Centreboard
Encyclopedia
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
which pivots out of a slot in the hull
Hull (watercraft)
A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...
of a sailboat
Sailboat
A sailboat or sailing boat is a boat propelled partly or entirely by sails. The term covers a variety of boats, larger than small vessels such as sailboards and smaller than sailing ships, but distinctions in the size are not strictly defined and what constitutes a sailing ship, sailboat, or a...
, 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 to offset changes to the sailplan that move the centre of effort aft and to reduce drag when the full area of the centreboard is not needed or when removing the boat from the water as when trailering
Trailer (vehicle)
A trailer is generally an unpowered vehicle pulled by a powered vehicle. Commonly, the term trailer refers to such vehicles used for transport of goods and materials....
. In case the centreboard consist of just a pivoting metal plate it is called a centerplate. A daggerboard
Daggerboard
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...
is similar but slides vertically rather than pivoting.
Function
A centreboard is used to provide liftLift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
to counter the lateral force from the sail
Sail
A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...
s. This is required for sailboats to move in directions other than downwind, since the force of the sail
Sail
A sail is any type of surface intended to move a vessel, vehicle or rotor by being placed in a wind—in essence a propulsion wing. Sails are used in sailing.-History of sails:...
is never closer than 45 degrees to the apparent wind. Since most sailboats are symmetric along their axis of motion, 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.
Design
The efficiency of a centreboard improves with increasing aspect ratioAspect ratio (wing)
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth . A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings....
. 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 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
Keel
In boats and ships, keel can refer to either of two parts: a structural element, or a hydrodynamic element. These parts overlap. As the laying down of the keel is the initial step in construction of a ship, in British and American shipbuilding traditions the construction is dated from this event...
, and most take up space inside the hull of the boat that could otherwise be used for passenger accommodations. Other types feature a casing under the boat, which thus does not take up space but now has the problem of increased drag. For this reason, it is not uncommon to find boats with 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
Sailing ballast
Ballast is used in sailboats to provide moment to resist the lateral forces on the sail. Insufficiently ballasted boats will tend to tip, or heel, excessively in high winds. Too much heel may result in the boat capsizing. If a sailing vessel should need to voyage without cargo then ballast of...
is usually provided in the keel, keeping the centreboard lighter and easier to handle.
Centreboards are often ballasted. 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.
On larger sailing vessels, a similar design is sometimes incorporated to enable navigating into shallower 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.
See also
- DinghyDinghyA dinghy is a type of small boat, often carried or towed for use as a ship's boat by a larger vessel. It is a loanword from either Bengali or Urdu. The term can also refer to small racing yachts or recreational open sailing boats. Utility dinghies are usually rowboats or have an outboard motor,...
- Dinghy sailingDinghy sailingDinghy sailing is the activity of sailing small boats by using five essential controls:* the sails* the foils ....
- Dinghy racingDinghy racingDinghy racing is the competitive sport of sailing dinghies. Dinghy racing has affected aspects of the modern dinghy, including hull design, sail materials and sailplan, and techniques such as planing and trapezing.-Organisation of dinghy racing:...
- LeeboardLeeboardA leeboard is a lifting foil used by a sailboat, much like a centerboard, but located on the leeward side of the boat. The leeward side is used so that the leeboard isn't lifted from the water when the boat heels, or leans under the force of the wind....
- BilgeboardBilgeboardA 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...
- DaggerboardDaggerboardA 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...