Summer Freeboard is the distance between the uppermost continuous deck i.e. deck line, and the waterline. The larger it is the more reserve buoyancy the vessel has, and the less chance there is of waves breaking over the deck.
It is common to base references to deadweight on what can be carried when a ship is loaded to ‘summer marks’, a ship’s ‘summer deadweight’, occasionally expressed as ‘summer freeboard’.
When a DWCC (Deadweight Cargo Capacity) is quoted this is customarily also based on summer marks and a full quantity of bunkers etc. In practice, the DWCC (Deadweight Cargo Capacity) is infinitely variable depending on the weight of bunkers and spares and freshwater on board. Consideration also has to be given to any waste oil and water the ship is carrying since in most parts of the world these can no longer be dumped at sea with impunity.
Load Lines may be referred to as ‘Plimsoll Marks’ or ‘Plimsoll Lines’, after the British politician Samuel Plimsoll who, in the late 19th century campaigned against shipowners who loaded their ships to a depth which jeopardized crew members’ lives. Eventually, in 1890, a system of calculating and marking a safe ‘freeboard’, the distance from the water line to the weatherdeck, was devised and adopted in England, although it was not until 1930 that this finally became international law.
On ‘Plimsoll Marks’, there are, in fact, six ‘load-lines’. This is because the account is taken of the world’s geography and weather load-lines assessing the hazards of any particular voyage, as well as whether a ship is transiting a salt or, technically safer, freshwater area. The actual load line mark (the disc with a line through it) lines up with the summer load line. On this ‘Plimsoll Marks’, the letters ‘A’ and ‘B’ are related to the classification society (American Bureau) which surveyed the ship to determine the positioning of her marks and thereafter arranged for them to be ‘cut-in’ and painted on the side of the hull on behalf of the nation in which a ship is registered. Other classification societies initials LR (for Lloyd’s Register), BV (Bureau Veritas), NV (den Norske Veritas), GL (Germanischer Lloyd), and so on.
Load-line 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 a shipowner 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 load-line 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. For example, lumber carriers are granted a second set of load lines for when carrying a deck load of lumber, ‘lumber load-lines’, 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. Load-line zones are displayed on a special Load Line Chart. 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 tropical zones, but extra allowance can be made when trading in what are assumed to be safer freshwater conditions.
Ships with an overall length of 100 meters 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 (International Maritime Organisation) resolution. This applied immediately to all new-building ships, and from 1994 all existing ships had to conform to its provisions.