A centerboard is a type of movable keel which is fitted into the centerline of a sailing vessel. Centerboards are used when sailors want access to a retractable keel so that they can use a keel when they need it, and tuck it away when they do not. Some sailing vessels come fitted with a centerboard, and in other cases, a boat may be retrofitted to add one.
Keels, like other types of foils used on sailing craft, surfboards, planes, and so forth, are designed to add stability and control. When weighted, a keel will help a ship stay upright, which is desirable, and it will also allow sailors to move into the wind without sliding to a leeward direction. This increases the flexibility and maneuverability of the boat, making a keel an advantage in many settings.
However, keels can also cause problems. A keel makes it difficult to navigate in shallow water, and means that a boat cannot rest on its bottom without tipping to one side. Furthermore, the keel adds drag, which may be desirable in some settings, but it can be problematic in others, slowing the boat down when sailors want to focus on speed.
The centerboard design balances these conflicting needs, allowing sailors to enjoy the best of both worlds. When a keel is needed, the centerboard is pivoted through a slot in the bottom of the boat, and may be weighted or locked in place. When the centerboard is not contributing to the function of the craft, it can be raised, allowing the craft to do things like navigating in the shallows without worrying about damage to the keel.
The daggerboard, a closely related concept, slides up and down into a sheath inside the boat, rather than pivoting as the centerboard does. Centerboards and daggerboards come in a variety of styles which can be fitted to various boats, allowing sailors to select the design which will be most effective for their craft. One drawback to the centerboard design to be aware of is that it can be difficult to service from inside the boat; the boat needs to either be hauled onto land, or someone needs to dive to access the centerboard.
Ship yards and marine suppliers stock centerboards and accessories, including replacements for parts which may wear down over time. Skilled sailors can install their own, or use the services of the staff at a shipyard, whether they are refitting and repairing a craft with an existing centerboard, or retrofitting a boat to add a centerboard or dagger board.