Experimental Results.—Some records of the experiments made at the various tanks have appeared from time to time; a few of those made at the Admiralty tanks have been published through the I.N.A. The "Greyhound" experiments have already been mentioned ; other papers of fundamental importance to which reference can be made are those recording the "Merkara" results 1876; experiments on the effect produced on wave-making resist ance by varying the length of parallel middle body, 1877 ; papers in 1888 and 1892 on the "constant" system of notation of results of model experiments used at the Admiralty Experimental Works; and some results of a systematic series of model experiments by R. E. Froude in The rapid growth in numbers, size and speed of merchant ships in the present century led to the establishment of the Teddington Tank. Many valuable papers dealing with the resistance, form and other properties of merchant ship types are recorded by G. S. Baker and his staff in the Trans. I.N.A. and other societies. Other experimenters in this country and abroad have also published valuable results. The experiments made cover a wide field and include not only resistance and propulsion but such matters as manoeuvring, rolling and pitching; and in general give guidance on a number of special points in design, information regarding which could only be obtained on full scale with great expense and labour.
When the resistance has been obtained from a model experi ment, or deduced by the law of comparison from data obtained with a vessel of similar type, the effective horse-power required is known; and by adopting a suitable value for the propulsive coef ficient based on the model screw propeller efficiency and other factors the indicated or shaft horse-power is determined. If model experiments or data for exactly similar ships are not available, one approximate method of estimating the power which is com monly used is expressed by the f ormula C being called the Admiralty coefficient. The value of C varies considerably and care must be taken that the values selected for comparison are appropriate to ships of similar type and of corre sponding lengths and speeds and also type of machinery.
Another method of obtaining approximate estimates of the power required for ships of ordinary types is from curves of re sistance drawn on a base of simple functions of the speed, length and displacement, the curves being passed through the spots obtained from a large number of results of model experiments with different classes of ships. Curves of this character have been con structed by Admiral D. W. Taylor, U.S.N. and A. W. Johns (see Trans. I.N.A. 1907). By the use of • such curves the residuary resistance, or residuary horse power is obtained ; the frictional resistance being calculated independently.
Apart from model experiments on the actual design under consideration the most serviceable method of estimating power, and one capable of very general application is to use the results of a methodical series of model experiments. These consist of a series of experiments on systematic variants of a parent form or forms. The first of such a series was carried out by R. E. Froude, and the results published in Trans. I.N.A. 19°4. Subsequently
Taylor in his book "Speed and Power of Ships," and G. S. Baker in papers in Trans. I.N.A. 1913 and later, greatly increased the range of type ship. The curves thus obtained, used with discre tion, are most valuable to a designer of all types of ships. Refer ence can be made to the records referred to for very interesting details and particulars of various sizes and forms of ships. The value of such curves is greatly enhanced by the fact established experimentally that the resistance of a ship of given length and displacement depends primarily on the shape of the curve of transverse areas, the maximum beam and the shape of the water line. If these features are maintained the shape of section, profile and other features can be varied over wide limits without appreciable change of resistance.
In addition to the foregoing considerations, which relate to the shape of the ship, the restricted nature of the flow also affects the performance of the propeller, often prejudicing its efficiency; further the speed of rotation of the shaft and consequently the power exerted may be affected, resulting in an increase in the loss of speed. In general, shallowness of water causes a reduction in speed, but for high speed ships such as torpedo boat destroyers, there may be a reduction in resistance at top speeds due to the change in wave formation, and as a result on certain shallow water speed trial courses there is an appreciable increase in the obtainable speed. The phenomenon is so complex that recourse must be made to experiment data to estimate the quantitative effect in any specific case. Model experiments have proved most useful in this respect and results of experiments covering a large range of conditions are available. Some results of trials of actual ships in various depths of water have also been published and reference should be made to papers in Trans. I.N.A. and other shipbuilding societies for further information.