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flat or curved finlike device, attached by struts to the hull of a watercraft, that lifts the moving watercraft above the water's surface. The term is often extended to include the vessel itself. Like an aircraft wing in its appearance and function, the foil develops lift as it passes through the water; the hull is raised above the surface, and the drag caused by the vessel's contact with the water is thereby reduced. Hydrofoil vessels are capable of traveling faster than 70 mi (113 km) per hr. They are used for ferries in many countries in Europe and Asia; in the United States they are used mostly for military purposes. In addition to offering greater speeds, such vessels do not pitch and roll as do conventional watercraft. Foils may be of the submerged or the surface-piercing type. On oceangoing passenger ships a type of hydrofoil called a stabilizer is used to minimize wave action on the vessel. The first hydrofoil vessel was built in 1905 by the Italian engineer Enrico Forlanini. In 1918, Alexander Graham Bell built the HD-4, a vessel 60 ft (18.3 m) long that attained a speed of 70.86 mi (114.03 km) per hr.


See C. Hook, Hydrofoils (1967); R. McLeary, Jane's Surface Skimmers (annually, 1968–); W. T. Gunston, Hydrofoils and Hovercraft (1969).



a ship constructed with submerged winglike devices (foils) that generate lift during forward motion, raising the vessel’s hull above the level of the water. A patent for a hydrofoil was awarded in Russia in 1891, but such vessels were not placed in service until the second half of the 20th century. The theory of hydrofoil transport was formulated by the Soviet scientists M. V. Keldysh, M. A. Lavrent’ev, N. E. Kochin, and A. I. Vladimirov, among others.

When not in motion, a hydrofoil is supported by buoyancy in its water-displacing position. When the vessel reaches a certain velocity, the hull rises above the level of the water. As the area of the hull in contact with the water decreases, only the foils, struts, rudders, propeller shafts, and propellers remain immersed, and the resistance of the water to forward motion is reduced. Hydrofoils attain higher speeds for a given specific fuel consumption than either conventional displacement craft or hydroplanes. Hydrofoils are stabilized after takeoff by the automatic varying of the immersion area or depth of fixed foils or by varying the attack angle of adjustable foils; these same techniques are used to ensure longitudinal and transverse stability.

Hydrofoils with fixed foils that are shallowly submerged and that do not intersect the surface of the water are used primarily on rivers; arched or V-shaped surface-piercing foils that run deeply submerged are more suitable for turbulent waters. Adjustable foils are increasingly used for seagoing hydrofoils. These vessels are equipped with a device that tracks the shape of oncoming waves so that the attack angle of the foils may be controlled.

Hydrofoils are used to transport passengers and small express cargoes. In the military they serve as torpedo boats and rocket carriers; other official functions include service as guard and dispatch boats.

Hydrofoil hulls are usually constructed of light alloys, and stainless steel is used for the foils. The power plants are internalcombustion engines or gas turbines, and propellers are used for propulsion. Hydrofoils are capable of speeds of 80–100 km/hr and more. Passenger hydrofoils are usually equipped with airplane-type seats.

The first Soviet Raketa-type hydrofoil, which was built in the USSR in 1957, had 66 seats. In 1976, the shipbuilding industry of the USSR manufactured both river-service and seagoing hydrofoils that accommodated 150 to 250 passengers.


Murugov, V. S., and O. V. Iaremenko. Morskie suda na podvodnykh kryl’iakh. Moscow, 1962.
Zaitsev, N. A., and A. I. Maskalik. Otechestvennye suda na podvodnykh kryl’iakh, 2nd ed. Leningrad, 1967.



(naval architecture)
A flat or airfoil-shaped plate attached to a ship to stabilize it in roll.
Foils attached by struts to the bottom of a hydrofoil boat to lift the hull out of the water as its speed is increased.


a fast light vessel the hull of which is raised out of the water on one or more pairs of fixed vanes
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