Sailboat ratio calculator
Hull speed, displacement/length, sail area/displacement, ballast ratio, capsize screening and motion comfort. Six published formulas, your numbers, and a plain reading of what each result means.
Use the designer's numbers, not the broker's. Listings inflate sail area by quoting a genoa, and quote "displacement" that predates thirty years of accumulated ground tackle, batteries, tools and cruising gear. A boat that has been lived on floats several inches below its design waterline, which changes every ratio on this page.
Ratios rank boats, they do not survey them. A comfortable, stiff, well-balanced hull with a wet deck core and gate valves for seacocks is a worse buy than an ordinary hull that has been maintained. These numbers narrow the shortlist; the inspection decides it.
What each formula is and what it misses
| Ratio | Formula | Reading it |
|---|---|---|
| Hull speed | 1.34 × √LWL | The classic displacement-speed limit in knots. A light, easily driven modern hull exceeds it downwind; a heavy full-keel cruiser rarely reaches it upwind. |
| Displacement / length | (D / 2240) / (0.01 × LWL)³ | Under 100 ultralight, 100–200 light, 200–300 moderate, 300–400 heavy, over 400 very heavy. Heavy carries stores and motion well and accelerates poorly. |
| Sail area / displacement | SA / (D / 64)2/3 | Under 16 undercanvassed for light air, 16–20 typical cruiser, over 20 performance. This is the single best predictor of whether the boat moves in 8 knots of breeze. |
| Ballast ratio | ballast / displacement | Roughly 30–40% is normal. On its own it says little: ballast low in a deep keel does far more than the same weight in a shallow one. |
| Capsize screening | beam / (D / 64)1/3 | Below 2.0 was the offshore threshold in the formula's original use. It is a crude beam-versus-weight screen, not a stability calculation, and it rewards narrow heavy boats regardless of how they are actually built. |
| Motion comfort | D / (0.65 × (0.7 LWL + 0.3 LOA) × beam1.33) | Ted Brewer's own description was that he devised it partly in jest. Higher means an easier, slower motion in a seaway. Useful for comparing two boats, meaningless as an absolute score. |
The pre-purchase inspection kit for your boat
Ratios shortlist boats. The kit is what you take aboard the one you shortlisted: the documented failures for that exact model, what each repair costs, and the findings that end the deal.
See the inspection kits