IOM class rules 2002

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[The following article, now edited a little, appeared in the February 2001 edition of Marine Modelling International.]

As the IOM class is popular at all levels from local club Sunday sailors up to the highly competitive "pot hunting" Championship skippers, your Editor has asked me to consider the key points which will appear in the revised ISAF-RSD rules due to be adopted on March 1st 2001 [and finally adopted in March 2002!] and to give you an insight into the reasoning behind the changes and the effects they may have on your building, checking, and registration for this summer's racing programme.

The philosophy of the new rules

The old 1995 rules labelled the hull and appendages as "development" items, and the rig and sails as "one-design" items. The new 2002 rules now label all of the boat as "closed", meaning that if something isn’t explicitly permitted by the rules, that something is automatically prohibited. This approach to defining the IOM now implies that, if there is anything about the boat that is not clearly permitted, the boat is presumed to be out of class. Guilty until proven innocent. This element of the Napoleonic Code can be a bit of a shock to Anglo-Saxons, but much of the rest of the world operates this way quite cheerfully.

The advantage of closed rules is that they do not need updating every time some new materials, methods, or ideas appear. Closed rules also bring measurable consistency and conformity. The other advantage is that, in some sense, the interpretation and application of closed rules should become less adversarial – less "us versus them" – and more naturally cooperative – more "us with them".

The disadvantage of closed rules is that there has to be lots of them, and there has to be lots of things that must be checked. This is the price that must be paid if the IOM class is to remain fully international, popular, stable, and inexpensive. In my opinion, it is certainly a price worth paying to keep the IOM class true to its original vision and original purpose.

New arrangements for measurement and certification

Measurement of the boat is now split into four quite separate areas. Previously, all of a boat was measured "at once" and a single certificate was issued. Thereafter, the boat could be checked at an event. We now have four distinct occasions of measurement: "Fundamental measurement", "Event measurement", possible "Sail manufacturer certification", and "Owner preparation".

The table gives a very rough estimate of the number of measurement questions that are answered during the complete measurement of a boat with three rigs, comparing measurement under the old rules with that under the new rules.

Table 1: Approximate number of measurement questions

  Hull &
Rig & sails Sail identi-
fication marks
Old rules





New rules:        
    Fundamental measurement





    Sail manufacturer certification





    Owner preparation





    Event measurement





"Fundamental measurement" consists of the bulk of the old rules measurement to do with the rigs, hull, and appendages. The remainder of the old rules boat measurement is split between "Event" and "Sail" measurement. Most of the measurement of the sail identification marks has become the owner’s responsibility to comply with the rules to do with sail numbers.

There is a new provision which allows a sail manufacturer to do most of the sail measurement, and to supply a certified sail. This sail would have the sail manufacturer’s certification at the tack. The sail manufacturer would have to apply for, and receive a licence from, the RYA (in the UK) in order to be specially authorised under this provision. If the sail is not certified by its authorised manufacturer, then it would need to be measured and signed by a measurer in the usual way.

The completed rig/sail measurement form from a measurer, along with a completed boat measurement form, now allows the boat to be registered and issued with a certificate (called a "hull" certificate in the rules).

Almost the whole issue of sail identification marks has been removed from the class rules and placed into the Racing Rules of Sailing. This means that the sail numbers are no longer measured for the purpose of completing a sail marks measurement form, and so a boat can be registered and receive a certificate without having sail numbers. (Indeed, the boat can receive a certificate even if its sail numbers are a total mess.) It is now up to the owner to place sail numbers on his sails correctly. If, at an event, your numbers do not conform to the required measurements, you may be protested by the Race Committee, and the issue will be dealt with then and there by the usual Protest Committee procedures. I’m just guessing here, but this should lighten the burden of sail numbering for a skipper, since you are unlikely to be protested unless your numbers are genuinely unreadable. Small errors in positioning and size are unlikely to be penalised, and I understand that the sail numbering rules themselves have been eased a little as well.

The most important of the boat measurements have been moved into a new process of measurement called "Event measurement". The significant measures of boat weight, length, draft, keel and rudder weights, set of the sails against the rig limit marks, and so on, are now handled at an event. Like sail numbers, it is up to the owner to ensure that the boat meets the class rules when the boat is entered for an event. It is then up to the Race Committee to arrange for Event measurement, checking that the boats conform to these key measures at their event. These event measurements are not made at the time of Fundamental measurement, and, as mentioned earlier, the boat can be registered and issued a certificate without these event measurements being made.

The new arrangements reduce the work load of a measurer, especially if the sails come from a licensed manufacturer. Previously, the 172 questions for a full measurement of an IOM and its three rigs took me about 4 hours, and I didn’t want to do another one for quite some time! Now, this is reduced to around 46 questions, and I’m guessing that these can be done in around half an hour. The burden of responsibility on an individual measurer is thus eased somewhat, and shared out more evenly with other measurers at event and sail measurement, and with the owner. In my opinion, all of these new arrangements are very much to be welcomed.

I’ve heard some skippers say that these new arrangements will constitute a "Cheater’s Charter". I’m more optimistic. I think that the new arrangements will in fact reduce what little cheating there may be, either deliberate or inadvertent. In the past, a Race Committee has usually not seen fit to check the boats at an event, since the owner showed a certificate which said that all the key measurements had already been checked. In future, the certificate will say no such thing, and there will be a clear and pressing requirement for the RC to set up some sort of system for quickly checking the key parameters of most boats at an event. I’m confident that this will have the effect of reducing the temptation to cheat, rather than increasing it. It will certainly reduce the temptation to "forget" that replacing a 6-cell AA NiCd pack with a 5-cell Li-ion AAA pack could take the boat under the weight limit, for example.

There is, of course, no such thing as a free lunch, and these changes have costs as well as benefits. The major costs are that the Race Committee will have more work than they have become accustomed to, and skippers will have to take more responsibility for their boat and sail numbers. The benefits are that an IOM will be much easier to measure and register, the burden of sail numbering will ease for a skipper, and any cheating will be reduced.

Major rule changes

  • A "deck datum point" has been introduced. Like a mast band, it defines a position on the hull, the "deck", from which rig measurements are taken. Within a reasonable tolerance, the owner can position the deck datum point wherever convenient. It avoids having the rules define the "deck", or the having the measurer decide where the "deck" is. The purpose of the deck datum point is to limit the variation in height of the rig, and the new rules prohibit the rig being lifted higher for light airs and lowered for heavy airs by more than 5mm in total.
  • The mainsail must have either three battens, or three batten pockets. If it has batten pockets, there is no requirement that the pockets actually have battens, so battens are now optional if you have batten pockets. This avoids an argument about what is a "batten".
  • There is no longer "any method of attachment of the sail to the mast is permitted". Instead, only bolt rope, eyes, jackstay attachments, and track slides are mentioned as permitted construction at the luff, and mast rings, jackstay, and jackstay attachments are mentioned as permitted mast fittings. This change is unlikely to cause any problems for existing boats, however.

Other new and changed rules

  • Check stays are permitted only if no mast ram is used, but now regardless of how the mast is stepped. However, having check stays with a keel-stepped mast becomes effectively pointless because the check stay rigging point must be not more than 100mm away from the mast heel. The rules thus no longer bother with whether a mast is keel stepped or deck stepped.
  • If the mainsail has a bolt rope or slides, it must be bent to the internal track of a mast; it cannot be bent to a mast that does not have a luff groove, for example. (The bolt rope is, after all, specifically excluded from the cross width measurements.)
  • The prohibition on materials denser than lead now only applies to rudder and keel. Any metal is permitted in the hull. The point of using depleted Uranium, for example, is that it takes up less volume for its weight, but this is only significant for the appendages. Otherwise, it doesn’t really matter.
  • The only metal permitted for spars remains aluminium alloy. The adjustment to the permitted aluminium content is in the form of now specifying which aluminium alloys are permitted.  Note that the list is different for masts and booms.
  • Local cut-aways are now permitted in the mast. A number of skippers have been using out of class spars which had cut-aways, but these should now fall into class.
  • The wind indicator is optional and, if used, is now explicitly excluded from any measurements.
  • There is now a wall thickness permitted variation. The spars have always had to have a "constant" section, and this now defines what is meant by "constant".
  • In anticipation of the formation of an IOM International Class Association, an ICA sticker could be required on the hull.
  • Repair of a sail is now explicitly permitted, where previously any boat repairs required the repair to be checked by a measurer. Further, the appendages may be altered without the requirement for re-measurement, so long as they still conform to the rules. While the skipper now has increased responsibility to make sure his boat conforms to the rules, a repair to the hull or spars is still subject to check by a measurer.
  • Eyelets are now explicitly restricted in number at head, tack, and clew.
  • Draft stripes are now explicitly permitted, but are restricted in number, size, and materials.

Issues that could be misunderstood (I misunderstood ‘em!)

  • Running a luffline through the mainsail luff tabling does not seem to be permitted at first sight, whereas it is explicitly permitted for the jibstay. However, the rules permit jackstay attachments at the luff, and in this case the luff tabling provides the permitted attachment of the mainsail to the luffline.
  • It is not immediately clear that a mast head fitting could be used to define the upper mast band measurement point, since the fitting introduces a change in the mast section below the upper point. However, the Equipment Rules of Sailing define the mast as the "spar, its rigging, spreaders, fittings and any corrector weights." The head fitting is thus not part of the spar as such, and it is the spar which must have a constant section.
  • Neither string nor elastic is permitted "standing rigging" material. But a piece of elastic or cotton as the topping lift restraint line, for example, is permitted, because this is "running rigging".

Other details

  • The new rules are due to come into force on 1 March 2001, having been ratified by ISAF-RSD in January.  [Erm, it didn't work out that way!  The new rules finally came into force on 1 March 2002.]
  • Ball-raced goosenecks are still permitted.
  • There was little support for a restriction on the fin thickness, and so this proposal does not appear in the new rules.
  • Any IOM that measures under the old rules and that has not been altered in any way will retain a valid certificate. If the boat is altered, however, it will need to be checked by a measurer, when it must conform either to the rules in force at the time of its initial (Fundamental) measurement, or to the new rules currently in force.
  • A builder can be licensed under special circumstances to mass-produce hulls which do not fully comply with the hull rules. It is clear that the sort of non-compliance envisaged involves matters such as the internal hull construction being obscured by pigmented resin.


I’ve discussed the new rules with Graham Bantock, Chairman of the RSD Technical Committee, at some length. Henry Farley and Larry Robinson have also offered helpful comments, though Henry nevertheless remains in fundamental though cheerful disagreement with my opinions. Whatever errors remain in the article are, of course, entirely my responsibility. It reflects my own understanding, and does not represent any "official" view.


2011 Lester Gilbert