High Performance Ships—Promises and Problems ll 
- Fig. 11. Artist’s concept of the antisubmarine hydrofoil craft “PC(H)” 
of attack of the foils, which causes cavitation to occur at lower speeds. Fig. 12 shows the 
variation of angle of attack with craft speed for a state 5 sea. Fig. 13 shows the degrada- 
tion of cavitation-inception speed with increasing wave height for a thin hydrofoil. Consid- 
erable work has been done on section shapes to delay cavitation, but only limited gains are 
possible in this direction. If we are to attain the promise of high speeds, it must be by 
utilizing supercavitating sections. 
In the supercavitating regime we find that very high speeds or large angles of attack 
are required for true cavity flow. In practical foil configurations, the cavity is likely to 
vent to the atmosphere, either through tip vortices or down a strut. This leads to the con- 
clusion that superventilation rather than supercavitation is the more practical mechanism 
to consider. 
It appears that in the subcavitating range we may expect overall lift/drag ratios of about 
9 to 12, and in the superventilated range, lift/drag ratios of about 5 to 7. While the lower 
lift/drag ratios require more power and demand more economy in structural and other weights, 
this does not appear to be prohibitively restrictive. 
In the course of designing PC(H) a number of problems were successfully resolved on 
paper, while others required considerable test and reevaluation effort. For example, calcu- 
lated predictions of lift and drag at top speed were in close agreement with model tests. 
