1 An early type of keel or centreboard, usually made of wood. Attached by a pivot at their upper end, they were positioned at approximately the midpoint on either side of the hull of a flat-bottomed or shallow-draught sailing vessel. There is no evidence that leeboards existed in Europe before 1570, but when they were introduced—almost certainly from China where rafts and ships had used them since at least the 8th century—they became particularly popular in the Low Countries where they are still in common use on the boeier and other Dutch craft. They can also be seen on the few remaining examples of the Thames sailing barge. The earliest representation of them in Europe is a marine painting of Amsterdam harbour in 1600.
When the board on the lee side is lowered it increases the effective draught, thereby reducing the leeway made when sailing close hauled. When the vessel has to turn through the wind to change from one tack to the other, the board on the lee side is lowered as the sails fill on the new tack while the board on the weather side is hauled up.
When constructed for use on inland waters and canals leeboards are generally very broad in relation to their length, in shape not unlike an opened fan, so as to present the greatest practicable area to resist leeway within the limits of the depth of water. In coastal waters, where rough seas can blow up with great rapidity, the leeboards are usually long and narrow, more like dagger-boards.
2 A board or other preventive fitted to the side of a bunk on board ship to keep the occupant from rolling out of it when the vessel is lively in a rough sea.