ONR Hydrofoil Boat XCH-6 221 
Thrust available is dependent upon the size of the shear pin located in the horizontal 
transmission shafting, aft of the engine gear box. These pins are hollow. The choice of 
inside diameter and pin material alloy allows selection of maximum transmission and pro- 
peller torque. This, in turn, determines available propeller thrust at each forward speed. 
Normal vehicle operation utilizes a shear pin which is designed to fail when propeller 
torque reaches 2100 inch-pounds. 
Component drag prediction for operation in smooth water has been based upon methods 
outlined in Refs. 4 and 5. Such prediction appears to be in good agreement with test results 
obtained to date. The total smooth water drag values given in Fig. 8 can be considered to 
correspond to normal operation, but the low-speed region can be altered depending upon pilot 
adjustment of tail foil incidence in pitch with respect to the keel. Minimum takeoff speed, 
the speed at which the keel just clears the smooth water surface, is approximately 20.0 
knots at a gross weight of 2550 pounds. 
The propeller was designed to develop 774.0 pounds of thrust at a speed of 62.0 knots 
and at the maximum rotative speed of 6000 rpm. Predicted propeller efficiency, under these 
conditions, is 73.9 percent. Associated propeller torque is 2100 inch-pounds. 
For operation in smooth water at 60.0 knots, the total drag is estimated to be 660.0 
pounds. Only 20.0 percent of this value corresponds to drag due to lift; 8.0 percent is 
attributed to spray drag; 16.0 percent is due to air drag; the remaining 56.0 percent is due to 
friction plus pressure drag of wetted elements. This distribution of drag is important when 
considering vehicles of much larger size with the same cruise speed. Larger craft can be 
expected to incorporate suitable aerodynamic streamlining, not employed on the XCH~, and 
will benefit from considerably reduced friction drag coefficients associated with larger 
chords and consequently higher Reynolds numbers. 
INITIAL TRIALS 
Several features of the present configuration have resulted from initial testing. 
The nylon liner between the propeller shaft and the propeller hub was provided after an 
early run wherein the propeller struck a piece of floating debris. The transmission was pro- 
tected by a shear pin in the drive shaft for conditions of excessive torque from the engine. 
When excessive torque was suddenly applied from the propeller, however, the inertia of 
transmission components allowed the bevel gears within the pod to feel the overload prior to 
shear-pin failure. The result was a tooth failure on one of the bevel gears. Damage to the 
propeller, consisting of a few nicks on blade leading edges, was very slight and easily 
corrected by filing. - 
A reduction in drag, associated with forced air ventilation, was first established by 
providing air to the blunt base of the tail strut. It allowed the maintenance of given forward 
velocities with reduced propeller speeds. Provision for forced air supply was then extended 
to include the blunt base regions of the forward hydrofoils. 
During initial trails, a slight porpoising was encountered at approximately 40.0 knots. 
The oscillation was of constant amplitude, without either build up or decay. It was theorized 
that ventilation behind the blunt base of the tail strut extended over the sides of the pod 
near its aft end so as to cause a small decrease in total lift of the tail assembly. This 
change in lift caused a small increase in vehicle trim and a corresponding increase in pod 
646551 O—62——_16 
