How to use the hull speed calculator
- Enter the waterline length of the boat and choose feet or meters. This is the length of the hull where it meets the water, not the overall length.
- Read the hull speed in knots, miles per hour and kilometers per hour.
- Use the table to see the speed at several speed-length ratios, from a slow cruise at 0.5 to the hull speed at 1.34.
- Enter your boat speed in the advanced options to see your speed-length ratio.
Formula
hull speed in knots = 1.34 x sqrt(waterline length in feet)
speed-length ratio = speed in knots / sqrt(waterline length in feet)
1 knot = 1.150779 mph = 1.852 km/h
waterline length in feet = meters x 3.280840
The 1.34 figure is the rule of thumb for the speed at which the waves made by the bow and stern match the length of the hull.
A 30 foot waterline
A boat with a waterline length of 30 feet has a hull speed of 1.34 x sqrt(30) = 7.34 knots, which is 8.45 mph or 13.59 km/h. At a speed-length ratio of 1.0 it travels at 5.48 knots, and at 0.5 it goes 2.74 knots. A boat doing 5 knots at 30 feet of waterline has a ratio of 5 / sqrt(30) = 0.913.
A 24 foot waterline gives 6.56 knots. A boat with a waterline of 10 meters is 32.81 feet, so its hull speed is 7.68 knots. Longer boats are faster because a longer hull makes a longer wave, and a longer wave travels faster.
What hull speed means
A displacement hull pushes water aside as it moves. At low speed it makes a short wave pattern, but near the hull speed the bow wave and stern wave merge into one wave as long as the boat, and the boat sits in a trough. Pushing past that point takes a lot more power, so a heavy, round-bottomed boat rarely goes much faster than its hull speed.
Boats that break the rule
Light boats can climb onto their own wave and plane, so they can exceed the number by a wide margin. Narrow boats, catamarans and racing sailboats are driven past it, and surfing down a wave can briefly exceed it. The figure is a handy guide for a heavy cruising boat and not a limit set by law or by safety. For other speed calculations, see the Average Speed Calculator.
More tools for courses, tides and surveys
For related work with courses, positions and boats, try the Wind Triangle Calculator, Traverse Closure Calculator and Tide Rule of Twelfths Calculator.
Assumptions and limits
- The boat is a displacement hull that is not planing.
- The waterline length is the length at the water line with the boat at rest and loaded as usual.
- The 1.34 constant is a rule of thumb, not an exact law.
- The tool does not account for waves, wind or current.
- The waterline length in the form is an example. Use the figure for your own boat.
Frequently asked questions
Is hull speed a limit?
No. It is where drag from the wave system rises sharply for a displacement hull. Some boats exceed it by planing or surfing, and light boats do so easily.
Why is waterline length used and not overall length?
The wave the boat makes depends on how much of the hull is in the water. The overhangs at the bow and stern are usually above the water when the boat is at rest.
What is a speed-length ratio?
It is the speed in knots divided by the square root of the waterline length in feet. It lets you compare boats of different sizes, since boats at the same ratio make similar wave patterns.
Is a knot the same as a mile per hour?
No. A knot is one nautical mile per hour, and a nautical mile is 1.852 km, so a knot is about 1.151 statute miles per hour.