A hovercraft (air-cushion vehicle, ACV) is a craft capable of traveling over surfaces while supported by a cushion of slow moving, high-pressure air which is ejected against the surface below and contained within a "skirt." Although supported by air, hovercraft are not considered an aircraft.
Hovercraft are used throughout the world as specialized transports. Because they are supported by a cushion of air, hovercraft are unique among all other forms of ground transportation in their ability to travel equally well over land, ice, and water. Small hovercraft are used for sport, or passenger service, while giant hovercraft have civilian and military applications, and are used to transport cars, tanks, and large equipment in hostile environments and terrain.
In 1915 Austrian Dagobert Müller built the world's first air-cushion vehicle. Shaped like a section of a large aircraft wing, the craft was propelled forward by four aero engines, with a fifth that blew air under the front of the craft to increase the air pressure under it. In motion, the craft also trapped air under the front, increasing lift. Thus the Versuchsgleitboot is half way between the ram-air vehicles similar to later Soviet designs, and the modern hovercraft that uses air forced into a skirt. Designed as a fast torpedo boat, the Versuchsgleitboot had a top speed over 32 knots.
The idea of the modern hovercraft is most often associated with Sir Christopher Cockerell. Cockerell had built several models of his hovercraft design in the early 1950s, featuring an engine mounted to blow from the front of the craft into a cavity below it, combining both lift and propulsion. In spite of tireless efforts to arrange funding the military proved uninterested, as he later noted, "The Navy said it was a plane not a boat; the RAF said it was a boat not a plane; and the Army were ‘plain not interested’". As a result the work was allowed to be declassified, and he finally able to convince the National Research Development Corporation to fund development of a full-scale model. In 1958 the NRDC placed a contract with Saunders Roe for the development of what would become the SR.N1, for "Saunders Roe, Nautical 1". Lacking a skirt, the SR.N1 required a powerful 450 hp engine for lift. It made a successful crossing of the English Channel on 25 July 1959.
Hovercraft can be powered by one or more engines. Small craft, such as the SR.N6, usually have one engine with the drive split through a gearbox. On vehicles with several engines, one usually drives the fan (or impeller), which is responsible for lifting the vehicle by forcing high pressure air under the craft. The air inflates the "skirt" under the vehicle, causing it to rise above the surface. Additional engines provide thrust in order to propel the craft. Some hovercraft use ducting to allow one engine to perform both tasks by directing some of the air to the skirt, the rest of the air passing out of the back to push the craft forward.
American inventor Norman B. McCreary of Little Rock, Arkansas, is credited with inventing and patenting the "Double-Walled Flexible Skirt". The design first appeared in the Arkansas Gazette Newspaper on January 25, 1960 and later in Science and Mechanics Magazine in June 1960. Later known as the "Bag Skirt", it inflated around the edge of the hovercraft, and was a major technological development enabling hovercraft to more effectively travel over uneven terrain or waves. The "Bag Skirt" would raise and lower the hovercraft off the surface by inflation and deflation of the "Double-Walled Flexible Skirt". Later, fingers were added to the bottom of the skirt to compensate for wear and reduce drag. After this concept was made public in 1960, all hovercraft began utilizing a "Double-Walled Flexible Skirt" system for practical hovercraft operations, and additional development of the skirt would continue in the U.K under the supervision of British engineer Cecil Latimer-Needham. Initially the skirt was of equal length around the base of the hovercraft. In the mid-1960s it was discovered that the ability of hovercraft to overcome small obstacles was enhanced by adjusting the vehicle's pitch 15 degrees upward. This resulted in excess wear on the trailing skirts, which dragged along the surface and lasted only 20% as long as the bow skirt sections. The problem was overcome, beginning with the SR.N6, by angling the lower edge of the skirt 15 degrees so that it rested even with the surface while the vehicle maintains a 15 degree upward pitch.
In Finland, small hovercraft are widely used in maritime rescue and during the rasputitsa ("mud season") as archipelago liaison vehicles. In England, hovercraft of the Burnham-on-Sea Area Rescue Boat (BARB) are used to rescue people from thick mud in Bridgwater Bay. Avon Fire and Rescue Service became the first fire service in the UK to operate a hovercraft. It is used to rescue people from thick mud in the Weston-super-Mare area and during times of inland flooding. Numerous fire departments around the U.S./Canadian Great Lakes operate hovercraft for water and ice rescues, often of ice fisherman stranded when ice breaks off from shore.
The Soviet Union was the world's largest developer of military hovercraft. Their designs range from the small Czilim class ACV, comparable to the SR.N6, to the monstrous Zubr class LCAC, the world's largest hovercraft. The Soviet Union was also one of the first nations to use a hovercraft, the Bora, as a guided missile corvette, though this craft possessed rigid, non-inflatable sides. With the fall of the Soviet Union most Soviet military hovercraft fell into disuse and disrepair. Only recently has the modern Russian Navy begun building new classes of military hovercraft.
Topics of Interest
Hoverbarge: A real benefit of air cushion vehicles in moving heavy loads over difficult terrain, such as swamps, was overlooked by the excitement of the British Government funding to develop high-speed hovercraft. It was not until the early 1970s that the technology was used for moving a modular marine barge with a dragline on board for use over soft reclaimed land.
Hovertrain: Several attempts have been made to adopt air cushion technology for use in fixed track systems, in order to utilize the lower frictional forces for delivering high speeds. The most advanced example of this was the Aérotrain, an experimental high speed hovertrain built and operated in France between 1965 and 1977. The project was abandoned in 1977 due to lack of funding, the death of its lead engineer and the adoption of TGV by the French government as its high-speed ground transport solution.
The Hovercraft Museum, located in Hampshire, England, is dedicated to hovercraft.
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