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USPS® Nautical Glossary © 1999 United States Power Squadrons abcde fghij klmno pqrst uvwxy z

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A aback To have the on the forward side of the . abaft Behind, of. abandonment An abandoned race is one that is declared void by the race committee at any time; can be resailed at the race committee’s discretion. abeam Off the boat at right angles to centerline. aboard Upon or in the boat. about See come about, ready about. absolute humidity The mass of per unit volume of air. Generally expressed in grams per cubic meter. absolute temperature Scientific temperature scale with 0° as the at which mol- ecules cease their motion. See Kelvin temperature. absorption (of ) The process by which is retained by (i.e., not reflected from or transmitted through) a substance.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard accelerator pump A small pump in the carburetor, operated by a throttle linkage that supplies additional fuel needed for acceleration of the boat. active device A device that requires and controls electrical power to per- form its function, such as a transistor. adapter A device for connecting two dissimilar connectors. adiabatic process A process in which no heat is transferred between the sur- roundings and the system being changed. The temperature, pressure, and volume of a parcel of air (or any gas) are inter- related. A change in volume or pressure can thus produce a change in temperature without the addition or removal of heat. See general gas law, temperature. adrift Unattached to shore or bottom, floating out of control. advection Transport of atmospheric conditions such as temperature, pres- sure, and humidity, from one place to another by the motion of air (wind). advection fog Fog formed when warm, moist air is transported over a cold surface that lowers the temperature of the air below the dew point. aft After; at, near, or toward the .

Return to Keyboard USPS® Nautical Glossary Return to Keyboard aground Touching or stuck on the bottom. ahead 1. The direction toward the or a boat (relative to the position of the observer). 2. In front of a boat. 3. Moving in a forward direction. Compare astern. ahoy Term used in hailing a boat. ahull See lie ahull. aids to navigation Charted objects available to assist in determination of position or safe , or to warn of danger (e.g., buoys, , fog signals, , beacons, range marks). Also, any elec- tronic devices used for navigation. airfoil A device designed to deflect an air stream, thereby deriving energy from it. Examples: an aircraft wing, elevator, or rud- der; a kite; a . air-fuel ratio Ratio by weight of fuel compared to air in the carburetor mix- ture. air-gap Space between spark plug electrodes.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard air mass Air with uniform properties over a large surface area. These properties will vary vertically within the air mass, but be nearly uniform horizontally. air-mass thunderstorm A thunderstorm that is not associated with a weather front that develops locally owing to heating of humid air near the surface (such as by hot asphalt in a parking lot), causing the air to rise, cool, and form storm clouds. alee To leeward, away from the wind. alignment The bringing together of two parts of a mechanical coupling in near-perfect horizontal and vertical agreement. aloft Above , usually in the . alongside Along or close to another boat or a pier. (AC) An that reverses direction of flow at regularly recurring intervals of time. alternator A device that generates an alternating voltage by rotating a coil through a , or by a rotating magnetic field cutting the turns of a stationary coil. Diodes may be attached to convert the AC voltage to DC voltage.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard altitude Angular distance above the horizon, measured along a verti- cal circle from the horizon to a celestial body. altitude difference See intercept. altocumulus A form of middle cloud with many dome-shaped or pillow-shaped elements, indicating some vertical development. altostratus A form of middle cloud that is -like, or layered, in shape. ambient General current conditions of the atmosphere such as tem- perature and pressure in a region. amidships In or near the middle of the boat. ammeter A meter connected in series with a circuit to measure the current flowing through the circuit. It is also possible to have a -on meter. ampacity The current-carrying capacity of a conductor, expressed in amperes. ampere (I) The unit of measurement of current flow. The symbol for cur- rent in amperes in equations is (I).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard ampere-hour (Ah) One ampere flowing in a circuit for one hour, or 0.1 ampere for 10 hours. Also used to rate the capacity of a storage bat- tery, stating the number of hours the battery will supply a given number of amperes before discharge. amplification Increasing the output voltage, current, or power of a device over that which is supplied to its input terminals. Any device that produces amplification. amplitude 1. The magnitude of a voltage, current, or power. 2. The arc, measured along the observer’s horizon, between an observed (celestial) body on the horizon and the observer’s prime vertical. amplitude (AM) The process of impressing intelligence on a radio- signal by varying its amplitude in accordance with the ampli- tude of the information to be transmitted. analemma curve A graduated scale of the declination of the sun and the equa- tion of time for each day of the year, located in the Torrid Zone on the terrestrial globe. analog Value of a measurement displayed graphically or by move- ment of a meter pointer; pertains to data in the form of con- tinuously variable physical quantities.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard analog display Method of presenting physical variables such as speed or di- rection, indicated by a pointer on a scale, usually a graduated dial. Device used to secure a boat to the bottom of a body of wa- ter. anchorage Suitable or designated place where boats anchor. anchor watch A security lookout maintained while a vessel is anchored or docked. anemometer An instrument for measuring wind speed. aneroid barometer An instrument that measures atmospheric pressure via the rise or fall of a flexible metal disk that is part of a sealed chamber containing a partial vacuum. angle of attack The angle between the apparent wind and the sail chord, or the water flow and the . anode The positive electrode, toward which the negative are drawn. A wire, rod, or other assembly designed to transmit or receive radio signals.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Ratio of power radiated from a complex antenna system or array to that radiated by a simple reference antenna, such as a dipole. anticyclone A high-pressure area (highest pressure at the center) in which the wind is spiraling outward—clockwise in the Northern Hemi- sphere. Referred to as a high. See cyclone. antifreeze A chemical that lowers the freezing point of water. It may also contain anti-rust and lubricating compounds. anti-friction bearing A bearing constructed with balls or rollers between journal and bearing surfaces to provide rolling instead of sliding fric- tion. anti-siphon valve A valve that admits air to a and prevents siphonic action. anvil Common appearance of the cloud tops of well-developed thun- derstorms. This portion of the cloud is often blown downwind by strong air currents at high altitudes, causing the top to look like an anvil. apparent altitude (ha) Resulting value after sextant altitude (hs) is corrected for in- dex error and dip.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard apparent time Time based on the rotation of the earth relative to the appar- ent (true) sun. apparent wind The vector (speed and direction) sum of the actual wind and the wind created by the movement of the boat. The wind di- rection and speed experienced by an observer on the moving boat. aqualift An engine exhaust system and silencing system in which cool- ing water is injected into the exhaust and carried out by the exhaust gases. arc 1. The discharge of electrical current across a gap. 2. Gradu- ated scale of a marine sextant. 3. Part of a curved line, as of a circle. arc to time The converting of arc in degrees to time in hours (hh), min- utes (mm), and seconds (ss). arctic air mass The coldest of air masses originating in high latitudes. arctic smoke See steam fog.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Aries See first point of Aries. armature Part of an electric motor or generator that includes the main current-carrying winding. artificial horizon A device, such as a pan of dark oil, used to indicate a horizon- tal reference when a true horizon is not available to take sights with a sextant. aspect ratio 1. of the luff length of a sail divided by the sail area. 2. Square of the depth of a divided by the keel area. assumed latitude (Asm L) The latitude at which an observer is assumed to be for a cal- culation of computed altitude (Hc) in some methods of sight reduction. assumed longitude (Asm Lo) The longitude at which an observer is assumed to be for cal- culation of computed altitude (Hc) in some methods of sight reduction. assumed position (AP) A point at which a vessel is assumed to be located, particu- larly one used as a preliminary to establishing certain naviga- tional data, as a point on the surface of the earth for which the computed altitude (Hc) is determined in the solution of a ce- lestial observation. An arbitrarily chosen position near the DR (See dead reckoning), used for computation of Hc in some methods of sight reduction.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard astern 1. The direction toward the bow of a boat (relative to the position of the observer). 2. In back of or behind a boat. 3. Moving in reverse (backwards). Compare ahead. athwart At right angles to the fore-and-aft line of a vessel. athwartship Direction at right angles to the centerline of a boat. atmosphere The envelope of air surrounding the earth. atmospheric pressure The force per unit area exerted by the weight of the atmo- sphere. Expressed in several units such as millibars, inches of mercury, and pounds per square inch. atoll cloud A tall pillar of cloud caused by the sea breeze flowing toward an isolated island. The smallest part into which an element can be divided and still retain the characteristics of that element. atomizer See injector. attached flow The movement of a fluid (i.e., water or air) along a surface (i.e., a , keel, or sail) in a smooth, sheet-like fashion with- out turbulence.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard attenuate To make thin or to reduce in force. attenuation The reduction in magnitude during transmission from one point to another. augmentation (A) The apparent increase in semidiameter of a celestial body as its altitude increases, due to reduced distance from the ob- server. aurora (borealis and australis) The northern and southern lights. Created by reactions be- tween charged particles emitted by solar storms and the at- mosphere at high altitudes near the magnetic poles. Much of the energy released by the reaction is visible radiation (). autotransformer A type of transformer with a single winding having one or more taps. Part of the winding is common to both primary and secondary circuits. auxiliary The engine of a ; a sailboat with an auxiliary engine. awash Mostly under water, but not completely submerged. aweigh Position of anchor when raised off the bottom of a body of water.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard azimuth (Zn) The true direction of an observed body, measured clockwise along the visible horizon from 0° to 360°. azimuth angle (Z) The computed direction of an observed body from a point on the horizon toward the elevated pole (north or south), mea- sured along the visible horizon (eastward or westward) to- ward the geographic position (GP) of the body. B babbitt A soft, white metal alloy frequently used to line replaceable shell-type engine bearings. back 1. Change in direction of the wind in a counterclockwise di- rection in the Northern Hemisphere, and a clockwise direc- tion in the Southern Hemisphere. 2. To cause to move backwards. See veer. backfire Ignition of the air-fuel mixture in the intake manifold by a flame from a cylinder. backing plate A plate used inside the hull to assure that fittings attached to the outside with bolts will not pull loose. backing wind A wind whose direction changes progressively in a counter- clockwise direction (e.g., W-SW-S-SE). Opposite of veering wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Part of , usually cable, that supports a from aft. backwinded A sail is backwinded when another sail to leeward of it is trimmed so closely as to cause increased pressure on the lee side of the windward sail, thus causing the windward sail to bulge to windward. baffle (baffleplate) An obstruction for checking or deflecting the flow of gases, liquids, or sound. Anti-chafe material made from old yarns woven between strands of marline. Bahamian moor A way of anchoring using two to allow for reversing tidal currents. bail 1. To remove water by scooping it out with a bucket. 2. A curved fitting for securing a line or a to a . balance The ability of a sailboat to stay on a steady course without having to apply helm. ballast Weight carried low in a boat to improve trim or stability.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard ball bearing An anti-friction bearing consisting of a hardened inner and outer face with hardened steel balls set between the two faces. ball A visible electrical discharge in the form of a concentrated ball. May appear to roll along for a short time before disap- pearing (sometimes explosively). band width 1. Frequency limits between which electrical signals are con- fined. 2. The part of the spectrum occupied by the carrier and side bands. barber hauler A trimming line for positioning the athwartship lead of the . bare poles Under way with no sail set; usually a heavy-weather precau- tion. barge An unpowered vessel used to carry cargo. barnacle A crustacean often found attached to the bottom of a boat, especially in salt water. barograph An aneroid barometer that records pressure versus time. barometer An instrument used to measure atmospheric pressure. See aneroid barometer and mercurial barometer.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard base metal A metal chemically active in an electrolyte. Anodic metals as opposed to cathodic or passive metals. bass boat An open-, outboard-powered fishing boat used prima- rily on inland waters. battcars® A system of attaching battens to the mast slide to reduce fric- tion while raising and lowering the sail, and to improve sail shapes compared with traditional sail slugs and slides. batten 1. A thin, narrow strip of , plastic, or fiberglass used to stiffen the leech of a sail. 2. A wooden strip fastened over a seam to stop leakage. batten down To secure all hatches and ports in preparation for heavy weather. batten pocket An envelope-like enclosure stitched into the trailing edge of a sail to a batten. battery A group of cells that produce an electrical potential by chemi- cal reaction. beach To run the bow of a boat up on a beach.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Anything that serves as a signal or indication for guidance or warning. A fixed (non-floating) aid to navigation. beam 1. Maximum width of a boat. 2. A horizontal athwartship sup- port for the deck. beam ends A boat heeled completely on her side. beam reach Sailing with the wind at a right angle to the boat. beam sea Sea that is abeam. beam width Applied to or other directional antennas. The width or sharpness of the emitted radiation in degrees of angle, hori- zontally or vertically. Usually measured at the halfpower points. bearing 1. Horizontal direction of an object from an observer, ex- pressed as an angle from a reference direction, e.g., bearing, true bearing, relative bearing. 2. A device for sup- porting a rotating shaft with minimum friction. May take the form of a metal sleeve (a bushing), a set of balls (a ball bear- ing), or a set of pins around a shaft (a roller or needle bear- ing). bear off 1. Steer away from the wind, shore, or any object. 2. To steer on a new course farther off the wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard beat A close-hauled course. Beaufort scale An arbitrary set of numbers assigned to describe various esti- mated wind velocities from calm to hurricane force. The Weather course Student Manual includes a modified Beaufort scale. See pilot chart. becket An eye in the end of a block, used for securing an end of a line; an eye in the end of a line. before the wind Sailing the same direction the wind blows; sailing downwind. belay 1. Secure a line without a knot or hitch. 2. Make fast by wind- ing a line around a or . 3. An order to cease action. below Beneath the deck. bend 1. To make fast a sail to a spar or stay. 2. A knot to secure a line to another line or object. bendix gear or drive A gear mounted on a screw shaft attached to the starting- motor armature; automatically engages and disengages the electric starting motor. Bermuda (Bermudan or Bermudian) rig Same as Marconi rig.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Berth 1. A place to sleep on a boat. 2. A boat’s position at a pier or . 3. A margin of safety, as in “give it a wide berth.” BHP (brake horsepower) The actual power output of an engine at the flywheel. bight 1. Open or closed loop in a line. 2. A small bay off a sea- coast. Lowest part of a boat’s interior. One of a pair of centerboards on either side of the centerline. bilge keel Twin , similar to . bilge strainers Screens to keep debris out of the bilge-pump suction hoses. binnacle Pedestal in which a compass is mounted. bitt A post through the deck, used for securing lines and cables. bitter end The inboard end of a line, chain, or cable; the end made fast to the vessel, as opposed to the “working end,” which may be attached to an anchor, cleat, or other vessel. Compare work- ing end.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard blanket To come between the wind and a sail so the sail is not full. bleeding The process of purging air from a fuel or hydraulic system. block A sheave that rotates on a pin; a nautical pulley. blocking The effect of a stalled high in slowing or deflecting an eastward-moving weather system. blooper A full sail set opposite a . blow-by The escape of combustion gases past piston rings or closed valves. board boat Small (car-top) centerboard sailing with very low topsides and virtually no cockpit. boat hook A pole with a blunt hook on the end, used to pull in lines or push things away from the boat. boatswain A senior seaman in charge of a ship’s boat, and skilled in rope and maintenance work. (Pronounced “bo’sun.”) boatswain’s chair A seat, often including a harness, hauled aloft by a to facilitate work on a mast. (Pronounced “bo’sun’s” chair.)

Return to Keyboard USPS® Nautical Glossary Return to Keyboard boat wind The so-called wind produced by the movement of the boat through the water. A stay from the to the outer end of the . bollard Stout post on a deck, wharf, or pier for securing mooring lines. boltrope A rope sewn along the edge of a sail to reinforce, to limit stretch, or to allow the edge to slide into a cove in the or mast to eliminate slides. bonding The process of electrically tying together all major fixed metal items on a boat. bonding ground A conductor connecting all metallic structures, tanks, and other equipment on board to maintain a common local (earth) ground. Used to control galvanic corrosion and induced lightning cur- rents. It should not carry current for equipment operation. boom Spar used to extend and control the foot of a fore-and-aft sail. boom crutch A temporary support for the boom when the sail is not hoisted. A spar projecting aft of the stern with a boomkin stay to sup- port a backstay.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A tackle or hydraulic system that removes twist from the sail by restraining the boom from lifting. boot top Distinctive stripe on exterior of hull just above the waterline. bore The diameter of a cylindrical hole such as an engine cylinder. boss An extension or strengthened section, such as projections within a piston, that supports the piston pin or piston pin bush- ings. bottom 1. Land beneath a body of water. 2. A boat's underbody or the portion of the hull under the water. bottom dead center (BDC) The position of a crankshaft when the piston is at the very bottom of its stroke. bow Forward end of a boat. bow chocks Fittings (usually metal) on deck at the bow, through which mooring and anchor lines are led. bow line A line used at the bow of a boat.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard bowline Knot used to form an eye or loop in the end of a line. (Pro- nounced “bo-lin.”) bowsprit A spar that extends forward from the bow. Sails and rigging are attached to the bowsprit. A line used to set the angle of a on a square-rigged ves- sel. brake horsepower Actual horsepower delivered by the crankshaft, measured by means of a dynamometer or prony brake. branch One of several paths for current flow in a circuit. breaker A sea-surface that has become too steep to be stable. breaker arm Movable part of a pair of contact points in a distributor or magneto. breaker points Two separable points, usually faced with silver, platinum, or tungsten, that interrupt the primary circuit in the distributor or magneto for the purpose of inducing a high-tension current in the ignition system. bridge rectifier An arrangement of diodes for converting alternating current (AC) to direct current (DC).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard bridle A length of line or wire rope with both ends secure (or a se- cured loop), to the middle of which another line is attached. brightwork 1. Polished metal fittings. 2. Varnished woodwork. Bristol fashion In good sea-keeping order. British thermal unit (BTU) The quantity of heat required to raise the temperature of 1 pound of water 1°F. broach Turn a boat broadside to wind or , subjecting it to pos- sible . broad reach A point of sail with the wind more or less over either quarter. Bruce® anchor A plow anchor designed to right itself no matter how it lands on the bottom. brummel hooks Patented type of rigging devices that attach to each other easily and quickly and remain secure even when not under load. brushes Bars of carbon or other conducting material that contact the commutator of an electric motor, generator, or alternator. BTU See British thermal unit.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard bulkhead Vertical partition separating compartments in a boat. bulwark Portion of hull extending above the deck. bunk boards Canvas cloths installed to keep a person from falling out of a bunk. buoy Anchored floating device used as an aid to navigation. May carry a light, horn, whistle, bell, gong, or combination for iden- tification. Also may be used to mark a mooring (e.g., anchor buoy). buoyancy The upward force exerted by a fluid on a body or object in that fluid. The upward force that keeps a boat floating. burgee A tapered flag with a device denoting a yachting organization. bus bar A heavy copper strap used in breaker boxes and circuit panel boxes for carrying high current and multiple connections. bushing A removable liner that forms a bearing surface. butterfly valve A hinged flap that serves to control the air entering a carbure- tor or the air entering the air inlet manifold of an engine. May also be used to shut down a diesel engine in an emergency.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Joint formed by squared ends or edges of two planks or plates. buttock Underside of boat’s stern above waterline. See also counter. Buys-Ballot’s Law A method for approximating the location of a storm center. When an observer faces the wind, the storm (low-pressure) center is roughly 135° to the observer’s right in northern lati- tudes; to the observer’s left in southern latitudes. (Pronounced like “Bice BaLots.”) by the lee Sailing on a run with the wind on the same side as the main boom, making a sudden jibe possible. C cabin The enclosed or decked-over living space of a boat. cable 1. A number of conductors individually insulated and bound together as a group. 2. A heavy-stranded conductor such as is used for battery connections. 3. A strong, heavy rope or chain used for mooring. 4. A unit of length equal to 720 feet. calorie The quantity of heat required to raise the temperature of 1 gram of water 1°C.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cam 1. A moving piece of machinery that imparts motion to an adjacent component. In engines, used to refer to a camshaft or one of the cams on a camshaft. 2. An elliptical protrusion on a camshaft. See also lobe. camber 1. or fullness in a sail. 2. Athwartship arching of deck or cabin top. cam cleat A fitting that grips a rope in the teeth of two rotating cams. cam or breaker cam Multi-lobed cam rotating in the ignition distributor; serves to interrupt the primary circuit to induce a high-tension spark for ignition. camshaft A shaft with cams, used to operate the valve mechanism on an engine. capacitance The property that permits the storage of electrically separated charges when potential differences exist between two con- ducting materials. capacitive reactance The amount of opposition to the flow of an alternating current due to the presence of capacitance in a circuit. Symbol XC; measured in ohms.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard capacitor Also called a condenser. A device that can store an electrical charge on conducting plates separated by an insulating me- dium. capsize To turn over, upset. car A sliding bracket to which a block may be attached. The car slides along a track to provide a range of positions for the block. carburetor A device for automatically mixing fuel in proper proportion with air to produce a combustible mixture. carrier The center or reference upon which informa- tion is impressed in radio transmissions. carry away To break free and become lost; said of gear that is stressed beyond its strength or its fastenings. carry her way, to The movement of a boat through the water as a result of mo- mentum. carvel Smooth-planked hull construction. cast off To let go a line; to set loose, unfasten; to undo all mooring lines in preparation for departure.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cat’s paw A small, isolated puff of wind that ripples the water surface in a small area. catamaran Boat with twin, narrow hulls connected by a deck or crossbeams, resulting in a wide beam and good stability. Sailboat with a mast stepped near the bow, and no jib. cathode The negative pole or electrode of an electrolytic cell. The el- ement from which the current leaves the electrolyte. cathodic protection Reduction or prevention of corrosion by the use of sacrificial anodes. caulk Fill seam with a material to prevent leakage. cavitation A partial vacuum created in the water around a rapidly re- volving propeller that loses contact with the water in which it is turning. CB radio See . ceiling 1. The height above the ground at which there appears to be six-tenths or more cloud cover. 2. Inner sheathing of a boat.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard celestial equator A great circle located at the intersection of the extended of the earth’s equator and the celestial sphere, thus 90° from the celestial poles. Declination of celestial bodies is measured from the celestial equator. celestial horizon A great circle at the intersection of an extended plane through the center of the earth and the celestial sphere, parallel to the observer’s horizon and perpendicular to the zenith-nadir line. celestial poles The intersections of the extended axis of the earth with the celestial sphere; labeled Pn and Ps. celestial sphere An imaginary globe of infinite radius, concentric with the earth, on which the stars seem to be fixed and the sun, moon, and planets appear to wander. celestial triangle A spherical triangle on the celestial sphere, especially the navi- gational triangle. cell A single unit that either converts chemical energy to electri- cal energy or the reverse of this process. Celsius (C) A temperature scale (formerly called centigrade) on which 0° represents the freezing point of water and 100° the boiling point at standard pressure (1,013 millibars). Formula for con-

verting to Fahrenheit: 9 F° = /5 C° + 32

Return to Keyboard USPS® Nautical Glossary Return to Keyboard centerboard A pivoted board that can be lowered through a slot in the keel to reduce leeway. centerline Fore-and-aft line that runs along the exact center of a boat. center of buoyancy The center of gravity of the water displaced by a floating boat. center of effort Static: the geometric center of a single sail or the combined geometric centers of several sails. Dynamic: the point where all the lateral forces of the sails of a boat under sail could be concentrated without changing their effect. center of gravity The point from which a body could be freely suspended with- out rotating in any direction. center of lateral resistance Static: the exact middle of the lateral plane. Dynamic: the point where all lateral forces acting on a hull moving through the water could be concentrated without changing their effect. Centigrade temperature scale See Celsius. centrifugal force A force that tends to move a body away from its center of rotation. Also, the apparent force that deflects wind outward in a rotating system. cetane A measure of the ignition quality of diesel engine fuels.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard chafe Wear on a sail, spar, or line. Prevented by chafing gear— sacrificial coverings, such as leather, cloth, or rubber. chafing gear Sacrificial wrapping around lines, rigging, or spars to prevent wear. chain plate Metal strap on a sailboat, usually secured to the hull or bulk- , to which a or stay is attached. change of state The process causing a material to change form from one state (solid, liquid, gas) to another (e.g., melting or boiling). channel Portion of a waterway that is navigable, usually marked, and having a known depth of water. chart A map intended primarily for navigational use by aircraft or vessels. charter To rent a boat for a predetermined price and time. cheater A low-cut sail set below a high-flying spinnaker. check valve A gate or valve that allows passage of gas or fluid in one direction only.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cheek block A pulley with one flat side so that it can be mounted on a spar or structure. chine Line formed by intersection of sides and bottom of a flat- or V-bottomed boat. Chinook wind Name given to a warm, dry wind on the east side of the Rocky Mountains, caused by a rush of cold air down the mountainside; the air warms due to an increase in pressure produced by decreasing altitude. See foehn wind, Santa Ana wind. chock 1. Fitting to guide a line or cable. 2. Wedge or block to keep an object from moving. choke 1. A reduced passage. 2. In an engine, the valve used in the carburetor air inlet to reduce the volume of air admitted. choke coil A coil of wire or inductor used to oppose changes in current flow. Normally used to impede an unwanted alternating cur- rent from a circuit for interference suppression. chord The straight line between the luff and leech of a sail. chronometer A timepiece with a nearly constant rate of gain or loss, made especially to keep accurate time.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard chute Slang for spinnaker (because it resembles a parachute). circle of equal altitude A circle on the surface of the earth from any point of which the altitude of a celestial body is the same at a given instant. circle of position Circular line of position determined by a distance from an ob- ject or point or celestial body. circuit Path of electric current, fluids, or gases. circuit breaker An automatic device that, under an overload condition, will open a current-carrying circuit to prevent damage. It can be re-set to reestablish the circuit. circulation Primary wind circulation at various latitudes when unaffected by storms and local phenomena (e.g., northeast trade winds). circumpolar Revolving about the elevated pole without setting. cirrocumulus A form of high cloud made up of small, rippled, scale-like or pad-like elements. It does not show gray shadows at the bases of thicker spots as altocumulus does. cirrostratus A form of high cloud appearing as a whitish veil.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cirrus A form of high cloud composed of detached white filaments, patches, or narrow bands. Sometimes called mares’ tails. citizens band radio An unlicensed (non-marine) type of 2-way radio. civil twilight The period of incomplete darkness when the upper limb of the sun is below the visible horizon, and the center of the sun is not more than 6° below the celestial horizon. Useful in taking sights on stars or planets. claw off To beat off of a lee shore under difficult conditions. clear astern; clear ahead; overlap Sailing terms describing the relative positions of two boats close enough to each other to possibly restrict their maneuverabil- ity. A boat is clear astern of another when her hull and equip- ment in normal position are behind an imaginary line abeam from the aftermost point of the other’s hull and equipment in normal position. The other boat is clear ahead. They over- lap when neither is clear astern of the other or when a third boat between them overlaps both. cleat Fitting, usually with two projecting horns, to which lines are made fast. cleat hitch A figure-eight hitch used to attach a line to a cleat.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard clevis pin An unthreaded locking pin, with a head at one end and a hole at the other end, through which a cotter pin or split ring may be inserted to prevent the clevis pin from being withdrawn. clew After lower corner of a triangular sail. Of a spinnaker, the lower corner attached to the sheet. climate Average weather conditions of a place or region based on years of observation. -built Type of hull construction where the lower edge of each plank overlaps and is fastened to the upper edge of the plank below it. Also called lapstrake. clinometer An instrument, usually a bubble in a curved tube filled with water, that indicates the degree of heel angle. close aboard Close enough, in the judgment of the skipper, to a charted ob- ject to consider the object's position a fix. close-hauled Sailing as close to the wind, or as directly into the wind, as possible. Also on-the-wind or by-the-wind, beating. close reach The point of sail between close-hauled and beam reaching. close-reefed When all possible reefs have been taken in.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cloud A visible cluster of minute water droplets or ice crystals in the atmosphere above the earth’s surface. clove hitch A knot used to secure a line temporarily to a bitt, spar, or rope. club-footed jib A jib with a boom. coalescence The process by which raindrops merge and grow in size. co-altitude (co-alt) Ninety degrees minus the altitude of a body above the celes- tial horizon. Same as zenith distance. The side of the naviga- tional triangle between the AP or DR and the GP of the body. coaming Vertical structure around a hatch, cockpit, or skylight to repel water. A in which one conductor completely sur- rounds the other, the two being coaxial and separated by a dielectric. There is no external radiated field and the line is not susceptible to other external fields. cockpit Well or sunken space in the deck.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard co-declination (co-Dec) Ninety degrees minus the declination of a body; when decli- nation and latitude have the same name; co-declination is the same as polar distance measured from the elevated pole; the side of the navigational triangle between the GP and the el- evated pole. coil 1. To arrange a line in loops. 2. An electrical device com- posed of a number of turns of wire in spiral form for electro- magnetic effect or for providing electrical resistance. col The intersection of a ridge of high pressure and a trough of low pressure. An area of little pressure gradient and light wind. co-latitude (co-L) Ninety degrees minus the latitude of the observer. The side of the navigational triangle between the AP or DR and the el- evated pole. cold cranking amperes (CCA) The number of amperes a battery will supply to a load for 30 seconds at 0°F with the terminal voltage not dropping below 7.2 volts in a 12-volt system. cold front The boundary between the forward edge of an advancing cold air mass and the rear edge of a retreating warm air mass. cold sector Location of the area surrounding a low-pressure frontal sys- tem (usually behind the cold front), where the colder air mass is in contact with the earth’s surface.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard collision mat A tarpaulin with lines leading from its corners to cover, from the outside, a breach in the hull below the waterline. Also, anything usable as a substitute for this. color code Different colors of insulation used on conductors to identify their use in wiring circuits. Also used to identify the value of certain types of components. COLREGS International Regulations for Preventing Collisions at Sea, in- cluded as the International Rules in the Navigation Rules pub- lished by the United States Coast Guard and distributed by the Government Printing Office. These Rules are applicable on waters outside of established navigational lines of demarca- tion. The lines are called COLREGS Demarcation Lines and delineate those waters upon which mariners shall comply with the Inland and International Rules. combustion chamber The space left in a cylinder (and cylinder head) when a piston is at the top of its stroke. come about Significant change in direction; in sailing, changing course for the bow to swing through the eye of the wind and pay off on the other . common rail A type of fuel injection in which fuel circulates to all injectors all of the time. Each injector contains its own injection pump with this system.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard commutator Copper segments on the rotor of a DC motor or generator used to conduct current through brushes to or from the rotat- ing coils. companionway Stairway or ladder leading below from the deck. compass conversion A standard system employed to convert True values to Com- pass values, and vice versa, by the addition or subtraction of variation and deviation. compass course The course by boat’s compass. The angle between the boat’s keel and the north point of the compass card when the boat is on course. compass error Combined effect of variation and deviation. compass rose A circular diagram on a chart showing directions, usually to the nearest degree. The outer ring (rose) shows directions relative to the geographic north pole (true direction). A con- centric inner rose shows directions relative to the magnetic north pole at that location. The offset between the two roses is the variation at that location. The numerical value of the variation and its rate of change are often indicated in the cen- ter of the rose.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard compressible Capable of being reduced in volume without loss of mass. This property is, for all practical purposes, limited to gases as op- posed to liquids and solids, and is due to the lack of any mo- lecular structure in gases. compression post The post between the keel and cabin top supporting a deck- stepped mast. compression ratio The volume of a cylinder with the piston at the bottom of its stroke, compared to the volume of the cylinder when the pis- ton is at the top of its stroke. compulsorily equipped A vessel required by FCC rules or International Law to have radio transmitting and receiving equipment on board. computed altitude (Hc) Altitude of a body above the celestial horizon from a dead reckoning position (DR), an assumed position (AP), or a known position (KP) at a given time, as determined by computation, usually with the aid of a sight-reduction table or calculator. computed course (Cn) Course angle computed from true north. concentric Having a common center, as circles or spheres. condensation The process by which water vapor is changed to liquid water.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard condensation nuclei Minute, usually water-absorbent particles on which water va- por condenses or sublimes. condenser See capacitor. conditional instability State of the atmosphere in which rising air will stop rising (sta- bility) if it is not saturated, and will continue to rise if it is (instability). A condition under which air-mass thunderstorms might form. conduction A process of involving heat flow through a body or from one body to another in physical contact with it. This is a mechanical transfer of molecular thermal energy. conductor A material that has many free and will readily carry an electrical current. cone of protection The area surrounding a grounded conducting mast that is the center of a cone whose inner surface is at an angle of 60° to the mast. This area will be more or less free from lightning strikes, protected by the mast. conformal projection A projection in which all angles around any point are correctly represented. connecting rod The rod connecting the piston to the crankshaft.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard connecting rod bearing The bearing at the crankshaft end of the connecting rod. connecting rod cap The housing that bolts to the end of a connecting rod, holding it to a crankshaft. conservation of angular A body moving in a circular path has a mass (m), a speed (v), and a radius or distance from the center of rotation (r). Once the mass is set in motion, its angular momentum (mvr) re- mains constant unless some unbalanced outside torque exists. conservative property A property that remains unchanged while certain other prop- erties vary (e.g., dew point, which remains virtually unchanged even though the temperature of the air changes). constellation One of the 88 recognized groups of fixed stars, or the region of the sky in which the group appears. contact points See breaker points. continental air mass An air mass that originated over land. continuous The refraction due to increasing density of air with decreas- ing altitude. convection The transfer of heat by fluid motion; in a meteorological sense, generally by vertical air currents. See advection.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard convergence The phenomenon of air flowing to a point within a layer of the atmosphere. Since this air cannot be collected at the point of convergence, a vertical exhaust current develops. Coordinated Universal Time (UTC) For purposes of marine navigation, equivalent to Greenwich mean time. cordage A general term for all types of rope. Coriolis effect Apparent deflection of a moving object due to rotation of the observer’s of reference. In meteorology, the Coriolis effect is important in that it explains the apparent deflection of wind relative to the observer, whose reference, the earth, is rotating beneath the moving air. corona A narrow, luminous ring around the sun or moon resulting from of light through thin, water-droplet clouds. Bands of colors can sometimes be distinguished, like those of a pri- mary rainbow, but fainter. See halo. corrosion Chemical change or displacement of a metal at an anodic pole from which current flows to enter an electrolyte. cotter pin A split pin, the ends of which can be bent after insertion through a hole.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard counter Above-water stern extending back from the after end of the waterline. See also buttock. coupling A connecting means for transferring movement from one part to another. course (C) The direction in which a boat is steered through the water. course made good (CMG) The resultant direction from a point of departure to a point of arrival. (This is a term in common usage, although course is a misnomer. The path of a boat with respect to the bottom is its track.) course over ground Actual direction of travel of a boat over the bottom. (This is a term in common usage although course is a misnomer. The path of a boat with respect to the bottom is track.) course steered The direction in which the bow of the boat is pointed when underway. cove A or slot built into the boom, the airfoil, or after edge of a mast. cowl A hooded opening in the deck for purposes of ventilation. CPR Cardio-pulmonary resuscitation.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard CQR® anchor A plow anchor with a single fluke shaped like a plowshare, which digs itself deeper under heavy horizontal strain. cradle A fitted frame by which a boat is supported during storage or shipping. crank An offset section of a crankshaft to which a connecting rod is attached. crankshaft The main part of an engine that changes the reciprocating motion of the pistons into rotary motion. craze To develop small cracks emanating outward from a central point. crimp terminals Metallic terminals that are designed to clamp on a conductor by compression. A ring sewn into a sail through which a line can be passed. See grommet. critical table A table in which values of the quantity to be found are tabu- lated for limiting values of the entering argument. No interpo- lation is required.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard crown The point on an anchor where the shank joins the arms. Also, deck camber. crystal A device to establish an electrical signal of precise frequency, in radio, radar, etc. Usually a precisely ground quartz chip that will vibrate at a constant frequency when subjected to a volt- age. Crystals of other materials are sometimes used for spe- cial applications, such as the barium titanate crystals used in depth sounders. cuddy A shelter cabin in a small boat. cumulonimbus A form of cloud with extensive vertical development, usually with a visible anvil top and showers falling from the base. The thunderstorm cloud. cumulus A form of cloud showing vertical development, generally dome-shaped on top and flat on the bottom. (rig) An arrangement of gear used to adjust the luff tension in a sail without using a halyard or moving the boom. current 1. The horizontal movement of water; a horizontal motion of such set and drift as to account for the difference between a dead reckoning position and a fix at the same time. 2. The movement of electrons through a conductor.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cut The angle of intersection of the lines of position in a fix. cutless bearing A rubber-sleeved bearing that supports the propeller shaft in the stern of a boat. cutter A single-masted boat with and usually two headsails, with the mast stepped close to amidships (from 40% to 50% aft of the bow, versus 33% for a ). cyclone A low-pressure area (lowest pressure near the center) in which the wind is spiraling inward in a counterclockwise pattern (Northern Hemisphere). This may be an extratropical cyclone, usually involving fronts and contrasting air masses, or a tropi- cal cyclone (hurricane). See anticyclone. cylinder A round chamber bored to receive a piston. cylinder block The largest single part of an engine. Basic or main mass of metal in which cylinders are bored or placed. cylinder head A detachable portion of an engine fastened to the top of the cylinder block. cylinder head Seal between engine block and cylinder head.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard cylinder liner or sleeve A machined sleeve that is pressed into a block, and within which a piston moves up and down. Provides a renewable wearing surface. D A sliding board that can be lowered through the keel to re- duce leeway. Danforth® anchor An anchor with long, pointed, hinged flukes with great holding power. (Fortress®, West Marine®, Hooker®, and Suncor® are other examples of this type of anchor.) dangerous semicircle In a tropical cyclone (Northern Hemisphere), the dangerous semicircle is the side to the right of the storm track as you face the direction the storm is moving. Wind in this semi-circle is the sum of the cyclonic wind plus the velocity along the storm track. In the Southern Hemisphere, the left side is known as the dangerous semicircle. datum 1. The reference plane from which depths of water are mea- sured and recorded on charts (as “charted depths” or “sound- ings”) and, in coastal waters, to which height of tide is added algebraically to determine depth of the water. 2. A mathemati- cal representation of the irregular, not-quite-spherical shape of the earth; defines latitude and longitude for the area cov- ered.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard davit Crane-like device for hoisting small boat, anchor, or cargo. Davy Jones’ locker The bottom of the sea. daybeacon Unlighted fixed aid to navigation. daylight saving time A variation of standard time in order to make better use of daylight. daymark Unlighted fixed aid to navigation. daysailor Sailboat without amenities, such as a cabin, used for short duration sailing or racing. day shape An object of specified shape and size used to describe a ves- sel and its activity in daytime. day tank A small tank used to supply fuel or water to be used immedi- ately. dead reckoning (DR) Calculating a boat’s position based on its course, speed, and time run from a previous position. dead reckoning track The intended course line of a vessel as plotted on a chart.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard deadrise The athwartships angle between a line horizontal to the top of the keel and a line from the top of the keel to the chine or the turn of the bilge above it. Usually expressed in inches for each foot of distance from the keel. dead run The point of sail on which a boat has the wind blowing straight over the stern. decarbonizing The process of removing carbon deposits from the inside of an engine. decibel (db) The standard unit for measuring relative power levels. When an electrical signal is “attenuated” or “amplified,” the loss or gain is measured in decibels. It is also used to indicate power levels above or below a standard reference level. The unit is based on a logarithmic function so that every time the power is doubled, three decibels are added. deck The portion of the boat that covers the hull. deck beam Horizontal support running athwartships on which the deck rests. deck drain Openings in the deck or cockpit to drain water overboard. deck house A partially enclosed structure erected on the deck.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard deck log Journal kept on board to record all information pertaining to boat movement, position, and important events. declination (Dec) Angular distance of a celestial body north or south of the ce- lestial equator; the arc of an hour circle between the celestial equator and a point on the celestial sphere. decompression levers Levers that hold the exhaust valves open so that no compres- sion pressure is built up, making it easy to turn the engine over. deep cycle Term used for a battery designed to supply a relatively low current for an extended period as opposed to an automotive type battery that supplies very high starting currents for short periods. deepening The lowering of the central pressure within a cyclone. Deep- ening is often associated with increased wind velocity and in- creased size of the low. Opposite of filling. The process of recovering information from a radio signal. density The mass per unit volume. At sea-level pressure, air is com- pressed and therefore has a higher density than it would have aloft under lower pressure. (At sea level, air density is ap- proximately 1.2 ounces per cubic foot.)

Return to Keyboard USPS® Nautical Glossary Return to Keyboard departure (p) Term used in sailing for distance made good to the east or west on any course, expressed in nautical miles. Do not con- fuse with difference of longitude (DLo). depressed pole The celestial pole below the horizon; it is of contrary name to the latitude. depth sounder An electronic device that uses the time lapse between a trans- mitted ultrasonic pulse and its received echo to determine the depth of water. desalinator A device used to remove salt from seawater to make it po- table. detonation (pinging) A too-rapid burning or explosion of the mixture in the engine cylinders (gas engine); usually audible upon acceleration of the engine. deviation (D, Dev) Disturbing effect of boat’s magnetic field upon its compass. Numerical difference in degrees, measured east (E) or west (W), between the magnetic value and the compass value of a given direction. dew Water condensed on objects the temperature of which has fallen below the dew point of the surrounding air.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard dew point The temperature to which the air must be cooled at constant pressure and moisture content to reach saturation. dew-point spread The difference between the air temperature and the dew point. Small dew-point spreads are an indication radiation fog may form. DGPS Differential Global . An extension of GPS where a correction signal is transmitted from a surveyed lo- cation to improve the positional accuracy of the GPS fix. See differential GPS receiver. diaphragm A flexible partition or wall separating two cavities used in cer- tain type pumps. dielectric A non-conducting material with few free electrons. A mate- rial with insulating properties. dieseling The continued running of an internal combustion engine after ignition is turned off. difference of latitude (l) The shorter arc of a meridian between the parallels of two places, expressed in angular measure.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard difference of longitude (DLo) The shorter arc of a parallel between the meridians of two places, expressed in angular measure. Numerical value is the same at any latitude. Do not confuse with departure (p). differential GPS receiver A receiver of signals from ground-based that can- cels out the random errors introduced into Global Positioning System signals through “selective availability.” See DGPS. diffraction The optical phenomenon caused by light from a bending around an obstacle. The resulting shadow is outlined with multiple bands of light and dark (diffraction bands). diffusion of light The scattering and crisscrossing of light rays, producing gen- eral illumination rather than direct radiation. digital 1. Values of measurements displayed as numbers. 2. Circuits that perform their operations using binary mathematics, as dis- tinct from analog circuits. digital display A number flashed on the display unit representing a physical variable. dinghy Small open boat used as a lifeboat or tender.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard diode An electronic device that permits current to flow in one direc- tion. dip (D) The angle of depression of the visible sea horizon from the horizontal due to the elevation of the eye above sea level. dip short (DS) A sight in which the body is brought down short of the natural horizon. direct current (DC) A current of either constant or variable magnitude that flows in one direction only. discontinuity The surface between two transparent substances, such as air and water or air and glass, at which refraction occurs. dismast To lose a mast on a sailing craft. dispersion An optical phenomenon in which white light is separated into spectral (color) bands on passing through a nonuniform me- dium. This occurs because different are bent (re- fracted) at different angles, spreading out into a spectrum. displacement 1. The total swept volume of an engine’s cylinders, i.e., through which the pistions actually move, expressed in cubic inches or liters. 2. The weight of water displaced by a floating vessel; hence, the weight of the vessel itself.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard displacement hull A hull that maintains its full displacement of water whether at rest or moving at various speeds (as opposed to a planing hull, which generally decreases its displacement with increased speed). distance (D) The spatial separation of two points and the length of the line joining them. Navigators use nautical miles. See nautical mile. distress call See Mayday. distribution system A system for the distribution or routing of power to various branch circuits. It may contain switches and overload devices for each circuit. distributor See ignition distributor. distributor points See breaker points. distributor pump A type of fuel injection pump using one central pumping ele- ment with a rotating distributor head that sends fuel to each cylinder in turn. ditty bag A small bag for stowage (e.g., sewing kit, or small tools). diurnal Daily phenomena that occur in 24-hour cycles—such as maxi- mum and minimum temperatures—are said to be diurnal.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard diurnal pressure change A regular daily change in the pattern of barometric pressure. This change, usually small and not due to moving highs or lows, is most prominent in lower latitudes and in settled weather. divergence Expansion in the volume of a mass of air over a region, result- ing in a net horizontal outward flow. This may be caused by the sinking of the air aloft. Opposite of convergence. dock 1. Area of water, within which a vessel can be made fast, between two landing piers or wharves. 2. To guide a vessel alongside a float, pier, or wharf. documentation The registration of a vessel with the United States Govern- ment (U.S. Coast Guard) to establish ownership and national- ity. dog watch A watch of half usual duration; used to stagger the watch periods from day to day. doldrums The belt of comparatively low pressure, calm or light winds, and high temperatures within a few degrees of the equator. dolphin A small group of piles, in the water, tied together into a single structure, generally used for mooring or as part of an aid to navigation.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A short spar fitted below a bowsprit, over which the bobstay is fitted. Doppler radar Radar system that not only detects the presence of moisture (or other objects) but also measures their quantity and direc- tion, enabling measurement of rainfall and determination of serious conditions such as tornadoes. Doppler shift Change in the frequency with which waves (e.g., sound, light, or radio) from a given source reach an observer, caused by the change in the relative speeds of the source and observer. dorade vent A ventilator that does not permit water or spray to pass below with the fresh air. double sheet bend A sheet bend with a second round turn on the standing bight. double side band Usually refers to amplitude moderated (AM) radio transmis- sion signals containing both upper and lower side-band com- ponents. douse To lower quickly. downburst A strong downdraft that induces an outburst of damaging winds at or near the surface of the earth.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard downdraft 1. A downward current of air, comparatively strong and lim- ited in horizontal extent, commonly found within thunderstorms and other areas of atmospheric turbulence. 2. Type of carbu- retor in which the fuel-air mixture flows downward to the en- gine. A line or tackle used to exert a downward pull on a sail or spar. draft 1. Depth of water needed to float a boat. 2. The fullness or “belly” of a sail. drainage winds Cold air accumulated inland, usually at high elevations, that flows down through valleys under the influence of gravity rather than pressure gradient. drift (DFT) 1. Movement of a boat due to wind and current. 2. Velocity (speed) of current. drizzle Precipitation consisting of numerous tiny droplets, sometimes called mist. drogue Drag deployed off the stern of a boat to create resistance and slow the boat’s speed in heavy weather, but still allow steer- ageway. Often conical or a series of conical shapes. Com- pare sea anchor.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard DR position A position determined by dead reckoning. See dead reckoning. dry bulb Name given to an ordinary thermometer, or the thermometer without wicking in a psychrometer. Also, the air temperature taken with a dry-bulb thermometer, as distinguished from the wet-bulb temperature. DSC Digital . A semi-automated system for estab- lishing communication with selected radio stations. ducting A phenomenon affecting the transmission of VHF signals when they become trapped between upper-air temperature inversions. The signals may travel for great distances before returning to earth. Dutchman® A mainsail furling system that automatically flakes the sail on the boom, using vertical polymer lines that pierce the sail on alternate sides. duty cycle The length of time a piece of equipment may be turned on as compared to the total time it is on and off. dwell angle/period (cam angle) Number of degrees of rotation of the distributor shaft during which contact points are closed.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard E A line that secures a cringle to the boom. earth ground A point that is at the same electrical potential as the surround- ing earth. ease To let out a line gradually. easterlies Belts of persistent easterly winds such as the trade winds and the polar easterlies. easterly wave A zone, or trough, of low pressure and convergence embed- ded in the trade-wind belt, drifting from east to west. ebb current A tidal current in which the flow of water is generally away from the major land mass toward the open sea (often incor- rectly termed “ebb tide”). ecliptic The great circle in which the sun appears to move on the ce- lestial sphere by reason of the annual revolution of the earth around the sun. This great circle is inclined at about 23°27' to the celestial equator, because the axis of rotation of the earth is not perpendicular to its orbit. This makes the sun seem to move north and south during the year and gives the earth its seasons. See equation of time.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard eddy A small whirl or circling movement of a fluid, such as air or water, embedded within a larger current. electrolysis 1. Chemical effect caused by the flow of an electric current through a conducting fluid. 2. The decomposition of metal caused by stray electrical current. electrolyte Any liquid solution that is capable of conducting an electric current. electrolytic action A plating and eroding action that occurs when dissimilar met- als are immersed in an electrolyte such as salt water. A field of force produced around a conductor when a current flows through it. The entire range of wavelengths of energy (including light) reaching the earth’s outer atmosphere from the sun. The at- mosphere, ozone, and ionized gas layers, as well as the earth’s magnetic field, shield us from much of this radiation. Of the energy that reaches the earth, 90% is in the visible portion of the spectrum. electromotive force (E or EMF) Voltage or electrical difference of potential generated by a battery or a generator. See potential difference.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard electrons The negatively charged particles surrounding the nucleus of an atom. These particles determine the chemical and electri- cal characteristics of the atom. electrostatic field Electric force field set up in the vicinity of electrically charged matter. elevated pole The celestial pole above the horizon, agreeing in name with the latitude. elevator A wedge, either attached to or built into the bottom of a ves- sel, for the purpose of keeping its stern up when underway. Called “trim tabs” when adjustable. El Niño A slackening of the (easterly) trade winds accompanied by a flow of warm sea water from the western Pacific back to the east. These changes in water temperature are known to alter air temperature, rainfall, and storm formation patterns over much of the earth. See La Niña. ellipse The curve formed when a cone or circular cylinder is cut across at an angle. When the angle is other than perpendicular to the axis of the body being cut, the curve is not a circle. embark To go aboard.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Emergency Position Indicating See EPIRB. energy The capacity for performing work. ephemeris A collection of tables or data showing the position of the plan- ets or heavenly bodies for every day of a given period; also, an astronomical almanac containing such tables. EPIRB Emergency Position Indicating Radio Beacon. A small trans- mitter operating on aircraft, marine, or satellite emergency channels used in cases of distress. equation of time (EqT) The amount of time by which the mean sun leads or be- hind the true sun at any instant. equatorial air mass An air mass that originated and derived its temperature and moisture characteristics from the region of the equator. equilibrium The state of air in vertical balance with other surrounding air— that is, having no tendency to move vertically with respect to its surroundings. equinoctial See celestial equator. estimated position (EP) Most probable position of a boat based on a single line of po- sition (LOP) or based on incomplete or questionable data.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard evaporation The physical process of transformation of a substance from the liquid to the gaseous state. Opposite of condensation. even keel A boat is on an even keel when it is floating level. exhaust manifold The assembly that conducts exhaust gases out of the combus- tion chambers of an engine. extratropical cyclone A low that develops outside the tropics. Involves two or more air masses, with the fronts that separate them. eye The center of a tropical storm (a hurricane or a typhoon). The eye is a roughly circular area of low pressure, with compara- tively light winds and fair weather. eyelet A small grommet through which lacing is passed. eye of the wind Directly upwind. eye splice A permanent loop in the end of a rope.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard F Fahrenheit (F) A temperature scale on which 32° represents the freezing point of water and 212° the boiling point at standard pressure. For- mula for converting to Celsius (Centigrade): 5 C° = /9 (F° – 32) fairing block An underwater block mounted on the hull of a vessel and shaped to provide a laminar flow of water around an obstruc- tion. Fitting to route controls, lines, or cables. fairway Navigable channel in a body of water. fake down 1. To coil a line so that it will run free. 2. The mainsail may be faked or faked down onto the boom. fall The line in a tackle. fall off To change course so as to bring the relative wind direction farther aft. An unlicensed (non-marine) type of two-way radio.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard farad The unit of capacitance. A unit of measurement of the amount of electrostatic charge that can be stored. fast Secure: to make something fast is to secure it. fastenings Screws or nails used to hold parts of a boat together. fathom Nautical linear measure equal to 6 feet. FCC Federal Communications Commission. fender Protective device between a boat and another object. fender rail A fixed on the hull to keep a vessel from scraping the wharf. fetch The unobstructed distance over the water on which the wind may act to build wind waves. F-head engine An engine designed with one valve in the cylinder block at the side of the piston and one valve in the cylinder head above the piston. fiberglass A glass fiber usually impregnated with a synthetic resin such as polyester or epoxy; may be woven or random fibers.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard fid Tapered tool used in splicing. See marlinspike. field coil A coil of insulated wire surrounding the field pole of a genera- tor or starter. figure-eight knot A stopper knot; used to keep the bitter end of a line from running through a block. filling The process of normalizing or increasing the central pressure of a low so that it becomes less intense or disappears. Filling is the opposite of deepening. filter (oil-water-fuel) A unit containing an element or screen to retain foreign par- ticles in the fluid. rudder A rudder fastened directly to the after edge of a keel. first point of Aries An imaginary point at the intersection of the celestial equator and the ecliptic, where the declination of the sun is zero as it passes over the equator on the way northward at the begin- ning of spring (about 21 March). Also, the instant when the sun crosses the equator. Often called Aries. Same as vernal equinox. fisherman’s bend A knot used to connect a rode to an anchor.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard fix Relatively accurate position determined without reference to any former position. Usually determined by nearness to a known charted object or by crossed (intersecting) lines of position. flake To fold a sail on the boom. flame arrestor A screen-like metal fitting fastened over a carburetor air in- take to keep backfire flames from flashing out into an engine compartment where they could ignite gasoline fumes. flare 1. Outward curve of the hull towards the deck. 2. A blaze of fire or light used to attract attention. 3. The device that pro- duces the blaze. flatboat High-speed, outboard-powered fishing boat used in shallow coastal waters. flax A natural fiber used in packing, e.g., around a shaftlog or rud- derpost at the point of hull penetration. float level The predetermined height of the fuel in a carburetor bowl, usually regulated by a needle valve or float.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard float plan An outline plan of a cruise left at one’s home port or with friends to provide a description of the boat, list of persons on board, list of safety equipment carried, and, most importantly, where the boat will be cruising and when it is expected to return to home port. flood current A tidal current in which the flow of water is generally to- wards the major land mass from open sea (often incorrectly termed flood tide). fluke Flattened end of an anchor arm that bites into the ground. fly 1. A masthead wind pennant. 2. Length of a flag or burgee perpendicular to the luff. 3. Part of flag farthest from the hoist. flybridge (flying bridge) A steering position located atop the cabin of a powerboat. flyweight A small pivoted weight used in mechanical governors. flywheel A heavy wheel (usually steel or iron) in which energy is ab- sorbed and stored by means of momentum. foehn wind Name given to a warm, dry wind caused by a rush of cold air down the mountainside; the air warms due to an increase in pressure produced by decreasing altitude. (Pronounced like “foon wind.”) See chinook, Santa Ana wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard fog Minute water droplets suspended in the atmosphere. Differs from a cloud only in that it is at the surface instead of aloft. Ice fog (minute ice crystals) may occur in extremely cold regions. foot The lower edge of a sail. force 1. A mechanical push or pull or an electromagnetic attraction or repulsion. 2. A unit of measure of wind velocity. fore Denoting at, near, or toward the bow. fore-and-aft Parallel to the centerline or keel. foredeck The deck of a boat located near the bow. foreguy A line led from the foredeck to a to limit its upward swing. forepeak A stowage compartment in the very forward part of the boat. forereach 1. To carry way (continue to move by momentum) upwind as a boat heads into the wind with sails luffing. 2. To overtake another vessel while on the same course. 3. To make head- way while hove to.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard forerunner An observable phenomenon such as cirrus clouds or a long, low swell moving ahead of a severe storm. In the case of a hurricane, the swell is marked by an abnormally long period. The sail set from a schooner’s foremast. A stay below and aft of the headstay on a sailboat. forestaysail A sail, similar to a jib, set on the forestay. foretriangle The area formed by the headstay, mast, and deck. forward Toward the bow. foul Jammed, entangled, not clear; being hindered or impeded. foul ground Bottom that is unsuitable as an anchorage because holding qualities for an anchor are poor or obstructions exist. foul-weather gear Rain suit, sometimes including boots and/or hat. founder To swamp and sink to the bottom. four-cycle engine See four-stroke-cycle engine.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard four-stroke-cycle engine Engine in which explosion in a given cylinder occurs every other revolution of the crankshaft. A cycle, also known as 1 Otto cycle, is considered /2 revolution of the crankshaft. Strokes are intake, compression, power, and exhaust. fractional rig A sailboat with the jibstay attached to the mast at a point other than the masthead. frame Athwartship structural member () that gives shape and stiff- ening to the hull and to which planking is attached. freeboard Vertical distance from the water surface to the lowest point where unwanted water could come aboard. free Electron that is loosely bound to the nucleus of an atom and is free to circulate among the other of a substance. frequency Number of cycles per second. (FM) Transmission of intelligence by varying a radio frequency about its center frequency or “carrier.” frequency range A portion of the electromagnetic spectrum—for example, the range of for , visible, or radia- tion, or the frequencies used for radio and radar.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard front The zone of transition between two air masses of different temperature characteristics. frontal fog Fog associated with a front, usually formed by rain from warmer air aloft falling through and saturating a cooler layer of air next to the earth. frontal depression An area of slightly lower pressure along a front. This often indicates the formation of a new low-pressure center. frontal surface The three-dimensional surface separating two air masses of contrasting characteristics, extending from the surface posi- tion of the front up over the colder air mass. frost A deposit of crystals of ice on cold objects by direct sublima- tion from the vapor. fuel injection pump A device for metering precise quantities of fuel at precise times and raising them up to injection pressures. full and by Close-hauled with all sails full and drawing. funnel cloud A aloft, not touching the ground. furl Gather and lash sails to a spar.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard fuse A protective device containing a metal alloy of low melting point designed to melt and break a circuit under conditions of excessive current flow. fusion Transformation of a substance from the solid state to the liq- uid state by the induction of heat, as in the melting of ice. fusion temperature The temperature at which a solid melts. G gaff 1. Spar that supports the upper side of a fore-and-aft four- sided sail. 2. Long-handled hook to bring fish aboard. Any sailboat with a four-sided mainsail. gale A wind in the range of 39 to 54 miles per hour or 34 to 47 knots. See Beaufort scale. galley Area where food is stored and cooked; nautical kitchen. gallows A frame to hold the boom when the mainsail is down or is being reefed.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard galvanic action Decomposition of metals in an electrolyte due to their differ- ent electrical potentials. Although salt water is usually consid- ered as the electrolyte, rivers and lakes will have similar action due to pollutants in the water. galvanic corrosion Chemical destruction of metal due to galvanic action causing a current flow between two different metals connected to- gether and immersed in an electrolyte. galvanic series A scale of potentials developed between different metals bonded together and immersed in an electrolyte. For marine purposes the electrolyte is assumed to be salt water. galvanometer A meter or other device used for measuring very small cur- rents. garboard The longitudinal plank next to the keel. gasket 1. A strip of cloth or line used to secure a furled sail to a boom (also, stop). 2. Anything used as a packing to form a seal between two metal surfaces. Wire size. American Wire Gauge (AWG) is a numerical siz- ing system based on the cross section of a conductor.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard gear General name for all non-permanent nautical equipment, in- cluding crew’s clothing and personal effects. gear ratio The number of revolutions made by a driving gear compared to the number made by a driven gear of different size. gelcoat The outside color coat used in fiberglass construction. General gas law The relationship of the temperature, pressure, and volume of a gas, including the atmosphere. One form of this law,

P1V1 ÷ T1 = P2V2 ÷ T2, predicts how changes in atmospheric properties occur. generator Similar to an alternator with a wound coil rotor but provided with a commutator to obtain direct current output. block The movable block that enables adjustment of the sail’s luff curvature to optimize sail performance. Genoa jib A large overlapping headsail. geographical position (GP) A point on the earth’s surface aligned with the center of the earth and a body’s position on the celestial sphere. geoidal horizon The intersection of the celestial sphere and a plane tangent to the earth at the observer’s position.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard geometrical horizon Intersection of the celestial sphere and a cone tangent to the surface of the earth, with the apex at the eye of the observer. ghosting Sailing in very light winds. giga (G) Metric prefix meaning one billion (109). gimbals Mounting supports permitting an object, such as the compass, to remain level when the boat heels. give-way vessel A term, from the Navigation Rules, used to describe the ves- sel that must yield in meeting, crossing, or overtaking situa- tions. Compare stand-on vessel. glaze A clear, solid coating of ice created by the impingement of supercooled liquid raindrops onto exposed surfaces. Glaze is usually associated with large water drops such as those found in cumuliform precipitation. Global Positioning System (GPS) A satellite-based world-wide navigation system using simul- taneous signals from three or more to establish highly accurate positioning. GLONASS Global Navigational Satellite System. The Russian Global Po- sitioning System, with better performance near the polar re- gions.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard glow plug A heating element installed in diesel engine combustion cham- bers to aid in cold starting. GMDSS Global Maritime Distress and Safety System. Part of the In- ternational Maritime Organization (IMO) to define radio pro- cedures. go about To tack or come about. A mechanism used to attach a boom to a mast. To be effec- tive it must permit the unattached end of the boom to swing in any direction without restraint. goosewing jibe A jibe in which either the top or bottom of the sail has failed to jibe with the rest of the sail. governor A device to regulate or control engine speed, regardless of the load. It may be mechanical, hydraulic, or electrical. gradient In weather terminology, change in an atmospheric property, such as pressure with horizontal distance. grapnel anchor An anchor with four or five arms to snag projections on the sea bed; often called a grappling hook in smaller sizes.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard graticule The network of lines representing parallels and meridians on a map, chart, or plotting sheet. gravity switch A switch, usually electrical, that is closed when the contact is pulled down by gravity. great circle A circle on the surface of a sphere, produced by the intersec- tion of a plane that passes through the center of the sphere. great circle chart A gnomonic-projection chart on which a great circle appears as a straight line. flash A momentary flash of green color seen just as the sun’s upper limb dips below the horizon. When seen, it is usually over ocean waters in the tropics or subtropics, in fine, clear weather. greenhouse effect Much of the atmosphere is transparent to short- heat energy from the sun. As solids and liquids on the earth’s surface absorb this energy, they heat up and also give off heat energy, but at longer wavelengths. Certain gases, such as car- bon dioxide, reflect or absorb this longer-wavelength energy and trap it, causing further warming. Glass in a greenhouse also does this; hence, the term.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Greenwich apparent time (GAT) Local apparent time at the Greenwich meridian. The angle at the celestial pole between the lower branch of the Greenwich celestial meridian and the hour circle of the apparent or true sun. Greenwich hour angle (GHA) The angular distance of the hour circle of a body westward from the celestial meridian of Greenwich, measured from 0° to 360° along the celestial equator. GHA values for the sun, moon, planets, and stars are tabulated in the Nautical Alma- nac. Greenwich mean time (GMT, UT) Zone time at the Greenwich meridian. Greenwich hour angle of the mean sun, expressed in time units, plus 12 hours. Used in the Nautical Almanac for tabulation of GHA and declina- tion of celestial bodies. Greenwich meridian (G) The meridian through Greenwich, England, serving as the prime meridian and the reference meridian for Greenwich time. Greenwich sidereal time (GST) Local sidereal time at the Greenwich meridian, the upper branch of the Greenwich celestial meridian, and the hour circle of the vernal equinox. grommet A metal ring fitted into a hole in a sail or in canvas. See cringle.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard ground fault circuit interrupter (GFCI) An AC circuit-breaker-like safety device that interrupts the power conductor of an AC circuit when there is leakage cur- rent to earth over 5 milliamperes, which would indicate that an appliance housing had shorted to the energized conductor and could shock anyone touching it. grounding Connecting by a conductor to a point of zero voltage, such as the earth. ground swell Steepening and increasing severity of ocean swells as they cross a shallow area such as a reef or bar. ground tackle Anchor with rode and related gear. ground wave A traveling essentially parallel to the earth’s sur- face. group velocity The speed at which an entire group of waves moves. gudgeon One half of a rudder hing. The other half is the pintle. gudgeon and pintle A metal eye and matching pin used to mount a rudder on the or the of a boat.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Gulf Stream A well-defined, narrow, swift-moving, warm ocean current flowing from close along the Florida coast northeasterly along the continental slope off Cape Hatteras, then east of the Grand Banks to become the North Atlantic Current. Upper edge or rail of a boat. (Pronounced “gun’el.”) gust A sudden, brief increase in wind speed. gust front A cooled downdraft pushed ahead of a thunderstorm. The cold air forces the lighter, warmer air at the surface up rapidly and cools it to saturation, adding to the cloud formation and rain- fall. The effect is similar to that of a cold front. The control line connected to the tack (windward clew) of a spinnaker. H hack watch A watch used for timing observations of celestial bodies.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hail Frozen water droplets that are formed by strong updrafts in thunderstorms and fall toward earth when they become heavy. If they do not melt in the warmer air below, they reach the earth in the form of pea-sized balls. In strong storms, several trips may be made up to freezing level and down to gather more water, and much larger hailstones (up to four inches in diameter) may form. halo A broad ring around the sun or moon, caused by the refrac- tion of light passing through cirrus-type ice-crystal clouds. halyard Line for hoisting sails or flags. ham radio , licensed for non-commercial use only. hand 1. A member of the crew. 2. To lower a sail. handrail A grab rail provided for the safety of personnel. hank Clip sewn to the edge of a sail and used to attach it to wire rigging, such as a forestay. hard alee The command given when the helmsman pushes the to leeward to tack. harden To haul the sheets in and sail closer to the wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hard over To put the helm over as far as possible. harmonic Multiple of a basic or fundamental frequency. harness Webbing with D-rings worn around the torso and attached to a strong point on a boat to prevent falling overboard. The web- bing may be incorporated into a flotation device. hash Electrical interference that produces a scratching or frying sound, usually caused by vibrating or rotating electrical ma- chinery. hatch Closable opening in the deck. haul 1. The wind hauls when it draws more ahead. 2. To remove the boat from the water. hawsepipe A hole in the bow through which an anchor rode is led. hawser A heavy rope or cable used for various purposes, such as towing or mooring large vessels. haze Fine salt, dust, or other particles in the air, too small to be seen individually, reducing visibility slightly or moderately but sel- 3 dom below /4 mile.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard head 1. Marine toilet and its compartment. 2. Upper corner of a triangular sail or upper edge of a four-sided sail. 3. Upper end of the mast, masthead. 4. Foremost part of a boat, bow. header A wind that shifts more ahead of a boat sailing close-hauled, forcing it to change course to avoid pinching or luffing. Usu- ally a good time to tack. header tank A small tank set above an engine on heat-exchanger cooled systems. The header tank serves as an expansion chamber, coolant reservoir, and pressure regulator (via a pressure cap). head gasket The gasket between a cylinder head and a cylinder block. heading Direction in which a boat is pointing at a given moment. head off Head away from the wind, bear off. headsail Any sail flown forward of the mast. headstay A stay that runs from the top of the mast to the bow or bow- sprit and is the outermost stay on boats with more than one headsail; a jibstay. head up To sail closer to the wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard headway Boat’s forward motion. heat Energy in the process of being transferred from warmer to colder bodies by virtue of their temperature difference. Heat may be converted to or from other forms of energy, or stored within a system as internal energy. heat exchanger A device containing a number of small tubes through which raw cooling water is passed, while engine cooling water is circulated around the outside of the tubes. heat of condensation The number of calories per gram given off by a substance when it changes from vapor to liquid (for water, 540 calories per gram—numerically same as heat of vaporization). heat of fusion The number of calories per gram required to melt a given sub- stance (for water, 80 calories per gram). heat of vaporization The number of calories per gram required to vaporize a sub- stance (for water at sea-level pressure, 540 calories per gram). heave 1. Rise and fall of a boat due to wave action. 2. To pull on a line to tighten it. 3. To throw an object (e.g., a line). heave to To bring a boat into a position where there is little or no head- way, usually with the bow into the wind or current.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard heaving line A light line used for throwing. heavy weather Storms and rough seas. heel Incline to one side due to force of wind or waves. height of tide The vertical distance between the surface of tidal water at a given moment and the datum (reference plane) from which depths are measured and recorded on charts of the area. Val- ues may be + or – and are added algebraically to the charted depth to give the depth of water. helm The tiller or wheel and related steering gear. helmsman The one who steers the boat with the tiller or wheel. henry (H) Unit of measurement for inductance. (Hz) The unit of frequency of an alternating current. One hertz equals one cycle per second. high An area of comparatively high pressure with winds spiraling outward in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. An anticyclone.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard high aspect Having much greater height than width. hiking out Moving a crew’s weight outside the boat to the windward side. The technique is used on small boats to decrease the heeling angle. hitch 1. A knot used to secure a rope fast to another rope or other object. 2. A connection between a vehicle and a trailer. 3. To connect a trailer with a vehicle. knot used to secure a rope fast to another rope or other object. hockle Short tight bend or kink in a rope or line that has been twisted too hard or drawn off coil too fast. hogged Describes a boat whose bow and stern have sagged. hoist To raise a sail or flag. Also, the length of a sail’s luff. holed An accidental hole in the hull, tending to cause flooding. hole-type nozzle An injector nozzle with one or more very fine holes generally used in direct (open) combustion chambers. homing Piloting a vessel in the direction of the source of a signal or reflected signal by use of an electronic device (e.g., RDF or radar).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hook echo Condition seen on Doppler radar as a result of strong, mois- ture-bearing winds blowing in opposite directions in close prox- imity to each other. The appearance on radar is similar to a fishhook and is a very strong indication of tornado formation. horizon See visible horizon. horizontal parallax (HP) The parallax of a body—usually the moon—when it is on the celestial horizon. See parallax. horse latitudes Either of two belts of high pressure centered roughly at 30°N and 30°S latitude, characterized by fickle winds and lack of rain. So named because becalmed sailing ships often jettisoned horses to conserve drinking water. horsepower (HP) The amount of power or the rate of doing work. One horse- power is equal to the energy required to lift 550 pounds 1 foot in 1 second. One electrical horsepower equals 746 watts. horseshoe buoy A horseshoe-shaped life preserver, generally to be thrown to someone in the water. hot wire A wire not at ground potential.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hour angle (HA) The angular distance from a celestial meridian to an hour circle, measured westward through 360°. Designated local or Green- wich, according to origin. Called sidereal hour angle if mea- sured from the hour circle of the vernal equinox, or first point of Aries, to the hour circle of a body. hour circle A great circle through the celestial poles and a point or body on the celestial sphere. Moves with the celestial sphere, whereas a celestial meridian remains fixed with respect to the earth. hour glass The twisted shape of a spinnaker that has not been hoisted properly. hour meter A meter that records engine running time. hull Basic structure and shell of a boat. humidity A measure of the water-vapor content of the atmosphere. hunting (rolling, lopping) A rhythmical cycling up and down in speed of a governed engine. hurricane A tropical cyclone with wind speeds of 73 miles per hour or greater or 64 knots or greater.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hurricane surge A sudden rise in the level of the sea along a coast line as a result of the approach and passage of a hurricane. See storm surge. hydrofoil 1. A device designed to deflect a water stream, thereby gen- erating a force from it, e.g., rudder, keel, centerboard. 2. A vessel designed to have its hull raised clear of the water, when moving at high speed, by hydrofoils extended below the hull on struts. hydrolant Urgent radio notice of dangers to navigation, Atlantic Ocean. hydrologic cycle The cyclical process of evaporation, transportation, conden- sation, and precipitation that maintains a water balance in the atmosphere. hydrometer A float type instrument used to measure the specific gravity of the battery electrolyte to determine the percent of charge. hydropac Urgent radio notice of dangers to navigation, Pacific Ocean. hygrograph An automatic hygrometer that measures atmospheric humid- ity and continuously records readings on a clockwork-driven chart.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard hygrometer A device for measuring atmospheric humidity. The hair hy- grometer and sling psychrometer are most common. hygroscopic The property of attracting or absorbing moisture from the air. hyperbolic system A navigation system, such as loran, in which the hyperbola- shaped lines of position (LOPs) are determined by the differ- ence between the arrival times of pulses from two transmitters in different locations. I ice Water in its solid form. ICW See Intracoastal Waterway. ignition distributor An electrical device usually containing a circuit breaker (points) for the primary circuit and providing a means for conveying the secondary or high-tension current (from the rotor and dis- tributor cap) to the spark plug wires. ignition system Means for igniting fuel in cylinders; includes spark plugs, wir- ing, ignition distributor, ignition coil, and source of electrical current supply.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard impedance (Z) The total opposition that a circuit offers to the flow of an al- ternating current due to the combined effects of resistance and reactance. Symbol Z; measured in ohms. impeller A rotor or wheel with blades used to pump water or other fluids. impressed current system An alternate method of providing cathodic protection by the selective application of controlled electrical potentials using electronic circuitry. This method is common to large metal- hulled vessels. inboard Toward the centerline of the boat; inside the boat. inboard/outboard (I/O) Propulsion system consisting of an inboard engine connected through the transom to an outboard drive unit. Also see stern drive. inches of mercury (Hg) A unit of barometric pressure measurement related to the height of the column in a mercurial barometer, usually ranging between 27.00 and 31.00 inches reduced to sea level. 1 inch Hg = 25.40005 millimeters Hg 1 inch Hg = 33.86395 millibars index correction (IC) Correction applied to a sextant reading (hs) to compensate for the sextant index error.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard index error (IE) Error in a sextant reading caused by sextant misalignment. induced current Current caused to flow in a conductor due to its presence in a changing magnetic field. inductance Property of a circuit that tends to oppose any change in the current flow in a conductor because of a magnetic field caused by that current. Symbol I; measured in henrys. induction The influence of magnetic fields of different strength not elec- trically connected to one another. induction coil Essentially a transformer that, through the action of induction, cre- ates a high-tension current by means of an increase in voltage. induction motor An AC motor usually without brushes or commutator, the ro- tor being energized by transformer action. inductive reactance The amount of opposition offered to the flow of an alternating current due to the presence of inductance in a circuit. Symbol XL; measured in ohms. in irons Headed directly into the wind with no headway, the boat can- not be trimmed to fill on either tack. Also, “in .” injection pump See fuel injection pump.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard injection timing The relationship of the beginning point of injection to the rota- tion of the crankshaft. injector A device for atomizing diesel fuel and spraying it directly into a combustion chamber. injector nozzle That part of an injector containing the needle valve and its seat. injector nut The nut that holds a fuel line to an injector. International Marine Satellite Organization, commercial com- munications system. inlet valve (intake valve) A valve that permits a fluid or gas to enter a chamber and seals against exit. inline pump A series of jerk pumps in a common housing, operated by a common camshaft. insolation Radiant energy received from the sun at the earth’s surface. INcoming SOLar radiATION.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard instability 1. The state of the atmosphere when the vertical temperature distribution is such that an air parcel, if set in vertical motion, tends to move away from the original position of the parcel with increasing speed. 2. The inability of a boat to return natu- rally to its original waterline position after being disturbed. intended track Intended or expected direction of travel of a boat over the bottom, with consideration given to current effects. insulator A material with few free electrons; a nonconductor. intake manifold The assembly on an engine that conducts air or fuel-mix mix- ture into an engine. intercept (a) The difference in minutes of arc between computed altitude (Hc) and observed altitude (Ho), representing the difference in length of the radii of the computed and observed circles of altitude. Labeled T (toward) if observed altitude is greater, A (away) if computed altitude is greater. Also called altitude dif- ference and altitude intercept. internal combustion Burning of fuel within an enclosed space. International Date Line (IDL) The boundary between the –12 and +12 time zones, corre- sponding approximately with the 180th meridian.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Intertropical Convergence Zone (ITCZ) A zone within the tropics where the northeasterly trade winds of the Northern Hemisphere meet the southeasterly trade winds of the Southern Hemisphere. The zone lies within the equatorial air mass, near but usually not on the equator. Intracoastal Waterway A section along the Atlantic seaboard that allows a mostly sheltered, close-to-shore way for boats to travel between New York to Florida. A phenomenon in which an overlying layer of air is warmer than the underlying layer. inverter A mechanical or electronic device used to convert direct cur- rent to alternating current. An electrically . The process of imparting an to a particle. The atmospheric shell characterized by high ion density. Its base is about 50 miles in height and it extends to outer space. isallobar A line connecting points on a weather map with the same values of three-hour barometric pressure change.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard isobar A line connecting points on a weather map with the same values of barometric pressure. isolation transformer A transformer having two or more separate windings that are not electrically connected. isotherm A line connecting points on a weather map with the same values of temperature. J jack stay A stay that runs forward from the masthead over a jumper strut and back to the mast to stiffen the upper portion of a fractional-rigged mast. jack wire/line A line run from the cockpit to the foredeck, and firmly at- tached on both ends, to permit a crew member’s safety har- ness lanyard hook to have uninterrupted fore-and-aft travel. jerk pump A type of fuel-injection pump that uses a separate pumping element for each cylinder.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard jet stream A meandering, tube-like system of high-velocity winds (50 to 300 knots). The jet stream flows west to east and may be 100 miles wide and one or more miles high. It usually exists be- tween 30,000 and 40,000 feet of altitude and is responsible for steering air masses and weather fronts across the middle lati- tudes (20° to 60° N and S). Two jet streams exist in the North- ern Hemisphere—subtropical and polar. jetty A small pier or landing place. jib Triangular sail set on a stay forward of the mast. jib downhaul A line from the head of a jib to a block near the tack and then leading aft. Used to douse the jib without going forward. jibe To change from one tack to another by turning the stern through the wind. jib-headed (sail) A triangular sail. jibstay The forwardmost stay supporting the mast, extending from the bow or bowsprit to the upper part of the mast. jiffy A method of reducing the area of a sail by means of led through reef in the sail and through on the boom to or cleats.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard jigger The mizzen or the mizzenmast. jon boat A type of simple flat-bottomed fishing boat. journal The part of a shaft or crank that rotates inside a bearing. jumper stay A stay that runs forward from the masthead over a jumper strut and back to the mast, to stiffen the upper portion of a fractional-rigged mast. jumper strut Horizontal spar on the forward side of the mast to which the jumper stay is attached to keep the upper part of the mast straight; used with fractional rigs. junction A connection common to two or more components in a cir- cuit. A contact between two dissimilar metals as in a transis- tor. jury rig Makeshift repair or substitute. K kedge 1. Move a boat by pulling it up to its anchor. 2. Type of an- chor.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard keel Main centerline structural member (backbone) of a boat. Also, downward extension of hull to increase lateral resistance and stability. keeper 1. The hand that can be set to indicate an initial reading on an aneroid barometer dial. 2. Small metal, dish-shaped piece that holds a valve spring assembly on a valve stem. Kelvin temperature A temperature scale using Celsius units based on a theoreti- cal absolute zero. K = C + 273.16° or 0°C = 273.16°K ketch A two-masted sailboat with the after mast stepped forward of the rudder post. kilo (k) Metric prefix meaning one thousand (103). kilowatt One thousand watts. kilowatt hour A measure of power consumed or generated at the rate of 1,000 watts per hour. knife switch An open, pivoted-blade-type switch. knock A general term used to describe various engine noises caused by loose internal parts, or from pre-ignition detonation of the fuel.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard knockdown A temporary condition of excessive heel resulting from a sud- den increase of wind force. knot 1. General term for securing a line to an object, another line, or itself. 2. Unit of speed of one nautical mile per hour. known position (KP) Triangular symbol with a dot in the center used to identify a position ashore, on a dock or pier, or boat tied to them. Used only in sight folders for USPS celestial navigation courses. L or lead The displacement in time or in electrical degrees (0°-360°) between two electrical signals of the same frequency. lake breeze A local wind blowing from a lake to the land as a result of more rapid warming of the land; similar to sea breeze. lake effect Changes in temperature, humidity, stability, and momentum of an air mass as it blows across the Great Lakes, and the result- ing weather phenomena over downwind areas. land breeze A local wind blowing from the land to the sea. It is caused by rapid cooling of land surface as compared to water surface. Generally begins near midnight and lasts till daylight.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard lane A band approximately 8 nautical miles wide or one-half the wave length of an Omega navigational signal operating at 10.2 kHz. La Niña A strengthening of the (easterly) trade winds off South America accompanied by the retention of warm sea water in the western Pacific, causing the waters in the eastern Pacific, off Peru, to cool. These changes in water temperature are known to alter temperature, rainfall, and storm formation pat- terns over much of the earth. See El Niño. lanyard Short line used as a handle or to secure an object. lapstrake Type of hull construction with overlapping planks. See clin- ker-built. latent heat Heat absorbed or given off by a substance during a change of state (as from a solid to a liquid). Since no temperature change occurs, the heat is hidden or latent. lateral plane Any surface or appendage that serves to inhibit lateral motion of a boat through the water. latitude Angular distance on the earth's surface, measured north and south from the earth’s equator.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard launch 1. To place a boat in the water. 2. A boat used to carry people between shore and a moored vessel. lay 1. To lay a mark is to be able to reach it in a single tack, close-hauled. 2. The lay of a line is the direction in which its strands are twisted. lay line An imaginary line that brings a closehauled boat directly to the mark. lay to To lie without headway either to a sea anchor or to lines streamed over the side, or merely drifting (lying ahull). layer instability The condition of a layer of air —usually moist in the lower portion, but dry in the upper—that leaves the layer stable as long as it is not lifted but results in instability if the layer un- dergoes a general uplift. lazarette Storage compartment in stern of boat. lazy jacks A series of lines attached on both sides of the boom, allowing the mainsail to flake down in a neat pile. lee (lee side) Side away or opposite that from which the wind blows. Also, area sheltered from the wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A pivoting board attached to the side of a boat to provide lateral resistance. leech The after edge of a sail between the clew and the head. A condition of balance that requires the tiller to be held to leeward to keep a boat on a straight course. lee shore One onto which wind or current can force a boat; shore to leeward of boat. leech line A light line sewn through the tabling in the leech of a sail, used to control leech flutter. leeward Direction away from the wind; downwind. (Pronounced “loo’ard.”) leeway 1. Sideways movement of a boat through the water caused by wind. 2. The angular difference due to wind between the true course steered and the true course over ground. leg 1. The distance covered on one tack when sailing to wind- ward. 2. On a race course, the course between two succes- sive marks. lever arm The distance through which a force acts.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard license A government authorization for operation of a radio, radar, or other type of signal-generating station. lie ahull To lie without headway, more or less broadside to the wind, with no sail set, drifting freely. A means of with heavy weather. lie to See heave to. life jacket A buoyant vest; called a Personal Flotation Device (PFD) in the United States. lifeline Plastic-coated wire rigged to stanchions around the periphery of the deck to help keep personnel aboard. life preserver A buoyant coat, suit, vest, ring, horseshoe buoy, or cushion; called a Personal Flotation Device (PFD) in the United States. lift 1. A force generated on the leeward side of a sail by air pass- ing over the airfoil shape. It is the force that drives a sailboat. 2. When sailing close-hauled, a wind that shifts aft to allow a change of course toward the wind direction. lift pump A low-pressure pump that feeds fuel from a tank to an injec- tion pump.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard lightning A visible electrical discharge, generally produced during a thun- derstorm. light sails Sails made of lightweight fabric for use in light wind. limber chain A chain run through the limber holes for the purpose of loos- ening clogged debris. limber holes Openings through frames to permit water to drain to the low- est part of the bilge. line of position (LOP) 1. A line, usually plotted on a chart, along which the boat lies, as determined from a single observation. 2. A visual or elec- tronic reference line from or about a known navigational aid. line of sight Line of direct visual observation from one point to another. linkage Any series of rod, yokes, levers, etc., used to transmit motion from one unit to another. line stopper (rope clutch) A quick-release, clamp-type device that contains a cam for securing a line. list Inclination of a boat to one side due to weight distribution.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard LOA Length overall. load A device that uses power from a source to perform a useful function. A coil placed in a circuit to lower its natural resonant fre- quency. Used to increase the electrical length of an antenna. lobe An off-center or eccentric enlargement on a shaft to convert rotary motion to reciprocating motion. Also called a cam. local apparent noon (LAN) The instant at which the apparent (true) sun is over the upper branch of the local meridian. Time of sun’s maximum altitude at the observer’s location. local apparent time (LAT) The angle at the celestial pole, between the lower branch of the local celestial meridian and the hour circle of the apparent or true sun. local hour angle (LHA) The angular distance of the hour circle of a celestial body measured westward from the celestial meridian of the observer, measured from 0° to 360° along the parallel of declination. local mean time (LMT) The time based on the mean sun with reference to the local celestial meridian (not the zone meridian).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard local safety ground A terminal or conductor in direct contact with or at the same potential as local earth or water. local sidereal time (LST) Local hour angle of the first point of Aries (vernal equinox) expressed in time units; the angle at the celestial pole, be- tween the upper branch of the local celestial meridian and the hour circle of the vernal equinox. LST is LHA of Aries ex- pressed in time units. locker A storage or stowage compartment. log 1. Instrument to measure distance or speed through the wa- ter. 2. A book in which all matters pertaining to a vessel’s navigation are recorded. log book See deck log and log. longitude Angular distance on the earth's surface, measured east or west from the prime meridian (000°) extending through Greenwich, England. longshore current A current along a shoreline, caused by waves approaching the bottom contours of the coast at an angle. long splice A method of permanently joining the ends of two with- out increasing the diameter.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard loom Flexible insulating wrap for enclosing wiring. looming An optical effect due to a marked increase in air density near the surface, causing objects or lights that are actually below the horizon to appear above it. loose-footed A sail that is attached to the boom only at the tack and clew. loran An electronic navigation system for establishing lines of posi- tion by using the time difference between the reception of signals from different terrestrial locations. low An area of comparatively low barometric pressure with winds spiraling inward in a counter-clockwise direction in the North- ern Hemisphere and a clockwise direction in the Southern Hemisphere. A cyclone. lower branch (m) The half of the observer’s meridian, measured from pole to pole, passing through the nadir of the observer, i.e., the point vertically below on the opposite side of the celestial sphere. lower limb (LL) The lower part of the circumference of the sun or moon. lubber’s line A compass reference oriented parallel to the fore-and-aft keel line of the vessel.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard luff 1. To alter course towards the wind; to head so close to the wind that the sails flutter. 2. The forward edge of a fore-and-aft sail. luffing (to luff or luff up) Head into the winds causing sails to flutter. luff wire A piece of wire rope sewn into the leading edge of a jib. lunch hook A light anchor used for short stops in settled conditions. LWL Load waterline; the length of a vessel from bow to stern at the waterline. See waterline length. M mackerel sky A sky covered with cirrocumulus clouds with shapes that re- semble the scales of a mackerel. maelstrom An extremely violent storm, usually a hurricane. Also, a whirl- pool. Mafor code Weather forecasts on the Great Lakes and St. Lawrence Sea- way are broadcast in Mafor code. A special chart is needed to decode the forecasts. magnetic flux Magnetic lines of force.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard magnetic field The flow of a magnetic force, or magnetism between the op- posite poles of a magnet. magnetic meridian An irregular curve on the earth’s surface, passing through the magnetic poles and representing the direction of the magnetic field at that location. magneto An electrical device that generates current when rotated by an outside source of power. magnitude Relative brightness of a celestial body; the smaller the num- ber indicating magnitude, the brighter the body. main bearing A bearing within which a crankshaft rotates and which sup- ports the crankshaft within an engine block. mainsail Boat’s principal sail, set aft of mainmast. (Pronounced “mains’el.”) mainsheet A line that controls the mainsail. mammatus Clouds with hanging protuberances, like pouches, on their lower surface. Typical of turbulent, tornado-forming conditions. Marconi rig A rig having a three-sided or jib-headed mainsail.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard mares’ tails Long, well-defined wisps of cirrus clouds resembling a horse’s tail. marina A facility in which recreational boats may be berthed. maritime air mass An air mass that originated over water. mark Any object required by sailing instructions to be passed by a boat on a specific side. marline Two-stranded twine used for whipping and seizing. marlinespike A pointed steel tool used in splicing to open the strands of a rope or cable. See fid. marlinespike seamanship General knowledge of knots, bends, hitches, splices, and care of the lines (rope). mast 1. A vertical spar on a sailboat, on which sails may be set. 2. A vertical spar on a powerboat, from which may be flown a steadying sail or flags and burgees, and which can be used with a boom to lift gear. master station The station of a loran chain from which the first group of eight pulses plus a ninth signalling pulse are transmitted.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard master switch A switch that disconnects a battery or generator from the elec- trical system. masthead The top of the mast. masthead rig A sailboat with the jibstay attached to the top of the mast. mast hoops Traditional wood rings used to secure a gaff sail to the mast; seized with marline to grommets in the sail. mast step Fitting into which the butt of the mast sits. mat Fiberglass material formed of irregular chopped filaments of glass. maximum draft The greatest amount of fullness in a sail. Mayday The term used to signify an urgent distress communication under international procedures; the highest pri- ority transmission, indicating there is immediate danger to a vessel or to someone on board. mean sun An imaginary sun traveling westward around the celestial equator at a uniform rate equal to the average rate of the true sun.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard mean time Time based on the rotation of the earth relative to the mean sun. med mooring To secure a boat with an anchor at the bow and the stern at a dock. mega (M) Metric prefix for one million (106). mercurial barometer An instrument used to measure barometric pressure. Consists of an evacuated tube, closed at one end, about 36 inches long. The open end is inverted into a bowl of mercury; atmospheric pressure forces the mercury up the tube. The distance the mercury is raised is the barometric pressure in inches of mer- cury. meridian A great circle of the earth passing through both poles in the direction of true north or true south. meridian angle (t) The angular distance, measured east or west (whichever dis- tance is shorter) through 180°, between the upper branch of the local celestial meridian and the hour circle of a celestial body. Labeled E or W. meridian transit The passage of a celestial body across a celestial meridian, usually that of the observer.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard meridional difference (m) The difference in meridional parts between two given lati- tudes, expressed in minutes of arc. meridional parts (M) The length of the arc of a meridian between the equator and a given parallel of latitude on a Mercator chart, expressed in units of one minute of longitude at the equator. mesopause The interface or boundary layer between the mesosphere and the thermosphere, approximately 55 miles high. mesosphere The atmospheric region from approximately 30 to 55 miles high. Air temperature normally rises with altitude in this re- gion. messenger A light line used to haul a heavier working line. meteorology Study of atmospheric phenomena; particularly, the study of changing conditions in the lower atmosphere. meter 1. An analog dial or digital type of instrument for measuring voltage, current, pressure, etc. 2. A metric linear measure approximately equal to 39.37 inches or 3.28 feet. micro(µ) (Greek letter mu) Metric prefix meaning one one-millionth.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard microburst A strong downdraft that induces an outburst of damaging winds on or near the surface of the earth. The outflow at the sur- face is normally less than 4 kilometers across, with peak winds lasting only two to five minutes. micro processor The control and processing portion of a containing large numbers of transistors, integrated circuits, diodes, and other components. mid-latitude The middle latitude; half the sum of the latitudes of two places on the same side of the equator. milli (m) Metric prefix meaning one one-thousandth. millibar (mb) A unit of barometric pressure measurement; atmospheric pres- sure usually ranges between 950.0 and 1,050.0 millibars re- duced to sea level. 1 millibar = 0.0295299 inch of mercury 1 inch of mercury =33.86389 mb millimeter (mm) A linear unit of barometric pressure measurement (height of the mercury column) usually ranging between 700 and 800 millimeters reduced to sea level. 1 millimeter = 0.039370 inch 1 millimeter mercury = 1.33322387 millibars

Return to Keyboard USPS® Nautical Glossary Return to Keyboard mirage An optical effect due to refraction in the atmosphere, causing the image of an object to be displaced from its true position. mist Name for drizzle in North America. In Great Britain and some other countries, light fog. mizzen The fore-and-aft sail set on the mizzenmast. Also called a jigger. mizzenmast The mast immediately aft of the mainmast, sometimes called a jigger. mizzen A triangular sail set from the mizzen masthead to the deck forward of that mast. modification A change in the character of an air mass or a portion of the mass. This usually occurs when the air mass stalls over a sur- face with different temperature and humidity characteristics from those of the mass. When the underlying surface causes change, the air mass is said to be modified. modulation The impression of intelligence on a signal or . The unique combination of atoms that constitutes the basic chemical building block of a substance.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard moment A force acting through a distance, tending to cause rotation. monel metal Corrosion resistant alloy of nickel, copper, iron, and manga- nese. Sometimes used for propellers and shafts. monsoon A seasonal wind flowing between ocean and continent, caused mainly by the temperature disparity between them. When the seasons change, the disparity reverses, as does the wind di- rection. mooring Permanent ground tackle to which a boat is attached or moored. A place where vessels are kept at anchor. motor A rotating machine that converts electrical or combustible energy into mechanical energy. mountain wind Air flowing down the side of a mountain owing to nighttime radiational cooling of the air next to the slope. mushroom anchor An anchor shaped like an inverted mushroom, often used to anchor permanent structures in soft sea beds. It tends to sink deeply into the bottom if left for long periods.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard N nadir (Na) The point on the celestial sphere directly below the observer, or 180° from the zenith. nano (n) Metric prefix meaning one one-billionth. Napier diagram A graphic plot of compass deviation values. naturally aspirated Refers to an engine that draws in air solely by the action of its pistons, without the help of a supercharger or turbocharger. Nautical Almanac (N.A.) A yearly publication tabulating the position of celestial bodies and other astronomical data useful to marine navigators. nautical mile (nm) A unit of distance equal to 1 minute of latitude and equal to approximately 6,076.1 feet or 1.15 statute miles. nautical twilight (NT) The period of incomplete darkness that ends in the evening and begins in the morning when the center of the sun is 12° below the celestial horizon. navigable semicircle In a tropical cyclone in the Northern Hemisphere, the navi- gable semicircle is the side to the left of the storm track as you face the way the storm is moving. Wind in this semicircle is diminished by the velocity of the storm track. In the Southern Hemisphere, the right side is known as the navigable semicircle.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard navigation lights Lights shown by a vessel that indicate course, position, and status, such as fishing, towing, or at anchor. Navigation Rules The Rules of the Road of the United States, governing navi- gation lights, rules for vessels meeting or passing, sound sig- nals, and distress signals. navigational triangle The spherical triangle solved in computing altitude and azi- muth and great circle problems. needle bearing An anti-friction bearing using a great number of rollers of small diameter in relation to their length. needle valve A type of valve in which a pointed shaft (the needle) moves to control the flow of fluid; used in injector nozzles and carbure- tors. negative Polarity of a cathode. Electrode from which electrons flow to the anode. negative pole Point from which an electrical current flows as it passes through a circuit; designated by a minus sign (–).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard neutral A conductor that is grounded at the marina service and car- ries return current from the appliance served by the energized conductor. Must be white or gray in color, or both ends can be taped white. Newton’s laws of motion 1) If the resultant force acting on a body is zero, it will con- tinue to do whatever it is doing: (a) If it is at rest, it will remain at rest; (b) If it is in motion, it will continue to move in a straight line with constant velocity. 2) If the resultant force on a body is not zero, it will have an acceleration proportional to and in the direction of the resultant force. 3) For every action there is an equal and opposite reaction. That is, if body A exerts a force on body B, then body B exerts an equal and opposite force on body A. nicopress fitting A sleeve that is crimped onto wire rope to form a connection. nicopress tool A tool used to compress cable fittings. nimbostratus Classified as a middle cloud but also occupies lower levels. Formed by the downward thickening of altostratus. Charac- terized by its grey color and no well-defined base. Normally associated with steady rain. NOAA National Oceanic and Atmospheric Administration.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard noble metal A metal that is passive in the galvanic series and, therefore, is resistant to corrosion. Gold, platinum, etc. noise Unwanted disturbances, superimposed upon a useful signal that tend to obscure its information content. North Atlantic current A continuation of the Gulf Stream as a broad, slow-moving drift of comparatively warm water toward the British Isles and the Iberian Peninsula. norther A wind in the southern United States, especially in Texas and the Gulf of Mexico, occurring from November through April. This strong, cold, northerly wind frequently causes a rapid drop in temperature. Northill® anchor A lightweight anchor similar to the Danforth® anchor, but with the flukes opposite each other as on a kedge anchor. NOS National Ocean Service. Notice to Mariners A weekly newsletter, published by the United States Coast Guard, advising of changes or discrepancies relating to navi- gation. no wake zone An area where boats are prohibited from causing a wake.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard O oblate spheroid A sphere flattened or depressed at the poles—such as the earth. observed altitude (Ho) Sextant altitude after all corrections have been applied. occluded front A zone in which an advancing cold air mass overtakes a re- ceding cold air mass and, in the process, forces the warm air previously separating them up above the surface. Two types: 1) Cold-front occlusion, in which the advancing cold air mass is colder than the receding cold air mass. 2) Warm-front oc- clusion, in which the advancing cold air mass is warmer than the receding cold air mass. occlusion Same as occluded front. octane number A unit of measurement on a scale intended to indicate ten- dency of a fuel to detonate or knock in a gasoline engine. offing A position on the sea at a safe distance from land. offshore A direction away from the shore, as offshore wind. off the wind Sailing on a reach or run.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard ohm (Ω) (Greek letter omega) The unit of electrical resistance. ohmmeter Instrument used to measure resistance. Ohm’s law Basic law for circuit analysis: E = IR; I = E ÷ R; R = E ÷ I. oil pumping A term used to describe an engine that is using an excessive amount of lubricating oil. oil-skins Foul weather gear. omnidirectional In all directions. Omnirange Navigation system in general use for aircraft. onshore Towards the shore, from seaward, as onshore wind. on the wind Close-hauled. open circuit A break or opening in an electrical circuit that stops passage of a current.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard optical haze A restriction to visibility caused by refraction of light while passing through layers of air with differing densities. The re- sult is a haze-like distortion. Atmospheric particulates are not involved in optical haze. orographic lifting Moving air mass forced upwards by the presence of moun- tains. This rising air cools and saturates, often causing rainfall upwind of the mountains. This now drier air warms as it de- scends the downwind side, often creating desert conditions in such areas. oscillator Circuit used to produce an alternating signal of desired shape and frequency. oscilloscope An electrical measuring instrument that can display the ampli- tude of rapidly varying signals as a function of time on a cath- ode ray tube similar to a tv picture tube. outboard Toward the outside of a boat. An engine with propeller attached, designed to be fastened to the transom of a boat. Device or line used to tension the foot of a sail.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard out of phase Two or more wave forms that have the same frequency but do not pass through corresponding values at the same time. overboard Off the boat and in the water. ozone layer a rarefied layer or layers of triatomic oxygen, or ozone, in the upper stratosphere. This is an unstable form of oxygen cre- ated and sustained by ultraviolet radiation from the sun. The ozone layer shields the earth from a large part of this damag- ing radiation. P Line tied to the bow of a dinghy to tow or secure it. palm 1. See fluke. 2. Leather glove-like device used as a thimble to push a needle through a line or a sail. Pan-Pan The term used to signify an urgent communication under in- ternational radiotelephone procedures; the second highest pri- ority transmission, indicating that the safety of the vessel or someone on board is involved, but the need for assistance is not immediate. (Pronounced “pahn-pahn”) parabolic path The curve followed by a stone thrown into the air; found in many other applications as well.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard parallactic angle (X) The angle of the navigational triangle at the celestial body— i.e., the angle between the hour circle and the vertical circle. parallax (P) The difference in the apparent direction of a celestial body when viewed from a point on the earth’s surface as compared to viewing it from the center of the earth. parallel circuit A circuit that has two or more branches or paths for current flow. passive device A device that does not radiate any electrical signals but uses external signals to perform its function. passive metal A metal that does not react readily in an electrolyte. pay off To head away from the wind. pay out Slacken or let out a line gradually. peak 1. The corner of a gaff-headed sail formed by the head and the leech. 2. The aft end of a gaff to which the peak of a gaff sail is set. peak envelope power (PEP) Power of an alternating current at the maximum amplitude of its cycle.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A snap often used for lifeline gates and to attach or remove an inner forestay. pendant Short rope serving as an extension of a line, chain, or cable with descriptive name based on use, e. g., mooring pendant. (Usually pronounced “pen’ant.”) pennant A tapering flag. Compare burgee. personal flotation device (PFD) A buoyant coat, suit, vest, ring, horseshoe buoy, or cushion. personal watercraft (PWC) A Class-A boat using an inboard motor to drive a water-jet pump for motive power, designed to be operated by a person sitting, standing, or kneeling on the vessel. phase One of the stages in the appearance of the moon during its cycle. phase or phase angle Number of electrical degrees by which one wave form leads or lags another. pico (p) Metric prefix meaning one-trillionth. pier Structure extending into the water from shoreline to provide dockage.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard pig-tail adapter A short electrical cord, used to connect to shore power. pile A pole or post driven vertically into the bottom, usually to sup- port a pier or float or to moor a boat. piling A structure of piles often used to protect wharves and piers. Pilot Chart A planning chart of a major ocean area published by the De- fense Mapping Agency. It includes as much information on weather, seas, and winds as can be put on a single sheet of paper. piloting A means of navigating using reference points that may be seen or determined from information on charts—such as depths, heights, or ranges. pilot ladder A ladder with steps joined with ropes, lowered from a ship’s rail. pin 1. A metal peg used to fasten rigging together, usually se- cured with a cotter or circular pin. 2. A floating flag on a pole used as a racing mark, particularly to mark the end of the start-finish line. To sail too close to the wind.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard pin end The end of the sailboat race starting or finishing line, marked by a float with flag (pin) or other suitable mark, opposite the race committee (boat). pintle See gudgeon and pintle. pintle nozzle An injector nozzle with one central hole; generally used in engines with pre-combustion chambers. pip Image of a reflected signal on a radar screen. piston A pumping device used to generate pressure in a cylinder. piston collapse An abnormal reduction in diameter of the piston skirt due to heat or stress. piston crown The top of a piston above the rings. piston displacement The volume of air moved or displaced by moving the piston from one end of its stroke to the other. piston hanks A hank with a spring-loaded plunger sewn to the luff of a sail and used to attach sail to a stay. piston lands Those parts of a piston between the piston rings.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard piston pin The part that connects the piston to the small end of an engine connecting rod; also known as wrist pin. piston ring Spring-tensioned ring set in a groove in the circumference of a piston; this ring pushes out against the cylinder wall to make a gas-tight seal. piston ring gap Clearance between ends of piston rings. piston ring groove The slot in the circumference of a piston into which a piston ring fits. piston skirt Part of piston below the rings and bosses. piston slap Rocking of a loose-fitting piston in a cylinder, making a hol- low, bell-like sound. pitching Oscillation around the transverse axis; the rising and falling of the bow in waves. pitchpole Capsize end-over-end. pivot point The theoretical fulcrum of a vessel around which it revolves in making a turn.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard plan position indicator (PPI) Radar display on a cathode ray tube as the terrain would ap- pear if viewed from above. plane sailing Method of computing courses, distances, and positions by treat- ing the earth as flat and applying plane trigonometry. See sailings. planing hull A hull designed to climb toward the surface of the water as sufficient power is applied and to skim along the water at a greatly reduced-displacement attitude. planking Boards used for covering the bottom, sides, and deck of a vessel. plow anchor An anchor shaped like a farmer’s plow. plug A piece of wood fitted into a hole in a vessel, usually below the waterline. point 1. To sail effectively close to the wind. 2. Any of the 32 equal divisions, at 11°15’ intervals, marked at the circumference of a mariner’s compass card that indicate direction.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard polar air mass (truly subpolar) An air mass having its origin over the colder, more northern portions of North America, Eurasia, and the oceans. Differ- entiated from Arctic air mass, which forms over the Arctic Ocean and ice-covered polar regions. polar distance (p) The angular distance from the observer’s elevated pole to the parallel of declination of a body. See co-declination. polar easterlies Circumferential bands of surface air flowing from the polar highs toward the subpolar lows. These winds are deflected westward (making them easterly) by the Coriolis effect. The polar easterlies occur in both hemispheres between the poles and roughly 60° latitude. polar front The southern limit of the polar air mass. This boundary, which fluctuates widely with the jet stream, is the turbulent interface between the warm prevailing westerlies and the polar easterlies. The polar front is a spawning ground of extratropi- cal cyclones. polarity indicator A device for determining the polarity of an electrical terminal such as the “hot” side of an AC power outlet. polarity of voltage A method of describing a voltage as either positive or nega- tive with respect to some reference point such as the earth.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard poppet valve A valve structure consisting of a circular head with an elon- gated stem attached in the center; designed to open and close a circular hole or port. port 1. Left side of a boat when facing the bow. 2. Toward the boat’s left. 3. Opening in a boat’s side (e. g., portlight). 4. Harbor. 5. In engines, the openings in the cylinder head or block for valves, exhaust and inlet pipes, or water connec- tions; in two-stroke cycle engines, openings for inlet and ex- haust purposes. port of entry A port where a vessel may legally enter a country. port tack Any heading where the wind is on the port side and the mainsail is carried on the starboard side of the boat. potential difference (E) Difference in voltage (V) or electromotive force (EMF) be- tween two points. potentiometer A resistor placed across a source of voltage and having a vari- able tap to allow intermediate values of voltage to be taken off. power (P) The rate of doing work. power-driven vessel Any vessel propelled by machinery, including sailing vessels under both sail and power.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard Power Law P = E I = I2 R = E2 ÷ R Where P = power in watts E = voltage in volts I = current in amperes R = resistance in ohms pram Flat-bottomed dinghy with blunt bow. precipitation Forms of water particles, solid or liquid, that fall from the at- mosphere and reach the ground: drizzle, rain, snow, sleet, or hail. precursory cirrus Forerunner clouds of a tropical cyclone. These clouds occur at a high level and may be visible 400 to 500 miles in advance of the eye of the hurricane. preignition Ignition occurring earlier than intended. pressure Force per unit area. In weather, the weight of the air, often given in millibars, or inches of mercury. See millibar, inches of mercury. pressure gradient A change in atmospheric pressure per unit of horizontal distance. pressure tendency The change in atmospheric pressure within a given time, usu- ally three hours.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard prevailing westerlies A zone of winds flowing from the high pressure of the horse latitudes to the low pressure of the subpolar region. These winds are deflected by the Coriolis effect (to the right, in the Northern Hemisphere) so that they become westerly. A line run forward from the boom to a deck fitting to prevent accidental jibes. primary cell A single galvanic cell producing a flow of current by chemical reaction. The reaction is not reversible. It cannot be recharged. primary winding A wire that conducts low-tension current and will create, by induction, a high-tension current in a secondary winding of an ignition coil or magneto armature. primary wires Wiring circuit used for conducting low tension or primary cur- rent to points where it is to be used. prime vertical (PV) The vertical circle that passes through the observer’s zenith and the east and west points of the horizon. prognostic chart A weather map projected into the future, showing pressure systems, fronts, and other features as they are expected to be at a given time. prony brake A machine for testing power of an engine while running against a friction brake. Return to Keyboard USPS® Nautical Glossary Return to Keyboard propeller A multi-bladed, rotating wheel that furnishes propulsion. propeller shaft A rod that transmits power from engine and transmission to propeller. proton A particle that is the nucleus of the hydrogen atom and is present in the nucleus of other atoms. It has a positive charge equal and opposite to the charge of an electron. pseudo-cirrus Thin, wispy, cirrus-like clouds blown from the anvil top of cu- mulonimbus clouds by upper winds. The pseudo-cirrus clouds trail off the anvil top in the direction of movement of the cell. psychrometer An instrument for measuring humidity. Consists of two identi- cal thermometers, one with wet muslin around its bulb to al- low cooling by evaporation, the other with a dry, uncovered bulb. The instrument is ventilated by fan, or by manually whirl- ing it (sling psychrometer). The difference in the readings of the two thermometers, called the wet-bulb depression, is used in calculating dew point and relative humidity. puff A short gust of wind. pulpit A guardrail around the bow or stern.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard pulse A momentary increase in the amplitude of a voltage or cur- rent. purging The process of removing air from a fluid system, such as purg- ing air from the fuel system of a diesel engine. push-to-talk A spring-loaded switch used to energize a radio ; in the released position, the receiver is energized. pushrod A connecting link in an operating mechanism. Example: rod interposed between valve lifter and rocker arm on an over- head valve engine. pyrometer A gauge that measures high temperatures; often used to mea- sure the temperature of engine exhaust gases. Q quarter 1. After part of a boat’s side, e.g., port quarter. 2. Direction 45° abaft the beam.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard R racing The technical status of a boat entered in a race from the time of her preparatory signal until she has finished and cleared the finish line and marks, or has retired, or the race committee has signalled a general recall, postponement, or abandonment of the race. racon A radar set of known position, usually shore-based. When ac- tivated by a ship’s radar signal, it will transmit an identifiable signal that appears on the ship’s receiver screen. radar An electronic navigation system using reflected radio pulse signals to determine the relative bearing and distance of an object from the vessel. radar reflector An arrangement of three metallic surfaces exactly at 90° angles to each other. Acts as a passive reflector to radar sig- nals. cut Typical shape of spinnakers (radial head or tri-radial) where tapered, parallel panels are sewn together to form a convex shape toward the head or all three corners. radiate To emit energy.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard radiation The transfer of energy by electromagnetic wave motion. No medium is necessary for the process. Energy from the sun is transmitted to the earth by this method. radiation fog Fog formed by the radiational cooling of the earth at night and the resultant conductive cooling to dew point of the adjacent layer of air. radio beacon (Rbn) An electronic aid to navigation of known position emitting coded radio signals for use by shipboard radio direction finders. radio direction finder (RDF) A equipped with a compass rose and a loop or other type of directional antenna to determine the direction of the source of a received signal. radiofax Transforms weather broadcasts into readable weather charts. Also known as Weatherfax. radiometer An electronic instrument used on a satellite to detect radia- tion. radiosonde A device consisting of a radio transmitter and sensors, carried aloft by a balloon, to measure pressure, temperature, and hu- midity. Information from the sensors, converted to radio sig- nals, gives the receiving station a profile of the pressure, temperature, and humidity aloft.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard raft A floating platform. rafting up Tying a group of boats together for a social gathering, using only a few anchors. rail A boat’s side above the deck line. rainbow An arc in the sky featuring a spectrum of color caused by the refraction, , and dispersion of the sun’s rays in rain- drops. rain catcher Canvas used to catch rain to augment a vessel’s water supplies. rain gauge An instrument for measuring the amount of rainfall. rake The slope from the vertical, in the fore-and-aft direction, of a mast, transom, or deckhouse. range 1. Two or more objects in line. 2. Maximum distance at which an object may be seen. 3. Distance between any two points, such as a radar range to a navigational aid. 4. Difference in elevation (height) between any successive pair of high and low tides. rating rule A mathematical formula to establish handicaps for racing boats of diverse size and design.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard ratings Manufacturer’s specifications regarding electrical and other characteristics of equipment. raydist An electronic system for hydrographic surveying that is char- acterized by short distance and high accuracy. RDF See radio direction finder. reach 1. A point of sail between close-hauled and a run. 2. A dis- tance, or fetch. reacher A light-weight jib used for reaching. reactance Opposition to the flow of an electric current due to inductance or capacitance. ready about The helmsman’s warning that he intends to tack. reciprocal In navigation, a bearing or direction 180° from another bear- ing or direction. rectifier An electrical device for changing AC current to DC current. recurve The generally parabolic path of the center of a tropical cy- clone, as it veers through west to northwest and north to north- east, is known as recurvature. Return to Keyboard USPS® Nautical Glossary Return to Keyboard reed valve A flat, springy valve covering ports between carburetor and crank chamber in a two-stroke cycle engine. reef 1. To reduce the area of the sail. 2. A part of a sail that is taken in or let out to regulate sail area. 3. An underwater barrier, such as rock or coral. reef knot Same as a square knot. reef points Short pieces of line in a sail, placed in a line parallel to the boom and used to secure unused material when the sail is reefed. reeve To pass a line through a block. reflection Generally obtained by a highly polished mirror surface, but any surface of any medium may return a portion of incident light at an angle of reflection equal to the angle of incidence. The remainder of the light may be refracted or absorbed within the medium. refraction (R) An optical phenomenon in which light is bent at an angle upon undergoing a change in speed, as when passing from one me- dium to another. In effect, light is slowed upon entering a denser medium and refracted or bent toward the normal. An example is the focusing of light rays by a camera lens.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard relative bearing Direction of an object relative to a boat’s heading, measured clockwise from dead ahead. relative humidity The ratio of the amount of moisture in the air to the amount that would be present at the same temperature and pressure if the air were saturated, expressed as a percentage. relay A switching device activated by an electromagnetic coil or solenoid. reserve power The number of minutes a battery can deliver 25 amperes ef- fectively to a load, at 80°F with no charging current applied. resistance (R) The opposition an appliance or wire offers to the flow of elec- tric current, measured in ohms. A condition that exists when an electrical circuit or mechani- cal device oscillates at its natural frequency. resultant force The vector sum of two or more forces. retard To cause a spark to occur at a later time in the cycle of engine operation. Opposite of spark advance. rheostat A continuously variable electrical resistor, used to regulate current.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard rhumb line A straight line on a Mercator chart intersecting all meridians at the same angle. rib Landsman’s term for frame. ridge of high pressure Elongated region of high pressure. rig 1. To prepare a boat for sailing. 2. Arrangement of spars and sails, e.g., as on a sloop. rigging All the lines and gear used to support the spars and control the sails. right ascension (RA) An angular measurement used by astronomers to locate stars, measured eastward from the first point of Aries, from 0° through 360° (24 hours). righting arm Horizontal distance between the vertical line through the cen- ter of buoyancy and the vertical line through the center of gravity of the vessel. righting moment The righting arm multiplied by the vessel’s displacement.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard rime A white, granular deposit of ice created by the impingement of small, supercooled drizzle or fog droplets onto exposed sur- faces. Associated with small droplet size such as is found in stratiform precipitation. ring buoy A ring-shaped life preserver, generally to be thrown to some- one in the water. roach The part of a sail that extends beyond a straight line from the head to the clew. rocker arm In engines, a lever located on a fulcrum or shaft, one end bearing on the valve stem, the other end on a pushrod. rode Anchor line or chain. rod rigging Standing rigging made of connected rods rather than wire to reduce stretch; often used in racing boats. roll cloud Part of the cloud base along the leading edge of a cumulonim- bus cloud. Formed by the rolling action in the wind-shear re- gion between the cool downdrafts within the cloud and the warm updrafts outside the cloud. roller furling Reducing the area of a jib by rolling it around its luff wire.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard roller reefing Reducing the area of a sail by rolling it around the boom. rolling Oscillation of a boat around a longitudinal axis. rope Cordage made of natural or synthetic fibers; can be made of steel wire. rotor The rotating part of an electrical or mechanical device, such as motor, generator, or distributor rotor. round turn A complete turn of line around a cleat, bitt, or post. RPM Revolutions per minute, usually of an engine. rudder A flat board, blade, or plate hung on the aft end of a boat and used to steer the boat. rudder post The shaft to which a rudder blade is attached. The tiller or other steering apparatus is affixed to the other end. run (or running) Point of sailing with the wind astern. Stays that counteract forces from an inner forestay; must be tended as the boat tacks or jibes.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard running fix (R Fix) Position derived by the crossing of lines of position that have been observed 20 minutes or more apart and one of which has been advanced or retired to a common time. The lines used to raise, set, and trim the sails. S sacrificial anode A galvanically active base metal, such as zinc, placed in elec- trical contact with a more noble metal, such as steel, to pre- vent corrosion or loss of the nobler metal due to galvanic action. SAE Society of Automotive Engineers; also a term used to describe certain nuts, bolts, and wrenches, or other parts meeting the standards of the SAE. safety ground A terminal or conductor at the potential of the immediate local earth or water. It is not used to carry current for the operation of equipment. sailing off An intentional change of course away from the wind. sailings Mathematical solutions of navigational problems involving course, distance, difference in latitude, difference in longitude, and departure.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sailing vessel Any vessel under sail, provided the propelling machinery, if any, is not being used. sail slides Metal or plastic fasteners that are attached to the edge of a sail and ride on a track on the mast or boom. sail slugs Small metal or plastic cylinders that are sewn to the edge of a sail and fit into a spar cove to attach the sail to the spar. sail twist Tendency for the top of a fore-and-aft sail to sag farther to leeward than the foot of the sail. salinity A measure of the percentage of salts dissolved in water. Sea water has about 3.5% salinity in the open ocean. samson post A bitt in the bow of a boat. It often projects downward to the keel and is secured to it for greater strength. Santa Ana wind A foehn-type wind that drains air from the western high desert areas to the coastal regions of southern California. saturation In weather terms, air that contains all the water vapor it can hold is saturated. In the absence of condensation nuclei, air can hold much more water vapor and is said to be supersatu- rated.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard saturation temperature See dew point. scattering A process in which light rays are diffused in their movement by tiny particles, including air , much smaller than the wavelength of light. scavenging The process of replacing the spent gases of combustion with fresh air in a two-stroke cycle engine. schooner A vessel with two or more masts rigged fore-and-aft, with the forward mast being shorter or equal in height to the aft mast. scope Ratio of the length of the payed out anchor line (rode) to the height of the chock above the bottom of the body of water. screw In nautical usage, an alternate name for a propeller. SCUBA Self-Contained, Underwater-Breathing Apparatus; a type of diving gear. scud 1. Thin, low, fast-moving clouds. 2. To run off before a strong wind. scull To propel a boat with a single oar at the stern or with the rudder.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard scupper An opening in the rail or bulwark to permit water to drain overboard. sea A system of wind waves that are still being blown by the wind force that created them. Usually irregular and choppy, unlike swell. sea anchor A parachute-like device deployed off the bow to hold a boat head-to-wind with minimum sternway; often used in heavy weather to keep the bow into the wind and seas. Compare drogue. sea breeze An onshore breeze; one that blows from over the water to- ward the shore. Caused by the more rapid heating of the land in the daytime, with resulting rising air currents over the land and an inflow of air from the sea to replace it. seacock A valve to close through-hull openings below the waterline. seakindly Describes a boat with easy motion. sea-level pressure Local barometric pressure corrected by adding or subtracting an amount to adjust the local pressure reading to what it would be at sea level. seams The spaces between planks in a vessel’s deck or sides.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sea return (or sea clutter) Return or echo of a radar signal from the water surface close to the radar station. Waves cause interference and loss of resolution near the center of the screen. sea room Navigable water sufficient for safe maneuvering. sea smoke See steam fog. seaworthy Said of a boat that is in fit condition to put to sea. secant A straight line intersecting a curve at two or more points. secondary cell An electrolytic cell in which the chemical process of charging and discharging is reversible. A component of a storage bat- tery. secondary low A low that forms along either the cold front or the warm front of a mature -tropical cyclone. The secondary low forms at some distance from the central low. secondary stations Two to four stations in the loran chain that transmit in se- quence at precisely predetermined intervals after the master station’s transmission.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard secondary winding In an ignition coil or magneto armature, a wire in which a secondary or high tension current is created by induction due to the interruption of the current in the adjacent primary wind- ing. secure To cover openings and make movable objects fast. Security The term used to signify a message concerning the safety of navigation under international radiotelephone procedures; the third priority transmission. (Pronounced “sea-cur-i-tay.”) seiche A rapid change in the water level of a landlocked body of water, usually associated with a rapid increase in the velocity of wind or a change in its direction, and a rapid change in barometric pressure. Water may oscillate following the initial rise or fall. seize 1. To bind by many wrappings of small line. 2. To secure the pin of a shackle using non-corrosive wire. seizing Small stuff for binding. seizure The process by which excessive friction brings an engine to a halt (may be caused by lack of lubrication).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard selectivity Ability of a receiver to reject unwanted signals, designated by band width. self-destructing metals Alloys, such as brass, in which one or more of the constituent metals is destroyed by galvanic action when immersed in an electrolyte. self-tending A sail that will change position by itself during a tack or jibe. semidiameter One half the diameter of an observed celestial body, mea- sured in minutes of arc. semi-diesel A semi-diesel engine operates on a comparatively high com- pression ratio and utilizes fuel similar to diesel engines; how- ever, it does use an electrical ignition system rather than depend solely upon heat generated by compression to furnish ignition. semi-displacement hull A boat hull that performs effectively at low speeds as a dis- placement hull and at high speeds as a planing hull. sensible horizon The circle formed on the celestial sphere by the intersection of a plane through the eye of the observer, perpendicular to the zenith-nadir line.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sensitivity A receiver’s ability to receive weak signals. Measured by the amount of signal in microvolts required at the antenna termi- nals to provide a designated output from the receiver. sentinel A weight suspended from the rode to help keep the pull on the anchor as horizontal as possible to prevent dragging. separation Detachment of air in a smooth laminar flow from the leeward side of a sail. See also stall. series circuit A circuit with only one path through which current can flow. set 1. True direction toward which a current flows. 2. True di- rection of movement of a boat due to wind and current. sextant A precision hand-held instrument used to measure angles to a high degree of accuracy, e.g., altitude angles of celestial bod- ies, heights of objects, and horizontal angles between objects. sextant altitude (hs) The angle indicated by a sextant before corrections are ap- plied. shackle A U-shaped metal device with removable pin, used to connect a sail, chain, line, or fitting.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard shaft A cylindrical rod. A rotating shaft is used to transmit power from the engine to the propeller. shaft horsepower (SHP) The actual power output of an engine and power train mea- sured at the propeller shaft. sheave The roller over which a line passes as it goes through a block. (Pronounced “shiv.”) sheer The fore-and-aft vertical curvature of the deck. sheer strake The topmost plank on the sides of a boat, just under the gun- wale. sheet Line used to adjust a sail relative to the wind, e.g., a jibsheet is used to adjust a jib. sheet bend A knot used to join two ropes, usually of unequal diameter. sheet stopper A clamp-type device that contains a cam for securing a sheet or line. It can be released quickly. shim A specially cut piece of shim stock used as a spacer in spe- cific applications, generally engine alignment.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard shim stock Very thin, accurately machined pieces of metal. ship’s papers A boat’s registration papers, and often radio licenses, cus- toms papers, and other documents needed for international travel. shoal A localized shallow spot, often built up of sand or other loose material. shoaling waters Shallow water over a sandbar or rocks. short circuit A path with little or no resistance connected across a power source. short splice A method of permanently joining the ends of two ropes. shroud Standing rigging that supports a mast laterally. shroud roller A tube installed on the shrouds and designed to turn freely in order to minimize chafing of the sheets and sails. shunt 1. May be used interchangeably with “parallel” when speak- ing of circuits. 2. A low resistance placed across meter ter- minals to limit current flow through the instrument.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sidelobes Side echoes detected by a radar set, resulting in multiple tar- gets. sidereal hour angle (SHA) The hour angle of a body, measured westward from the first point of Aries. sidereal time Time based on the rotation of the earth relative to the first point of Aries. May be designated as Greenwich or local, ac- cording to the reference meridian. sight A sextant observation of altitude of a celestial body. sight reduction Process of deriving from a sight, along with data from the Nautical Almanac, the information needed to establish a line of position (LOP). An extension of the keel, or a keel-like projection at the aft end of the hull, for protection of propeller and rudder. Tendency of radio frequency currents to travel along the sur- face of a conductor rather than be distributed equally through- out its cross section. skipper Captain, or often the boat owner. sky wave Radio wave refracted back to earth from the ionosphere.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard slack 1. Not carrying any load, loose. 2. To ease or pay out a line. slack water Minimum velocity of a tidal current (sometimes abbreviated “slack”). sleet Generally transparent, globular, solid grains of ice formed from the freezing of raindrops (North American definition). In Great Britain, a mixture of rain and snow. sling psychrometer An instrument consisting of a wet-bulb thermometer and a dry-bulb thermometer mounted on a frame that is connected to a handle through a swivel so that the instrument may be spun to fan the wet-bulb thermometer. Accurate wet-bulb and dry-bulb readings are then taken to calculate relative humidity and dew point. slip 1. The difference between the theoretical movement of a pro- peller through the water and the actual movement. 2. A berth for a boat between two piers or floats. slip rings Continuous contact rings used for the transfer of current, through brushes, to the rotor coils of an alternator. sloop Single-masted sailboat with the mast stepped less than 40% of boat length aft of the bow and setting a mainsail and, nor- mally, one triangular headsail.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard slot effect The effect on air flow between two sails, caused by the mu- tual interaction of the sails. small stuff Light line, marline, waxed twine, etc., used for whipping, seiz- ing, and serving. smog Fog contaminated by industrial pollutants; a mixture of smoke and fog. snap shackle A shackle with a retractable spring-loaded pin. snatch block A block with a hinged hook that can be opened to receive or release a line. snow Precipitation in the form of white or translucent ice crystals often clustered in large snowflakes, formed by sublimation. snub To stop a line running out by taking a turn around a bitt or cleat. sodium silicate Any of various soluble silicate glass compounds, used as a preservative for eggs. Also called “water glass.” SOLAS Safety Of Life At Sea conventions, which set international standards for life rafts, life preservers, flares, etc.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sole A cockpit or interior cabin floor. solenoid An iron core, surrounded by a coil of wire, that moves due to magnetic attraction when electric current is fed to the coil. Often used to actuate a mechanism by electric means, e.g., starter solenoid. solid injection System used in full diesel and semi-diesel, where fuel in fluid state is injected into a cylinder rather than a mixture of air and fuel drawn from a carburetor. solid state Pertaining to components and circuits using semiconductors. solstice The point on the ecliptic at which the sun reaches maximum declination—that is, farthest north (at summer solstice, about 21 June) or farthest south (at winter solstice, about 22 De- cember) of the celestial equator. Also, the instant at which the sun reaches one of the solstices. sounding A measured or charted depth of water, or the measurement of that depth. source region The region in which an air mass originates and acquires its initial characteristics. spade rudder A rudder attached to the boat only by its stock.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard spar Any shaft or pole for the attachment of a sail, such as the mast, boom, yard, or sprit. spark A momentary arc of electrical current across a gap. spark advance To cause spark to occur at an earlier time in the cycle of engine operation. Opposite of spark retard. spark gap The space between the electrodes of a spark plug across which the spark jumps. spark plug An electrical or electronic device inserted into the combustion chamber of an engine to deliver a precisely-timed spark to detonate a fuel-air mixture. specific gravity 1. The measure of the density of the electrolyte in a battery; that is, the strength of the acid and therefore the battery’s state of charge. 2. The ratio of the weight of a given volume of a substance to the weight of the same volume of water. specific heat The amount of heat in calories necessary to warm 1 gram of a particular substance 1°C. Water has a specific heat value of 1 cal /g /°C—higher than most land-mass substances. speed Rate of motion.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard speed gradient A marked difference in speed between two adjacent winds. speed made good The resultant speed from a point of departure to a point of arrival. speed of advance Intended or expected speed along the track. speed of electromagnetic waves The speed of electromagnetic waves in space is the same as the . However, the speed of propagation of elec- trical signals along wires is somewhat slower. For example, it 3 is /5 of the speed of light along coaxial cable. speed of light Approximately 186,000 miles per second, or 300,000 kilome- ters per second. speed of sound 1,076 feet per second through air at standard pressure and temperature; 4,800 feet per second through water. speed over the ground Actual speed being achieved relative to the ground. spinnaker A large, lightweight nylon headsail set forward of the headstay and used when reaching or running. Spinnakers may be sym- metrical or non-symmetrical.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard spinnaker net A web of light line hung in the foretriangle to prevent a - naker wrap where the spinnaker tends to wrap itself around the forestay. spinnaker pole A pole of fixed length to hold the tack of a spinnaker to wind- ward. spinnaker sock The fabric tube pulled over a spinnaker to safely furl the sail. splice Joining lines by interweaving the strands. spray rail Device built into or attached to the hull of a boat, forward and above the waterline, to deflect spray outboard and downward. A strut fitted to the side of the mast to hold one or more shrouds away from the mast. spring line A line leading forward or aft from a vessel to a piling or fitting on a pier to prevent the boat from moving ahead or astern. Compare breast line. A quadrilateral sail with its upper aft corner held by a shaft that extends from the mast diagonally across the sail to the corner.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard spurious emissions Any radiation of radio energy outside the authorized channel or band. squall A sudden, violent burst of wind, often accompanied by rain or snow. Persists longer than a gust, usually two minutes or more. squall line Any non-frontal line or narrow band of active thunderstorms. A squall line that is associated with and precedes a cold front— and likely parallels it—is a pre-frontal squall line. square knot A knot used to join two lines of similar size; also called a reef knot. squelch A circuit that mutes the output of a receiver in the absence of a received signal. SSB radio Single-side-band radio used in marine communications over long distances on the high seas. stability 1. A state in which the vertical temperature distribution of the atmosphere is such that an air parcel resists displacement from its original level. 2. The ability of a boat to resist heeling and overturning. stable Describes the tendency of a parcel of air to resist vertical motion and, if displaced, to return to its original level.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard staff An upright pole to which a flag or light is fixed. stainless steel General term for the many alloys of steel, chromium, nickel, and other metals that are corrosion-resistant. stall The condition that arises when the airfoil (sail) or hydrofoil (rudder) is turned too far off and the flow of air or water separates from the ; there is no longer an attached flow. stanchion A vertical post or column used to support a deck, cabin top, or lifelines. stand That phase (or portion) of the tidal cycle during which there is no vertical change in the height of tide. standard time The legal established time for a given zone, often used with adjectives, such as Eastern Standard Time (EST). standing part The part of a rope that is made fast. standing rigging Shrouds and stays supporting the mast. stand-on vessel A term, from the Navigation Rules, used to describe the ves- sel that continues its course in the same direction at the same speed during a crossing or overtaking situation, unless a colli- sion appears imminent. Compare give-way vessel.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard starboard Side of a boat, or direction, to the right when facing toward the bow. starboard tack Any heading where the wind is on the starboard side and the mainsail is carried on the port side of the boat. starter An electric motor attached to an engine to provide the means to turn the engine over by gearing to start the engine. static electricity Stationary electricity, a charge in equilibrium. station model The arrangement of symbols used by weather observers and forecasters when posting station observations on a weather map. stationary front The line of discontinuity between a cold air mass and a warm air mass when neither is moving. stator The stationary armature on an alternator within which the ro- tor spins. stay Standing rigging that supports a mast fore and aft. staysail A fore-and-aft triangular sail normally set upon a stay other than the headstay or jibstay.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard steam Popularly, the visible condensation that occurs when water vapor passes from a warm to a colder environment; an ex- ample is steam fog. steam fog Fog formed when cold air moves over a relatively warm wa- ter surface. Also called sea smoke, or arctic smoke. steerageway Enough speed to steer the boat. St. Elmo’s fire A discharge of static electricity, usually from pointed objects such as masts and spars, into the moist, highly ionized air sur- rounding them. The visible energy released by recombining electrons with ionized air particles produces the glow. stem The upright structural member or post of the bow, attached to the foremost part of a vessel’s keel. stemhead The top of the timber that forms the bow of the boat. stemhead fitting A metal casting made to fit over the top of a stemhead. step The act of installing a mast on a boat or removing (unstepping) a mast. stern After end of a boat.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard stern drive An inboard/outboard drive system with the engine inside the boat. See Inboard/Outboard. stern line A mooring line that runs from the aft end of a boat to a float, a piling, or a pier. sternway Movement stern first, as opposed to headway. stiff Stable, resistant to heeling. Opposite of tender. stock The crossbar of an anchor. Also, a rudder shaft. stop Strap, line, or shock cord used to lash a rolled sail. storm jib A very small, strongly built jib set in heavy weather. storm surge A great dome of water, often 50 miles wide, that comes sweep- ing across the coastline near the area where the eye of a hur- ricane will make landfall. This abnormal rise of the sea is primarily due to the winds of the storm. storm A small, strongly-built sail used in place of a mainsail in storm conditions. stow To put something in its proper place.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard strake A continuous longitudinal line of planking on the bottom or side of a boat. strand 1. To drive a vessel ashore or aground. 2. One of the lays of a rope (the wound yarns or fibers that are woven with other strands to make a rope). stranded wire Helically laid small conductors forming a flexible cable or con- ductor of larger cross section. stratocumulus A form of low cloud composed of a continuous or broken layer, with rounded top elements or undulations giving the appear- ance of a simple wave system. Often caused by the flattening and spreading of cumulus clouds. stratopause The interface or boundary layer, approximately 28 miles above the earth’s surface, between the stratosphere and the mesos- phere. stratosphere The region from approximately 6 to 28 miles above the earth’s surface. It is characterized by increasing temperature with altitude. The lower third of the stratosphere is the upper limit for weather clouds, cirrus and cumulonimbus. stratus A form of low cloud appearing as a gray, sheetlike layer with a uniform base, characteristic of stable air.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard stray current Leakage of a current through a path other than the one in- tended. stray-current corrosion Corrosion caused by stray or unwanted current action. streak lightning Ordinary lightning of a cloud-to-ground discharge that appears to be concentrated in a single, relatively straight lightning chan- nel. stringer A fore-and-aft structural member of a hull. strobe light A brilliant, flashing white light. stroke Distance traveled by a piston from bottom dead center to top dead center. strut A bracket that supports the propeller shaft; located outside of the hull. stud A rod with threads on both ends, such as a cylinder head stud. stuffing box Fitting through which the propeller shaft or rudder post pen- etrates the hull; designed to prevent water from entering the bilge and to provide lubrication for the shaft or post.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard sublimation The transition of a substance directly from the solid state to the gaseous state, or vice-versa, without passing through the liquid state. See snow. subordinate Term applied to a secondary station of a loran chain. subpolar lows Circumferential low-pressure belts with varied weather char- acteristics, located at approximately 60°N and 60°S latitude. subsidence Descending motion of a layer of air in the atmosphere, usually over a substantial area. supercharger A mechanically driven blower used to compress the inlet air. supercooled A liquid, such as water, is supercooled if its temperature is below the temperature at which the liquid normally freezes. This is usually due to lack of crystallization nuclei. supersaturation A condition in which the relative humidity exceeds 100%, usu- ally due to lack of sufficient condensation nuclei. superstructure Cabins, deckhouses, etc., above the deck. surf The activity between the outermost line of break- ers and the shore.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard surface dispersion Spread of current over earth or water surface resulting from a lightning strike. swab A seagoing name for a mop. You “swab down,” not “mop up.” swamp To fill with water, but not to founder. sweat The soldering of two surfaces together by the application of heat, which causes molten solder to flow between the sur- faces by capillary attraction. sweat up To tighten a line as much as possible. swell A wave system that has outrun or is no longer being acted upon by the wind that created it. A swell is characterized by regular, smooth-crested wave forms, usually of long wave length. switch A device used to open or close a circuit, stopping or allowing current to flow. synchronous speed Turning or vibrating at the same speed as another device. As applied to electric motors or generators, it means that the speed of the motor is “locked in” with the powerline frequency.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard synoptic chart or map A map showing the state of the weather over a large area at a given moment. systematic wind A wind created by flow from one air mass into another as opposed to wind created by localized heating and cooling. T tabernacle A hinge at the base of a mast; permits lowering the mast. tachometer A device for measuring and indicating the rotational speed of an engine. tack The lower forward corner of a mainsail or jib or the corner of a spinnaker that is near the spinnaker pole. Also, with “star- board” or “port,” describes the side over which the wind passes. Also, to change course by turning the bow through the eye of the wind. tackle An arrangement of line and blocks used to provide increased mechanical advantage. The bulwark or rail across the stern of a vessel. tail To haul on a sheet around a being cranked by another crew member.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard tallboy A tall, narrow sail set close to leeward of the main, usually when a spinnaker is set. tang A metal strap used to attach standing rigging to a mast. tap As applied to a coil or resistor, a connection somewhere be- tween the start and finish of the winding. tappet A cylinder, rod, or lever that moves or is moved by intermit- tent contact, as with a cam in an engine or machine. telltale A wind direction indicator made of a bit of cloth or other light material. temperature (T) A measure of hotness. In a gas, temperature is proportional to the average random kinetic energy of the molecules, but not a measure of heat or total energy. The thermometer measures some property that varies with temperature. The thermom- eter can then be calibrated by assigning arbitrary values to fixed temperatures such as the freezing point and boiling point of water at standard pressure. tender 1. A small boat accompanying a vessel, used to transport per- sons, gear, and supplies, a dinghy. 2. Lower than average re- sistance to heeling.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard terminal A connecting point on a device or component. thermal expansion Physical property of a material by which it expands as it is heated. Conversely, it contracts as it cools. thermoplastic A plastic that softens when heated. Used as an insulator for electrical wiring. thermosphere The region above 55 miles altitude, characterized by an in- creasing temperature with altitude, but containing an air den- sity so low that sound is not carried. thermostat 1. A heat controlled valve used in the cooling system of an engine to regulate the flow of water. 2. An element, such as a bimetal strip, responding to ambient temperature changes, to make or break a circuit. thimble A grooved metal loop, around which a rope or wire rope may be spliced, thus making the spliced eye more resistant to chaf- ing. third brush In DC generators, an auxiliary brush placed on a commutator to control current output of the generator. throat Upper, forward corner of a quadrilateral fore-and-aft sail.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard throttle See butterfly valve. throw Distance from the center of a crankshaft main bearing to the center of the connecting rod journal. thunder The sound emitted by rapidly expanding air along the channel of a lightning discharge. thunderstorm A storm produced by a cumulonimbus cloud. The thunder- storm is accompanied by lightning and thunder, usually with strong gusts, heavy rain, and sometimes hail. thwart A transverse seat in a boat. See thwartships. thwartships At right angles to the fore-and-aft line. See athwartship. tidal current The horizontal movement of water caused by tidal action. See also ebb current and flood current. tidal range Difference in height of tide between any successive pair of high and low tides. tide The vertical rise and fall of ocean water (most noticeable in coastal regions) resulting mainly from the gravitational attrac- tion of the moon and sun.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard tiller A lever attached to the upper end of a rudder stock, used by the helmsman to turn the rudder. timber A general term for the rectangular pieces of wood used in ship building. time 1. The instant of the day reckoned by the position of a celes- tial reference point, such as the mean sun, relative to a refer- ence celestial meridian. 2. A measure of the elapsed time interval between two events. time diagram (TD) A diagram in which the celestial equator, as viewed from the south celestial pole, appears as a circle, and the hour circles appear as radial lines. time difference (TD) The difference in time of arrival, measured in microseconds, of two loran signals, one from the master station and the other from one of the secondary stations. time to arc The converting of time in hours (hh), minutes (mm), and sec- onds (ss) to arc in degrees. time zone A zone, usually 15° of longitude wide, in which the same time is kept throughout.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard timing chain Chain used to drive camshaft and accessory shafts of an en- gine. timing gears A group of gears that are driven from the engine crankshaft to cause the valves, ignition, and other engine-driven appara- tus to operate at the desired time during the cycle. toe rail A small raised section around the perimeter of the deck. toggle 1. A U-shaped fitting linking a turnbuckle with a chainplate; permits movement so the turnbuckle is not bent when the stay is pulled out of line. 2. A small wooden crosspiece at the end of a line, passed through an eye or loop in another line as when bending a flag onto a halyard. top dead center (TDC) The position of the crankshaft when the piston is at the very top of its stroke. A line used to support a boom end and/or spinnaker pole. topside On or above the weather deck (wholly exposed to the ele- ments). topsides The sides of a vessel between the water line and the deck.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard tornado A violently rotating column of air attended by a funnel-shaped or tubular cloud hanging beneath a cumulonimbus cloud. A cyclonic storm of devastating destruction in the Northern Hemi- sphere, the tornado varies from about 20 yards to a mile wide at the surface. Wind speeds are estimated from 100 to over 300 miles per hour. torque An effort devoted toward twisting or turning. torque wrench A special wrench that measures the turning force applied to a nut, bolt, or shaft. trace To analyze or follow current flow or signals through a circuit; also, the fluorescent line or image on an oscilloscope screen. track 1. A metal piece on the after edge of a mast or top of a boom on which sail slides run. Also, a metal strip on deck for cars with blocks for adjusting sheet leads. 2. The actual path of a vessel over the ground, such as may be determined by track- ing. See intended track. trade winds Circumferential bands of surface winds flowing from the horse-latitude highs toward the low-pressure doldrums at the equator. These winds are deflected westward (making them easterly) by the Coriolis effect. Trade winds occur in both hemispheres between 30° and 0° latitude.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard A radio set that combines transmitting and receiving modes in one package. It may have circuits or components common to both. Element of a depth sounder that translates electromagnetic pulses into ultrasonic pulses and vice versa. Usually contains a barium titanate crystal. transfer port In two-stroke cycle gasoline engines, an opening in the cylin- der wall permitting the fuel mixture to enter from the crank- case. transformer An alternating current device to magnetically couple one cir- cuit to another. It can provide higher or lower output voltage relative to the input voltage. transistor A semiconductor device having three or more elements. It has the capability of amplification, rectification, or switching voltage or currents. transit (Tr) The passage of a celestial body across a celestial meridian. See meridian transit. transmitter An electronic unit capable of generating a signal for propaga- tion through a medium.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard transom The portion of the hull at the stern that is at right angles to the centerline of the boat. A line from the mast that can be used by the crew of a small boat to suspend themselves outboard of the boat, thereby in- creasing the stability of the boat. traveler A sail-positioning system composed of a track on which slides a car attached to blocks to permit positioning sheet leads un- der load. traverse A series of directions and distances, such as those taken by a sailboat tacking to reach a destination windward. traverse sailing A method of determining course and distance made good by a traverse. trim 1. To adjust the set of the sail. 2. Refers to the attitude of a boat at rest in the water. trimaran A boat with three hulls connected by a deck or crossbeams, resulting in a wide beam and good stability; the middle hull is usually larger than the two outer hulls.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard trip-free A circuit breaker that will function to interrupt a circuit under overload conditions even though the reset mechanism may be set against the trip. trip line 1. A buoyed line attached to the crown of an anchor for the purpose of freeing it when fouled. 2. A line fastened to the small end of the cone of a sea anchor to collapse it, thus spill- ing the water from the cone. trochoid Curve traced by a point on or inside the rim of a wheel rolling along level ground. tropical air mass Warm air that has its source in the low latitudes, chiefly in the regions of the subtropical high-pressure systems. tropical cyclone An intense, violent storm spawned on the outer fringes of the doldrums (6° to 10° latitude) and fed by the warm, moist air of the maritime tropics. Also called hurricane, willy-willy, typhoon, baguio, and/or Mexican or Indian Ocean cyclone. See hurricane. tropical depression A rotary circulation at the surface; highest constant wind speed not more than 38 miles per hour (33 knots). tropical disturbance A moving area of thunderstorms that maintains its identity for 24 hours or more. This is a common phenomenon in the tropics.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard tropical storm A distinct rotary circulation; constant wind speed of 39 to 73 miles per hour (34 to 63 knots). tropopause The boundary between the lowest layer of the atmosphere, the troposphere, and the next layer, the stratosphere. Ranges in altitude from 5 to 12 miles above the surface. troposphere The lowest layer of the atmosphere, characterized by a gen- erally decreasing temperature with increased elevation. All ordinary, visible weather phenomena occur in this layer. Its thickness varies from about 5 miles at the poles to about 12 miles at the equator. trough of low pressure A flattened or elongated zone of low pressure that has not developed into a cyclone. The top of the mast. true course (TC) The course as plotted on the chart, measured from true north. true wind Actual direction and speed of horizontal air motion in relation to the earth’s surface. Also, strength and direction of the wind felt on a boat not moving. trysail See storm trysail.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard tsunami A seismically generated sea wave. Not observable at sea, but can be extremely destructive when it strikes a coastline. tumblehome The inward curve of a boat’s sides from bilge to deck line, particularly near the stern. turbocharger A blower driven by an engine’s exhaust gas; used to com- press the inlet air. turbulence Small-scale irregularities in the horizontal or vertical flow of air. turbulent flow Irregular, detached flow along a surface such as a sail or rud- der, which reduces the lifting characteristic of the foil. turnbuckle A tension-adjusting device for tightening wire rigging or cable, composed of threaded rods extending from within a threaded barrel. turning block A block used to change the direction of a line, such as a sheet or halyard, to make hauling more convenient. turn of the bilge The lower outer part of the hull where the sides meet the bottom.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard twilight The period of incomplete darkness following sunset or pre- ceding sunrise. See civil twilight and nautical twilight. twist The change in the angle of a sail’s chord with respect to its foot, which increases from tack to head. twist-lock A connector or plug that is mechanically locked into its socket by twisting after insertion. two block A tackle that has been pulled to bring the blocks together. Also, a halyard that has been hoisted all the way. type acceptance Approval by FCC of a manufacturer’s design of a transmitter that, when properly installed, will meet FCC technical stan- dards of performance. typhoon A severe tropical storm in the western Pacific or the China Sea; same as a hurricane except for location. U ultrasonic Signal or pulse whose frequencies are above the limits of au- dibility.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard under bare poles Sailing (underway) with no sail set and being driven only by the force of the wind on hull, spars, and rigging; usually a heavy-weather precaution. under power A boat being propelled by an engine, even though sail may be set. underway 1. Making progress through the water. 2. Afloat but not at anchor, aground, or made fast to the shore. universal motor An electric motor that will operate on either AC or DC power. Universal Time (UT) See Coordinated Universal Time (UTC). unship To remove an object from its regular place of use. unstable Applied to an air mass or parcel of air, this term refers to the tendency of the air to remain in vertical motion once set in motion. updraft An upward current of air, usually generated by convection near or within a developing cumuliform cloud, or by turbu- lence within a layer of air. Also, a type of carburetor wherein the fuel-air mixture flows upward to the engine.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard upper branch (M) The half of the observer’s meridian, measured from pole to pole, passing through the zenith of the observer. upper limb (UL) The upper part of the circumference of the sun or moon. upslope fog Fog formed when warm, moist air is lifted by moving over higher ground and is cooled to its saturation point. urgent call See Pan-Pan. USYRA United States Yacht Racing Union. V valley wind Air flowing up the side of a mountain as the result of heating of the mountain slope and the air next to it during the daytime. valve Device for allowing gases in and out of an engine cylinder at precise moments. valve clearance The gap between a valve stem and its rocker arm or tappet when the valve is fully closed. valve face The part of a valve that mates with and rests upon a seating surface.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard valve grinding A process of mating the valve seat and valve face, performed with the aid of an . valve key or lock Key, keeper, washer, or other device that holds the valve spring cup or washer in place on the valve stem. valve lifter See tappet. valve overlap The period of time in which an exhaust valve and inlet valve are both open in a cylinder at the same time. valve seat Mating surface upon which valve face rests. valve spring A spring attached to a valve to return it to its seat after the lifting force is released. valve stem A portion of a valve that rests within a guide. valve stem guide A bushing or hole in which the valve stem is placed that al- lows longitudinal motion only. valve timing Indicates the position of the valves (open or closed) relative to the travel of the piston, in crankshaft degrees. vang A line or purchase used to assist in positioning a boom or gaff.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard vapor A substance in the gaseous state. vaporization The change of state from a liquid to a gas (i.e., water to water vapor). vapor pressure That portion of the total atmospheric pressure (gas pressure) caused by water vapor. This value is correctly called “partial pressure” of water vapor. The vapor pressure given in dew-point tables refers to the maximum, or saturation, pres- sure at a specific temperature. variation (Var) The angle between the geographic meridian and the magnetic meridian at a given locality. Variation is easterly or westerly, as the direction of the magnetic meridian is toward the east or west of geographic north. vector A quantity that has both magnitude and direction, commonly represented by an arrow. The length of the arrow represents the magnitude; the direction in which the arrow flies repre- sents the direction in which the quantity acts. vector sum The “addition” in which the tail of one vector is placed at the tip of a second vector, and a new vector is taken from the tail of the second vector to the tip of the first.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard veer 1. Change in direction of the wind in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. 2. To change direction or course. See back. veering wind A wind whose direction changes progressively in a clockwise manner (for example, N-NE-E-SE) in the Northern Hemi- sphere; opposite of a backing wind. velocity A vector quantity that includes both rate of motion and direc- tion of motion—as contrasted with speed, which does not in- clude direction. venturi Two tapered, streamlined tubes joined at their small ends so as to reduce the internal diameter. Also, a constriction in the air passage of a carburetor, which causes an increase in ve- locity and a decrease in pressure. vernal equinox See first point of Aries. vernier A short auxiliary scale alongside the graduated scale of a marine sextant to allow precise reading of fractions of a minute. vertical circle A great circle of the celestial sphere, running through the observer’s zenith and nadir in a plane perpendicular to the horizon. See prime vertical.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard vessel engaged in fishing Any vessel fishing with nets, lines, trawls, or other fishing ap- paratus that restrict maneuverability. But the term “vessel en- gaged in fishing” does not include a vessel fishing with trolling lines (or other fishing apparatus) if these lines do not restrict maneuverability. vessel not under command A vessel that, through some exceptional circumstance, is un- able to maneuver as required by the Navigation Rules and is, therefore, unable to keep out of the way of another vessel. vibration damper A device to reduce the torsional vibration that occurs along the length of the crankshaft of an engine. viscosity The measure of a fluid’s resistance to flow. visibility The greatest distance at which one can see and recognize prominent objects. visible horizon Junction where earth and sky appear to meet. volt A unit of electrical force that will cause a current of one am- pere to flow through a resistance of one ohm. voltmeter An instrument for measuring voltage in an electrical circuit.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard volumetric efficiency The efficiency with which an engine replaces the spent gases with fresh air (diesel) or gasoline and air (gasoline engine). voluntarily equipped Applies to vessels having radio or other safety equipment in- stalled even though not required by government regulations. vortex The funnel-shaped profile formed by a spinning fluid. W wake Water surface turbulence left by a moving boat. warm front The discontinuity or boundary at the trailing edge of a retreat- ing cold air mass that is being displaced by an advancing mass of relatively warm air. warm sector That sector, usually the smaller portion of the area surround- ing a wave cyclone, in which the warmer air mass is in con- tact with the earth’s surface. warning In weather advisories, a bulletin indicating that a hazardous condition, such as a tornado or flash flood, is in existence and may threaten some portion of the warning area.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard warp 1. To maneuver a boat by hauling on a line fastened to a fixed object, such as a cleat, pile, bollard, or anchor. 2. The line used in warping. 3. A line streamed behind a boat to slow it down. wash 1. Turbulent flow of water left by a moving boat or as a result of a boat’s turning propeller(s). 2. The rush or sweeping of waves on a bank, shore, or vessel. watch (W) 1. In weather advisories, a bulletin indicating that a hazardous condition, such as a tornado or flash flood, may develop. 2. A period of time during which a crew member is on duty. watch time (WT) The hour of the day as indicated by a watch or clock; usually expressed in 24-hour time, or a 12-hour cycle and labeled AM or PM. water vapor Water in its gaseous state. waterline length (LWL) The measurement along the waterline from the point where the bow touches the water to the point where the stern emerges from the water. waterspout A whirlwind or spiraling uplift of sea or lake surface water into the lower atmosphere, caused either by a tornado that has moved out over the water or by convective air motion (either cyclonic or anticyclonic).

Return to Keyboard USPS® Nautical Glossary Return to Keyboard watt The unit of electrical power. watt-hour meter Instrument used to indicate total electrical energy consumed; usually designated in kilowatt hours. wave 1. In water, a series of undulations that move along the sur- face. 2. Other periodic occurrences, such as the succession of compressions and rarefactions of the air in sound waves or the cyclical radiation of energy in electromagnetic waves. wave height The vertical distance from the trough of a wave to the crest. wave length 1. The distance between successive crests of a wave. 2. Dis- tance occupied by one full cycle of an alternating current or electromagnetic wave. wave period The time interval between passages of two successive crests. wave system A collection of waves all due to the same disturbance, having similar heights, lengths, and periods. way Movement of a vessel through the water, such as headway, sternway, or leeway. waypoint A defined point, such as a buoy or latitude/longitude position, entered in a Loran-C or GPS device.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard wear Changing tacks by jibing instead of tacking. weather 1. The condition of the atmosphere at any given time and place. 2. The windward side. weather deck A deck wholly exposed to the elements. weather helm A condition of imbalance that requires the tiller to be held to weather to keep the boat on a straight course. well-found Having all necessary equipment on board, in good condition. westerlies See prevailing westerlies. wetted surface The portion of a vessel’s exterior that is in contact with the water. wharf Man-made structure parallel to the shoreline for loading, un- loading, or making fast. whip To bind the end of a rope with light line. whisker pole A pole used to hold a jib to windward when running wing-and-wing.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard winch A geared drum turned by a handle and used to pull lines such as sheets and . wind Horizontal motion of the air, reported in both speed and direc- tion. windage Resistance to the wind. windlass A winch, powered by hand or motor, for hauling anchors. wind’s eye The exact direction from which the wind is blowing. wind shear The rate of change of wind velocity (direction and speed) per unit distance. This term is conventionally expressed as verti- cal or horizontal wind shear. wind shift A sudden change in wind direction, usually occurring with the passage of a front. wind vane A type of autopilot that uses the apparent wind to steer the boat. windward The direction from which the wind is blowing. (Pronounced “wind’ard.”)

Return to Keyboard USPS® Nautical Glossary Return to Keyboard wing-and-wing Sailing on a run with the jib and mainsail set on opposite sides. working end The outboard or free end of a line, chain, or cable, used to make knots, bends, and splices and to attach to an anchor, cleat, other vessel, etc. Compare bitter end. working jib A normally-used jib that has an area about equal to the area of the foretriangle. woven roving Loosely woven fiberglass cloth. wrist pin The journal for the bearing in the small end of an engine con- necting rod that also passes through piston walls. X Y yardarm The end of a spar crossing a mast horizontally. yaw To swing off course due to wave action or bad steering, usu- ally to one side and then the other. yawing moment A force tending to rotate the hull about a vertical axis, usually due to displacement of the center of effort of the sails out- board of the center of resistance of the hull.

Return to Keyboard USPS® Nautical Glossary Return to Keyboard yawl A two-masted vessel whose mizzen is stepped abaft the rud- der post. Z zenith (Z) The point on the celestial sphere directly overhead of the ob- server. zenith distance (z) Angular distance along a vertical circle, from the zenith to a body. Same as co-altitude. The side of the navigational tri- angle between the assumed position (AP), dead reckoning position (DR), and the geographical position (GP) of the body. zodiac Band of the sky extending about 8° to either side of the eclip- tic. Each of the zodiac’s 12 areas is named for a constellation and represented by a sign. Important to astrologers but of no significance for navigation. zone description (ZD) A number, including a positive or negative sign, added to or subtracted from zone time, to obtain Greenwich mean time. zone time (ZT) Local mean time kept throughout a designated zone, usually based on the nearest meridian with a longitude exactly divis- ible by 15°.

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