A daggerboard is a retractable keel 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.
Daggerboards are often longer and thinner than pivoting centerboards, thus providing a better lift to drag ratio. Daggerboards are usually found in small craft such as day sailers, where their size is easily handled by a single person. Unlike centreboards, daggerboards are not usually ballasted, but are locked in place by a clip.
When a daggerboard is extended through the keel, it improves a ship's stability. Daggerboards can be raised when the a ship enters a shallow harbor, allowing the boat (for example) to load and unload cargo in locations that would not otherwise be accessible to larger ships.
One problem with such boats if that if one were to hit shore, it would be quite easily damaged. Centreboards, by comparison, are less easily damaged.
Boats equipped with it
The Mirror Dinghy, for example, uses a plywood daggerboard.
In 2008, a 55-foot long daggerboard ship was found in Lake Ontario, using deep scan sonar equipment off the lake's southern shore, the only one known to have been found in the Great Lakes. Vessels of this type were used for a short time in the early 1800s.
According to  there are only 4 non-custom manufacturer of catamarans which use daggerboards: Catana, Outremer, Sud Composites, and Dolphin. One reason might be its construction cost, estimated to be about $30,000 for a 45-foot catamaran.