¸‘S¤ð›S =à›SAir Cushion VehiclesThe air-cushion vehicle (ACV), also known as a ground-effect machine and often popularly called a Hovercraft, is a conveyance that rides on a cushion of air at a pressure slightly above atmospheric. The difference between atmospheric and air-cushion pressure is so small that a large ACV can be safely driven over a person lying prone. Air must be constantly pumped into the cushion, and this consumes at least as much power as is needed to propel the vehicle. A great advantage of the ACV, however, is its versatility; it can be routed over water, ice, mud, quicksand, marsh, and many other surfaces that are inaccessible to traditional vehicles. The largest of today's ACVs is the SR.N4, a class of vehicle that is used for ferry service across the English Channel. When originally built, the N4 class carried 34 cars and 174 passengers; the Super 4 type, with an extra section amidships, can transport 60 cars and 416 passengers. Powered by four 3,800 horsepower gas-turbine engines that drive four lift fans and four swiveling air propellers, these big craft cruise smoothly at up to 77 knots, making them the fastest class of seaworthy water vehicles in the world. Some purely marine ACVs have rigid sidewalls extending down into the water along each side to help contain the air cushion. This reduces the power needed to keep the cushion inflated but increases drag (resistance to motion) because part of the vehicle is immersed in the water. Most ACVs have a flexible skirt of tough, rubberized fabric around the underside. Ducts direct air inward and downward, which helps to contain the air cushion and replace air that escapes from the cushion. The lower part of the skirt is especially flexible, enabling the vehicle to move smoothly over waves. In the SR.N4, the cushion height is 2.75 m (9 ft) so that the craft can pass over a projecting obstacle of that height. The skirted ACV is, in theory, fully amphibious; but the N4, with its beam of 28 m (92 ft), is too wide to be driven over land areas containing obstructions, although it can cross deserts or ice floes. Many ACVs are used by the military. They can be armed with missiles, guns, antisubmarine weapons, and other special equipment. In Vietnam, British ACVs developed further by Bell Aerospace were used for patrolling various rivers and estuaries. Since they ride above the water, they are less vulnerable to torpedoes and undetectable by sonar. Many countries use small ACVs as high-speed frontier or customs enforcement vehicles, and thousands are used as amphibious, sporting runabouts. LCAC2.PCX