Tropical Zone

The time of the year, however, is concerned with the ship having a greater freeboard (higher out of the water) for safety’s sake in winter when rougher weather may be encountered.  For this reason the greatest freeboard is demanded for Winter North Atlantic whilst the least freeboard in salt water is the line marked ‘T’ for tropical. The universal marking of all ships with a Plimsoll line dates from an international convention concluded in 1930.  That was forty years after the philanthropist, Samuel Plimsoll persuaded the British government to take a more positive view of safety of life at sea by adopting the statutory marking of all British ships in this way.  It seems that reaching international agreement on shipping matters took even longer in those days than perhaps it does now. At the same time as the measurement and marking of ships was agreed, the convention devised a method of dividing the world into zones to correspond to the lines on the Plimsoll mark. There is a wide band around the world extending several degrees either side of the Equator, marked ‘T’ for Tropical Zone which permits the deepest draft.  Obviously, if one can trade one’s ships within that area, the additional depth means more cargo and so more income.  Conversely trading, say, between northern Europe and North America in winter will mean loading to the lightest mark (WNA) and care will always have to be exercised so as to avoid an area in that Zone where winter restrictions are imposed from October to April. Most voyages, however, are likely to involve more than one zone which is why ship managers need to have a picture of the loadline zones clearly in their minds (or, better still, on their computer or framed and hung on the office wall).  The main impact of these zones is usually on the bunker planning.