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72
THE SUBMARINE TORPEDO BOAT
the increased volume of displacement and wetted surface.
This means that the indicated thrust per square inch of
disc area is increased and consequently the slip is made
much greater. To overcome this it would be necessary to
increase the area of the blade in order to obtain a greater
projected area otherwise a greater projected area must be
obtained by altering the pitch and turning the screw up
to a greater number of revolutions to get the speed.
Changeable pitch propellers of the size used on sub-
marines have never been found to give satisfaction and in
fact are usually less efficient in both conditions than a
single fixed propeller would be. We are forced then to
decide which condition, whether the surface or submerged,
we wish to favor. The best method is to select a com-
promise propeller to fit as nearly as possible the two
conditions.
This may be done by designing separately a propeller
to meet each condition and then effecting a compromise
between them by taking an intermediate pitch for the final
propeller. It will always be found to be good policy to
allow an excess of area to favor the deficient side. I he
final adjustment of the blades should not be made until
after a number of trials.
A further consideration to be taken in designing the pro-
pellers, is the wide difference in the speed-power curves of
the internal combustion engines, the motors and the char-
acteristic E.H.P. curve for various speeds. The R.P.M.
power curve of the reciprocating engine is high in point of
power at low speeds and is very nearly a straight line, while
the E.H.P. curve is low in point of power at low speeds
and comparatively higher at high speeds than the engine.
The corresponding speed-load curve of the motors must