A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains speed, the hydrofoils lift the boat's hull out of the water, decreasing drag and allowing greater speeds.
The hydrofoil usually consists of a wing like structure mounted on struts below the hull, or across the keels of a catamaran in a variety of boats (see illustration). As a hydrofoil-equipped watercraft increases in speed, the hydrofoil elements below the hull(s) develop enough lift to raise the hull out of the water, which greatly reduces hull drag. This provides a corresponding increase in speed and fuel efficiency.
Wider adoption of hydrofoils is prevented by the increased complexity of building and maintaining them. Hydrofoils are generally prohibitively more expensive than conventional watercraft above the certain displacement, so most hydrofoil craft are relatively small, and are mainly used as high-speed passenger ferries, where the relatively high passenger fees can offset the high cost of the craft itself. However, the design is simple enough that there are many human-powered hydrofoil designs. Amateur experimentation and development of the concept is popular.
Since air and water are governed by similar fluid equations—albeit with different levels of viscosity, density, and compressibility—the hydrofoil and airfoil (both types of foil) create lift in identical ways. The foil shape moves smoothly through the water, deflecting the flow downward, which, following the Euler equations, exerts an upward force on the foil. This turning of the water creates higher pressure on the bottom of the foil and reduced pressure on the top. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flow field 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 lifts clear, turbulence and drag act increasingly on the much smaller surface area of the hydrofoil, and decreasingly on the hull, creating a marked increase in speed.
Early hydrofoils used V-shaped foils. Hydrofoils of this type are known as "surface-piercing" since portions of the V-shape hydrofoils rise above the water surface when foilborne. Some modern hydrofoils use fully submerged inverted T-shape foils. Fully submerged hydrofoils are less subject to the effects of wave action, and, therefore, more stable at sea and more comfortable for crew and passengers. This type of configuration, however, is not self-stabilizing. The angle of attack on the hydrofoils must be adjusted continuously to changing conditions, a control process performed by sensors, a computer, and active surfaces.
The first evidence of a hydrofoil on a vessel appears on a British patent granted in 1869 to Emmanuel Denis Farcot, a Parisian. He claimed that "adapting to the sides and bottom of the vessel a series or inclined planes or wedge formed pieces, which as the vessel is driven forward will have the effect of lifting it in the water and reducing the draught.". Italian inventor Enrico Forlanini began work on hydrofoils in 1898 and used a "ladder" foil system. Forlanini obtained patents in Britain and the United States for his ideas and designs.
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 kn (65 km/h; 40 mph).
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 those designs, which led 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 (47 kn; 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 hp) engines. On 9 September 1919 the HD-4 set a world marine speed record of 114 km/h (62 kn; 71 mph), which stood for two decades. A full-scale replica of the HD-4 is viewable at the Alexander Graham Bell National Historic Site museum in Baddeck, Nova Scotia.
In the early 1950s an English couple built the White Hawk, a jet-powered hydrofoil water craft, in an attempt to beat the absolute water speed record. However, in tests, White Hawk could barely top the record breaking speed of the 1919 HD-4. The designers had faced an engineering phenomenon that limits the top speed of even modern hydrofoils: cavitation disturbs the lift created by the foils as they move through the water at speed above 60 kn (110 km/h; 69 mph), bending the lifting foil.
German engineer Hanns von Schertel worked on hydrofoils prior to and during World War II in Germany. After the war, the Russians captured Schertel's team. As Germany was not authorized to build fast boats, Schertel 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; 40 mph). In 1968, the Bahraini born banker Hussain Najadi 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 Sicily, 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 1950s era high speed hydrofoil designs. Later, circa 1970s, 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 US Domestic and Foreign Commerce". Commercial use of hydrofoils in the US 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.
A 17-ton German craft VS-6 Hydrofoil was designed and constructed in 1940, completed in 1941 for use as a mine layer, it was tested in the Baltic Sea, producing speeds of 47 knots. Tested against a standard E-boat over the next three years it performed well but was not brought into production. Being faster it could carry a higher payload and was capable of travelling over minefields but was prone to damage and noisier.
In Canada during World War II, Baldwin worked on an experimental smoke laying hydrofoil (later called the Comox Torpedo) that was later 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. These 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), which culminated in the high-speed anti-submarine hydrofoil HMCS Bras d'Or in the late 1960s. However, 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 that 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 US Navy began experiments with hydrofoils in the mid-1950s by funding a sailing vessel that used hydrofoils to reach speeds in the 30 mph range. The XCH-4 (officially, Experimental Craft, Hydrofoil No. 4), designed by William P. Carl, exceeded speeds of 65 mph (56 kn; 105 km/h) and was mistaken for a seaplane due to its shape. The US Navy implemented a small number of combat hydrofoils, such as the Pegasus class, from 1977 through 1993. These hydrofoils were fast and well armed.
The Italian Navy has used six hydrofoils of the Sparviero class since the late 1970s. These were armed with a 76 mm gun and two missiles, and were capable of speeds 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 1 nautical mile (1852 m) category with a speed of 50.17 knots (92.91 km/h).
Another trimaran sailboat is the Windrider Rave. 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.
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.
Quadrofoil Q2 is a two-seater, four-foiled hydrofoil electrical leisure watercraft. Its initial design was set in 2012 and it has been available commercially since the end of 2016. Powered by a 5.2-kWh lithium-ion battery pack and propelled by a 5.5 kW motor, it reaches the top speed of 40 km/h and has 80 km of range.
The Manta5 Hydrofoiler XE-1 is a Hydrofoil E-bike, designed and built in New Zealand. Initially designed in secret by Guy Howard-Willis in 2011, it has since been available commercially for pre-order in New Zealand since late 2017  Propelled by a 400 watt motor, it can reach speeds exceeding 14 km/h with a weight of 22 kg. A single charge of the battery lasts an hour for a rider weighing 85kg. 
Soviet-built Voskhods are one of the most successful passenger hydrofoil designs. Manufactured in Russia and Ukraine, they are in service in more than 20 countries. The most recent model, Voskhod-2M FFF, also known as Eurofoil, was built in Feodosiya for the Dutch public transport operator Connexxion.
Current operators of hydrofoils include:
Hydrofoils had their peak in popularity in the 1960s and 70s. Since then there has been a steady decline in their use and popularity for leisure, military and commercial passenger transport use. There are a number of reasons for this:
The Batboat, Batstrike, or Batsub is the fictional personal aqua-dynamic hydrofoil/submersible watercraft of the DC Comics superhero Batman.Boeing 929
The Boeing 929 Jetfoil is the name for a passenger-carrying waterjet-propelled hydrofoil design by Boeing.
Boeing began adapting many systems used in jet airplanes for hydrofoils. Robert Bateman led development. Boeing launched its first passenger-carrying waterjet-propelled hydrofoil in April 1974. It could carry from 167 to 400 passengers. It was based on the same technology pioneered by the patrol hydrofoil Tucumcari, and used some of the same technology used in the Pegasus class military patrol hydrofoils. The product line was licensed to the Japanese company Kawasaki Heavy Industries.Boeing hydrofoils
Boeing has designed and built several hydrofoil craft for both military and civilian use.Flyak
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 (7.6 m·s−1, 16.9 mph) can be achieved on calm water.Foilboard
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. Laird Hamilton, a prominent figure in the invention of big wave tow-in surfing, later discovered the foilboard's capability to harness swell energy with the use of a jet ski, pulling the rider into a wave. Mango Carafino, a big wave tow surfing athlete and water sport instructor from the Hawaiian Island of Maui, is the original innovating developer of the hydrofoil board design for stand-up hydro foil boarding (kitesurfing) and behind boat applications. The stand-up design allows the rider to glide with the moving wave by harnessing the kinetic energy with the underwater swell. Hydrofoil kiteboards allow the rider to achieve the same result with the use of a kite. The hydrofoil minimizes the effects of choppy or rough conditions. Due to the hydrofoil's underwater characteristics, the rider can angle higher into the wind than on traditional kiteboards which ride on the surface of the water.
Foils are used on wind-surfboards through design development from Neil Pryde Maui, inventors of hydrofoil sailing "windsurfing" boards. Using a moderately sized sail, a foil windboard can achieve speeds over 6 knots faster than the apparent wind.
In 2017, Lift Foils, a small company from Puerto Rico, introduced to the world to the first commercially available electric powered hydrofoil surfboard.HD-4
HD-4 or Hydrodome number 4 was an early research hydrofoil watercraft developed by the scientist Alexander Graham Bell. It was designed and built at the Bell Boatyard on Bell's Beinn Bhreagh estate near Baddeck, Nova Scotia. In 1919, it set a world marine speed record of 70.86 miles per hour (114.04 km/h).HMCS Bras d'Or (FHE 400)
HMCS Bras d'Or (FHE 400) was a hydrofoil that served in the Canadian Forces from 1968 to 1971. During sea trials in 1969, the vessel exceeded 63 knots (117 km/h; 72 mph), making her the fastest unarmed warship in the world at the time.
The vessel was originally built from 1960 to 1967 for the Royal Canadian Navy, as a project for the testing of anti-submarine warfare technology on an ocean-going hydrofoil. The RCN was replaced on 1 February 1968 by the unified Canadian Armed Forces, and Bras d'Or was commissioned into that service several months later. Changes in priorities and cost overruns later led to the project's cancellation.
Bras d'Or was named in honour of Bras d'Or Lake on Nova Scotia's Cape Breton Island, where inventor Alexander Graham Bell performed hydrofoil experiments in the early 20th century near his estate and new laboratory at Beinn Bhreagh, setting the world watercraft speed record in the process. In 1909 the lake was also the historic site of the first flight of an aircraft in Canada and the British Commonwealth; the airplane, named the Silver Dart, was built by the Aerial Experiment Association under Dr. Bell's tutelage. The lake's name was thus fitting for a hydrofoil vessel which could 'fly' above an ocean's surface.Human-powered hydrofoil
A human-powered hydrofoil is a small hydrofoil watercraft propelled entirely by the muscle power of its operator(s). Hydrofoils are the fastest water-based vehicles propelled solely by human power. They can reach speeds of up to 34 km/h (21 mph; 18 kn), easily exceeding the world records set by competitive rowing which stand at about 20 km/h (12 mph; 11 kn). This speed advantage is achieved since hydrofoils lack a submerged body to provide buoyancy, greatly reducing the drag force.Kyushu Railway Company
The Kyushu Railway Company, also referred to as JR Kyushu (JR九州, Jeiāru Kyūshū), is one of the constituent companies of Japan Railways Group (JR Group). It operates intercity rail services in Kyushu, Japan and the JR Kyushu Jet Ferry Beetle hydrofoil service across the Tsushima Strait between Fukuoka and Busan, South Korea. It also operates hotels, restaurants, and drugstores across its service region. JR Kyushu's headquarters are in Hakata-ku, Fukuoka.Lun-class ekranoplan
The Lun-class ekranoplan is a ground effect vehicle (GEV) designed by Rostislav Evgenievich Alexeyev in 1975 and used by the Soviet and Russian navies from 1987 until sometime in the late 1990s.It flew using the lift generated by the ground effect of its large wings when within about four metres (13 ft) above the surface of the water. Although they might look similar to regular aircraft, and have related technical characteristics, ekranoplans like the Lun are not aircraft, seaplanes, hovercraft, nor hydrofoils. Rather, "ground effect" is a distinct technology. The International Maritime Organization classifies these vehicles as maritime ships.The name Lun comes from the Russian for harrier.Muravey-class patrol boat
The Muravey class is the NATO reporting name for a class of hydrofoil patrol boats built for the Soviet Navy between 1983 and 1989. The Soviet designation was Project 133 Antares.Nacra 17
The Nacra 17 is a performance catamaran used for sailing. It was designed in 2011, went into production in 2012 and has been the focus of multihull sailing at the Olympic Games since its conception.
The Nacra has been converted to a sailing hydrofoil for the 2020 Tokyo Olympics.Pegasus-class hydrofoil
The Pegasus-class hydrofoils were a series of fast attack patrol boats employed by the U.S. Navy. They were in service from 1977 through 1993. These hydrofoils carried the designation "PHM" for "Patrol, Hydrofoil, Missile." The Pegasus-class vessels were originally intended for NATO operations in the North Sea and Baltic Sea. Subsequently, participation by other NATO navies, including Germany and Italy, ceased and the U.S. Navy proceeded to procure six PHMs, which were highly successful in conducting coastal operations, such as narcotics interdiction and coastal patrol, in the Caribbean basin.Raketa (hydrofoil)
Raketa (Russian: Раке́та, Rocket) was the first type of hydrofoil boats commercially produced in the Soviet Union. They were manufactured from 1957 until the early 1970s. The chief designer was Rostislav Alexeyev and the project 340 vessels had been planned already in the late 1940s.
The first model, Raketa-1, was built by Krasnoye Sormovo (Красное Сормово) shipbuilding plant in Sormovo, Nizhny Novgorod (Нижний Новгород). On its maiden voyage, on 25 August 1957, it carried 30 passengers from Nizhny Novgorod to Kazan (420 km) in seven hours.
Raketa boats were soon in wide commercial service on the Volga River and elsewhere in the Soviet Union. To this day, the name is often used generically in Russian for all hydrofoil river boats. Later designs include the Meteor and Kometa types, among many others.
There were several versions of project 340 vessels: project 340 (проект 340), project 340E (проект 340Э) and project 340ME (проект 340МЕ).Sailing hydrofoil
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 twice the wind speed.
Both monohull and multihull sailboats can be retrofitted with hydrofoils, although greater stability can be achieved by using the wider planform of a catamaran or trimaran.Sarancha-class missile boat
The Sarancha class is the NATO reporting name for a hydrofoil missile boat built for the Soviet Navy. The Soviet designation was Project 1240 Uragan (Серия 1240 Ураган- Hurricane).TurboJET
TurboJET (Chinese: 噴射飛航; Cantonese Yale: Panseh fēihòng) is the brand name for the operations of the Hong Kong-headquartered Shun Tak-China Travel Ship Management Limited (Chinese: 信德中旅船務管理有限公司), which was established from the joint venture between Shun Tak Holdings Limited and China Travel International Investment Hong Kong Limited in July 1999. It operates hydrofoil ferry services between Hong Kong, Macau, Shenzhen and Zhuhai, in the Pearl River Delta area.Turya-class torpedo boat
"Turya class" is the NATO reporting name for a class of hydrofoil torpedo boats built for the Soviet Navy and Soviet allies. The Soviet designation was Project 206M.USS High Point
USS High Point (PCH-1) was a High Point-class patrol craft of the United States Navy, and was launched 17 August 1962 by J.M. Martinac Shipbuilding Corp. in Tacoma, Washington as a sub-contractor to Boeing in Seattle, Washington at a cost of $2.08 million, together with Vickers, Inc. Marine and Ordnance Department in Waterbury, Connecticut, which provided hydraulic components and controls. Electronic equipment aboard, including automatic stabilization equipment, dead reckoning navigation system, was developed and manufactured by the United Aircraft Corp. Hamilton Standard Division.She was placed in service 15 August 1963 with Lieutenant H. G. Billerbeck in charge. High Point was named after High Point, North Carolina.High Point was the first of a series of hydrofoil craft designed to evaluate the performance of this kind of craft for the US Navy. She has three submerged foils containing propulsion nacelles and propellers, and was also capable of riding on her hull like a more conventional ship. On her foils, High Point obtained very high-speed and was evaluated for mobility and flexibility as an antisubmarine force. The craft carried out tests in Puget Sound during 1963-1967.High Point was decommissioned in March 1975 from the US Navy and transferred to the United States Coast Guard on 4 April 1975 and commissioned as USCGC High Point (WMEH-1). The Coast Guard acquired the ship to evaluate hydrofoil characteristics for use in law enforcement, search and rescue and marine environmental protection missions. After a first round of tests in Puget Sound, she was transferred to San Francisco. While attempting to moor at Treasure Island Naval Base, the turbine exploded. The $300,000 cost of repair was not in the Coast Guard budget so she was again decommissioned 5 May 1975 and returned to the Navy. High Point was stricken from the Naval Vessel Register sometime in 1980. In 2002 she was purchased by a private owner intent on restoring the craft, but the effort did not succeed. In 2005 Terence Orme purchased High Point to save it from scrap. Currently docked at Tongue Point near Astoria, Oregon, volunteers are restoring the vessel as a future museum.