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A centreboard or centerboard (US)  is a retractable hull appendage which pivots out of a slot in the hull of a sailboat, known as a centreboard trunk (UK) or centerboard case (US). The retractability allows the centreboard to be raised to operate in shallow waters, to move the centre of lateral resistance (offsetting changes to the sailplan that move the centre of effort aft), to reduce drag when the full area of the centreboard is not needed, or when removing the boat from the water, as when trailering. A centreboard which consists of solely a pivoting metal plate is called a centerplate. A daggerboard is similar but slides vertically rather than pivoting.
The analog in a scow is a bilgeboard: these are fitted in pairs and used one at a time.
Lt. John Schank (c. 1740 – 6 February 1823) was an officer of the British Royal Navy and is credited with the invention of the centerboard. Schank, however, gave credit for the idea to British Brigadier General Earl Percy.
A centreboard (often called a lifting foil in a modern racing dinghy) is used to provide lift to counter the lateral force from the sails. This is required for sailboats to move in directions other than downwind, since the force of the sail is never closer than 45 degrees to the apparent wind. Since most sailboats are symmetric along their axis of motion, when sailing upright, the lateral force can come from either side, which means that centreboards must use symmetric foil shapes so they will operate with equal efficiency on either tack. The more a yacht heels the more asymmetric its hull shape becomes.
The centerboard, daggerboard or bilgeboard can be used as a recovery platform upon which to stand, providing increased leverage, in the event the dinghy overturns via a capsize or turtle.
The efficiency of a centreboard improves with increasing aspect ratio. A long narrow centreboard produces less drag than a short, wide one for a given amount of lift, resulting in a faster boat that can point closer into the wind. A fore and aft, pivoting centreboard can also be used to move the centre of lateral resistance aft to match a change in sail plan, such as furling or dropping the jib. A retracting centreboard is more complex than a fixed keel, and most take up space inside the hull of the boat that could otherwise be used for passenger accommodation. Other types feature a casing under the boat, which does not take up space but instead has the problem of increased drag. For this reason, it is not uncommon to find boats with a combination of shallow keel and centreboard (e.g. Randmeer). The keel provides the housing for the centreboard, moving it out of the hull, but adds only a small amount of draft to the boat. The centreboard can then be lowered in deeper waters to increase the amount of lift. Ballast is usually provided in the keel, keeping the centreboard lighter and easier to handle.
Centreboards are often ballasted in keelboats and called "lifting keels" because like keels they are centred under the hull, although unlike keels, they are retractable  . Ballasted centreboards are generally not locked in place when lowered; the mass of the ballast keeps them down. This also provides a measure of safety should the boat run aground—the force of impact will push the foil back into the centreboard trunk, rather than breaking it, as might happen if the board were locked in place. The mass of a ballasted foil means that a system of pulleys may be required to allow the sailor to lift the foil, and a method of latching the board in the upward position is needed. A centreboard differs from a ballast keel in that centreboards do not contribute to the stability of the vessel; their purpose is to provide lateral resistance. In small sailing dinghies it is rare to find a ballasted centreboard.
On larger sailing vessels, a similar design is sometimes incorporated to facilitate better navigation in shallow water than a fixed keel would allow. In these situations the appendage is generally referred to as a "lifting keel" (which is usually pivoted but occasionally retracted like a daggerboard) or a "swing keel".
In such installations on offshore vessels, the keel should ideally be lockable in any position, so that it does not fall back into the keel well if the vessel is inverted.
When the craft is moving, the centreboard acts as a lifting foil.
A hull is the watertight body of a ship, boat, or flying boat. The hull may open at the top, or it may be fully or partially covered with a deck. Atop the deck may be a deckhouse and other superstructures, such as a funnel, derrick, or mast. The line where the hull meets the water surface is called the waterline.
Sailing employs the wind—acting on sails, wingsails or kites—to propel a craft on the surface of the water, on ice (iceboat) or on land over a chosen course, which is often part of a larger plan of navigation.
A sailboat or sailing boat is a boat propelled partly or entirely by sails and is smaller than a sailing ship. Distinctions in what constitutes a sailing boat and ship vary by region and maritime culture.
The keel is the bottom-most longitudinal structural element on a vessel. On some sailboats, it may have a hydrodynamic and counterbalancing purpose, as well. As the laying down of the keel is the initial step in the construction of a ship, in British and American shipbuilding traditions the construction is dated from this event.
A point of sail is a sailing craft's direction of travel under sail in relation to the true wind direction over the surface.
A jibe (US) or gybe (Britain) is a sailing maneuver whereby a sailing vessel reaching downwind turns its stern through the wind, which then exerts its force from the opposite side of the vessel. For square-rigged ships, this maneuver is called wearing ship.
A daggerboard is a retractable centreboard used by various sailing craft. While other types of centreboard may pivot to retract, a daggerboard slides in a casing. The shape of the daggerboard converts the forward motion into a windward lift, countering the leeward push of the sail. The theoretical centre of lateral resistance is on the trailing edge of the daggerboard.
Planing is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift (buoyancy).
A leeboard is a form of pivoting keel used by a sailboat largely and very often in lieu of a fixed keel. Typically mounted in pairs on each side of a hull, leeboards function much like a centreboard, allowing shallow-draft craft to ply waters fixed keel boats cannot. Only the leeward side leeboard is used at any time, as it submerges when the boat heels under the force of the wind.
A bilgeboard is a lifting foil used in a sailboat, which resembles a cross between a centerboard and a leeboard. Bilgeboards are mounted between the centerline of the boat and the sides, and are almost always asymmetric foils mounted at an angle to maximize lateral lift while minimizing drag. They are most often found on racing scows.
Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voyage without cargo, then ballast of little or no value will be loaded to keep the vessel upright. Some or all of this ballast will then be discarded when cargo is loaded.
A canting keel is a form of sailing ballast, suspended from a rigid canting strut beneath the boat, which can be swung to windward of a boat under sail, in order to counteract the heeling force of the sail. The canting keel must be able to pivot to either port or starboard, depending on the current tack.
A sailing hydrofoil, hydrofoil sailboat, or hydrosail is a sailboat with wing-like foils mounted under the hull. As the craft increases its speed the hydrofoils lift the hull up and out of the water, greatly reducing wetted area, resulting in decreased drag and increased speed. A sailing hydrofoil can achieve speeds exceeding double and in some cases triple the wind speed.
Weather helm is the tendency of sailing vessels to turn towards the source of wind, creating an unbalanced helm that requires pulling the tiller to windward in order to counteract the effect.
A trailer sailer is a type of sailboat that has been designed to be easily transported using a boat trailer towed by an automobile. They are generally larger than a sailing dinghy. Trailer sailers include day sailers and small cabin cruisers, suitable for living on.
Forces on sails result from movement of air that interacts with sails and gives them motive power for sailing craft, including sailing ships, sailboats, windsurfers, ice boats, and sail-powered land vehicles. Similar principles in a rotating frame of reference apply to wind mill sails and wind turbine blades, which are also wind-driven. They are differentiated from forces on wings, and propeller blades, the actions of which are not adjusted to the wind. Kites also power certain sailing craft, but do not employ a mast to support the airfoil and are beyond the scope of this article.
The CS 22 is a Canadian trailerable sailboat, that was designed by John A. Butler and first built in 1971. The design is out of production.
The fin keel is a stationary foil positioned amidships and projecting downwards under the hull of a sailing vessel. A fin keel is relatively short in a fore-aft direction, and relatively deep, located near the center of the boat. A fin keel is a fixed element, unlike a centerboard, which is retractable. The design purpose of the fin keel is to provide lateral resistance to wind forces applied to the boat via the sails and to facilitate the placement of ballast below the hull while presenting less wetted surface area than a full keel, which helps to reduce drag and leeway. The fin keel was invented by James Brown Herreshoff (1834–1930).
The Neptune 24 is an American trailerable sailboat that was designed as a cruiser and first built in 1978.
The Poacher 21, also called the Parker Dawson Poacher, is an American trailerable sailboat that was designed by W. Richardson as a racer-cruiser and first built in 1980.