SINCE THE END OF THE WAR amateur sailors have been taking to the navigable waters of the world in numbers that rival a migration of lemmings. Some of the newcomers to yachting are bringing-along a strong interest in science. In consequence the aerodynamics of sails and the hydrodynamics of hulls are coming in for increased attention from amateurs. In fact, yachtsmen in England recently organized an Amateur Yacht Research Society, which has a U. S. secretary—Robert Harris of Great Neck, N.Y.

That amateurs can make experiments of great interest to boat builders was pointed out in this department last August by Randolph Ashton, an engineer associated with the Experimental Towing Tank at the Stephens Institute of Technology. Since a towing tank and its associated apparatus for experiments with models is expensive, he urged amateurs to undertake experiments with full-sized boats.

Cyrus Hamlin, a naval architect of Manset, Me., agrees that full-scale tests can provide much needed information and a-lot of fun for the experimenter. He suggests, however, that interesting tests of models can be made without a towing tank.

"Naval architecture," he writes, "may be poorly endowed with funds for research, but I find it heavily endowed with fascination. For me it is a vocation, but even an amateur can, at little financial cost, make important contributions to research into the scientific design of small vessels and boats.

"The radical differences of vessel design and construction, found not only in widely separated areas but within the same port, indicate the sizable area of investigation open to the amateur. For instance, a conventional 85-foot Gloucester dragger (a fishing vessel exactly like a trawler but smaller) is heavily built, full-bodied, and has a beam of from 20 to 22 feet. Yet the lightly built, finebodied World War I subchaser, with a beam of only 15 feet on an over-all length of 110 feet, also is popular as a dragger at Gloucester. Even taking into account the personal (and frequently illogical) preferences of skippers and owners, it seems unlikely that two such widely divergent types can perform the same work equally efficiently.

"An amateur contemplating research in naval architecture might give consideration to three background thoughts. First of all he can hardly expect to compete with the formal establishments in precision of measurements of forces, angles, velocities and so on. There are so many variables involved in vessel testing and design (due largely to the fact that boats operate in two fluids of very different densities, separated by what can be a very obstreperous boundary layer) that small errors of measurement can build up a large accumulated error. Thus a beginner might better concern himself with qualitative rather than quantitative testing and content himself with examining large differences in the characteristics of the structure being tested.

"Secondly, an amateur should not concern himself with relatively slow vessels. The ratio of a boat's speed to its length (conventionally calculated as the velocity in knots divided by the square root of. the waterline length) is a measure of the wave-making characteristics of a hull and is perhaps the most important and fundamental coefficient in naval' architecture. Slow vessels can be defined for our purposes as those whose speed-length ratio at normal travel speeds is one or less than one.

"The length of a water wave, crest to crest, is a rigid function of its speed of advance, whether it is generated by wind, an earthquake or the passage of a vessel. A body traveling through the water generates a wave system which travels with it and hence has the same velocity of advance as the moving body. At a speed-length ratio less than unity, a vessel is supported on three or more of the wave crests it is itself generating. Such a slow vessel remains essentially horizontal. A faster vessel, generating longer waves, is very sensitive to the location of the wave crests near its bow and its stern. At a speed-length ratio of about two, the vessel is trying to climb up the back of the bow wave and its stern is in the trough of the wave. This is a very . difficult position for any but a very light vessel to attain and is ruinously costly to h maintain. At considerably higher values t: of the speed-length ratio the center of gravity of a vessel may get ahead of its bow-wave crest, at which point she begins to coast down the front side of the wave and consequently requires less horsepower. For all practical purposes this condition is possible only for light planing hulls (a planing hull is one whose center of gravity rises, relative to its at-rest position, due to the hydrodynamic lift that is exerted on the hull's bottom) .

"A vessel whose speed-length ratio is around one (e.g., a 100-footer at 10 knots or more, or a 36-footer at six knots or more), generates waves which are about its own length. A vast number of vessels, in all kinds of service, operate in this one-wavelength speed range. These small vessels, because of the high cost of testing relative to their building cost, and the traditional arid individualistic approach of their owners and operators, have not received research attention by any means proportionate to their importance.

"Thirdly, an amateur model experimenter interested in qualitative rather than quantitative results can make his tests in open water under any simulated type of weather he may choose, from flat calm to a gale. Most towing-tank facilities are equipped with wave-making machines, but the waves generated are smooth, regular and quite unlike the breaking and confused seas met in practice. It is my opinion that tests with machine-made waves are valuable only on a rarefied theoretical level, and are of little or no practical value in investigating a particular model. In fact Randy Ashton informs me that the Stevens Towing Tank is installing equipment which will generate confused storm seas in reproducible patterns and permit testing models at various angles of attack. After all, although simple resistance to forward motion is of considerable importance, for a small vessel it is perhaps more important that she be able to operate efficiently and obediently under adverse. conditions, and in the final. analysis be able to carry her crew and cargo safely through the worst conditions she might possibly meet. Alan Villiers' new book, Posted Missing, is a moving account of a disaster which emphasizes the need for improvement in design. I recall a shocking experience that brought this home to me: a few hours after listening to the skipper of a dragger. tell his wife over the radiotelephone that he was encountering rough weather but expected to be home the next day, I heard the bad news that the vessel had rolled over and was lost with all hands.

"An amateur taking up model-testing will find an endless variety of fields to tackle and ample scope for the most vivid imagination. Besides the conventional types of power-driven and sailing craft, there are now many innovations to investigate: hydrofoils which lift the hull partly or completely out of the water, boats with more than one hull (such as catamarans), jet propulsion, improved propellers, wing sails, new construction materials and methods, and so on.

"I believe that sailboats offer one of the most profitable areas of investigation. Although it may seem, as Ashton wrote, that the days of commercial sailing ships are gone forever, still a return of wind-driven marine commerce is not by any means unthinkable. Nearly all present water-borne travel depends upon fossil fuels. Depletion of these fuels may make them prohibitively costly before long. Nuclear reactors are so expensive and bulky that their use will probably be limited to large or specialized vessels.

"The one source of power that will remain unchanging and always available is the sun. This power manifests itself in the form of wind, which can be used day and night. The sailing craft of the future, setting an arrangement of wing-sails, or their yet-to-be-discovered counterpart, will divert the wind's force for the production of forward motion and do it without the necessity of complex machinery. Such a vessel, with a small auxiliary engine for negotiating calms and narrow waters, and a small crew assisted by power winches, should be able to compete favorably, both in speed and economy of operation, with fully powered craft. It is here, in the improvement and development of efficient sailing hulls and rigs, that I believe amateur experimenters will find the most imaginative, challenging and rewarding field for their energy.

"Model-testing procedures and instrumentation present many problems of the difficult kind that amateurs seem to delight in solving. I venture to set forth a few suggestions. The model should be at least three feet in length over all; and it should be finished to accurate and uniformly smooth lines, without humps or hollows. The simplest form of test is to tow a model by a string from a power boat. The towing string should represent an extension of the propeller shaft line. The towing point must be rigged out to one side so that the model is clear of the wake of the power boat. The speed of the vessel can be estimated by judging the wavelength of the wake generated by the model or the power boat: the velocity, in knots, is 1.34 times the square root of the length of the wave. Such a test can give a good indication of the qualities of the model's motion, its directional stability, the formation of waves and other properties of the motion, in smooth water and at various angles of attack to waves through a range of sizes.

"The experimenter can refine his results by incorporating a dynamometer in the tow line, and by adding a vertical turbulence-inducing wire just ahead of the model but not attached to it. The total resistance, or drag, of the model can then be measured. The precision of this measurement will depend upon the accuracy of form and finish of the model, the sensitivity of the dynamometer and the accuracy of the speed estimate. From the total resistance of the model you subtract its computed frictional resistance. You then expand the remaining figure to the dimensions of the full-sized ship and now add the computed frictional resistance of the ship. The result is the total resistance of the ship for the speed-length ratio at which the model was tested.

"A simpler test is to compare the model's performance with that of another model of the same size and type whose characteristics are known. You can do this by attaching both models to the opposite ends-of a pivoted and balanced yoke and towing the two together. You should hook up a spring, or locate the pivot point ahead of the towing points, so that as the difference between the drags of the two models increases, the resistance of the yoke to turning also will increase.

"Sailing-hull testing, as Ashton made so clear, is an extremely complex and difficult problem and should be approached with some trepidation. To help clarify my own thinking, I once began to make a list of all the factors and relationships affecting sailboat performance. When my list had grown to more than 40 items, I gave up, lest I discourage myself into becoming a chicken farmer.

"The only logical plan for this method of testing sailboat models is to tow the model in the same way the wind drives it, i.e., from the center of effort of the -sails along the line of the resultant wind force. Although there are many unsolved practical questions connected with this method of testing, it seems reasonable.

"One area of ignorance which the amateur can help illuminate is the question of sailboat rigs. We have only spotty data on forces, pressures, effects of sail shapes, flow lines and the interrelationships of various sail combinations. I have always been intrigued by the possibilities of recording sail shapes stereoscopically by two cameras spaced on a wide baseline. Scale-model tests of rigs are good subject matter for an experimenter with access to a low-velocity wind tunnel. I would like to see some ambitious amateur look into the practicality of examining rigs, si4gly and in groups, in a smoke tunnel and analyzing the results by the streamline method set forth in "The Amateur Scientist" for May, 1955, and October, 1955.

"Finally, an amateur can make important contributions by experimenting with and observing full-sized boats. Ashton: recently had this to say on the subject: 'Designers and model-testing establishments continually deplore the lack of full-size observations of sufficient accuracy to show definitely the refutation or verification of their ideas and predictions. A week spent on a fishing vessel, measuring periods of pitch and roll, horsepower, steering characteristics and similar matters under various conditions of loading would be a memorable experience.

"A great deal of knowledge, as well as satisfaction, is to be derived from model testing. A beginner, towing his model with a length of butcher's string, can turn up-as much provocative information as a professional, with his fine models and intricate, sensitive equipment."

Ernest Hunter Wright, the retired former head of the Department of English at Columbia University, is baffled by an odd phenomenon which came to his attention through chance observation.

"With the luck of a layman'" he writes, "I have had the novel experience of seeing several of the men who plucked the heart out of the atom's mystery scratch their heads in vain for the solution of a problem which I now submit to a wider audience.

"We all think we know what happens when we skip a stone across a water surface. We believe that it bounces over the water in a series of successively shorter leaps until it finally stops and sinks. Of course the number and the length of the leaps vary with the smoothness of the water, the size and shape of the stone, the speed and skill of the throw, and other conditions, but by and large the missile seems to act about as I have said.

"I am fairly sure it does no such thing. I think a stone does not behave on water in the way described, because I know it does not on sand–the hard, wet sand at the water's edge. So first let me tell you what the stone does on the sand, and how I came to know it.

"I found it out in the course of a long walk along the beach. I had been skipping pebbles over the water. (By the way, I wish someone could tell me precisely why beach pebbles are always flattened rather than spherical–no geologist has given me a satisfactory answer. ) Because the water rolling into the beach was too rough for good performance, I decided to see how a pebble would behave on the hard, wet sand. On this surface, of course, it would leave little marks recording its travel.

"When I saw the marks left in the sand by my first pebble, I think I must have been as astonished as old Crusoe on beholding the first footprint of his man Friday! The first bounce of the pebble was only four inches long; the next was nearly seven feet; then came another short hop of only four inches; then a leap of about five feet; then again the four-inch hop, and so on for seven big hops punctuated by the four-inch ones. Each short skip was unmistakably recorded by two neat little marks in the sand. After the seventh repetition the pebble ceased this strange behavior and merely jumped along with successively shorter strides until it stopped. The total number of hops was about 20–my average on hard sand.

"I kept skipping pebbles all the afternoon, for mile after mile along the beach. I tried them in all shapes and sizes, over every contour of terrain that I could find, at every angle at which it was possible to launch them. I tried all the variations I could think of, and I went back a second day and tried them all again. With never an exception the result was, just about the same.

"Now I fancy the same thing happens on the water, though in the water there is no imprint left to tell the story. A proper record with a camera would give us the answer. In the sand, at least, the story is quite clear, and a very pretty story it is–as pretty as the tracks of some little animal in the fresh snow.

"As yet I have no explanation for these facts. I have put the problem before several physicists of high distinction, but so s; far have received no answer in return. Somewhere there must be an answer for my little riddle. Who will find it?

"I may save some trouble if I say that two or three suggestions have already been tested and found wanting. One was that each of the double marks is the result of the pebble's turning over when it strikes the sand. I can imagine no reason why the pebble should turn over, and at all events I can certify from scrutiny that the stone does not turn over– and so can the companions who have watched it with me.

"Another suggestion was that the pebble, striking the sand at a tilt, might hit h with its rear end first, do a little flop and strike with its front end before taking off for the long leap. But this supposes a precision of timing and a uniformity of tilt at each landing which are beyond all credence, and besides, why would a big stone make its flop within the same space (four inches) as a tiny one? As a matter of fact the marks in the sand show that the stone usually strikes the sand flat, and all the observers agree with me that there is no flop.

"The only other suggestion so far is that the pebble's spin around its vertical h axis may account for its strange action. Why such a spin should make it behave the way it does is not clear to me, but we do not need to labor the question, because I have thrown pebbles without any spin (it can be done) and they all left the same mincing steps in the sand.

"So what scientist, professional or amateur, wants to go down to the beach with all the needful instruments and find the answer to my riddle? I shall be glad to go along if I am wanted; I can throw pretty well."

Bibliography

PRINCIPLES OF NAVAL ARCHITECTURE. Edited by Henry E. Rossell and Lawrence B. Chapman. The Society of Naval Architects and Marine Engineers, 1939.

WIND WAVES AT SEA–BREAKERS AND SURF. Henry B. Bigelow and W. T. Edmondson. Hydrographic Office Publication 602, U. S. Navy Department, 1947.

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