Estimate displacement hull speed from waterline length: about 1.34 × square root of waterline in feet.
| Speed (kn) | – |
| Speed | – |
Use this calculator to estimate the theoretical hull speed of a displacement sailing boat from length on the waterline. Enter waterline length in metres (m) or feet (ft); the unit selector converts the value to metres before the calculation.
Speed in knots = 1.34 × √(waterline in feet). Waterline in metres is divided by 0.3048 inside the square root. An 8 m waterline is about 7.3 knots, or about 13.5 km/h. Planing boats, foils, and light racing hulls can go faster than this rule.
This is a displacement-hull rule of thumb, not a polar diagram, rating certificate, or a prediction for motorboats that plane.
Waterline: Length on the waterline, not overall length. Enter m or ft.
Speed (kn): Theoretical hull speed in knots.
Speed (km/h): The same speed in kilometres per hour (or mph if the waterline unit is imperial).
Related calculators