⛵ Hull Speed Calculator
Determine the theoretical maximum speed of displacement hulls using the classic 1.34 × √LWL formula.
Use LWL measurement (waterline length).
1 long ton = 2240 lb (used for speed-to-length ratio).
Hull Speed
7.34 kn
Speed (mph)
8.45 mph
Speed (km/h)
13.59 km/h
How to Use This Calculator
Measure waterline length
Use LWL (waterline length) rather than overall boat length.
Enter displacement
Optional: use long tons to check speed-to-length ratio.
Review hull speed
Compare knots, mph, and km/h outputs.
Plan performance expectations
Use hull speed to set realistic cruising speeds for displacement boats.
Formula
Hull Speed (kn) = 1.34 × √LWL (ft)
1.34 is derived from wave dynamics (√(g/(2π))) for displacement hulls.
The formula applies to displacement hulls (sailboats, trawlers). Planing hulls can exceed hull speed when generating lift on top of the water.
Sailing Insights
Hull speed reflects a speed barrier where the boat must climb its bow wave to go faster. Displacement boats require disproportionate power to exceed it.
Improving Speed
- Reduce weight and clean hull to minimize drag.
- Optimize sail trim or prop pitch to deliver efficient thrust.
- Surf following seas or ride waves to exceed theoretical speed temporarily.
Frequently Asked Questions
Why does hull speed matter?
It indicates the efficient cruising limit for displacement boats; extra power yields diminishing returns beyond this speed.
Can a boat exceed hull speed?
Planing or semi-displacement hulls can exceed it by generating lift. Displacement hulls need surfing conditions or significant power to surpass hull speed.
Does waterline length include bowsprit?
No. Use actual waterline length at load condition, not overall length (LOA).
What about catamarans?
Catamarans have longer effective waterlines and lower displacement; they often exceed hull speed due to reduced wave drag.
Is hull speed relevant for powerboats?
Yes for displacement trawlers; less so for fast planing powerboats, which can exceed hull speed with proper power/trim.