Hull speed calculator
Hull speed = 1.34 × √LWL(ft). A practical cruising ceiling for displacement hulls, not a hard limit.
How hull speed is calculated
Hull speed (kn) = 1.34 × √LWL (ft)The 1.34 constant comes from wave physics: a displacement hull cannot easily outrun the bow wave it creates, and that wave's speed depends on its length. The formula dates back to 19th-century naval architecture and remains the standard quick estimate.
What hull speed does not tell you
Hull speed says nothing about how a boat sails in light air, how she handles, or what she costs to keep. Two boats with identical hull speed can behave completely differently; the other design ratios and, above all, the boat's actual condition matter far more when buying.
For the fuller picture, read the sailboat design ratios guide or compute everything at once with the all-ratios calculator.
Paste a listing URL and Knotworth's free pre-check computes hull speed and every design ratio for that boat, plus what a boat of her age typically needs.
Hull speed questions
What is a typical hull speed for a cruising sailboat?+
Most 30 to 45 foot cruising sailboats have hull speeds between about 6.5 and 8.5 knots. A boat with a 9.6 m (31.5 ft) waterline, typical for a 38-footer, works out to roughly 7.5 knots.
Can a sailboat exceed its hull speed?+
Yes, in the right conditions. Surfing down waves, planing-capable light hulls and strong reaching winds can all push a boat past the 1.34 figure. Hull speed is a practical cruising ceiling for heavy displacement hulls, not a law of physics that stops the boat.
Why does waterline length set the speed limit?+
A displacement hull makes a wave as long as its waterline. Past hull speed the boat is trying to climb its own bow wave, and the power needed rises steeply. Longer waterline means a longer, faster wave, which is why bigger boats are faster.
Is higher hull speed better when buying a used boat?+
It mostly reflects size: more waterline means more speed and usually more boat to maintain. For passage planning it matters, but for judging a specific used boat the condition of rig, sails and engine moves the needle far more than a few tenths of a knot.