Sailboat design ratio calculator
Formulas per the published conventions (imperial internally). Numbers describe the design, not the condition of a specific boat.
What each number tells you
- SA/D (sail area / displacement). Power-to-weight under sail. Below ~16 reads under-canvassed, 16 to 20 moderate, above 20 powered-up.
- D/L (displacement / length). How heavy the boat is for her waterline. Under ~200 light, 200 to 300 moderate, above 300 heavy: think load-carrying cruiser.
- Ballast ratio. Share of total weight in the keel. Higher generally means more righting muscle, though ballast depth matters as much as quantity.
- Comfort ratio. Ted Brewer's motion-comfort estimate: higher means a slower, easier motion in a seaway. Under 20 lively, 20 to 30 coastal cruiser, over 30 easy offshore motion.
- Capsize screening. Post-Fastnet quick screen: 2.0 or below is conventionally offshore-favorable.
- Hull speed. The displacement-hull cruising ceiling from waterline length.
Each of these has its own guide with the formula, history and honest limitations, all collected in the design ratios guide.
Design numbers first, condition second
Ratios shortlist the design; they say nothing about the specific boat's rigging age, sail condition or engine hours. That is the half of the picture listings tend to leave out, and it is where the real money hides on a used boat.
Paste a URL and Knotworth computes these ratios automatically, matches the design data, checks mission fit against your plans and flags what the ad doesn't say.
Design ratio questions
Which design ratio matters most when choosing a sailboat?+
It depends on your mission. For light-air performance look at SA/D, for motion at sea the comfort ratio and D/L, for offshore suitability the capsize screen alongside ballast ratio. No single number decides a boat; the pattern across all of them describes her character.
Where do I find the specs to enter?+
The listing usually has LOA, beam and displacement. LWL, ballast and sail area often come from the designer's specs on sites like sailboatdata.com. If the listing is missing numbers, that itself is worth noting: Knotworth's pre-check pulls design data automatically for matched models.
Are these ratios reliable for modern boat designs?+
They were calibrated on traditional hull forms, so modern wide-stern, twin-rudder designs read as more extreme than they behave. The ratios remain useful for comparing boats of a similar era and for reading older cruisers, which is most of the used market.
Do design ratios change with the boat's age or condition?+
No, and that is the point to remember: ratios describe the boat on paper. A well-designed boat with 20-year-old rigging is still a boat with a rigging bill attached. Use ratios to shortlist the design, then investigate the condition of the actual boat.