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A hydrofoil, not to be confused with a hovercraft, is a waterborne vessel that uses underwater wings to generate lift in the same way that a plane uses wings to generate lift in air. Foil is another word for wing. A foil generates a region of negative pressure immediately above it, producing lift. This lift elevates the hull from the surface of the water, supporting it on vertical struts attached to the foils underneath.
When the hydrofoil reaches a critical speed, the entire body of the hull sails along without touching the water. When the hull is suspended, it is not necessary for the engine to expend much energy to overcome water resistance. The foils are the only part of the ship which continue to produce drag through the water. Because they literally fly through the water, hydrofoils are capable of high speeds, having set worldwide records for both conventional and human-powered boating.
A hydrofoil held the world water speed record between 1919 and 1929. Interestingly enough, this hydrofoil was built by telephone inventor Alexander Graham Bell in 1908, after he became interested in hydrofoils. Bell and his collaborator, Casey Baldwin, had seen the first popular article on hydrofoils in a 1906 Scientific American article by William E. Meacham and were inspired to build one. By 1919, they experienced great success and built a craft which hit a top speed of 70.86 mph (114 km/h) thanks to engines given to them by the U.S. Navy.
Early hydrofoils had U-shaped foils, called surface-piercing foils, which essentially skipped across the top of the water. In modern hydrofoils, the foils are entirely submerged, making the craft less susceptible to surface turbulence. The downside is that there is no smooth layer of surface tension to ride on, so advanced computer-driven control systems must continuously update roll, pitch, and yaw to keep the craft stable. Sometimes multi-tiered foils are used, in a similar style to that of a biplane, for the purpose of providing a smoother ride.
It was not until 1952 that a commercial hydrofoil route opened. Baron von Schertel, a German hydrofoil pioneer, was active in the area before and throughout World War II. After WWII, he fled to Switzerland, where he founded Supramar company, launched the first commercial hydrofoil route, and went on to design and build dozens of new hydrofoils throughout 1951-1971. Pegasus class hydrofoils served in the U.S. Navy between 1977 and 1993, where they were used to suppress the trade of narcotics. Today, hydrofoils are still used in China, the U.S., Greece, Japan and Russia, among other countries.