chain girth skin girthL = waterline length (LWL)d = difference between skin girth and chain girthS = sail area F = freeboard ( area between the waterline and a specified deck edge )2,37 = a mathematical constant
The International Rule differs between classes, the smallest called 5-m and the biggest has a 23-m rating. Best known is probably the 12-Metre Class sailing in 8 America’s Cups from 1958 to 1987. They followed as far less expensive yachts the impressing and costly J-Class of the 30’s . Their construction has been stopped by the Second World War. 12-m does not mean the length of a yacht in metres. The “m” is the term for a mathematical equitation only (a recommendation to use metric units.). The 12 Metres of the AC have a LWL of about 14 m, a total length of about 20 m and displacements of around 20 tons. The formula keeps a balance between the below parameters. A change of one of the figures requires the change of the others - in order not to exceed the number 12. The rule has been changed three times, the last version has following rule
“The International Rule” ( Metre Rule )Around 1900 the European Yacht Clubs were looking for an international rating rule to assure that a yacht can race in the same class and under the same specifications in every country. Initiated by the British YRA (Yacht Racing Association ) and created in several international conferences ( final ones 1906 in London and Berlin ) the International Rule (Metre Rule ) has been ratified 1907 in Paris. On this occasion the international delegates formed the IYRU ( International Yacht Racing Union ) now replaced by the ISAF ( International Sailing Federation). As rating bodies the Lloyd’s Register (GB) the German Lloyd (D) and the Bureau Veritas (F) have been applied. At that time the USA has sent observers but continued with the Universal Rule based on a formula of N.Herreshoff.