Professionals across the shipping industry may be familiar with the terms like “gross tonnage”, “deadweight tonnage”, “net tonnage” and many more terms we regularly meet in shipping-related sources, but those are not always easy to distinguish, especially when being new in the industry. Have you ever wondered what is the difference between gross and deadweight tonnage?
Broadly speaking, ‘tonnage’ measures a ship’s size and can be expressed in terms of either volume or weight.
Gross tonnage / Counted in: No unit
Gross tonnage (GT or gt) is a measure of a ship’s overall internal volume and is determined by dividing by 100 the contents, in cubic feet, of the vessel’s enclosed spaces. GT applies to the vessel, not to cargo. It measures the ship’s volume and has nothing to do with weight. It is based on two variables: V, the ship’s total volume in cubic metres (m3), and K, a multiplier based on the ship volume.
Net Tonnage / Counted in: No unit
GT is often confused with Net Tonnage, which is a ship’s gross tonnage minus the space occupied by accommodations for crew, by machinery, for navigation, by the engine room and fuel. This means a vessel’s net tonnage represents the available space for accommodation of passengers and stowage of cargo.
Note: GT and NT are dimensionless so they cannot be counted in physical units of tonnage. The word “tons” is no longer in use in reference to ships’ tonnage. So, we typically say “the ship has Gross Tonnage of 12,345” without the addition of any units.
Deadweight tonnage / Counted in: metric tonnes (1,000 kg)
Simply put, deadweight tonnage (DWT) is a measure of how much weight a ship can carry, so it includes the sum of the weights of cargo, fuel, fresh water, ballast water, provisions, passengers, and crew. This measurement does not include the empty weight of the ship, which means that DWT represents the difference between the number of tons of water a vessel displaces “light” and the number of tons it displaces when submerged to the “load line.” As you probably remember, load lines (also known as Plimsoll lines) on a ship’s hull indicate the maximum depth to which the vessel may be safely immersed when loaded with cargo.
‘Net Tonnage’ = useful capacity of a ship
‘Deadweight tonnage’ = carrying capacity of a ship in tonnes
A ship’s displacement, as the word indicates, is the volume of water it displaces when it is floating and is measured in cubic metres (m3). The displacement tonnage is the weight of the water that it displaces when it is floating with its fuel tanks full and all stores onboard, and is measured in metric tons (1,000 Kg). The term “displacement tonnage” maybe seen describing the weight of the vessel and its contents in tons of 2,240 pounds. Displacement “light” is the weight of the vessel without stores, bunker fuel, or cargo, while displacement “loaded” is the weight of the vessel including all the above.
The Gross and Net Tonnage measurement was established by the IMO’s International Convention on Tonnage Measurement of Ships. First adopted in 1969 and set into force in 1982, the Convention was the first successful attempt to introduce a universal tonnage measurement system. Previously, various systems were used to calculate the tonnage of merchant ships but there were significant differences in them, so the Convention addressed the need for a globally accepted system.
The Convention provides for gross and net tonnages, both of which are calculated independently. The rules apply to all ships built on or after 18 July 1982 – the date of entry into force. The Convention meant a transition from the traditionally used terms gross register tons (GRT) and net register tons (NRT) to gross tonnage (GT) and net tonnage (NT).
Why are these useful?
GT is not a really usable measure in everyday life, but it forms the basis for authorities, PSC and flag states, as well as classification societies to set manning regulations, safety rules, as well as registration fees. Both gross and net tonnages are also used to calculate port dues. According to Steamship Mutual P&I Club, Tonnage Measurement is used in the assessment of the following:
- Harbour Dues – which can be based on either Gross or Net Tonnage
- Pilotage Dues – which can be based on either Gross or Net Tonnage
- Light Dues – usually based on Net Tonnage
- Canal Dues – usually based on Net Tonnage
Sometimes, the term can also be used to compare ships in terms of their size, even though a larger ship does not necessarily have also a bigger gross tonnage. To put Gross Tonnage into context, currently the world’s largest container ships by GT have a GT of just over 230,000 and the largest cruise ships by GT have just over 228,000.
Before the IMO Convention, the traditionally used terms were gross register tonnage (GRT), measuring the volume of only certain enclosed spaces, typically above the deck of a merchant ship which were available for cargo, stores, fuel, passengers and crew. Accordingly, the net register tonnage (NRT) used to measure the volume of cargo the vessel can carry, minus the spaces that do not hold cargo.
How do I calculate the actual weight of the vessel?
each ship has displacement tonnage tables. that is, the amount of water it carries when the ship is empty. You can find this on the ship chart by looking behind the plimsol m.