Sail area-displacement ratio explained

The sail area-displacement ratio (SA/D) is a calculation used to express how much sail a boat carries relative to its weight.^[1]

SA/D = (SailArea ( ft ^{2)})/([ Displacement ( lb )/ 64 ] ^{(2)/(3)}) = (SailArea ( m ^{2)})/(Displacement ( m ^{3 )} ^{(2)/(3)})

In the first equation, the denominator in pounds is divided by 64 to convert it to cubic feet (because 1 cubic foot of salt water weights 64 pounds). The denominator is taken to the 2/3 power to make the entire metric unit-less (without this, the denominator is in cubic feet, and the numerator is in square feet).

It is an indicator of the performance of a boat.^[2] The higher the SA/D, the more lively the boat's sailing performance:^[3]

Boat Type	SA/D
Motorsailers	13 - 14
Slow auxiliary sailboats	14 - 15
Average offshore cruisers	15 - 16
Coastal cruisers	16 - 17
Racing yachts	17 - 19
Ultra light racers, class racers, daysailers	20+

The SA/D, however, does not provide information about a boat behavior in a storm or upwind. A polar diagram from a velocity prediction program gives a more precise view.^[4]

See also

• Displacement–length ratio

Notes and References

1. Web site: Paris . Jay E. . Comparing Design Ratios . Sail Magazine . 31 May 2018 . 9 July 2020 . en-us.
2. Web site: McGeary . Jeremy . How Sailboats Measure Up . Cruising World . 17 October 2012 . 9 July 2020 . en-us.
3. Web site: Sponberg . Eric W. . The Design Ratios . 9 July 2020 . Sponberg Yacht Design Inc. . en-us.
4. Web site: Reisberg . Lars . Sail Area-Displacement Ratio: Skipper´s Basics . No frills sailing . 11 July 2020.