Estimate the theoretical displacement hull speed from waterline length (LWL). Uses the classic 1.34 × √LWL (ft) formula in knots. Not for planing hulls.
Hull speed is the approximate efficient speed for a traditional displacement hull, when the wavelength of the bow wave matches the waterline length.
hull_speed_kn = 1.34 × √(LWL_ft)
or hull_speed_kn ≈ 2.43 × √(LWL_m)
mph ≈ knots × 1.15078 · km/h ≈ knots × 1.852
Always use LWL (length at waterline), not LOA. Overhangs do not count. The 1.34 constant is an empirical average — modern hulls, light displacement, and planing forms can cruise above or below this estimate. Volume/KD for this query: unknown.
| LWL (ft) | Hull speed (kn) |
|---|---|
| 20 | 5.99 |
| 28 | 7.09 |
| 36 | 8.04 |
| 40 | 8.47 |
No. It is a classical estimate of efficient displacement speed. Planing hulls, semi-displacement forms, surfing, and current can all read higher on GPS. Treat it as a planning reference, not a redline.
Waterline length only. A 35 ft LOA boat may have a 28 ft waterline. Using LOA overstates hull speed.
It comes from deep-water gravity-wave speed expressed in nautical units. Designers sometimes use roughly 1.1–1.5 depending on hull form. This page sticks to the common textbook constant and labels it as an approximation.
Yes — heavier trim can lengthen the immersed waterline slightly. Use the LWL that matches how you actually float, if you have it.