ONR Hydrofoil Boat XCH-6 219 
stainless steel with an ultimate tensile strength of 140,000 psi. A design feature is a nylon 
liner between the propeller hub and the propeller shaft. The shaft is tapered and propeller 
attachment is by means of a locknut and a brass key. Friction between the hub and shaft is 
reduced, by the nylon liner, to a very low value. This reduced friction, together with a 
torque limit on the locknut, has enabled specification of key size such that key failure in 
shear occurs at a predetermined value of propeller torque. Protection to transmission compo- 
nents is thereby provided for a condition wherein the propeller might strike floating debris 
or some solid object. Laboratory tests were undertaken to determine proper key size; failure 
of the key occurs at a torque of 2500 inch-pounds. 
INSTRUMENTATION 
Instrumentation has been incorporated to provide cockpit indication of water speed, pro- 
peller torque, propeller rotative speed, propeller thrust, keel trim in pitch, tail foil incidence 
in pitch with respect to the keel, tail strut position in yaw, transmission and engine oil 
temperatures and pressures, compressor and power turbine speeds and temperatures, and fuel 
pressure. 
The propeller torque measuring device is installed in the horizontal transmission shaft 
aft of the engine gear box. It is a torque transducer, Model No. TG-5-3000A, manufactured 
by the Crescent Engineering and Research Company, El Monte, California. The core or 
sensor section of the transducer incorporates a solid shaft which acts as a torsional spring. 
It is splined at its ends to mating sections of the drive shaft, and is enclosed by a cylindri- 
cal, fixed pickup unit. Cylindrical crowns are attached to the shaft of the sensor section at 
two locations which are separated along its longitudinal axis. The crowns have inter- 
meshing teeth which extend around the center region of the solid shaft. Torque causes a 
change in the size of air gaps which exist between the intermeshed teeth of the two crowns, 
and this change modifies the magnetic reluctance. The variation in reluctance effects pro- 
portional changes of impedance in two coils of the enclosing pickup unit. The coils perform 
as two legs of an ac bridge to reflect electrical unbalance which is proportional to torque. 
This unbalance is amplified, demodulated and applied to a microammeter which reads torque 
directly. 
Propeller thrust is calculated by employing strain gages affixed to the linear actuator 
which attaches the lower yoke support point to the hull. Data is recorded on a portable 
oscillograph. Measured compression loads in the actuator enable the calculation of propeller 
thrust, as required for moment equilibrium about the retraction axis, based on estimated tail 
foil lift and tail assembly drag components, together with associated centers of pressure. 
The calculation of thrust is therefore approximate at the present time. It is planned to per- 
form towing-tank tests of the tail assembly to accurately measure estimated quantities and to 
thereby provide more accurate calculation of propeller thrust. 
FLOW CONDITIONS 
Flow conditions involve cavities over the upper surface and behind the blunt base of 
each forward cruise foil, behind the blunt base of each forward diagonal foil, along the after- 
body sides of the forward struts, and behind the blunt base of the tail strut. All other 
regions of foils and struts experience subcavitating flow. 
Hydrofoil and strut design provides several air paths to cavity regions. Forced ventila- 
tion is provided through tubes to the blunt base of the tail strut and to the blunt trailing edge 
regions of the diagonal and cruise foil elements of each forward hydrofoil assembly. Air 
supply for this purpose is obtained from two bleed ports located aft of the compressor section 
