What is a hydrofoil? A hydrofoil is a watercraft that is supported on ski-like pontoons while in motion, with the bulk of the hull remaining entirely above the water (Encarta Encyclopedia 2002). The picture below shows a basic idea of how hydrofoils work as well as some different kinds of hydrofoils.
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Hydrofoils were first seen about in 1869. Emmanuel Denis Farcot was issued a patent on a boat that he had developed to go faster through the water because of less resistance. If you look at his design, he was using many little foils along the side of his boat to lift it out of the water in order to reduce the drag on the hull of the boat. The drawing of Farcot's boat is seen below.
More pictures and information at http://www.foils.org/gallery/early.htm
Hydrofoils are not only used on boats. Guidoni and Croco took the idea of hydrofoils and put then on seaplanes. Their main reason for puting foils on the plane was to make the landings smoother and to make takeoffs much quicker. The plane can get out of the water much faster with foils because there is less drag inhibiting the plane from gaining speed to get airborn. This is useful when taking off of short lakes or in rough water.
More pictures and information at http://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html
This diagram shows how most modern hydrofoils are designed. The drawing is slightly exagerated in order to emphasize the curvature on the top of the foil. This curvature of the top portion of the foil provides extra lift, even when the angle of attack is zero.