A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other conveyance that moves through a fluid medium. On an aircraft the rudder is used primarily to counter adverse yaw, a rudder operates by redirecting the fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft. In basic form, a rudder is a plane or sheet of material attached with hinges to the crafts stern, tail. 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 turned by a helmsman. In larger vessels, cables, pushrods, or hydraulics may be used to link rudders to steering wheels, in typical aircraft, the rudder is operated by pedals via mechanical linkages or hydraulics. Generally, a rudder is part of the apparatus of a boat or ship that is fastened outside the hull, that is denoting all different types of oars, paddles. More specifically, the gear of ancient vessels can be classified into side-rudders and stern-mounted rudders. A third term, steering oar, can denote both types, in a Mediterranean context, side-rudders are more specifically called quarter-rudders as the later term designates more exactly the place where the rudder was mounted. Stern-mounted rudders are uniformly suspended at the back of the ship in a central position. S, a steering oar was used at this time because the rudder had not yet been invented. With a single sail, a frequent movement of the oar was required to steer a straight course. The steering oar or steering board is an oar or board to control the direction of a ship or other watercraft prior to the invention of the rudder. It is normally attached to the side in larger vessels, though in smaller ones it is rarely, if ever. Rowing oars set aside for steering appeared on large Egyptian vessels long before the time of Menes, in the Old Kingdom as much as five steering oars are found on each side of passenger boats. The tiller, at first a small pin run through the stock of the steering oar, both the tiller and the introduction of an upright steering post abaft reduced the usual number of necessary steering oars to one each side. In Iran, oars mounted on the side of ships for steering are documented from the 3rd millennium BCE in artwork, wooden models, the strength of the steering oar lay in its combination of effectiveness, adaptability and simpleness. Roman quarter steering oar mounting systems survived mostly intact through the medieval period, by the first half of the 1st century AD, steering gear mounted on the stern were also quite common in Roman river and harbour craft as proved from reliefs and archaeological finds. A tomb plaque of Hadrianic age shows a harbour tug boat in Ostia with a long stern-mounted oar for better leverage, interestingly, the boat already featured a spritsail, adding to the mobility of the harbour vessel. Further attested Roman uses of stern-mounted steering oars includes barges under tow, transport ships for wine casks, also, the well-known Zwammerdam find, a large river barge at the mouth of the Rhine, featured a large steering gear mounted on the stern
Modern ship rudder (the long red rectangle behind the propeller)
Stern-mounted steering oar of an Egyptian riverboat depicted in the Tomb of Menna (c. 1422-1411 BC)
Steering oar of a Roman boat, 1st century AD (RG-Museum, Cologne).
An early Song Dynasty (960–1279) painting on silk of two Chinese cargo ships accompanied by a smaller boat, by Guo Zhongshu (c. 910–977 AD); notice the large sternpost-mounted rudder on the ship shown in the foreground