The Submarine Torpedo Boat
Its Characteristics And Modern Development

DESIGN OF THE SUBMARINE TORPEDO BOAT 67 way may be realized by considering the greater rudder moment gained and a correspondingly less upsetting mo- ment caused by the force of the water impinging against the top of the hull forward. It will obviously then help to overcome the inherent tendency of the boat to dive at its critical speed. The flow of water to the propellers would also be much more free and the influence of the wake be sensibly decreased. Speed and Power Estimation In estimating the speed and power required for the pro- pulsion of a proposed design the three factors entering into the propulsive efficiency are: the engine efficiency, the propeller efficiency, and the hull efficiency. The propul- sive efficiency is the ratio between the E.H.P. or tow rope horse power, including the resistance due to all appen- dages, and the I.H.P. taken at the cylinders of the engine, and generally averages about 50 per cent of the I.H.P. In actual practice however this value ranges from 42 per cent to 62 per cent, and it becomes necessary to fix this coefficient with some degree of precision in order to obtain any very accurate results. This may be done by assigning to each factor which enters into the composition of the propulsive efficiency a value which experience or experi- ment has shown to be what might be expected in a new problem. The engine efficiency may vary from 75 to 90 per cent according to the type and characteristics of the engine selected and the efficiency may be assumed for the calculations according to past experience with a similar type of engine, or the guaranteed efficiency of the engine builders may be taken. The E.H.P. necessary to drive the bare hull through