The Twelve Metre Class has operated under the so-called Third Rule since 1933. Using an unchanged rating formula for more than 80 years, although with some changes in secondary technical details, Twelve Metre designs have evolved significantly within the Rule’s envelope.
The basic structure of the (First) Rule was created in 1906, with a similar formula as the Third Rule, but having additional terms that were subsequently discarded and changes made in the weighting of other terms.
Rating = (L + B + 1⁄2G + 3d + 1⁄3√S – F)/2 = 12.000 m
The basis of L, d, S and F are close to the way that they are used in the current Rule, although S was based on gaff rigs. L included girth corrections for the overhangs at the ends of L, similar to now. G was a midships girth measurement taken under the keel, which forced keels to reduce their draft forward of the rudder post – a poor design influence.
12, by the way, was meant to be the approximate waterline length, but LWL has grown toward 14 m in evolutionary design development.
Shortcomings of the Rule were apparent early on, and a lot of effort went into improvements after WW I. The Second Rule in 1920 abandoned maximum beam B and replaced it with a minimum beam limit. The weighting of G was halved, the factor on d was reduced to 2 and √S was taken at full value. The denominator was changed to 2.5 to keep existing yachts’ ratings close to 12.
Also, the reference plane for L was raised from the LWL to 11⁄2% of Class rating above the LWL, the lower end for measuring d was set at 121⁄2% of rating below LWL. The fore triangle area was discounted by a factor of 85%. And a formula for minimum displacement was introduced, along with prohibiting hollows in the topsides.
The Second Rule is actually quite close to the Third Rule except for one term. The girth G under the keel was eliminated in the Third-Rule formula, and the denominator re-set to 2.37 to keep ratings close to 12. With these most recent changes in the rating formula in 1933, the Third Rule became the familiar:
Rating = (L + 2d – F + √S)/2.37 = 12.000 m
There are many detailed requirements and restrictions within the text of the Class Rule, but separate rudders and trim tabs, and then winged keels, have nonetheless broken through design barriers while fitting within these same, basic Rule parameters. Even Appendix E, created in preparation for the America’s Cup Jubilee in 2001, is built on this basic Rule formula with credits for age, design type and propulsion.
The Rule’s girth measurements in the overhangs and the separate displacement-length formula cause the long overhangs and U’d end shapes common to all Twelves. The specified location of the midships d measurement, also coupled with displacement-length, forces the hull’s slack-bilged shape. And the fixed, maximum heights of the mainsail and fore triangle dictate the rig. All pretty simple and durable since the Second Rule of 1920, yet leaving liberty for a century of distinctly different designs.