Because of the different densities of fresh and salt water, the displacement of a vessel and hence her draught will change from one area to another. The density of fresh water is 1 tonne/cubic metre and is used as the basis against which the relative densities of other substances are measured, ie salt water has a density of 1.025 tonnes/cubic metre and therefore a relative density of 1.025. As a result of this, a vessel moving from fresh water to seawater reduces her draught by an amount that can be calculated in advance.
In order to ensure that vessels are not overloaded, an international convention on loadlines established loadline rules for compliance by all vessels using the high seas for commerce. The rules contain two tables which show the minimum permitted freeboards for two types of ship, each of which is assumed to be of standard form. One table covers ships designed to carry bulk liquids, ie tankers, the other covers all other ships. The British Merchant Shipping (Loadlines) Act of 1967 lays down penalties to which the master of a vessel is liable if he allows his vessel to be overloaded. Other signatory nations to the Loadline Rules have similar legislation.
A ship will be marked on each side with a deck line, loadlines and a loadline mark as shown in the figure below. The marks must be plainly visible and either cut into or welded onto the side of the hull.
- Deck-line. This is marked amidships and indicates the position of the surface of the freeboard deck.
- Loadline mark. This is marked amidships, vertically below the deck line and consists of a ring with the upper edge of a horizontal line passing through its centre. The vertical distance between the centre of the ring and the upper edge of the deck-line is called the summer freeboard. A letter marked on either side of the loadline mark indicates the name of the assigning authority.
- Summer loadline. This is marked with the letter “S” and placed forward of and on the same level as the level of the loadline mark.
- Winter loadline. This marked with the letter “W” and is placed 1/48th of the summer loadline draught below “S” and the Tropical Loadline marked “T” is placed the same distance above “S”.
- Fresh water Loadline. This is marked “F” and is placed above the summer loadline a distance equal to D/4T millimetres, where D is the displacement in salt water at the summer loadline in tonnes and “T” is the tonnes per centimetre immersion in salt water at the waterline.
- Tropical fresh water loadline. This is marked “TF” and is placed the same distance above “F” as “T” is above “S”.
- Winter North Atlantic. This marked “WNA” and is placed only on vessels of 100 metres or less in length at a distance of 50 mm below “W”.