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