Loadline Certificates are issued based on the surveyor’s calculations which must follow the rules laid down in the Loadline Convention and without such a document a ship cannot trade legitimately. Most shipowners prefer the marks to allow the ship to load as deep as possible but there are occasions when an owner will opt for a “surplus freeboard” and put the mark slightly lower down on the hull. This might be because the ship will trade to ports where charges are based on summer loadline draughts but the type of cargo envisaged will mean that the ship will never load that deep. Reducing the draught thereby reduces the port costs to be incurred. Lumber carriers referred to in Lesson One, are granted a second set of load lines for when carrying a deck load of lumber – ‘lumber loadlines‘ – and can sail with reduced freeboard when so laden. By international agreement, the oceans and waterways of the world are divided into ‘load line zones‘ – either permanent summer, winter or tropical, or seasonal summer, winter or tropical, depending upon the prevailing weather conditions likely to be experienced at different times of the year. These zones are shown on a special Load Line Chart (published in the United Kingdom, for example, by the Hydrographic Office). A ship passing through a summer load line zone can load down to but no further than the top of the summer load line. The same arrangements apply for trading in winter or in tropical zones, but extra allowance can be made when trading in what are assumed to be safer fresh water conditions. Ships with an overall length of 100 metres or less, are further restricted when trading in the North Atlantic Ocean in winter. Great care must be taken when planning a voyage to think ahead and to avoid transiting a load line zone when too deeply laden to be able to comply with these international regulations. In 1982 a new international system of measurement for ships came into force, under an IMO (The International Maritime Organisation – part of the United Nations) resolution. This applied immediately to all newbuildings, and from 1994 all existing vessels had to conform to its provisions.