Sail area-displacement ratio

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

${\displaystyle {\mathit {SA/D}}={\frac {{\mathit {SailArea}}({\text{ft}}^{2})}{[{\mathit {Displacement}}({\text{lb}})/64]^{\frac {2}{3}}}}={\frac {{\mathit {SailArea}}({\text{m}}^{2})}{{\mathit {Displacement}}({\text{m}}^{3})^{\frac {2}{3}}}}}$

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