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7°
THE SUBMARINE TORPEDO BOAT
remainder added to the E.H.P. given by the model test.
Assuming for the proposed design a beam-length ratio of
.84, take 8.5 per cent as the appendage resistance and the
total E.H.P. becomes
640 X.085 +640 = 695 E.H.P.
The hull efficiency or the thrust deduction factor as it
is called, is the next factor to be considered in computing
the propulsive efficiency. This factor is evidenced by
the difference in resistance shown when towing a model
with no propellors behind and when towing the same model
at the same speed with propellors of the same proportion
to the ship’s screw as the model is to the ship working
behind at a rotary speed such as to give a thrust equal to
the resistance of the model. With the propellers working
behind the resistance is found to be increased a definite
amount, termed thrust deducton, and is caused by the
suction influence of the propellers upon the after part of
the ship. This suction influence extends far enough
forward of the screws to cause a marked diminution of the
pressure against the after part of the ship thereby causing
a virtual increase in resistance. The propellers must
therefore exert a thrust equal to this resistance.
* This thrust deduction factor has been determined by
numerous experiments and is found to vary directly with
the value of the block coefficient of a model having a stand-
ard set of lines, and varies from 1 with a block coefficient
of .5, to 1.6 with B.C. = .9. The curve from 1.17 the value
of the thrust deduction factor corresponding to a block
coefficient of .69, is found to be flat all the way up to 1.6
* The Dyson Method. See “Design of Screw Propellers” by Capt.
C. W. Dyson, U. S. N.