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`How do hovercraft work? - Explain that Stuff
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`Hovercraft
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`by Chris Woodford. Last updated: December 2,
`2018.
`
`-40%
`
`I
`
`s it a boat or a plane? Maybe it's a flying
`saucer? Back in 1959, when the world's first
`hovercraft, SR.N1, floated out across the
`windy English Channel, people must have
`
`Buy New Timberland®
`Footwear
`wondered exactly what they were
`seeing. Like a boat, a hovercraft
`moves across water, but like a
`plane, it also pushes through the
`air with the help of propellers . The
`"big idea" is that a hovercraft can glide
`just as easily over water, land, or, ice.
`That makes it a perfect vehicle for
`getting round some of the world's most
`inaccessible areas—places where
`ordinary boats can't beach and planes
`can't land. How exactly does this
`unique and rather remarkable craft actually work? Let's take a closer look!
`
`Photo: A US Navy hovercraft (LCAC) photographed in 2008. Picture by Chad R. Erdmann courtesy of US Navy.
`Much of the deck is empty space, suitable for carrying huge amounts of drive-on, drive-off military cargo.
`
`What is a hovercraft?
`
`One part boat, one part airplane, and one part helicopter a hovercraft is a vehicle that traps a
`cushion of air underneath itself and then floats along on top of it. The air cushion holds it high
`above waves and land obstructions, making the craft superbly amphibious (equally capable
`of traveling on land or water or gliding smoothly from one to the other). That's why military
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`hovercraft, designed for swift beach landings, are often called LCACs (Landing Craft Air
`Cushion).
`
`
`
`
`Photo: A coastguard hovercraft photographed in 1971. Photo courtesy of NASA Ames Research Center (NASA-
`ARC)
`
`Hovercraft come in all shapes and sizes, from one-person fun machines and small beach
`rescue craft to giant passenger ferries capable of carrying over 400 passengers and 50 cars.
`Where boats are slowed by hulls that drag deep in the water, hovercraft ride fully clear, which
`means they use less fuel and can reach blistering speeds of up to 145kph (90mph). From ice
`and water to mud and sand, from floodplains and river deltas to mangrove swamps and
`frozen glaciers, the great advantage of a hovercraft is that it can glide with ease to places
`ordinary boats struggle to reach, and land people safely even where there are no harbors or
`landing stages.
`
`In practice, hovercraft have four broad applications: large commercial hovercraft are mostly
`used as high-speed people and car ferries; slightly smaller military LCACs are used as tried-
`and-tested beach landing craft; smaller niche craft are used for things like oil and gas
`prospecting, inshore search and rescue, and scientific surveys; and small, one-person
`recreational craft are often raced round courses like flying go-karts!
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`How do hovercraft work? - Explain that Stuff
`
`How does a hovercraft work?
`
`At first sight, you might think a hovercraft works in much the same way
`as a helicopter: it throws air down underneath itself and then simply
`rides along on top. But where a helicopter balances its own weight (the
`force of gravity pulling it down) with a massive down-draft of air
`(pushing it back up again), a hovercraft works in a much more subtle
`way that allows it to use far less air, far more efficiently, so getting by
`with a much smaller engine and considerably less fuel.
`
`The basic mechanism of a hovercraft is very simple: there's an engine
`(diesel or gasoline) that powers both a large central fan, pointing
`downward, and one or more other fans pointing backward. The central
`fan creates the lift that holds the craft above the waves; the other fans
`propel the craft backward, forward, or to the side. A rubber skirt (with or
`without fingers) traps a cushion of air under the craft. Side-wall
`hovercraft have only partial skirts: with solid sides and a skirt only at
`the front and back, they can be powered by quieter propellers or water-
`jet engines, making them quieter.
`
`Photo: A typical hovercraft has two or more fans. The main fan in the center blows air
`downward to push the craft upward, above the water. Two or more other fans at the back
`blow air backward to make the craft go forward. This is an example of action-and-
`reaction (Newton's third law of motion) at work.
`
`How much can a hovercraft carry?
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`How do hovercraft work? - Explain that Stuff
`A fan of a given power will create a certain amount of pressure under
`the craft. Now since:
`pressure = force / area
`
`it follows that a bigger hovercraft (one with a bigger overall area) can
`carry more weight than a smaller hovercraft with a fan the same size.
`Moreover, as Christopher Cockerell, the inventor of the hovercraft,
`quickly discovered, bigger hovercraft are more efficient than smaller
`ones:
`
`"In such vehicles, the lift or load carrying capacity is
`proportional to the plan area of the gas cushion or cushions.
`The energy required to contain the cushion or cushions is
`proportional to the peripheral dimension of the cushion or
`cushions. Thus for an increase in size of a vehicle, the lift
`increases proportionally to the area of the cushion or
`cushions whilst the energy requirements increase linearly
`with the periphery of the cushion or cushions. The efficiency
`of a vehicle therefore increases with the plan area of the
`cushion or cushions, and hence with the plan area of the
`vehicle."—Christopher Cockerell, US Patent 3,177,960,
`1965.
`
`Types of hovercraft
`
`Artwork: Hovercraft work in three main ways. Top: In an open-plenum design, the air effectively just pumps straight
`down under the craft. This requires a massive airflow and a very powerful engine. Middle: In Christopher Cockerell's
`peripheral jet design, a ring of fast-moving air, created by outer (peripheral) jets makes a "momentum curtain" that
`traps high pressure air inside it. The fan needs to move much less air to create the same lifting force, so it's a more
`efficient design than the open plenum. Right: Adding a skirt makes the air cushion higher, so the craft can safely
`clear bigger ocean waves and land obstacles. Skirts are either simple, flexible bags or more complex arrangements
`of individually moving segments called fingers.
`
`Now it's certainly possible to build a simple hovercraft with a giant fan that blows air down
`into a container of some kind (you'll find plenty on YouTube—a couple of them are linked in
`the references at the bottom of this article); that design is called an open plenum ("plenum"
`being another word for the hollow region underneath the craft where the air gathers).
`However, most hovercraft work in one of two other ways.
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`10/29/2019
`The original hovercraft design used a
`vertically mounted fan to blow air
`between its outer shell and a slightly
`smaller inner container, creating
`what's called a "momentum curtain":
`a ring of fast-moving, inward-pointing
`air that trapped a bigger cushion of
`air inside it. This type of design is
`called a peripheral jet and its big
`advantage over an open plenum is
`that the fan needs to move much
`less air (or, to put it another way, it
`can create more lift with less power).
`Unfortunately, it still only produces a
`relatively modest hover height unless
`the fan is extremely powerful.
`
`Later, engineers discovered it was
`more effective (and efficient) to trap a
`much bigger air cushion with a
`rubber skirt that could flex around
`waves and other obstructions, giving
`a greater hover height and a better
`seal. Hovercraft with skirts could
`clear bigger waves and land
`obstacles with no loss of stability or
`the all-important air cushion
`underneath them, so the ride was
`generally quite smooth. Eventually, the flexible skirt evolved into a more intricate design,
`with hundreds of independently moving "fingers" attached to the bottom that could maintain
`the airflow even more effectively. A modern hovercraft combines elements of the peripheral
`jet and flexible skirt designs by directing many jets of air inward through the skirt.
`
`Other important parts
`
`What else do you need to make a hovercraft? A downward-pointing fan can only blow air
`underneath, so hovercraft typically have one or more propeller fans on top of the hull,
`pointing backward to propel them forward. Usually, there's a rudder positioned just behind
`each fan to swivel the air it produces and steer the hovercraft in the appropriate direction. An
`alternative method of steering is to divert some of the down-draft from the fan through air
`nozzles that point horizontally—and the very first hovercraft prototype, SR.N1, effectively
`worked this way. Although hovercraft usually have separate fans (to create the cushion) and
`propellers (to drive them along), the same engines typically drive both, using gearboxes and
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`transmissions to turn the engine's power through ninety degrees. Bigger hovercraft like the
`US military LCACs typically use several very hefty engines, such as powerful gas turbines.
`Then there's the hull itself. Most large hovercraft are built from light, rustproof, and highly
`durable aluminum, though hobby craft are often molded from tough composite materials such
`as fiber glass. Finally, you need a cockpit to keep your pilot safe and sound—and some
`cargo space (either enclosed, for passengers and cars, or a large "open well" deck for
`carrying military cargo).
`
`Left: Close-up of a hovercraft skirt making a tight seal with the water beneath. Photo by Cody D. Lund courtesy of
`US Navy.
`Middle: Vertical rudders behind the fans steer the hovercraft by directing air to the side. Photo by Brian P. Biller
`courtesy of US Navy.
`Right: The fans are driven from engines in the side by giant axles. Photo by Christopher A Newsome courtesy of US
`Navy.
`Click the US Navy links to see further details and download hi-res versions of these photos.
`
`Advantages and disadvantages
`
`Hovercraft can launch and land anywhere, travel over almost any kind of surface, race along
`at high speeds, and efficiently carry large numbers of passengers and equipment or hefty
`military cargos. They compare favorably with all kinds of rival vehicles. Since they produce
`an air cushion more efficiently than a helicopter, they're cheaper to operate, simpler, and
`easier to maintain (safer too). Where boats waste energy dragging through water and waves,
`a hovercraft riding smoothly on top creates little in the way of either drag or wake, so it's
`generally more efficient (and less disruptive to the marine environment than a propeller-
`driven ship).
`
`But if hovercraft are so wonderful, why aren't they used everywhere? They're expensive
`initially and, though cheaper than helicopters, considerably more costly to maintain than
`ships and boats of similar cargo capacity (because they're essentially aircraft, not boats, and
`mechanically more complex). Although hovercraft successfully carried tens of millions of
`people between Britain and France for just over 30 years, they eventually stopped operating
`following the opening of the Channel Tunnel and the arrival of low-cost ferry ships and fast,
`wave-piercing catamarans. Hovercraft are also fairly tricky to pilot: more like helicopters, in
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`this respect, than simple-to-operate boats. They're very noisy too, which can be a problem
`both for passengers and people living near the ports where they operate, and is certainly a
`drawback for "covert" military operations.
`
`Who invented the hovercraft?
`
`The basic idea behind the hovercraft can be traced back at least to the early 18th century: in
`1716, Swedish philosopher Emmanuel Swedenborg (1688–1772) conceived a kind of
`overturned rowing boat in which each stroke of the oars pumped air under the hull, floating it
`happily over the waves. Unfortunately, it soon became obvious to Swedenborg that
`generating an air cushion by human muscle power wasn't going to work, so the craft was
`never built. In the 1870s, British marine engineer Sir John Thornycroft (1843–1928) figured
`out that a boat that could make an air cushion and carry it underneath itself would be able to
`avoid the problem of dragging its hull through the water. But his experiments to generate the
`cushion simply by pumping air with bellows were unsuccessful: technology was not on his
`side.
`
`It wasn't until the early 1950s that the theory of the hovercraft moved into practice, thanks to
`the work of another British engineer, Dr (and later "Sir") Christopher Cockerell (1910–
`1999). Famously, he carried out an experiment with a coffee can and an empty tin of cat
`food, putting one inside the other to create an ring of empty space between them. Firing air
`from a blower down into this space from above, he found he could generate what he called
`the momentum curtain—a downward ring of high-pressure air that would effectively trap a
`much bigger cushion of air under a hovercraft, producing more lifting force for the same
`engine power. He measured the lift his "craft" produced using a simple pair of kitchen scales.
`
`Initially, Cockerell thought his idea would be of huge benefit to the military and offered it to
`the British government, who promptly classified it. Unfortunately for Cockerell, the military
`weren't that interested, and the "top secret" classification also prevented any further
`commercial development. In the late 1950s, the frustrated inventor managed to get his idea
`declassified again and, in 1959, formed the Hovercraft Development Company. With
`£150,000 backing (equivalent to several million dollars or pounds today) from a British
`government agency called the National Research Development Corporation (NRDC), he
`commissioned a full-scale prototype, which took eight months to build. Constructed at Cowes
`on the Isle of Wight, England by a marine company called Saunders Roe, the SR.N1
`(Saunders-Roe Navigation 1) was roughly the size of a small truck, but almost square (8.8m
`by 7.3m or 29ft by 24ft). Its most distinctive feature was a large, vertically mounted, white fan
`(powered by a 450 horsepower engine) that produced both the air cushion (by the peripheral
`jet principle) and steering (using directional channels that diverted some of the fan's airflow).
`On July 25 that year, Cockerell and pilot Peter Lamb took the SR.N1 across the English
`Channel (from England to France) in just over two hours, marking the 50th anniversary of
`Louis Bleriot's pioneering cross-channel airplane flight (but taking about 1.5 hours longer).
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`How do hovercraft work? - Explain that Stuff
`
`Artwork: This early sketch of a hovercraft by Christopher Cockerell shows all the essential components of a modern
`machine—except the skirt, which he added later. Following Cockerell's original numbering: 1 is the hovercraft itself;
`2 is an opening at the front through which air enters; 3 is a double, four-bladed propeller; 4 is the engine; 5 is the
`drive shaft by which the engine powers the propeller; 6 is a chamber through which air flows; 7 is a tunnel into which
`air flows beneath the machine; 10 is the cockpit; 11 is the cargo bay; 12 are the bay doors; and 14 is the steering
`rudder at the back. Artwork from US Patent #3,363,716: Vehicles for travelling over land and/or water by Christopher
`Cockerell, filed on 12 December 1956 and granted on 16 January 1968. Courtesy of US Patent and Trademark
`Office.
`
`Although Cockerell and his engineers continued to tinker with the design of the SR.N1 and
`made small improvements, the next big advance came with the development of the flexible
`skirt, invented by British aircraft engineer Cecil Latimer-Needham (1900–1975). Thanks to
`this innovation, the SR.N1 prototype was soon superseded by much bigger and more
`practical craft. The first commercial passenger hovercraft service began in 1962, with a
`Vickers-Armstrong VA3 operating between Rhyl in North Wales and Merseyside, England
`carrying 24 passengers at up to 110kph (70mph). By 1968, technology had advanced to the
`point where Saunders Roe could build two giant, cross-channel, SR.N4 hovercraft ferries.
`These huge machines successfully ferried tens of millions of people from England to France
`until 2000, when the service was closed for good. Although Britain pioneered the hovercraft,
`the only passenger service now operating in the UK is a relatively modest ferry shuttling
`passengers from Portsmouth on the English mainland to the nearby Isle of Wight (fittingly, the
`island where hovercraft first buzzed into life). Even so, hovercraft continue to be widely used
`by military forces throughout the world, and in all kinds of niche applications where they
`outperform boats and helicopters.
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`How do hovercraft work? - Explain that Stuff
`
`SHOP NOW
`
`Find out more
`
`On this site
`
`Ships and boats
`
`Other websites
`
`Burnham on Sea Rescue Hovercraft: A small seaside hovercraft used to rescue
`people in danger.
`Griffon Hoverwork: This site has some great technical data and specifications and
`offers an interesting insight into the range and diversity of modern hovercraft, from
`one-person military vessels to giant firefighting craft.
`The Hovercraft Museum: A collection of 60 historic hovercraft.
`Hover Club of America: An organization for recreational hovercraft enthusiasts.
`
`News articles
`
`Hovercraft still afloat 50 years on by Brian Milligan, BBC News, 10 June 2009. A news
`report celebrating 50 years of the hovercraft, with video footage of early hovercraft
`experiments.
`
`https://www.explainthatstuff.com/hovercraft.html
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`10/29/2019
`
`How do hovercraft work? - Explain that Stuff
`It's a Boat! It'a Plane! It's a... Hovercraft! by Steven Zeitchik. The New York Times, 23
`April 2004. The growing amateur enthusiasm for hovercraft.
`Hovercraft genius dies BBC News, 3 June 1999. An obituary for hovercraft inventor
`Sir Christopher Cockerell.
`Everyone By Hovercraft: Christopher Cockerell demonstrates some of his designs in
`this 1964 Pathe News clip.
`
`Books
`
`For older readers
`
`The Hovercraft: Photographs from the Archives of the World's Only Hovercraft
`Museum by Jim Gray. Amberley Publishing, 2017. Wonderful photos for hover buffs.
`The Hovercraft: A History by Ashley Hollebone. The History Press, 2012. The
`fascinating history of water-skimming hovercraft.
`Discover the Hovercraft by Kevin Jackson. Flexitech, 2004. This short, hands-on book
`about hovercraft technology includes experiments and activities you can try.
`
`For younger readers
`
`Hovercraft (Speed Machines) by Matt Scheff. ABDO, 2015. A 32-page introduction
`covering the history of hovercraft, how they work, and what they're used for. Suitable
`for ages 7–9.
`Ships and submarines by Chris Woodford. Facts on File, 2004. My own book about
`the history of ships, from ancient wooden craft to the very latest wave-piercing
`catamarans. Ages 10+.
`
`Activities
`
`NASA: Hovering on a Cushion of Air: Make a simple balloon-and-CD hovercraft—and
`learn the science that makes it skim over your desk.
`Styrofoam tray hovercraft: A simple-to-build hovercraft model you can make in
`perhaps an hour or less.
`Adam Savage: Build a Hovercraft With Your Kids: You'll need a leaf blower, shower
`curtain, and a few more bits and pieces.
`
`Patents
`https://www.explainthatstuff.com/hovercraft.html
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`How do hovercraft work? - Explain that Stuff
`10/29/2019
`If you're looking for more technical explanations, patents are always a good place to start.
`Here are four of Christopher Cokerell's pioneering designs:
`
`US Patent #3,363,716: Vehicles for travelling over land and/or water by Christopher
`Cockerell, granted 16 January 1968. This appears to be the first US patent Cockerell
`filed (in 1956), though it wasn't published until 1968 (after the patents listed below).
`US Patent #3,177,960: Vehicles for travelling over land and/or water by Christopher
`Cockerell, granted 13 April 1965. A modified hovercraft design from the 1960s.
`US Patent #3,334,609: Vehicles operable over water by Christopher Cockerell,
`granted 8 August 1967. This early design used paddles for propulsion rather than
`fans.
`US Patent #3,318,405: Flexible skirt deflecting means for ground effect vehicles by
`Christopher Cockerell, granted 9 May 1967. This patent explains the design and
`operation of the basic hovercraft skirt.
`
`Please do NOT copy our articles onto blogs and other websites
`
`Text copyright © Chris Woodford 2000, 2016. All rights reserved. Full copyright notice and terms of use.
`
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`How do hovercraft work? - Explain that Stuff
`
`Cite this page
`
`Woodford, Chris. (2000/2016) Hovercraft. Retrieved from
`https://www.explainthatstuff.com/hovercraft.html. [Accessed (Insert date here)]
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