A hovercraft (or "air cushion vehicle") is a machine that "hovers" on a cushion of air above the surface it is traveling over.
The air cushion is "trapped" under the hovercraft by a skirt around the edge of the craft (a popular misconception is that the skirt holds the hovercraft up - it doesn't - it just stops most of the cushion air escaping). The skirt constantly leaks air at the surface as it can never make a perfect seal - this air acts as a lubricant to make the skirt slide easily over the surface. Because a hovercraft is suspended on an air cushion and has no direct contact with the surface it is travelling over it's a virtually frictionless machine. One effect of this is that the only way a hovercraft can push itself along is by using air propulsion - usually a propeller of fan (it can't use wheels or a water propeller). Because there is little or no friction, very little power is needed to push a hovercraft along.
Where can it go?
The big advantage of a hovercraft is that, due to the very low pressure 'footprint' it can travel over almost any type of flat surface - it makes no difference how solid the surface is - grass, water, mud, sand, gravel - it's all the same to a hovercraft! As a hovercraft is "floating" above the surface it can travel up a fast flowing river at the same speed as it can travel downriver!
Hovercraft are often the only possible transport in areas that are part land and part water (mud flats, soft sand, swamps, shallow rivers, thin ice, soft snow etc).
Where can't it go?
A hovercraft can't climb steep hills (much over 20 degrees or so) or travel over extremely rough surfaces (bushes or small shrubs or tall stiff grass for example). They can't travel over surfaces that contain long sharp objects (branches, roots or large sharp rocks) that could damage the skirt or hull. The air based steering system also makes it difficult for a hovercraft to travel in very confined spaces (it's difficult to drive along a narrow track or roadway). Cambered road surfaces are also very difficult - the lack of friction makes the hovercraft drift off to one side of the other because of the slope of the camber).
Does it damage the Environment?
The hovercraft air cushion exerts a surface pressure less than a small bird (about 4gms/cm2). Hovercraft can safely pass over delicate vegetation (and even small animals) without causing damage. There are no damaging devices (wheels or water propellers) in contact with the surface.
The inherent efficiency of a hovercraft means it consumes far less fuel (and therefore generates less atmospheric pollution) than almost any other form of transport. Exhaust pollution levels are roughly equivalent to a small family car and far less than agricultural, off road vehicles or boats. The fuel systems in a hovercraft are fully contained within the hull and pose virtually zero risk of spillage or leakage into the environment. Most cruising hovercraft are fitted with 4 stroke engines rather than the water and air polluting 2-strokes outboard engines fitted to most boats.
When traveling over water hovercraft generate virtually no wake and therefore do not cause river bank erosion or bed disturbance. Because all of the mechanical components are installed on the top side of the hull and the hull is not in direct contact with the water surface there is almost no noise transmitted underwater. Hovercraft have absolutely no effect on fish and other marine life or on the bed of a river.
Launching hovercraft onto water does not need a slipway or other environmentally damaging facilities - they can be driven directly from land onto water (and can even travel overland to reach the water).
How fast can a hovercraft go?
The current official world speed record for a light hovercraft stands at 84mph (although there are reports of large craft exceeding 100mph!). Powerful racing craft can exceed 70mph and regularly operate at over 45mph. Most cruising craft operate at around 20-35mph although some are capable of speeds in excess of 70mph! At speeds in excess of 50mph or so, a hovercraft starts to turn into a low flying aircraft! Generally, for cruising, 20-30mph is about optimum - any faster and you don't get much chance to enjoy the scenery!
What type of hovercraft are there?
There are a huge variety of hovercraft designs. Most people are aware of the type of hovercraft used commercially - like the out of service UK cross channel ferry for example. These craft are similar to ships or aircraft in their construction and operation. Small recreational hovercraft are completely different machines.
In the UK, recreational hovercraft fall into two main groups, cruising or racing. Racing hovercraft are usually small, single seat machines for operation at speed on custom designed land/water courses racing against similar craft. They are designed for maximum speed and maneuverability. Cruising hovercraft are designed to operate efficiently at lower speed, carry two or more people and be used on river, estuary, lake or land. A racing craft can be compared to a racing car or motorbike and a cruising hovercraft a family car or MPV.
The Hovercraft Buyers Guide has information on choosing a suitable cruising hovercraft.