Hull speed calculator
Calculate the theoretical maximum speed for a displacement hull from its waterline length.
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Worked example
A typical 30-foot cruising yacht with a 9 m (29.5 ft) waterline:
- Hull speed = 1.34 × √29.5 ft = 1.34 × 5.43 = 7.3 knots
- This is the speed at which the bow wave wavelength equals the waterline length
- Pushing beyond this requires significantly more power for marginal gain
- For passage planning, use 5.5–6.5 knots as a realistic average
What is hull speed?
Hull speed is the theoretical maximum speed of a displacement hull. Beyond this speed, the vessel must climb its own bow wave, requiring disproportionately more power.
Hull speed (knots) = 1.34 × √LWL(feet)
Frequently asked questions
Can a boat exceed its hull speed?
Yes — but it requires enormous power. Some beamy motorboats and modern lightweight racing sailboats can "plane" over their bow wave and go considerably faster. Displacement cruisers and most sailing yachts cannot plane and are effectively limited to hull speed in practical terms.
Is hull speed the same as maximum speed?
No. Hull speed is a theoretical limit for displacement hulls. Your actual maximum depends on engine power, sail area, wind strength, sea state and loading. Most cruising sailboats reach hull speed only in ideal downwind conditions.
Why does waterline length matter more than overall length?
The bow wave length is determined by the waterline length — the length of the hull that is actually in the water. Overhangs and bow/stern extensions above the waterline don't count. A boat with a long bowsprit has the same hull speed as one without.
What is a realistic cruising speed?
Most displacement sailboats cruise at 55–75% of their hull speed. For a 9 m waterline boat with 7.3 kn hull speed, expect 4–5.5 knots as a realistic day-passage average, depending on wind and swell.
Typical hull speeds
| Waterline length | Hull speed (knots) | Hull speed (mph) |
|---|---|---|
| 6 m (19.7 ft) | 5.9 kn | 6.8 mph |
| 8 m (26.2 ft) | 6.8 kn | 7.9 mph |
| 10 m (32.8 ft) | 7.7 kn | 8.8 mph |
| 12 m (39.4 ft) | 8.4 kn | 9.7 mph |
| 15 m (49.2 ft) | 9.4 kn | 10.8 mph |