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DESIGN OF THE SUBMARINE TORPEDO BOAT 73
be higher in point of revolutions than the engines in order
to make up for the loss of the increased slip submerged.
The reciprocating engine is generating more power at low
speeds than is absorbed by the propellers; this is due to
the translation of straight line motion into rotary motion.
Therefore it would seem that the best practice would be to
design the motor with correspondingly lower speeds for
equal power than the engine in the range below half load
and with higher speeds than the engine in the range above
half load.
On Chart II are shown typical speed-power curves for
internal combustion engines, motors and E.H.P. The
existing relations are clearly defined.
The formulae to be used in the computations are:
Speed in feet per min. = SX 101.33
. . . S X 1O1.33
Pitch X revolutions = PX R =
S = Speed in knots per hour.
C = thrust deduction factor for standard B.C. cor-
responding to design.
P.C.1= propulsive coefficient standard corresponding
to P.A.+ D.A.
E.II.P.
Estimated I.H.P. for speed S = I.H.P.= C—-
1.0.1
. . , „ ,/291.8 X I.II.P. X C.2
Diameter of propeller = V — -
Y P. X R. X I.T.
P.C.= actual propulsive coefficient =2
P. X T.S. x D. x I.T.d
I.H.P. = -----------------------
916.7 x C2
I.T.d= indicated thrust per square inch of disc area.