part of the steering apparatus of a boat or ship that is fastened outside the hull, usually at the stern. The most common form consists of a nearly flat, smooth surface of wood or metal hinged at its forward edge to the sternpost. It operates on the principle of unequal water pressures. When the rudder is turned so that one side is more exposed to the force of the water flowing past it than the other side, the stern will be thrust away from the side that the rudder is on and the boat will swerve from its original course. In small craft the rudder is operated manually by a handle termed a tiller or helm. In larger vessels, the rudder is turned by hydraulic, steam, or electrical machinery.

The earliest type of rudder was a paddle or oar used to pry or row the stern of the craft around. The next development was to fasten a steering oar, in a semivertical position, to the vessel's side near the stern. This arrangement was improved by increasing the width of the blade and attaching a tiller to the upper part of the handle. Ancient Greek and Roman vessels frequently used two sets of these steering paddles. Rudders fastened to the vessel's sternpost did not come into general use until after the time of William the Conqueror. In ships having two or more screw propellers, rudders are fitted sometimes directly behind each screw.