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.

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.

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,and did not have any skirt at first

trials. 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 true passenger-carrying hovercraft 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.

Bell licenced and sold the 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.

As well as Saunders Roe and Vickers (which combined in 1966 to form the British Hovercraft

Corporation), 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).

In the late 1960s and early 1970s, Jean Bertin developed a hovercraft train dubbed the

Aérotrain in France. His I-80 prototype established the world speed record for overland air

cushion vehicles with a mean speed of 417.6 km/h (260 mp/h) and a top speed of 430 km/h (267

mp/h).

By 1970 the largest British hovercraft were in service, the Mountbatten class SR-N4 model,

each powered by four Rolls-Royce Proteus engines, regularly carrying cars and passengers

across the English Channel from Dover or Ramsgate to Calais. This service ceased in 2000

after years of 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.

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.

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.