SA/D
Sail area / displacement ratio
SA/D = SailArea_ft2 / (Displacement_lb / 64)^(2/3) = SailArea_m2 * 101.65 / (Displacement_kg)^(2/3)
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) = SailArea_m2 * 101.65 / (Displacement_kg)^(2/3)
Imperial: Sail area in square feet, displacement in pounds. 64 is seawater density in lb/cu.ft; dividing weight by 64 yields displaced volume in cubic feet, and raising it to 2/3 gives area dimensions (sq.ft), making SA/D purely dimensionless. Metric: Sail area in m2, displacement in kg, seawater density 1025 kg/m3.
Try your numbers
SA/D
Result
Enter positive numbers in all fields.
Rule of thumb
Typical cruising monohull bands: Under 15 indicates an undercanvassed motorsailer or heavy expedition boat needing motor assistance in sub-12 knot winds. 16-19 represents the classic family cruiser band (Hallberg-Rassy 36, Najad 390, Bavaria 36) offering predictable offshore handling. 20-23 is the performance cruiser band (First 31.7, Linjett 40) with fast light-air drive and early reefing needs. Above 24 marks club racers and ultra-light planning hulls.
Limits
Listing trap: brokers frequently quote sail area with a 135% or 150% overlapping genoa rather than the standard 100% foretriangle (I * J / 2), inflating calculated SA/D by 20-30%. Second, lightship brochure weight ignores fuel, water, ground tackle, and cruising stores, which can reduce real-world SA/D by 1.5 to 2 points. Crucially, SA/D says nothing about stability, righting moment, or motion comfort.
Boat examples
- Hallberg-Rassy 36
Centre-cockpit cruiser: SA/D around 17.3 provides relaxed ocean passage making and forgiving helm balance in heavy weather.
- Beneteau First 31.7
Performance cruiser: SA/D around 22.1 delivers quick acceleration in light Baltic breezes, requiring reefs once true wind hits 16 knots.
- Linjett 40
Swedish coastal cruiser: modern powerful rig with SA/D around 22.6 combines light-air speed with a deep lead bulb for stiffness.
History
Pioneered by 20th-century naval architects including C.A. Marchaj and popularized in cruising literature by Ted Brewer, SA/D provided a quick way to compare driving power against immersed hull volume across radically different hull shapes. Unlike rating rules that penalised sail area, SA/D helped sailors understand whether a production boat was a responsive sailing yacht or an undercanvassed motorsailer.
FAQ
- Should I compare boats using 100% foretriangle or overlapping genoa?
- Always use 100% foretriangle (Main + 0.5 * I * J) for fair comparisons. Calculating SA/D with a 150% genoa makes a heavy motorsailer look like a sports racer on paper.
- Does a high SA/D make a sailboat dangerous in heavy weather?
- No, but it demands active sail management. You will need to reef earlier, often at 15-18 knots of wind, whereas a low SA/D yacht carries full canvas up to 20 knots.
- How does cruising payload affect the real SA/D?
- Adding 1.5 tons of crew, anchors, chain, dinghy, and fuel to a 7.5-ton boat increases displacement by 20%, dropping real SA/D by nearly 2 points.