To many of us who take an interest in ships and their operations, tonnage is simply another method of comparing the size of various vessels. It is something that we check by looking in Registers and we know that the description of a ship is incomplete without it, but few of us know what these measurements are and how they are obtained. We present here a few useful definitions, courtesy of Canada Steamship Lines Ltd.'s house organ "C.S.L. News" which ran them a few years ago.
Most of the definitions refer to two particular decks in a ship and we should first explain these two terms. The tank top of a ship is the lowest point in which cargo can be stowed and amounts to the floor of the holds immediately above ballast and fuel storage areas. The tonnage deck is the uppermost complete deck extending the full length and breadth of the ship except in ships having more than two decks, in which case it becomes the second complete deck from below.
UNDERDECK TONNAGE is the total volume of the ship from stem to stern above the tank tops and below the tonnage deck, measured from inside the frames and beams and expressed in units of 100 cubic feet, such units called "tons."
GROSS REGISTER TONNAGE is the underdeck tonnage plus the same measurement applied to all permanently enclosed spaces (with certain minor exceptions) above the tonnage deck, expressed in units of 100 cubic feet. In short, it is the measurement of all enclosed spaces, on the ship. The fitting of as little as one nut and bolt to a "temporary closing appliance" may change an "open" space to an enclosed space which is measurable and similar ships may thus have greatly different gross tonnages.
NET REGISTER TONNAGE is the gross register tonnage less those spaces not considered to be earning spaces, such as crew's quarters, bunkers, machinery space, etc. Net tonnage is not indicative of either cargo capacity or earning ability today since it is still figured using a machinery space allowance to which is added a proportional addition to allow for fuel space which used to vary from trip to trip. This measurement is antiquated in view of technological progress effecting the efficency of modern propulsion machinery.
TOTAL DEADWEIGHT is a measure of the total number of long tons of material that the ship can carry and includes cargo, fuel, crew and effects, stores, fresh and feed water, passengers and baggage. It is usually quoted for a ship at mid-summer draft.
Reproduced for the Web with the permission of the Toronto Marine Historical Society.