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A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, airship, or other vehicle that moves through a fluid medium (usually aircraft or watercraft). On an airplane, 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 to the craft's stern, tail, or afterend. Often rudders are shaped 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, , or hydraulics may link rudders to steering wheels. In typical aircraft, the rudder is operated by pedals via mechanical linkages or hydraulics.
Although some classify a steering oar as a rudder, others argue that the steering oar used in ancient Egypt and Rome was not a true rudder and define only the stern-mounted rudder used in ancient Han dynasty China as a true rudder. The steering oar can interfere with the 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, and first appeared in ancient China during the 1st century AD.Tom, K.S. (1989). Echoes from Old China: Life, Legends, and Lore of the Middle Kingdom. Honolulu: The Hawaii Chinese History Center of the University of Hawaii Press. . Page 103–104.Adshead, Samuel Adrian Miles. (2000). China in World History. London: MacMillan Press Ltd. New York: St. Martin's Press. . Page 156.Needham, Joseph. (1986). Science and Civilization in China: Volume 4, Physics and Physical Technology, Part 3, Civil Engineering and Nautics. Taipei: Caves Books Ltd. Pages 627–628.Fairbank, John K. and Merle Goldman. (1998). China: A New History, Enlarged Edition. Cambridge: Harvard University Press. . Page 93.Block, Leo. (2003). To Harness the Wind: A Short History of the Development of Sails. Annapolis: Naval Institute Press. . Page 123. In regards to the ancient (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, 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 as a brake. The second sail, located forward, could be trimmed to offset the turning tendency of the mainsail and minimize the need for course corrections by the steering oar, which would have substantially improved sail performance.Block, Leo. (2003). To Harness the Wind: A Short History of the Development of Sails. Annapolis: Naval Institute Press. . 8–9.
The steering oar or steering board is an oversized oar or board to control the direction of a ship or other watercraft before the invention of the rudder. It is normally attached to the starboard side in larger vessels, though in smaller ones it is rarely if ever, attached.
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.
By the first half of the 1st century AD, steering gear mounted on the stern were also quite common in Roman Empire river and harbour craft as proved from and archaeological finds (Zwammerdam, Woerden 7). A tomb plaque of age shows a harbour tug boat in Ostia Antica with a long stern-mounted oar for better leverage.Lionel Casson: Harbour and River Boats of Ancient Rome, The Journal of Roman Studies, Vol. 55, No. 1/2, Parts 1 and 2 (1965), pp. 31–39 (plate 1) The boat already featured a spritsail, adding to the mobility of the harbour vessel.Lionel Casson, Harbour and River Boats of Ancient Rome, The Journal of Roman Studies, Vol. 55, No. ½, Parts 1 and 2. (1965), pp. 35 (Pl. I); 36, Fn.43 (Pl.II,1) Further attested Roman uses of stern-mounted steering oars includes barges under tow, transport ships for wine casks, and diverse other ship types.Lawrence V. Mott, The Development of the Rudder, A.D. 100–1600: A Technological Tale, Thesis May 1991, Texas A&M University, p.84, 95f.Lionel Casson: “Ships and Seamanship in the Ancient World”, , S.XXVIII, 336f.; Fig.193Tilmann Bechert: Römisches Germanien zwischen Rhein und Maas. Die Provinz Germania inferior, Hirmer, München 1982, , p.183, 203 (Fig.266) A large river barge found at the mouth of the Rhine near Zwammerdam featured a large steering gear mounted on the stern.M. D. de Weerd: Ships of the Roman Period at Zwammerdam / Nigrum Pullum, Germania Inferior, in: Roman Shipping and Trade: Britain and the Rhine Provinces. (The Council for) British Archaeology, Research Report 24, 1978, 15ff.M. D. de Weerd: Römerzeitliche Transportschiffe und Einbäume aus Nigrum Pullum / Zwammerdam, in: Studien zu den Militärgrenzen Roms II (1977), 187ff. According to new research, the advanced Nemi ships, the palace barges of emperor Caligula (37–41 AD), may have featured 14-m-long rudders. Deutschlandfunk: Römische Schiffsversuchsanstalt in den Albaner Bergen
Chinese rudders are 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.Needham, Volume 4, Part 3, 362. Also, many junks incorporated "fenestrated rudders" (rudders with holes in them, supposedly allowing for better control). 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 wrote of the junk design and its use of the rudder with enthusiasm and admiration.Needham, Volume 4, Par634. Paul Johnstone and Sean McGrail state that the Chinese invented the "median, vertical and axial" sternpost-mounted rudder, and that such a kind of rudder preceded the pintle-and-gudgeon rudder found in the West by roughly a millennium.
While earlier rudders were mounted on the stern by the way of rudderposts or tackles, the iron hinges allowed the rudder to be attached to the entire length of the sternpost in a permanent fashion. Lawrence V. Mott, The Development of the Rudder, A.D. 100-1600: A Technological Tale, Thesis May 1991, Texas A&M University, S.2, 92f. 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. Lawrence V. Mott, The Development of the Rudder, A.D. 100-1600: A Technological Tale, Thesis May 1991, Texas A&M University, S.118f.
Historian Joseph Needham holds that the stern-mounted rudder was transferred from China to Europe and the Islamic world during the Middle Ages.
Rudder post and mast placement defines the difference between a ketch and a yawl, as these two-masted vessels are similar. Yawls are defined as having the mizzen mast abaft (i.e. "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. Transom hung rudders or far aft mounted fin rudders generate greater moment and faster turning than more forward mounted keel hung rudders. Rudders on smaller craft can be operated by means of a tiller that fits into the rudder stock that also forms the fixings to the rudder foil. Craft where the length of the tiller could impede movement of the helm can be split with a rubber universal joint and the part adjoined the tiller termed a tiller extension. Tillers can further be extended by means of adjustable telescopic twist locking extension.
There is also the barrel type rudder, where the ship's screw is enclosed and can be swiveled to steer the vessel. Designers claim that this type of rudder on a smaller vessel will answer the helm faster. "Barrel Type Rudder Makes Quicker Turns" Popular Mechanics, October 1934, bottom-right pg.495
Unlike a ship, both aileron and rudder controls are used together to turn an aircraft, with the ailerons imparting roll and the rudder imparting yaw and also compensating for a phenomenon called adverse yaw. A rudder alone will turn a conventional fixed-wing aircraft, but much more slowly than if ailerons are also used in conjunction. Sometimes pilots may intentionally operate the rudder and ailerons in opposite directions in a maneuver called a slip or sideslip. This may be done to overcome crosswinds and keep the fuselage in line with the runway, or to lose altitude by increasing drag, or both.
Another technique for yaw control, used on some tailless aircraft and , is to add one or more drag-creating surfaces, such as split ailerons, on the outer wing section. Operating one of these surfaces creates drag on the wing, causing the plane to yaw in that direction. These surfaces are often referred to as drag rudders.
Rudders are typically controlled with Rudder pedal.
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