Hull Speed Calculator

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Forward: enter LWL to find hull speed. Reverse: enter a target speed to find the waterline length you need.
The length of the hull at the waterline, measured bow to stern at the waterline when the boat is floating at its designed displacement. Abbreviation: LWL.
ft
The standard coefficient (1.34) applies to typical cruising displacement hulls. Fine-entry or light-displacement designs can reach a slightly higher ratio - up to about 1.51 - before wave resistance peaks.
Hull speedModerate wave resistance
8.04knots

Theoretical maximum speed before wave resistance rises steeply

Hull speed (mph)9.25mph
Hull speed (km/h)14.89km/h
Speed-length ratio1.34
Froude number0.399
0.399 Fn
Low resistance<0.35Rising resistance0.35-0.4Hull speed zone0.4-0.5Planing transition0.5+

Theoretical hull speed: 8.04 knots

  • At this waterline length the hull reaches approximately 8.04 knots (9.3 mph / 14.9 km/h) before wave-making resistance becomes prohibitive.
  • The Froude number at hull speed is 0.399, near the classic threshold of 0.40 where transverse and divergent bow waves align.
  • Displacement hulls - cruising sailboats, trawlers, traditional workboats - are effectively capped here. Planing hulls, semi-displacement designs, and hydrofoils can exceed it by riding up onto or above the water surface.
  • To raise hull speed on a displacement vessel, the only practical levers are increasing waterline length (longer hull or greater displacement to sink the bow) or switching to a planing or hydrofoil design.

Next stepHull speed is a theoretical ceiling, not a guaranteed top speed. Engine power, sail area, windage, hull form, and loading all affect how close a real vessel comes to it.

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