Waterline

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 as warm water is more buoyant than cold water. Even the salinity of the water will affect the level because freshwater is more buoyant 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.

Load line

Plimsoll 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. Politician Samuel Plimsoll advocated for the improved safety standards and in his honor the mark bears his name.

The letters on the Plimsoll line 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 are used because changes in density between these different water masses affect ship buoyancy.

See also

• Coffin ship
• Ballast tank
• Sailing ballast
• Plimsoll shoe

External links

• UN Atlas of the Oceans article on waterline
• Water density calculator