Universal Rule

The Universal Rule determined a yachts eligibility to race in the America's Cup from 1920 to 1937. Ships built according to the rule reached their peak in the large J-class yachts. Before 1920 the The International Rule was used, and after 1937 smaller boats were desirable, and so the International Rule regained its popularity and became the standard. The 6, 8 and 12 Metre Classes were the most popular and the latter used for the America's Cup until 1987, the last year the America's Cup was sailed in 12-meter yachts.

Universal Rule formula

Mathematically, the Universal Rule formula was: $R=frac\; \{0.2\; cdot\; L\; cdot\; sqrt\{S\; \{sqrt\; [3]\; \{D$

Variables:
*L - Boat length
*S - Sail area
*D - Displacement
*R - Rating

The numerator contains a yacht's speed giving elements, length and sail area, while the retarding quantity of displacement is in the denominator. Also the result will be dimensionally correct, R will be a linear unit of length (such as feet or meters).

The listing for single-masted boats, namely classes I through S.
*Class I: Rating = 88 feet
*Class J: Rating = 76 feet
*Class K: Rating = 65 feet
*Class L: Rating = 55 feet
*Class M: Rating = 46 feet
*Class N: Rating = 38 feet
*Class P: Rating = 31 feet
*Class Q: Rating = 25 feet
*Class R: Rating = 20 feet
*Class S: Rating = 17 feet

There were multi-mast classes too, running something like A through H.

The original versions of Elements of Yacht Design by Norman L. Skene (1937?) should have more detail on this.