What is hull speed?
A displacement hull creates a bow wave whose length grows with speed. When the wave length matches the waterline length, the boat is effectively climbing its own wave and needs much more power to go faster. That speed is the hull speed.
Hull speed (knots) = 1.34 × √LWL (feet)
Example: 30 ft waterline
1.34 × √30 = 7.3 knots (8.4 mph).
Hull speed by waterline
| LWL | Hull speed |
|---|---|
| 20 ft | 6.0 kn |
| 25 ft | 6.7 kn |
| 30 ft | 7.3 kn |
| 35 ft | 7.9 kn |
| 40 ft | 8.5 kn |
| 50 ft | 9.5 kn |
Frequently asked questions
Can a sailboat exceed hull speed?
Light, modern hulls can surf or plane beyond it, especially downwind. Heavy cruisers rarely exceed it.
Should I use LWL or LOA?
Waterline length (LWL). Overall length includes overhangs that aren't in the water at rest.
What speed is most fuel-efficient for a trawler?
Around a speed/length ratio of 1.0–1.1, well below hull speed.