Length water line

Length water line (LWL) is the fore-and-aft length of a vessel where the hull intersects the water when floating at its normal load. It is measured along the waterline, not the overall deck length, so overhangs, bowsprits and transoms may be excluded.

LWL matters because it strongly influences displacement hull speed, efficiency, motion and manoeuvring. A longer LWL generally gives a higher comfortable cruising speed and smoother pitching, while changes in load can alter the effective Waterline. Consider it alongside Hull shape, Displacement and draught when estimating passage times or comparing boats.

For PredictWind users, accurate LWL is useful when setting up boat performance assumptions for Weather Routing and polars. It helps the routing engine reflect realistic speeds in different wind and sea states, especially for cruising yachts and displacement powerboats, improving departure decisions and fuel or sail planning.