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Hydrofoils

Hydrofoil craft are boats which typically possess a more or less conventional planing boat hull and which have one or more support arms extending from beneath the hull into the water. One or more foils for supporting the hull are connected to the lower ends of the support arms. When the hydrofoil craft has accelerated to a sufficient velocity through the water, the lift created by the foils raises the hull above the water's surface, thus eliminating the hull's resistance.

Attempts to create a hydrofoil had been made in England as early as 1861. The first hydrofoil to be tested successfully was built by Enrico Forlanini from Italy. It used a ladder system of foils and a 60 hp engine driving two counter-rotating air props. During testing on Lake Maggiore in 1906, this craft reached a top speed of 42.5 mph.

A March 1906 Scientific American article by American hydrofoil pioneer William E. Meacham explained the basic principle of hydrofoils. After reading this article Alexander Graham Bell, famous for inventing the telephone, began to sketch concepts of the "heavier than water craft", which he called the "hydrodrome". With Casey Baldwin he began hydrofoil experimentation in the summer of 1908. Bell's first hydrofoil, the HD-1, achieved speeds of 72 km/h in 1911 and 80 km/h in 1912. The HD-2 broke up. The HD-3 was built in 1913, but further work on hydrofoils slowed during the Great War. On 09 September 1919 Bell's HD-4 set a world marine speed record of 114 km/h (70.86 mph), when the world's fastest steamships travelled at only 50 km/h. This record stood for ten years.

Baron Hanns von Schertel worked on hydrofoils prior to and during World War II in Germany. The first hydrofoil built to carry passengers was made in 1936. Five different types of hydrofoils were ordered by the German Armed Forces during World War II. After the war Schertel's team was captured by the Russians. Schertel himself went to Switzerland, where he established the Supramar company. In 1952, Supramar launched the first commercial hydrofoil, PT10, in Lake Maggiore, between Switzerland and Italy.

The surface piecing hydrofoils were commercially developed in the mid fifties and produced in series by Rodriquez in Italy. As known, these are based on a monohull fitted with a forward- and aft-located surface piecing hydrofoil arrangement, which in the transverse section features a V-like shape of the foil span. Thus parts of the foil span protrude the waterline on both sides, and provide a transverse righting momentum when the craft heels causing the surface piercing foil span to be submerged. The hull is completely lifted out of the water at higher speed, being self stabilized in roll and pitch by the surface piecing hydrofoil arrangement. It is propelled by fully submerged propellers, mounted on inclined shafts. The advantages of this conceptual design over traditional monohulls of similar size were improved seakeeping and power-to-speed efficiency at service speed around 35 knots. The disadvantages were larger complexity, building costs, weight- and speed restriction. Therefore the design is generally limited to an overall length of around 40 m and maximum displacement of around 150 tons.

The design became very popular as passenger ferries, and today there is still a large number of these operating around the world, particularly in previous Russian countries, Japan and the Mediterranean countries. According to the inventor's opinion, there is probably no other high-speed concept that has transported the same amount of passengers so far. Compared to modern type high speed crafts, like catamarans and monohulls, the design has lost it's popularity, and can no longer compete in terms of speed requirements and passenger comfort, though it still maintain an edge over these regarding speed-to-power efficiency.

On of the most significant steps in high-speed marine technology development came in the mid seventies when Boeing, USA, developed the Jetfoil. As the Rodriquez hydrofoils, the conceptual design is based on a monohull that is lifted clear of the waterline at higher speed, However, the foil system is based on the fully submerged type, which consists of a substantially plane fully submerged foil span supported by three vertical struts. As opposed to the surface-piercing hydrofoil, it is not self-stabilizing and therefore depends on controllable flaps integrated to the following edge of the foilspan. The primary foil is located aft extending to the full width of the craft, and provides the primary lift and roll stabilization. A lesser foil (T-foil) is located in the centerline forward and supported by a vertical strut. This provides a secondary lifting force as well as the required pitch controlling momentum. All foils can be tilted upward when the craft is in a fully displacement mode. The 27.4 m and 117 tons displacement Jetfoil design has a normal service speed in foil-born mode of around 45 knots. This design has the advantage of excellent seakeeping and high speed-to-power efficiency. The disadvantages, however, are high building costs, technical complexity, overall weight- and payload capacity restrictions.

In the early nineties, Kvaerner Fjellstrand and Westamarin, both Norway, developed the Foilcat concept. This is basically a catamaran fitted with a fully submerged foil system that lifts the craft clear of the waterline, and operates at a service speed of around 45 knots. The largest design is 35 m and has a maximum displacement of around 175 tons. The design has basically the same advantages and disadvantages as the Jetfoils. This has limited its commercial acceptance. In order to reduce the large frictional resistance related to catamarans operating at speeds of around 45 knots, there is a resent trend towards developing foil assisted catamarans that is fitted with a fully submerged foil system for the purpose of lifting the hull partially out of the water. At the same time it is controlling the pitch, and to less degree roll and hive. Since they are operating in a partial displacement mode, and as such still have two hulls submerged in the water, they are still left with a major frictional drag.



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