Polymatroid

The Universal Rule determined a yacht's eligibility to race in the America's Cup from 1914 to 1937. Boats built according to the rule reached their peak in the large J-class yachts. Before 1914, the Seawanhaka 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 was used for the America's Cup until 1987, the last year the America's Cup was sailed in 12-meter yachts.

Yacht designer Nathanael Herreshoff devised the rule in 1903. Herreshoff had designed winning America's Cup yachts which fully exploited the Seawanhaka rule, which was based only on a yacht's waterline length and sail area, to create narrow boats with long overhangs. This reached its peak with Reliance, the defender of the 1903 America's Cup, which was described as a "racing freak", suitable only for certain conditions. This prompted Herreshoff to propose a rule which also took into account the displacement of the boat.

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). Sailing craft are thus rated when their $R$ rating falls within a certain range. J-Class boats, for example, are any single masted craft with an $R$ between 65 and 76 feet.

The listing for single mast boats, namely classes I through S.

• Class I: Rating = 76- 88 feet
• Class J: Rating = 65 - 76 feet
• Class K: Rating = 55 - 65 feet
• Class L: Rating = 46 - 55 feet
• Class M: Rating = 38 - 46 feet
• Class N: Rating = 31 - 38 feet
• Class P: Rating = 25 - 31 feet
• Class Q: Rating = 20 - 25 feet
• Class R: Rating = 17- 20 feet
• Class S: Rating = 0 - 17 feet

There were multi-mast classes too, running Class A to Class H.