A rudder is a device used to steer a ship, boat, submarine, hovercraft, or other conveyance that move through a fluid (generally air or water). On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane. 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 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.
Generally, a rudder is "part of the steering apparatus of a boat or ship that is fastened outside the hull", that is denoting all different types of oars, paddles and rudders. More specifically, the steering gear of ancient vessels can be classified into side-rudders and stern-mounted rudders, depending on their location on the ship. 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, but the term has historically been found wanting because it does not take into account that the stern rudders were attached to the ship hull in quite a different way: While the European pintle-and-gudgeon rudder was attached to the sternpost by pivoting iron fastenings, the Arabs used instead a system of lashings. Chinese stern rudders also featured tackles, but, unlike their medieval and Arab counterparts, had no sternpost to which to attach them. Roman and particularly ancient Egyptian stern rudders featured again a different method of fastening where the stock, having a single point of contact with the stern, was additionally secured to the ship body by an upright rudderpost or braced ropes.
Although Lawrence Mott in his comprehensive treatment of the history of the rudder, Timothy Runyan, the Propyläen History of Technology, the Encyclopedia Britannica, and The Concise Oxford Dictionary of English Etymology classify a steering oar as a rudder, Joseph Needham, Lefèbre des Noëttes, K.S. Tom, Chung Chee Kit, S.A.M. Adshead, John K. Fairbank, Merle Goldman, Frank Ross, and Leo Block state that the steering oar used in ancient Egypt and Rome (and even ancient China) was not a true rudder; the steering oar has the capacity to interfere with handling of the sails (limiting any potential for long ocean-going voyages) while it was fit more for small vessels on narrow, rapid-water transport; the rudder did not disturb the handling of the sails, took less energy to operate by its helmsman, was better fit for larger vessels on ocean-going travel. A specific form of the stern-mounted rudder which was not attached to a sternpost appeared in ancient China during the 1st century AD. In regards to the ancient Phoenician (1550–300 BC) use of the steering oar without a rudder in the Mediterranean, Leo Block (2003) writes:
A single sail tends to turn a vessel in an upwind or downwind direction, and rudder action is required to steer a straight course. 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 steering oar was required to steer a straight course; this slowed down the vessel because a steering oar (or rudder) course correction acts like a break. The second sail, located forward, could be trimmed to offset the turning tendency of the main sail and minimize the need for course corrections by the steering oar, which would have substantially improved sail performance.
Rowing oars set aside for steering appeared on large Egyptian vessels long before the time of Menes (3100 BC). In the Old Kingdom (2686 BC-2134 BC) 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, can be traced to the fifth dynasty (2504–2347 BC). Both the tiller and the introduction of an upright steering post abaft reduced the usual number of necessary steering oars to one each side. Apart from side-rudders, single rudders put on the stern can be found in a number of tomb models of the time, particularly during the Middle Kingdom when tomb reliefs suggests them commonly employed in Nile navigation. The first literary reference appears in the works of the Greek historian Herodot (484-424 BC), who had spent several month in Egypt: "They make one rudder, and this is thrust through the keel", probably meaning the crotch at the end of the keel (see right pic "Tomb of Menna").
In Iran, oars mounted on the side of ships for steering are documented from the 3rd millennium BCE in artwork, wooden models, and even remnants of actual boats.
In China, miniature models of ships that feature steering oars have been dated to the Zhou Dynasty (c. 1050 BC – 256 BC). Stern mounted rudders started to appear on Chinese ship models starting in the 1st century AD. However, the Chinese continued to use the steering oar long after they invented the rudder, since the steering oar still had limited practical use for inland rapid-river travel. One of oldest known depiction of a stern-mounted rudder in China can be seen on a 2 ft. long tomb pottery model of a junk dating from the 1st century AD, during the Han Dynasty (202 BC-220 AD). It was discovered in Guangzhou in an archeological excavation carried out by the Guangdong Provincial Museum and Academia Sinica of Taiwan in 1958. Within decades, several other Han Dynasty ship models featuring rudders were found in archaeological excavations. The first solid written reference to the use of a rudder without a steering oar dates to the 5th century.
Chinese rudders were not supported by pintle-and-gudgeon as in the Western tradition; rather, they were attached to the hull by means of wooden jaws or sockets, while typically larger ones were suspended from above by a rope tackle system so that they could be raised or lowered into the water. 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. Detailed descriptions of Chinese junks during the Middle Ages are known from various travellers to China, such as Ibn Battuta of Tangier, Morocco and Marco Polo of Venice, Italy. The later Chinese encyclopedist Song Yingxing (1587-1666) and the 17th century European traveler Louis Lecomte would write of the junk design and its use of the rudder with enthusiasm and admiration.
Paul Johnstone and Sean McGrail state that the Chinese invented the "median, vertical and axial" stern-mounted rudder, and that such a kind of rudder preceded the pintle-and-gudgeon rudder found in the West by roughly a millennium. However, Mott points out that the Chinese rudder worked by a very different suspension system, and that it was not permanently attached to a sternpost, leaving little point in comparing two such different types of rudder. As a matter of fact, the method of mounting steering gear from the stern was well known in Mediterranean navigation by the time the practice appeared in Chinese ships.
Roman navigation used primarily quarter rudders which went in the Mediterranean through a long period of constant refinement and improvement, so that by Roman times ancient vessels reached extraordinary sizes. The strength of quarter rudder lay in its combination of effectiveness, adaptability and simpleness. Roman quarter rudder mounting systems survived mostly intact through the medieval period.
By the first half of the 1st century AD, rudders mounted on the stern were also quite common in Roman river and harbour craft as proved from reliefs and archaeological finds (Zwammderdam, Woerden 7). 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 rudders includes barges under tow, transport ships for wine casks, and diverse other ship types. Also, the well-known Zwammerdam find, a large river barge at the mouth of the Rhine, featured a large rudder mounted on the stern. According to new research, the advanced Nemi ships, the palace barges of emperor Caligula (37-41 AD), may have featured 14 m long stern-mounted rudders.
Arabs used instead a system of lashings. Chinese stern rudders also featured tackles, but, unlike their medieval and Arab counterparts, had no sternpost to which to attach them. According to Lawrence V. Mott, the "idea of attaching the rudder to the sternpost in a relatively permanent fashion, therefore, must have been an Arab invention independent of the Chinese."
Oars mounted on the side of ships evolved into quarter rudders, which were used from 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 unwieldy and were replaced by the more sturdy stern-mounted rudders with pintle and gudgeon attachment. While stern-mounted rudders were found in Europe on a wide range of vessels since Roman times, including light war galleys in Mediterranean, the oldest known depiction of a pintle-and-gudgeon rudder can be found on church carvings of Zedelgem and Winchester dating to around 1180.
Historically, the radical concept of the medieval pintle-and-gudgeon rudder did not come as a single invention into being. It presented rather a combination of ideas which each had been long around before: rudders mounted on the stern, iron hinges and the straight sternpost of northern European ships. While earlier rudders were mounted on the stern by the way of rudderposts or tackles, the iron hinges allowed for the first time to attach the rudder to the entire length of the sternpost in a really permanent fashion. However, its full potential could only to be realized after the introduction of the vertical sternpost and the full-rigged ship in the 14th century. From the age of discovery onwards, European ships with pintle-and-gudgeon rudders sailed successfully on all seven seas.
Contrary to an older hypothesis, all evidence indicates that the European hinged stern-mounted rudder, whose technical specifications considerably differ from the Chinese one, was invented independently:
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.
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. The rudder is usually attached to the fin (or vertical stabilizer) which allows the pilot to control yaw in the vertical axis, i.e. 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 most simple type of 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 (though since the 1930s, many aircraft have used frise ailerons or differential ailerons, which compensate for the adverse yaw and require little or no rudder input in regular turns). 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 maneuver 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.