A form of high-speed ship that supports its weight by means of wings (properly called hydrofoils, or simply foils) beneath the surface of the water. The hydrofoils generate lift by movement in the same manner as an airplane wing. The hydrofoil was conceived in order to produce faster ships. The most effective means of developing a faster ship is to find a way to lift the ship's hull clear of the water. This greatly reduces the drag on the hull, in turn greatly reducing the power required to drive the ship. The hydrofoil ship is one means to this end. See also Airfoil; Ship powering, maneuvering, and seakeeping.
There are two basic types of hydrofoils: fully submerged and partially submerged.
The most efficient hydrofoil craft, from a powering standpoint, are those with fully submerged foils; that is, the wings are completely below the surface of the water. They are thus not subject to surface interactions such as air drawing, or to the danger of broaching through small waves. They are also relatively unaffected by wave action, resulting in excellent ship ride comfort. However, because they are fully submerged, they require an active control system which senses the craft's height and attitude and adjusts the wings as necessary to maintain the desired condition. See also Control systems.
In a partially submerged hydrofoil, the foils are generally V-shaped when viewed head on, with the apex of the V below the water and the tips of the wings above the water. In this case the amount of lift generated will increase if the foils are more deeply submerged, and it will decrease if the foils are less submerged. (The increase and decrease in lift are due to the change in the wetted area of the foils.) This behavior results in a steady ride attitude at an equilibrium balance of lift. A disadvantage of partially submerged foils is that the ride is not as smooth, especially in rough water, since the foils will react to each wave they encounter.
