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: seawater density physics, 100% foretriangle vs genoa traps, metric formulas, and typical cruiser bands.
- D/L
Displacement / length ratio
This page is the displacement to length ratio (D/L): it compares a cruising sailboat's immersed weight with the cube of its waterline length.
- B/D
Ballast / displacement ratio
Ballast to displacement ratio (B/D) for cruising sailboats: why 33% deep lead bulb beats 42% shallow iron, righting levers, and cruising gear weight drop.
- CSF
Capsize screening formula
Capsize screening formula CSF: Fastnet 1979 lessons, why 64 is seawater density, metric calculation, the 2.0 offshore benchmark, and inverted stability.
- CR
Comfort ratio
Ted Brewer comfort ratio: formula and units first, working bands for similar LOA, limits of the number, and Baltic reading examples.
- HS
Theoretical hull speed
Hull speed = 1.34 × √LWL (ft) or 2.43 × √LWL (m). Free calculator, 80% cruise rule, LOA vs LWL trap, when Baltic cruisers surf past it.
- SA/WS
Sail area / wetted surface
Sail area to wetted surface ratio (SA/WS) for cruising yachts: light-air skin friction physics, appendage drag, ORC data sources, and antifouling impact.
- L/B
Length / beam ratio
Length to beam ratio (L/B) for cruising sailboats: marina salon volume vs snappy roll accelerations, LOA vs LWL slenderness, and berth fee thresholds.
The Open European and Nordic Sailboat Benchmark lists hull speed, CSF, comfort ratio, D/L and SA/D for 93 production sailboats. Archived 14 October 2026 on Zenodo.
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.