To IOM or not to IOM

I’m considering building an IOM. I now have US1M and I like the simple rules. I’ve glossed over the IOM rules and have a “few” questions. If someone would like to help me I would appreciate it. The rules I am reading are here

C.7.4.a Lower point to deck limit mark 60 to 100 mm.

Is this saying that the bottom of the mast can only go 100mm below the deck? Why is there a minimum? Is deck stepping a mast illegal?

C.8.4.a.1 A sail of one rig can’t be used with another rig.

Does this mean you can’t use your ‘a’ main and your ‘b’ jib or other such combos?

D.1.5 Just what is this"Deck Limit Mark" all about?

That’s just a start, their may be more


Hi Don,
Hope you do go on to build an IOM and join the thousands all over the world who have ‘built’ the class. The IOM rules are really all common sense, but they are sometimes documented in such a way that questions are inevitably asked.

Ref.C.7.4 a this is a measurement tolerence of 40mm for the distance between the selected “Deck Limit Mark” and the lower (sail) limit band on every rig which is to be used on the individual hull. The idea is that every hull has a selected ‘spot’, be it where the mast emerges from the mast box at the level of the ‘Well’ deck on a skiff type hull or the position of the Mast Step on a flush deck or by choise at the highest point of a ‘Peaked’ deck where the ‘partners’ are located. Every rig on the certificate for this hull must then have its lower sail ‘Limit Band’ within this tolerence. This rule prevents say two No1 rigs being available, one with the sails positioned as low as possible for top of the range winds and another rig having the sails positioned away above the deck for tickling along in whisper conditions where the wind gradient gives such a rig an advantage. 40mm tolerence still gives plenty scope.

D.1.5 this is simply a ‘spot’ of paint about 5mm in diameter at the chosen point on the deck so that, as and when there is a measurement exercise, the measurer has a reference point for the foot of the vertical measurement up to the lower (sail) limit band.

You have correctly assumed that sails of the three different sizes of rigs cannot be mixed.


Ralph Knowles
(Senior Measurer Scottish District MYA)

Thanks Ralph
I think I understand but “Lower sail limit Band”? Can’t they just measure to the bottom of the sail? I’m not trying to be a jerk, I’m just making sure I’m clear.

Hi Don,

As in most full size dinghy and OD keel classes, where there is a measurement regime, there is usually sail height restrictions. In the IOM, when building a rig, it is usual to start measuring from the top band which is actually the masthead fitting, usually black ‘Delrin’ with an exposed height of about 3mm. Below this is an intermediate band, above which the extended line of the forestay must not cut, and then there is the lower limit band below which the mainsail must not come. So the full ‘fall’ of the main must be between the upper and lower bands and the headsail cannot be attached above the intermediate band.

Hope this clarifies the situation.



Hi Don,

I hear that you may be at Long Lake this Sunday. If so we will finally get to meet! I’m looking foprward to it and will be happy to talk about the IOM rules; and sailing in general.


Hi John
Maybe next Sunday, I’m all tied up tomorrow.

Are checkstays the same as lower shrouds?

Do I have this right? If you are keel-stepped you can have uppers, deck-stepped you can have uppers and lowers?

In the IOM, the item called a checkstay is set very low on the mast and provides an effect similar to a mast ram. As the height above the deck limit mark is limited, there is now way that a checkstay can be used as lower shrouds.

In IOM, lower shrouds are not allowed.

If building an IOM it is very important to have a lower mast control to resist the forward force of the vang/boom, and also to straighten the lower setion of the mast (forcing the bend into the upper mast).

The easiest way to do this is to install a raised foredeck and inset a mast ram. This is easier to install and control than checkstays or an external (above-deck ram)

Hi Don,

This is right off the top of my head, and I will stand corrected if I am wrong.

Check stays are not lower shrouds, as in full size yachts, where the stay has an upper termination just underneath the spreaders. On IOM’s and other deck stepped classes, check stays are limited in height above the deck. On the IOM, I think they are limited to 100mm above the deck limit mark. Their function can be equal to the operation of a deck strut which is fitted forward of the mast, and which gives an aftwards push to the mast to keep the lower half straight when backstay tension is increased to give forestay tension. On a keel stepped mast, the mast ram provides this function. Check stays are installed such that they are run outboard and aft of the mast to additional chainplates or deck eyes, and are individually adjusted to pull the mast aft, using small rigging screws. Hope this is not too complicated a description.
By the way, Don, where about in the Island do you live. I am asking because my wife spent six years of her childhood living in Victoria, and when I told her I was posting information to an Islander she insisted I ask. The last remainder of her family still live in Sydney.

Edit: Looks like John got in before me. Drat!! Too slow!



I live on the Island in Parksville (about 145 klms north of Victoria) and Don is in Cambell River (about 275 klms north of Victoria). (and we have not yet met!!! But soon!)

Thanks to both. I had a feeling towards the way you described but I thought I’d better check. It’s obvious now that keel stepped with a ram is the way to go as long as you know for certain where the mast will be. Unfortunately I have had problems with this but I’m closeing in on the solution.

Ralph, I was born and raised in Campbell River and never left, been here 60 years now. My son lives in Victoria. Theres a large model sailing club there
I get down a few times a year


Between you and John you have about covered the answer.

There is a good sized portion of our West Coast IOM fleet located in and around your wife’s old stomping grounds. I am located in Victoria and our club sails at least a couple of times a month at Beaver Lake which is about halfway between Victoria proper and Sydney. So now you have a reason to come visit someday!!!

-Barry Fox

I agree on the ‘keel stepped’ and ram.

From my observations, most IOMs seem to step their mast about 10 to 15 mm ahead of the leading edge of the fin. As the sail plan is ‘fixed’ by the rules there does not seem to be a lot of variation from one hull design to another.

A good solution for the mast is to build an oval/rectangular tube placed immediately in front of the fin box. The length of the oval should be about 30 mm.

Make a shoe out of a flat piece of plastic or aluminum to fit in the bottom of the oval tube and drill about four holes at 5 mm along it. Glue it in the bottom of the mast tube.

Put a plug in the bottom of the mast with a pin in it to engage in one of the holes.

Now you can move the mast heel forward and backward from hole to hole in the tube until you get the right amount of ‘helm’. Then use mast rake to fine tune.

The pin in the hole should hold the mast heel in place to resist the pressure of the ram.