⛵ Hull Speed Calculator
Enter your boat's waterline length to get its theoretical hull speed — the natural speed limit of a displacement hull — in knots, mph, and km/h.
🌊 Waterline Length to Top Speed
What is a Hull Speed Calculator?
It estimates how fast a displacement hull can sail before it's held back by its own wave. As a boat speeds up, the wave it makes stretches until its length matches the hull; at that point the boat sits in the trough and pushing harder mostly makes a bigger wave, not more speed. The formula 1.34 × √LWL captures that limit from a single number: the waterline length.
Use it to set realistic passage-planning speeds, compare two boats, or understand why a longer hull is simply faster. It's a theoretical figure — real speed depends on the wind, sea state, and how heavily the boat is loaded, and light or planing hulls can exceed it — so treat it as a guide, not a promise.
❓ Frequently Asked Questions
How is hull speed calculated?
It uses the displacement hull-speed formula: hull speed in knots equals 1.34 multiplied by the square root of the load waterline length (LWL) in feet. So a 25-foot waterline gives roughly 1.34 × 5 = 6.7 knots. If you enter the length in metres it is converted to feet (× 3.28084) first, and the answer is also shown in mph and km/h.
What is hull speed, and can a boat go faster?
Hull speed is the speed at which a displacement hull's bow and stern waves lengthen until the boat sits in the trough of its own wave and can't easily climb out. Heavier displacement cruisers rarely exceed it, but light, planing, or multihull designs can break through and surf well past hull speed off the wind.
Which length do I enter — overall length or waterline length?
Use the load waterline length (LWL), not the length overall (LOA). Only the hull that's actually in the water makes the wave that limits speed, and overhangs at the bow and stern don't count. LWL is on the boat's spec sheet and is always shorter than LOA.
Why does the speed-length ratio use 1.34?
1.34 is the classic speed-length ratio for a displacement hull in salt water — it comes from the physics of the wave a hull makes at speed. Some designers quote a range from about 1.1 for heavy hulls up to 1.5 for slippery ones, but 1.34 is the standard rule-of-thumb this calculator uses.