Freeboard

Freeboard is a measure of how much of a ship's watertight hull stands above the water. Defined as the vertical distance from the waterline up to the freeboard deck (generally the uppermost continuous watertight deck) at the side of the ship, it is the visible expression of the vessel's reserve buoyancy: the volume of intact, enclosed hull that is not immersed and that provides the extra flotation and righting energy a ship needs to ride safely over waves and to survive flooding. The greater the freeboard, the greater this reserve, and the more resistant the ship is to being overwhelmed by the sea.

Because freeboard is directly related to safety, it is regulated. The International Convention on Load Lines assigns each ship a minimum freeboard according to its length, type, form and subdivision, and this minimum is marked on the hull by the load line, the Plimsoll mark. Loading the ship deeper than the assigned line reduces the freeboard below the safe minimum and is prohibited. Because the safe depth of loading varies with the density of the water and the severity of the season, the load line includes several marks, and the freeboard the ship must maintain changes accordingly for tropical, summer and winter conditions.

Freeboard connects to several other concepts of ship safety and loading. It is inseparable from draft, since increasing draft by loading reduces freeboard, and from stability and damage survivability, because reserve buoyancy governs how a ship behaves if it is holed and flooded. Ships intended to carry deck cargo or to operate in severe conditions may be assigned greater freeboard for extra safety, while certain types with high standards of subdivision and watertight integrity may qualify for reduced freeboard. In all cases the principle is the same: adequate freeboard is a fundamental guarantee that a ship has enough buoyancy in reserve to remain safe at sea.