Hull Speed & Sail Ratio Calculator
Hull speed from waterline length, plus the sail-area and displacement ratios that describe how a boat will actually behave.
Waterline length, not overall length — the two can differ by several feet, and only the waterline matters here. Sail area is usually quoted as main plus 100% foretriangle.
What the ratios say about this boat
Why a displacement hull has a speed limit
A boat pushing through water makes a wave, and as it speeds up that wave gets longer. When the wave is as long as the hull, the boat is sitting in a trough of its own making with a crest at the bow and another at the stern — and going faster means trying to climb its own bow wave. Power goes up steeply for almost no speed, which is where the 1.34 × √waterline figure comes from.
It is a soft limit rather than a wall. Light modern hulls exceed it downwind in a breeze, and genuinely light boats can climb over the wave altogether and plane. But for a normal cruising boat under sail it is a good description of the day.
Note what the square root does. Four times the waterline gives only twice the speed. This is why long boats are fast and why an extra couple of feet on a small boat buys more than an extra couple on a big one — and why waterline length, of all the numbers on a spec sheet, is the one that most predicts passage times.
Hull speed by waterline
| Waterline | Hull speed |
|---|---|
| 16 ft | 5.4 kt |
| 20 ft | 6.0 kt |
| 25 ft | 6.7 kt |
| 30 ft | 7.3 kt |
| 35 ft | 7.9 kt |
| 40 ft | 8.5 kt |
| 50 ft | 9.5 kt |
| 65 ft | 10.8 kt |
How to use
- Enter the waterline length, not the overall length.
- Read the theoretical hull speed.
- Add displacement and sail area for the ratios.
- Treat hull speed as a guide, not a hard limit.
Frequently asked questions
What is hull speed?
Roughly 1.34 times the square root of the waterline length in feet, giving knots. It is the speed at which the boat's own bow and stern waves are one boat length apart, so the hull effectively sits in a trough of its own making and further power mostly makes a bigger wave.
Is it a hard limit?
No, and calling it one is a persistent oversimplification. Light displacement hulls, planing hulls and multihulls exceed it routinely — some by a wide margin. It is a meaningful constraint for heavy displacement hulls and a poor description of anything designed to plane or surf.
Why waterline length rather than overall?
Because it is the wave-making length that matters, and overhangs contribute nothing while the boat is upright. This is also why a heeled boat with long overhangs can sail faster than its upright figure suggests — heeling immerses the overhangs and lengthens the waterline.
What is displacement to length ratio?
A measure of how heavy a boat is for its length. Low figures indicate a light boat that accelerates readily and may plane; high figures indicate a heavy boat that carries way well and is less affected by crew and stores. It describes character more usefully than any single dimension.
What does the sail area to displacement ratio tell me?
How much power the boat has for its weight — essentially its power-to-weight ratio. Higher figures mean better light-air performance and an earlier need to reef; lower figures mean a boat that needs breeze to move but is easy-going when it blows.
Can I compare these ratios across very different boats?
With care. They are most useful within a type — comparing cruising monohulls with each other — and become misleading across types, because a multihull or a planing dinghy is governed by different physics. Ratios describe tendencies, not rankings.
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