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Hovercraft

by Chris Woodford. Last updated: March 21, 2016.

Is 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 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 hovercraft, designed for swift beach landings, are often called LCACs (Landing Craft Air Cushion).

Photo of a hovercraft by NASA Photo of a hovercraft by NASA
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!

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.

Artwork showing how a hovercraft works
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?

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 hovercrafts

Diagram comparing three types of hovercraft: an open plenum, a peripheral jet, and a finger skirt.

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.

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.

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.

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 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).

Closeup of a hovercraft skirt, fan, and engine axle

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 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).

Once of Christopher Cockerell's original hovercraft designs from US Patent 3,177,960 granted in 1965

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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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:

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Woodford, Chris. (2000/2016) Hovercraft. Retrieved from http://www.explainthatstuff.com/hovercraft.html. [Accessed (Insert date here)]

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