Hull speed calculator
Enter the length at the waterline of a displacement hull. The calculator gives the hull speed, the speed at which the bow wave's length matches the waterline and the boat starts climbing its own wave, from the classic rule of 1.34 times the square root of the waterline length in feet.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose feet or metres.
- Enter the waterline length (LWL), not the length overall.
- Keep 1.34 or set another speed/length constant.
- Read the hull speed in knots, km/h and mph.
Hull speed (kn) = 1.34 × √LWL (ft)
1 knot = 1.852 km/h ≈ 1.151 mph. For a waterline in metres: LWL (ft) = LWL (m) ÷ 0.3048.
Worked example
A sailboat with a 30 ft waterline:
- √30 = 5.477;
- 1.34 × 5.477 = 7.34 knots;
- 7.34 × 1.852 = 13.6 km/h, or 8.4 mph.
A 9 m waterline is 29.5 ft and gives 7.28 knots.
Tips
- Use hull speed to plan passages and to judge how hard an engine is working: near hull speed, extra throttle mostly makes waves.
- A clean bottom, good sail trim and calm water get you close to it; a heavy load and a foul hull keep you below.
- For planing motorboats the rule does not describe top speed; there power and weight decide.
Frequently asked questions
What is hull speed?
A displacement hull makes a bow wave whose length grows with speed. When the wave is as long as the waterline, the stern sits in the trough and the boat is effectively climbing its own wave; pushing faster takes a steep rise in power. That speed is called hull speed. It is a practical limit, not a law: planing hulls and light, narrow boats go faster.
What is the formula?
Hull speed (knots) = 1.34 × √LWL, with the waterline length in feet. The 1.34 is a speed/length ratio; in metres the same rule is about 2.43 × √LWL. The calculator converts metres to feet first and uses the constant you set.
Why is the factor 1.34?
A wave in deep water travels at about 1.34 knots × √(wavelength in feet). When the bow wave's length equals the waterline, the boat moves at that wave's speed. Light displacement boats can exceed it somewhat; heavy ones may struggle to reach it, so some sailors use a constant between 1.1 and 1.5.
Is LWL the same as the boat's length?
No. The waterline length is the length where the hull meets the water at rest, usually shorter than the length overall (LOA) because of overhangs at the bow and stern. Check the boat's specifications for LWL.
Does heeling change it?
On many sailboats with overhangs the waterline gets longer when the boat heels or sits deeper, which raises the hull speed slightly. The calculator uses the length you enter.