Do you know the difference between Net Tonnage and Lightweight Tonnage? By my count there are at least eight different ways a vessel’s “tonnage” is measured in regards to ships, MODUs, and other types of commercial vessels (Displacement Tonnage, Standard Displacement Tonnage, Deadweight Tonnage, Lightweight Tonnage, Gross Tonnage, Net Tonnage, Gross Registered Tonnage, and Net Registered Tonnage).
If you’re a little confused about what all these different units of measurement mean, here’s a quick guide to help straighten you out the next time someone asks you “how much your ship weighs”.
What is Displacement Tonnage:
Displacement tonnage is nothing more than the total weight of the volume of water a ship “displaces” when it is sitting in the water.
Imagine you had a large bucket filled to the brim with water. Now, suppose you gently placed a basketball in the bucket causing some of the water in the bucket to spill out. Once the basketball had stabilized and was freely floating in the bucket, the weight of the water that was “displaced” is equal to the “displacement” (or weight) of the basketball.
What is Standard Displacement Tonnage:
Standard displacement tonnage is basically the same thing as “displacement tonnage” with one minor difference. When calculating standard displacement tonnage, you subtract the weight of any fuel and potable water carried on board the ship.
What is Deadweight Tonnage:
Deadweight tonnage is the weight (in tons) of all the cargo, fuel, dry provisions, supplies, etc. carried on board the ship. In other words, it is the “displacement tonnage” of the vessel minus the “lightweight tonnage” (see lightweight tonnage below). Deadweight tonnage is a good indication for ship owners and clients of how much revenue the vessel is capable of generating.
What is Lightweight Tonnage:
Lightweight tonnage is best described as the weight of the ship when it was built in the shipyard including all framing, machinery, decking, etc. However, lightweight tonnage does not include the weight of any consumable such as fuel, water, oil, or supplies.
What is Gross Tonnage:
Gross Tonnage is a measure of the ships total interior volume and is calculated by multiplying the interior volume “V” of the ship in cubic meters by a variable known as “K” (which varies depending on the ships overall volume).
Gross Tonnage is not to be confused with Gross Registered Tonnage which is explained below.
What is Net Tonnage:
Similar to Gross Tonnage, Net Tonnage is a measure of the total interior volume of a ship’s cargo spaces and is calculated in much the same way. The total volume of designated cargo spaces in cubic meters is then multiplied by myriad factors resulting in an official net tonnage value. The actual calculation of Net Tonnage is one of the more complicated tonnages to calculated and beyond the scope of this article but takes into account factors such as moulded draft and the number of passengers a vessel is rated to carry.
Net Tonnage is not to be confused with Net Registered Tonnage which is explained below.
What is Gross Registered Tonnage (No Longer Used):
Gross registered tonnage is a measurement of volume of all enclosed spaces on a ship with 100 cubic feet = to one ton. For instance, if the total cubic volume of all the enclosed spaces on a ship were 340,000 cubic feet, the gross registered tonnage will most likely be 3400 gross tons (340,000 cu. feet/100 cu feet/ton = 3400 Gross Tons).
The use of the term “Gross Registered Tonnage” was phased out beginning in 1969 with passage of The International Convention on Tonnage Measurements of Ships and hasn’t been used officially sense 1982.
What is Net Registered Tonnage (No Longer Used):
Net registered tonnage is also a measurement of volume however you only consider the volume of actual cargo storage areas when dividing the cubic volume in feet by 100 to get your “tonnage”. This includes any tanks, cargo holds, etc. that are normally used for transporting cargo.