International Offshore Rule

The International Offshore Rule (IOR) was a measurement rule for racing sailboats. The IOR evolved from the Cruising Club of America (CCA) rule for racer/cruisers and the Royal Ocean Racing Club (RORC) rule.

The IOR was superseded (in the early 1990s) by the International Measurement System (IMS) and CHS (since renamed IRC). While some IOR yachts race at club level under IRC 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 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.

The IOR rule encouraged wide short boats with limited stability. A narrow waterline and large beam on deck, combined with a high centre of gravity, meant that crew weight provided a significant proportion of stability at small heel angles, and boats had a relatively low angle of vanishing stability. This developed into the situation about 1977 when the boats winning in most smaller IOR categories (up to the half tonners - about 10m LOA) had all internal ballast, often with an unballasted daggerboard. The managers of the rule realised that this was not a suitable direction for seaworthy yachts, and heavily penalised boats with lifting keels.

Apart from the girth measurements, all measurements were basically point measurements. This meant that the hull was often locally distorted to maximise or minimise a measurement locally, with minimal effect to the surrounding hull. This gave a characteristic bumped look to many boats, particularly at the point of maximum beam and in the stern. Also, as stability was only measured at very low heel angles (less than 5 degrees), boats were designed with a very narrow waterline and low stability in measurement trim, but a hull form that gained stability with the weight of the crew and other equipment, and with increasing angles of heel. Interestingly, low stability was encouraged (up to a point) because the initial assumption was that low stability indicated a well fitted out interior, and so more of a cruising boat than a stripped out racer.

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