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