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.
Daggerboards are often long and thin, 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. Daggerboards are not usually ballasted, but are locked in place by a clip. They are raised vertically unlike a centreboard which can be set at different angles to the hull of the boat.
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 is that if one were to hit shore, it would be quite easily damaged.
 Boats equipped with it
In 2008, a 55-foot-long (17 m) 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 19th century.
According to  there are only four non-custom manufacturers 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 (14 m) catamaran.
- "Upstate NY explorers ID rare boat in Lake Ontario". Associated Press. December 13, 2008.
- "Daggerboards vs. Keels".