The hydrofoil is very similar to the hovercraft, because it moves in the boundary between air and water. It avoids drag by lifting itself out of the water, using wing-shaped structures called hydrofoils that extend into the water from the craft. These hydrofoils function like the wings on a plane, creating lift and flying the hull above the surface of the water.
The first person to work on this idea was a French priest, Ramus, in the mid-1800s. However, there was no engine that could supply sufficient thrust. In the 1890s, another Frenchman, the Count de Lambert, tried and failed to make a working model using a gasoline engine.
The first successful hydrofoil boats were created in the early 1900s. Enrico Forlanini, an Italian airship designer, built a small boat with hydrofoils in 1905. He showed Alexander Graham Bell a later model that impressed the famous American. Bell built one himself, based on Forlanini's patented design and set a water-based speed record of 71 mph (114 kph) with it in 1918. This record stood until the 1960s.
Although there were small improvements made over the next few decades, hydrofoils did not see commercial use until the 1950s, when Hans von Schertel, a German scientist, developed his designs for passenger hydrofoils. Italy created their Supramar boats, and Russia and the United States developed hydrofoils with both commercial and military applications.
There have been experiments with various types of foils and different types of engines, including the gas turbine, diesel, gasoline, and jet engines.
The foils themselves have two distinct shapes. The surface-piercing models are V-shaped, so that part of the foil stays out of the water. This type is good for calm surfaces like rivers and lakes. The other foil is completely submerged. It usually consists of three foils extending straight down beneath the boat. Hydrofoils with this configuration need autopilots to keep them level. Whenever the boat shifts to one side, sensors send messages to flaps on the foils, which then adjust automatically to bring the boat back to a normal position.
Hydrofoils today are used by commuter services, fishery patrols, fire fighters, harbor control, water police, and air-sea rescues. For the military, hydrofoils can be excellent small submarine chasers and patrol craft.
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