IRC Changes for 2013 and Already Known Changes for 2014

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For 2013, the IRC Technical Committee has made a number of changes to the calculation of IRC TCCs.

Most of these have small effects, or affect only a small minority of boats. In a number of cases, they either simplify the IRC software or make it more logical, without intent to change TCCs. Any effects in these cases are therefore also small and are not discussed below.

In other cases, the intent of the changes is either to directly improve how IRC treats particular features on boats, or to address actual or potential abuse of IRC; these are discussed below. In all cases however, the Technical Committee is constrained in the level of detail that can be given, due to the unpublished nature of IRC calculations.

1.         Running backstays and Checkstays

Historically, running backstays have been treated slightly more severely than checkstays. With the full adoption of the ISAF Equipment Rules of Sailing (ERS), the practical distinction between them has reduced. It had also become apparent that in many cases of older boats, some rated with runners probably actually had checkstays.

Noting all of this, and also developments in rig design generally, the Technical Committee concluded that it was appropriate that for IRC purposes runners and checkstays should be treated equally. The effect of this change is that a boat fitted with runners will see a small rating reduction.

Additionally, the effect on TCC of runners and/or checkstays for boats with in-line spreaders has been slightly reduced. These changes will be reflected by a reduction in the effect of Rig Factor.

2.         Mainsails

During 2012, a minor error became apparent in the treatment of mainsails. The effect of correcting this is a small increase in TCC for the minority of boats affected.

3.         Flying Bows

In recent years, the Technical Committee has worked to improve the treatment of flying bows, particularly for boats with 'classic' style bow shapes, and the results of this work have now been implemented. For the majority of the IRC fleet the effects will be undetectable, but boats with very long flying bows, usually because of a classic bow shape, will see a reduction in TCC. The extent of this reduction will vary from boat to boat.

4.         Headsails and HSA

During 2012, the Technical Committee noted an increasing number of headsails designed with high upper roach allied to a reduction in luff length. The effect was a reduction in rated sail area and TCC without corresponding reduction in actual performance; this is contrary to the intent of IRC Rules 2.4 and 2.5.

In parallel, the ERS definition of aft head point (used in the measurement of HHB) had been found to be ambiguous. Sailmakers could thus avoid IRC's intent of incorporating HHB into the calculation of HSA.

The Technical Committee therefore concluded that HHB should be deleted from the calculation of HSA and replaced with HUW, headsail upper width, defined as the 7/8th width of the sail. The calculation of HSA was also appropriately changed.

The effects of this change for boats with headsails with high upper roach and short luff length are an increase in TCC. For other boats, the change is undetectable.

To avoid the necessity for all boats to supply HUW, the Technical Committee also defined default figures for HUW based on actual measured data for a significant number of sails as:

For overlapping headsails with LP/J greater than 1.10, the default is HTW x 0.50.

For non‐overlapping headsails with LP/J of 1.10 or smaller, the default is HTW x 0.56.

In the great majority of cases, any actual variation in HUW from the default will have no effect on TCC.

5.         Gaff Mainsails

The treatment of gaff mainsails has changed significantly, to improve recognition of the inherent inefficiency of these sails. The effect will be a reduction in TCC for all gaff rigged boats, which in some cases will be significant.

6.         Already Known Changes for 2014

There is one already known change for 2014. Currently, boats using kevlar in their hull construction see a small resulting increase in TCC compared with a comparable glass sandwich construction. With effect from 1/1/2014 (1/6/2014 in South countries), this will disappear.

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John Crawley (CAN IRC)  
Jinno (c/o JPN IRC)
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