|Thunderbolt (photo Larry Moran)|
The origins of the IOR came from the two main rating rules in use in the 1960s on either side of the Atlantic, the North American CCA rule (Cruising Club of America) and the European RORC rule (Royal Ocean Racing Club). The growing popularity of international ocean racing events, such as the Admiral's Cup and Southern Cross Cup, and the revival of level rating for the various 'Ton' cups, combined to drive the two rules towards the need for a single international rule. An International Technical Committee, headed by Olin Stephens, devised the new rule over a three year period, and the Mk II version of the IOR came into effect in January 1970.
|Some of the hull measurements involved in an IOR measurement (diagram by Bob Perry)|
|The Dubois designed Vanguard shows the downwind rolling characteristics of early IOR typeforming that arose from pinched stern sections which sought to minimise measurement penalties around the after girth stations|
The basic formula of IOR Mk II was expressed as a combination of MR (measured rating) and R (rating). Under the IOR a yacht's rating was expressed in feet, and based on the outcome of the following formulas:
MR = 0.13(L x S1/2) / (B x D)1/2 + 0.25L + 0.2S1/2 + DC + FC
R = MR x EPF x CGF
|The measurement of B, and limits to hull radius|
|The pushing of 'B' to the extreme, as seen here on the aptly named Stephen Jones Half Tonner Tumblehome|
|The 'basic' parameters involved in finding 'L'|
Determining one of the components of L involved finding the forward overhang component (FOC). The measurement of the forward girths needed to find this line was relatively straightforward, with bow shapes being generally similar across the design spectrum, but with variances between the deep forefoot favoured by proponents of moderate to heavier displacement hull forms (e.g. Britton Chance, Doug Peterson), and the 'knuckle', which borrowed from the metre boat typology but was married to a lighter displacement style, particularly that of Bruce Farr and Paul Whiting. English designer Stephen Jones also pursued another approach, with a bow shape designed to maximise waterline length for'ard with a concavity around FGS.
|The 'crease' in the stern of the Davidson designed 1985 One Tonner Pendragon can be seen here while the yacht is being constructed, with a long run aft that provided additional sailing length when heeled|
|Part of the mathematical challenge for an IOR yacht designer can be seen in this diagram|
|The Peterson 'pintail' approach on Moonshine|
L = LBG - FOC - AOCC
The trend from soon after the IOR was born was to work the two aft girth stations to shorten the rated length. By artificially steepening the profile slope of the hull and the ‘slope’ of the girths between the two after girth stations, it was possible to shorten the rated length, and the way the rule treated girths in the way of skegs allowed that effect to be even stronger if the skeg was correctly sized at AIGS. Moving the top corner of the transom forward also forced the girth stations forward from their natural position which further magnified the effect. This is what lead to the steep run up to the ‘crease’ or bustle in that area.
|The continual effort to cheat the measured length rules is taken to the extreme here in the tortured shape of the David Thomas designed Three-Quarter Tonner Liz of Lymington VI|
|The early Farr approach - wider and cleaner stern shapes, but with a consequently longer measured length that to achieve a competitive rating had to be offset with a modest sail plan|