Yacht performance formulas
These pages explain the classic sailboat ratios brokers and surveyors still quote. FairHelm view: use them to compare like with like, then check the boat on the hard. A number never replaces a survey.
Open one formula when a listing or brochure quotes a ratio. Read the units first, then the rule of thumb and the limits. Cross-check against FairHelm model guides for Baltic cruisers before you shortlist.
Inputs must match the published formula (imperial vs metric). Brochure displacement often differs from measured light-ship. Model pages link these formula guides under Calculations and ratios; live numeric calc waits on displacement and sail-area fields.
Eight formulas
- SA/D
Sail area / displacement ratio
What SA/D measures for cruising sailboats, the imperial formula, typical bands, and Baltic boat examples. Limits of brochure sail areas.
- D/L
Displacement / length ratio
D/L formula for sailboats, what heavy vs light means for cruising, and how to read Baltic cruiser examples without over-trusting brochure tons.
- B/D
Ballast / displacement ratio
Ballast ratio formula for sailboats, what percent means for stiffness claims, and why keel type still matters more than the brochure percentage.
- CSF
Capsize screening formula
Capsize screening formula after Fastnet 1979: Beam / (Disp/64)^(1/3), the under-2 rule of thumb, limits, and Baltic cruiser examples.
- CR
Comfort ratio
Ted Brewer comfort ratio for sailboats: formula, typical bands, what motion comfort claims mean, and Baltic cruiser examples with clear limits.
- HS
Theoretical hull speed
Theoretical hull speed 1.34 * sqrt(LWL): what it means for displacement sailboats, when surfing exceeds it, and how Baltic cruisers use the number.
- SA/WS
Sail area / wetted surface
Sail area over wetted surface as a light-air drive index. Why wetted area is hard to get right, and how to read the ratio on cruising sailboats.
- L/B
Length / beam ratio
LOA/beam and LWL/beam ratios for sailboats: how beam affects form stability and interior volume, with Baltic cruiser examples and limits.
FAQ
- Why eight formulas and not a single score?
- Each ratio answers a different question (power, stiffness, motion, form). Collapsing them into one score hides trade-offs that matter on the Baltic and North Sea.
- Do FairHelm model pages show these numbers yet?
- Model pages link each formula under Calculations and ratios. Specs still show LOA, beam, and production years. Live computed ratios wait on displacement and sail-area fields in the model export.
- Can I trust a broker SA/D or CSF?
- Only if units and the exact formula variant are stated. Ask for sail area (main + genoa or 100% foretriangle) and displacement basis before comparing boats.