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.

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

1985 Australian Admiral's Cup Trials

Intrigue

These photographs of the 1985 Australian Admiral's Cup trials series are from Australian yachtsman Chris Furey's personal collection. 

After winning the Admiral's Cup in 1979, Australia slumped to eighth in 1981 but bounced back somewhat in 1983 with a fourth. The 1985 trials series was held on Melbourne's Port Phillip Bay and in Bass Strait, and featured a number of new boats, including Alan Bond's new Apollo VI, the Dubois 40 Black Magic, Peter Kurt's new Farr 43 Drake's Prayer and Lou Abraham's latest 'Challenge' yacht, the Frers 43 Challenge III. Following his disappointing 1981 campaign in his original Frers 42 Hitchhiker (but with better success in 1983), Peter Briggs was back with a new Frers 40, Hitchhiker II. 

Drake's Prayer, which had earlier retired from the 1984 Sydney-Hobart with cracks for'ard, was one of the top boats in the eight race series, and showed devastating speed reaching and running in moderate to fresh winds, and backed this with better than average performance elsewhere around the track. Her worst place was tenth in the first race when the minimum raters finished quickly on a freshening breeze. After that she won two and never finished lower than fifth.

Challenge III

Don Calvert's new Tony Castro design Intrigue, rating 30.4ft, proved a real flyer in light airs and flat water, also won two races. She had to retire from the long race finale with rig damage but still finished a clear second on points. Abrahams finally made the team at long last with Challenge III, rating 33.2ft. She started the series well, slumped slightly in the mid-stages but recovered at the finish to clinch selection with a third in the final long ocean race.  

Sagacious (Farr 40) with Police Car to the left, and ex-German yacht Sudpack to the right

 The Farr 40 Sagacious, formerly Huon Spirit, won the final long race and one other race. But she slumped badly in the middle distance races and was unlucky to miss out, finishing fourth on points

Hitchhiker II

 Hitchhiker II, rating above the One Ton limit at 31.1ft, suffered drastically from a lack of preparation.  Apollo VI, with a nucleus of Australia II talent aboard, was expertly sailed and won the first race. Thereafter however she struggled to match the speed of the other minimum raters. The much vaunted Margaret Rintoul IV was never in the hunt and returned to Sydney without completing the trials.

Black Magic

So for 1985 Australia bucked the trend of many other nations who had selected minimum rating One Tonners for their teams. It was considered that the team would be big enough to clear the ruck of One Tonners, but still be small enough to be in the hunt if the Channel and Fastnet races turn into small boat benefits, as had often been the case.

Scallywag II (Farr 40)

The Australian team proved to be reasonably competitive, finishing fourth overall, and were not far behind the third placed New Zealand team. Their overall result was, however, hampered by a retirement by Challenge III in the Channel Race when a spreader collapsed soon after the start. Drake's Prayer also suffered from a broken main halyard in the Fastnet Race, but Kurt's and his crew returned to repair the halyard and headed back out to sea again. Intrigue, which had performed only averagely in the earlier One Ton Cup, was fully competitive and finished as the top yacht in the team in tenth place overall.

Dry White (right) in a close tussle with Intrigue

Apollo VI

Kialoa III (S&S Maxi)

Kialoa III was one of the most well known of Jim Kilroy's maxis to wear the famous 'Kialoa' name, and photos of her under full sail downwind are still impressive even today. Kialoa III is perhaps best remembered for her line honours victory in the 1975 Sydney-Hobart race when she broke the race record with a time of 2 days, 4 hours and 37 minutes. This was a time that wasn't beaten for 21 years.

Kialoa III was a 79 footer designed by the New York design office of Sparkman & Stephens and a maxi-rater under the IOR (70.0ft). She was a heavy displacement design but a powerful boat that was at home in many of the offshore classics of the era, including the Southern Cross Cup and Clipper Cup. She also made an appearance in New Zealand for the 1976 Dunhill Cup after her record setting Sydney-Hobart effort, and later on underwent significant alterations at the Auckland yard of McMullen & Wing with a new stern and keel. Kialoa III went on to win a number of major races, including setting a new record in the St Petersburg-Ft Lauderdale race during the 1977 SORC.

Kialoa III on launching day in 1974 at the Palmer Johnston yard (photo Facebook)

What 10,000 square feet of sail looks like - spinnaker, blooper, mainsail, mizzen and mizzen spinnaker set.

The video below shows Kialoa III and her arch rival Windward Passage enjoying the downwind conditions during the 1975 Sydney-Hobart race. The video starts with an interview with Jim Kilroy and the sailing footage commences at 3:50. 

 

Kialoa III chases Australia's Ballyhoo during the 1976 Big Boat Series in San Francisco (photo Facebook)

Kialoa III seen here finishing the 1977 Transpac (photo Phil Uhl)

Kialoa III during the 1977 Southern Cross Cup series (photo Guy Gurney)

Kialoa III during the 1977 Southern Cross Cup series (photo Guy Gurney)

Kialoa III running down the Sydney coastline during the 1977 Southern Cross Cup series (photo Guy Gurney)

Kialoa III with all sails set during the 1977 Southern Cross Cup (photo Guy Gurney)

A magnificent shot of Kialoa III running downwind off Sydney Heads with all sails set during the 1977 Southern Cross Cup (photo Guy Gurney

Kialoa III and Windward Passage in a close battle off Sydney Heads at the start of the 1977 Sydney to Hobart race (photo Phil Uhl)

Kialoa III soon after the start of the 1977 Sydney-Hobart race (photo Phil Uhl)

Kialoa III seen here soon after the start of the 1977 Sydney to Hobart race (photo Facebook)

Gybing required plenty of advance warning!

Kialoa III, as a sloop, racing Condor of Bermuda

Kialoa III racing in the 1980 SORC where she finished fifth in Class A and 20th overall (photo Guy Gurney)

Kialoa III in her modern configuration

Kialoa III as seen recently above (2022) and below (2023) (Facebook)

Jim Kilroy has written a book about the Kialoa days called Kialoa US-1: Dare to Win, available here Kialoa Book

Kialoa III seen here in Cascais during an extended cruise during the 2025 summer (photo Facebook)

Article updated October 2025