128 Transactions of the Royal Canadian Institute 



Law of Comparison requires that at corresponding speeds of ship and 

 model, the pressures of the fluid around ship and model should be in the 

 ratio of the linear dimensions of the models. This is substantially 

 fulfilled when we tow a model through a model tank. The pressures of 

 water are proportional to the linear dimensions, and the atmospheric 

 pressure, which is not proportional to the linear dimensions, being 

 constant for model and ship, has no effect upon the resulting phenomena. 

 In fact, with minor exceptions, the disturbances set up by model and 

 ship in the water are the same as if there were no air pressure, as if 

 model and ship were operating in a vacuum. When we come to a 

 propeller, however, the case is different. This operates completely sub- 

 merged, and the pressure of the air being equivalent to some 33 feet of 

 water, affects the model propeller differently from its effect upon the 

 operation of the full-size propeller. The practical result is that the full- 

 size propeller experiences "cavitation," as it is called, while the model 

 propeller at corresponding speed does not show cavitation. Cavitation 

 is the name that we give to the condition where the water around a pro- 

 pellor operating at high speed, under high pressure, is unable to follow the 

 blades which tear holes or cavities in the water, resulting in a rapid and 

 pronounced loss of efficiency. If one takes a stick and draws it rapidly 

 through the water of a stream, it will be observed that the water is unable 

 to follow closely the stick which leaves a cavity behind it. The cavitation 

 of a propeller is somewhat the same kind of phenomenon. If we could 

 test a model in a vacuum or partial vacuum, we could fully reproduce 

 the phenomenon of cavitation on the model. We can also place a model 

 propeller in somewhat the required condition by testing it in hot water. 

 There are obvious difficulties in such experiments, however, and in 

 dealing with propellers we are compelled to restrict the application of 

 the Law of Comparison to conditions where cavitation does not appear 

 in the full-size propeller. Incidentally this covers the practical field, 

 since we of course avoid having propellers which show cavitation with the 

 resulting loss of efficiency. Unfortunately our knowledge of the border 

 line between cavitating and non-cavitating propellers is not exact, the 

 cases of cavitation of full-sized propellers being always unintentional. 



I need not enlarge upon the obvious value of the tool which Froude 

 supplied the naval architect. We have here a most beautiful illustration 

 of scientific methods applied in practice. We do not have to build a 

 ship or resort to elaborate calculations to determine its resistance. We 

 can build a small model ten or twenty feet, and months or even years 

 before the ship is finished, from the tests of this model, estimate with 

 great accuracy the performance of the ship. But the outstanding value 

 of the method is the fact that we can quickiy and cheaply modify our 



