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By Sid of Stone Marmot
Aug. 1, 2008
A hovercraft is a vehicle that floats above whatever surface it is over on a cushion of air. This surface can be land, water, ice, or anything else that can trap air. An example of a surface that doesn't trap air is an elevated metal grate. Hovercraft are considered to be amphibious vehicles.
Hovercraft are also known as air cushion vehicles and ground effect machines (GEMs).
A hovercraft relies on air pressure to lift it off the ground, as opposed to aerodynamic lift, which lifts airplanes and helicopters off the ground, or thrust, which lifts rockets off the ground. A typical hovercraft will float at a height that is about 5 % of its length, or about 8 inches (20 cm) for a 15 foot (4.7 m) long hovercraft.
The lift mechanism for a hovercraft must supply enough air to:
1) pressurize the air under the hovercraft enough to lift the vehicle off the surface;
2) replace the air that leaks out from under the hovercraft so as to maintain the desired air pressure.
A flexible gasket, called a skirt, usually hangs around the outside bottom edge of a hovercraft to seal most of the gap between the bottom of the hovercraft and the surface it is passing over. This significantly reduces the amount of air leaking from under the hovercraft, which in turn dramatically reduces the amount of power and, hence, the size of the motor and fan needed to provide the air necessary to lift the hovercraft to the desired height. This is why a hovercraft can successfully hover with an engine and fan blades that are very significantly smaller than those needed by an equivalent sized helicopter.
A significant advantage to using a hovercraft over other vehicles is its ability to cross a wide variety of surfaces. To maintain this advantage, whatever is propelling the hovercraft across the surface shouldn't touch the surface or in any other way be dependent upon the characteristics of the surface the hovercraft is over. Consequently, most hovercraft use propellers similar to those on an airplane or airboat to move the hovercraft across the surface.
Hovercraft cause less damage to surfaces it passes over. All the weight of a wheeled vehicle is concentrated in the small areas where the wheels touch the ground. For an 800 pound (364 kg) four wheeled vehicle with about 0.5 square foot (46 square cm) of tire contact area, this is about 1600 lb./square ft. (7900 kg/ square meter) of pressure on the ground. The weight of a hovercraft is distributed over most of the bottom surface of the craft. For a 800 pound (364 kg) hovercraft that is about 15 ft by 7 ft (4.7 m by 2.1 m), this is about 7.6 lb./square ft. (37 kg/square meter). The result is that the hovercraft doesn't cause ruts in the surface and causes a lot less damage to the vegetation than a wheeled vehicle.
Most of the manatee and many of the dolphin in the waters around Florida have scars from being hit by boat propellers. When a hovercraft is hovering, no part of the craft is in the water and only its skirt is near the water's surface. Getting hit by a hovercraft skirt is less traumatic than getting hit by a toy balloon, since the air pressure under a hovercraft and in its skirt is typically much less than the pressure in a balloon.
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