The Study of Sailing Yachts

'The design of their hulls is assisted by experiments with models. Now tests of the full-scale hull of a racing yacht in a towing tank are aiding the e'aluationof results obtained froii work with models

by halsey C. 1-lerreshoff and J. N. Newman

modernsailingyachthas Society of Naval Architects and Ma- In discussing the mechanics of sail- evolved froni one of man's oldestnue Engineers and the availability ofing we shall consider the two extreme Theefforts to harness natural forces large towing-tank facilities at the Navy's conditions of operation: when a yacht is ¡or the purpose of locomotion. It is also David Taylor Model Basin near Wash- running before the wind, that is, sailing a unique vehicle in that its performance ington. vith the wind at its stern, and when it depends on fluid flow in two mediums: is sailing to windward, or against the water and air. For centuries the design sailing yacht must be designed towind. The mechanics of intermediate of sailing vessels proceeded on the basis perform with maximum speed andheading anglesareessentiallycom- of practical experience, shrewd observa- seaworthiness over a wide range of con-binations of these two extremes.In tion aiid intuition. Nonetheless, the suc- ditions. The wind can blow from anyrunning before the wind the sails act cessful design of fastsailingvessels direction with strength ranging fromsimply as an aerodynamic drag to the played an important part in the fate of calms to storms, and the sea conditionswind; it follows that the boat speed will nations and the founding of empires. are equally variable. Thus the boat must be less than the wind speed. The only ' Now that the sailing vessel is used al-be fast not only when it is sailing down- relevant hydrodynamic force exerted by most solely for recreation and racing wind (when its course coincides with the water on the hull is a drag, which on1petition one can appreciate anew the wind direction) but also when it is obviouslyshould be minimizedfor he complexity of the problem of de-sailing on a "reach," or at right anglesmaximum speed. In sailing to wind- signing a wind-driven craft that willto the wind [see illustration on page ward, on the other hand, the sails and move through both water and air with63]. In the latter condition thrust from hull both act as wings: lifting surfaces high efficiency. Nations that have nothe sailsisaccompanied by a large that meet the oncoming wind and water difficulty making the most advancedsidewise force that tends to heel theat various angles of attack and develop technological devices have discoveredboat over. Above all, a racing yachtnot only adverse aerodynamic and hy- that building a superior sailing yacht ismust be able to perform well sailingdrodynaniic drag forces but also favor- a task of considerable subtlety. upwind, when its course is at an angle able lift forces. The principal means of reducing the of perhaps 40 degrees to the direction In principle there is no limit to the unknown factors insailing-yacht de- of the wind and when a delicate balance boat's speed in sailing to windward, sign isto test scale models of yachtof circumstances determines the small particularly because the relative wind hulls by towing them in an experimental difference between racing success and speed is increased by the boat's own tank. This procedure can be checkedfailure.Itis not uncommon for well-speed. In practice, however, the sails against the ultimate performance of the matched yachts, such as those that com- are most efficient and the boat's speed full-scale vessel at sea. Such confirma- pete for the America's Cup, to sail ais greatest when the course is approxi- tion is never entirely satisfactory, how-four-mile leg of a course against themate]y at right angles to the wind; in ever, because of the many variables, wind with a variation in elapsed time of that condition the forward component such as wind and waves, that must beonly a few seconds, which amounts to aof the lift force on the sails is at a maxi- taken into account. In the case of a sail- difference in speed of only a few partsmum. In sailing to wiñdward there is an ing yacht this is particularly true be- in several thousand. optimum heading angle with respect to cause the craft's ultimate performance Indeed, an important stimulus to re- the wind, generally 40 to 45 degrees. is so largely controlled by the conditions search in yacht performance is the pros- Accordingly for a boat to proceed in a of wind and sea, the skill of the crew pect of continued challenges to U.S.direction opposite to the wind itis and the quality of the sails. possession of the America's Cup bynecessary to sail a zigzag course, which Recently a full-sized sailing yacht has Britain, Australia and probably otheris termed "beating to windward," In beei tested in a towing tank, allowingcountries. Since 1&51 U.S. Cup defend- sailing before the wind the hull is es- foi the first time a direct comparison of ers have successfully met all challengers, sentially upright and moving straight the yacht's behavior under controlledbut a close match is in prospect forahead, whereas in sailing to windward conditions with that of its scale model. September, 1967, when newly designedthe large sidewise component of sail The experiment was made possible U.S. and Australian 12-meter boats will force causes the boat to heel on its through the interest and support of the compete in the waters off Newport, R,I. longitudinal axis. In order for the hull to

61 develop an equal and opposite sidewiseit is generally less than three degrees,that producing vertical lift in an air- force it must move through the waterand in highly developed boats itis as plane. The main body of a sailing-yacht with a small angle of attack, or yaw small as one degree. Nevertheless, it ishull, like the fuselage of an airplane, is angle, defined as a rotation around theof fundamental importance to the en-in itself an inefficient lifting surface; life' hull's verlical axis. This angle is anal- tire process of sailing to windward [seeis provided by a thin keel or retractable ogous to the angle of attack of the illustration below]. centerboard, which functions as a wing. wind acting on the sails. The yaw angle The mechanism by which a hull gen- Also of fundamental importance is is usually so small as to escape notice; erates a horizontal lift is identical withthe equilibrium of hydrodynamic and aerodynamic moments: the turning in- fluences exerted by water and wind. The aerodynamic heeling moment, ans- ANGLE OF ATTACK ing from the sidewise force acüng on the sails, is substantial. In order to ne- .ßOATMOTION VECTOR sist this moment the hull must have a large restoring moment. This is provided FORE iii many large sailing yachts by a heavy TRUE WIND DIRECTION lead ballast at the bottom of the keel and in small, light craft by a shifting of the crew's weight, which acts as "live ballast." The yaw, or vertical, moments are also significant; if they are unbal- anced, the boat will tend to turn rather APPARENT WIND than sail a straight course. Small turn- DIRECTION_- ing moments around the vertical axis are inevitable because the centers of RECIPROCAL OF hydrodynamic and aerodynamic pres- VECTOR sure cannot be predicted with certainty and are subject to variation with sailing conditions. In general small angular SAIL LIFT adjustments of the rudder are required to maintain a steady course. It is necessary, however, to have the boat well balanced to obviate the need for larg rudder angles, which induce additional drag forces. Moreover, the sign (plus oi HULL LIFT minus) of the turning moment, or the direction of the corrective rudder angle, is of crucial importance. It is desirable to design the hull and sails to produce a small turning moment in the directioi that makes the bow head into the wind. This requires a small compensating rudder angle that increases the sidewise force on the hull-a force pushing the hull to windwardby giving an effec- tive camber, or favorable curvature, to the combination of keel and rudder. A goodsailor appreciates the importance of this balance and adjusts the fore-and-aft position of his mast and sails accordingly. The improvement of sailboat hulls requires a quantitative study of the hydrodynamic forces and moments act- AFT ing on the hull at various speeds and attitudes. Yacht design has advanced to FORCES ON SAILING YACHT are produced by the flow of air past the sails and theflow the point where extremely small differ- of water around the hull, including the effect of keel and rudder. The lift and drag pro- ences between yachts are important. duced by the sails are conventionally represented as being respectively perpendicular to Improving the average speed and parallel to the apparent wind direction. Similarly, the lift and drag produced by the of an hullare shown as being perpendicular to and parallel to the direction of boat motion. WhenAmerica's Cup contender by .01 knot the yachtissailing at constant speed, the resultant forces produced by the sails and hull are would be a significant achievement; .1 equal and opposite. In this diagram the yacht is sailing to windward, or within about45 knot would be a major breakthrough. degrees of the true wind direction. Because of the boat's forward motion the angle be- Since no satisfactory theoretical pre- tween the boat's course and the apparent wind is only about30degrees. The hull meets the dictions are available, hull shapes can. oncoming water at a small angle of attack, necessary to produce a lift force on the hull. be evaluated only by the observation o

£2 WIND

SAILING TO WINDWARD SAILING TO WINDWARD (PORT TACK) (STARBOARD TACK)

SPEED MADE GOOD TO WINDWARD

CLOSE REACHING CLOSE REACHING I 900

BEAM REACHING BEAM REACHING (SPINNAKER OPTIONAL) (SPINNAKER OPTIONAL)

1800

BROAD REACHING BROAD REACHING (WITH SPINNAKER) (WITH SPINNAKER)

RUNNING BEFORE THE WIND (WITH SPINNAKER)

SAILING CONDITIONS depend on the intensity of the wind and (iently when sailing to windward. Typical speeds for all possible the heading the boat has in relation to the wind. An angle of 40 headings are indicated by the two curves (color) ; the farther a to 45 degrees to the left or right of the direction from which the point on a curve from the center of the circle, the higher the wind is coming is as close a heading as a sailboat can achieve effi speed. The spinnaker is the large, ballooning sail at the bow. actual sailing performance or by theof the test facilities used for thislu- gauges for measurement of drag and direct measurementof therelevant pose are on the order of 100 feet long, sidewise forces must be extremely sen- forces in a towing tank. eight feet wide and four feet deep, and sitive; on this scale the forces and mo- The first intensive use of a towingthey are equipped with carriages thatments are very small and nearly infini-- tank for sailing craft was the pioneer- span the tank and move along accurate-tesimal differences between models are ing effort of K. S. M. Davidson at the ly aligned rails with controlled speeds ofbeing sought. Stevens Institute of Technology in the a few feet per second. The model hulls, Even assuming a perfect experimen- 1930's. Davidson's tank studies playedusually about five feet long, are con- tal regime, towing-tank studies present a central role iii the development of the nected to the towing carriage through theiiivestigator with a fundamental highly successful America's Cup yachtprecision linkages and dynamometersproblem: There is no way to duplicate Ranger, which in 1937 defeated thethat restrict the yaw and heel angles to in miniature and in proper scale all the British challenger Endeavour li in fourspecified values. The linkages must be dynamical relations that affect the full- straight races. Since that time modeldesigned so as not to restrain the verti-sized hull of a ship or yacht. In es- tests have been a key factor in develop- cal position and attitude of the hull, sence the problem is that the motion of ing the America's Cup contenders andwhich change slightly with speed be- the water moving along the hull is goy- many other racing yachts as well. Mostcause of dynamic pressure effects. Theerned not oriiy by the frictional proper- ties of water but also by wave effects, which depend on gravity. When a hull is scaled down to model size, there is no way to scale down in equal degree the frictional effects and the wave effects. The frictional effects are scaled according to the Reynolds number (named for the British engineer Osborne Reynolds), which states that such effects can be scaled accurately only if the velocity of a body moving through a fluid is inversely proportional to its length (assuming that the viscosity of the fluid is held constant). The Reynolds number is more precisely defined as the product of the body's speed and length, divided by the kinematic viscosity coefficient o4 the fluid. This nondimensional param- eter must have the same value for the

full-scale vessel and its model 4 Wave effects, on the other hand, are scaled according to the Fronde number (named for another British engineer, William Froude), which states that the wave drag cati be scaled only if the velocity of a hullisproportional to the square root of the length of the hull(assuming thattheacceleration of gravity is held constant). The Froude number is defined as the ratio of the hull speed to the square root of the product of the hull length and gravita- tional acceleration. Thus if one wants to carry out an experiment with ship models in which the dynamical forces are in proper scale, one must be able to adjust either the viscosity of the fluid or the force of gravity. Altering gravity is clearly im- practical, and there is no fluid whose viscosity corresponds to that needed to represent vater for models of practical size. Therefore one is forced to choose between test measurements that preserve the Reynolds number by varyill g the speed of the model inversely with the length of the model hull or that preserve the Fronde number by vary- ANTIOPE RACING sails io wjnd;ard in an international rare. The wind is about 40 dc.ing the speed directly with the squarc grees off the bow; the sails are trimmed almost flat with a small angle of attack to the wind. root of the hull length.

64 - this is added the frictional drag of the full-scale hull, calculated as in the case of the model but for the full-scale Reyn- L olds number, to obtain a predicted value for the total resistance. Unfortunately there is some uncer- tainty about the calculationoffric- tional drag. This is particularly true of yachts, because their hull forms differ so greatly from those of the conventional ship hulls for which the em- pineal formulations of frictional drag were derived. Moreover, since the frictional drag constitutes the largest portion of the total drag produced by a towed model, an accurate prediction is necessary for the overall Froude method to be valid. The scaling of the lift, or sidewise force, due to yaw is also a source of potential trouble, but this is usually assumed to scale down strictly according to Froude's law for wave effects, with frictional effects being of sec- SCALE MODEL of the Antiope is fitted for tests in the ship-model towing tank of the De- ondary importance. partment of Naval Architecture and Marine Engineering at the Massachusetts Institute of Technology. Forces and moments are measured by means of the sensing devices mounted Such wasthe state of the art when fore and aft; device at midships imparts heel. The model is about one-sixth the size of Antiope. two years ago the Society of Naval ArchitectsandMarine Engineers formed Panel H-13 (Sailing Yachts) as part of its technical and research pro- gram. This group, consisting primarily of yacht designers and towing-tankop- erators, immediately focused its atten- tion on the scale-effect problem and possible experiments that might provide acorrelation between full-scale and model results. It had been amply demonstrated that there was scant hope of obtaining reliable dynamic measurements on a real boat at sea or even in protected waters. The alternative was clear: to find some way to study a full- scale boatinalarge towing tank. Through a chain of fortunate events this seemingly unlikely project quickly materialized. The Navy's large experimental facilityatthe David Taylor Model Basin is directed by charter to perform commercial testing services for the American maritime industiy. The proposed experiment met the requirements of the charter. The towing tank selected for the experiment is about 1,700 feet long, 51 feet wide and 20 feet deep. Normally this facilityis used for towing scale models of ships and submarines. It is big enough, however, to accommodate the hull of a full-sized sailing yacht up to about 30 feet in length. Although larger hulls could be placed in the tank, they would be subject to wall effects (for example wave reflections from the TEST RUN is conducted with the model of the Antiope in the M.I.T. tank. A typical test sides of the tank) that might influence consists of 90 runs covering all the practical sailing attitudes and speeds. The tests pro- the measurements. a basis for predicting how the full-scale vessel will perform under various conditions. After considering various possibili- Late in the 19th century Fronde de- vised a practical procedure foi' sepa- rating the hull drag, or hull resistance, into two distinct components, one frictional and one residual. This procedure has made it possible to carry out tow- iìg-tank studies in spite of what appear to be the conflicting requirements set by the Reynolds number and the Froude number. Although the two components in Froude's method are not completely independent, they can be regarded as such for engineering purposes. r [he frictional component of drag is derived from the transmission of vis- cousshearingforcesthatactina boundary layer of fluid adjacent to the surface of the hull. This component contributes nearly all the drag at low speeds and about half of the total at the highest speeds of conventional ships and yachts. The residual component of drag is composed primarily of the wave resistance, which is the force required to transfer energy to the familiar system of waves that forms behind the hull of a moving ship. There are smaller secondary components, such as the drag produced if the flow of water over the hull becomes "separated" (that is, no longer flows smoothly). Other secondary components include the normal pressure forces that originate with the viscosity of water and the induced drag associated with the ]ift on a yawed hull. These components ai-earbitrarilyin- cluded in the residual component. For a given hull shape and attitude the residual componentofresistanceis considered to depend solely on the Froude number, whereas the frictional component is assumed to depend solely on the Reynolds number. Before this approach can be applied otte must k-now how to separate the total drag into its frictional and residual components. Extensive research on flat plates and ship hulls has led to a useful technique for predicting the frictional component; this component is regarded as being simply proportional to the sub- merged surface area of a ship or its model and independent of geometrical form. Accordingly models are run at a speed set by the Froude scaling, so that the waves generated by the model are geometrically similar to those of the real ship. Frictional resistance is calculated from empirical formulas and subtracted from the total model drag to provide the residual component. This compo- RUDDER ANGLE affects drag, as shown by strips of nylon yarn attached to the hull of nent is then multiplied by the cube of the Antiope. The hull is in the David Taylor Model Basin's flow tank, in which the water the scale ratio (yacht length dividedmoves but the vessel does not. At top the rudder angle is zero and the strips indicate an by model length) to yield the residual even flow. At middle a slight stalling effect appears with a rudder angle of five degrees; at component for the full-scale hull. To bottom it is intensified with a 17.degree angle. Visible portion of hull is below waterline. 65'' 400 experiments and analyses are under way in the study of sails. The scaling of force measurements on sails in a wind 350 tunnel is simpler than the scaling of / hydrodynamic forces on hulls; wave effects are no longer a factor and Reyn- 300 olds scaling is valid. In the case of the sails, however, there is the additional problem that the shape is not constant: 250 it varies with wind conditions because o(I) of the elasticity of the sail fabric and the z distortion of the rigging that holds the o sails. Moreover, the number of variables 5,200 w governing the choice of sails is large. (J Tests of jib, mainsail and hull combina- Q::

LLo tions are now under way at the M.I.T. 150 Wright Brothers Wind Tunnel and at the U.S. Naval Experimental Station in Philadelphia [see illust ration on pre- 100 ceding page]. As in the towing-tank tests, dynamometers are used to measure the aerodynamic forces and mo- 50 ments on the sails. The test sails are made of aluminum or of a rigid fiber- glass-reinforced plastic in order to main-

o tain control over their shape.

o 2 3 4 6 7 SPEED (KNOTS) j\though itis useful for design and research to separate the treatment DATA FROM TESTS of Antiope's hull in the model basin include relation of drag to speed of the hull from that of the sails, the two (color) and side force to speed (black). Each set of curves shows, from bottom, yaw angles are closely interrelated. A sail that is of zero, 2.65 and 6.45 degrees; heel angles vary from zero to 30 degrees. Small side force optimum for one boat is not optimum occurs even with no yaw angle because of asymmetry of hull resulting from heel angle. for boats of a different class or type. As an extreme example, an iceboat is capa- Ie Of Thgh speeds, and ít can exert 'I 7 throughts runner bfades a high side- RUDDER 30 RUDDER 00 vise force «h practically no drag. Tnder thesecircumstancesflatsails I RU DDE R 6° 6 with small angles of attack are appro- priate; such sails produce rather small I driving forces but have a high ratio of liftto drag. In contrast, a 12-meter 5 sailing-yacht hull is subject to a considerable hydrodynamic resistance to forward motion and can develop a side- I

4 A wise force of a large (bat limited) ex- tent. The sail design used in an iceboat wouldprovideinsufficientforward thrustfor a 12-meter yacht, which /-- 4ZONEOFPOSSIBLE needs a more cambered sail set at a - WINDWARDSAILINGCONDITIONS / larger angle of attack in order to produce the necessary drive. LIKELY WINDWARDSAILINGCONDITION The tremendous popularity of recrea- tional boating and in particular the re- cent upsurge of interest irs sailing vessels have stimulated renewed interest in yacht development. Yacht designers I have long been aware that much research is needed to fill the existing gaps in information about the dynamics of

O 2 3 4 6 7 sailing craft. It is hoped that model ex- YAW ANGLE (DEGREES) periments andfull-scalecorrelation, such as the Antiopc tests, will lead to a LIFT.TO-DRAG RATIO of Antiope's hull is plotted against yaw angle for windward sailing better understandingofthe funda- at a speed of five knots and various rudder angles. Color shows possible and likely wind. mental mechanics of sailing-yacht per- ward sailing conditions as derived from liftto-drag ratio of sails ona 5.5.meter yacht. formance. ties it was decided that a yacht from thehighlycompetitiveInternational 5.5-Meter Class would make a fine test subject.Designations suchas"5.5- meter" and "12-meter" are derived not from the actual length of the boats but from international rules based on an empirical formula. The formulain- cludes length, sail area, displacement and other factors to yield a figure, ex- pressed as length, that is assumed to be a fair measure of the speed of the boat. Unlike "one design" yachts, each of which has a standard hull form and sailplan, the "meter" classes permit variations of design that lead to the development of new hull shapes and sailcombinations. An analogousde- velopment process contributes to the improvement of ocean racing yachts. One of the leading designers in the 5.5-meter class, A. E. Luders, offered his own boat the Antiope for the proposed tank tests. Her length at the waterline is 23 feet; her length overall, 31 feet. Financial support for the tests was obtained not only from the Society of Naval Architects and Marine Engineers but also from interested yachtsmen and naval architects. In November of last year the Antiope was shipped to the David Taylor Model Basin, where she was fitted onto the towing carriage by means of specially prepared linkages and equipped with gauges to record the three components of force at each end of the boat [see illustrations on page 60]. Yaw angle was adjusted by setting the bow off-center;heel angle was SAIL TEST is conducted in the flow.visualization wind tunnel at the U.S. Naval Experi- varied by moving 400 pounds of lead mental Station in Philadelphia. This is a 1:37-scale model of a 12.meter boat. The jib, or ballast along a 14-foot aluminum plat- forward sail, is made of Mylar, the mainsail of solid plastic. Jets of smoke demonstrate forni that extended from one side of thethat the flow of air begins to deflect well forward of the leading edges of the two sails hull. Testing then proceeded for a week, both day and night, with runs spacedproviding a detailed comparison withand photographed through underwa- at half-hour intervals to ensure calm exactscale modelsofthe Antiope, ter windows in the sides and bottom of conditions of the tank water. Speed, which are being tested under identicalthe channel [sec illustration on page heel and rudder angle were varied sys- conditions in the smaller towing tanks 65]. The flow studies revealed a rather tematically, and some tests were dupli-at the Massachusetts Institute of Tech-regular, unseparated flow over most of cated with small brass studs fastenednology, the Stevens Institute of Tech-the hull for the practical range of sail- near the leading edge of the keel tonology and other laboratories in thising conditions. Significant separation of stimulate turbulence. These tests pro-country and abroad. The resulting ex-flow did not occur until the yaw angle duced 100 sets of data, which were act comparison will undoubtedly leadof the hull was increased to 10 degrees, automatically converted to digital fornito detailed refinements in future test far beyond the angles normally encoun- and reduced on a computer to give for procedures. tered in sailing. ALjs extreme yaw each test condition the magnitude of the characteristic of normal sailing drag and sidewise forces and the center the Antiope was at the David con. itions, considerable cross flow wis of the sidewise force. WhileTaylor Model Basin she was alsoobserved at the bottom of the keel. The results obtained with the Anti-studied in another large test facility: a This flow is indicative of the "tivor- ope are substantially the same as pre- flow-observation channel. In this facility tex" associated with all lifting surfaces. dictions based on testing small-scalethe hull is held stationary while the vortex is sometimes visible at models of similar hulls in the 5.5-meter water is made to flow at specified veloci- the wing tip of an airplane when the class, and a preliminary comparisonties, in a manner analogous to the testing reduced pressure at the core of the shows no sign of drastic scale effects on of airplanes in a wind tunnel. Strips ofvortex causes the condensation of wa- either the resistance or the sidewiseyarn were fastened to the hull to show ter vapor. force. Now in progress is the step ofthe flow pattern, which was observed Parallel to the research on yacht hulls, 67, XACHT A)VTIOPE undergoes tests in the to%sirlg tank of the U.S. been inìutated in to ways: the hull has been given a "beeF o Navy's David Taylor Model Basin near Washington. Above is a starboard partly by means of weights in a box on the platform at general view of the hull in the 1,700-foot tank; below, a closer view midships, and the bow has been offset toward the camera to simu- of a test in progress. In the close.up actual -ai1ing conditions have late yaw angle. Several sensing devices are attached to the hull. PoI

Polaroid hasa new way to get peoplein focus We used a little anthropology. This is our Model 04. lt's the Color ject (whose dimensions are known) as it reference measure became the over- Pack Camera that costs under $60. appears in your field of view. A stadim- age adult's head. lt works with any- Which makes it less than half the price eier becomes a focusing device when bodyexceptchildrenunder fhre of the original model. How did we do it's linked to a cameras lens mount. (with them you measure from the tof it? By inventing The Peoplefinder, for Once we decided that the stadim-of the head instead of from the hair one thing. eter would make a comeback in our line). There were a number of problems 04, the next question was What do How do you take this measure wher designing a camera for everybody. we use for a reference object?lt had you take a picture?It's very easy We had ta devise economies down the to be something of constant size and When you look through the rangefinde line. And still come up with a fine in- something people would want in the window (its also the viewfinder wfr st ru ment. picture. (One of the old stadimeters dow) you see two horizontal lines. Thi One of the crucial problems was the used flagpoles which didn't exactly move the bottom line up and down b focusing system. A system like the one meet our requirements.) The ideal an- pushing the focusing buttons back an< on the original Color Pack Cameroswer for us was to use people because forth. When you get your subject o superimposed-image rangefinder-people mostly take pictures of people. head between the lines, you shoot. was just too costly. The04 had to But how? We couldn't use the whole Of course, an anthropological rei have something a lot simpler, yet accu- body for two reasons. People come in erence wont work when your subjec rate and easy to ute. all sizes. And close-ups would be im-isn't a person. Then you use the d The Peoplefinder was our unconven- possible. tance scale and the arrow at the left c tional answer. This focusing system is This is where a little anthropologythe finder window, estimate your di actually a very old idea combined with came in. We found that peoples head tance and move the arrow to the esti some new ones of our own. Basically, dimensions, from hairline to chin, known mated range with the focusing button5 its a stadimeter: a type of rangefind- osthe Crinion-Gnathion dimension, lt takes a couple of seconds eithe ing device that's been around sincedon't vary much. In fact, there's less way. And 60 seconds later you have 890 and scarcely used in modern than two inches difference between a color picture. In perfect focus. cameras. tots head and a mans head. So, our A stadimeter measures distances by Polaroid Corporatior measuring the size of a reference ob- Cambridge, Massochusetts