Prospects for Very-High-Speed Hydrofoils
Severe hydrodynamic problems remain to be solved before hydrofoils can be designed to operate on the open ocean at speeds between 50 and 80 knots. These problems stem from two sources, cavitation and the free-water surface, complicated by the action of waves. Recently, a natural air venting phenomenon has been investigated which shows promise of being a very satisfactory mode for the operation of highspeed hydrofoils up to speeds at least as high as 80 knots. "Hyperventilation" is the complete breaking open to the atmosphere of a vapor or partially air-filled cavity, giving rise to an underwater planing condition. What was previously thought of as a transient, extremely shallow submergence phenomenon has been shown to be a stable and predictable operating condition with a wide "window" encompassing speed, depth, angle of attack, foil section, and planform geometry. Model experiments also showed that the discovery explains a problem that has been encountered with present-generation subcavitating hydrofoil boats; that is, if a non-surface-piercing foil broaches the water surface and then reenters, it fails to pick up lift immediately and the hull consequently impacts the water heavily.