Hull speed calculator for sailboats and displacement hulls
This hull speed calculator uses the rule of thumb taught at the U.S. Naval Academy: hull speed in knots = 1.34 × √(waterline length in feet). It is the speed at which the boat’s own bow wave is as long as the hull. A sailboat with a 30 ft waterline reaches about 7.34 knots (13.59 km/h).
Example: Unit of length: feet · Waterline length (LWL) 30 → Hull speed: 7.34 kn (13.59 km/h). Source: U.S. Naval Academy – EN400 Principles of Ship Performance, Chapter 7 “Resistance and Powering of Ships”: hull speed rule of thumb V = 1.34 √L (kn, ft), speed-to-length ratio (read 09.10.2026). Updated: .
How it is calculated
Hull speed (kn) = 1.34 × √LWL (ft). The U.S. Naval Academy’s course “Principles of Ship Performance” explains that resistance rises sharply at a speed-to-length ratio of 1.34: at that speed the wave the hull makes is exactly as long as the ship, so the stern drops into the trough. In metric units the same rule reads about 2.43 × √LWL (m), because 1.34 ÷ √0.3048 = 2.427.
Examples
| Waterline length | Hull speed | km/h |
|---|---|---|
| 20 ft (6.1 m) | 5.99 kn | 11.10 km/h |
| 25 ft (7.6 m) | 6.70 kn | 12.41 km/h |
| 30 ft (9.1 m) | 7.34 kn | 13.59 km/h |
| 36 ft (11.0 m) | 8.04 kn | 14.89 km/h |
| 40 ft (12.2 m) | 8.47 kn | 15.70 km/h |
One knot is 0.514 444 m/s (NIST), so 7.34 kn are 3.78 m/s. The waterline length counts, not the length overall: overhangs at bow and stern add nothing until the boat heels or the wave lifts them.
Speed-length ratio
Enter your actual speed to see V ÷ √L. A 30 ft waterline at 6 knots gives 1.10, i.e. 82 % of hull speed; 8 knots would be 1.46 and only reachable surfing or planing. For unit conversions use knots to mph or km/h to mph; the wave relation behind it is in the wavelength calculator.
Frequently asked questions
How do you calculate hull speed?
Multiply the square root of the waterline length in feet by 1.34. A 30 ft waterline gives 1.34 × 5.477 = 7.34 knots.
What is the hull speed formula in metric?
About 2.43 × √LWL in meters (exactly 1.34 ÷ √0.3048 = 2.427). An 8 m waterline gives 6.87 knots.
Why can’t a displacement boat go much faster?
At a speed-length ratio of 1.34 the bow wave is as long as the hull, and resistance rises steeply – more power adds little speed.
Do I use length overall or waterline length?
Waterline length (LWL). A 36 ft waterline gives 8.04 knots.
What does a speed-length ratio of 1.10 mean?
You sail at 82 % of hull speed, e.g. 6 knots on a 30 ft waterline.
Does hull speed apply to catamarans and planing boats?
Only roughly. It is a rule of thumb for displacement hulls; slender multihulls and planing boats can exceed it.
Sources and legal basis
- U.S. Naval Academy – EN400 Principles of Ship Performance, Chapter 7 “Resistance and Powering of Ships”: hull speed rule of thumb V = 1.34 √L (kn, ft), speed-to-length ratio (read 09.10.2026)
- NIST SP 811, Appendix B.8 – conversion factors: 1 ft = 0.3048 m, 1 knot = 0.514 444 m/s (read 09.10.2026)
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