Calculate the theoretical hull speed of a displacement boat
Common small cruiser LWL
Typical mid-size sailboat LWL
Long-range trawler LWL
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Download NowHull speed is the theoretical maximum speed a displacement hull can travel before it has to climb over its own bow wave. For a DIY boatbuilder it is the single most useful number in early design, because it sets a realistic ceiling on how fast your sailboat or trawler will go for the power or sail area you plan to install.
The limit exists because a moving hull creates a wave whose length grows with speed. When the wavelength equals the waterline length, the boat sits in the trough of its own wave and is effectively trapped. Pushing past that point demands exponentially more energy, which is why displacement hulls are described as 'wave-making' rather than 'planing' forms.
This calculator takes your waterline length (LWL) in feet and returns the classic hull-speed figure in knots along with the speed-to-length ratio. Enter your LWL above, or load one of the quick samples, to see the number for your own design.
Hull Speed (kn) = 1.34 × √LWL| Waterline Length (ft) | Hull Speed (kn) | Efficient Cruise @ S/L 1.1 (kn) |
|---|---|---|
| 16 | 5.4 | 4.4 |
| 20 | 6.0 | 4.9 |
| 23 | 6.4 | 5.3 |
| 27 | 7.0 | 5.7 |
| 32 | 7.6 | 6.2 |
| 36 | 8.0 | 6.6 |
| 42 | 8.7 | 7.1 |
Approximate hull speeds for common waterline lengths (rounded).
Formula verified against standard naval-architecture references · Updated October 2026
Hull speed is the fastest a displacement hull can move through the water without having to climb over the wave it creates at the bow. As the boat goes faster, that wave gets longer. Once the wave length matches the waterline length, the boat settles into the trough and is held back. Reaching about 1.34 times the square root of the waterline length (in knots) is where this effect becomes strong.
The standard formula is Hull Speed (knots) = 1.34 × √(LWL), where LWL is the waterline length in feet. The 1.34 comes from the physics of deep-water wave propagation. If you work in metres, the constant changes to roughly 2.43 for knots, so always confirm your units before comparing results with a design or a chart.
Yes, but it depends on the hull form. Planing and semi-displacement hulls are designed to lift and skim over the wave rather than plough through it, so they routinely exceed classic hull speed. A true displacement hull can also exceed it, but only with a steep, often impractical rise in power. Modern fin-keel sailboats with flat runners surf down waves and beat the theoretical number in the right conditions.
It is based on waterline length (LWL), the length of the hull that is actually in contact with the water when floating at your designed waterline. Overall length (LOA) includes overhanging bow and stern that do not influence the wave-making system. Using LOA will give a falsely high hull speed, so measure LWL at your intended load waterline.
An S/L ratio of 1.1 to 1.2 is a comfortable, fuel-efficient cruising range for a displacement hull. At 1.34 you are right at the hull-speed wall, where resistance climbs sharply. Ratios near or above 1.5 mean the hull is planing or semi-displacement rather than truly displacing. For daily cruising, aim low and enjoy the fuel savings.
The only direct way is to lengthen the waterline. This is why modern cruising designs use raked bows, wide transoms, and canoe sterns to squeeze more LWL from a given overall length. Weight reduction does not raise hull speed, though it can help you reach your existing hull speed with less power and improve light-air performance under sail.
It absolutely does. The wave-making limit applies to any displacement hull regardless of power source, which is why racing shells are extremely long and narrow to maximize LWL per unit of drag. A wide, short dinghy has a low hull speed and will feel like it is ploughing once you exceed a few knots, no matter how hard you row.