Hull speed calculator
The theoretical top speed of a displacement hull, from its length on the waterline.
Result
1.34 × √16 ft = 5.36 knots.
How this is worked out
Hull speed = 1.34 × √LWL, with the waterline length in feet and the answer in knots.
A boat moving through the water makes a wave at the bow, and the length of a wave depends on its speed. At about 1.34 times the square root of the waterline length the bow wave stretches to the length of the hull, so the stern settles into the trough and the boat is in effect sailing uphill. A displacement hull can be pushed past it, but each extra knot takes a great deal more power, so in practice it works as a ceiling.
Metric: convert meters to feet by multiplying by 3.28, or use about 2.43 × √LWL in meters. For km/h multiply knots by 1.852, and for mph by 1.151.
It is a rule of thumb, not a law. Light, narrow hulls (racing kayaks, rowing shells, many multihulls) beat it, and planing powerboats leave it far behind. Most cruising boats and most paddlers spend their time well under it. For the sailboat designs and canoes and kayaks people are building here, the build logs say how long they took to build, which is the other number everyone wants.
Hull speed by waterline length
| LWL, ft | LWL, m | Knots | km/h |
|---|---|---|---|
| 10 | 3.0 | 4.2 | 7.8 |
| 12 | 3.7 | 4.6 | 8.6 |
| 14 | 4.3 | 5.0 | 9.3 |
| 16 | 4.9 | 5.4 | 9.9 |
| 18 | 5.5 | 5.7 | 10.5 |
| 20 | 6.1 | 6.0 | 11.1 |
| 24 | 7.3 | 6.6 | 12.2 |
| 28 | 8.5 | 7.1 | 13.1 |
| 32 | 9.8 | 7.6 | 14.0 |
| 36 | 11.0 | 8.0 | 14.9 |
| 40 | 12.2 | 8.5 | 15.7 |
Questions people ask
What is the hull speed formula?
Hull speed in knots is about 1.34 times the square root of the waterline length in feet. For waterline length in meters, multiply by 3.28 to get feet first, or use about 2.43 times the square root of the length in meters.
Can a boat go faster than hull speed?
Yes. Hull speed is where a displacement hull starts climbing its own bow wave and each extra knot costs far more power. Planing hulls, light multihulls and long, narrow hulls like racing kayaks and rowing shells can go past it. Heavy displacement boats rarely do.
Should I use length overall or waterline length?
Waterline length, the length of the hull where it sits in the water. It is shorter than length overall, often by 10 to 20 percent, and it changes with load. Using length overall overstates the speed.
What is the speed-length ratio?
Speed in knots divided by the square root of the waterline length in feet. Hull speed is a speed-length ratio of about 1.34. Cruising displacement boats are usually most comfortable between about 1.0 and 1.2.