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. The Hovercraft is designed for traveling over land or water on a supportive cushion of slowly moving, low-pressure air.
Hovercraft have one 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 (the impeller) which is responsible for lifting the vehicle by forcing air under the craft. The air therefore must exit throughout the "skirt", lifting the craft above the area on which the craft resides. 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. Demonstrated at the Farnborough Airshow in 1960, it was shown that this simple craft could carry a load of up to 12 marines with their equipment as well as the pilot and co-pilot with only a slight reduction in hover height proportional to the load carried. The SR.N1 did not have any skirt instead using the peripheral air principle that Sir Christopher has patented. It was later found that the craft's hover height 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, Latimer-Needham, who sold his idea to Westland (parent company of Saunders-Roe), and 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 1962 carried passengers regularly along the North Wales Coast from Moreton, Merseyside 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 by Hovertravel 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.
In 1966 two Cross Channel passenger hovercraft services were inaugurated using hovercraft. Hoverlloyd ran services from Ramsgate Harbour to Calais and Townshend Ferries also started a service to Calais from Dover.
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 with 'normal' hovercraft skirts at the bow and stern).
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 gas turbine engines, were used to start the regular car and passenger carrying service across the English Channel from Dover, Ramsgate, where a special hoverport had been built at Pegwell Bay by Hoverlloyd, and Folkestone in England to Calais and Boulogne in France. The first SR-N4 had a capacity of 254 passengers and 30 cars, and a top speed of 83 knots (96 mph). The Channel crossing took around 30 minutes and was run rather like an airline with flight numbers. The later SR-N4 MkIII 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, and was used intermittently for a few years on the cross-channel service due to technical problems. The service ceased in 2000 after 32 years, due to competition with traditional ferries, catamaran, 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, this could potentially break the ice and create hazards for villagers using their snowmobiles 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.
In Finland small hovercraft are widely used in maritime rescue and during the rasputitsa ("mud season") as archipelago liaison vehicles.
The scandinavian airline SAS used to charter an AP. 1-88 Hovercraft for regular passengers between Copenhagen Airport, Denmark and the SAS Hovercraft Terminal in Malmo, Sweden.
A real benefit of air cushion vehicles in moving heavy loads over difficult terrain, such as swamps, was overlooked by the excitement of the 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.
In the 1970s UK company Mackace (Mackley Air Cushion Equipment) produced a number of successful Hoverbarges, such as the 250 ton payload "Sea Pearl" which operated in Abu Dhabi and the twin 160 ton payload "Yukon Princesses" which ferried trucks across the Yukon river to aid the pipeline build. Hoverbarges are still in operation today. In 2006, Hovertrans (formed by the original managers of Mackace) launched a 330 ton payload drilling barge in the swamps of Suriname.
The Hoverbarge technology is somewhat different to high-speed hovercraft, which has traditionally been constructed using aircraft technology. The initial concept of the air cushion barge has always been to provide a low-tech amphibious solution for accessing construction sites using typical equipment found in this area, such as diesel engines, ventilating fans, winches and marine equipment. The load to move a 200-ton payload ACV barge at 5 knots would only be 5 tons. The skirt and air distribution design on the high-speed craft again is more complex as they have to cope with the air cushion being washed out by a wave and wave impact. The slow speed and large mono chamber of the hover barge actually helps reduce the effect of wave action giving a very smooth ride.