rudder, mechanism for steering an airplane or a ship. In ships it is a flat-surfaced structure hinged to the stern and controlled by a helm. When the ship is on a straight course, the rudder is in line with the vessel; if the rudder is turned to one side or the other it offers sufficient resistance to the water to deflect the stern, thus changing the direction of the ship. In earliest times, as in small boats today, a paddle or oar hand-operated at the stern served to turn a boat. Later, Greek and Roman vessels required two rudders, one at each end, in order to maintain course when the prow or stern lifted out of the water. Vikings placed the rudder not directly on the stern but on the right side near it; thus the term starboard (steerboard) is used for the right side of a vessel. By the early 14th cent. the stern rudder had generally replaced the side rudder, and in the latter half of the 19th cent. wooden rudders gave way to iron and steel. Large modern liners have rudders that are 60 ft (18 m) or more in height and weigh 100 tons.
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