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From-Multihull International, January, 1988 by : RICHARD WOODS

LIKE MOST multihull designers we are getting an increasing number of enquiries from people who want to race their boats in coastal races, in fact, as we own two performance multihulls ourselves, we also want to race. Unfortunately we often have to tell people that there is no racing in their area because local clubs do not want to be given an arbitrary handicap, nor do they like being penalised if they can sail well. PY type systems, which although very successful when there is a large statistical base to work from, have major limitations when, used with small fleets, particularly if many boats are one-off designs. Therefore a design/measurement-rating rule is essential.

Rating rules are only really successful if: a) They give an accurate and fair assessment of speed under all conditions - say within 2% of reality for all types of boat. b) Boats can be easily and quickly measured and there are few, if any, rule-cheating loopholes for designers to exploit. c) Boats are encouraged to race, resulting in big fleets, international competition, etc. In monohulls the IOR scores well with point a) poorly with point b) and is becoming progressively less successful with point c).

The existing IYRU approved multihull rule is the IOMR but unfortunately it fails on all three above points mainly for the following reasons:

A) The rule’s complexity gives the impression that it must be accurate, but in fact it is easy to find major flaws in the assumptions used to produce the rule. Appendix 1) gives several examples of faults in these assumptions and shows that ratings can be at least 2% in error. (Now although we said that a rule could be called accurate if, like the IOR, it predicted speeds to within 2% of reality, anyone who has sailed under a yardstick system knows that in fact a 2% difference in speed (or yardstick) is a very difficult margin to overcome for example the 420 is always faster than an Enterprise, yet an accurate rating rule might give them the same rating).

The IOMR assumes that races are held in 10 knots of wind. The graph in appendix 2) shows predicted boat/true wind speeds for four different multihulls that have raced together regularly. The majority of existing multihulls (and certainly all those which the IOMR was originally designed to rate) fit between curves 1) and 2) so it is clear that in winds up to 10, maybe even 15 knots, it is feasible to predict performance variations between different boats. However this is not the case for stronger winds as the curves diverge rapidly, nor is it feasible to compare boat 3), and certainly not 4) with 1) and 2).

In practice these theoretical curves have been shown to hold good for winds between Fl and F7. One can understand the rule makers arguments that led them to ignore hull shapes, type of keels etc., but it does mean that a round bilge boat with daggerboards is rated the same as a deep V boat with no keels.

B) The IOMR is perhaps better than the IOR when it comes to actually measuring the boats as it only uses hull length measurements and relies on weighing the boat to assess the potential drag characteristics of the hulls. Sail areas still have to be accurately measured, a tedious but simple task, common to all rating rules. However we believe that it is better to measure actual sail areas rather thin measure the total space in which a sail could fit.

Unfortunately the method that the IOMR uses for measuring length is wide open for abuse, as appendix 3) shows. This aspect of the rule has to be urgently looked at by the IYRU if the IOMR is to retain any credibility. If one takes all the above points into consideration it is perhaps no surprise to find that two typical modern performance multihulls, i.e. Strider and Banshee, should have IOMR ratings that bear no relation to their actual relative speeds, for the IOMR predicts that Banshee should be about 5% faster than Strider, yet in practice the reverse is correct.

In other words the IOMR incorrectly estimates their relative speeds by a massive 10% - even an educated guess would be closer to the truth.

C) The IOMR is now only used in parts of Scandinavia (chiefly Denmark) and the East Coast of Australia. It has died out completely on the West Coast of the USA, where it originated, was never really used in the UK or France and has certainly never encouraged international competition. The IOMR is now 15 years old and although the multihulls now sailing have changed radically in that time the rule has not, so it is high time for at least a major revision.

Even if there is no real international competition at the moment, more and more multihulls are being marketed on a worldwide basis, so the same designs are being raced in several countries. The problem at the moment is that, with no uniform world wide handicapping system, the handicappers can only use the results from their own country. If there was a uniform system, then handicappers would have access to many more results and so accurate handicaps could be calculated more quickly and rule discrepancies discovered earlier.

Thus it is clear that the only reasons why the IOMR is still in use are because no one has officially asked the IYRU to change it, and no one has offered a viable alternative. Although international competition between cruising multihulls may be almost nonexistent, the same cannot be said about smaller multihulls for hundreds of beach cats race under rating rules in Europe and the USA, specifically the PMA rule in the USA and the Texel Yardstick (TY ) in many parts of Europe.

Interestingly both rules are based on the IOMR, but because beach cats are largely similar (e.g. same freeboard/windage, same rig type, short overhangs etc.) the problems with the IOMR described earlier are less noticeable. However, the results under both the PMA and TY improved when the Aspect Ratio correction factor for sails (see appendix 4) was used instead of the one used by the IOMR. Furthermore a ‘fudge’ factor of 2% was used on boats not fitted with daggerboards, again producing more accurate results.

More recently the TY committee has begun to use a new basic formula: although only based on empirical and statistical evidence is claimed to give more accurate results than the modified IOMR, despite being considerably simpler.

If the TY3 is indeed more accurate than the IOMR for smaller boats then it could imply. that the TY3 is also fairer for larger ones as well, indeed the limited evidence available from race results seems to indicate that this is the case.

Unlike monohull owners who race to a rating rule, (IOR, 12m rule etc.) the keenest multihull racers race boat-for-boat either in offshore races (CSTAR etc.) or inshore in F40 Grand Prix etc. Realistically, therefore, the majority of multihulls that will race using a rating rule will be cruiser racers which also implies that many will be stock production boats so the rule makers can give toilers, schooner rigs etc. a low priority, while most owners will not take their racing too seriously.

It is therefore felt that the Channel Handicap approach would be more favoured by these owners than a pure measurement rule. In other words the most sensible rating rule would be one where the rating formula was used as a basis for a rule and the resulting ratings were modified by g series of ‘fudge’ factors to cover items that are difficult to quantify analytically - e.g. windage, daggerboard size, rig type etc.

Again like CH. penalties could also be given for exotic hull or sail materials, wing masts etc., though it is debatable whether the actual rule need be kept as secret as the CH rule. Perhaps the main underlying problem with the IOMR is that it tried to cater for too big a range of’ boat types - the curves in appendix 2) show how difficult it would be to rate all boats fairly - we hope that by concentrating on only one type of boat a more successful rule will emerge.

Outline details of such a rule together wit some example ‘fudge’ factors are given in appendix 5). Clearly this proposal still need careful refinement but we feel that it is a goo starting point for discussion. Multihulls Magazine in America have tried t stimulate international cooperation formulating a new rating rule, indeed several of the ideas in this article are as a result of the Multihull-Magazine-inspired correspondence, but unfortunately a magazine does not have sufficient authority to force countries to agree on a new uniform rating system.

That is why the IYRU, as the only impartial international Organisation, has to b involved with any new rule and so we hope that it will find these notes helpful and that the multihull committee will give discussions on finding a new multihull rating rule its highest priority. We also hope that anyone else who reads this article will contact their own national multihull association and ask them to contact the IYRU on their behalf and offer their suggestions.

Appendix 1

Analysis of assumptions made when IOMR was created.

The IOMR calculates the driving forces created by the rig and the total hull drag forces and then compares them to decide on a boat speed/wind speed ratio. The driving force term includes a hull windage factor while the drag force is split into friction, wave making and rough water drags. Unfortunately several anomalies were found when the IOMR-calculated forces were compared to those of two real boats - specifically the Strider and the Banshee. These two boats have raced against each other enough times over the last two seasons for it to be clear that the Strider is at least 5% faster than the Banshee in ten knots of wind.

Friction drag is the easiest to calculate and comparing actual friction drag with the IOMR figures we see that there is on average a 6% error. This error is probably due to the fact that, because the only dimensions measured on the boat are length, weight and sail area, the IOMR calculates wetted surface area as SQRT(WxL). Wave drag is calculated as a proportion of friction drag by the IOMR, unfortunately this method does not seem to have any sensible correlation with other methods, thus its accuracy must be treated with suspicion.

Windage drag is effectively ignored by the IOMR as it assumes that for a given rig type and given Vb/Vt the ratio between air drag and sail area is a constant. Again, comparing Strider and Banshee which have radically different windages and sail areas (but similar rig types), yet sail at almost the same speed, it is clear that such an assumption will lead to major errors. In fact it can be shown that, even assuming that the frontal area is a fair indicator of windage drag (which is unlikely), the Banshee sail forces are effectively reduced by around 5% with a corresponding decrease in rating of 2%.

Appendix 2

IOMA method of measuring rated length. Two boats that have raced against each other in 1987. were the 7.3m STRIDER and 8m MY-CAT In practice the Strider has an effective sailing length about equal to its waterline length (6.7m), while the My-cat has an effective sailing length of about 7.5m. Yet the rated length of the Strider was 7.0m and of the much bigger Mycat 6.4m!

Clearly the IOMR RL bears no relation to the real sailing length of either of the two boats.

NOTE 2004

I haven't included my rule ideas here because I did not get ANY response from the IYRU (now ISAF) not even a "thank you" letter!

If you are interested in rating rules I suggest you look at the Texel Rating web site in Holland, or the CTC web site