IOM numbers?

hi all…
got a question for all the IOM drivers out there
I have been drawing my own IOMs now for the past 20 years… and was at a meeting this past weekend…
OAL of a IOM is to be 1000 cm. ( or 1 meter) but the forward part of the boat has to be bumper…so does that mean the WTL can be no longer that 980? or does the WTL stay at 1000…
i even got confused… I have been drawing my boats for a WTL of 980… and a deck of 1000…never had a problem with it…nobody has argued with it…

what can you guys tell me what you have expenienced? or what do you read from the rules??.. 980 WTL or 1000 cm?


I assume you mean WTL is waterline length? (LWL).
The IOM rule does not care about waterline length. The front 10mm of the hull must be elastomeric materials. So the actual fiberglass/wood portion must be under 990mm. You can have the boat sit with the bumper immersed, so LWL is 1000, or you can have your boat sit with the bow slightly out of the water. The rule does not care.

Looking at many of the latest designs, they all seem to sit with the waterline about 20 to 30 mm back from the front of the bumper. As they come under sailing pressure, the bow seems to settle a little, but not to the point that the bumper is cutting the water. Pushing the bumper into the water seems to slow down the boat.



no part of an IOM can exceed 1000mm and the bow bumper must be at least 10mm, so it leaves you with 990mm for the actual boat as John says and as long as you comply to the other class rules like the 420mm from the waterline to the bottom of the keel then you’re ok


Recently I had my IOM measured and it just scraped in at exactly 1000mm, unfortunately the bow bumper only measured 9.5mm
So I had to shorten it by cutting of the bow, etc, etc. :rolleyes:
Now I plan for a 985mm boat plus a 12mm bumper.

i have drawn all my IOM to be 1000 OAL… and never had a issues… but i also included the bumper in all my designs… after a reggata last year. a few skippers were talking this. and it was brought up that the 1 meter . also included the lwl… so it got a few guys confused. have anybody thought about onsite measure? it would be easier to bring the boat to a regatta. and measure in the morning. those that have got new boats…

but just an idea