rudder

A rudder is a primary control surface used to steer a boat, ship, or aircraft. On watercraft, the rudder is a flat plate or blade attached to the stern and hinged to a rudder stock. Deflecting the rudder changes the direction of the vessel by redirecting the flow of water around the hull, creating a sideways force that yaws the hull.

In ships, the rudder is typically operated from a wheel or tiller and is driven by a steering gear, which in modern vessels is often hydraulic or electric. Rudders can be full or balanced; a balanced rudder has a portion in front of the rudder stock to reduce the steering effort required. Common shipboard arrangements include transom rudders and post rudders mounted behind a stern post or within a skeg. The effectiveness of a rudder depends on speed, hull form, depth of water, and sea state, and it may be supplemented by trim devices and fins.

In aviation, the rudder is located on the vertical tail surface and is used to control yaw. It is typically actuated by foot pedals, and in most aircraft is supplemented by ailerons and elevators to maintain coordinated turns and stability. At high speeds or in strong crosswinds, rudder use is limited by structural and aerodynamic limits.

Historically, rudders evolved from steering oars and have become a standard control element across watercraft and aircraft, with variations adapted to propulsion, hull form, and control systems.