GT and DWT measure

Tonnage 

Broadly speaking, ‘tonnage’ measures a ship’s size and can be expressed in terms of either volume or weight. 

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 meters (m³), and K, a multiplier based on the ship volume. 

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.  

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 know, 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.  

‘Gross Tonnage’ = overall size of a ship 
‘Net Tonnage’ = useful capacity of a ship
‘Deadweight tonnage’ = carrying capacity of a ship in tonnes 

Displacement 

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.  

Regulatory landscape at a glance: 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.