The waterline length (originally Load Waterline Length, abbreviated to LWL) is a measurement of ships and boats. The term denotes the length of the vessel at the point where it sits in the water. It excludes the total length of the boat, such as features that are out of the water. Most boats rise outwards at the bow and stern, so a boat may be quite a bit longer than its waterline length. In a ship with such raked stems, naturally the waterline length changes as the draft of the ship changes, therefore it is measured from a defined loaded condition.
Length at the waterline is often abbreviated as lwl, w/l, w.l. or wl.
This measure is essential in determining a lot of properties of a vessel, such as how much water it displaces, where the bow and stern waves are, hull speed, amount of bottom-paint needed, etc.
It should be noted that in sailing boats, as the wind increases heeling angle, the length on the waterline will typically increase. This will usually enable a greater maximum speed.
See also 
- "Speed of an average sailboat - Boat Design Forums". Boatdesign.net. Retrieved 2012-08-16.
- Hayler, William B.; Keever, John M. (2003). American Merchant Seaman's Manual. Cornell Maritime Pr. ISBN 0-87033-549-9.
- Turpin, Edward A.; McEwen, William A. (1980). Merchant Marine Officers' Handbook (4th ed.). Centreville, MD: Cornell Maritime Press. ISBN 0-87033-056-X.
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