SA/D
Sail area / displacement ratio
Sail area / displacement (SA/D) is a power-to-weight index. Higher SA/D usually means stronger light-air drive and earlier reefing pressure. FairHelm judgment: for Baltic weekend cruisers, mid-teens SA/D is common; chase extremes only if you accept sail handling cost.
Formula
SA/D = SailArea_ft2 / (Displacement_lb / 64)^(2/3)
Sail area in square feet. Displacement in pounds. The constant 64 converts pounds to long tons for the classical imperial form.
History
Yacht designers and magazine writers popularised SA/D through the late 20th century as a quick comparison of drive versus immersed volume. It sits alongside D/L and ballast ratio in brochure tables. Variants exist when sail area is main + 100% foretriangle versus main + overlapping genoa.
Rule of thumb
Rough bands for monohull cruisers: under about 14 leans toward underpowered in light air; 16-20 is a typical modern cruiser band; above about 22 often reads as sporty or race-oriented. Always pair with D/L before judging 'stiff' or 'tender'.
Limits
Brochure sail area may omit or include overlapping genoas inconsistently. Displacement may be light-ship, half-load, or marketing. Metric conversions without rewriting the constant invalidate comparisons. SA/D says nothing about stability or motion comfort.
Boat examples
- Hallberg-Rassy 36
Centre-cockpit cruiser: expect mid-band SA/D aimed at passage making, not out-and-out light-air racing.
- Beneteau First 31.7
Performance cruiser/racer: typically higher SA/D than heavy Scandic cruisers of similar LOA.
- Linjett 40
Swedish coastal cruiser: check published sail plan basis before quoting a number from a listing.
FAQ
- Should I use main + genoa or 100% foretriangle?
- State which you used. Comparing a 140% genoa boat to a 100% foretriangle number inflates SA/D. Prefer the same convention across your shortlist.
- Does a high SA/D mean the boat is unsafe?
- No. High SA/D means more power for the displacement. Safety depends on stability, scantlings, crew, and weather. Use CSF and survey findings for capsizing risk questions.
- Can I compute SA/D from LOA alone?
- No. You need sail area and displacement. LOA without those inputs is not enough.