Hull Speed Calculator — Displacement Speed Limit for Boats & Kayaks
Enter your boat or kayak waterline length to find the theoretical hull speed — the practical maximum speed in displacement mode before the vessel needs to climb its own bow wave to go faster.
Units
m
Theoretical displacement-mode speed limit for this waterline length
- 1
Convert LWL to feet
8 m × 3.28084 = 26.2467 - 2
Square root of LWL (ft)
√26.2467 = 5.1232 - 3
Hull speed
1.34 × 5.1232 = 6.87The constant 1.34 comes from the gravity wave speed formula evaluated in feet and knots.
How does this calculator work?
Hull speed (knots) = 1.34 × √(waterline length in feet), or equivalently 2.43 × √(LWL in metres) km/h. This is the practical displacement-mode speed ceiling where the bow wave wavelength equals the waterline length, corresponding to a Froude number ≈ 0.4.
Formula
How this is calculated
A displacement hull moves through the water rather than over it, and as speed rises the vessel generates a bow wave whose wavelength increases. Hull speed is reached when the wavelength of the bow wave equals the waterline length — at that point the boat sits in a wave trough between its own bow and stern waves, and propelling it faster requires a disproportionate increase in power as the hull tries to climb its own wave. The empirical formula V = 1.34 × √LWL (with LWL in feet, V in knots) comes from the relationship between wave speed and wavelength in open water and corresponds to a Froude number of approximately 0.4.
The formula is a practical guideline rather than a hard physical limit. Modern light displacement hulls and catamarans can exceed it because they semi-plane or fully plane at high speed ratios. Heavy cruising keelboats rarely reach it in practice. Kayaks and canoes with longer, slender hull forms come closer to their hull speed than short fat dinghies. The Froude number (dimensionless speed) normalises across vessel sizes; displacement mode is typically below Fn = 0.4, semi-planing between 0.4–1.0, and planing above 1.0.
Frequently asked questions
Yes — light, beamy, or powered vessels can enter semi-planing or planing mode above hull speed, but it demands a large increase in power or sail area. Long, narrow hulls (like racing kayaks or multihulls) are better at exceeding it than short, heavy ones.
No. Waterline length (LWL) is the boat length measured at the water surface when the vessel sits at its designed displacement. Overhanging bows, transoms, and bowsprits extend the overall length but do not count for hull speed unless they are submerged.
The constant derives from the gravity wave speed formula c = √(gλ/2π), set equal to the wave speed at a wavelength matching LWL. Evaluating this in customary units (feet, knots) gives approximately 1.34. Some sources use 1.30–1.35 depending on hull form and displacement.
Also known as
TG we-Calculate Editorial Team. (2026). Hull Speed Calculator — Displacement Speed Limit for Boats & Kayaks [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/hull-speed-calculator
TG we-Calculate Editorial Team. "Hull Speed Calculator — Displacement Speed Limit for Boats & Kayaks." TG we-Calculate. 2026. https://we-calculate.com/calculator/hull-speed-calculator.
TG we-Calculate Editorial Team, "Hull Speed Calculator — Displacement Speed Limit for Boats & Kayaks," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/hull-speed-calculator
@misc{wecalculate_hull_speed_calculator, title = {Hull Speed Calculator — Displacement Speed Limit for Boats & Kayaks}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/hull-speed-calculator}}, year = {2026}, note = {TG we-Calculate} }
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