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DESIGN OF THE SUBMARINE TORPEDO BOAT 71
the thrust deduction factor corresponding to a block
coefficient of .9. To lind the thrust deduction for a pro-
posed design which departs in form from the standard
set of lines it is necessary to find the slip block coefficient
corresponding to the block coefficient of the standard
form. The slip block coefficient of the new design may be
assumed to equal the standard block coefficient times the
standard midship section coefficient divided by the actual
M.S. coefficient and is the B.C. to be used in determining
the thrust deduction and the propeller calculations.
The total E.II.P. then becomes E.H.P.XC, and the
I.II.P. =
E.II.P. X C.
P.C.
P.C., the propulsive coefficient, is determined by the
propeller computation. To make the propeller computa-
tions it is necessary to know the allowable tip speed and
this is calculated by the formula,
T.S. = 7 D.R.
the number of revolutions per minute and the diameter
having first been selected and being within the proper limi-
tations for the proposed design. It has been found by ex-
periment upon various model screws that the P.A. +D.A.
ratio, to obtain the best performance, varies accordingly
with the T.S., and the P.C. is also found to be dependent
to a great extent upon this ratio.
In making the selection of the propellers we are con-
fronted with a problem of which there is no exact solution.
It must be borne in mind that we have two distinct condi-
tions under which to operate, namely, when on the surface
and when submerged. The resistance submerged is greatly
increased over that for the surface condition because of