The International Offshore Rule - Part 1: Measuring Length

Thunderbolt (photo Larry Moran)

This is the first in a series of articles designed to provide a brief overview of the workings of the International Offshore Rule (IOR). The rule is of course no longer in use, having disappeared from the offshore racing scene in the early to mid 1990s. But its influence on the design of yachts over the period in which it was in effect was wide and pervasive, and the purpose of this series of articles is to give an insight as to the reasons for some of the more interesting and peculiar features of yachts that were designed to its parameters.

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)

Broadly speaking, the IOR was formulated on the basis of the CCA approach to sail measurement and the RORC method of hull measurement. But both these aspects of a yacht’s rating under IOR borrowed aspects from both rules, while balancing the use of profile (CCA) and girth (RORC) in the measurement of length.    

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 S^1/2) / (B x D)^1/2 + 0.25L + 0.2S^1/2 + DC + FC 
    R = MR x EPF x CGF

The measurement of B, and limits to hull radius

The determination of ‘L’, rated length, was the most complex within the MR formula. This is not surprising as it attempted to fairly rate the length, and speed-producing characteristics of a three-dimensional body. It sought, at least in its central objective if not in the result, to avoid the typeforming that arose under the CCA and RORC rules, and the distortions which arose from those simpler and earlier rules. 

The pushing of 'B' to the extreme, as seen here on the aptly named Stephen Jones Half Tonner Tumblehome

The measurement of L began with finding the length between girths (LBG) which firstly required the determination of rated beam (B). This measurement was taken at the point of maximum beam (BMAX), but measured at a distance one-sixth of maximum beam below the sheer. Most boats designed to the rule sported a pronounced hard point along the topsides in the position of the rated beam measurement, as maximising this dimension was critical to obtaining a favourable rated length measurement.

The 'basic' parameters involved in finding 'L'

The measurement of rated beam allowed the position of four hull ‘girth stations’ to be determined, these being the after girth station (AGS) and after inner girth station (AIGS), and the forward girth station (FGS) and forward inner girth station (FIGS). These girth stations were those points aft and for'ard where the girth was equal to a certain fraction of rated beam – for example, AGS was the point at which the hull girth was equal to 0.75B, while FGS was equivalent to 0.5B. In broad terms, the squeezing of the stern at AGS, and a raked bow to force FGS aft, enabled a designer to minimise rated length relative to actual length. 

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

The second component of L required the measurement of the after overhang component corrected (AOCC). Much greater variety was possible in the design of the stern area, and the most complex part of the rated length calculations related to this part of the boat. This complexity was driven by the stated objective to rate many different types of stern shapes equitably, and with the knowledge that designers would be more tempted to distort stern shapes than the bow in order to achieve a lower rating. The initial approach, as promoted under the RORC rule, saw designers pursue a reduction in rated length through pinched stern sections, in combination with a heavier displacement and beamier hull forms. This measured well in static trim, but allowed little in the way of performance increases once the yacht was sailing, apart from a modest gain (often very modest when combined with a steep counter profile) in waterline length. It also offered little in terms of form stability when running downwind.

Part of the mathematical challenge for an IOR yacht designer can be seen in this diagram

Any yacht with a broad stern would of course be unable to accommodate the AGS girth measurement anywhere within her stern sections. In this event, AGS was placed as far aft as possible, and the ‘girth difference’ was taken into account within the after overhang correction (AOC) calculation. The calculations required throughout the rule, and for length in particular, were complex enough, but the formulae for calculating AOC, and its derivative AOCC, were the most formidable looking in the rule.  

The Peterson 'pintail' approach on Moonshine

With LBG, FOC and AOCC known, it was then possible to calculate L thus:

    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 

In addition, designers began to accept the penalties of increased width in the stern and to extend the length of the transom to obtain ‘free’ length beyond AGS, which was normally at the point where the transom intersected the deck. Because waterline length was measured while floating upright, large overhangs and broader flatter sections were used to increase actual waterline length when sailing at speed. Hull flare was also used near the transom partly in response to the vagaries of the AOC calculation and partly to widen the deck to maximise the contribution of the crew to overall stability.

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

The next article will tackle the rated depth (D) and draft corrected (DC) measurements, and the loophole that gave rise to the controversial and short-lived era of the ocean racing centreboarder.

A scanned copy of the IOR rule (Mk III 1985) can be viewed here.

Part of the text used for this article is adapted from the book A Lighter Ton - The Champion NZ Yachts of the 1970s, 2012