Gross Tonnage (GT) is a measure of the total enclosed volume of the ship in cubic meters multiplied by a constant, and the Net Tonnage (NT) or Register Tonnage is the total enclosed volume available for cargo in cubic meters multiplied by a constant.
Port dues, canal dues, and various government dues are usually levied on Gross Tonnage (GT) and Net Tonnage (NT). Gross Tonnage (GT) and Net Tonnage (NT) are measurement tonnages. Shipowners are eager to keep the dues to a minimum. Therefore, shipowners instruct naval architects to study the rules thoughtfully so as to build a ship of a certain capacity with the smallest Gross Tonnage (GT) possible. In order to encourage safety, governments would offer various exempted spaces as an incentive for good building practices such as the double bottom was exempted from Gross Tonnage (GT) if it was used only for water ballast. The accurate definitions of measurement tonnage had therefore become complicated, and somewhat obscure. Furthermore, different countries had developed different rules for making these measurements. Prior to 1982, there were about five (5) basic systems in use:
- Other Maritime Nations
- Suez Canal Authority
- Panama Canal Authority
However, ships built before 1982 were allowed to retain their original Gross Registered Tonnage (GRT) and Net Registered Tonnage (NRT), if the owners so wished, until 18 July 1994.
Gross Registered Tonnage (GRT) was the total internal capacity below the uppermost continuous deck, the ‘tonnage’ deck for the most ship, plus all permanently enclosed spaces above this deck, less exempted spaces. Examples of exempted spaces were double bottom tanks if used only for water ballast, and crew accommodation places above the upper deck. Net Registered Tonnage (NRT) was the gross tonnage less deducted spaces. Examples of deducted spaces included machinery spaces, crew accommodation, chart room, and radio room.
History of International Tonnage Certificate:
In 1694 the Thames Tonnage Measurement was used. In 1849 a royal commission originated the basic concept that assessment of dues should be based on a ship’s potential earning capacity. It was known as the Moorsom System after the secretary of the commission, George Moorsom. This came into force in 1854. In 1873 an international tonnage commission was held at Istanbul (Constantinople). Its findings were not followed, except by the authorities of the newly opened Suez Canal. In 1930 the League of Nations tried to obtain universal agreement but it was not followed by either the British or Americans, though it was adopted by most other countries.
In 1967 the Merchant Shipping (Tonnage) Regulations were passed. In 1969 the IMO (International Maritime Organisation) held the International Convention on Tonnage Measurement of Ships. This convention at long last brought in a universally accepted system of Gross Tonnage and Net Tonnage on 18 July 1982. Note that as these tonnages are independent of the nationality of the ship they no longer need to be linked to the registration of the ship, so their official title is Gross Tonnage (GT) instead of Gross Registered Tonnage (GRT). Likewise, since 1982 Net Tonnage is abbreviated to NT instead of NRT. When initially measured, ships are issued with an International Tonnage Certificate (1969).
What is Shelter Deck Ship?
If the shipowner built a ‘shelter’ deck over his uppermost continuous deck and left a small ‘tonnage’ opening shipowner had greatly increased the ship’s cargo-carrying capacity but not the ship’s Gross Tonnage (GT). Such a ship was known as a shelter deck ship. The uppermost continuous deck was the deck from which the freeboard was measured in both cases. Shelter Deck Ship was not as structurally sound as a ship of equal carrying capacity, where the top deck was the uppermost continuous deck.