A rudder is a device used to steer ships, boats, submarines, aircraft, hovercraft or other conveyances that move through air or water. Rudders operate by re-directing the flow of air or water past the hull or fuselage, thus imparting a turning or yawing motion to the craft. In basic form, a rudder is a flat plane or sheet of material attached with hinges to the craft's stern, tail or after end. Often rudders are shaped so as to minimize hydrodynamic or aerodynamic drag. On simple watercraft, a tiller—essentially, a stick or pole acting as a lever arm—may be attached to the top of the rudder to allow it to be steered by a helmsman. In larger vessels, cables, pushrods and hydraulics may be used to link rudders to steering wheels. In typical aircraft, pedals operate rudders via mechanical linkages.

Invention of the rudder

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Oars mounted on the side of ships for steering are documented from the 3rd millennium BCE in Ancient Egypt in artwork, wooden models, and even remnants of actual boats. These evolved into quarter rudders, which were used in Antiquity until the end of the Middle Ages in Europe. As the size of ships and the height of the freeboards increased, quarter-rudders became less satisfactory and were replaced in Europe by the more sturdy stern-mounted rudders with pintle and gudgeon attachment from the 12th century. The West's oldest known depiction of a stern-mounted rudder can be found on church carvings dating to around 1180.

The world's oldest known depiction of a stern-mounted rudder can be seen on a pottery model of a Chinese junk dating from the 1st century CE, predating their appearance in the West by a thousand years. The Chinese stern-mounted rudder is hung from the stern, held in place and controlled with a rope mechanism. The invention of the stern-mounted rudder in the West may have been independent, as its technical specifications certainly differ, although the idea may have been transmited through trade exchanges with the East. Detailed descriptions of Chinese junk (sailing)|junks during the Middle Ages are known from various travellers to China, such as Ibn Battuta and Marco Polo.

Also, many junks incorporated "fenestrated rudders" (rudders with holes in them, allowing for better control), an innovation adopted in the West in 1901 to increase the manoeuvrability of torpedo boats.

Boat rudders may be either outboard or inboard. Outboard rudders are hung on the stern or transom. Inboard rudders are hung from a keel or skeg and are thus fully submerged beneath the hull, connected to the steering mechanism by a rudder post which comes up through the hull to deck level, often into a cockpit.

Some sailors use rudder post and mast placement to define the difference between a ketch and a yawl, similar two-masted vessels. Yawls are defined as having the mizzen mast abaft (ie. "aft of") the rudder post; ketches are defined as having the mizzen mast forward of the rudder post.

Small boat rudders that can be steered more or less perpendicular to the hull's longitudinal axis make effective brakes when pushed "hard over." However, terms such as "hard over," "hard to starboard," etc. signify a maximum-rate turn for larger vessels.

Aircraft rudders

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On an aircraft, the rudder is called a "control surface" along with the rudder-like elevator (attached to horizontal tail structure) and ailerons (attached to the wings) that control pitch and roll respectively. The rudder is usually attached to the fin (or vertical stabilizer) which allows the pilot to control yaw in the horizontal axis, ie change the horizontal direction in which the nose is pointing. The rudder's direction is manipulated with the movement of foot pedals by the pilot.

In practice, both aileron and rudder control input are used together to turn an aircraft, the ailerons imparting roll, the rudder imparting yaw, and also compensating for a phenomenon called adverse yaw. Adverse yaw is readily seen if the ailerons alone are used for a turn. The downward moving aileron acts like a flap, generating more lift for one wing, and therefore more drag. Initially, this drag yaws the aircraft in the direction opposite to the desired course. A rudder alone will turn a conventional fixed wing aircraft, but much more slowly than if ailerons are also used in conjunction. Use of rudder and ailerons together produces co-ordinated turns, in which the longitudinal axis of the aircraft is in line with the arc of the turn, neither slipping, (under-ruddered) nor skidding (over-ruddered). Improperly ruddered turns at low speed can precipitate a spin which can be dangerous at low altitudes.

Sometimes pilots may intentionally operate the rudder and ailerons in opposite directions in a manouver called a forward slip. This may be done to overcome crosswinds and keep the fuselage in line with the runway, or to more rapidly lose altitude by increasing drag, or both.

See also

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• Flight controls
• Ship's wheel

Conventional ship and boat rudders

• Fully balanced rudder
• Semi balanced rudder
• Spade rudder

Specialist ship and boat rudders

• Jet flap rudder
• Kitchen rudder
• Pleuger rudder
• Rotating cylinder rudder
• Schilling Rudder
• Voith Schneider Propeller (a combination propulsion and steering unit}

Aircraft controls | Sailboat anatomy | Sailing ship elements | Ship construction

Ruder | Gouvernail | Richtingsroer | ラダー (飛行機) | Ster | Roder