The term “waterline” describes the point where water reaches the hull of a ship, which is the external body of a ship located below the superstructure. Waterline is also used to indicate the plimsoll line or the national load line, both of which indicate the legal cargo limit for a ship based on calculations that take into consideration the temperature and salinity of the water. Some of the considerations used to calculate the waterline or where to place the plimsoll mark on the ship include factors such as the length of the ship, the height of the bow, the type of ship (passenger or cargo), and other factors like the number of superstructures.
The waterline was pioneered by Samuel Plimsoll who was motivated by the increasing rate of ship accidents attributed to overloading. The load line mark was introduced to indicate safe levels to which ships may be loaded to prevent accidents. Load lines were first internationally adopted during the 1930 Load Line Convention. Since then, subsequent amendments have been made to the first regulations, with the last one occurring in 2003.
The loadline or plimsoll mark was originally shaped like a circle that was bisected by a horizontal line. Over the years, other marks have been included to the original one to compensate for various expected sea conditions and water densities. Warm water is generally less buoyant than cold water, due to the fact that it is not as dense as cold water. Fresh water is also not at the same level of density with seawater due to the higher level of salinity in marine waters.
The adoption of the international load lines resulted in cargo ships having to carry a load line certificate in addition to the plimsoll lines, which must be painted onto the two sides of the ships. This allows anyone to see if the ship is maintaining a legal waterline just by glancing at the level of the water in relation to the markings on the hull of the ship. The ship itself must be examined in order to determine the exact waterline for that particular ship, since the waterline of different ships vary.
When determining the watermark, considerations about the effects of fresh water and salt water lead to the placement of two plimsoll marks; one for salt water and one for fresh water. In freshwater, the hull of the ship will sink farther with the same cargo than it would in salt water. The plimsoll mark for freshwater is consequently higher than that for salt water.