The first posts in this series covered certification that identified a ship, the next group of posts cover those that define a ship.
Defining the size of a ship
What is it?
A certificate stating the Gross and net tonnages of a ship.
Why is it needed?
It is required by the International Convention on Tonnage measurement of Ships 1969.
Gross tonnage is used to determine which regulations apply to which ships and Net tonnage is often used to determine the size of harbor and canal dues.
The UK legislation is The Merchant Shipping (Tonnage) Regulations 1982
What is gross and net tonnages?
Gross tonnage is a the measure of the overall size of a ship. It is derived from a formula that multiplies the internal volume of a ship in cubic metres by a constant (K) contained in a table in the tonnage convention.
Net tonnage is the measure of the useful capacity of a ship determined in accordance with the Convention. It is also derived from multiplying the internal volume by the constant K, but then multiplies it by the square of 4 x ships summer moulded summer draft/3 x moulded depth. An additional factor is used for passenger ships.
What ships does it apply to?
Ships engaged on international voyages, but not to ships of war and ships of less than 24 meters in length.
What is contained on the certificate?
- Name of Ship
- Distinctive Number or Letters
- Port of Registry
- Moulded depth amidships
- The Gross and net tonnages
A table showing the spaces included within the tonnage. The table lists for each space:
When is it Issued?
On build or alteration after a tonnage survey has been conducted.
What are the Suez and Panama tonnages?
These are issued on behalf of the canal authorities. The canals use a different measurement system from the international certificate, and the certificate is not mandatory, unless the vessel intends to use the canals. Most ships are issued with the certificates on build.