The J Class of racing yachts (sometimes called "J-boats") were built to the specifications of Nathanael Herreshoff's Universal Rule. The J Class is considered the apex of the era when the Universal Rule determined eligibility in the America's Cup. Universal Rule The J Class is one of several classes deriving from the Universal Rule for racing boats. The rule was established in 1903 and rates double-masted racers (classes A through H) and single-masted racers (classes I through S). From 1914 to 1937, the rule was used to determine eligibility for the Americas Cup. In the late 1920s, the trend was towards smaller boats and so agreement among American yacht clubs led to rule changes such that after 1937 the International Rule would be used for 12 Metre class boats. Universal Rule formula The Universal Rule formula is: Where: is boat length (a number itself derived from a formula that includes Load Waterline Length L.W.L in feet) is sail area is displacement is rating Herreshoff initially proposed an index of .2 but ratifying committees of the various yacht clubs changed this to, at various times, .18 or .185. This is, essentially, a 'fudge factor' to allow some boats designed and built prior to the adoption of the Universal Rule to compete. 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). J Class boats will have a rating of between 65 and 76 feet. This is not the overall length of the boat but a limiting factor for the variables in the equation. Designers are free to change any of the variables such as length or displacement but must reduce the other variables to compensate. J Class examples A table of well-known J Class yachts demonstrating that application of the Universal Rule could result in a rating between 65 and 76 feet. (Colours: Red: America's Cup challengers; Blue: defenders; Green: non-competitors). History and evolution of the J Class Prior to the adoption of the Universal Rule, the Seawanhaka Rule was used to govern the design of boats for inter club racing. As the Seawanhaka Rule used only two variables: Load Waterline Length () and Sail Area, racing boats at the time were becoming more and more extreme. Larger and larger