hydrofoil

A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.

Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the wings of swimming seabirds, or other creatures like the sand dollar.

The term "hydrofoil" is commonly used for the wing-like structure mounted on struts below the hull of a variety of boats (see illustration), which lifts the boat out of the water during forward motion, in order to reduce hull drag. As a synecdoche, the term "hydrofoil" is often used to refer to boats using hydrofoil technology. Most of this article is about this type of hydrofoil.