The IOR was superseded (in the early 1990s) by the International Measurement System (IMS) and CHS While some IOR yachts race at club level in more or less their original form, others had major surgery to make them competitive within the new rules
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 length, in feet. A typical IOR 40 footer (a one tonner) rated 30.55 feet.
In a handicapped race, the IOR length was used to compute a time allowance, in seconds per nautical mile (s/M) which was multiplied by the distance of the race, and subtracted from the boat's actual time, to compute the boat's corrected time. Longer IOR length gave a smaller time allowance.
The IOR is also used to define level classes, which instead of time correction, a boat in a given class-level has an IOR-length less than a specified value. The Ton Classes (Mini Ton, 1/4 Ton, 1/2 Ton, 3/4 Ton, 1 Ton, and Two Ton)--as well as 50-footer, ULDB 70, and Maxi classes--are examples.
To account for improvements in design and materials, boats are given an old age allowance, which decreases their IOR length as time passes. In spite of the old -age allowance, about 3/4 s/M/year on a 40 footer, boats over 2 years old were usually not competitive, which is why IOR handicap racing died off. The boats did not offer exciting performance off the wind, which was another reason for evolution away from the IOR.
Often accused of prohibiting great boats by encouraging mediocrity, the IOR was run by an arcane branch of international sailing called the ITC, or International Technical Committee, chaired largely by the late Gary Mull of San Francisco. Early boats penned by Mr. Mull with material information regarding rule changes were Improbable, a custom Mull 40 and Munequita, a production Ranger 37. Very well sailed, both won their respective SORC classes in the early '70s and featured heavy amounts of tumblehome and relatively low initial stability.
Peculiarities of IOR designs result from features that increase actual performance more than they increase IOR length, or other odd rules; IOR hulls bulge at girth measurement points; a reverse transom moves a girth measurement point to a thicker part of the hull; waterline length is measured while floating upright, so large overhangs are used to increase waterline sailing at speed; the stability factor ignores crew, so IOR designers assume lots of live ballast; after the 1979 Fastnet race excessive tenderness was penalized; full-length battens were prohibited to prevent mainsail roach area, but short battens became strong enough that the IOR had to start measuring and penalizing extra mainsail girth; mainsail area adds less IOR length than jib area, so new IOR boats are fractionally rigged; The IOR encourages high freeboard and high booms and prohibits keels wider at the bottom than at the top (bulbs). The IOR, in sum, encouraged heavy boats that lacked fair lines and clean hull forms.
The introduction of VPPs, or Velocity Prediction Programs, morphed the science of yacht performance measurement. Inherent to the IOR was the concept of a measurement officer taking discrete hull measurements and the IOR formula assumed the hull lines behaved continuously between measurement points. The IMS took the actual hull lines and analysed their continuum, essentially eliminating funny bumps or hollows in the ensuing yachts and generally rendering much cleaner, faster lines that were far more exciting, safer to sail, and had higher resale value.