The IOM class

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The International One Metre class was created by Jan Dejmo, with the support of Graham Bantock and the Permanent Committee of the ISAF-Radio Sailing Division, and adopted by ISAF-RSD in 1988.  The intent was to pull together a variety of "One metre" class rules from different countries, and provide for a common, restricted, inexpensive international class of stable, easily-built designs.  The result is the most popular RC racing yacht class in the world.

The IOM rules

In rough order of importance, the performance of a RC yacht is a function of the following parameters.  It is interesting to note that, of all the major performance factors, only the beam of the hull has been left free by the IOM class rules.  It is therefore primarily differing beams, and positions of maximum beam, which currently distinguish the different IOM designs.
Parameter Restricted  
Mono/multi-hull  yes Mono-hull
Sail area yes  3 rigs only
"A" approx 6000 sq cm
"B" approx 4100 sq cm
"C" approx 2700 sq cm
Bulb draft yes  420mm max
Ballast ratio yes  62.5% max (2500g bulb max)
Hull length yes  1000mm
Sail aspect ratio  yes "A" rig approx 4.8
Displacement  yes 4000g minimum
Rig plan  yes Bermuda sloop, 7/8 fractional rig
Beam  no  

In addition to performance factors, the class rules explicitly restrict certain materials and construction methods in order to keep the class inexpensive and able to be built by an amateur builder "on his kitchen table".  Radio control is restricted to two channels.  It is interesting to note that the appendages and sails can be made of any material, and it is currently doubtful whether an amateur builder can realistically manufacture a fin to the same quality and standards as a professional builder.  The rest of a competitive boat, though, can be built at home.

Item Restricted  
Hull materials  yes Wood, glass fibre, resin
Sail materials no  
Sail construction  yes "Soft" sails
Limits on panels, seams, reinforcements, battens, etc
Bulb materials yes Lead
Spar materials yes  Wood, aluminium
Fin, rudder no   
Radio control  yes 2 channels
Rig construction yes Limits on shrouds, spreaders, stays, gooseneck, kicking strap, etc
Rig materials yes Steel wire, polymer


IOM success

The class has enjoyed enormous success.  I'd suggest two main reasons.

First, a competitive IOM can be built by an ordinary club skipper on his kitchen table.  Part of this is that there is just sufficient freedom in the hull rules to encourage an amateur builder to "give it a go" and see if he can turn something out that'll beat the "professional" designs.  Part of this is that the weight limits and materials limits make home building quite feasible.  (It is not a push-over, however.  A professionally-built boat can carry around 400g of corrector weight, while you'll have to work damn hard to get your kitchen-table boat down onto the weight limit, never mind light enough to require correction.)

Second, and possibly most importantly, a competitive boat remains competitive for several years.  Mostly this is due to the restrictions on the boat and on the rig in particular.  This provides very considerable benefits of reduced life-time costs and maintained resale values.


Amongst a number of development decisions taken in the IOM class in the last 10 years or so, I feel two have been particularly significant.

The first was the decision to allow fins and rudders to be unrestricted as to materials and dimensions.  Now, blessed with perfect hindsight, we realise that a competitive IOM must have a thin, carbon fibre fin with a precise and very carefully designed cross-section. Such a fin cannot be realistically built on the kitchen table by an amateur builder.  Jan Dejmo has straightened me out on this one.  I thought the rule had been changed, but Jan tells me that it always permitted any materials for fin and always permitted any thickness, though there was considerable debate at the start as to whether either freedom was the right way to go.

The second was the finding that a very simple head fitting, a piece of bent wire, was outside the 1995 class rules.  An apparently carefully drafted and uncontentious rule led to the banning of a very desirable method (simple, cheap) of head attachment.

For me, the lessons are, first, that despite a lot of thought and consideration, the best-intended rule and rule change can still be wrong.  And second, it is not possible to be certain whether a proposed rule or rule change is for the best.

Where might we go from here?

Any development of the IOM class must maintain its success features, and there is surely little reason to meddle with a formula that apparently works so well.  But there do seem to me to be two areas of development that might be worth pursuing.

My first thought was to simplify the measurement of an IOM, and thus to simplify the class rules.  After a couple of years of trying to simplify the rules, I realised they were about as simple as they could possibly be and still achieve a "home-build one-design".  [Yes, indeed!  For those doubters out there, I invite you to have a go yourselves at simplifying the IOM rules and still end up with essentially the same boat.  Jan always reminds me that the apparently "simple" IOM class is achieved by "complex" rules.  There seems to be something of an inverse relationship between the "simplicity" of a class and "complexity" of the rules.]  The idea, of course, is to maximise the appeal of the class to the club skipper, to encourage its increased adoption in countries where it has not yet achieved a significant position, and to minimise regulatory and measurement problems and overhead.  I now think that the IOM class association should introduce an IOM design which is a strict one-design hard-chine hull (the Boxkite is on offer from Graham Bantock) with materials restricted to wood only in both hull and appendages.  This kind of IOM would use "normal" measured IOM rigs and sails, but ball-raced goosenecks or jib swivels would not be permitted.  Using an idea of Charlie Coventry, whenever this kind of design races with other IOMs, the other IOMs would have 2 points added to their score in every race of that event.  We might call this kind of IOM the BOM -- the Box One Metre and provide it with its own class rules.

Following this idea, and taking it one step further, is to introduce a "manufacturer" IOM.  This design would consist of a complete kit of materials and fittings, needing only radio control to go racing.  Its rules would be very simple -- you can use only what is in the kit, and you have to use it the way the kit instructions say.  This would be the MOM class, and it would have its own class rules.

My second thought a while ago was to improve the standard of sailing at IOM events.  We now have umpiring at International events, and this I think is an excellent development.  So my second thought now is to give experimenters and tinkerers a class to play in as well -- a "Marblehead" IOM, if you like.  So I think the IOM class association should introduce a kind of One Metre, where there are no restrictions on materials or fittings, while keeping the "normal" IOM length, weight, and overall draught limits for a mono-hull, and the "normal" IOM dimensions for the sails.  Everything else is "free".  Extending Charlie's idea, every such "unlimited" One Metre would have 1 point added to its score when racing.  We might call this kind of One Metre the FOM -- the Free One Metre, and again provide it with its own class rules.

While the BOM and MOM could measure as an IOM, the FOM obviously would not.  The FOM's attraction is that it conforms to the IOM "core" idea -- a minimum displacement in a one metre length with standard size sails -- while allowing itchy fingers and enquiring minds to enjoy freedom...  Neither the BOM, MOM, nor the FOM would be "International" classes, but who knows what time might bring us if boats were built and raced in these classes around the world?  It might be worth being clear that none of these ideas actually affect the IOM class and the IOM class rules -- these continue exactly as they are.  If you are interested in reviewing and commenting upon the proposed BOM, FOM, and MOM class rules, e-mail me and I can send you a document.


2011 Lester Gilbert