Why do sailing boats use centreboards or dagger boards?

Sailing boats use centreboards and daggerboards primarily to increase lateral resistance and prevent the boat from sliding sideways, a phenomenon known as sideslip, when sailing against the wind.1

These devices function as retractable or removable keels. By lowering a centreboard or sliding a daggerboard into the water, the sailor provides the hull with a vertical surface that resists the sideways force of the wind on the sails. This allows the boat to maintain its intended course more effectively. Because they are retractable or removable, they also offer the practical advantage of allowing a boat to operate in shallow waters where a fixed, deep keel would be impractical or prone to grounding.123