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.
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, but not before the 1979 Fastnet race ended in disaster.
Writes John Rousmaniere:[2] In the '70s, the IOR helped produce a new breed of racing and racing-cruising boat that was little more than a big, wide dinghy with a stability range as low as 90 degrees (meaning it would capsize completely when heeled only that far).
Good work led to the new IMS rule, which turned out boats that, while fast, resist capsize to beyond 120 degrees.
The IOR was run by the ITC, or International Technical Committee, of the Offshore Racing Congress, chaired between 1979 and 1987 by the late Gary Mull of San Francisco.
[3] As with all published handicapping formulae, there was an ongoing game between the designers finding ways of designing boats that took advantage of shortcomings in the measurement system and handicapping formulae, and the rule makers closing the loopholes to ensure fair racing and a reasonable competitive life for the boats.
These were powerful boats for sailing to windward, but had limited performance offwind as well as often having an alarming tendency to broach - designers included Sparkman & Stephens and Dick Carter.
The ultimate in this stage was the various lightweight centerboard designs with internal ballast, but these were not sufficiently seaworthy for offshore racing and so were penalised so heavily that they were effectively banned.
A good reference on the IOR Rule, including an analysis of the different formulae and measurements used, can be found in the book The Offshore Yacht.