The purpose of an outrigger or a catamaran-type of construction may simply be to keep the boat stable, but the modern use of this type of vessel for sailboats is for an entirely different reason: because it can "plane" on top of the water, and thus have less drag and achieve greater speed than a boat which merely floats in the water. In order to plane, a boat must obtain hydrodymanic "lift" similar to the aerodynamic lift which supports an airplane, and at low speeds "aspect ratio" (q.v.) is an important factor in obtaining such lift. Thus, the same considerations which dictate the large wingspan of a glider dictate great beam (breadth) for a boat which must plane at low speeds. The effective beam of a catamaran is the distance between its two hulls. A simple sailing catamaran, for example, on reaching a speed of about 10 knots, may rise up on to the surface of the water and immediately obtain a considerable increment of speed. Such a vessel may sail at 20 knots in a 10-knots wind. Because of their great advantage over conventional boats, catamarans are banned from most sailing races.
....The most primitive form of catamaran consists of three logs, the middle one longer, lashed together. Sush boats have been used by natives of th Coromandel coast, particularly Madras, and in the West Indies and on the coast of South America. The Fiji Islanders developed this idea in their war canoes, two parallel logs joined together with alpatform, on which a mast is placed. These boats bave a speed of about 14 miles an hour. The flying prao of the Ladrone Islanders, a boat which two hulls of unequal size, surpasses the Fiji canoe in speed. The larger of its hulls, which carries all the rigging, is flat on one side and rounded on the other; on it are placed bamboo poles projecting beyond the rounded side, and to their ends is fastened a boatshaped log one half or one third the size of the larger hull. This arrangement prevents capsizing as effectually as that of the Fiji double canoe. Both ends of the proa are made alike, and the boat is sailed with either end first, but the outrigger is always to windward. Proas from 40 to 65 feet long and 6 or 7 feet wide, are said to have a speed of 20 miles an hour.
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CATAMARAN
