High Speed Displacement-Type Hulls 585 
an attempt to obtain further results is being made by the Ship Division, NPL. For this 
purpose two portable torque meters, of strain gauge type, have been designed and construc- 
ted at NPL to fit propeller shafts having diameters from 1-1/4 inches to 6 inches. A 
description of the first of these is given in Appendix A. 
Using this torque meter, good trial results have recently been obtained for two twin- 
screw launches, one 7] feet in length and the other 51 feet. Resistance experiments were 
made at NPL with both forms, and propulsion experiments with the model of the smaller 
vessel. 
The results of the trials with the 71-foot launch are given in Fig. 21; this shows the 
measured power for one shaft (only one torque meter was available) and also calculated 
DIMENSIONS OF VESSEL 
71'BP X 19-75 BMX5-42 DRAFT X 70 TONS 
MEASURED TRIAL POWER 
SHAFT HORSE POWER 
KNOTS 
Fig. 21. Results of trials with a 71-foot round-bilge launch in a water 
depth of 50 feet 
values of the power absorbed by the propeller for assumed wake fractions wy of 0, 0.05, and 
0.10. Closest agreement between measured and calculated power occurs for wy = 0.05; the 
estimated value was 0.03. The measured power at the highest speed is greater than the 
absorbed power calculated for ws = 0.05; this may have been caused by a 2-1./2 percent 
difference in the shaft revolutions. Had both shafts run at the higher speed of that with the 
torque meter, the total power, and consequently the speed, would have been slightly higher. 
In this case ws = 0.05 would have given close agreement at the highest speed. 
