TONNAGE AND MEASUREMENTS

Your explanation of ‘gt’ in the abbreviations (Chartroom, SM Jan) is somewhat misleading, although the business of calculating a ship’s tonnage is complicated. Before 1982, various systems were used to calculate the tonnage of merchant ships. Although all went back to the method devised by George Moorsom of the British Board of Trade in 1854, there were considerable differences between them (particularly the United Statesbased and British-based systems) and it was recognised that a single international system was needed. To simplify the explanation, under these old systems the ship’s volume in cubic feet was divided by 100 to produce the tonnage figure.

The Tonnage Convention adopted by the IMO in 1969 was the first successful attempt to introduce a universal tonnage measurement system. The rules