International Offshore Rule - Part 4: Level Rating and Handicapping

Earlier articles about the International Offshore Rule ('IOR') have looked at some of the key components of measuring a yacht under the rule, and deriving a handicap with the aim of allowing different size yachts to compete against each other at any venue around the world (Part 1 is here). Level rating was the non-handicap form of racing under the IOR, where each boat was designed to the same handicap rating, or ‘Ton’ class. 

Part of the big 43-boat fleet that contested the 1976 One Ton Cup in Marseille, France (photo Bateaux magazine)

Looking at the level rating classes first, the glamour fleet were the One Tonners, which had the ‘Coupe Internationale du Cercle de la Voile de Paris’, or the One Ton Cup, as their holy grail. The Cup was so named because it was presented in 1899 by the Cercle de la Voile de Paris ('CVP') for competition by boats with fixed keels, rated at one ton or less under the French tonnage rule of 1892. These small boats, approximately 5m on the waterline, raced in Cowes or on the Siene until 6-metre class yachts were used for the trophy following the advent of the IYRU International Rule. The impressive trophy was just under a metre high and was carved out of a 10kg block of solid silver by a Parisian goldsmith.

The One Ton Cup

The last One Ton Cup in 6-metres was held in 1962. In 1965 the CVP declared that it should be used for yachts racing boat for boat at a maximum specified RORC rating (22ft). The clubs at La Rochelle soon after gave the Half Ton Cup for boats with an 18ft rating, and the Quarter Ton Cup for boats with a 15ft rating. The additional international racing that occurred under this arrangement became a major influence on the whole process for a change towards an international rating rule. With the arrival of IOR, it was agreed to use the same level rating concept. Thus in 1971 the One Ton class was defined at 27.5ft IOR, Half Ton at 21.7ft, and Quarter Ton at 18ft. There was also a Two Ton class which was not well supported at a 33ft rating and was changed to 32ft in 1974 (and later to 34.5ft). In 1974 the US delegates to the ORC presented a Three-Quarter Ton Cup for racing at 24.5ft, and the Mini Ton class was introduced in 1978 for racing at 16.0ft. In 1983 the rating limit for One Tonners was raised to 30.5ft, after a drop in support for the One Ton Cup at the old limit, and giving the class a dual purpose as they would now meet the lower rating limit for the Admiral's Cup.

The popularity of the Half Ton Class was evident from this photograph of a start during the 1976 Half Ton Cup in Trieste (photo Bateaux magazine)

These limits were further adjusted to two decimal figures as measurement techniques became more precise (e.g., the One Ton limit became 30.54ft). The 50-Foot Class, followed a similar model using a limit of 40ft (increased slightly to 40.54ft in 1991), but used individual and variable ratings within this limit for some regattas, and no ‘Ton’ classification was applied to this fleet. 

The 50-Footers enjoyed close racing on a level-rating basis for many years (photo Guy Gurney)

In the same vein, Maxi yachts were those that raced under the IOR's rating limit of 70.0ft (which was subsequently eased in the late 1980s), but given the sometimes large variance in actual ratings for these large yachts their racing was usually based on handicap results, although line honours remained a keen focus. 

Part of the Maxi fleet competing in the 1988 Kenwood Cup in Hawaii, with Sorcery (70.82ft IOR) leading Il Moro di Venezia (70.05ft), Ondine VII (70.04ft) and Matador (69.99ft)

Associated with the Quarter, Half and One Ton Cups were race-specific trophies. For the One Ton Cup, these included the Trophee de la Societe Nautique de Marseilles, for the winner of the short offshore race, and the White Horse Trophy, for the winner of the long offshore race.

One challenge in changing Pendragon from her original configuration as a Three-Quarter Tonner to a One Tonner in 1979 was meeting the increased accommodation requirements for One Tonners stipulated by the ORC 'Green Book' regulations (photo Paul Mello)

Each level rating class was subject to specific ORC regulations that governed the world championships (the ‘Ton Cups’) for each of the level rating classes (the ‘Green Book’). This set out the rating limits for each class, the rules under which each championship would be raced (such as the length and type of courses to be raced) and the maximum number of entries for any given championship. The Green Book also specified certain design parameters not otherwise covered by the IOR rule itself, such as the minimum height and area of headroom, freshwater capacity, the minimum number of berths, and even the minimum thickness of squabs to be carried.

A closely packed fleet of One Tonners during the 1984 One Ton Cup, the first held under the new rating limit of 30.5ft

Level-rating yachts were also campaigned in mixed fleet regattas (where they might further optimise performance and fall in or out of the class limit), which necessitated the use of handicapping and the application of time correction factors.

Close racing between a group of One Tonners during the 1987 Admiral's Cup (photo Bateaux magazine)

A yacht’s IOR rating was a length measurement, expressed in feet. For a yacht designed with an eye to the rule, its rating might typically be around 8-10ft less than its LOA. In a handicapped race, the IOR rating was used to compute either a Time Correction Factor ('TCF') expressed as a decimal figure, and which for 'Time-on-Time' handicapping was computed against elapsed time to provide a corrected time; or as a Time-on-Distance allowance, expressed in seconds per nautical mile. Time-on-Time is similar to a general handicap or PHRF system. Time-on-Distance was used by multiplying the distance of the race by a time factor, and subtracting from the boat's actual time, to compute the boat's corrected time (the current IRC rule dispenses with the conversion between rating and handicap, by simply expressing a yacht’s rating by way of a TCF).

A crowded start line at the 1985 Admiral's Cup, with 57 yachts (representing 19 national teams) rating between 30 - 40ft IOR in a single fleet (photo Guy Gurney)

This was a further complication in the level of acceptance achieved by the IOR, at least in mixed-fleet racing. This is because no matter how accurate the measurement system, if the time correction formula was seen to unduly favour one size of yacht over another, the system would lose support.

Startline action during the 1978 Quarter Ton Cup in 1978, held in Japan (photo Guy Gurney)

Under IOR, and as with the former CCA and RORC, the USA continued to use Time-on-Distance while others used a single figure TCF, and this also resulted in different perceptions of the rule. A committee of ORC sat for several years attempting to reach a single compromise on time allowances, and duly dissolved itself without finding a solution. However, both approaches were based on the same principle, in that a higher rating gave a smaller time allowance, and vice versa.

The diminutive Farr-designed KevFlyer, rating just 20.0ft, racing in the 1980 Clipper Cup (photo Phil Uhl)

The generally adopted formula for the TCF adopted by the RORC for IOR racing was, at least initially, a relatively simple calculation: TCF = (R1/2 + 2.6) / 10. For example, a One Tonner, with a rating of 27.5ft, had a TCF of 0.774, while a Half Tonner, with a rating of 21.7ft, had a TCF of 0.716.

International mixed fleet racing under IOR, with Half Tonners (eg Swuzzlebubble, KZ3494) racing alongside the Maxi Kialoa III (background) during the 1977 Southern Cross Cup (photo Phil Uhl)

As with the IOR rule itself, the TCF formula underwent further evolution, with yacht clubs able to adjust the formula to suit their preferences. For example, in 1975 the RORC produced the following amendment: TCF = 0.2424(R)1/2/1 + 0.567(R)1/2. The only significance of this was to create a ‘hard point’ in the time correction factor graph at the place of a ‘base’ yacht of 29ft rating (TCF of 1.000). This formula withstood pressure for further change until the mid-1980s, where bigger yachts were given some easement following the dominance of One Tonners in the 1985 and 1987 editions of the Admiral’s Cup, but the swing of the pendulum lead to something of a rout by the fastest 50-footers in 1989. Such problems with finding the 'perfect' formula were, at least for the Admiral's Cup, subsequently avoided by changes to the regatta format from 1991 with boats racing in level rating divisions. 

Changes to the RORC's TCF in 1989 saw the ascendancy of the 50-footers in that year's Admiral's Cup, with Jamarella taking individual honours overall, followed by Will (second) and Stockbrokers Container in fourth (photo Peter Ludlow)

The Farr-designed Shockwave on her way to winning the 1992 Two Ton Cup (held in conjunction with the 1992 Kenwood Cup in Hawaii)

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

The International Offshore Rule - Part 3: Measuring Sail Area

In this third part of the series about the International Offshore Rule we look at the measurement of sail area (S), the engine room of any yacht and an important component of the measured rating (MR) component of the overall IOR formula.  

A startline in a closely fought 50-Footer regatta

The assessment of sail area was subject to its complex series of measurements, although much of that complexity arose by the myriad of formulas that had to be established to equitably rate a wide range of rig types (fractional versus masthead, and single masts versus two masts). In its most simple form, the rule component S was the square root value of total rated sail area (RSAT), itself a combination of mainsail and foretriangle measurements.

The 1986 Half Ton Cup winner Cofica - the broken spinnaker pole was not a rating device!

l'Effraie (left and above)

Complex as the rated sail area formulas were, they still yielded some significant rating gains when applied to unusual rigs. Such opportunities were first exposed by Professor Jerry Milgram (US) with his 38ft cat-rigged ketch Cascade which rated just 21ft IOR (less than a Half Tonner). She was slapped with an arbitrary 10% rating penalty for the 1973 SORC, but still won three of the races in the series. The design took advantage of a loophole which rated the yacht's 800 sq ft of sail as 300 sq ft. At the April 1973 meeting of the ORC a new rating for such rigs increased her rating to around 27ft (near the One Ton limit). The loophole had not quite been closed, however, and in 1976 the French cat-rigged Mini Tonner l'Effraie ("The Owl") featured an enormous mainsail and walked off with the Mini Ton Cup that year. But Cascade still managed to maintain a reasonably favourable rating, through the addition of a small headsail (off a Star class yacht), and competed with some distinction as late as 1983 in the SORC of that year, scoring a first and second in Class F in two legs of the series. Gear failure saw her retire from the final two races, but she still finished ninth in class.   

Cascade during the 1983 SORC

Sail measurement loopholes were again demonstrated to even greater effect 13 years later by the ketch rigs employed on three Farr designed maxi yachts in the 1989-90 Whitbread Round the World Yacht Race, a concept which was taken even further in the 1993-94 event where the mizzen rigs became ever larger.

The ketch rigged New Zealand Endeavour during the 1993-94 Whitbread Round the World Yacht Race

The typical rig of the early IOR years

While the IOR reduced the emphasis of earlier rules on the mainsail and provided fractional rigs with some advantages, the large foretriangle and small/ribbon main arrangement remained popular in the early years of the rule. This is because only the foretriangle area for'ard of the mast was measured (i.e. ['I' x 'J] x .5), so large overlapping headsails achieved a high level of unmeasured sail area. Mainsails were measured as a function of luff length (P) and foot (E), and as leech length was unmeasured, yachts usually sported drooping booms in order to gain additional unrated sail area in the foot of the sail. The rule controlled various components of the rig such as battens, boom depth and spinnaker pole lengths by relating these to certain base dimensions, and applying 'corrections' to the primary sail dimensions should those components exceed limitations.

The J41 Dazzler dropping her spinnaker at a leeward mark during the 1984 SORC

Wai Aniwa and the infamous 'shooter'

Spinnakers were subject to normal limitations of girth, related to the J measurement. The distinctive shooter sail (or 'blooper' in US parlance) was not something contemplated by the rules, and first came on the scene aboard Chris Bouzaid's One Tonner Wai Aniwa during the 1971 Southern Cross Cup series. Bouzaid courted controversy when he set the drifter headsail to leeward of the spinnaker and its sheets, with a loose halyard. While subject to a protest, this was dismissed by the New South Wales Yachting Association. The practice was later ratified by the International Technical Committee, to the chagrin of many, and the shooter went on to become an essential, if unloved, part of the IOR sail wardrobe before finally disappearing from the scene in the mid to late 1980s. 

The ubiquitous fractional rig first came to the fore on light displacements yachts from the mid-1970s, and had their first real success on the Farr 727 in the 1975 Quarter Ton Cup. But masthead rigs still remained popular through the 1980s, and maintained parity with fractionally rigged yachts for many years before technological advances and greater reliability saw the more flexible and adjustable fractional set up become integral with success at the top end of IOR racing in the late 1980s and 1990s.   

A fleet of IOR 50s displaying a mix of fractional and masthead rigs

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

The International Offshore Rule - Part 2: Measuring Depth

In this article we look at the rated depth (D) and the draft correction (DC) aspects of the IOR formula for measured rating (MR). 

Light air bow down trim aboard Jiminy Cricket (photo Farr YD)

The D value represented the rated depth of the yachts hull. This was based on mid and forward depth stations (MD and FD respectively), which were located at points at half and quarter the length of LOA from the bow, and then measured from specified widths from the centreline. These points were eventually expressed through visible 'flats' between these measurement points as designers sought to shed unnecessary hull volume and unmeasured displacement from a yacht's hull. Additional depth measurement points were added to the rule as early as April 1971 in order to discourage the emergence of these flat sections in the garboard area of the hull in particular. The effect of this change was limited, however, and flat sections between the depth measurement points were considered to be fast under the rule (relative to rating), and persisted throughout its life.

Noticeable flats between measurement points displayed here on the 1976 Farr Two Tonner Uin-na-Mara

Because all depth measurements were in the centre and the forward part of the yacht, boats typically featured a bow-down static trim when properly optimised, sometimes assisted by a more for'ard location for the yachts engine than might otherwise be ideal. The mathematics involved in determining D looked like this:

The B x D component of the MR formula thus provided an estimate of the boats volume through an approximation of the cross-sectional area of the hull. A larger value for one or both factors would result in a reduction in rating, all else being equal.

Bow down trim is evident here on the Davidson One Tonner Pendragon, circa 1988

The centreboard on Resolute Salmon

The DC component allowed for differences from 'base' draft (DB) and rated draft (RD) values, calculated as a function of rated length, with deviations on either side of the base value becoming a positive or negative factor for the resulting rating. The rule did not initially measure centreboards in their fully lowered position, and this conferred a negative, and therefore advantageous, DC value to the MR formula. This was not so much a loophole as a recognition of centreboard craft popular on the east coast of the US due to the shallow nature of the cruising grounds prevalent in that area. However, US designer Britton Chance recognised a potential advantage under the IOR if the centreboard was designed solely as a useful lifting foil, in combination with internal ballast. The result was Resolute Salmon, a big heavy One Tonner that was ideally suited to the conditions off Marseilles, France, to win the One Ton Cup in 1976. 

Comparative rating statistics between some of the top boats at the 1976 One Ton Cup

The issue of centreboards became a hot topic at the next meeting of the International Technical Committee at their annual meeting after the 1976 series, but it was left for another year while the Committee focussed on what was seen as the bigger issue of broad sterns that were evident on some of the new light displacement yachts that had come on the scene. With the centreboard option still open, the 'Ton' Cup events for 1977 were set to be dominated by a new generation of offshore centreboarders.

Another advantage of a centreboarder was the ability to launch at low tide!

The advantages of the centreboard lay not just in the DC measurement benefit. With ballast mostly internal, the new vanguard of centreboard yachts took advantage of lightweight construction techniques that provided for a high ballast ratio - with that ballast held in the bilges their pitching moment in waves was much reduced. Use of a centreboard also allowed a designer to incorporate a little more displacement into the keel area of the hull because the centreboard displaced less than a keel which provided a depth measurement increase. The boats were measured with the centreboard up and this also allowed the boat to sink a little more and thus further reduce rating.

With her centreboard retracted Gunboat Rangiriri is prepared for launching (mid-1977)

However, the use of centreboards, in combination with light displacement and light construction, led to a great deal of controversy through the 1977-78 Southern Hemisphere season, and new stability tests (via a new 'Screening Value' promoted by the Cruising Yacht Club of Australia) were devised in attempt to outlaw such yachts from competing from events such as the Sydney to Hobart.

The Farr One Ton centreboarder Hecate undergoes a pull down test before the 1977 One Ton Cup to assess the effect of the new CYCA Screening Value formula

The ITC belatedly acted in its November 1977 meeting to change the rules relating to centreboards, and to introduce a moderated form of the Screening Value test, but it still remained an option to use them, but unless it was pinned in place the appendage would carry a penalty. Their short reign practically ended in the 1978 Northern Hemisphere season, and it was notable that in the 1978 Half Ton Cup, and even though Waverider won, there was no real difference between centreboarders and keelers. Indeed, a full scale experiment between two otherwise identical Holland Half Tonners showed that the keeler was faster. 

An unusual sight in offshore sailing, the crew of Silver Shamrock III install the centreboard before racing in the 1977 Half Ton Cup in Sydney

A centreboard made its most prominent re-apperance at the 1985 SORC on the Ted Irwin design Razzle Dazzle, a heavy masthead yacht with an elliptical centreboard shape that attracted a 0.3ft rating penalty. In comparison to earlier approaches, the board weighed some 5,000lbs and required hydraulics to lift it. Razzle Dazzle's modest effort in the series did not lead to any further development of the concept.

Waverider, with her centreboard pinned in place, taking the gun in the first race of the 1978 Half Ton Cup

The next article will look at the measurement of rigs, which was an area that yielded some of the more radical looking rule cheaters of all. 

 
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

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