Waterline

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Waterline refers to an imaginary line marking the level to which ship or boat submerges in the water. To an observer on the ship the water appears to rise or fall against the hull of the ship.

Many factors determine the waterline. A heavy ship, or one heavily laden with cargo will submerge deeper in the water. Temperature will affect the level because warm water provides less buoyancy as it is less dense than cold water. Even the salinity of the water will affect the level because freshwater provides less buoyancy as it is less dense than salty seawater.

For vessels with displacement hulls, the hull speed is determined by the waterline length. In sailing the length of the waterline can change as the boat heels and can dynamically impact the speed of the boat.

[edit] Load line

Many commercial ships have a symbol called a Load line painted on each side of the ship. This symbol, also called a International load line or Plimsoll line, marks the level to which the ship can be safely loaded. As cargo is brought on board the ship submerges and the symbol dips farther into the water.

Before these symbols were made compulsory many ships were lost due to overloading. Sometimes ships were intentionally 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 in his honour the mark bears his name.

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

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.