A hydrofoil is a boat with wing-like foils mounted on struts below the hull. As the craft increases its speed the hydrofoils develop enough lift for the boat to become foilborne - i.e. to raise the hull up and out of the water. This results in a great reduction in drag and a corresponding increase in speed.
A sailing hydrofoil or hydrosail is a sailboat with wing-like foils mounted under the hull. As the craft increases its speed the hydrofoils lift the hull up and out of the water, greatly reducing wetted area, resulting in decreased drag and increased speed. A sailing hydrofoil can achieve speeds exceeding two times the wind speed.
Hydrodynamics: Since air and water are governed by similar fluid equations, albeit with different levels of viscosity, density, and compressibility, the hydrofoil and airfoil create lift in identical ways (see foil (fluid mechanics)). The foil is shaped to move smoothly through the water with faster flow over the top face of the foil, creating a pressure drop and consequently an upward force on the foil. This upward force lifts the body of the vessel, decreasing drag and increasing speed. The lifting force eventually balances with the weight of the craft, reaching a point where the hydrofoil no longer lifts out of the water, but remains in equilibrium. Since the force of the waves acts over a smaller area of the hydrofoil, there is a marked decrease in turbulence drag.
Foil configurations: The 2 types of hydrofoilsEarly hydrofoils used V-shape foils. Hydrofoils of this type are known as surface-piercing since portions of the V-shape hydrofoils will rise above the water surface when foilborne. Some modern hydrofoils use inverted T-shape foils which are fully submerged. Fully submerged hydrofoils are less subject to the effects of wave action, and are therefore more stable at sea and are more comfortable for the crew and passengers. This type of configuration, however, is not self-stabilizing. The angle of attack on the hydrofoils needs to be adjusted continuously in accordance to the changing conditions, a control process that is performed by sensors, computer and active surfaces.
A March 1906 Scientific American article by American hydrofoil pioneer William E. Meacham explained the basic principle of hydrofoils. Alexander Graham Bell considered the invention of the hydroplane a very significant achievement. After reading this article Bell began to sketch concepts of what is now called a hydrofoil boat. With Casey Baldwin, he began hydrofoil experimentation in the summer of 1908. Baldwin studied the work of the Italian inventor Enrico Forlanini and began testing models based on his designs. This led him and Bell to the development of hydrofoil watercraft. During Bell's world tour of 1910-1911 he and Baldwin met with Forlanini in Italy. They had rides in the Forlanini hydrofoil boat over Lake Maggiore. Baldwin described it as being as smooth as flying. On returning to Baddeck a number of designs were tried culminating in the HD-4. Using Renault engines a top speed of 87 km/h (54 mph) was achieved, accelerating rapidly, taking wave without difficulty, steering well and showing good stability. Bell's report to the United States Navy permitted him to obtain two 260 kW (350 horsepower) engines. On September 9, 1919 the HD-4 set a world marine speed record of 114 km/h (70.86 mph). This record stood for ten years. A full-scale replica of the HD-4 can be seen in the museum on the Alexander Graham Bell National Historic Site in Baddeck.
Commercial use of hydrofoils in the U.S. first appeared in 1961 when two commuter vessels were commissioned by Harry Gale Nye, Jr.'s North American Hydrofoils to service the route from Atlantic Highlands, New Jersey to the financial district of Lower Manhattan.
Military applications: The Canadian Navy built and tested a high-speed anti-submarine hydrofoil, the HMCS Bras d'Or, in the late 1960s, but the program was cancelled due to a shift away from ASW by the Canadian Navy. The Bras d'Or was a surface-piercing type which performed well during her trials, reaching a maximum speed of 63 knots (117 km/h).
The Soviets introduced several hydrofoil-based fast attack craft into their navy.
The U.S. Navy operated combat hydrofoils, such as the Pegasus class, from 1977 through 1993. These hydrofoils were fast and well armed, and were capable of sinking all but the largest surface vessels. In their narcotics interdiction role, they were a nightmare for drug runners, being very fast, and having missiles and guns to stop anything they could not catch, as well as the ability to call in air support.
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Voskhod (Russian: Sunrise) (Design 352 and Design 03521) is a type of passenger hydrofoil boats built in the Soviet Union and later in independent Ukraine. They were intended for use in rivers and lakes. Good seaworthiness allows Voskhod boats to enter coastal sea areas as well.
Raketa (Russian: Rocket) was the first type of hydrofoil boats commercially produced in the Soviet Union. They were manufactured from 1957 until the early 1970s.
Boeing has designed and built several hydrofoil craft for both military and civilian use.
A sailing hydrofoil, hydrofoil sailboat, or hydrosail is a sailboat with wing-like foils mounted under the hull. As the craft increases its speed the hydrofoils lift the hull up and out of the water, greatly reducing wetted area, resulting in decreased drag and increased speed. A sailing hydrofoil can achieve speeds exceeding two times the wind speed.
The sit-down hydrofoil is a fairly new water sport. Unlike water skiing and wakeboarding you do not ride on the water, instead your body and the board rise above the water, supported by the hydrofoil wing which is still under the water. This is done by being towed by a towboat with a driver and spotter. The skier sits on the seat of the hydrofoil and is strapped in with a seatbelt; their feet are strapped into bindings on the front of the board. These precautions are to prevent the skier and board from separating while being towed. When the skier is strapped in and ready to ride the boat will start to pull the skier. This will cause the hydrofoil to lift the rider and board above the surface of the water. This is the point where balance becomes crucial. The rider must be centered over the post of the hydrofoil; small body movements will cause great reactions with the hydrofoil. Once general balance and control of the board has been achieved, the possibilities of the sport are endless. Because the board is above the water surface and generally not contacting it, the ride is largely unaffected by the wake or chop of the water. The ride is relatively smooth, as if floating on air. The air board is a modified hydrofoil where the skier stands up. Manufacturers of sit-down hydrofoils include Air Chair and Sky Ski.
The Flyak is a hydrofoil adaptation to the conventional kayak. It uses twin hydrofoils designed to raise the hull out of the water to increase the speed. Speeds of up to 27.2 km/h (16.9 mph) can be achieved on calm water.
The Disco Volante is a fictional ship in the James Bond novel Thunderball (1961) and its 1965 film adaptation of the same name. It was a hydrofoil craft owned by Emilio Largo, an agent of SPECTRE. It was purchased with SPECTRE funds for £200,000. The craft plays a pivotal role in the seizure and transportation of two nuclear warheads. It is a high-tech ship that possesses a number of smaller underwater submarine craft.
Trampofoil, also known as Hydro-bike, HydroSlide, Hydrothopter or Aquaskipper, is a light, one-person hydrofoil. The athlete moves it over the water by jumping up and down with both feet together on a footboard. The Trampofoil is kept afloat on a centrally-placed wing, once the correct speed is achieved. Another wing in the front maintains a constant height of the Trampofoil in the water.
A human-powered hydrofoil is a small hydrofoil boat propelled entirely by the muscle power of its operator. A common design for human powered hydrofoils consists of a large hydrofoil at the stern end that is used both for propulsion and keeping the vehicle above the water, connected to a smaller foil at the bow used for steering. Riders operate the vehicle by bouncing up and down on a small platform at the stern, whilst holding onto a steering column.
A foilboard or hydrofoil board is a surfboard with a hydrofoil that extends below the board into the water. This design causes the board to leave the surface of the water at various speeds.
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