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68
THE SUBMARINE TORPEDO BOAT
the water at a certain speed is best obtained by the aid of
an experimental model in a towing basin. The method
of reasoning and the determination of resistances by this
means is carried out in the following manner. Suppose
we have a paraffin model 12 feet long constructed on a
scale of i inch to 1 foot and it is desired to find the E.H.P.
for the full sized vessel at a speed of 16 knots. We must
first find the corresponding speed at which the model must
be pulled to give the corresponding tow rope resistance.
The corresponding speed of the model is found by the ratio,
„ 16 X 12 ,
Vm = --------=1.333 knots.
144
When towed at this speed the resistance is measured and
found to be 4.9435 pounds.
The wetted surface of the vessel is 3700 square feet,
therefore the wetted surface of the model is
a 3700 X 122 ,
Sm = s. ---------= 25.7 square leet.
144
The friction factor and the exponent given in Froude’s
Tables (see Table 1) are
/ = .00908
and
1 = 1.04
Therefore the frictional resistance is
.00908 X 25.7 X i.3331M = -4075 lbs.
The friction factor and exponent for the full sized vessel
taken from Table 2 is
/= .00910
and
n = 1.825