SA/WS
Sail area / wetted surface
SA/WS = SailArea / WettedSurfaceArea (same area units)
Sail area / wetted surface (SA/WS) compares driving sail area to skin friction area. Higher values favour light-air acceleration. FairHelm judgment: valuable when wetted area is honest; many listings omit wetted surface entirely, so treat orphan SA/WS quotes with care.
Formula
SA/WS = SailArea / WettedSurfaceArea (same area units)
Strictly dimensionless. Both sail area (100% foretriangle plus mainsail) and total wetted surface area (hull plus keel and rudder) must use identical units: square metres over square metres, or square feet over square feet.
Try your numbers
SA/WS
Result
Enter positive numbers in all fields.
Rule of thumb
Cruising monohull benchmark bands (using 100% sail area): Under 2.0: Heavy traditional full-keel cruisers (Contessa 26, Albin Vega 27) and motorsailers; require a gennaker or auxiliary engine below 7-8 knots of true wind. 2.0 to 2.4: Balanced offshore and Scandinavian fin-keel cruisers (Hallberg-Rassy 36 around 2.15, Najad 390, Omega 34); steady ghosting from 6 knots of breeze on a clean bottom. 2.4 to 2.7: Performance cruisers and club racers (Sweden Yachts 390, Linjett 40, Beneteau First 31.7, X-372); lively acceleration in 4-5 knots of evening air. Above 2.8: Light sportboats and racing hulls with high-aspect foils and tall rigs.
Limits
Two practical buyer realities: First, yards almost never print wetted surface in sales brochures. Reliable figures come from ORC or IMS measurement certificates, designer lines plans, or hydrostatic approximations (Larsson-Eliasson / Schenzle formulas from LWL, beam, draft, and displacement); when wetted area is missing, compare SA/D instead of guessing from LOA. Second, hull condition overwhelms paper ratios: a mid-summer slime film raises skin friction by 15%, while rough antifouling, barnacles, and a fixed three-blade propeller increase drag by 40-50%, erasing any 5% advantage in SA/WS.
Boat examples
- Beneteau First 31.7
Performance cruiser: SA/WS around 2.55 with a clean Finot fin keel and tall rig delivers immediate acceleration in 5 knots of breeze.
- Sweden Yachts 390
Fast Nordic cruiser: SA/WS near 2.45 pairs a high-aspect lead fin with generous sail area; verify wetted surface on ORC sheets rather than brochure estimates.
- Hallberg-Rassy 36
Moderate ocean cruiser: SA/WS around 2.15 moves reliably in light air with a clean bottom and folding propeller, though heavier than club racers.
History
Naval architects developed SA/WS (sometimes labelled S# on rating sheets) as the light-air counterpart to SA/D. Traditional long-keel cruisers with attached rudders carried massive wetted surface along the full keel run. The shift in the 1970s and 1980s to separate fin keels and spade or skeg rudders cut skin friction dramatically, letting moderate cruisers ghost in faint airs. Modern wide-transom hulls with twin rudders add wetted surface when upright, which is why experienced crews shift weight to leeward in light air: inducing 8 to 10 degrees of heel lifts the windward chine and one rudder clear of the water, trimming wetted area.
FAQ
- Is this the same as SAAR?
- Some sites use SAAR or S# for sail-area-based power indices. FairHelm locks the URL slug to sail-area-wetted to mean sail area over wetted surface, not a different acronym.
- Where do I find wetted surface?
- Yard data sheets, ORC or IMS certificates, or a designer lines plan. If none exist, use a hydrostatic calculator from LWL, beam, draft, and displacement rather than inventing a number from LOA.
- Should I prefer SA/WS over SA/D?
- Use SA/WS to judge light-air ghosting under 8 knots when both areas come from a consistent source. Use SA/D for all-around power in moderate to fresh breeze when only displacement is published.