Freeboard. This is the distance measured from the deck to the waterline. The minimum freeboard of a vessel is the vertical distance from the freeboard-deck to the loadlines, indicating the maximum permissible draught, measured at the middle of the ship’s length.
The freeboard deck is the uppermost continuous deck equipped with permanent means of closing all openings, which are exposed to the elements. This uppermost continuous deck is indicated on the ship’s side by a small horizontal line (deckline) above the loadlines mark.
The rules for the minimum freeboard for ocean vessels start from the principle that a certain “reserve buoyancy” is one of the fundamental requirements to ensure that the ship is seaworthy. The reserve buoyancy is that portion of a vessel above the water line, which is weather tight (including superstructures).
When assigning the freeboard, several factors enter into the picture, such as hull, form, depth, length of superstructures, and other ship features. Freeboards are assigned by governments or Classification Societies on their behalf. The rules are in the Loadlines Convention 1966.
When a vessel is issued with an International Loadline Certificate 1966 (“ILLC”) the minimum freeboards are indicated for the different loadlines. The minimum statutory freeboard is based on the summer, salt water loadline. The vessel may also be assigned a “greater than minimum freeboard” if the owner applies and the vessel meets certain criteria.
The statutory freeboard shown on the ILLC is the minimum. In practice the vessel may load to a lower draught and the “actual freeboard” may be more than the statutory freeboard. This is taken to be the distance from the deck to the waterline.
Under the International Convention on Load Lines of Ships 1966, the countries which were party to the Convention introduced their own rules on the Convention details. The basic freeboards that are calculated according to these rules depend on the length and type of vessel. “Type A vessel” is designed to carry only liquid cargoes in bulk and in which the cargo tanks have only small access openings closed by watertight gasketed covers of steel or equivalent material. These vessels are assigned with the lowest freeboards owing to the fact that because they ‘are nearly sealed, their buoyancy is unlikely to be lost unless they are damaged.
All other ships are “Type B vessels”. Basic Type B freeboards are larger, which means a lower draught. The Type B freeboard can be increased or reduced depending on the type of ship and its fittings. If vessels have old-fashioned hatchways covered with portable beams and covers, their freeboards are increased. Vessels having steel weathertight covers fitted with gaskets and clamping devices, improved crew protection, better arrangements for freeing water on deck and good subdivision characteristics, are considered to be safer and can have a freeboard reduced from the basic Type B freeboard. A greater reduction can be enjoyed by such vessels up to the total difference between the Type A and basic Type B freeboards. The Type B vessel which is assigned a Type A freeboard is called a “Type B – 100” vessel. Its subdivision requirements will be very severe. Other Type B vessels may be assigned a freeboard based on 60 per cent of the difference between Types A and B freeboards if their subdivision requirements are less severe.
Bulk carriers will be most advantaged if they can reduce their freeboards and load to deeper draughts. Ore carriers with two longitudinal bulkheads may be as sealed as tankers and can enjoy the B – 100 freeboard.