daggerboard n : a removable centerboard on a small sailboat that can be lowered into the water to serve as a keel
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.
The Mirror Dinghy, for example, uses a plywood daggerboard.
daggerboard in Swedish: Sticksvärd