Frequently Asked Questions

Why all the different types of size information?
Part of the AIS information includes the ship's size (length, width (beam) and draught) but for many (around 1 in 8) ships this information is either not entered or wrong. For instance zero or entered in feet instead of meters. The standard international measurement of a ship's size under the Universal Tonnage Measurement System (UMS), defined by the 1969 Tonnage Regulations, is the Gross Ton (GT). The 'ton' in gross tonnage is not a measure of weight but of volume (2.78 cubic meters) and is more closely related to the old English word 'tun' used as a measure of wine. It must not be written as 'tonne' which is an internationally accepted unit (under the SI system) of weight. However, volume (GT) is only a useful guide for certain types of vessel, mainly conventional cargo ships and passenger ships. For tankers and bulk carriers, lifting capacity is a more useful guide and this is measured by Deadweight Tonnage (dwt) in tonnes of weight. So as you can see the size of ship can be measured in a variety of ways. When possible, I try to use GT even though it is not broadcast on AIS as this information is most readily available from other sources. If GT is not available, I use the broadcast dimensions from AIS.