for example, proceeding running a tough inlet.) This task will be vastly simplified if at a slower speed provides the nav- Chapter 9 makes the point that a simple record of estimated speeds igator time to “catch up” with the any confusion in interpreting a rel- and courses was maintained. Such a physical progress of a vessel. It is ative motion plot (RMP) from radar record may not be exact for a vari- not unusual for three or more min- observations can be eliminated if ety of reasons; speeds may be very utes to be required to take bearings you stop. After stopping, aside from approximate (particularly for a sail- and plot a fix. At 20 knots the ves- the effects of possible drift, the ing vessel running in a storm under sel will be 1 M, or approximately radar plot appears much the same as bare poles) and leeway only a 2,000 yards, beyond the fix by the if a true-motion device were used. guess. But even an approximate time it is plotted, at 5 knots only This comment is not intended to DR record is better than no infor- 500 yards. The “maritime casual- relieve the mariner of any obliga- mation. ties” section of Professional tion under the navigation rules, Get in the habit of using check- Mariner provides numerous exam- such as the responsibility of the lists. Start out with obvious check- ples that show the benefits of slow- stand-on vessel (if defined) to lists, such as the pre-underway ing down. For example (Anon maintain course and speed. checks, cruise checks, approaching 1995b), the tanker Kentucky went Slowing down or stopping may rough weather checks, and the like. aground and ripped a 10-foot-long be prudent or necessary in circum- Put experience to work by gradual- gash in its hull by running over a stances of reduced visibility. And it ly developing and revising these rocky bottom in the Delaware River should be considered whenever the checklists for maximum utility on south of the Commodore Barry vessel is in hazardous waters and your vessel. And remember that bridge. Rain squalls and a malfunc- you are unsure of your position. checklists are particularly useful in tioning radar set created difficulties Leaving the vessel running will just emergency situations because it is for the pilot. However, Coast

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Guard officials concluded that the culated in advance. to know the vessel’s fuel state and pilot was negligent because he con- Stopping gives you time to think safe cruising radius at all times, tinued ahead without slowing the and to use other means of fixing the together with feasible alternate des- ship. Before grounding, the vessel’s position. Stopping gives tinations where fuel is available. A Kentucky passed the Marcus Hook you silence to listen. Stopping few minutes time spent with the anchorage, an area where it could allows you time to regain com- U.S. Coast Pilot (USCP), or equiv- easily have pulled aside to await mand. alent cruising guide, are all that is better visibility. required to identify these alternates. PRINCIPLE 7: Preplanning is particularly PREPLAN AS MUCH important for approaches to unfa- AS POSSIBLE miliar harbors. Of course, the rele- vant charts and reference publica- George S. Morrison, in dis- tions should be aboard, but it is cussing the planning for the so- even easier if the navigator summa- called Culebra Cut, one of the most rizes this information as extra chart difficult parts of the Panama Canal, annotations or as separate lists. (For is quoted as saying: example, a list of which ATONs “It was a piece of work that have horns, gongs, or other sound- reminds me of what a producing devices would be a If you elect to drift or anchor, teacher said to me when I handy reference in circumstances of remember that, while this gives you was at Exeter over forty reduced visibility.) The mere act of valuable time to assess your situa- years ago, that if he had five writing this information down tion, it confers no immunity from minutes in which to solve a increases the navigator’s retention. accidents. It is still important to stay problem he would spend Precalculated turning bearings alert and maintain a proper lookout. three deciding the best way (even if buoys are available) on The majority of recreational boat- to do it.” what are expected to be prominent ing accidents (nearly three-quar- So it is with the practice of nav- visual landmarks save work during ters) occur while vessels are cruis- igation. Time spent in planning the the actual approach when time may ing, engaged in special operations details of a voyage is time well be at a premium. (e.g., water-skiing, towing, being spent. Simply put, if you fail to Preplanning is also very impor- towed), or maneuvering, but an plan, you plan to fail. tant for operators of fast boats. This appreciable fraction (15 percent) Search and rescue experience is one of the key points made by occur while drifting or at anchor. If indicates that too many mariners authors who write about fast boat circumstances permit (e.g., absence neglect to plan fuel needs adequate- navigation (e.g., Bartlett, 1992; of rocks, shoals, bars, crab or lob- ly. They behave as though they Kettlewell and Kettlewell, 1993; ster pots), it may be advisable to were driving automobiles in easy Pike, 1990). Fast boats, typically, move a shallow-draft vessel out of range of the next service station. On have smaller helm areas (which the main channel to avoid traffic the water, refueling opportunities renders plotting more difficult) and conflicts with deep-draft vessels. are often more limited. Moreover, are lively (which also makes plot- When it comes to stopping, ves- other factors combine to make fuel ting a chore). Moreover, there is sels have an extraordinary advan- consumption per nautical mile quite less time to take and plot bearings. tage over aircraft. A boat can be variable: unanticipated currents, a Waypoints should be carefully allowed to drift or the anchor can be diversion to assist a disabled vessel, entered (and checked) into naviga- set to provide ample time for care- etc. For these reasons, formal fuel tion receivers before the trip, plau- ful consideration of options. An planning, as discussed in Chapter sibility checks on visual fixes aircraft can be stopped only when it 11, is important. Fuel planning should be preplanned to reduce the lands or crashes. The maximum should not be limited to before-the- time required for taking and inter- possible flight duration can be cal- fact calculations. A mariner needs preting these, and routes should be

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L FIG. 12-1–The destroyers S. P. Lee (right) and Nicholas (left) seen against the rock-strewn coast. Photograph originally appeared in Tragedy at Honda, and is reproduced with the kind permission of the Lockwood family. selected to maximize the number of fixes is likely to be much more pro- opment of contingency plans gives navigation marks along the way ductive than the same time spent at the navigator a real edge in the bat- (the distance penalty is of less con- the helm of a pitching vessel. tle with panic when trouble devel- sequence on a fast boat). As noted in other chapters of ops. It’s much more calming (to the The mariner should devote this text, it is impossible to plan out navigator and crew alike) to be able some time to thinking through pos- everything in advance; unanticipat- to say “Let’s put plan B into opera- sible contingencies during the voy- ed events can and do arise. tion,” rather than “This sure doesn’t age planning process. For example, However, planning can go a long look like Kansas, Toto; any ideas?” the mariner might plan to make a way towards reducing the likeli- Planning, like checklists, serves to landfall by offsetting the intended hood and/or consequences of an make contingencies routine. aim point so as to arrive definitely unpleasant surprise. Arrival at the either north or south of the harbor proper time to take advantage of a PRINCIPLE 8: entrance (a trick of long-ago and slack or favorable current (Chapter BE OPEN TO DATA OR present-day navigators alike) and to 8), the onboard availability of the INFORMATION AT VARIANCE turn one way or the other to follow appropriate large-scale nautical a particular depth contour to the charts for alternate destinations, WITH YOUR UNDERSTANDING main channel. However, this plan is adequate fuel margins (Chapter 11), OF THE SITUATION certain to need revision if the depth distance of visibility arcs (Chapter Robert Jervis, a political scien- sounder becomes inoperative en 10) scribed on charts, and an array tist writing on misperception in route. Time spent in the comfort of of prominent pre-identified fix international politics, writes, your den or office thinking about objects, for example, will not be “actors tend to perceive what they how to handle this contingency and happenstance events if a little care expect.” Humans often tend to the precalculation of alternate is exercised in the planning phase form fixed ideas based upon limited check points based upon visual of a voyage. Moreover, the devel- information. Once formed, these

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L FIG. 12-2–Port side of the destroyers Young (middle distance) and Chauncey (foreground). The Woodbury (309) can be seen in the distance. Photograph originally appeared in Tragedy at Honda, and is reproduced with the kind permission of the Lockwood family. ideas are often difficult to displace occurred on numerous occasions in track was maintained, predicated on and contrary evidence may be dis- the history of navigation. Perhaps the assumption of a 20-knot speed. missed as incorrect or overlooked the most dramatic example of this RDF was then a new system and, in entirely. error occurred on the night of 8 the opinion of the many mariners, The same phenomenon also September 1923 when nine navy unproven. Nonetheless, RDF bear- occurs with navigators. In this con- destroyers ran aground and seven ings were obtained from a shore text, the fixed idea might be the were stranded/sunk off the coast of station. These bearings gave LOPs appropriateness of a particular California. that were inconsistent with the lead method of navigation (compared to In brief, the story is as follows. destroyer’s (the Delphy) DR plot, alternatives), or even the accuracy (See Alden, 1965; Hadaway, 1957; indicating that the squadron was not of a particular position estimate Lockwood and Adamson, 1960; making good the estimated speed of developed by this method. Often Mueller, 1980, for details.) The advance (SOA) and/or the squadron these ideas are soundly based; for destroyers were part of a navy train- was closer to the shore than example, a visual fix taken on posi- ing exercise involving a high-speed planned. tively identified fixed ATONs is endurance test run down the coast An anticipated sighting of a light likely to be more accurate than one of California from San Francisco to at Point Arguello was not observed, based solely on buoys. The danger San Diego. The lead destroyer had and so no visual LOPs or running arises when conflicting information three experienced navigators fixes were obtained from this source. from other sources is simply reject- aboard, one fresh from teaching Visibility was sharply reduced by ed out-of-hand without more care- navigation at the U.S. Naval fog. Given this situation, a prudent ful examination. Academy. mariner should have reduced speed Misperception errors have During the run, a careful DR to try to sort things out and, in this

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L FIG. 12-3–Another photograph of Chauncey (296) at right and Woodbury (309) at left on what is now known as Woodbury Rock. The mast of the Fuller (297) can be seen projecting above Woodbury Rock. Photograph courtesy of the Lockwood family. case, to obtain soundings that might neous. Indeed, it was assumed that a learned from this tragedy—e.g., confirm or reject the DR position. reciprocal bearing was obtained. The some navigators on the following (One of the destroyers, the squadron altered course to an easter- destroyers had a very different Thompson, did just that but, as this ly heading and, one-by-one in appreciation of the situation, yet did was the last destroyer in the column, sequence, crashed onto the rocks not question the order to alter this did not help the others.) This appropriately named “the Devil’s course to the east, etc. But the action was not taken by the lead Jaw” well north of the DR position. errors that stand out in this context ship, in part because of the demands Figures 12-1, 12-2, and 12-3 show are the reluctance and, ultimately, of the training exercise to maintain photographs taken after the disaster. failure of the lead navigation team speed. In any event, a further RDF Remarkably, although nine vessels to consider information at variance bearing was taken which also indi- were sunk or damaged, only 23 with their preconceived DR-based cated that the squadron was well sailors of the 800 at risk were killed. estimate of position (Principle 8), north of its DR position at the The full version of this abbreviated their unwillingness to seek addi- entrance to the Santa Barbara chan- story can be found in the excellent tional depth information (Principle nel. Despite this evidence, the navi- (but, unfortunately, out-of-print) 3), or their failure to slow down to gators in the lead destroyer clung to book, Tragedy at Honda, listed sort things out (Principle 6). the idea that the DR was correct and among the references. Many observers have drawn the that the RDF bearings were erro- There are many lessons to be analogy between navigation and

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more stringent if components of IT’S COMFORTING WHEN THE PIECES FIT… the landing system are inopera- tive. Recreational boaters are not fed- DRIFT erally licensed and have much greater flexibility of operation. DEPTH Nonetheless, the idea of self- imposed limits is valuable. It is SET ATONS obviously foolish, for example, to SEEN deliberately undertake an ocean DR PLOT voyage without, at least, a working GPS knowledge of celestial navigation and the appropriate tools, reference publications, and nautical charts. LORAN (This notwithstanding, the story published in one of the, otherwise fine, boating magazines tells of a BUT EQUALLY IMPORTANT IF THEY DON’T major ocean crossing in which the navigator brought books and embarked on a “crash”, on-the-job solving a puzzle. The pieces of the example, cannot legally under- self-training program in celestial puzzle are the various bits of infor- take a flight unless the weather navigation!) mation, or misinformation, and the conditions along the route of Some harbors may require navigator has to assemble these into flight satisfy certain constraints extensive local knowledge for safe a coherent picture. It’s comforting in terms of ceiling and visibility. operation, while others have easy when the pieces fit—but this princi- Additionally, the pilot must sat- and well-marked approaches. Some ple reminds us that it is equally isfy recent experience and harbors may be easy to enter during important if they don’t. retraining requirements. conditions of daylight and good vis- J Destination weather: In addi- ibility, but tricky at night or in fog. PRINCIPLE 9: tion to requirements on the In this latter example, a prudent KNOW AND OPERATE demonstrated proficiency of the mariner would plan to arrive at the WITHIN YOUR LIMITS pilot, each type of instrument harbor during times of good visibil- In commercial aviation, the con- approach to a destination airport ity and either divert to an alternate cept of limits and standards for safe has unique weather minimums. destination or anchor out if this operation are well-developed and So-called precision approaches proved impossible. codified. For example, there are to an airport enable the pilot to A prudent navigator gives explicit regulatory limits on: descend lower and operate in thought to these and other factors in J the aircraft: Commercial air- lower visibility than do non- deciding whether or when to craft have minimum equipment precision approaches to the attempt a particular voyage. Table lists (MELs) which determine same airport. And even for the 12-1 provides a candidate list of exactly what equipment must be same type of instrument factors to consider in establishing on board and operational before approach (e.g., ADF), the personal limits to operation. There a flight is permitted. weather minimums may vary is no attempt to spell out detailed J from airport to airport depend- go-no-go rules. This is left to the the pilot: Pilots are, likewise, ing upon terrain and obstruc- discretion of the navigator. subject to a variety of explicit tions. Finally, the legal weather However, it is suggested that the limits/requirements. Pilots with- minimums for a particular navigator think carefully about out an instrument rating, for instrument approach become establishing personal limits for

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these and other relevant factors. impaired judgment. Fatigue is not accidents attributed to sleep depri- Over time, these limits are likely to often explicitly cited as a cause of vation, including the Exxon Valdez change. As the navigator gains vessel or aircraft accidents, in part (the third mate had slept only 6 experience, local knowledge, because data may be lacking and/or hours in the previous 48), space and/or operates a better-equipped the effects of fatigue are difficult to shuttle Challenger (officials of the vessel, these personal limits can be quantify. Nonetheless, it is general- National Aeronautics and Space modified or stretched. ly recognized that a well-rested Administration (NASA) suffered One of the “personal mini- crew is essential to safe operations. from severe sleep deprivation, mums” often overlooked by Here are a few examples of the con- according to the Human Factors mariners and aircraft pilots alike are sequences of fatigue: Subcommittee), and 1995 crash of limits related to fatigue. Mariners J Fatigue was one of the causes an American Airlines flight in and aircraft pilots can suffer two listed in the crash of a Learjet on Colombia. overlapping types of fatigue, acute final approach to Byrd Field in J The Golden Gate, a San skill fatigue and chronic fatigue. Richmond, Virginia, on 6 May Francisco pilot boat, ran into the Acute skill fatigue is that diminu- 1980. At the time of the acci- bridge of the same name and tion of acuity resulting from task dent, the pilot had been awake suffered hull and engine damage repetition during an extended voy- for 20 hours, and the copilot for (Anon, 1994a). It was alleged age. Symptoms include lassitude, 18 hours. The crash came after that the pilot fell asleep or was an inability to concentrate, and an the pilots misjudged the daydreaming when the accident aversion to further activity. Fatigue approach. occurred. The vessel was being leads to degradation of skills, Dement and Vaughn (1999) guided by an autopilot at the increased reaction time, and relate several other examples of time of the accident.

1. General knowledge and recent experience of navigator and crew 2. Local knowledge of navigator and crew 3. Navigation equipment (including up-to-date and corrected nautical charts and navigation reference publications) and communications gear aboard 4. En route and destination weather (visibility and sea conditions) and daylight/darkness at ETA 5. Features of destination harbor and en route: –hazards to navigation –availability of ATONs 6. Availability and characteristics of nearby alternate destinations 7. Health and fatigue status of navigator/crew 8, General seaworthiness and type/size/speed of vessel

L TABLE 12-1–A suggested list of factors to consider for setting personal limits.

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J The 642-foot freighter Pacific correctly. As noted in many places (Anon, 1993d) after becoming Breeze ran aground at 16 knots in this text, the marine environment confused over the identity of in the Chesapeake Bay near is notoriously hostile (or, at least, buoys in calm weather and clear Sharps Island Light. The pilot is unkind) to electronics and power visibility. Coast Guard investi- alleged to have fallen asleep and supplies alike. Barely a month goes gators said he had not fixed his did not make a necessary course by without one or more articles position on a chart but was pro- correction. The chief mate was appearing in the boating press ceeding entirely by seaman’s “just too polite to wake the pilot; reporting instances of shipboard eye. I don’t think he realized how failures of electronic components J The third mate of the 989-foot fast asleep the pilot was,” on recreational, commercial, and freighter Indiana Harbor was observed a Coast Guard investi- naval vessels. Fuses blow, antennas blamed after the ship drove gating officer. When the pilot are knocked down, green water directly into the Lansing Shoals fell asleep he reportedly had had breaks over the bow and saturates Light on Lake Michigan. Coast only six hours of sleep in the the supposedly waterproof loran, Guard investigators believed previous 24 hours. and batteries or alternators fail. G. that the mate was navigating Mariners should establish per- R. Kane, for example, in his inter- solely by radar without plotting sonal fatigue standards as part of esting book, Instant Navigation positions and “just failed to their rules for go-no-go decisions. (Kane, 1984), writes of a racing notice the lighthouse ahead of yacht lost on a Cuban shoal after a the ship.” loran failure which occurred after PRINCIPLE 10: J The 525-foot cruise ship starting the engine to recharge the MAINTAIN A DR PLOT Starward ran aground on the batteries. This was an unpleasant Arguably, the need to maintain island of St. John (U.S. Virgin event, to be sure, and one with crit- an accurate DR plot falls under sev- Islands) in 1994 (Anon, 1994c). ical consequences because the nav- eral principles enumerated above. In this case, GPS positions were igator apparently did not maintain a But, to lend emphasis to this impor- plotted on a semiregular basis, DR track or record of fixes. tant point, it is made a separate but no radar ranges or bearings There are numerous instances principle. Were this text written a were taken. No DR positions where well-equipped ships have run few years ago, it would probably were plotted and no determina- aground and failure to maintain a not have been necessary to under- tion of the current set and drift DR plot was cited as a reason for score the need for DR plotting. were made. The ship drifted loss of situational awareness. Here However, the “electronic revolu- onto rocks in the vicinity of is a small sample of cases: tion”, with its proliferation of ever Dittlif Point, a spit on the south J more lightweight, lower cost, and The 207-foot cruise ship coast of the island. The ship more powerful navigation tools Nantucket Clipper grounded on could be backed off the coral, (such as GPS, radar, electronic the rocks in Penobscot Bay, but only after leaving parts of its charting, and integrated navigation Maine, in 1992 (Anon, 1993c). starboard propeller at the scene packages), has rendered DR all but Coast Guard investigators of the grounding. obsolete in the minds of many nav- pressed charges against the first If these accidents can happen to igators. mate after determining that he experienced mariners, they can Electronic navigation is fast, had referred solely to the radar happen to you. accurate, convenient, and powerful. for navigation and did not plot Redundancy is a partial solution The system reliability of the princi- positions or maintain a DR plot to the availability/reliability prob- pal electronic navigation aids that on the chart. lems of electronic systems. Carry require external signals (e.g., J The mate on the tug Janice Ann extra fuses, bulbs, batteries, an Loran-C, GPS) is very high. Yet, for Reinauer drove a loaded fuel emergency antenna, and even com- these systems to function, the ship- barge up onto rocks outside of plete systems, such as a portable board components must also work New London Harbor in 1993 GPS. But this solution is expensive

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and, in any event, still imperfect. A this chapter. CONCLUDING COMMENTS cheaper and ultimately more satis- Another very useful source of Probably most readers of this factory solution to the problem is to information is provided in a series text will never elect to make navi- maintain a DR plot and update this of accident reports written by the gation a profession, in the sense of by frequent visual fixes as dis- National Transportation Safety a paying occupation. But all readers cussed in Chapters 5 and 6 of this Board (NTSB), an independent of this text have the opportunity to text. Unless the mariner is voyaging agency of the U.S. Government that become a professional navigator in investigates aircraft, marine, high- near homeport in well-known the sense of developing a profes- way, railroad, and pipeline acci- waters and the actual and forecast sional attitude and professional cal- dents. NTSB maintains a Web Site weather is benign, a DR plot should iber skills and judgment. (http://www.ntsb.gov/) which con- be maintained and updated, as new Graduation from high school, col- tains copies of these accident data become available. lege, or university is often called As the story of the tragedy at reports. Periodically, access this site and download copies of recent commencement, because this marks Honda illustrates, DR positions can reports for later study. NTSB the point when the graduate com- still be in error. But these are marine accident (click on the mences to gain experience, maturi- undoubtedly superior to no position Marine or Publications icons) ty, and insight—in a word wisdom. at all. Remember the proverb that in reports include both commercial Go forth, learn, enjoy, stay humble the land of the blind, the one-eyed ship and recreational boat mishaps. (remember, it is arrogant to sail to a man is king. This site also contains copies of destination and appropriately hum- other relevant reports. For example, ble to sail towards a destination), KEEP READING the report Evaluation of U.S. continue to study (the chapter refer- Numerous sea stories are Department of Transportation ences are a good starting point), and included in this chapter to illustrate Efforts in the 1990s to Address write to tell us of your experiences. the points made in the text. These Operator Fatigue provides an inter- Better still, join the USCGAUX are taken from the literature illus- esting summary of the operator and consider writing the next edi- trated in the references at the end of fatigue problem. tion of this book!

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SELECTED REFERENCES

Alden, J. D., (1965). Flush Decks and Four Pipes, Bachand, R. G., (1989). Northeast Lights: Lighthouses United States Naval Institute, Annapolis, MD. and Lightships Rhode Island to Cape May, New Jersey, Sea Sports Publications, Norwalk, CT. Anon, (1990). A Spin of the Dial, Ocean Navigator, No. 30, January/February, pp. 67-68. Bartlett, T., (1992). Navigation at Speed, Fernhurst Books, Brighton, UK. Anon, (1993a). Fishing Vessel Hits Beach in Navigational Blunder, Professional Mariner, No. 1, Bloomfield, H. V., (1966). The Compact History of the p. 29. United States Coast Guard, Hawthorn Books, New York, NY. Anon, (1993b). Navigation Lapse Sent Chesapeake Freighter Aground, Professional Mariner, No. 3, Bright, C., (1987). A Confusing Night Off Cape Oct/Nov, p. 31. Hatteras, Ocean Navigator, No. 14, July/August 1987, pp. 6, 44. Anon., (1993c). First Mate Defends License After Grounding Cruise Ship, Professional Mariner, No. Cahill, R. A., (1985). Strandings and Their Causes, 1, p. 28. Fairplay Publications, London, UK.

Anon., (1993d). Navigational Lapse Caused Chichester, Sir F. (1967). Gypsy Moth Circles the Grounding off New London, Professional Mariner, World, Coward-McCann, Inc., New York, NY. No. 1, p. 24. Coote, J. O., (1988). Yacht Navigation—My Way, W. W. Anon., (1994a). New San Francisco Pilot Boat Crashed Norton and Company, New York, NY. into Golden Gate Bridge, Professional Mariner, No. 8, Aug/Sept, p. 48. Creamer, M., (1986). Dead Reckoning during the Onslaught, Ocean Navigator, January/February, p. Anon., (1994b). Third Mate Charged With Negligence 39. after Piloting Ship into Lighthouse, Professional Mariner, No. 6, p. 49. Dement, W.C. and Vaughan, C., (1999). The Promise of Sleep, Delacorte Press, New York, NY. Anon., (1994c). What Did Happen That Morning Aboard the Starward?, Professional Mariner, No. Hadaway, R. B., (1957). Course Zero Nine Five, 7, p. 52. United States Naval Institute Proceedings, Vol. 83, pp. 40-48. Anon, (1995a). Space-age Cruise Ship Goes Aground, Ocean Navigator, No. 70, p. 38. Haines, T. B. (1990). Retrospective, AOPA Pilot, Vol. 33, No. 2, February, p. 102. Anon, (1995b). Delaware Pilot Charged with Negligence After Study of Grounding, Professional Hall, G. E., (1989). The Correct Use of Corrections, Mariner, No. 13, June/July, p. 63. Ocean Navigator, No. 23, January/February, pp. 15-16. Anon, (1995c). Freighter Runs Aground in Chesapeake as Pilot Nods off in Pilot Seat, Professional Hoffer, W., and Hoffer, M. M., (1989). Freefall, 41,000 Mariner, No. 10, pp. 40-41. Feet and Out of Fuel, St. Martin’s Press, New York, N.Y. Anon, (1987). Nantucket LNB Goes Adrift, Ocean Navigator, No. 11, January/February, p. 12. Howell, S., (1985). Aground in the Bahamas, Ocean Navigator (then called The Navigator), July/August, No. 2, pp. 8 - 10.

12-19 12 Reflections

SELECTED REFERENCES

Jervis, R., (1968). Hypotheses on Misperception, United States Department of Transportation, United World Politics, Vol. 20, No. 3, pp. 454 - 479. States Coast Guard. Boating Statistics, COMDT- PUB P 16754.1, Washington, DC, Various years. Jones, R. V. (1967). The Theory of Practical Joking; Its Relevance to Physics, Bulletin of the Institute of United States Department of Transportation, United Physics, June, p. 7. States Coast Guard. (1988). Shiphandling D 42601-0, U.S. Coast Guard Institute, Oklahoma Kane, G. R., (1984). Instant Navigation, Second City, OK. Edition, Associated Marine, San Mateo, CA. United States Power Squadrons®, (1986). The Advanced Kettlewell, J. J. and Kettlewell, L., (1993). Reed’s Piloting Course, USPS, PAP-85/86, Appendix SM Nautical Companion, The Handbook to B-2. Complement Reed’s Almanacs, North American Edition, Boston, MA. Walsh, G., (1987). Poor Navigation Leads to Disaster, Ocean Navigator, No. 13, May/June, p. 5, 6. Leonard, B.A., (1998). Navigating Safely with GPS, Sail, Vol. 29, No. 1, January, pp. 96 et seq. Waters, J. M., Jr., (1989). Rescue at Sea, Second Edition, Naval Institute Press, Annapolis, MD. Lockwood, C. A., and Adamson, H.C., (1960). Tragedy at Honda, Chilton Company, Philadelphia, PA. Wright, F. W., (1982). Celestial Navigation, Cornell Maritime Press, Centreville, MD. Lord, W., (1988). A Night To Remember, Bantam Books, New York, NY, p. 135.

MacLeish, W. H., (1989). The Gulf Stream, Houghton Mifflin Co., Boston, MA.

McClench, D., and Millar, D.B., Eds., (1979). Mixter’s Primer of Navigation, Fifth Edition, Van Nostrand Reinhold Co., New York, NY.

McCullough, D., (1977). The Path Between the Seas, Simon and Schuster, New York, NY.

Milligan, J. E., (1974). The Amateur Pilot, Cornell Maritime Press, Centerville, MD.

Mueller, W., (1980). Seven Out at Pt. Honda, Sea Classics, Special Edition, Spring, p. 40 et seq.

Pike, D., (1990). Fast Boat Navigation, Adlard Coles, London, UK.

Quinn, W. P., (1988). Shipwrecks Along the Atlantic Coast, Parnassus Imprints, Orleans, MA.

Rousmaniere, J., (1989). The Annapolis Book of Seamanship, New Revised Edition, Simon and Schuster, NY, p. 264.

12-20 Glossary A

GLOSSARY

This section contains a glossary of nautical terms for convenient reference. Many, but not all, of these terms are used in this Advanced Coastal Navigation (ACN) text or on the 1210-Tr chart. These definitions are brief, of necessity. Definitions are also included for some specialized terms of art that space and scope constraints prevented from being discussed at any length in the text. The reader is sometimes referred to the appropriate chapter in the text and the associated refer- ences for a more complete discussion. Certain of these definitions have been taken from the Light List, the Tide Tables, the United States Coast Pilot, and Bowditch. Many specialized terms that are used on nautical charts are included. The 1210-Tr chart is an important part of this course, and it is appropriate to provide the technical definitions of many of the terms used on this chart. To many, the distinctions among such terms as rock awash, rock, and sub - merged rock may seem pedantic. But, these distinctions could be important to the navigator. As it is, aside from Bowditch (which, though excellent, is sometimes found to be intimidating by many begin- ning stu dents of navigation), there is no single, generally accessible, and easily readable source for these spe cialized terms. It is hoped that the ACN student finds this a worthwhile source.

Adrift—Loose, not on moorings or towline. A Advanced LOP—A line of position which has been Abeam—At right angles to the keel of the boat, but not moved forward, parallel to itself, along a course on the boat. line to obtain a line of position at a later time. If the same procedure is followed to move the line to an Aboard—On or within the boat. earlier time, the LOP is said to be retired. Abreast—Side by side; by the side of. Aft—The stern or back of the vessel. Absolute Accuracy—Term often used in connection Agger—See Double Tide. with a navigation system to mean the difference between the system’s estimate of position and the Agonic Line—The imaginary line connecting points of actual position. Also called geodetic accuracy. For zero variation. example, within the designated coverage area, the Aground—A vessel touching or fast to the bottom. absolute accuracy of Loran-C is 0.25 NM or better. Aid to Navigation (ATON or NAVAID)—Any device Acquisition—Loran-C: The reception and identifica- external to a vessel or aircraft specifically intended tion of trans mitted navigation signals from master to assist navigators in determining their position or and selected secondaries to permit reliable mea- safe course, or to warn them of dangers or obstruc - surement of posi tion. GPS: The reception and tions to navigation. identification of transmitted signals from various Alee—Away from the direction of the wind. Opposite satellites to permit reliable measurement of posi- of windward. tion. Align—To place objects in line. Additional Secondary Factors—Land path factors due to variation in the conductivity of the earth’s sur- face that alter the speed of propagation of loran sig- nals over land compared to over water. ASFs degrade the absolute accuracy of a loran system (unless com pensated for) but do not affect the repeatable accu racy.

A-1 A Glossary

Allision—n. [L. allisio, Fr. Allidere, to strike against or Arc of Visibility—The portion of the horizon over dash against; ad + laedere to dash against.] The act which a lighted aid to navigation is visible from of dashing against, or striking upon. Term used in sea ward. admiralty law to denote the act of striking or colli- Armed Lead—A weight that has a hollowed bottom sion of a moving vessel against a stationary object. and is filled with tallow, grease, wax, chewing Thus, for example, a vessel could have an allision gum, or bedding compound to bring up a sample of with a bridge, dock, or vessel tied to a dock. This the bot tom. differs from a collision, which occurs if a moving Articulated Beacon—A beacon-like buoyant structure, vessel strikes another moving object—particularly tethered directly to the seabed and having no watch a vessel. circle. Called articulated light or articulated Alternating Light—A rhythmic light showing light of daybea con, as appropriate. alternating colors. Assigned Position—The latitude and longitude position Amidships—The center of the boat with reference to its for an aid to navigation. length or breath. Astern—Direction of movement, opposite of ahead; Anchor Alarm—Feature of many navigation receivers toward a vessel’s stern. that can be set to warn the user that the vessel has Athwartships—Across or at right angle to the center- moved outside the swing circle of the anchor. This line of a boat; rowboat seats are generally placed is also termed an anchor watch. athwartships (thwarts). Anchor Rode—See Rode. Automated Notices to Mariners System—Computer Anchor Scope—See Scope. system that can be accessed by authorized users to Anchorage—A place suitable for anchoring in relation obtain chart corrections and notices to mariners. to the wind, seas and bottom; and permitted by reg - Users need a Teletype, computer terminal, or other ulations. device, and an access code available from NIMA. Anchorage Mark—A navigation mark, which indicates Autopilot—Device for automatic steering of a vessel. an anchorage area or defines its limits. Depending upon the sophistication of the autopilot, Angle of Cut—The smaller angular difference of two these can be used to maintain a heading, or to inter - bearings or lines of position. See also crossing face with a loran or other electronic navigation sys - angle. tem. Sometimes informally called “George” or “Iron Mike.” Annotation—Any marking on illustrative material for the purpose of clarification, such as numbers, let - Axis of Rotation—The imaginary line connecting the ters, symbols, or signs. poles of the earth and on which the earth supposed - ly spins or rotates. Annual Inequality—Seasonal variation in the water level or current, more or less periodic, due chiefly to meteorological causes. B Apogean Tides or Tidal Currents—Tides of range or currents of decreased speed monthly as the result of Back Range—A range observed astern, particularly one the moon being in apogee (farthest from the earth). used as guidance for a craft moving away from the objects forming the range. Apogee—Point in the lunar cycle when the moon and the earth are furthest apart. Tides have decreased Bar—A ridge or mound of sand, gravel, or other uncon - range when the moon is in apogee. solidated material below the high water level, espe - cially at the mouth of a river or estuary, which may Arc Measure—The angle included between radii con - obstruct navigation. necting the ends of an arc with the center of the cir - cle of which it is a part. Bare Poles—Term used to describe a sailboat that is underway with no sails set.

A-2 Glossary A

Bare Rock—A rock that extends above the mean high Blink—An indication that the master or secondary sig - water datum in tidal areas or above the low water nals in a loran chain is out of tolerance and not to datum in the Great Lakes. See also rock awash, be used. Loran-C receivers have a blink alarm that sub merged rock. warns the user that the indicated positions may not Baseline—The segment of a great circle that joins the be reliable. Blink conditions warn that the signal master and a secondary station in a loran chain. power or TD is out-of-tolerance (OOT) and/or that an improper phase code or GRI is being transmit- Baseline Extension—The extension of the baseline ted. GPS receivers have analogous integrity checks. beyond the two joined stations. Loran positions in baseline extension areas are problematic and Bluff—A headland or stretch of cliff having a broad, ambiguous. nearly perpendicular, face. See also cliff. Bay—A recess in the shore, on an inlet of a sea or lake Boat—A fairly indefinite term. A waterborne vehicle between two capes or headlands, that may vary smaller than a ship. One definition is a small craft greatly in size but is usually smaller than a gulf and carried aboard a ship. Submarines, however, are larger than a cove. universally referred to as boats Beacon—A lighted or unlighted fixed aid to navigation Bobbing a Light—Quickly lowering the height of eye attached directly to the earth’s surface. (Lights and several feet and then raising it again when a naviga - daybeacons both constitute “beacons.”) tional light is first sighted to determine whether or not the observer is at the geographic range of the Beam—The greatest width of the boat. light. If he/she is, the light disappears when the eye Beam Sea—Waves act directly on the vessel’s sides is lowered and reappears when it is restored to its (coming from abeam) and, in rough water, could original position. roll some boats over on their side. Commonly Bolt Holes—Safe places to anchor or moor in the event known as “in the trough.” that the weather worsens or mechanical difficulties Bearing—The horizontal direction of a line of sight occur. between two objects on the surface of the earth. Boulder—A detached water-rounded stone more than Beat—To sail to windward, generally in a series of 256 millimeters in diameter, i.e., roughly larger tacks. Beating is one of the three points of sailing, than a basketball. also referred to as sailing “close-hauled” or “by the Bow—The forward part of a boat. wind.” Breakwater—Anything that breaks the force of the sea Bell—A sound signal producing bell tones by means of at a particular place, thus forming protection for a hammer actuated by electricity or, on buoys, by vessels. Often an artificial embankment built to sea motion. pro tect the entrance to a harbor, or to form an arti- Bifurcation—The point where a channel divides when ficial harbor. See also jetty. proceeding from seaward. The place where two Bridge–1. An elevated structure extending across or trib utaries meet. over the deck of a vessel or part of such a structure. Bight—1. A long and gradual bend or recess in the 2. A structure erected over a depression or an obsta - coastline that forms a large open receding bay. 2. A cle, such as a body of water, railroad, etc., to pro- bend in a river or mountain range. vide a roadway for vehicles or pedestrians. Binnacle—A stand holding the steering compass. Broach—The turning of a boat parallel to the waves, Binocular—An optical instrument for use with both subjecting it to possible capsizing. eyes simultaneously. Broad on the Beam—At right angles to the keel or cen - terline. Broad On the Bow—A direction midway between abeam and dead ahead.

A-3 A Glossary

Broad On the Quarter—A direction midway between Characteristic—The audible, visual or electronic signal abeam and dead astern. displayed by an aid to navigation to assist in the Broadcast Notice to Mariners—A radio broadcast identification of an aid to navigation. Characteristic designed to provide important marine information. refers to lights, sound signals, racons, radiobea- cons, and daybeacons. Building or House—One of these terms, as appropriate, is used on nautical charts when the entire structure Chart—A nautical map for use by mariners or aviators, is a landmark, rather than an individual feature of which depicts features and displays information of it. interest to these groups. Bulkhead—A transverse, vertical partition separating Chart No. 1—A booklet prepared by the National compartments. Ocean Survey that contains symbols and abbrevia- tions that have been approved for use on nautical Buoy—A floating object of defined shape and color, charts pub lished by the U. S. Government. which is anchored at a given position and serves as an aid to navigation. Chart Scale—The number of distance units on the earth’s surface represented by the same distance Buoy System—IALA (See IALA) Maritime Buoyage unit on the chart. Charts are typically partitioned on System B applies to buoys and beacons in U.S. the basis of scale. Sailing charts have scales of waters that indicate the later al limits of navigable 1:600,000 and greater. General charts have scales channels, obstructions, dan gers (such as wrecks), between 1:150,000 and 1:600,000. Coast charts and other areas or features of importance to the have scales between 1:40,000 and 1:150,000. mariner. Harbor charts have scales larger than 1:40,000. Burdened Vessel—That vessel which, according to the Chart Symbol—A character, letter, or similar graphic applicable Navigation Rules, must give way to representation used on a chart to indicate some another (privileged) vessel. The terms have been object, characteristic, etc. May be called map sym - superseded by the terms “give-way” and “stand- bol when applied to any map. on.” Chimney—A relatively small, upright structure project - ing above a building for the conveyance of smoke. C Clay—See Mud. Cable Area—Area shown on charts transited by subma - Clear—To leave or pass safely, as to clear port or clear rine cables. Formal anchorage restrictions may a shoal. apply in cable areas. Clearing Bearing—British term for danger bearing. Cairn—A mound of rough stones or concrete, particu - Cliff—Land arising abruptly for a considerable dis- larly one serving, or intended to serve, as a land- tance above water or surrounding land. See also mark. The stones are customarily piled in a pyra- bluff. midal or beehive shape. Close Aboard—Not on, but near to, a vessel. Cape—A relatively extensive land area jutting seaward Close-hauled—Sailing with the boom hauled as close from a continent, or large island, which prominent - to the centerline of the vessel as is possible, thus ly marks a change in, or interrupts notably, the sail ing as much into the wind as is possible. Also coastal trend. known as beating, or “by the wind”, one of the Channel—1. That part of a body of water deep enough three points of sailing. for navigation through an area otherwise not suit - Closest Point of Approach—The closest distance that a able. It is usually marked by a single or double line target will pass clear of the reference vessel. This of buoys and sometimes by ranges. 2. The deepest distance is estimated from the relative motion plot. part of a stream, bay, or strait, through which the The estimated time that this occurs is called the main current flows. 3. A name given to large straits, time to closest point of approach (TCPA). for example, the English Channel.

A-4 Glossary A

Clutter (Radar)—Unwanted radar echoes reflected Compass Heading—The direction a vessel is heading at from heavy rain, snow, waves, etc., which may any one instant as shown by its compass. obscure relatively large areas on the PPI—and, Compass Point—One of 32 points of the compass equal thus, targets of interest. Related terms: sea clutter, to 11-1/4 degrees. sea return, rain clutter. Compass Rose—The resulting figure when the com - Cobble—See Stones. plete 360-degree directional system is developed as Coastal Confluence Zone—A zone extending seaward a circle with each degree graduated upon it, and 50 nautical miles from shore or to the 100-fathom with the 000 indicated as true north. Also called curve, whichever is greater. true rose. This is printed on nautical charts for Coastal Navigation—Navigation in coastal (sometimes determin ing direction. called pilot) waters, where the opportunity exists to Composite Group-Flashing Light—A group-flashing determine or check the vessel’s position by refer - light in which the flashes are combined in succes - ence to navigational aids and observations (by sive groups of different numbers of flashes. either visual or electronic means) of the coast and Composite Group-Occulting Light—A light similar to a its fea tures. group-occulting light, except that the successive Cocked Hat—Error triangle formed by lines of position groups in a period have different numbers of that do not cross at a common point. So named eclipses. because of the characteristic appearance of these Conformal Projection—A map or chart projection that lines in the vicinity of the fix. The size of the preserves correct angular relationships. cocked hat is an indication of the precision of the Contact—Any echo detected on the radarscope not fix—and is valuable information to the navigator. eval uated as clutter or as a false echo. The term For this rea son, conservative navigators term a “con tact” is used in a general sense, whereas “tar- position a fix only if at least three objects are used get” (q.v.) is used in a more particular sense to to determine the fix. Fixes determined from only denote a contact about which more information two LOPs would be relegated to the status of esti- (such as CPA, TCPA, course, or speed) is desired. mated posi tions in this view. Thus, radar targets would typically be plotted. Of Cockpit—An opening in the deck from which the boat course, the difference is not clear-cut—one naviga- is handled. tor’s con tact might be another’s target. COLREGS, 72—The international rules for preventing Contour Lines—Lines that connect points of equal collisions at sea, called the International Rules. depth on a nautical chart. Coming About—The changing of course when close- Conventional Direction of Buoyage—The general hauled by swinging the bow through the eye of the direc tion taken by the mariner when approaching a wind and changing from one tack to another; har bor, river, estuary, or other waterway from sea- reverse course or nearly so. ward, or proceeding upstream or in the direction of Commissioned—Specialized term of art to denote the the main stream of flood tide, or in the direction action of placing a previously discontinued aid to indi cated in appropriate nautical documents (nor- navigation back in operation. mally, following a clockwise direction around land Compass Card—Part of a compass, the card is graduat - mass es). ed in degrees, to conform with the magnetic merid - Coriolis Force—The deflective effect of the earth’s ian-referenced direction system inscribed with rota tion on an object in motion, which causes it to direction which remains constant; the vessel turns, divert to the right in the Northern Hemisphere. See not the card. Rotary Current. Compass Errors—Generic term used to describe all Correcting—Converting a compass heading or a mag - compass errors including variation, deviation, netic heading to its equivalent true heading. northerly turning error, acceleration error, heeling error. These are discussed at length in Chapter 2.

A-5 A Glossary

Course (C)—Course is the average heading and the Current—Term used in two senses: It is used to refer horizontal direction in which a vessel is intended to either to the horizontal motion over the ground, be steered, expressed as the angular distance rela- including ocean current, tidal, and river currents, or tive to north, usually from 000 degrees at north, more generally to these factors together with the clock wise through 359 degrees from the point of effect of wind and seas, steering error of the helms - depar ture or start of the course to the point of man, compass error, speed curve error, and other arrival or other point of intended location. factors. (See Chapter 7.) Course Deviation Indicator—An indicator, shown on Current (alternate definition)—Generally, a horizontal some navigation receivers, that graphically dis- movement of water. Currents may be classified as plays whether or nor the vessel is on course and, if tidal and nontidal. Tidal currents are caused by not, the direction to return to course. grav itational interactions between the sun, moon, Course LOP—An LOP situated approximately directly and earth and are a part of the same general move- ahead or behind the vessel, so named because the ment of the sea that is manifested in the vertical rise LOP provides a good indication of the vessel’s and fall, called tide. Nontidal currents include the CMG. per manent currents in the general circulatory sys- tems of the sea as well as temporary currents aris- Course Made Good (CMG)—This indicates the single ing from more pronounced meteorological variabil- resultant direction from a point of departure to a ity. point of arrival at a given time. (Synonym: Track Made Good) Current Correction Angle—The difference between the intended track and the calculated course to steer to Course Of Advance (COA)—This indicates the direc - compensate for the estimated current. tion of the intended path to be made good over the ground. Current Difference—Difference between the time of slack water (or minimum current) or strength of Course over the Ground (COG)—This indicates the cur rent in any locality and the time of the direction of the path actually followed by the ves- correspond ing phase of the tidal current at a refer- sel over the ground, usually an irregular line. ence station, for which predictions are given in the Cove—A small sheltered recess or indentation in a Tidal Current Tables. shore or coast, generally inside a large embayment. Current Drift Angle—The difference in angle between Cross Rate (Cross Chain) Interference—Interference in the course steered and the resulting CMG in the the reception of radio signals from one loran chain presence of current. caused by signals from another loran chain. Current Ellipse—A graphic representation of a rotary Cross-Track Error (XTE)—Distance between the ves - current in which the velocity of the current at sel’s actual position and the direct course between differ ent hours of the tidal cycle is represented by two waypoints. radius vectors and vectorial angles. The cycle is Cross-Track Error Alarm—Alarm that can be set on completed in one-half tidal day or in a whole tidal many navigation receivers that warns the navigator day accord ing to whether the tidal current is of the if the vessel’s cross-track error exceeds some pre - semidiurnal or the diurnal type. A current of the specified value. mixed type will give a curve of two unequal loops Crossing Angle—Generally, the angle between two each tidal day. LOPs which determine a fix. The closer this angle Current Sailing—The process of allowing for current in is to 90 degrees, the better the fix. Also used with determining the predicted course made good, or in loran LOPs. determining the effect of a current on the direction Cupola—A small dome-shaped tower or turret rising and speed of motion of a vessel. from a building.

A-6 Glossary A

Departure—A known location (fix) from which a dead D reckoning plot is initiated. Danger Bearing—The maximum or minimum bearing Depth Sounder—An electronic means of measuring of a point for safe passage of an off-lying danger. water depth by sound waves. As a vessel proceeds along a coast, the bearing of a Deviation—The effect of the vessel’s magnetic fields fixed point on shore, such as a lighthouse, is mea - upon a compass. Deviation is the difference sured frequently. As long as the bearing does not between the direction that the compass actually exceed the limit of the predetermined danger bear - points and the direction that the compass would ing, the vessel is on a safe course with respect to the point if there were no magnetic fields aboard the hazard in question. vessel. Danger Buoy—A buoy marking an isolated danger to Diaphone—A sound signal which produces sound by navigation, such as a rock, shoal or sunken wreck. means of a slotted piston moved back and forth by Datum—The technical term for the baseline from compressed air. A “two-tone” diaphone produces which a chart’s vertical measurements are made— two sequential tones with the second tone of lower heights of land or landmarks, or depths of water. pitch. Daybeacon—A fixed NAVAID structure used in shal- Dinghy—A small open boat. A dinghy is often used as low waters upon which is placed one or more day- a tender for a larger craft. marks. Direction (true)—The angle between the local true Daylight Saving Time—A time used during the sum- meridian and a line from the observer’s position to mer in some localities in which clocks are an object or another location. advanced one hour from the usual standard time. Direction of Relative Motion—Determined from the Daymark—A signboard attached to a daybeacon to rel ative motion plot, this is the apparent course of con vey navigational information presenting one of the target as inferred from observations on the radar sev eral standard shapes (square, triangle, or rectan- screen. gle) and colors (red, green, orange, yellow, or Directional Light—A light illuminating a sector or very black). Daymarks usually have reflective material narrow angle and intended to mark a direction to be indicat ing the shape, but may also be lighted. followed. Dead Ahead—A relative bearing of 000 degrees. Discontinued—To remove from operation (permanent- Dead Astern—Directly aft. ly or temporarily) a previously authorized aid to navi gation. Dead in the Water—Adrift, floating with the current. Discrepancy—Failure of an aid to navigation to main - Dead Reckoning—The practice of estimating position tain a position or function as prescribed in the Light by advancing a known position for courses and dis - List. tances run. The effects of wind and current are not considered in determining a position by dead reck - Discrepancy Buoy—An easily transportable buoy used oning. to temporarily replace an aid to navigation that is not watching properly. See Watching Properly. Dead Reckoning Plot—A DR plot is the charted move - ment of a vessel as determined by dead reckoning. Displacement—The weight of water displaced by a floating vessel, thus, a boat’s weight. Dead Reckoning (DR) Position—A position deter- mined by dead reckoning. Displacement Hull—A type of hull that plows through the water, displacing a weight of water equal to its Deck—A permanent covering over a compartment, own weight, even when more power is added. (See hull, or any part thereof. Chapter 11.) Demarcation Line—Boundary shown on nautical charts between areas where inland navigation rules and international navigation rules apply.

A-7 A Glossary

Diurnal—Having a period or cycle of approximately water consisting of two maxima of nearly the same one tidal day. Thus, the tide is said to be diurnal height separated by a relatively small depression, when only one high water and one low water occur or a low water consisting of two minima separated during a tidal day, and the tidal current is said to be by a relatively small elevation. Sometimes, it is diurnal when there is a single flood and single ebb called an agger. period in the tidal day. A rotary current is diurnal if Doubling the Angle on the Bow—A method of it changes its direction through all points of the calculat ing a running fix by measuring the distance compass once each tidal day. a vessel travels on a steady course while the rela- Diurnal Inequality—The difference in height of the two tive bearing (right or left) of a fixed object doubles. high waters or of the two low waters of each day; The distance from the object at the time of the sec- also, the difference in speed between the two flood ond bearing is equal to the distance run between tidal currents or the two ebb tidal currents of each bearings, neglect ing drift. day. The difference changes with the declination of Draft—The vertical depth from the bottom of the keel the moon and, to a lesser extent, with the declina- to the top of the water. tion of the sun. Drift—The speed in knots at which the current is mov - Dividers—An instrument consisting of two pointed ing. Drift may also be indicated in statute miles per legs joined by a pivot, and used principally for hour in some areas, the Great Lakes, for example. measur ing distances or coordinates. An instrument This term is also commonly used to mean the speed having one pointed leg and the other carrying a pen at which a vessel deviates from the course steered or pen cil is called a drafting compass. due to the combined effects of external forces such Dock—A protected water area in which vessels are as wind and current. moored. The term is often used to denote a pier or Drms—A term used to describe the statistical accuracy a wharf. of an electronic or other fix. Twice the Drms is the Dolphin—A minor aid to navigation structure consist - radius of a circle that should include the fix point ing of a number of piles driven into the seabed or with at least 95 percent certainty. riverbed in a circular pattern and drawn together Drogue—Any device streamed astern to slow a vessel’s with wire rope. speed, or to keep its stern to the waves in a follow - Dome—A large, rounded, hemispherical structure ris- ing sea. ing above a building or a roof of the same shape. Drying Heights—Heights above chart sounding datum Double Ebb—An ebb tidal current where, after ebb of those features which are periodically covered begins, the speed increases to a maximum called and exposed by the rise and fall of the tide. first ebb and then decreases, reaching a minimum Dumping Ground—Area shown on charts where dump - ebb near the middle of the ebb period (and, at some ing took place (the practice is no longer permitted) places, it may actually run in a flood direction for a and which may present a hazard to navigation. short period); it then again ebbs to a maximum Duration of Flood and Duration of Ebb—-Duration of speed called second ebb, after which it decreases to flood is the interval of time in which a tidal current slack water. is flooding, and the duration of ebb is the interval in Double Flood—A flood tidal current where, after flood which it is ebbing. begins, the speed increases to a maximum called Duration of Rise and Duration of Fall—Duration of rise first flood; it then decreases, reaching a minimum is the interval from low water to high water, and flood near the middle of the flood period (and, at duration of fall is the interval from high water to some places, it may actually run in an ebb direction low water. for a short period); it then again floods to a maxi - mum speed called second flood, after which it decreases to slack water. Double Tide—A double-headed tide; that is, a high

A-8 Glossary A

Dutchman’s Log—A buoyant object thrown overboard Emergency Light—A light of reduced intensity dis - to determine the speed of a vessel. The time played by certain aids to navigation when the main required for a known length of the vessel to pass light is extinguished. the object (assumed to be dead in the water) is mea - Emergency Position Indicating Radio Beacon sured. Speed can be computed from the two known (EPIRB)—A device which emits a continuous values of time and distance. The Dutchman’s log radio signal, alerting authorities to the existence of can also be used to measure the drift of a current if a dis tress situation and leading rescuers to the a vessel can be held stationary (keep station) with scene. respect to a fixed object. Endurance—The time in hours that the vessel can be operated at a given throttle setting until the en route E fuel is exhausted. En Route Fuel—Fuel intended for use in a voyage. Ebb Current—The movement of a tidal current away Numerically, the en route fuel is the fuel on board from shore or down a tidal river or estuary. In the minus an allowance for a fuel reserve. Also termed mixed type of reversing tidal current, the terms voyage fuel. greater ebb and lesser ebb are applied, respectively, Ensign—A national or organizational flag flown aboard to the ebb tidal currents of greater and lesser speed a vessel. of each day. The terms maximum ebb and mini- mum ebb are applied to the maximum and mini- Equator—Great circle formed by passing a plane per - mum speeds of a current running continuously ebb, pendicular to the axis of rotation of the earth. the speed alternately increasing and decreasing Equatorial Tidal Currents—Tidal currents occurring without coming to a slack or reversing. semimonthly as the result of the moon being over Echo—Term used in radar to denote an object that the equator. At these times, the tendency of the reflects the radar beam, often used interchangeably moon to produce a diurnal inequality in the tide is with the terms return, target, blip, contact, and pip. at a minimum. Properly speaking, however, there are subtle dis - Equatorial Tides—Tides occurring semimonthly as the tinctions among these. See, for example, contact. result of the moon being over the equator. At these Eclipse—An interval of darkness between appearances times, the tendency of the moon to produce a diur- of a light. nal inequality in the tide is at a minimum. Electronic Bearing Line—An adjustable bearing line, Establish—To place an authorized aid to navigation in which appears as a spoke radiating from the center operation for the first time. of the PPI. EBLs are used to measure the bearing Estimated Position (EP)—An improved position based of a target. Also called electronic bearing marker. upon the DR position and which may include, Electronic Bearing Marker—See electronic bearing among other things, factoring in the effects of wind line. and current, a single line of position, or all of the above. Electronic Chart—A device that can display a chart- like representation on a screen. Some electronic Extinguished—A lighted aid to navigation that fails to charts are very elaborate and allow the user to show a light characteristic. “zoom in” to examine an area at a larger scale. Existence Doubtful—Term used principally on charts Depth contours, NAVAIDS, and other chart fea- to indicate the possible existence of a rock, shoal, tures can be dis played—even down to individual or other obstruction, for which the actual existence docks at certain locations. Electronic charts can has not been established. interface with other shipboard electronics, such as a GPS or loran, and display the vessel’s current posi- tion, waypoints, and related information.

A-9 A Glossary

Flashing Light—A light in which the total duration of F light in each period is clearly shorter than the total duration of darkness and in which the flashes of Fair Current—Current moving in the same direction as light are all of equal duration. (Commonly used for the vessel. a single-flashing light that exhibits only single Fall Off—To turn the bow of the boat away from the flashes, which are repeated at regular intervals.) direction of the oncoming wind. Float Plan—A document that describes the route(s) and Fast—Said of an object that is secured to another. estimated time of arrival of a particular voyage. Fathom—A nautical measure of length, six feet, used The float plan generally includes a description of for measuring water depth and length of anchor the vessel, radio and safety equipment carried, rode. planned stops, names of passengers, and other per- Fix—A known position determined by passing close tinent information. aboard an object of known position or determined Floating Aid to Navigation—A buoy secured in by the intersection of two or more lines of position assigned position by a mooring. (LOPs) adjusted to a common time, determined Flood Current—The movement of a tidal current from terrestrial, electronic, and/or celestial data toward the shore or up a tidal river or estuary. In the (see Chapter 6). The accuracy, or quality of a fix, is mixed type of reversing current, the terms greater of great importance, especially in coastal waters, flood and lesser flood are applied, respectively, to and is dependent on a number of factors. the flood currents of greater and lesser speed of Fixed ATON—An aid to navigation placed on a fixed each day. The terms maximum flood and minimum structure such as a lighthouse, tower, etc. flood are applied to the maximum and minimum Fixed Bridge—A bridge that does not lift, swing, or speeds of a flood current, the speed of which alter- oth erwise open for vessel traffic. nately increases and decreases without coming to a Fixed Light—A light showing continuously and steadi - slack or reversing. ly, as opposed to a rhythmic light. (Do not confuse Fluxgate Compass—A compass that senses the earth’s with “fixed” as used to differentiate from “float - magnetic field electronically, rather than with mag - ing.”) nets. Fluxgate compasses can interface with other Fixed Range Markers—A series of concentric range shipboard electronics such as radar, GPS, and rings displayed on a PPI. A range switch can adjust loran. the spacing of these rings, but all FRMs are fixed in Flying Bridge—An added set of controls above the relation to each other. level of the normal control station for better visibil- Flag Tower—A scaffold-like tower from which flags ity. Usually open but may have a collapsible top for are displayed. shade. Flagpole—A single staff from which flags are dis - Fog Detector—An electronic device used to automati - played. The term is used when the pole is not cally determine conditions of visibility which war - attached to a building. rant the turning on and off of a sound signal or addi tional light signals. Flagstaff—A flagpole rising from a building. Fog Signal—See Sound Signal. Flash—A relatively brief appearance of a light, in com - parison with the longer interval of darkness in the Following Sea—Sea in which the waves move in a same character. direction approximately the same as the vessel’s heading. Opposite of head sea. Flash Tube—An electronically controlled high-intensi- ty discharge lamp with a very brief flash duration. Fore-and-Aft—In a line parallel to the keel. Forward—Toward the bow of the boat. Fouled—Any piece of equipment that is jammed entan - gled, or dirty.

A-10 Glossary A

Foul Current—Current moving in the opposite direc- Gradient—The ratio of the spacing between adjacent tion to the vessel. loran TDs, as measured in nautical miles, yards, or Foul Ground—An area unsuitable for anchoring due to feet, and the number of microseconds difference being strewn with rocks, boulders, coral, or between these lines. Generally speaking, the small - obstruc tions. Foul grounds are often shown on nau- er the gradient, the better the fix. tical charts. Graticule—The network of lines representing parallels Founder—When a vessel fills with water and sinks. and meridians on a map, chart, or plotting sheet. Freeboard—The minimum vertical distance from the Gravel—See Stones. surface of the water to the gunwale cap. Great Circle—The circle formed on the earth’s surface Frequency—The rate at which a cycle is repeated. when a plane is passed through the earth’s center. Fuel Consumption Chart—Chart or graph that relates Great Diurnal Range—The difference in height the engine throttle setting, speed through the water, between mean higher high water and mean lower and the fuel burn rate in gallons per hour. low water. The expression may also be used in its contracted form, diurnal range. Fuel Efficiency—The distance that a vessel can travel on each gallon of fuel. Fuel efficiency is a function Ground Swell—See Swells. of the throttle setting and several other factors dis - Ground Tackle—A collective term for the anchor and cussed in Chapter 11. its associated gear. Fuel Reserve—A quantity of fuel set aside for possible Ground Wave—A radio wave that travels near or along contingencies. the earth’s surface. Ground wave signals are used for the present loran system G Group Flashing Light—A navigational aid light that emits flashes in groups, specified repeated at regu- Gear—A general term for rope, block(s), tackle, and lar intervals. other equipment. Group Occulting Light—An occulting light in which a Geometric Dilution of Precision (GDOP)—Term used group of eclipses, specified in number, is regularly to include all geometric factors (gradient, crossing repeated. angle) that degrade the accuracy of position fixes Group Repetition Interval—Length of time (in from externally referenced navigation systems, microseconds) between the start of one transmis- such as GPS or Loran-C. GDOP can be calculated sion from the master station in a Loran-C chain and from an equation, which summarizes these effects the start of the next. For convenience, the GRI is in one single number. usual ly divided by 10. Thus, for example, the 9960, Gimbals—A pair of rings pivoted on axes at right or Northeast U. S. Chain, has a group repetition angles to each other, so that one is free to swing inter val of 99,600 microseconds. within the other; a ship’s compass, for instance, Gudgeon—The eye supports for the rudder, mounted will keep a horizontal position when suspended on on the transom and designed to receive the pintles. gimbals. Gulf—That part of an ocean or sea extending into the Give-way Vessel—A term, from the Navigational land, usually larger than a bay. Rules, used to describe the vessel which must yield Gulf Coast Low Water Datum—A chart datum. in meeting, crossing, or overtaking situations. Specifically, the tidal datum formerly designated Gong—A wave-actuated sound signal on buoys, which for the coastal waters of the Gulf Coast of the uses a group of saucer-shaped bells to produce dif - United States. It was defined as mean lower low ferent tones. water when the type of tide was mixed and mean GPS—Global Positioning System, an electronic navi - low water when the type of tide was diurnal. gation system using satellites for worldwide cover - Gunwale—The upper edge of a boat’s sides. age. See Chapter 9 of this text.

A-11 A Glossary

Higher Low Water (HLW)—The higher of the two low H waters of any tidal day. Hachures——Short marks on topographic maps or Hole—A small bay (or channel), particularly in New nauti cal charts to indicate the slope of the ground or England. the submarine bottom. These marks usually follow Homing—Process of moving towards a location by the direction of the slope. con tinually pointing the bow of the vessel in the Half-Tide Level—See mean tide level. direc tion of the station. In the absence of current, homing will lead to a ground track that is a straight Hand-Bearing Compass—Portable compass (magnetic line. With any current, however, the ground track or electronic) that is used aboard ship for taking will become curved, bowed in the direction of the bearings. pre vailing current. Head Sea—Sea in which the waves move in a direction Hook—Something resembling a hook in shape, particu - approximately opposite the vessel’s heading. larly, a spit or narrow cape of sand or gravel which Opposite of following sea. turns landward at the outer end; or a sharp bend or Heading (HDG)—The instantaneous direction of a ves - curve, as in a stream. sel’s bow. It is expressed as the angular distance Horn—A sound signal, which uses electricity or com - rel ative to north, usually 000 degrees at north, pressed air to vibrate a disc diaphragm. clock wise through 359 degrees. Heading should not be confused with course. Heading is a constant- Howgozit Chart—Chart that depicts the vessel’s actual ly changing value as a vessel yaws back and forth fuel quantity at various points in a voyage in com- across the course due to the effects of sea, wind, parison to the amount of fuel required to reach the and steering error. Heading is expressed in degrees destination with various levels of fuel reserves. with respect to true, magnetic, or compass north. Hug—To remain close to, as to hug the shore. Heading Flash—An illuminated radial line on the PPI Hull—The main body of a vessel. of a radarscope for indicating the reference ship’s Hull Speed—The maximum speed of a displacement heading on the bearing dial. Also called heading vessel. It is limited by the length of the vessel and marker. the shape of its underwater construction. Headway—The forward motion of a boat through the Hydraulic Current—A current in a channel caused by a water. Opposite of sternway. difference in the surface level at the two ends. Such Heave To—To bring a vessel up in a position where it a current may be expected in a strait connecting will maintain little or no headway, usually with the two bodies of water in which the tides differ in time bow into the wind or nearly so. To stop. or range. Heavy Iron—-Slang expression used to denote large Hyperbolic Grid—Lattice of curved (hyperbolic) lines ships. of position produced by a hyperbolic system. Heel—To tip or lean to one side. Hyperbolic System—Navigation system, such as Helm—The wheel or tiller controlling the rudder. Loran-C, that operates by measuring the time dif- ference between signals transmitted by two or more High Frequency (HF)—A special frequency band used trans mitters. in long-distance communications. High Water (HW)—The maximum height reached by a rising tide. The height may be due solely to the peri odic tidal forces or it may have superimposed upon it the effects of prevailing meteorological condi tions. Higher High Water (HHW)—The higher of the two high waters of any tidal day.

A-12 Glossary A

Details of the buoyage system and other relevant I information are available from the IALA web site (http://www.iala-aism.org/iala.htm) or another that IALA—Acronym for the International Association of provides several interesting documents Lighthouse Authorities, an organization founded in (http://www.beta.ialahq.org/). 1957 to gather together marine navigation authori- ties, manufacturers, and consultants throughout the Interrupted Quick Light—A quick flashing light in world. For details, see International Association of which the rapid alternations are interrupted at regu - Lighthouse Authorities. lar intervals by eclipses of long duration. IALA Region A—IALA region in which red is to port Intrusion Alarm—Alarm that can be set on a radar unit on entering. Region A includes Europe, Australia, to alert the radar operator that a target has penetrat - New Zealand, Africa, the Gulf, and some Asian ed a range ring. countries. Iron Genny—Slang expression to denote the engine of IALA Region B—IALA region in which red is to star- a sailboat. board when entering from seaward (i.e., the “red- Iron Mike—Slang expression for autopilot. right-returning” system). Region B includes the countries of North, Central, and South America, Isogonic Lines—Lines on a chart connecting points of Japan, Korea, and the Philippines. equal magnetic variation. Inclinometer—Device to measure the angle of roll of a Isolated Danger Mark—A mark erected on, or moored vessel. above or very near, an isolated danger which has navigable water all around it. Index Diagram—An inset in a nautical chart where con tiguous or related charts at different scales are Isophase Light—A rhythmic light in which all dura- noted. tions of light and darkness are equal. (Formerly called equal interval light.) Inoperative—Sound signal or electronic aid to naviga - tion out of service due to a malfunction. Isthmus—A narrow strip of land connecting two larger portions International Association of Lighthouse Authorities (IALA)—(In French, Association Internationale de Signalisation Maritime (AISM)), also known as the J International Association of Marine Aids to Navigation and Lighthouse Authorities. IALA is a Jetty—A structure built out into the water to restrain or non-profit international technical association estab- direct currents, usually to protect a river mouth or lished in 1957 and headquartered in St. Germain en harbor entrance from silting, etc. Laye, France. IALA comprises 200 members, 80 of Junction—The point where a channel divides when which are national navigation authorities and 60 are pro ceeding seaward. Also used to describe the commercial firms. Delegates to IALA have estab- place where a tributary departs from the main lished a uniform system of maritime buoyage now stream. being implemented by most maritime nations. Within the single system, there are two regions: denoted Region A and Region B, where lateral K marks differ only in the colors of port and starboard Keel—The main structural member of a vessel running hand marks. In Region A, red is to port on entering; fore-and-aft; the backbone of a vessel. in Region B, red is to starboard on entering. (Although this system—in which buoys have the Knot (kn. sometimes kt.)—A measure of speed equal to opposite lateral significance in the two regions— one nautical mile (6,076 feet) per hour. may appear confusing, it is an improvement. As late as 1976 there were more than thirty different buoyage systems in use throughout the world.)

A-13 A Glossary

Light Sector—The arc, over which a light is visible, L described in degrees true, as observed from sea- ward towards the light. May be used to define dis- Landmark—A conspicuous artificial feature on land, tinctive color difference of two adjoining sectors, other than an established aid to navigation, which or an obscured sector. can be used as an aid to navigation. Sometimes also used in a less technical sense to include natural fea - Lighted Ice Buoy (LIB)—A lighted buoy without a tures as well as artificial features. sound signal, and designed to withstand the forces of shift ing and flowing ice. Used to replace a con- Large Navigational Buoy (LNB)—Buoys developed to ventional buoy when that aid to navigation is replace lightships and are placed at points where it endangered by ice. is impractical to build a lighthouse. The unmanned LNBs are 40 feet in diameter with light towers Lighthouse—A lighted beacon of major importance. approximately 40 feet above the water. LNBs are Line of Position (LOP)—A line of bearing to a known equipped with lights, sound signals, radiobeacons, origin or reference, upon which a vessel is assumed and racons. LNBs are painted red, not for lateral to be located (see Chapter 6, Line of Position). An sig nificance, but to improve visibility. LOP is determined by observation (visual bearing) Lateral System—A system of aids to navigation in or measurement (radar). An LOP is assumed to be which characteristics of buoys and beacons indicate a straight line for visual bearings, an arc of a circle the sides of the channel or route relative to a con - (radar range), a sphere (GPS), or part of some other ventional direction of buoyage (usually upstream). curve such as hyperbola (loran). LOPs resulting from visual observations (magnetic bearings) are Latitude—Distance north or south of the equator generally converted to true bearings prior to plot- expressed in degrees from zero to ninety, north or ting on a chart. south; i.e., 073° N. Line of Sight—The straight line between two points. Lead Line—A line used to measure the depth of the This line is in the direction of a great circle, but water. does not follow the curvature of the earth. Leading Lights—British terminology for range lights. Local Notice to Mariners (LNM)—A written document Ledge—A rocky projection or outcrop on the sea floor. issued by each U. S. Coast Guard District to Lee—The side sheltered from the wind; the direction dissem inate important information affecting aids to toward which the wind is blowing. navi gation, dredging, marine construction, special Leeward—The direction away from the wind. Opposite marine activities, and bridge construction on the of windward. waterways within that district. Leeway—The sideways movement of the boat caused Log—A daily record of a ship’s progress or operations by either wind or current. and messages sent or received on its radio. A device Leg—That portion of a voyage track that can be repre - to measure a vessel’s speed. To record a ship’s sented by a single course line. A track could be progress in a journal. com posed of several legs. Longitude—Distance east or west of the prime meridi- Legend—A title or explanation on a chart, diagram, or an expressed in degrees from zero to 180 degrees illustration. east or west. Light—Lighthouses or beacons, fixed aids to naviga - Lookout Station (watchtower)—A tower atop a small tion, or a vessel’s navigation lights. On a vessel, house used for observation. lights are designed to help identify the size, direc - Loran—A contraction of long-range navigation, used to tion of movement, status of the vessel, and some - describe an electronic navigation system using a times the tasks being performed. chain of transmitting stations that allows mariners Light List—-USCG Publication discussed in Chapter (or aviators) to determine their position 10.

A-14 Glossary A

Loran-C LOP—Line of position as determined from Luminous Range Diagram—A diagram used to convert reception of the loran master signal and that of one the nominal range of a light to its luminous range secondary. Loran-C LOPs at convenient intervals under existing conditions. The ranges obtained are are plotted on NOAA charts. approximate. (See Chapter 10.) Loran Chain—Series of three to five transmitting sta - Lunar Day—The duration of one rotation of the earth tions consisting of a master station and two to four on its axis, with respect to the moon. Its average secondary stations used in the loran system. length is about 24 hours and 50 minutes. Loran Linear Interpolator—A small inset diagram shown on loran overprinted charts that enables M inter polation of time differences. Loran Pulse—Basic “building block” of the transmitted Magnetic Compass—A magnet, balanced so that it can loran signal. The loran pulse exhibits a characteris - pivot freely in a horizontal plane; a sailor’s most tic (and well-controlled) waveform which can be common and most reliable direction-indicating aid. identified and timed by a receiver. The loran signal Magnetic Direction (M)—A direction relative to the from a master station actually consists of nine puls - earth’s magnetic field and magnetic north. es. The first eight pulses are spaced 1,000 Magnetic courses are labeled with an “M” to signi- microsec onds (ms) apart, followed at an interval of fy “mag netic.” 2,000 microseconds by the ninth pulse. Secondary Magnetic Meridian—A system of “meridians” passing stations transmit only eight pulses, each separated through the earth’s magnetic poles. A compass by 1,000 microseconds. Pulsed transmission saves aligns with these “meridians” if there is no local on the power required for signal transmission and magnetic field on the vessel to cause deviation. facili tates signal identification. Multiple pulse transmis sion is used rather than single pulse trans- Make Fast—To secure by belaying or hitching; to fas- mission to increase the average power of the loran ten in place, as a sail; to secure a ship at a pier, etc., signal. by dock lines. Low Water (LW)—The minimum height reached by a Maneuvering Board—A printed compass rose that is falling tide. The height may be due solely to the used together with parallel rulers and dividers to periodic tidal forces or it may have superimposed solve problems of the movement of vessels relative upon it the effects of meteorological conditions. to each other, such as those that arise when the ves - Use of the synonymous term, low tide, is discour- sels change position relative to each other. Used, aged. for example, when another vessel is observed on a radarscope. Lower High Water (LHW)— The lower of the two high waters of any tidal day. Marine Radiotelephone—VHF-FM radio; an important safety device in emergencies. Lower Low Water (LLW)— The lower of the two low waters of any tidal day. Mark—A visual aid to navigation. Often called naviga - tion marks; includes floating marks (buoys) and Loxodrome—Any line on the earth’s surface (other fixed marks (beacons). than due east or due west) which cuts successive meridi ans of longitude at the same oblique angle. Masking—Obscuration of an object. Radar masking When extended, it spirals toward, but never reach- (also radar shadow) refers to a phenomenon in es, one of the earth’s poles. which a target is obscured (masked) by virtue of its location behind another larger target—such as a Lubber’s Line—A mark or permanent line on a com - mountain, structure, or other vessel. Visual mask- pass, which is used to read the compass heading of ing can also occur. a vessel. When properly mounted it is parallel to the vessel’s keel.

A-15 A Glossary

Master Station—Essential component of a Loran-C Mercator Projection—The projection technique most chain. This station broadcasts the signal that is used commonly used in navigational charts; shapes and to identify the chain (the GRI) and is the common distances are increasingly distorted as you move base against which all time differences are calculat - into extreme northern and southern areas. This is a ed. cylindrical projection ingeniously modified by Mean High Water (MHW)—The arithmetic mean of expanding the scale at increasing latitudes to pre - the high water heights observed over a specific 19- serve ship’s direction and angular relationships. year cycle. For stations with shorter series, simulta- Meridian (Geographic Meridian)—A great circle of the neous observational comparisons are made with a earth passing through both the geographic poles primary control tide station in order to derive the and any given point on the earth’s surface. equivalent of a 19-year value. Meteorological Visibility—The greatest distance at Mean Higher High Water (MHHW)—The arithmetic which a black object of suitable dimension could be mean of the higher high water heights of a mixed seen and recognized against the horizon sky by day, tide observed over a specific 19-year cycle. Only or, in the case of night observations, could be seen the higher high water of each pair of high waters, or and recognized if the general illumination were the only high water of a tidal day, is included in the raised to the normal daylight level. mean. Microsecond (µsec)—One millionth of a second. Mean Low Water (MLW)—A tidal datum. The arith - Midship—Approximately in the location equally dis- metic mean of the low water heights observed over tant from the bow and stern. a specific 19-year Metonic cycle (the National Mileage Number—A number assigned to aids to navi - Tidal Datum Epoch). For stations with shorter gation which gives the distance in sailing miles series, simultaneous observational comparisons are along the river from a reference point to the aid to made with a primary control tide station in order to navigation. The number is used principally in the derive the equivalent of a 19-year value. Mississippi River System Mean Low Water Springs (MLWS)—Frequently abbre - Mixed Tide—Type of tide with a large inequality in the viated spring low water. The arithmetic mean of the high and/or low water heights, with two high low water heights occurring at the time of the waters and two low waters usually occurring each spring tides observed over a specific 19-year cycle. tidal day. In strictness, all tides are mixed but the Mean Lower Low Water (MLLW)—The arithmetic name is usu ally applied to the tides intermediate to mean of the lower low water heights of a mixed those pre dominantly semidiurnal and those pre- tide observed over a specific 19-year cycle. Only dominantly diurnal. the lower low water of each pair of low waters, or Most Probable Position—Vessel’s probable position the only low water of a tidal day is included in the considering all available navigational information. mean. This is the vertical datum used on U.S. charts The term is generally used when there is position for portraying water depths. uncertainty as a result of conflicting or ambiguous Mean Range of Tide—The difference in height between information. mean high water and mean low water. Mud—A general term applied to mixtures of sediments Mean Sea Level (MSL)—The arithmetic mean of in water. Where the grains are less than 0.002 mil - hourly water elevations observed over a specific limeter in diameter, the mixture is called clay. 19-year cycle. Shorter series are specified in the Where the grains are between 0.002 and 0.0625 name, e.g., monthly mean sea level and yearly mil limeter in diameter, the mixture is called silt. mean sea level. See also sand, stones, and rock. Mean Tide Level (MTL)—Also called half-tide level. A Multiple Ranges—A group of two or more ranges hav - tidal datum midway between mean high water and ing one of the range marks in common. mean low water.

A-16 Glossary A

Mushroom Anchor—A stockless anchor with a cast North Geographic Pole—A reference for specifying a iron bowl at the end of the shank; used principally position on the earth’s surface, at the north end of in large sizes for permanent moorings. the earth’s axis. Also called true north. North Magnetic Pole—The central point of the north N end of the earth’s magnetic core to which a com- pass points when it is free of other influences. NAVCEN—The United States Coast Guard’s North Up—Type of relative motion radar display with Navigation Center. The center maintains a web site, own ship at center. This is linked to a gyrocompass which provides information on the organization and or fluxgate compass to display a continuous north its services (http://www.navcen.uscg.gov/). up picture on the PPI. NIMA—National Imagery and Mapping Agency (suc- Notch Filters—Filters in a loran receiver that are either cessor to the Defense Mapping Agency). For infor- fixed or capable of being tuned to reduce (“notch mation on NIMA, see its web site at out”) the effects of interfering signals. Some filters (http://www.nima.mil/). (termed “PacMan” filters) can automatically seek Nautical Chart—See Chart. and notch out interfering signals. Typical signals Nautical Mile (M)—One minute of latitude; approxi - that can cause loran interference are listed in the mately 6,076 feet—about 1/8 longer than the Loran-C Handbook. The notch filters on a loran statute mile of 5,280 feet. should be adjusted for the area of intended cruising to maximize the efficiency of the filtering. Nautical Slide Rule—Analog device for solving time- speed-distance calculations. In present manufac- Notice to Mariner (NTM or NM)—Publication, similar ture, these are typically circular slide rules with to Local Notice to Mariners that contains informa - three separate scales graduated in units of time, tion applicable to routes traversed by larger ships. speed, and distance. (See Chapter 10.) Navigation—The art and science of conducting a boat safely from one point to another. O Navigation Rules (NAVRULES)—Regulations govern - Oblate Spheroid—Sphere flattened at the poles, resem - ing the movement of vessels in relation to each bling a pumpkin. other, formerly “Rules of the Road.” Occulting Light—A light in which the total duration of Neap Tides or Tidal Currents—Tides of decreased light in each period is clearly longer than the total range or tidal currents of decreased speed occurring duration of darkness and in which the intervals of semi monthly as the result of the moon being in darkness (occultations) are all of equal duration. quadra ture. The neap range of the tide is the aver- (Commonly used for single occulting light, which age semidiurnal range occurring at the time of neap exhibits only single occultations that are repeat ed at tides regular intervals.) Neck—1. A narrow isthmus, cape, or promontory. 2. Ocean Data Acquisition System (ODAS)—Certain The land between streams flowing into a sound or very large buoys in deep water for the collection of bay. 3. A narrow strip of land, which connects a oceanographic and meteorological information. All peninsula with the mainland. 4. A narrow body of ODAS buoys are yellow in color and display a yel- water between two larger bodies. low light. NOAA—National Oceanic and Atmospheric Offshore Tower—Monitored light stations built on Administration. For details on this organization exposed marine sites to replace lightships. and its services, visit its web site at (http://noaa.gov/). Off Station—A floating aid to navigation not on its assigned position

A-17 A Glossary

Omega—Electronic navigation system (now no longer Perigean Tides or Tidal Currents—Tides of increased used). This is not discussed in the ACN text. See range or tidal currents of increased speed occurring ref erences listed at the end of Chapter 9 for details. monthly as the result of the moon being in perigee On Plane—As more and more speed is gained, a boat or nearest the earth. The perigean range of tide is feels as though it has “climbed out of its hole,” and the average semidiurnal range occurring at the time it rides up “on plane.” of Perigean tides. Ooze—A soft, slimy, organic sediment covering part of Perigee—Point in the lunar cycle when the moon and the ocean bottom. the earth are closest together. Tides have increased range when the moon is in perigee. Out of Tolerance—A condition in which a Loran-C sig - nal or time difference exceeds established toler - Period—The interval of time between the commence - ances. An out-of-tolerance (OOT) condition causes ment of two identical successive cycles of the char - the secondary transmitter to blink. acteristic of the light or sound signal. Outfall—The discharge end of a narrow stream, sewer Personal Flotation Device (PFD)—A life preserver drain, etc. which, when properly used, will support a person in the water. PFDs are available in several sizes and Overboard—Over the side. types. Phase Code Interval—That interval over which the P phase code repeats itself. For the Loran-C system, phase codes repeat every two GRIs. Palisades—A line of cliffs. Phase Coding—Not discussed in the text, this is a Paraline Plotter—Plotter that has a set of rollers scheme of changing the phase of the pulses in a attached to enable the device to be moved parallel transmitted loran signal to minimize pulse-to-pulse to itself and used for the same purpose as parallel skywave interference and to reject synchronous rules. interfering signals. Master and secondary transmit - Parallel of Latitude—Any of the imaginary lines paral - ters use different phase codes for signal identifica - lel to the equator and representing latitude. tion. Parallel Rules—An instrument for transferring a line Pier—A loading or mooring platform extending at an parallel to itself, used in chartwork for drawing and angle (usually a right angle) from the shore. measuring courses or bearings. Pile—A wood, metal or concrete pole driven into the Passing Light—A low intensity light, which may be bottom. Craft may be made fast to a pile; it may be mounted on the structure of another light to enable used to support a pier (see piling) or a float. the mariner to keep the latter light in sight when Piling—Support, protection for wharves, piers, etc., passing out of its beam during transit. constructed of piles (see pile). Pebble—See Stones. Pilot Waters—Areas in which the services of a pilot are Pelorus—A sighting device, marked off in degrees, recommended or required. Also used in a more gen - used to determine relative bearings. eral sense to denote waters in which navigation is Peninsula—A section of land nearly surrounded by done using pilotage/piloting. water. Frequently, but not necessarily, a peninsula Piloting—Piloting is navigation involving frequent or is connected to a larger body of land by a neck or continuous reference to charted landmarks, isth mus. ATONs, or charted objects, and depth soundings. Pivot Point—A point somewhat aft of the bow, some - where forward of the midpoint. To an observer on board, a vessel appears to turn about its pivot point.

A-18 Glossary A

Plan Position Indicator (PPI)—The screen display of a Position Approximate (PA)—Term used on nautical modern radar unit, so named because it presents a charts to denote an inexact position. This term is plan view of the area scanned. used princi pally on charts to indicate that the posi- Planing—A boat is said to be planing when it is essen - tion of a wreck, shoal, or other obstruction has not tially moving over the top of the water. been accurately determined or does not remain fixed. Planing Hull—A type of hull with flat surfaces (not nec essarily horizontal) which enable a vessel to Position Doubtful—Of uncertain position. This term is climb up its bow wave and to glide across the water used principally on charts to indicate that a wreck, when it has attained a sufficient speed. shoal, or other obstruction has been reported in var - ious positions and not definitely determined. Plotter—Device for drawing straight lines on a nautical chart, and measuring courses, bearings, and (with Position Line—See Line of Position. some plotters) distances. Predictable Accuracy—Term meaning the same as Plotting Sheet—A blank chart, usually on the Mercator absolute accuracy. projection, showing only the graticule and a com - Preferred Channel Mark—A lateral mark indicating a pass rose. The meridians are usually unlabeled by channel junction or bifurcation, or a wreck or other the publisher so that these can be appropriately obstruction which, after consulting a chart, may be labeled when the chart is used in any longitude. passed on either side. Plotting sheets are often used in lieu of charts when Primary Aid to Navigation—An aid to navigation the vessel is “off soundings” (in deep water). estab lished for the purpose of making landfalls and Point—A tapering piece of land projecting into a body coast-wise passages from headland to headland. of water. It is generally less prominent than a cape. Prime Meridian—The meridian from which longitude Point of No Return—The point of no return is the point is measured both east and west: 0° longitude. It beyond which there is not sufficient fuel on board passes through Greenwich, England, and divides to return on an out-and-back journey using the the earth into Eastern and Western Hemispheres. entire fuel on board, including the reserve. Privileged Vessel—Obsolete term for a vessel, which Point System—A nearly obsolete system of dividing a according to the applicable Navigation Rule has circle into 32 parts of 11 1/4 degrees each, for refer - right-of-way (this term has been superseded by the ence to direction. term “stand-on”). Polyconic Projection—A map or chart projection in Prohibited Area—An area shown on nautical charts which the earth is projected on a series of cones within which navigation, anchoring, or other activi - concentric with the earth’s axis and tangent to the ties are prohibited except as authorized by sphere of the earth. Charts of the Great Lakes are appropri ate authority. typically based on the polyconic projection. Prolate Spheroid—Sphere flattened at the equator, Port—The left side of a boat looking forward. A harbor. resembling a football. Port Hand Mark—A buoy or beacon which is left to the Promontory—High land extending into a large body of port hand when proceeding in the “conventional water beyond the line of the coast. Called headland direction of buoyage.” when the promontory is comparatively high and Position—On the earth, this refers to the actual geo - has a steep face. graphic location of a vessel defined by two parame - Protractor—An instrument for measuring angles on a ters called coordinates. Those customarily used are surface, such as a chart. Typically a protractor is latitude and longitude. Position may also be constructed of transparent plastic and has a semicir - expressed as a bearing and distance from an object, cular scale measured in degrees. the position of which is known. Pseudorange—Range as determined from a satellite to a GPS receiver that has not been corrected for the receiver’s clock error.

A-19 A Glossary

Pulse Repetition Frequency—Term used in radar to Radar Range—l. A range (distance) obtained with denote the average number of pulses per unit of radar. 2. The maximum distance at which a radar time. unit is effective in detecting targets. Radar Reflectors—Objects that reflect radar waves Q very well and which serve to increase the size or strength of the radar return. Some buoys, for exam- Quarter—The sides of a boat aft of amidships. ple, are equipped with a radar reflector to increase Quartering Sea—Sea coming on a boat’s quarter. the ease of detection and identification. Radar reflectors are also made to carry aboard fiberglass Quay—A structure of solid construction along a shore or wood vessels to increase the likelihood of detec- or bank which provides berthing and cargo han- tion by other radar-equipped vessels. dling facilities for ships. A similar facility of open con struction is called a wharf. Radar Transfer Plotting Sheet—Plotting sheet similar to a maneuvering board used for plotting radar tar- Quick Light—A light with more than 50, but less than gets. This sheet is discussed in Chapter 9. 80, flashes per minute. (Previously called quick flash ing light.) Radio Beacons—Transmitting stations used for radio direction finding. Radio Direction Finding—Older short-range radio nav - R igation system consisting of a series of land based stations broadcasting in the LF/MF band and on- Race—A rapid current or a constricted channel in board receivers with directional antennas. Use of which such current flows. the directional antenna enables relative bearings to Racon—Racons are devices placed on certain buoys or be determined and, by simple conversion, lines of other ATONs to increase the likelihood of detection position. For practical purposes, the RDF system and aid identification. Racons, when triggered by no longer exists in the United States. pulses from a vessel’s radar, will transmit a coded Radio Mast—A relatively short pole or slender struc- reply that is displayed on the vessel’s PPI. This ture for elevating radio antennas, usually found in reply identifies the racon station by exhibiting a groups. series of dots and dashes which appear on the PPI emanating radially from the racon. All racons oper - Radionavigation—Determining positions using radio ate in the marine radar XBand from 9,300 to 9,500 waves of known characteristics emitted from MHz. Some “frequency agile” racons also operate known locations. Forms include Loran-C, GPS, in the 2,900 to 3,000 MHz marine radar SBand. and DGPS. Radar—Self-contained navigation and collision avoid - Radio Tower—A tall pole or structure for elevating ance system consisting of a shipboard transmitter radio antennas. and receiver. The transmitter transmits briefly, then Radiobeacon—An electronic aid to navigation. shuts off to permit the receiver to “listen” for the Radius of Action—The greatest distance (in an out- reflected transmission or echo. and-back voyage) that the vessel can travel and still Radar Bearing—A bearing obtained with radar. leave sufficient fuel to return without drawing Radar Fix—A position fix determined by radar alone, down the fuel reserve. or by radar in conjunction with some other method Rake—The slant of a ship’s funnels, bow, or stern. The for determining a LOP. Conventionally, radar fixes fore-and-aft slant of a vessel’s mast. can be determined by a radar range and bearing Range—The distance in nautical miles that the vessel from an identified radar conspicuous object, by two can travel with the available fuel on board. The ranges from two such objects, or by two bearings range may or may not include an allowance for a from two such objects. fuel reserve. Range is a function of throttle setting and other factors discussed in Chapter 11.

A-20 Glossary A

Range Lights—Two lights associated to form a range, Reef—An offshore consolidated rock hazard to naviga - which often, but not necessarily, indicates a chan- tion at a depth of 16 fathoms (30 meters) or less. nel centerline. The front range light is the lower of Also used as a term for a low rocky or coral area— the two, and nearer to the mariner using the range. some of which is above water. The rear range light is higher and further from the Reference Station—A tide or current station for which mariner. independent daily predictions are given in the Tide Range of Tide—The difference in height between con - Tables and Tidal Current Tables, and from which secutive high and low waters. The mean range is corresponding predictions are obtained for subordi - the difference in height between mean high water nate stations by means of differences and ratios. and mean low water. Where the type of tide is diur- Regulatory Marks—A white and orange aid to naviga - nal, the mean range is the same as the diurnal tion with no lateral significance. Used to indicate a range. special meaning to the mariner, such as danger, Range Ring—Circular line on PPI (either fixed or vari- restricted operations, or exclusion area. able) denoting a particular distance from the Relative—See Relative Direction. observer. Relative Direction (Bearing)— A direction relative to Ranges—A pair of ATONs placed a suitable distance the fore-and-aft line of a vessel, expressed in apart, with the far daymark mounted higher than degrees and labeled “R.” the near one. When the range marks are in line, the Relative Motion Plot—Typical plot prepared on a ves sel is in the channel. Other charted objects can maneuvering board to determine the point of clos- also establish ranges. est approach and time of closest approach of a radar Rate—Generic term sometimes used to describe a tar get. See Chapter 9 for details. Loran-C LOP. Nautical charts, for example, will Relighted—An extinguished aid to navigation returned identify the “rates” shown, e.g., 9960 W, 9960 X, to its advertised light characteristics. 9960 Y, 9960 Z, 7980 W, etc. Repeatable Accuracy—Term used with the loran or Reach—The comparatively straight segment of a river GPS systems to measure the repeatability of the or channel between two bends. Lat/Lo at a fixed point. Repeatable accuracy is typ- Real Time Method—An alternative radar plotting ically much greater than absolute accuracy for the method, not discussed in this text, that provides a Loran-C sys tem, but not so for GPS. rapid means of plotting radar targets. Readers inter - Replaced—An aid to navigation previously off station, ested in radar plotting are well-advised to study this adrift, or missing, restored by another aid to method. naviga tion of the same type and characteristics. Rebuilt—A fixed aid to navigation, previously Replaced (Temporarily)—An aid to navigation previ - destroyed, which has been restored as an aid to nav - ously off station, adrift, or fussing, restored by igation. another aid to navigation of different type and/or Reciprocal Bearing or Course—A bearing or course characteristic. that differs from the original by 180 degrees. Reset—A floating aid to navigation previously off sta - Reciprocal Direction—Corresponding but reversed tion, adrift, or missing, that has been returned to its direction. assigned posi tion (station). Red Sector—A sector of the circle of visibility of a nav - Restricted Visibility—Any condition in which visibili- igational light in which a red light is exhibited. ty is restricted by fog, mist, falling snow, heavy Such sectors are designated by their limiting bear- rain storms, sandstorms, or other similar causes. ings, as observed at some point other than the light. Red sec tors are often located such that they warn of danger to vessels.

A-21 A Glossary

Reversing Current—A tidal current which flows alter - Rotary Current—A tidal current that flows continually, nately in approximately opposite directions with a with the direction of flow changing through all slack water at each reversal of direction. Currents points of the compass during the tidal period. of this type usually occur in rivers and straits where Rotary currents are usually found offshore where the direction of flow is more or less restricted to the direction of flow is not restricted by any barri- certain channels. ers. The tendency for the rotation in direction has Rhumb Line—A line that is formed that spirals around its ori gin in the Coriolis force and, unless modified the globe toward the nearer pole when a direction by location conditions, the change is clockwise in (other than due east or due west) is specified on the the Northern Hemisphere and counterclockwise in surface of the earth and followed for any distance, the Southern. so that each subsequent meridian is crossed at the Round of Bearings—A group of bearings observed same angle relative to the direction of the pole. simultaneously, or over a short period of time, such Also called a loxodrome. This appears as a straight as would he used to determine a visual fix. line on a Mercator chart. Rudder—A vertical plate or board, which can be pivot - Rhythmic Light—A light showing intermittently with a ed to steer a boat. regular periodicity. Running Fix (RFIX)—A fix obtained by means of two Right-of-way—An obsolete term. Under the 1972 or more LOPs taken at different times and adjusted COL REGS, no vessel has the “right-of-way” in a to a common time. This practice involves advanc- meet ing situation, and each is responsible for ing or retiring LOPs as discussed in Chapter 6. avoiding collision. Running Lights—Lights required to be shown on boats Riprap—Stones or broken rock thrown together with- underway between sundown and sunup; indicates out order to provide a revetment. location and orientation of vessel. Road—An open anchorage affording less protection than a harbor. Reefs, shoals, etc., may afford some S protection. Often used in the plural, e.g., Hampton Roads. Sand—Sediment consisting of small but distin - Rock Awash—A rock that becomes exposed, or nearly guishable separate grains between 0.0625 and 2.0 so, between chart sounding datum and mean high millimeters (mm) in diameter. It is called very fine water. In the Great Lakes, the rock awash symbol is sand if the grains are between 0.0625 and 0.125 used on charts for rocks that are awash, or nearly mm in diameter, fine sand if between 0.125 and so, at low water datum. 0.25 mm, medium sand if between 0.25 and 0.50 Rock—1. An isolated rocky formation or single large mm, coarse sand if between 0.50 and 1.0 mm, and stone, usually one constituting a danger to naviga - very coarse sand if between 1.0 and 2.0 mm. See tion. It may be always submerged, always uncov - also Mud, Stones, or Rock. ered, or alternately covered and uncovered by the Scalar—A scalar is a quantity that has magnitude tide. A pinnacle is a sharp pointed rock rising from only—in contrast to a vector, which has both quan - the bottom. 2. The naturally occurring material that tity and direction. Velocity, for example, is a vector forms the firm, hard, and solid masses of the ocean because, properly speaking, it has both quantity floor. Also, rock is a collective term for masses of (e.g., 10 knots) and direction (e.g., 045 degrees). hard material generally not smaller than 256 mil - Throttle setting, measured in revolutions per limeters. minute, for example, is a scalar quantity because it Rode—An anchor line and/or chain. has mag nitude only. Root Mean Square (RMS)—The square root of the Scope—The ratio of the length of anchor line deployed arithmetical mean of the squares of a group of num - to the depth of the water, including the distance bers. from the vessel’s bow to the water—see Chapter 8.

A-22 Glossary A

Screw—A boat’s propeller. Ship—A larger vessel usually thought of as being used Sea Anchor—Any device used to reduce a boat’s drift for ocean travel. A vessel able to carry a “boat” on before the wind. board. Sea Room—A safe distance from the shore or other Ship’s Head Up—Type of relative motion radar display haz ards. with own ship in center and instantaneous relative bearings of targets displayed. Seaman’s Eye—Navigation by informal means made possible by thorough familiarity with the area of Shoal—An offshore hazard to navigation at a depth of operations. 16 fathoms (30 meters) or less, composed of uncon - solidated material. Seaworthy—A boat or a boat’s gear able to meet the usual sea conditions. Shoal Water—Shallow water or water over a shoal. Secondary Coding Delay—Interval in microseconds Short Legs—Slang expression to denote a vessel with a between the reception of a loran signal at the sec - limited fuel capacity in relation to its fuel consump - ondary station and the time when the secondary sta - tion. Opposite: long legs. tion transmits a signal in the loran navigation sys - Signal-to-Noise Ratio (SNR)—The ratio of the signal tem. Secondary coding delays are published for strength to that of the electronic noise of a signal. each secondary station. Loran coverage diagrams are calculated so that the Secondary Station—One of the two to four other trans - SNR is at least 1:3, even though many receivers are mitters in the Loran-C chain (designated W, X, Y, capable of processing weaker signals. Signal-to- and Z) that transmits a signal, keyed in time to that noise is sometimes expressed in decibels (dB). The of the master, used to compute a time difference. At SNR in decibels is mathematically equal to 20 log one time, the secondary transmitter would transmit (SNR), so that art SNR of 1:3 works out to approxi - (after an interval known as the secondary coding mately 9.54. The higher the signal-to-noise ratio, delay) only on receipt of the master signal. Now, the better the signal. the secondary transmitters maintain their own time Silt—See Mud. standard, but the time of transmission relative to the Siren—A sound signal, which uses electricity or com - master signal is designed to be the same as before. pressed air to actuate either a disc or a cup-shaped Sector—See Light Sector. rotor. Secure—To Make Fast. Skeleton Tower—A tower, usually of steel, constructed Semidiurnal—Having a period or cycle of of heavy corner members and various horizontal approximate ly one-half of a tidal day. The predom- and diagonal bracing members. inating type of tide throughout the world is semidi- Skywave—Not discussed in this text, skywave is an urnal, with two high waters and two low waters indirect radio wave that reflects off the ionosphere, each tidal day. The tidal current is said to be semi- rather than traveling a direct path from transmitter diurnal when there are two flood and two ebb peri- to receiver. Because these waves travel a different ods each day. dis tance (in particular a longer distance), skywaves Set—The direction towards which the current is flow- will give an erroneous TD reading in a loran receiv - ing, expressed in degrees. This term is also com- er. The shape of the loran pulse and phase coding monly used to mean the direction towards which a are used to attempt to minimize or eliminate the vessel is being deviated from an intended course by effects of skywave contamination. the com bined effects of external force such as wind Skywave Delay—The time interval between the arrival and cur rent. of the groundwave and the various skywave reflec - Sextant—Device for precise measurement of horizontal tions. Typically, skywaves can arrive as early as 35 or vertical angles. microseconds, or as late as 1,500 microseconds after the groundwave.

A-23 A Glossary

Slack Water—The state of a tidal current when its speed Speed (S)—The rate at which a vessel advances relative is near zero, especially the moment when a revers - to the water over a horizontal distance. When ing current changes direction and its speed is zero. expressed in terms of nautical miles per hour, it is The term is also applied to the entire period of low referred to as knots (kn. or kt.). One knot equals speed near the time of turning of the current when approximately 1.15 statute miles per hour. it is too weak to be of any practical importance in Speed Curve—A curve relating the vessel’s speed nav igation. The relation of the time of slack water through the water to the engine’s throttle setting to the tidal phases varies in different localities. For expressed in revolutions per minute (RPM) See stand ing tidal waves, slack water occurs near the Chapter 5 for details. times of high and low water, while for progressive Speed LOP—An LOP situated at approximately right tidal waves, slack water occurs midway between angles to the intended track, so named because the high and low water. EP derived from this LOP provides a good indica - Slime—Soft, fine, oozy mud or other substance of sim - tion of the vessel’s SMG. ilar consistency. Speed Made Good (SMG)—Indicates the overall speed Small Circle—Any plane passing through the earth, but actually accomplished relative to the ground along not through its center, produces a small circle at its the course line. intersection with the earth’s surface. Speed of Advance (SOA)—The speed intended to be Solar Day—The duration of one rotation of the earth on made relative to the ground along the track line. its axis, with respect to the sun. Speed of Relative Motion—Apparent speed of the tar- Sound—A relatively long arm of the sea or ocean form - get on a radar display, determined from the relative ing a channel between an island and a mainland or motion plot. connecting two larger bodies of water, as a sea and Speed Over the Ground (SOG)—The actual speed the ocean, or two parts of the same body but usual- made good at any instant in time with respect to the ly wider and more extensive than a strait. The term ground along the course being steered. has been applied to many features that do not fit the accepted definition. Many are very large bodies of Speed Through the Water (STW)—The apparent speed water, such as Mississippi Sound and Prince indicated by log-type instruments or determined by William Sound; others are mere saltwater ponds or use of tachometer and speed curve or table, at a par - small passages between islands. ticular point in time, along the course line. Sound Signal—A device that transmits sound, intended Speed-Time-Distance—A formula to calculate speed, to provide information to mariners during periods time, or distance. of restricted visibility and foul weather. Spherical Coordinate System—The system used to Sounding—A measurement of the depth of water. define positions on the earth’s surface. South Geographic Pole—A reference for specifying a Spire—A slender pointed structure extending above a position on the earth’s surface, at the south end of building. It is seldom less than two-thirds of the the earth’s axis. Also called True South Pole. entire height of the structure, and its lines are rarely broken by stages or other features. The term is not South Magnetic Pole—The end of the earth’s magnetic applied to a short pyramid-shaped structure rising core opposite the North Magnetic Pole. (Located in from a tower or belfry. Antarctica.) Spit—A small tongue of land or a long narrow shoal Special Purpose Buoy—A buoy having no lateral sig - (usually sand) extending from the shore into a body nificance used to indicate a special meaning to the of water. mariner, such as one used to mark a quarantine or anchorage area.

A-24 Glossary A

Spoil Area—Area used for depositing dredged materi - Starboard Hand Mark—A buoy or beacon which is left als, usually near and parallel to dredged channels. to the starboard side when proceeding in the “con - Spoil areas are shown on charts because these may ventional direction of buoyage.” present hazards to navigation for even the smallest Station Buoy—An unlighted buoy set near a Large craft. Navigation Buoy or an important buoy as a refer - Spring Tides or Tidal Currents—Tides of increased ence point should the primary aid to navigation be range or tidal currents of increased speed occurring moved from its assigned position. semimonthly as the result of the moon being new or Station Pointer—See Three-Arm Protractor. full. The spring range of tide is the average semidi - Stern—The after part of the boat. urnal range occurring at the time of spring tides. Stones—A general term for rock fragments ranging in Stack—A tall smokestack or chimney. The term is used size from 2 to 256 millimeters. An individual when the stack is more prominent as a landmark water-rounded stone is called a cobble if between than accompanying buildings. 64 to 256 millimeters, a pebble if between 4 and 64 Stadimeter—An instrument for determining the dis- mil limeters, and gravel if between 2 and 4 millime- tance to an object of known height by measuring ters. These specialized terms of art are used on nau- the angle, at the observer, subtended by the object. tical charts to describe the quality of the bottom. The instrument is graduated directly in distance. Stow— To put an item in its proper place. Stand of Tide—Sometimes called a platform tide. An Strait—A relatively narrow waterway, usually narrow- interval at high or low water when there is no sensi - er and less extensive than a sound, connecting two ble change in the height of the tide. The water level larger bodies of water. is stationary at high and low water for only an instant, but the change in level near these times is Strength of Current—Phase of tidal current in which SO slow that it is not usually perceptible. the speed is a maximum; also the speed at this time. Beginning with slack before flood in the period of Stand-on Vessel—That vessel which continues its a reversing tidal current (or minimum before flood course in the same direction, at the same speed, in a rotary current), the speed gradually increases to dur ing a crossing or overtaking situation, unless a flood strength and then diminishes to slack before col lision appears imminent. (Was formerly called ebb (or minimum before ebb in a rotary current), “the privileged vessel.”) after which the current turns in direction, the speed Standard Time—A kind of time based upon the transit increases to ebb strength and then diminishes to of the sun over a certain specified meridian, called slack before flood, completing the cycle. the time meridian, and adopted for use over a con - Submerged Rock—A rock covered at the chart’s sound - siderable area. With a few exceptions, standard ing datum and considered to be potentially danger - time is based upon some meridian, which differs by ous to navigation. See also bare rock, rock awash. a multiple of 15° from the meridian of Greenwich. Subordinate Current Station—1. A current station from Standardized Color Coding (Charts)—Standardized which a relatively short series of observations is col ors used to show Loran-C lines of position on reduced by comparison with simultaneous observa - nauti cal charts. These color codes for the various tions from a control current station. 2. A station list- secon daries in the loran chain are W = blue, X ed in the Tidal Current Tables for which predic tions magen ta, Y = black, and Z = green. are to be obtained by means of differences and Standpipe—A tall cylindrical structure, in a waterworks ratios applied to the full predictions at a reference system, the height of which is several times the station. diameter. Starboard—The right side of a boat when looking for - ward.

A-25 A Glossary

Subordinate Tide Station—1. A tide station from which Three-Arm Protractor—An instrument consisting a relatively short series of observations is reduced essen tially of a circle graduated in degrees, to by comparison with simultaneous observations which is attached one fixed arm and two arms piv- from a tide station with a relatively long series of oted at the center and provided with clamps so that observations. 2. A station listed in the Tide Tables these can be set at any angle to the fixed arm, with- for which predictions are to be obtained by means in the limits of the instrument. It is used for finding of differences and ratios applied to the full predic- a ship’s posi tion when the angles between three tions at a reference station. fixed and known points are measured. Also termed Swells—After the deep-water waves are generated far a station pointer. out at sea, they move outward, away from their Thwartships—At right angles to the centerline of the wind source, in ever-increasing curves, and become boat. swells. Tidal Current Tables—Tables that give daily predic- Swing Ship—A systematic procedure for adjusting a tions of the times and speeds of the tidal currents. compass and/or developing a deviation curve for a These predictions are usually supplemented by cur- compass aboard a vessel. rent dif ferences and constants through which addi- Syzygy—Alignment of earth, moons, and sun where tional predictions can be obtained for numerous the earth, moon, and sun are aligned, and the moon other places. and sun are on the same side of the earth. Tides Tidal Difference—Difference in time or height of a have larger ranges (termed spring tides) when this high or low water at a subordinate station and at a condi tion exists. refer ence station for which predictions are given in the Tide Tables. The difference, when added or sub - tracted from the prediction at the reference station, T gives the corresponding time tide height for the Tachometer—An instrument that indicates the speed of sub ordinate station. the engine measured in revolutions per minute Tide—The periodic rise and fall of the water resulting (RPMs). from gravitational interactions between the sun, Tack—To come about; the lower forward corner of a moon, and earth. The vertical component of the sail; sailing with the wind on a given side of the par ticulate motion of a tidal wave. boat, as starboard or port tack. Tide Tables—Tables which give daily predictions of the Tacking—Moving the boat’s bow through the wind’s times and heights of high and low waters. These eye from closehauled on one tack to closehauled on predictions are usually supplemented by tidal the other tack. Same as coming about. differ ences and constants through which additional pre dictions can be obtained for numerous other Tank—A water tank elevated high above the ground by places. a tall skeletal framework. The expression “gas tank” or “oil tank” is used for the distinctive struc- Time Difference—In the loran system, the time differ - tures described by these words. ence (in microseconds) between the receipt of the master and secondary signals. Target—Object seen on a radar screen. If the object is known, it is so identified. If not, targets are often Time Meridian—A meridian used as a reference for given letter designations for plotting purposes, e.g., time. target alpha, bravo, charley, delta, etc. Time-To-Go (TTG)—Calculated time until the next waypoint is reached, obtained by dividing the dis - tance to go by the ground speed. Topmark—One or more relatively small objects of char acteristic shape and color placed on an aid to identi fy its purpose.

A-26 Glossary A

Tower—A structure with its base on the ground and True Rose—The resulting figure when the complete high in proportion to its base, or that part of a struc - 360-degree directional system is developed as a cir- ture higher than the rest, but having essentially ver - cle with each degree graduated upon it, and with tical sides for the greater part of its height. the 000 indicated as true north. Also called compass Track (TR)—The intended or desired horizontal direc - rose. tion of travel with respect to the ground. (Synonym: True South Pole—A reference for specifying a position Intended Track, Trackline.) on the earth’s surface, at the south end of the earth’s Tracking—Process of moving towards a location by axis. Also called South Geographic Pole. adjusting the heading to compensate for prevailing True Wind—The direction from which the wind is current, so as to travel to the station in a straight blowing. line. Turning Bearing—A bearing on a charted object, mea - Tracking (Loran)—The process of measuring time dif - sured in advance by the navigator, at which the ves - ferences from an acquired master-secondary Loran- sel should turn to reach the next leg of the course. C pair. The signal-to-noise ratio required for track - Turning Buoy—A buoy marking a turn, as in a channel. ing of a pre-identified signal is generally less than Twin Propellers (Screws)—A boat equipped with two that required for signal acquisition. For this reason, engines. it is sometimes the case that a vessel that has already acquired a loran signal can continue to nav- Type of Tide—A classification based on characteristic igate with this signal, although an identical receiv- forms of a tide curve. Qualitatively, when the two er turned on may be unable to acquire the signal. high waters and two low waters of each tidal day are approximately equal in height, the tide is said to Transducer—A device that converts one type of energy be semidiurnal; when there is a relatively large to another, as a loudspeaker that changes electrical diurnal inequality in the high or low waters or both, energy into acoustical energy. it is said to be mixed; and when there is only one Transit—British term for range. See Range. high water and one low water in each tidal day, it is Trawler—A general term to describe a vessel with a said to be diurnal. dis placement or semi-displacement hull designed for long distance cruising. Trawlers often resemble fishing vessels. U Trim—Fore-and-aft balance of a boat. Uncorrecting (a Magnetic Direction)—Converting a Tropic Currents—Tidal currents occurring semimonth- true direction to an equivalent magnetic or compass ly when the effect of the moon’s maximum decli- direction. nation is greatest. At these times, the tendency of Uncovered—Above water; the opposite of submerged. the moon to produce a diurnal inequality in the cur- Underway—A vessel not at anchor, made fast to a pier rent is at a maximum. or wharf, or aground. Tropic Tides—Tides occurring semimonthly when the Unmanned Light—A light, which is operated automati - effect of the moon’s maximum declination is great - cally. est. At these times, there is a tendency for an increase in the diurnal range. True North Pole—The north end of the earth’s axis. V Also called North Geographic Pole. The direction V-Bottom—A hull with the bottom section in the shape indi cated by 000° (or 360°) on the true compass of a “V.” rose.

A-27 A Glossary

Vanishing Tide—In a mixed tide with very large diur- Waterline—A line painted on a hull which shows the nal inequality, the lower high water (or higher low point to which a boat sinks when it is properly water) frequently becomes indistinct (or vanishes) trimmed. at time of extreme declinations. During these peri - Wave Height—The vertical distance between the crest ods, the diurnal tide has such overriding dominance and the trough of a wave. that the semidiurnal tide, although still present, Wave Length—The distance between consecutive can not be readily seen on the tide curve. crests of a wave. Variable Range Marker—An adjustable range ring in a Wave Shape—The height and length of the wave as it PPI that can be moved to measure the range of a travels. tar get. Way—Movement of a vessel through the water such as Variation—The angular difference between the headway, sternway, or leeway. magnet ic meridian and the geographic meridian at a partic ular location. Waypoint—Arbitrary geographic point entered into a navigation receiver (GPS or Loran-C) as a refer- Varsol—A liquid used in the bowl of a compass to ence point for navigational calculations. Typically, damp the card’s excessive motion and reduce voy ages are organized into a series of waypoints response to a slower, more readable, gentle rotation mark ing the legs of the trip. and to lubri cate the bearing on the pivot. Waypoint Sequencing (Route Option)—A feature Vector—See Scalar. incor porated into many navigation receivers that Very High Frequency Radio (VHF)—Radio frequency allows an operator to store a sequence of waypoints of 30 MHz to 300 MHz. The VHF system is essen - in the navigation receiver’s memory to describe a tially a line-of-sight system limited in range to only route. In this mode, whenever the vessel arrives at a little beyond the horizon. a waypoint, the next waypoint in a prestored route Vigia—A rock or shoal of uncertain position or exis - sequence automatically appears on the display tence. The same term is used to describe a printed screen. warning to that effect. Weighing Anchor—Raising the anchor when preparing Visual Aid to Navigation—An aid to navigation which to get underway. transmits information through its visual observa - Wharf—A man-made structure bounding the edge of a tion. It may be lighted or unlighted. dock and built along or at an angle to the shoreline, Voyage Fuel—See En Route Fuel. used for loading, unloading, or tying up vessels. Wheel—A circular frame with an axle attached to the W rudder of a vessel used for steering. Also, a slang expression for a propeller. WAAS—An acronym for Wide Area Augmentation Whistle—A wave-actuated sound signal on a buoy, System, a system similar to DGPS that transmits which produces sound by emitting compressed air correction signals from satellites, rather than a through a circumferential slot into a cylindrical bell ground-based system. chamber. Wake—Moving waves, track, or path that a boat leaves Windward—Toward the direction from which the wind behind it when moving across the waters. is coming. Watching Properly—An aid to navigation on its Winter Light—A light which is maintained during assigned position exhibiting the advertised charac - those winter months when the regular light is extin - teristics in all respects. guished; it is of lower candlepower than the regular Water Tower—A structure enclosing a tank or stand- light but usually of the same characteristic. pipe so that the presence of the tank or standpipe may not be apparent.

A-28 Glossary A

Winter Marker—An unlighted buoy without sound sig - nal, used to replace a conventional buoy when that Y aid to navigation is endangered by ice. Yaw—To swing off course, as when due to the impact Withdrawn—The discontinuance of a floating aid to of a following or quartering sea. navigation during severe ice conditions or for the winter season. Wreck—The ruined remains of a vessel which has been Z rendered useless, usually by violent action, as by Zeroing In—Approaching a point or object by use of the action of the sea and weather on a stranded or successive approximations such as in tacking. sunken vessel. In hydrography, the term is limited to a wrecked vessel, either submerged or visible, which is attached to or foul of the bottom or cast up on the shore. Wreck Buoy—A buoy marking the position of a wreck. It is usually placed on the seaward or channel side of the wreck and as near to the wreck as conditions will permit. To avoid confusion in some situations, two buoys may be used to mark the wreck. The pos sibility of the wreck having shifted position due to sea action between the times the buoy was estab - lished and later checked or serviced should not be overlooked. Also called wreck-marking buoy.

A-29 A Glossary

A-30 Index I

INDEX

CURRENT SAILING: FUEL PLANNING: Chart plotting conventions...... 7-6 Effect of current ...... 11-10 Common errors in current problems...... 7-8 Efficiency versus speed ...... 11-4 Current compensation when the Endurance ...... 11-3 SOA is prespecified ...... 7-14 En route time-RPM tradeoffs ...... 11-9 Current sailing terms ...... 7-2 Factors affecting fuel efficiency ...... 11-8 Current triangle ...... 7-3 Fuel consumption chart ...... 11-2 Currents and voyage planning ...... 7-18 Fuel efficiency ...... 11-3 Definition of current ...... 7-2 Gas versus Diesel...... 11-7 Determination of a course to steer Howgozit Chart ...... 11-17 given current ...... 7-12 Point of no return ...... 11-13 Determination of current set and drift ...... 7-7 Radius of action ...... 11-13 Distinctions between TR/CMG and SOA/SMG7-7 Range ...... 11-3 Maneuvering board for solution of current problems ...... 7-3 Reserves ...... 11-4 Vectors to determine the effect Sources of fuel consumption information ...... 11-7 of known currents ...... 7-3 INTRODUCTION: DEAD RECKONING: Conversion from true to magnetic directions Accuracy of DR plots ...... 5-14 and vice versa ...... 1-13 Definition ...... 5-3 Course overview ...... 1-2 DR plots for sailing vessels ...... 5-9 Great and small circles ...... 1-7 Elements of dead reckoning ...... 5-4 Longitude and latitude ...... 1-8 Illustrative plots ...... 5-4 Magnetic references ...... 1-9 Informal logs...... 5-9 Measurement of direction ...... 1-9 Nomenclature and plotting conventions ...... 5-2 Reciprocal bearings ...... 1-15 Rules for construction of DR plots ...... 5-15 Relative directions ...... 1-15 Ship’s log and DR ...... 5-8 Steps in voyage planning ...... 1-2 Speed curve ...... 5-18 Steps while underway ...... 1-4 Subtleties in definition of DR terms ...... 5-13 The planet earth ...... 1-6 Terms defined and illustrated ...... 5-11 Time-speed-distance (TSD) calculations ...... 5-17 Units, significant figures and precision ...... 5-2 Voyage plans and DR plots ...... 5-16 What is a position?...... 5-13

Index-1 I Index

GPS/DGPS: MARINE COMPASS (continued): Accuracy of GPS/DGPS ...... 9-10 Use of Pelorus ...... 2-12 Differential GPS...... 9-11 Use of range for deviation curve ...... 2-11 GDOP...... 9-11 NAUTICAL CHART: Introduced ...... 4-14 Principle of operation...... 9-8 ATONs ...... 3-26 GPS segments ...... 9-8 Basic information ...... 3-18 HDOP...... 9-11 Chart accuracy ...... 3-26 Precise Positioning Service...... 9-11 Chart as working document...... 3-31 Pseudorange ...... 9-8 Chart catalogs ...... 3-14 Standard Positioning Service...... 9-10 Chart projections ...... 3-3 Use correct chart datum with GPS receivers...9-10 Chart scales ...... 3-11 Chart storage ...... 3-30 LORAN: Chart terms have precise meanings ...... 3-21 Automatic coordinate conversion ...... 9-8 Chart types ...... 3-13 Crossing angle ...... 9-8 Coast charts ...... 3-13 Gradient ...... 9-8 Coordinates on Mercator projection ...... 3-5 Introduced ...... 4-13 Dangers to navigation ...... 3-26 Loran accuracy ...... 9-9 Direction on the Polyconic chart ...... 3-11 Loran charts ...... 9-6 Establishing position on Mercator projection ...3-7 Master stations ...... 9-2 General charts ...... 3-13 Principle of operation ...... 9-5 Harbor charts ...... 3-13 Secondary coding delay ...... 9-6 Heights ...... 3-17 Secondary stations ...... 9-3 Key elements of information ...... 3-2 Time differences ...... 9-5 Latitude and longitude scales on the Polyconic chart ...... 3-9 MARINE COMPASS: Measurement of direction on the Compass calculations ...... 2-18 Mercator chart ...... 3-7 Compass deviation ...... 2-6 Measurement of distance on the Mercator chart ...... 3-8 Compass dial design ...... 2-4 Measurement of distance on the Compass errors ...... 2-17 Polyconic chart ...... 3-11 Compass mounting ...... 2-6 Mercator projection ...... 3-4 Fluxgate compass ...... 2-20 Object detection and recognition ...... 3-23 History ...... 2-2 Other chart features ...... 3-26 Local magnetic disturbances...... 2-19 Polyconic projection ...... 3-8 Parts of the compass ...... 2-2 Sailing charts ...... 3-13 Swinging ship ...... 2-8 Small craft charts ...... 3-13 Use of deviation table ...... 2-14 Sounding and bottom characteristics...... 3-16

Index-2 Index I

NAUTICAL CHART (continued): NAVIGATORS TOOLS AND INSTRUMENTS: Story of U-853 ...... 3-31 Binocular ...... 4-7 Use of parallel rulers and dividers to Binocular (stabilized)...... 4-8 plot a position ...... 3-6 Charts and reference publications ...... 4-10 NAVIGATION RECEIVERS: Calculator/computer...... 4-5 Depth sounder ...... 4-11 Anchor watch or anchor alarm ...... 9-16 Drafting compass ...... 4-4 Arrival alarm...... 9-15 Hand-bearing compass ...... 4-6 Boundary alarm...... 9-16 Handheld lead line ...... 4-12 Checks necessary with electronic systems ...... 9-13 Loran-C ...... 4-13 Cross-track error ...... 9-14 Nautical slide rule ...... 4-5 Cross-track error alarm ...... 9-16 Night vision equipment...... 4-9 Defined...... 9-12 Optical range finder ...... 4-9 Integration with electronic charts ...... 9-15 Pencil...... 4-5 Passing alarm ...... 9-15 Plotters ...... 4-2 Waypoint defined ...... 9-13 Radar ...... 4-14 NAVIGATION REFERENCE PUBLICATIONS: Radio ...... 4-13 Sextant ...... 4-9 Abbreviations used in LNM ...... 10-9 VHF/FM direction finder ...... 4-12 Broadcast Warnings ...... 10-10 Wrist watch ...... 4-4 ANMS ...... 10-9 Coast Pilot ...... 10-2 PILOTING: Coast Pilot use ...... 10-3 Accuracy of LOPs ...... 6-7 Geographic range ...... 10-8 Buoys in piloting...... 6-3 Light List ...... 10-6 Danger bearings ...... 6-28 Local Notice to Mariners ...... 10-9 Defined ...... 6-2 Luminous range ...... 10-6 Distance by radar observations ...... 6-11 Luminous range diagram ...... 10-7 Distance by vertical angle ...... 6-11 Nominal range ...... 10-6 Electronic fix...... 6-23 Non-government sources ...... 10-10 Fix ...... 6-19 Notice to Mariners ...... 10-9 Fix by bearing and line of soundings ...... 6-22 Visibility of lights ...... 10-6 Fix by cross bearings ...... 6-19 Fix by distance and bearing to object ...... 6-22 Fix by passing close to charted aid to navigation ...... 6-23 Fix by range and bearing ...... 6-19 Fix by two distances ...... 6-20 Fix by two ranges ...... 6-20 Line of position ...... 6-4

Index-3 I Index

PILOTING (continued): RADIONAVIGATION: LOP by bearing from charted object...... 6-4 See under specific navigation systems LOP by directional range ...... 6-9 REFLECTIONS ON NAVIGATION: LOP by distance from charted object ...... 6-9 Most probable position ...... 6-28 Alert for the unexpected ...... 12-8 Practical pointers for determination of LOPs .6-11 Attention to detail ...... 12-2 Running fixes ...... 6-23 Avoid reliance on one method ...... 12-6 Running fix by angle on the bow calculations 6-26 Emphasize routine in times of stress ...... 12-9 Summary of DR and fix symbols ...... 6-28 Maintain a DR plot ...... 12-17 Worth of single LOP ...... 6-14 Necessity for practice ...... 12-4 New information ...... 12-12 RADAR: Operate within limits ...... 12-15 Automatic Radar Plotting Aid (ARPA) ...... 9-34 Preplan as much as possible ...... 12-11 Closest point of approach ...... 9-30 Slow down or stop vessel ...... 12-10 Displays ...... 9-22, 26 Seaman’s Eye: ...... 11-19 General information ...... 9-17 TIDES AND TIDAL CURRENTS: Interpretation of radar images ...... 9-18 Introduced ...... 4-14 Computer programs for tide and tidal current calculations...... 8-15, 24 Land targets ...... 9-19 Duration of slack ...... 8-24 North up display ...... 9-25 Errors in tide-height calculations ...... 8-14 Plotting ...... 9-21, 27 Importance of tidal information ...... 8-3 Radar shadow ...... 9-20 Rotary currents ...... 8-27 Regulations and use of radar ...... 9-21 Sources of tidal information ...... 8-4 RMP ...... 9-31 Strategies for using current data ...... 8-32 RTPS ...... 9-37 Tidal current ...... 8-15 Screens ...... 9-26 Tidal Current Charts ...... 8-31 Ship’s head up display ...... 9-22 Tidal Current Diagrams ...... 8-27 Special situations of interest ...... 9-39 Tidal current reference stations ...... 8-17 Steps in radar plotting ...... 9-39 Tidal current subordinate stations ...... 8-21 Target course and speed ...... 9-33 Tidal current worksheet ...... 8-22, 24 Time of closest point of approach ...... 9-30 Tide reference stations ...... 8-6 True motions display ...... 9-26 Tide subordinate stations ...... 8-8 Vector diagram ...... 9-32 Tide table worksheet ...... 8-10 Vessels as targets ...... 9-20 Tides and tidal patterns ...... 8-2 Worksheet for radar plotting ...... 9-27 Use of the Tide Tables ...... 8-6 Use of Tidal Current Tables ...... 8-2

Index-4