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 such, the term "hydrofoil" is often used to refer to boats using hydrofoil technology. Most of this article is about this type of hydrofoil.
As a hydrofoil-equipped watercraft increases in speed, the hydrofoil elements below the hull(s) develop enough lift to raise the hull up and out of the water. This results in a great reduction in hull drag, and a further corresponding increase in speed.
A wider adoption of the technical innovations of hydrofoils is prevented by the increased complexity of building and maintaining them. Hydrofoils are generally prohibitively more expensive than conventional watercraft. However, the design is simple enough that there are many human-powered hydrofoil designs. Amateur experimentation and development of the concept is popular.
A boat equipped with hydrofoils is also called a hydroplane (boat).
Read more about Hydrofoil: Hydrodynamics of Hydrofoil-equipped Boats ("hydrofoils"), Foil Configurations, Modern Passenger Boats