Sail Area to Displacement Ratio Calculator

The sail area to displacement ratio (SA/D) is a dimensionless performance index that indicates how powerful a sailboat is relative to its weight. It is calculated as SA/D = SA / (D / 64)^(2/3), where SA is sail area in square feet and D is displacement in pounds. A higher SA/D means more sail power per unit of boat weight, indicating a faster or livelier boat.

As a general guide: SA/D below 14 is a heavy, underpowered vessel; 14 to 16 is a comfortable cruiser; 16 to 18 is a performance cruiser; 18 to 20 is a lively racer-cruiser; above 20 is a racing yacht or very lively performance boat. These ranges come from standard naval architecture practice as summarised by Ted Brewer and other naval architects.

For the standard SA/D calculation, use the working sail area: mainsail area plus 100% foretriangle area (jib luff length times base divided by 2). Do not include downwind sails (spinnaker, gennaker) or reefed areas. Some designers use the upwind sail plan, others use 100% jib (not larger headsails). This calculator uses the working sail area as input.

Displacement raised to the power 2/3 converts the cube of weight (volume) to an area equivalent, making it comparable to sail area (which is in square feet). This dimensionless ratio allows meaningful comparison between boats of very different sizes: a dinghy and a 50-foot cruiser with the same SA/D have similar performance character relative to their size.

SA/D is a useful initial guide but is not the only performance metric. A high SA/D boat may be fast in light air but difficult to handle in heavy conditions. Displacement-length ratio, prismatic coefficient, and hull form also affect performance. Full performance analysis uses a velocity prediction program (VPP) that considers the complete set of hydrostatic and aerodynamic factors.