Units of Ship’s Weight

Displacement Tonnage

It is the weight in tons (2,240 lbs/ton) of the water displaced by the immersed part of the ship and is equal to the weight of the ship and everything on board (men, fuel oil, supplies, etc.) The density of sea water averages 64 lbs/cu. ft. or 35 cu. ft./ton, hence, the displacement in sea water is measured by the immersed volume in cubic feet, divided by 35. (For fresh water divide by 36.)

Dead Weight Tonnage

It is the weight of cargo, stores, fuel, water, personnel and effects that the ship can carry when loaded to a specific load draft. Dead weight is equal to the load displacement minus the weight of the equipped ship, commonly expressed as long tons (2,240 lbs), or the difference in weight of the ship when empty and fully laden.

Gross Tonnage

It is based on cubic capacity of the ship below the tonnage deck, plus allowances for certain compartments above, which are used for cargo, passengers, crew and navigating gear. One gross ton equals 100 cu. ft. of enclosed space.

Net Tonnage

It is gross tonnage minus deduction of spaces for propelling machinery, crew quarters, and other non-earning spaces. One net ton is equal to 100 cu. ft. of volume.

For tankers as a rough estimate, the dead weight tonnage figure is about 50% greater than the gross tonnage, also the loaded displacement is approximately one-third greater than the dead weight.