There are two particularly persistent problems faced by designers of hydrofoils: cavitation and ventilation. Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the foil. Since air is much less dense than water, the foil generates much less lift and the boat crashes down. Ventilation can occur at any air-water interface.
Ventilation occurs when air gets sucked down to the lifting surfaces. Although ventilation can occur on vertical struts, 'V' foils are particularly prone to this problem because of the shallow angle the foil makes with the water surface.
Cavitation occurs when the water pressure is lowered to the point where the water starts to boil. This frequently happens with propellors. When a propellor is turned fast enough, the blades generate so much lift (i.e. the pressure on the lifting surface of the blades goes down) that the water flowing over the propellor blades begins to boil. When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water.