586 W. J. Marwood and A. Silverleaf 
The full-scale power was estimated directly from the model results, taking 7p = 0.99, 
7, = 0.65, 7, = 0.83, and appendage resistance of 0.20 © corresponding to a factor of 
0.10. This gave a delivered horsepower (dhp) equal to 118 for a speed of 10.77 knots; wind 
resistance estimated from measured velocities increased this by 3 percent to dhp = 121.5. 
The measured power was 138; the correlation factor (ratio of measured to estimated power) 
is thus 1.135, slightly higher than the current NPL value 1.10 for this type of vessel. Part 
of the discrepancy may be due to a small difference in trim between model and full scale. 
These trials were run in water of depth about 50 feet, considered sufficient not to introduce 
any depth effect. 
The trials with the 51-foot launch were run in water of depth 20 feet or less while the 
model experiments were made in water of 45-foot equivalent depth. A possible shallow 
water effect thus confuses the ship-model comparison; Fig. 22 shows that the greatest 
discrepancy between measured and predicted powers occurs at speeds close to the critical 
speed for water of depth 17 feet. In this trial also only one torque meter was fitted, but it 
was here assumed that both propellers absorbed equal powers. A power estimate based on 
250 
POWER CALCULATED FROM 
GAWN CHARTS USING “SeN 
ipa MODEL 
200 EXPERIMENTS. 
fore sir (10 CORRELATION FACTOR. 
{50 
SHAFT HORSE POWER 
{OO 
DIMENSIONS OF VESSEL 
5\‘BP x (3-3 BM X 3-42’ DRAFT X 21-5 TONS 
10 if {2 3 18 
KNOTS 
Fig. 22. Results of trials with a 51-foot round-bilge launch in a water depth 
of 20 feet or less 
