Hovercraft are essentially boats which know a clever trick: by lifting the hull out of the water on a cushion of air they reduce their drag through the water, and by pushing themselves along using a propellor, or thrust fan, they can not only travel over shallow water, but also over rapids, mudbanks, sandbanks, ice, and land.
Air is forced under the hovercraft by a lift fan and trapped by a flexible skirt around the hovercraft. Air escapes from under the skirt creating a small gap at the bottom. As the skirt is flexible it can glide easily over the water or ground, with the main body of the hull lifted well clear. With very large hovercraft the underside of the hull can be a metre or more above the water, whilst the gap under the surrounding skirt remains only a few millimetres above the surface – just enough to glide freely as the air escapes.
Pushing the craft along with one or more thrust fans means there is no part of the hovercraft in the water, hence the capability to travel over many different surfaces.
Early hovercraft used fuel thirsty gas turbine engines, but modern craft use standard automotive or truck engines – with more efficient diesel engines being prefered for larger craft or in testing developing world environments.
The skirt is made from rubber, neoprene, hypalon, or nylon or even PVC depending on the application. The material has to be both lightweight and flexible, whilst remaining tough and durable, and the skirt is generally the most complicated part of a hovercraft to design. Whilst the idea of an air lubrictaed boat had been around since the mid nineteenth centuary it was the development of the flexible curtain like skirt which made the hovercraft truly work.
Hovercraft are primarily steered by rudders in the thrust air flow, which cause the craft to yaw and thereby make a turn. Alternative/complimentary steeing systems can create greater levels of control, such as elevators which can trim for pitch as seen on the Vortex 5 hovercraft; skirt shift systems which enable a craft to roll or bank into a turn as used on the Griffon 1500, or elevons which allow twin ducted craft such as the River Rover to achieve both pitch and roll control in a combined system.