Dead Weight Tonnage (DWT)… Gross Registered Tonnage (GRT)… Net Tonnage (NT). These terms could confuse those with scant knowledge of shipping, journalists, and even seafarers. I have often encountered news reports that describe a ship that has sunk or run aground as “weighing” so many tonnes. What exactly is the reporter referring to?
The following are the clearest definitions I have found of four tonnage terms. They are from the Tonnage Tax Manual of the Her Majesty’s Revenue and Customs (HM Revenue and Customs or HMRC), as published on the UK Government website:
‘Gross (registered) tonnage’ (GT or GRT) forms the basis for manning regulations, safety rules, registration fees and is also used to calculate port dues and fees for the transit of canals. Gross tonnage is the measurement of total volume of all enclosed spaces in a ship, with one gross ton equalling 100 cubic feet.
Gross tonnage of 100 tons or over is a requirement for a ship to be qualifying.
‘Net (or nett) (registered) tonnage’ (NT or NRT) represents the earning capacity of the vessel, as it is based on the amount of space available for cargo or passengers, excluding, for example, space occupied by the engines or the bunkers. It is based on a similar measurement to gross tonnage, i.e. 1 net ton equals 100 cubic feet.
Net tonnage is used in the calculation of the tonnage tax profit.
‘Deadweight Tonnage’ (DWT) is often used as a measurement of cargo-carrying capacity. Unlike GT and NT, which measure volume, it measures weight. The weight is of all cargo, stores, bunkers, fresh water, food, crew and passengers, which would bring a ship down to her summer load line. It is generally calculated by reference to 1,000 kg as one metric tonne.
‘Lightweight Tonnage’ (LWT) (or light displacement tonnage) is used to measure the scrap metal content of a ship destined to be broken up. Again, it is a measure of weight, this time of the ship’s hull and machinery.
Names may not have mattered to Shakespeare, who wrote in his play ‘Romeo and Juliet’: “That which we call a rose/ By any other name would smell as sweet.” But in the shipping world, they count for a lot.
~ Barista Uno