Hovercraft
2008/9 Schools Wikipedia Selection. Related subjects: Air & Sea transport; Engineering
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A Hovercraft, or Air-Cushion Vehicle (ACV), is an amphibious vehicle or craft, designed to travel over any sufficiently smooth surface - land or water - supported by a cushion of slowly moving, low-pressure air, ejected downwards against the surface close below it.
History
In the mid-1870s, the British engineer Sir Mark Daniel Webber 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 of chin.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.
The first fully functional, rigid-walled hovercraft was designed by Austrian Dagobert Müller von Thomamühl and built by the Imperial Austro-Hungarian Navy (Kaiserliche und Königliche Kriegsmarine) "Seearsenal" (Naval base) at Pola. The 'Versuchsgleitboot - System Thomamühl' was launched on 2/9/1915 and was 13 m long, 4 m wide, displaced about 6.5 tonnes, had a crew of 5 men, and had a top speed of 32+ knots. By 1916 it was undergoing testing as a fast-torpedo boat and was equipped with 2 torpedoes, one Schwarzlose machine gun and several 6kg 'water-bombs', intended for anti-submarine use. It had 2 propellers, each of which was driven by 2 x 6-cylinder 120 hp airplane egines, a fifth 4-cylinder 65 hp engine was used to blow air under the hull, creating the 'air-cushion or hover' effect. After wide ranging full scale sea trials, the vessel was eventually scrapped in 1917 and the engines returned to the naval air-arm (Luftfahrttruppe); no further testing or research into hovercrafts was undertaking by the Imperial Austro-Hungarian navy during the period up to its eventual capitulation.
Finnish engineer Toivo J. Kaario, head inspector of Valtion Lentokonetehdas (VL) airplane engine workshop, began to design an air cushion craft in 1931. He constructed and tested his craft, dubbed pintaliitäjä (Surface Glider), and received its Finnish patents 18630 and 26122. Kaario is considered to have designed and built the first functional ground effect vehicle, but his invention did not receive sufficient funds for further development.
The first to give scientific description of the ground effect and to provide theoretical methods of calculation of air cushion vehicles was Konstantin Tsiolkovsky in his 1927 paper "Air Resistance and the Express Train". Since then Soviet engineer Vladimir Levkov began to develop air cushion vehicles. In the mid 1930s, Soviet engineer Vladimir Levkov assembled about 20 experimental air-cushion boats (fast attack craft and high-speed torpedo boats). The first prototype, designated L-1, had a very simple design which consisted of two small wooden catamarans that were powered by three engines. Two M-11 radial aero-engines were installed horizontally in the funnel-shaped wells on the platform which connected the catamaran hulls together. The third engine, also an air-cooled M-11, was placed in the aft part of the craft on a removable four-strut pylon. An air cushion was produced by the horizontally-placed engines. During successful tests, one of Levkov's air-cushion craft, called fast attack L-5 boat, achieved a speed of 70 knots (130 km/h).
Air cushion on hovercraft principle
The first technically and commercially viable hovercraft was invented and patented by the English inventor Christopher Cockerell in 1955.
However, there had been numerous previous experimental attempts to design vehicles using the ground-effect principle, including prototypes built by Russian and German naval designers in WW1. In the US during World War II, Charles J. Fletcher designed his "Glidemobile" while he was a United States Navy Reservist. The design worked on the principle of trapping a constant airflow against a uniform surface (either the ground or water), providing anywhere from 10 inches (25 cm) to 2 feet (61 cm) 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 home town of Sparta Township, New Jersey, the design was immediately appropriated by the United States Department of War and classified, denying Fletcher the opportunity to patent his creation. As such Fletcher's work was largely unknown until a case was brought (British Hovercraft Ltd v. The United States of America) in which the British corporation maintained that its rights, coming from to Sir Christopher Cockerell's patent, had been infringed. British Hovercraft's claim, seeking US$104,000,000 in damages, was unsuccessful. In a case brought in 1985, Patent agents BTG successfully sued the US Department of Defence, being awarded $6M in damages in 1990.
However, Colonel Melville W. Beardsley (1913-1998), an American inventor and aeronautical engineer, received $80,000 from Cockerell for his rights to American patents. Beardsley worked on a number of unique ideas in the 1950s and '60s which he patented. His company built craft based on his designs at his Maryland base for the US Government and commercial applications. Beardsley later worked for the US Navy on developing the Hovercraft further for military use. Dr. W. Bertelsen also worked on developing early ACVs in the USA. Dr. Bertelsen built an early prototype of a hovercraft vehicle in 1959 (called Aeromobile 35-B), and was photographed for Popular Science magazine riding the vehicle over land and water in April on 1959. The article on his invention was the front page story for the July, 1959 edition of Popular Science.
In 1952 the British inventor Christopher Cockerell worked with air lubrication with test craft on the Norfolk Broads. From this he moved on to the idea of a deeper air cushion. Cockerell used simple experiments involving a vacuum cleaner motor and two cylindrical cans to create his unique peripheral jet system, the key to his hovercraft invention, patented as the 'hovercraft principle'. 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. He designed a working model vehicle based on his patent. Showing his model to the authorities led to it being put on the secret list as being of possible military use and therefore restricted. However, to keep Britain in the lead in developments, in 1958 the National Research and Development Corporation took on his design (paying £1000 for the rights) and paid for an experimental vehicle, the SR-N1 to be built by Saunders-Roe to Cockerell's design. It was launched on 11 June 1959. Shortly afterwards it made a crossing from France to the UK on the 50th anniversary of Bleriot's cross Channel flight. However, stability problems remained, and it was the invention of the segmented skirt by his close colleague and collaborator, engineer Denys Bliss in 1962 which solved these and made the hovercraft a commercial reality. According to patent agents BTG the Bliss patent was "the key factor for success". A further patent 1239745 "Anti-ditch shift of cushion C.P" was taken out jointly by Cockerell and Bliss in Jul. 1967:
Cockerell was knighted for his services to engineering in 1969. Sir Christopher coined the word 'Hovercraft' to describe his invention.
Design
2. Air
3. Fan
4. Flexible skirt
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 on the bottom of the hovercraft, (the impeller) which is responsible for lifting the vehicle by forcing high pressure 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 (these engines help push the hovercraft). Some hovercraft utilize 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.
Hovercraft
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 test 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 24 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 Townsend Ferries also started a service to Calais from Dover, which was soon superseded by that of Seaspeed.
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 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. These were both used by rival operators Hoverlloyd and Seaspeed to operate regular car and passenger carrying services across the English Channel. Hoverlloyd operated from Ramsgate, where a special hoverport had been built at Pegwell Bay, to Calais. Seaspeed operated from Dover, 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, the advancing age of the SR-N4 hovercraft 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. In England, hovercraft of the Burnham-on-Sea Area Rescue Boat (BARB) are used to rescue people from thick mud in Bridgwater Bay.
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 Malmö, Sweden.
An experimental service was operated in Scotland across the Firth of Forth (between Kirkcaldy and Portobello, Edinburgh), 16-28 July 2007. Marketed as Forthfast, the service used a craft chartered from Hovertravel Ltd and achieved 85% loadings. The possibility of establishing a permanent service is now under consideration.
From 1960s, several commercial lines were operated in Japan, without much success. In the country, the only commercial line still available is the one that links Ōita Airport and the central Ōita.
Military hovercraft
First applications of the hovercraft in military use was with the SR.N1 through SR.N6 craft built by Saunders-Roe in the Isle of Wight in the UK and used by the UK joint forces. To test the use of the hovercraft in military applications the UK set up the Interservice Hovercraft Trials Unit (IHTU) base at Lee-on-the-Solent in the UK (now the site of the Hovercraft Museum). This unit carried out trials on the SR.N1 from Mk1 through Mk5 as well as testing the SR.N2, 3, 5 and 6 craft. Currently the Royal Marines use the Griffon 2000TDX as an operational craft. This craft was recently deployed by the UK in Iraq.
In the US, during the 1960s, Bell licenced and sold the Saunders-Roe SRN-5 as the Bell SK-5. They 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. This was used in both the UK SR.N5 curved deck configuration and later with modified flat deck, gun turret and grenade launcher designated the 9255 PACV. The United States Army also experimented with the use of SR.N5 hovercraft in Vietnam. Three hovercraft with the flat deck configuration were deployed to Dong Tam in the Mekong delta region and later to Ben Luc. They saw action primarily in the Plain of Reeds. One was destroyed in early 1970 and another in August of that same year after which the unit was disbanded. The only remaining U.S. Army SR.N5 hovercraft is currently on display in the Army Transport Museum in Virginia. Experience led to the proposed Bell SK-10 which was the basis for the LCAC-class air-cushioned landing craft now deployed.
The former Soviet Union was one of the first few nations to use a hovercraft, the Bora, as a guided missile corvette.
The Finnish Navy designed an experimental missile attack hovercraft class, Tuuli class hovercraft, in the late 1990s. The prototype of the class, Tuuli, was commissioned in 2000. It proved an extremely successful design for a [littoral] fast attack craft, but due to fiscal reasons and doctrinal change in the Navy, the hovercraft was soon withdrawn.
The Hellenic Navy operates four Russian-designed Zubr class LCAC. This is the world’s largest military air-cushioned landing craft.
Other ACVs
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 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.
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.
Hovertrain
Several attempts have been made to adopt air cushion technology for use in fixed track systems, in order to take advantage of the lower frictional forces so as to deliver 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 main protagonist and the adoption of TGV by the French government as its high-speed ground transport solution.
A test track for a tracked hovercraft system was built at Earith near Cambridge, managed by Tracked Hovercraft Ltd., with Denys Bliss as Director in the early 1970's, only to be axed by the Aerospace Minister, Michael Heseltine Records on this project are available from the correspondence and papers of Sir Harry Legge-Bourke, MP at Leeds University Library. http://www.leeds.ac.uk/library/spcoll/handlists/080MS742_LBourke.pdf]and http://www.leeds.ac.uk/library/spcoll/handlists/084670M42_cambridge.pdf]. Heseltine was accused by Airey Neave and others of misleading the House of Commons when he stated that the government was still considering giving financial support to the Hovertrain, when the decision to pull the plug had already been taken by the Cabinet. Michael Heseltine
Despite promising early results, the Cambridge project was abandoned in 1973 due to financial constraints, but parts of the project were picked up by the engineering firm McAlpine, only to be finally abandoned in the mid 1980's. The tracked hovercraft project was in fierce competition for funding from Professor Laithwaite's Maglev train system and there was intense competition between the two prospective British systems for funding and credibility.
At the other end of the speed spectrum, the Dorfbahn Serfaus has been in continuous operation since 1985. This is an unusual underground air cushion funicular rapid transit system, situated in the Austrian ski resort of Serfaus. Only 1,280 m (4,199 ft) long, the line reaches a maximum speed of 25 miles per hour (40 km/h). The similar system also exist in Narita International Airport near Tokyo, Japan.
Records
- World's Largest Civil Hovercraft - The BHC SRN4 Mk III at 56.4 m (185 ft) length and 310 metric tons (305 tons) weight, can accommodate 418 passengers and 60 cars.
- English Channel crossing - 22 minutes by Princess Anne MCH SR-N4 Mk3 on 14 September 1995
- World's Hovercraft Speed Record - 18 September 1995 - Speed Trials, Bob Windt (USA) 137.4 km/h. (85.87 mph), 34.06 secs measured kilometre
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.
The Hovercraft Club of Great Britain organises inland and coastal cruising hovercraft races in various venues across the United Kingdom.
Modern Hovercraft Development
The real innovation in hovercraft development occured in 1957, and was revealed to the public in 1960. It was the invention of the "Double-Walled Flexible Skirt" by Mr. Norman B. McCreary in Little Rock, Arkansas, USA, (Patent No.3,532,179) and was published in the Arkansas Gazette Newspaper on Jan. 25, 1960 and in Science and Mechanics Magazine in June, 1960. This was the Conception and Technological Development that enabled hovercraft to travel over uneven terrain or waves of the sea. It later became known as the "bag skirt" as it inflated around the edge of the hovercraft. It would raise and lower the hovercraft off the ground 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 utalized a "Double-Walled Flexible Skirt" system for practical hovercraft operations, (see time line Naval Engineering Journal, Febuary 1985, page 261).