The Moorsom System
is a method created in Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
of calculating the tonnage
Tonnage is a measure of the size or cargo carrying capacity of a ship. The term derives from the taxation paid on tuns or casks of wine, and was later used in reference to the weight of a ship's cargo; however, in modern maritime usage, "tonnage" specifically refers to a calculation of the volume...
or cargo capacity of sailing ships as a basis for assessing harbour and other vessel fees. It was put into use starting in 1849 and became British law in 1854.
Previous methods of calculating tonnage, as Builder's Old Measurement
Builder's Old Measurement is the method of calculating the size or cargo capacity of a ship used in England from approximately 1720 to 1849. It estimated the tonnage of a ship based on length and maximum beam...
, were not being consistently applied and, because they were designed for sailing ships, could not be applied appropriately or fairly for steamships. Substantial portions of a steamship were required for boilers, machinery and coal, thus limiting the proportion of the ship's space available for cargo.
In 1849 Great Britain appointed a Commission with Admiral George Moorsom as secretary to resolve these problems. The Commission determined that fees should be proportional to the earning capacity of the ship, whether for cargo or passengers.
The result was called The Moorsom System, which set forth the rules for the measurement of the internal volume of entire ship. The total internal volume was then divided by 100 ft³ to produce the gross tonnage. Net tonnage was the volume remaining after subtracting the volume of the space used for machinery and other non-revenue producing functions.
The Commission sought to avoid a significant change in the fees charged to an existing vessel when the new system was implemented. George Moorsom ordered the entire fleet of British merchant ships to be measured according to the new System and then divided the total gross tonnage by the total registered tonnage. The result was 98.22 ft³ per gross ton, which was rounded to 100 ft³ per ton.
While the rules for measuring ships changed over the years, the standard of 100 ft³ per ton remained in effect until a new system was established by The International Convention on the Tonnage Measurement of Ships, effective for new ships in July 1982.