The area of oceans (70.8% of the earth’s surface) is far greater than the land area (29.2%). All the world’s oceans are saline (salt water), but the concentration of salt, which affects the density of the water, is not uniform.
The variations in salinity are caused by
A- changes due to mixing and circulation
B- water evaporation rates and
C- supplies of fresh water
Highest salinity is found near the Tropics due to the high evaporation. It is lower at the Equator due to the heavy rain and lower evaporation.
Towards the poles melting ice supplies fresh water, therefore low salinity levels. Inevitably climatic conditions will affect even the highly developed ships of today.
As a result of the campaigning of the social reformer Samuel Plimsoll at the end of the last century, a system of marking the position of the minimum free-board on all British ships was introduced.
Eventually, in the early 1930s a refinement of this system became an international convention. The Plimsoll Mark takes into account the difference in density as well as prescribing a greater Free-Board, the distance from the water line to the main deck, in areas where severe weather is more prevalent. Some such load-line zones are permanent whilst others are seasonal.
Netpas Distance shows the internationally agreed load-line zones of the world. The depth to which a ship is allowed to load will determine the weight of cargo it can carry, and this in turn will affect the freight payable. It is clearly important therefore that this factor must be taken into account in calculating voyage estimates.
During a voyage, a ship may cross from one load-line zone to another more than once, and its draft must not exceed the maximum limits for each zone. This is complicated by the changing limits in some zones as the seasons change, so the dates of crossing such zones must also be allowed for.