A hydrofoil is a foil which operates in water. They are similar in appearance and purpose to airfoils.[1]
Hydrofoils can be artificial, such as the rudder or keel on a boat, the diving planes on a submarine, a surfboard fin, or occur naturally, as with fish fins, the flippers of aquatic mammals, the wings of swimming seabirds, or other creatures like the sand dollar.
The term "hydrofoil" is commonly used for the wing-like structure mounted on struts below the hull of a variety of boat (see illustration), which lifts the boat out of the water during forward motion, in order to reduce hull drag. As a synecdoche, the term "hydrofoil" is often used to refer to any boat that utilizes hydrofoil technology in this way. Most of this article is about this type of hydrofoil.
As a hydrofoil-equipped watercraft increases in speed, the hydrofoil elements below the hull(s) develop enough lift to raise the hull up and out of the water. This results in a great reduction in hull drag, and a further corresponding increase in speed.
A wider adoption of the technical innovations of a hydrofoils is prevented by the increased complexity of building and maintaining them. Hydrofoils are generally prohibitively more expensive than conventional watercraft. However, the design is simple enough that there are many human-powered hydrofoil designs. Amateur experimentation and development of the concept is popular.[2]
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Since air and water are governed by similar fluid equations, albeit with vastly 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 causing the flow to be deflected downward which according to Newton's Third Law of Motion exerts an upward force on the foil. This turning of the water causes higher pressure on the bottom and reduced pressure on the top of the foil. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the foil has a higher average velocity on one side than the other.
When used as a lifting element on a "hydrofoil" boat, 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 wave resistance and other impeding forces such as various types of drag (physics) on the hull are eliminated as the hull is lifted clear, turbulence and drag act only on the much smaller surface area of the hydrofoil and there is a marked increase in speed.
Early hydrofoils used U-shaped foils. Hydrofoils of this type are known as surface-piercing since portions of the U-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.
Between 1899 and 1901, British boat designer John Thornycroft worked on a series of models with a stepped hull and single bow foil. In 1909 his company built the full scale 22-foot (6.7 m) long boat, Miranda III. Driven by a 60 hp (45 kW) engine, it rode on a bowfoil and flat stern. The subsequent Miranda IV was credited with a speed of 35 knots (65 km/h).[3]
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, and after reading the article began to sketch concepts of what is now called a hydrofoil boat. With his chief engineer Casey Baldwin, Bell began hydrofoil experiments in the summer of 1908. Baldwin studied the work of the Italian inventor Enrico Forlanini and began testing models based on his designs, which led them to the development of hydrofoil watercraft. During Bell's world tour of 1910–1911, Bell and Baldwin met with Forlanini in Italy, where they rode in his hydrofoil boat over Lake Maggiore. Baldwin described it as being as smooth as flying. On returning to Bell's large laboratory at his Beinn Bhreagh estate near Baddeck, Nova Scotia, they experimented with a number of designs, culminating in Bell's HD-4. Using Renault engines, a top speed of 87 km/h (54 mph) was achieved, accelerating rapidly, taking waves 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), a record which stood for two decades. A full-scale replica of Bell's HD-4 is viewable at the Alexander Graham Bell National Historic Site museum in Baddeck, Nova Scotia.
German engineer Hanns von Schertel worked on hydrofoils prior to and during World War II in Germany. After the war Schertel's team was captured by the Russians. As Germany was not authorized to build fast boats, Schertel himself went to Switzerland, where he established the Supramar company. In 1952, Supramar launched the first commercial hydrofoil, PT10 "Freccia d'Oro" (Golden Arrow), in Lake Maggiore, between Switzerland and Italy. The PT10 is of surface-piercing type, it can carry 32 passengers and travel at 35 knots (65 km/h). In 1968, Hussain Najadi the Bahraini born banker, acquired the Supramar AG and expanded its operations into Japan, Hong Kong, Singapore, the UK, Norway and the US. General Dynamics of the United States became its licensee, and the Pentagon awarded its first R&D naval research project in the field of supercavitation. Hitachi Shipbuilding of Osaka, Japan, was another licensee of Supramar, as well as many leading ship owners and shipyards in the OECD countries.
From 1952 to 1971, Supramar designed many models of hydrofoils: PT20, PT50, PT75, PT100 and PT150. All are of surface-piercing type, except the PT150 combining a surface-piercing foil forward with a fully submerged foil in the aft location. Over 200 of Supramar's design were built, most of them by Rodriquez in Italy.
During the same period the Soviet Union experimented extensively with hydrofoils, constructing hydrofoil river boats and ferries with streamlined designs during the cold war period and into the 1980s. Such vessels include the Raketa (1957) type, followed by the larger Meteor type and the smaller Voskhod type. One of the most successful Soviet designer/inventor in this area was Rostislav Alexeyev who some consider the 'father' of the modern hydrofoil due to his 1950's era high speed hydrofoil designs. Later, circa 1970's, Alexeyev combined his hydrofoil experience with the surface effect principle to create the Ekranoplan.
In 1961, SRI International issued a study on "The Economic Feasibility of Passenger Hydrofoil Craft in U.S. Domestic and Foreign Commerce".[4] 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.[5]
Canada - During World War II, Baldwin worked on an experimental smoke laying hydrofoil (later called the Comox Torpedo) which was superseded by other smoke-laying technology and an experimental target-towing hydrofoil. The forward two foil assemblies of what is believed to be the latter hydrofoil were salvaged in the mid-1960s from a derelict hulk in Baddeck, Nova Scotia by Colin MacGregor Stevens and were donated to the Maritime Museum in Halifax, Nova Scotia. The Canadian Armed Forces built and tested a number of hydrofoils (e.g. Baddeck and two vessels named Bras d'Or) culminating in the high-speed anti-submarine hydrofoil HMCS Bras d'Or in the late 1960s, but the program was cancelled in the early 1970s due to a shift away from anti-submarine warfare by the Canadian military. 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 USSR introduced several hydrofoil-based fast attack craft into their navy, principally:
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 illegal drug runners, being very fast, and also possessing armaments to stop anything they could not catch, as well as the ability to call in air support.
The Italian Navy has used six hydrofoils of the Sparviero class from the late 1970s. These were armed with a 76 mm gun, two missiles and were capable of speed up to 50 knots (93 km/h). Three similar boats were built for the Japan Maritime Self-Defense Force.
The French experimental sail powered hydrofoil Hydroptère is the result of a research project that involves advanced engineering skills and technologies. In September 2009, the Hydroptère set new sailcraft world speed records in the 500 m category, with a speed of 51.36 knots (95.12 km/h) and in the one nautical mile (1.9 km) category with a speed of 50.17 knots (92.91 km/h).[6][7]
Another trimaran sailboat is the Windrider Rave.[8] The Rave is a commercially available 17-foot (5.2 m), two person, hydrofoil trimaran, capable of reaching speeds of 40 kn (74 km/h). The boat was designed by Jim Brown.
The Moth dinghy has evolved into some radical foil configurations.
Hobie Sailboats once produced a production foiling trimaran. It was and is the fastest production sailboat in history. It is called the Hobie Trifoiler. Trifoilers have clocked speeds upward of thirty knots.
A new kayak design, called Flyak, has hydrofoils that lift the kayak enough to significantly reduce drag, allowing speeds of up to 27 km/h (17 mph). Some surfers have developed surfboards with hydrofoils called foilboards, specifically aimed at surfing big waves further out to sea.[9]
Soviet Union built Voskhods are one of the most successful passenger hydrofoil designs. It was developed by soviets and manufactured in Russia and Ukraine. Currently, they are in service in more than 20 countries. The most recent model Voskhod-2M FFF, also known as Eurofoil, was built in Feodosiya, Ukraine, for the Dutch public transport operator Connexxion.[10]
The Boeing 929 is widely used in Asia for passenger services between the many islands of Japan, between Hong Kong and Macau and on the Korean peninsula.
Some operators of hydrofoil include:
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