From Boat Design Wiki
Displacement is the nautical term for a boat's weight. As Archimedes discovered, a boat floating freely in water will 'displace' exactly the same weight of water as the boat weighs.
However, as fresh water and salt water have different densities, this weight of displaced water will be of a different volume in salt and fresh. Because of this difference, we are careful to distinquish between Displacement Volume (V) or Displacement Weight (W or Delta)
A boat's displacement in pounds divided by 62.4 will give her displacement volume in fresh water or divided by 64 to give her displacement volume in seawater.
In metric units the displacement in kilograms would be divided by 1000 to give her displacement volume in fresh water and 1025 in salt water.
ISO Standard 8666 gives details of what equipment should be included with the boat when describing it's 'displacement'. It's important to distinguish clearly which displacement condition is being quoted, especially when comparing boats. The degree of variation can be huge depending on whether full tanks, anchor gear, sails, etc are included.
Occasionally other forms of displacement will be met, especially in reference to older boats or commercial vessels, including terms such as Thames Tonnage, etc. These were often measures used for classing cargo vessels or for collecting taxation and don't really bear much correlation with the true displacement weight.
See also:
http://en.wikipedia.org/wiki/Buoyancy http://en.wikipedia.org/wiki/Displacement_(fluid) http://en.wikipedia.org/wiki/Displacement_hull http://en.wikipedia.org/wiki/Stability_conditions_%28watercraft%29
