The sail area-displacement ratio (SA/D) is a calculation used to express how much sail a boat carries relative to its weight.
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. The higher the SA/D, the more lively the boat's sailing performance:
+ 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.
See also Displacement–length ratio
References