The best known application of air cushion technology is the «hovercraft». The low pressure existing under this craft make it practically independent of the surface over which it operates.
The hovercraft is a vehicle of sizeable surface area in order to support its weight with a pressure of the order of 500 Pa (less than 1/10 of a pound per square inch) for small vehicles, to 3 000 Pa (about half a pound per square inch) for the largest one. Hovercraft are generally propelled with air propellers; marine hovercraft, with rigid sidewalls (SES), use water propellers or water jets.
The hovercraft is characterised by its air cushion system. This comprises fan, or fans, which absorb about 20 kW of power per tonne, and flexible skirts, whose height depends on the obstacles over which the hovercraft will operate (solid obstacles or water waves). Height of the skirt system is about 5 % of the length of the vehicle, a proportion slightly larger for the smaller vehicles.
Over water, at low speed, the cushion produces a cavity of depth equivalent to the cushion pressure, with a volume such that the weight of water displaced equal to that of the craft. As the vehicle accelerates, the same phenomenon occurs as with a fast boat: the craft climbs over the cavity edge and starts planing. However, as the speed increases the cushion effect on the water surface decreases with much reduced wave amplitude.
During acceleration the wave resistance increases to a maximum, at «hump» speed, decreasing afterward to a fraction of the hump resistance. For a mathematical development of this phenomenon, see, amongst other, Barratt, M.J., Journal of Fluid Mechanics (1965), vol. 22, part 1, pp. 39-47.
In practice, if the hydrodynamic resistance decreases, contact of waves with the skirts and the hard structure, the aerodynamic resistance, the fan air momentum drag, result eventually with an increasing resistance with speed. However, comparing with boats of equivalent payload, hovercraft permit speeds of 30 to 40 knot and over. The granddaddy of the fast ferries plying the Channel, the SRN-4 hovercraft, reach easily 60 knot. In addition, the hovercraft allows movement over water without having to worry about depth of water or shoals.
Hovercraft have been operated over mud, marshes, ice fields and, generally, wherever surfaces are too weak to support conventional vehicles. Operations «overland» have led to a slightly different breed of air cushion vehicles to cope with, in particular, manoeuvring over slopes and the lower speed of movement.
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on August 24, 1997 by MALINA
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Last update on December 4, 1997.