A Plimsoll line or mark shows the safe ‘freeboard’ (distance from water to weather deck). There are 6 load lines as follows:
List of Load Lines
TF Tropical Fresh
WNA Winter North AtlanticExample Plimsoll Line or Mark
* AB – This is the clasification society that surveyed the vessel in order to determine her marks. AB stands for American Bureau.
The world is divided into “load line zones” which correspond to the Plimsoll Line. A vessel running through a winter load line zone is only permitted to load down to her W mark. Ships of LOA (Length Overall) have further restrictions when running through the WNA zone.
The following should give you an idea of how the world is divided up into the 6 zones:
Example of Loadline Chart
The origin of the Plimsoll Line is from Samuel Plimsoll, an English Politician. He pushed for legislation to prevent the overloading of vessels in the 1870s and was incorportated in the Merchant Shipping Act of 1876. Many of the ships that were overloaded were heavily insured old ships in which Owners would profit if the vessel sank.