A displacement hull makes a bow wave whose length grows with speed. When the wave becomes as long as the boat, the stern sinks into the trough and the boat effectively hits a wall – the hull speed.
How to use the Hull Speed Calculator
- Find
- Waterline length (LWL)
- Target speed
- Speed-length ratio
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose what to find and enter the waterline length or the speed. Adjust the ratio for fine or heavy hulls.
Tips
- Check local conditions and safety rules before going out.
- Start with conservative equipment choices and adjust with experience.
- Every field has its own unit menu.
- Everything is calculated in your browser.
Common mistakes to avoid
- Using estimates instead of a proper check in the water.
- Mixing feet and metres, or knots and km/h.
- Overestimating your skill in strong wind.
Inputs explained
| Field | What to enter |
|---|---|
| Find | Choose one: Hull speed, Waterline length needed |
| Waterline length (LWL) | Units: ft, m |
| Target speed | Units: kn, km/h, mph |
| Speed-length ratio | A number |
What each result means
- Hull speed = [Speed-length ratio]×sqrt([Waterline length (LWL)])
- Waterline length needed = ([Target speed]÷[Speed-length ratio])^2
Example values
How the main result changes when Waterline length changes and everything else stays at the example values above:
| Waterline length (LWL) (ft) | Hull speed (kn) |
|---|---|
| 15 | 5.1898 |
| 22.5 | 6.3562 |
| 30 | 7.3395 |
| 37.5 | 8.2058 |
| 45 | 8.989 |
| 60 | 10.3796 |
Formula
Hull speed (knots) = 1.34 × √(waterline length in feet) · LWL = (speed ÷ 1.34)²
Worked example
Inputs — Find: Hull speed; Waterline length (LWL): 30 ft; Speed-length ratio: 1.34.
Results — Hull speed = 7.3395 kn.
Key terms
- LWL
- Length at the waterline.
Frequently asked questions
Can a boat go faster than hull speed?
Planing and multihull boats can; displacement boats need huge power to exceed it.
Why 1.34?
It is the speed-length ratio where wave length equals waterline length.
Is LWL the same as boat length?
No – it is the length at the waterline, usually shorter.
Does heeling change it?
Heeling can lengthen the waterline slightly.