Hull Speed Calculator
Calculate the theoretical maximum hull speed of a displacement yacht or boat from its waterline length in feet or metres.
What is hull speed?
Hull speed is the theoretical maximum efficient speed for a displacement boat — one that travels through the water rather than over it. It occurs when the wavelength of the bow wave equals the waterline length. At this point, the boat sits in the trough between its own bow and stern waves. Going faster requires the boat to climb its own bow wave, which demands a disproportionate increase in engine power or wind energy.
The formula
Hull Speed (knots) = 1.34 × √LWL(ft) Hull Speed (knots) = 2.43 × √LWL(m)
The constant 1.34 is empirically derived from the Froude number relationship. Specifically, hull speed corresponds to a Froude number of approximately 0.40.
Waterline length (LWL) vs overall length (LOA)
LWL is measured at the waterline — not the total boat length. For most cruising yachts, LWL is 75–90% of LOA. A 40-foot yacht might have an LWL of only 32–35 feet. Loading (fuel, water, stores, crew) sinks the boat slightly, increasing LWL and therefore hull speed by a small amount.
Passage planning with hull speed
As a rule of thumb for passage planning:
- Motoring in calm conditions: expect 90–100% of hull speed
- Sailing upwind: 60–80% of hull speed
- Sailing downwind in good breeze: may reach or slightly exceed hull speed
- Heavy weather: typically 50–70% (sea state limits progress)
A 35-foot cruising yacht (LWL ~28 ft) has a hull speed of about 7.1 knots. Plan passages at 5–5.5 knots average — this accounts for lighter winds, tacking, and slowing in waves.