# Hydrofoil

The U.S. Navy's XCH-4, with hydrofoils clearly lifting the hull out of the water

A hydrofoil is a lifting surface, or foil, which operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats using hydrofoil technology are also simply termed hydrofoils. As speed is gained, hydrofoils lift the boat's hull out of the water, decreasing drag and thus allowing greater speeds.

## Description

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 in order to greatly reduce hull drag. This gives a further corresponding increase in speed and efficiency of operation in terms of fuel consumption.

A wider adoption of the technical innovations of 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.[1]

## Hydrodynamic mechanics

The two types of hydrofoils: surface-piercing and fully submerged

Since air and water are governed by similar fluid equations, albeit with vastly different levels of viscosity, density, and compressibility, the hydrofoil and airfoil (both types of foil) create lift in identical ways. 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 increasingly on the much smaller surface area of the hydrofoil, and decreasingly on the hull, creating a marked increase in speed.

### Foil configurations

Early hydrofoils used V-shaped 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, a computer and active surfaces.

## History

Forlanini's hydrofoil over Lake Maggiore, 1910.

### Prototypes

Italian inventor Enrico Forlanini began working on hydrofoils in 1898 and used a "ladder" foils system. Forlanini obtained patents in Britain and the United States for his ideas and designs.[2][3]

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).[4]

Alexander Graham Bell's HD-4 on a test run, c.1919

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 9 September 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 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.[5] 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 knots (110 km/h), bending the lifting foil.[6]

A schematic illustration of self-stabilizing systems for fully submerged hydrofoils. Its computer gathers data for the boom position and current water level to determine the required flap position.

### First passenger boats

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.[citation needed] 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".[7] 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.[8]

## Military usage

A Project 206M "Shtorm" (NATO - Turya class) patrol fast attack craft hydrofoil of the Cuban Navy.
File:Sparviero DN-ST-84-03940.jpg
Aerial port beam view of the Italian Sparviero class hydrofoil-missile NIBBIO P-421 underway.
File:PHM-4.jpg
USS Aquila, a military hydrofoil. The T-shaped foils are visible just below the water.
HMCS Bras d'Or, a military concept hydrofoil.

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 U.S. 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.[9] The XCH-4 (officially, Experimental Craft, Hydrofoil No. 4), designed by William P. Carl, exceeded speeds of 65 mph and was mistaken for a seaplane due to its shape.[10] 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, and were capable of sinking all but the largest surface vessels.[citation needed]

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.

## Sailing and sports

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).[11][12]

Another trimaran sailboat is the Windrider Rave.[13] 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.[citation needed]

Hobie Sailboats produced a production foiling trimaran, the Hobie Trifoiler, the fastest production sailboat. 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.[14]

Soviet-built Voskhod on the North Sea Canal, the Netherlands
TurboJET's Urzela JetFoil
TurboJET's Barca Foilcat

## Modern passenger boats

File:MV Flying Poseidon (1982) and Bodrum Lines Kometas at Rhodes with MV Costa Fortuna 11am 5-8-2011.jpg
Flying Poseidon (built 1982[15]) had just berthed at Rhodes from Fethiye when the sister Kometas hydrofoil[16] from Bodrum also arrived from Turkey in 2011.

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.[17]

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.

### Current operation

Current operators of hydrofoils include:

Hydrofoil high-speed boat docking in St. Petersburg, Russia from a run to Peterhof Palace.
Passenger hydrofoil “Flying Dolphin Zeus” moving at high speed near Piraeus

### Discontinued operations

• Until 31 December 2013, Fast Flying Ferries operated by Connexxion provided a regular public transport service over the North Sea Canal between Amsterdam Central Station and Velsen-Zuid in the Netherlands, using Voskhod 2M hydrofoils. It was stopped due to a new speed limit.
• Between 1981 and 1990, "Transmediterranea" used to operate a service of hydrofoils connecting Ceuta and Algeciras in the Strait of Gibraltar. The crossing took half an hour, in comparison to the hour and a half of conventional ferries. Due to the common extreme winds and storms that take place in winter in the Strait of Gibraltar, the service was substituted in 1990 by catamarans, which were also able to carry cars. At the peak of the year, in summer, there was a service every half an hour in each direction. This high-speed connection had a big impact on the development of Ceuta, facilitating one-day business trips to mainland Spain.
• Between 1964 and 1991 the Sydney hydrofoils operated on Sydney Harbour between Circular Quay and Manly.
• During 70s and 80s there were frequent services between Belgrade and Tekija in Đerdap gorge. The distance 0f 220 km was covered in 3 hrs 30 minutes downstream and 4 hrs upstream.[18]

See also the history of Condor Ferries which operated 6 hydrofoil ferries over a 29-year period between the Channel Islands, south coast of England and Saint-Malo.

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:

• Hydrofoils are sensitive to impacts with floating objects and marine animals. On hitting something, a hydrofoil boat may fall off the foils, which often creates a perception of increased danger. However, hydrofoils operating in Hong Kong, reportedly some of the most littered waters, operate relatively trouble free. The Hong Kong Marine department maintains online records of all accidents in its waters.[citation needed]
• Hydrofoils have sharp edges that reside in the water while in operation. These edges can fatally injure marine mammals (e.g. whales) they hit.
• Hydrofoils are expensive to build. A vessel like the Boeing Jetfoil currently costs about three times the price of an equivalent catamaran passenger ferry. The increased costs are not always economically justifiable by consumers. Simpler designs such as those of Russian design and those by Rodriquez in Italy are more competitive in price and are still being produced today for operation on ferry routes.
• It is a very conservative industry. Hydrofoils are still considered exotic by many commercial operators of high-speed craft and many will not take the risk of trying such exotic vessels when they have no experience in operating them. Those that do have found benefit in their operation.[citation needed]
• They are technically complex and require high maintenance, which has put an end to most military hydrofoil projects. The US Navy, for example, developed some of the most technically advanced hydrofoils around but they could never get them to be reliable due to the complex propulsion systems and ride control required. This eventually led to the suspension of their hydrofoil projects. Most other Navy developments followed the same fate. The Russian Navy still operates very large hydrofoils and have been successful in their design, largely because they have opted for simpler systems that might not have the same performance as the US designs but are much more robust and reliable. In Hong Kong, the increasing maintenance cost of keeping the old Jetfoils running is resulting in these vessels slowly being phased out.

## References

2. "The Hydrofoil Resource Site". International Hydrofoil Society. Retrieved 2012-06-22.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
3. "Italian Hydroplane of Curious Type." Popular Mechanics, December 1911, p. 927.
4. hovercraft-museum.org. "Musthorn1". Retrieved 9 September 2009.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
5. "Jet Hydrofoil Shoots At World Record" Popular Mechanics, August 1953, pp. 70-71
6. The World Water Speed Record by Leo Villa and Kevin Desmond, 1976
7. SRI International (1961). "The Economic Feasibility of Passenger Hydrofoil Craft in U.S. Domestic and Foreign Commerce". Retrieved 9 September 2009.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
8. foils.org. "Enterprise". Retrieved 9 September 2009.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
9. "Sail Boat Stilts Boost Speed." Popular Mechanics, February 1956, p. 136.
10. "XCH4". International Hydrofoil Society. Retrieved 8 August 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>