Op de pagina over de
International Offshore Rule (IOR) staat de achterliggende rating wat uitgebreider beschreven. De rating-formule van de IOR levert een waarde op in
feet:
The IOR concentrated on hull shape with length, beam, freeboard and girth measurements, foretriangle, mast and boom measurements, and stability with an inclination test. Additionally, the IOR identified features which were dangerous, or it couldn't fairly rate, and penalized or prohibited them. The measurements and penalties were used to compute the handicap number, called an IOR rating, in feet. The higher the rating, the faster the boat was deemed to be able to sail. A typical IOR 40 footer (a one tonner) rated 30.55 feet.
De x-tonner-klassen hadden een maximale waarde voor de IOR-rating (bijv. 22'-0" voor een halftonner), en verder geen handicap binnen de wedstrijden:
The IOR rule was also used to define level rating classes, where each class had a maximum IOR rating, and the first boat to finish was the winner, with no handicapping. The first of these was the One Ton class, so named because there was a spare trophy from the defunct One Ton rating class, and this then spawned the Mini Ton, 1/4 Ton, 1/2 Ton, 3/4 Ton and Two Ton classes, as well as unofficial 50-footer, ULDB 70, and Maxi classes. The official classes each had an annual world championships.
