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The International Offshore rule (IOR) provided a handicapping system for racing yacht between 1972 and 1994. During this period great advances in both the materials used in construction and designs specifically to the rule, were made. The popular press discussed, at great length, how loopholes in the rules were exploited to gain a favourable rating. This led to the perception that the exploitation of geometric measurements was leading to boats with poor performance characteristics. This paper aims to address this, firstly through an analysis of the geometric parameters and their evolution through the early part of the IOR era. The paper concludes by undertaking a velocity prediction analysis of a series of boats, a technique that was in its infancy at the time these yachts were designed. The analyses show that the geometric parameters did evolve with time, but not necessarily in line with the understanding behind good performance. Penalties in the rule dictated the direction of design. However, the performance analysis did show that judging yachts based on rated characteristics could lead to misinterpretation, but in general, the performance data aligned reasonably well with assumed performance in specific sailing conditions. The velocity prediction analysis also concluded that the performance of yachts between 1972 and 1981 did increase regardless of the geometric form being dictated by the rules.