Hydrodynamics of High-Speed Hydrofoils 137 
— — — — For Max. Vertical Velocity in Waves 
—- —— - ——_ For |/3 Greatest Velocity in Waves 
For RMS Velocity in Waves 
70 
wn 
2 60 Danger of Buffeting 
= 
Uv 
® 
a 
a 
7p) 
= 50 
ra 
40 ‘ 
Sea State | 
| 2 3 + 4 5 6 
30 
(0) oy 10 15 20 25 30 
Wind Speed in knots 
Fig. 14. Wing cavitation inception vs sea state 
boundary in Fig. 14 outlines the region of forward speeds and sea state wherein severe foil 
vibrations might be expected; this prediction is entirely theoretical as far as operation in a 
seaway is concemed, being based on observations under steady angles of attack [20]. 
In order not to end this discussion of cavitation suppression on too despairing a note, 
I would like to present as Fig. 15 the results of calculations that we have carried out on 
the effect of the proximity of the free surface on the minimum pressures due to the thickness 
portion of a thin plane wing, and on the minimum pressures acting on spheroidal bodies of 
revolution. For the smaller depth-diameter ratios, the latter refer particularly to the pres- 
sures on the upper portion of the body. These calculations indicate that very modest gains 
in inception speed may result in practical cases due to the proximity of the free surface. 
SUPERCAVITATING OPERATION 
It has already been pointed out that the propulsion of high-speed hydrofoil boats 
depends upon the utilization of supercavitating propellers, and it would seem clear, in addi- 
tion, that the 70-knot-plus potential of these craft will depend upon the utilization of super- 
cavitating foils and struts. A great deal has recently been written about practical bodies 
operating in cavity flow, and particularly good references to this work are the collective 
papers presented at the second Office of Naval Research Symposium on Naval Hydrodynamics 
(1958), which was the immediate predecessor of the present Symposium. 
The Langley Laboratory of the (U.S.) National Aeronautics and Space Administration 
have in the past conducted especially valuable theoretical and experimental studies of 
supercavitating wings and struts operating in ventilated condition [17-19, 21]. These 
results provide at least a lower bound to the values of lift-drag ratios to be attained for 
high-speed craft, and it is believed that further significant increases in foil efficiency will 
be realized, if at all, only as the result of detailed and intensive research. 
