It is in solving the above kind of practical problem that Mr. Froude’s methods of experimenting with models in a tank are of such immense value. The first thing that the naval architect does in designing a ship is to prepare a series of drawings, showing the form of the hull of the vessel. From these drawings a model is constructed exactly to scale. In England, following Mr. Froude’s practice, these models are usually made of paraffin wax, about 12 or 14 feet long and 1 inch in thickness. In the United States wood is used. These models are constructed with elaborate care and by the aid of special machinery, and are generally 10 or 12 feet in length, and some proper fraction of the length of the real vessel they represent. The models are then placed in a tank and experiments are made, the object of which is to ascertain the force or “pull” required to drag the model through the water at various speeds.
The tank belonging to the British Admiralty is at Haslar, Gosport, near Portsmouth, and the experiments are now conducted there by Mr. E. Edmund Froude, who continues the scientific work and investigations of his distinguished father, Mr. William Froude. This Admiralty tank at Haslar is 400 feet in length. The well-known firm of shipbuilders, Messrs. Denny Bros., of Dumbarton, Scotland, have also a private experimental tank of the same kind. The Government of the United States of America have a similar tank at Washington, the Italian Government have one at Spezzia, and the Russian Admiralty has also made one. These tanks resemble large swimming-baths, which are roofed over ([see Fig. 40]).
Fig. 40.—An experimental tank for testing ship models (Washington). [18]
Over the water-surface is arranged a pair of rails, on which runs a light carriage or platform. This carriage is drawn along by a rope attached to a steam-engine, which moves at a very uniform rate, and its speed can be exactly ascertained and automatically recorded. This moving carriage has a rod or lever depending from it, to which the model ship is attached. The pull on this rod is exactly registered on a moving strip of paper by very delicate recording mechanism. The experiment is conducted by placing the model at one end of the tank, and taking a run at known and constant speed to the other end. The experimentalist is thus able to discover the total resistance which it is necessary to overcome in pushing the model ship at a certain known speed through the water. The immersed surface of the model being measured and the necessary calculations made, he can then deduct from the total resistance the resistance due to skin friction, and the residue gives the resistance due to wave-making. Suppose, then, that the experiment has been performed with a model of a ship yet to be built, the run being taken at a “corresponding speed.” The observations will give the wave-making resistance of the model, and from Mr. Froude’s second law the wave-making resistance of the real ship is predicted. Adding to this the calculated skin-friction resistance of the real ship, we have the predetermined actual total ship-resistance at the stated speed. For the sake of giving precision to these ideas, it may be well to give an outline of the calculations for a real ship, as given in a pamphlet by Mr. Archibald Denny.[19]
The tank at the Leven shipyard, constructed by Messrs. Denny Bros. for their own experiments, is 300 feet long, 22 feet wide, and 10 feet deep, and contains 1500 tons of fresh water. At each end are two shallower parts which serve as docks for ballasting and trimming models. As an example of the use of the tank in predicting the power required to drive a ship of certain design through the water, Mr. A. Denny gives the following figures: The ship to be built was 240 feet in length, and from the drawings a model was constructed 12 feet in length, or one-twentieth the size.
It was then required to predetermine the power required to drive the ship through the water at a speed of 13¹⁄₂ knots. A knot, be it remarked, is a speed or velocity of 1 nautical mile an hour, or 6080 feet per hour. It will be seen that this is not far from 100 feet per minute.
By Froude’s first law, the corresponding speed for the 12-foot model is therefore—
131/2 × 6080/60 × √12/240 = 306 feet per minute
The model was accordingly dragged through the tank at a speed of nearly 5 feet per second, and, after deducting from the total observed pull the resistance due to the calculated skin friction of the model, it was found that the resistance to the motion of the model at this speed due to wave-making was 1·08 lb. Hence, by Froude’s second law, the wave-making resistance of the ship was predetermined to be—