Modified tonnage. Sometimes a shipowner may find that the cargoes his ship carries rarely submerge the ship’s normal load lines.
This would be the case with light, bulky cargoes or with livestock or other lowdensity cargo, which can have a low deadweight. He can apply to the authorities for the load lines to be marked lower down the side of the ship. Calculations for the distance from the load lines to the deck (i.e., the “freeboard”) are made with reference to the second deck from above (in the usual general cargo ship type to which this applies) and not to the uppermost deck. This is called a “greater than minimum freeboard” under the Load Line Convention. (One advantage of larger freeboards may be that Classification Societies allow lesser “scantlings” or standard dimensions for the structural components of the vessel.)
The ship’s gross tonnage is measured, this time excluding the space between the second and upper decks. Naturally, the ship will also have a lower registered tonnage. This tonnage is known as “modified tonnage”.
Modified tonnage was a concept introduced in 1967, mainly by 1MG, because of the problem posed by the type of vessel known as a “shelter decker”. The scheme resulted in a special “Tonnage mark” placed on each side of the ship.
The traditionally designed general cargo vessels of the 1950s and 1960s (still found in some oldfashioned trading areas today) was the “three-island type” with raised forecastle, accommodation block and raised “poop” .
The spaces on deck between these structures were exempted from tonnage measurement but could be used for cargo. Owners did carry cargo in these “wells” and protected it by awnings and other shelters which gradually became more permanent. Eventually the spaces were totally enclosed with a permanent deck above. These spaces became known as the “shelter deck spaces.
However, to maintain the fiction that the space above the original upper deck was still “open”, the shelter deck spaces were interconnected by openings and a small “tonnage hatch” at the after end of the ship led to the shelter deck space. The tonnage hatch could not be permanently or substantially covered. This led to a concern for the safety of these vessels. For example, if there was fire or flooding, the danger could spread from one end of the ship to the other without hindrance.
The tonnage mark was introduced to encourage shipowners to close all these “tonnage openings” andrestore the vessel’s integrity without being penalised by having the shelter deck space included in the vessel’s tonnage. This method was not popular with port and other authorities which derive their revenue based on the ship’s tonnage.
For modified tonnage, the space between the second deck from above and the upper deck is not included in tonnage measurement. Where these tonnages apply, a “tonnage mark” is placed on the ship’s side, in line with the deepest load line. The tonnage mark functions as a badge rather than as a level to which the vessel may be loaded.