Hull Speed Calculator

A displacement hull pushes water aside and makes waves as it moves. The faster it goes, the longer its bow wave becomes. Once that wave is as long as the boat's waterline, the hull settles into the trough of its own wave and struggles to climb over the bow wave ahead of it. That speed is the traditional "hull speed."

hull speed (knots) ≈ 1.34 × √(LWL in feet) ≈ 2.43 × √(LWL in metres)

The coefficient comes from deep-water wave physics — a free wave's speed depends on its length — evaluated where the wavelength equals the waterline length.

• It is a rule of thumb, not a speed limit. Light, narrow or powerful boats routinely exceed it. Planing dinghies, performance keelboats and multihulls sail well past their "hull speed."
• It uses only waterline length. Displacement, beam, hull shape (prismatic coefficient) and the displacement-to-length ratio all shape the real drag curve — none of them appear in the formula.
• It assumes a displacement hull. It does not apply to planing or semi-displacement hulls, which climb over their own bow wave instead of being trapped behind it.
• The 1.34 coefficient is approximate. Real values range from about 1.1 for heavy, full-bodied hulls to 1.5 or more for light, fine-ended ones.
• Waterline length is not fixed. Heeling under sail and loading the boat both change the effective LWL, so the sailing figure differs from the static measurement.
• It marks where drag climbs steeply, not a wall. Near hull speed the power or wind needed to go faster rises sharply, but the boat can still be pushed past it with enough force.

Use it as a quick planning estimate — for passage times, comfortable motoring speed, or comparing similar boats — not as a precise performance prediction.