ONR Hydrofoil Boat XCH-6 213 
All aluminum components are 61S-T6 alloy; all steel elements are 416 stainless alloy 
with an ultimate tensile strength of 150,000 psi. The struts, which employ constant cross 
section, were externally contoured by a single pass through a Whaley Machine of 14-inch 
diameter. A Keller BL Model C Machine was used for obtaining the more complicated exter- 
nal contours of the forward cruise hydrofoils. 
CONTROLS AND RETRACTION 
The boat is steered by rotation in yaw of the entire tail strut-pod-foil assembly. Tail 
foil incidence with respect to the keel is adjustable while foilborne, and a trailing edge 
trim tab is provided on the cruise foil element of one forward hydrofoil assembly. Each of 
the three hydrofoil assemblies is separately retractable. 
Tail assembly retraction, steering, and incidence adjustment in pitch are accomplished 
by means of a unique, structural yoke. The yoke is an aluminum casting with machined sur- 
faces. As shown in Fig. 2a, the yoke provides support to the tail strut at two, vertically 
separated locations. Both of these supports are through sleeve bearings on cylindrical por- 
tions of the yoke structure. The bearings, in turn, support an aluminum tube which is 
rigidly bolted to the strut assembly at both support points. The sleeve bearings allow rota- 
tion of the tail assembly in yaw, about the tube and steering axis. This axis, extended, 
coincides with the 20.0-percent chord line of the lower strut. Each of the bearings can 
transmit loads in a horizontal plane. Up and down loads are transferred from the tube to 
the yoke at the lower and upper bearing supports, respectively. The upper end of the tube 
and the top of the strut are joined by a fitting. This fitting supports the upper gear box and 
has an arm extending out on its port side. Connected to the arm is a fore and aft push-pull 
rod of the steering system. Rod loads establish equilibrium of the tail assembly in regard 
to moments applied about the steering axis and motion of the rod provides steering action. 
Strut displacement in yaw is limited by stops to plus or minus 6.0 degrees. The remainder 
of the steering system provides for mechanical, irreversible control from the pilot’s wheel. 
At high speed, only small strut rotations are required; a wheel-to-strut rotation ratio of 
54.5 to 1.0 is employed. 
Yoke support to the hull is provided at three pickup points. Two points at the upper 
end of the yoke, separated athwartship, are hinged on a lateral axis. Rotation about this 
axis allows for both retraction and for tail foil incidence adjustment. Each of the two 
upper support points can transmit loads in all directions. The third support point is at the 
bottom of the yoke and is attached to the hull through an electrically operated, linear actu- 
ator manufactured by Lear, Inc., Instrument Division, Stamford, Connecticut (Model No. 
434-AJ). Variable length of this support, in the fore and aft direction, allows incidence 
change in pitch while foilborne. This attachment is designed to take only axial (fore and 
aft) loads. 
The vertical steering axis and the lateral retraction axis intersect at a point on the 
vehicle vertical plane of symmetry. Also passing through this point is the centerline of 
longitudinal, transmission shafting. A Rzeppa coupling in the transmission, at this three- 
axis intersection, allows for simultaneous transmission of power, tail assembly incidence 
change in pitch, and tail assembly steering. Manufacture of the flexible coupling is by 
Dana Corp., Con-Vel Division, Detroit, Michigan (Model No. OR). 
Three pickup points on the hull support each upper strut of the forward foil assemblies. 
Two points on top of each strut, separated in the fore and aft direction, are hinged on a 
longitudinal axis. Each of these supports can take loads in all directions. A third support 
