Hovercraft

Related Topics:
Hovercraft

A hovercraft, or air-cushion vehicle (ACV), is a vehicle or craft that can be supported by a cushion of air ejected downwards against a surface close below it, and can in principle travel over any relatively smooth surface, such as gently sloping land, water, or marshland, while having no substantial contact with it.

History

The first recorded design for a vehicle which could be termed a Hovercraft was in 1716 by Emanuel Swedenborg, a Swedish designer, philosopher and theologian. His man-powered air cushion platform resembled an upside-down boat with a cockpit in the center and manually operated oar-like scoops to push air under the vehicle on each downward stroke. No vehicle was ever built, no doubt because it was realised that human effort could not have generated enough lift.

In the mid-1870s, the British engineer Sir John Isaac Thornycroft built a number of ground effect machine test models based on his idea of using air between the hull of a boat and the water to reduce drag. Although he filed a number of patents involving air-lubricated hulls in 1877, no practical applications were found. Over the years, various other people had tried various methods of using air to reduce the drag on ships.

Early development of the modern "hovercraft" began with a design of American inventor Charles J. Fletcher, who designed his "Glidemobile" while in the United States Navy during World War II. The design worked on the principle of trapping a constant airflow against a uniform surface (either the ground or water), providing anywhere from ten inches to two feet of lift to free it from the surface, and control of the craft would be achieved by the measured release of air. Shortly after being tested on Beezer's Pond in Fletcher's hometown of Sparta Township, New Jersey, the design was immediately appropriated by the United States Department of Defense and classified, denying Fletcher the opportunity to patent his creation. Fletcher's claim as the original inventor was substantiated during the case of British Hovercraft Ltd v. United States, in which the British corporation which maintained the rights to Sir Christopher Cockerell's patent unsuccessfully sought to win $104,000,000 in lost royalties.

Col. Melville W. Beardsley (1913-1998), an American inventor and aeronautical engineer, along with Dr. W. Bertelsen worked on developing early ACV's in the USA.

In 1952 the British inventor Christopher Cockerell designed a vehicle based on his 'hovercraft principle'. He was knighted for his services to engineering in 1969. Sir Christopher invented the word 'Hovercraft' to describe his invention.

Cockerell used simple experiments involving a vacuum cleaner motor and two cylindrical cans. He proved the workable principle of a vehicle suspended on a cushion of air blown out under pressure, making the vehicle easily mobile over most surfaces. The supporting air cushion would enable it to operate over soft mud, water, and marshes and swamps as well as on firm ground.

Design

Hovercraft typically have two (or more) separate engines (some craft, such as the SR-N6, have one engine with a drive split through a gearbox). One engine drives the fan (aka the impeller) which is responsible for lifting the vehicle by forcing air under the craft. One or more additional engines are used to provide thrust in order to propel the craft in the desired direction. Some hovercraft utilise 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.

Civil commercial hovercraft

The British aircraft manufacturer Saunders Roe which had aeronautical expertise developed the first practical man-carrying hovercraft, the SR-N1, which carried out several test programmes in 1959 to 1961 (the first public demonstration in 1959), including a cross-channel run. The SR-N1 was powered by one (piston) engine, driven by expelled air, and could carry little more than its own weight and two men. It did not have any skirt in its first trials, but it was found that the craft's lift was improved by the addition of a 'skirt' of flexible fabric or rubber around the hovering surface to contain the air. The skirt was an independent invention made by a Royal Navy officer who worked with Sir Christopher to develop the idea further.

The first passenger-carrying hovercraft to enter service was the Vickers VA-3, which in the summer of 1961 carried passengers regularly along the North Wales Coast from Wallasey to Rhyl. It was powered by two turboprop aero-engines and driven by propellers.

During the 1960s Saunders Roe developed several larger designs which could carry passengers, including the SR-N2, which operated across the Solent in 1962 and later the SR-N6, which operated across the Solent from Southsea to Ryde on the Isle of Wight for many years. Operations commenced on 24th July 1965 using the SR-N6 which carried just 38 passengers. Two modern 98 seat AP1-88 hovercraft now ply this route, and over 20 million passengers have used the service as of 2004.

As well as Saunders Roe and Vickers (which combined in 1966 to form the British Hovercraft Corporation (BHC)), other commercial craft were developed during the 1960s in the UK by Cushioncraft (part of the Britten-Norman Group) and Hovermarine (the latter being 'sidewall' type hovercraft, where the sides of the hull projected down into the water to trap the cushion of air).

The world's first car-carrying hovercraft made their debut in 1968, the BHC Mountbatten class SR-N4 models, each powered by four Rolls-Royce Proteus engines, were used to start the regular car and passenger carrying service across the English Channel from Dover, Ramsgate and Folkestone in England to Calais and Boulogne in France. The first SR-N4's had a capacity of 254 passengers and 30 cars, and a top speed of 83 knots (96 mph). The later SR-N4 MkIII's had a capacity of 418 passengers and 60 cars. The French-built SEDAM N500 Naviplane with a capacity of 385 passengers and 45 cars, of which only one example entered service, was used for a few years on the cross-channel service. The service ceased in 2000, after 32, years due to competition with traditional ferries, catamarans, and the opening of the Channel tunnel.

In 1998, the US Postal Service began using the British built Hoverwork AP.1-88 to haul mail, freight, and passengers from Bethel, Alaska to and from eight small villages along the Kuskokwim River. Bethel is far removed from the Alaska road system, thus making the hovercraft an attractive alternative to the air based delivery methods used prior to introduction of the hovercraft service. Hovercraft service is suspended for several weeks each year while the river is beginning to freeze to minimize damage to the river ice surface. The hovercraft is perfectly able to operate during the freeze-up period, however, it could potentially break the ice creating hazards for the villagers using their snowmobiles for transportation along the river during the early winter.

The commercial success of hovercraft suffered from rapid rises in fuel prices during the late 1960s and 1970s following conflict in the Middle East. Alternative over-water vehicles such as wave-piercing catamarans (marketed as the Seacat in Britain) use less fuel and can perform most of the hovercraft's marine tasks. Although developed elsewhere in the world for both civil and military purposes, except for the Solent Ryde to Southsea crossing, hovercraft disappeared from the coastline of Britain until a range of Griffon Hovercraft were bought by the Royal National Lifeboat Institution.

Military hovercraft

Bell licenced and sold the Saunder Roe SRN-5 as the Bell SK-5. There were deployed on trial to the Vietnam War by the Navy as PACV patrol craft in the Mekong Delta where their mobility and speed was unique. Advanced AACVs were developed with automated turrets and slab sides, but use was eventually abandoned. Experience led to the proposed Bell SK-10 which was the basis for the LCAC now deployed.

Records

The fastest ever crossing of the English Channel by a commercial car-carrying hovercraft was 22 minutes, recorded by the Princess Anne BHC SR-N4 in September 1995.
World's Largest Civil Hovercraft - The BHC SRN4 Mk III at 56.4-m (185-ft) length and 310 tonnes (305 tons) weight, can accommodate 418 passengers and 60 cars.

Hobbyists

There are an increasing number of small homebuilt and kit-built hovercraft used for fun and racing purposes, mainly on inland lakes and rivers but also in marshy areas and in some estuaries.

References

Notes

External links

Amphibious vehicles | Hovercraft

Gourt Home::Gourt :: Hovercraft