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Load Line Convention 66 (Computation Of Marks)


Computation Of Load Line (Or Freeboard) Marks And Conditions Of Assignment (Load Line Convention 66)

Regulations for these matters are contained in the 1966 Load Line Convention, and take due cognizance of the regulatory strategy at Section 3(4) and (5).

They prescribe the computational method by which the load line or freeboard marks are fixed, and the conditions of their assignment. Freeboards are computed assuming the ship to be a completely enclosed and watertight/weathertight envelope. The Convention then goes on to recognize the practical need for openings in a ship and prescribes means of protection and closure of such openings. These are called conditions of assignment, since the assignment of the computed freeboards 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 bulwarks to prevent water accumulating on the deck, also guard rails and walkways to provide safe passage on deck for the ship’s personnel.

The philosophy is to provide sufficient height of the uppermost continuous deck above the waterline such that the breaking of seawater over it (commensurate with the size of hatchways and means of closure) will not constitute an unforeseen hazard and, provide the ship’s personnel with a working platform at a sufficiently safe height above the waterline.

The Regulations prescribe the computational method by which the freeboard marks are fixed. The prescribed method is related to the geometry of the ship. Hence, the minimum computed summer freeboard (corresponding to the maximum summer draught) is known as the minimum summer geometric freeboard. Before this can be assigned and, prior to establishing conditions of assignment regarding protection and closure of openings and protection of crew, as described above, it must be confirmed that the ship’s strength is adequate for the corresponding maximum summer draught.

This is not always the case, since a ship carrying a light density (or capacity) cargo may not attain the maximum obtainable draught when fully laden. In such a case, an owner may choose not to incur the expense involved in the provision of adequate strength for the deepest draught, and have a greater than minimum geometric freeboard assigned, corresponding to a maximum permissible draught for strength purposes, (i.e. not the maximum geometric) known as the scantling draught.

In most cases however, the maximum scantling draught is the same as that corresponding to the minimum geometric summer freeboard.

The minimum geometric summer freeboard is computed by first taking a freeboard for a standard ship of the same length (provided in tabular form in the regulations) and correcting it for those geometric properties of the ship which differ from those of the standard ship. The geometric corrections prescribed in the Regulations are those for block coefficient, depth, superstructure, trunks and sheer. There is also a regulation which stipulates a minimum bow height.

Once the summer freeboard mark has been established, it is a simple matter to further compute the additional marks for fresh water, tropical, tropical freshwater, winter and winter North Atlantic, as required.

Note: It is appreciated that the method of computing freeboards in accordance with the 1966 Load Line Convention is outdated, and that it would be more technically pure to replace it with a method based on stability and deck wetness.

However, thousands of ships in service have been assigned freeboard in accordance with the present statutory method. Any new method regardless of its technical purity, should provide for similar freeboards to those already assigned so that new ships neither benefit in terms of draught nor are penalized by comparison with ships already in service; unless there is compelling safety evidence indicating a need for change. In such a case, retroactive adjustments might also be indicated.

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