SA/WS
Sail area / wetted surface
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)
Square feet or square metres consistently. Do not mix units.
History
Designers use SA/WS (sometimes labelled S# in popular sites) as a light-air companion to SA/D. It emphasises friction drag rather than displacement volume. Public data quality is weaker than for SA/D because wetted area is rarely printed.
Rule of thumb
Higher SA/WS suggests better ghosting ability for a given sail inventory. Without a published wetted area from the yard or a reliable lines take-off, prefer SA/D for buyer comparisons.
Limits
Wetted area changes with displacement, heel, and appendages. Antifouling roughness dominates real light-air performance more than a 5% SA/WS difference. Brochure silence on wetted area is common for production cruisers.
Boat examples
- Beneteau First 31.7
Performance-oriented sail plans may quote stronger light-air indices; still verify sail area convention.
- Sweden Yachts 390
If wetted area is missing, skip SA/WS and compare SA/D plus owner light-air reports instead.
- Hallberg-Rassy 36
Heavier cruisers can still move in light air with sail area, but SA/WS without data is guesswork.
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 certificates, or a designer lines plan. If none exist, do not invent a number from LOA.
- Should I prefer SA/WS over SA/D?
- Use SA/D when displacement is known and wetted area is not. Use SA/WS when both areas are documented from the same source.