Waterline

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Waterline refers to an imaginary line marking the level at which ship or boat floats in the water. To an observer on the ship the water appears to rise or fall against the hull . Temperature also affects the level because warm water provides less buoyancy, being less dense than cold water. Likewise the salinity of the water affects the level, fresh water being less dense than salty seawater.

For vessels with displacement hulls, the hull speed is determined by, amongst other things, the waterline length. In a sailing boat, the length of the waterline can change significantly as the boat heels, and can dynamically affect the speed of the boat.

[edit] Load line

Plimsoll line

Many commercial ships have a symbol called a Load line painted on each side of the ship. As cargo is brought on board, the ship floats lower and the symbol descends farther into the water. This symbol, also called an International load line or Plimsoll line, indicates the maximum safe depth, and therefore the maximum amount of loading, for the vessel in various operating conditions.

The letters on the Load line marks have the following meanings:

TF - Tropical Fresh Water
F - Fresh Water
T - Tropical Seawater
S - Summer Seawater
W - Winter Seawater
WNA - Winter North Atlantic

Letters may also appear to the sides of the mark indicating the classification society that has surveyed the vessel's load line. The initials used include AB for the American Bureau of Shipping, LR for Lloyd's Register, and NV for Det Norske Veritas.

These season and zone marks are used to ensure adequate reserve buoyancy for the intended area of operation. Ships encounter rougher conditions in winter as opposed to summer, and in the North Atlantic as opposed to tropical waters, for example.

Fresh water marks make allowance for the fact that the ship will float deeper in fresh water.

Before these symbols were made compulsory many ships were lost due to overloading. Sometimes they were deliberately overloaded in the hope of collecting insurance money. Ships carrying emigrants from Europe to America were also lost. The British social reformer and politician Samuel Plimsoll advocated improved safety standards, particularly at sea, and the mark bears his name in his honour.