Evening Republican, Volume 21, Number 64, Rensselaer, Jasper County, 17 March 1917 — Tiny Warships Predict How Big Ones Will Speed [ARTICLE+ILLUSTRATION]

Tiny Warships Predict How Big Ones Will Speed

On the water of the model experiment basin at the PUashingt(fn~navyyard each new vessel of Uncle Sam’s fleets is tested before construction by means of.toy reproduction

~~~—— OU HAVE taken a patriotic pride in the sliced performances of American fight- - ing ships, and if you have followed these showings closely you hav<* no *: doubt been impressed with the steady . jBJL advance made. •Each year naval ves- . - seis have shown consistent improvement in speed, seaworthiness and engineering economy. but have you ever paused to ask how these results are primarily made possible'? This is the direct fruit of the model experimental basin.

The model tank, as it is generally called for brevity’s sake, is the establishment in which small models of prospective ships are made and tried on a reduced scale. When congress was asked to appropriate money for the plant which is now an interesting and important department of the navy yard at Washington, the proposition was looked upon as involving an expensive toy. Since then, however, the work done there preliminary to the building of warships has.represented a saving of many hundreds of thousands of dollars, besides Insuring the designed performance of the man-of-war.

Tills Is a not her exampleofthe old rule of thumb being supplanted by the certainties of science. Without going too far into the history of things, it i* sufficient to know that an Englishman, Dr. William Froude, discovered ffack in the seventies that there was a comparable relation between the power required to draw a small model through the water and the motive energy needed to propel a big craft of similar form. The principle which he discovered later became known as Froude’s law or the law of comparison. His studies were first made with miniatures and then verified by <he towing of a Ihrgq,vessel formed on Identical lines. Wltlun “the last twenty years there have been created and developed many vessels of novel types and the results obtained would have been virtually out of the question but for the model tank. Millions of dollars would have been wasted in blind groping instead of obtaining positive assurance of what the finished vessel would do before a single cent was spent in her building. dne illustration will suffice. Among the vessels in the naval review is the gunboat Nashville, designed in 1894. She had a displacement of about 1,375 tons and, with 2,500 horse power, makes 16 knots. Last year the model tank developed the hull form for the gunboat Sacramento, a vessel of 300 tons more displacement, but of a speed of 10 knots upon a development of only 2,000 horse power. This' saving of 500 horse power, calculated nt S6O a horse power, meant an economy JnJttrst cost of machinery of $30,000; but it also meant that the larger gunboat could be driven at a speed equal to her smaller sister ship. Upon 300 tons Thebigger Sacramento could carry a more powerful armament, go farther upon the same supply of fuel and be more weatherly in storiny seas; in brief, be a much better unit of the fleet. And all of this was made certain by the expenditure of a’ few hundred dollars for wooderf models and their testing, the lines of the hull being altered during these tests until the best length and shape were obtained. All of the models tested at Washington are of a uniform length of 20 feet, and the purpose of making them so large is to reduce the possible error in translating the performance figures of the model into those of the full-sized vessel. These models are made of wood and are fashioned in a machine especially designed for this work. Next the model is painted and carefully varnished, tn order to obtain a very smooth surface; and with tills done various waterlines are marked upon the white paint, so that instantaneous photographs taken during the towing tests will show just how and where the waves and the hollows produced by the miniature in motion are developed. When the model is ready it is carried to the basin and there loaded in the balancing tank with bags of shot until it has the right weight or displacement and proper trim or poise upon the water. With this done it is ready to be attached to the towing carriage and tested. . The towing carriage is a sort of a mobile bridge which straddles the main tank and it is driven by electricity, the different speeds being under very nice control. Upon the towing carriage are all of the operative switches and measuring instruments ■and also the recording mechahism which marks the speed of the model and its pull or resistance when drawn through the water. The alm is to obtain a ship form which will show the least pull or resistance at the desired maximum velocity. The _ actual speed of the model is a mathematical ratio o f rim relation between this miniature and tfiefullslzed ship, and therefore the model does not travel fast, if you consider what the big craft will really do. ' * • '■ '. ■ The towing basin is 370 feet long and 43 feet wide, and the maximum speed of the carriage is relatively far in excess of the probable speeds of anything but freak crafts. , J After a model of satisfactory form has been developed and tested, then comes the further task of making the figures of the trials applicable to ■the Intended ship. . Now there is just one part of this work which does not follow Fronde's law of comparison. This !• the factor of the friction set up between the

water and the wetted surface of the vessel’s underbody. This resistance follows a law of its own and it is necessary to tow another model which consists of a thin plate just as long as the -small craft and with a submerged surface exactly equal to that of the model. From these two tests, that of the plane and that of the model, the designer has the information he wants and this information covers various trials over a wide range of speeds, so that the naval architect knows both the maximum and the cruising speeds at which the vessel can be propelled most economically. The cruising speed is an important one, because it is at this ratio of travel that ships of war go most of the time. Full speed is really a battle reserve or something to be called for only when urgency demands. In this particular, fighting ships differ radically-from the ocean greyhound of commerce. But don’t think that the naval designer has an easy ttfsk even with the model tank at his dis-

posal. The speed trials of the full-sized ships are not less Important to him because they give him a check upon his model work. This check is very necessary inasmuch as it enables the designer to bridge over the gap between his model and the real vessel year by year with more exactness. The builder of the hull structure, the naval architect or naval constructor, has only a share in the final product. The naval engineer must take up the problem where his brother ends. That is to say, the engineer knows just what energy his propellers must exert effectively against the water in order to force the ship along at her several speeds. He, too, must work from the outside of the ship inward up to a point. Thfe position of the propeller in relation to the hull and the very form of the propeller must be suited to the particular craft in question. These can be tried in miniature in association with the model for the determination of some data, but after that- the engineer draw upon experience and his carefully tabulated records of other performances. —The average annual expenditures for the maintenance of the model basin at Washington are under ?25,000, and this is a trifling sum compared with the savings which have been effected through careful designing. In the case of the three scout cruisers, Birmingham, Chester and Salem, by merely lengthening their hulls it was found possible to save in those three ships a combined total of 17,000 horse power over that required for the original design. Figured at S6O a horse power, this represented an economy of machinery cost amounting to $1,020,000. The model experimental basin is no longer looked upon as an Investment of doubtful value; every first-class naval power has one of these establishments, and like that at Washington most of them lend their aid to the merchant marine as well. Such is the development of private experiments which Dr. William Froude began, so to speak, in his own backyard. All of the successful ships of today are Indebted to his pioneer work.