Freeboard is the distance measured from the waterline to the upper edge of the deck plating at side of the freeboard deck amidships.
Ships must have a load line mark located amidships on both sides to indicate the maximum allowable draught under specified conditions (geographical and seasonal).
The International Convention on Load Lines 1966 (ICLL 1966) with its Protocol of 1988 is a comprehensive set of regulations to determine the minimum allowable freeboard and defines the conditions of load line assignment. The minimum geometric summer freeboard is computed by taking a freeboard for a standard ship of the same length (provided in tabular form) and correcting it for those geometric properties of the ship which differ from those of the standard one. There are corrections for block coefficient, depth, superstructure, trunks and sheer. The result of this calculation – the load line mark – is permanently marked on the ship hull.
The calculation of the minimum freeboard must be approved by a classification society which defines the eventual location of the load line marks based on the measurement of built vessel. Marks cannot be welded to the sides of the hull during construction, even if certain shipyards would be eager to do that.
The assignment of the computed freeboard is conditional upon the prescribed means of protection and closure of openings such as hatchways, doorways, ventilation, air pipes, scuppers and discharges being complied with. Regulations are also included for freeing ports in bulwark to prevent water accumulating on deck, as well as guard rails and walkways to provide safe passage.