More balance questions

I have designed and built an IOM. It is very well behaved and seems fast but it tends slightly toward lee helm. What I mean by this is that in light winds I can’t get the mast/rig back far enough to get rid of the leehelm. It’s close, but I would like a little more adjustment. My problem is that the mast tube is already right up against the keel trunk. With this boat I can rake the mast way back and deal with it. What can I do on the next boat(same design) to correct this? I don’t want to use a deck stepped mast and IOM’s have a fixed sail plan. I suppose I could notch the keel trunk. I’m just wondering if there is a possibility that moving both the mast and fin back farther might solve this but I’m having a hard time getting my head around this. Any thoughts would be appreciated. I designed the boat in Hullform so if any centers etc are needed I can provide them.

hi don
my first IOM had the same problem… what i did was strap 3 mast tubes to the keel trunk… with all weather conditions…you are going to get different results…so my solution was to use the 3 mast tubes untill i figured out what i wanted…i could move the mast up to an 1 1/2 inch forward… of the fin…you cant move the fin unless you do surgery. where is the leading edge of the fin. is it at 50% or 45%. the best thing to do is experiment. my first 3 IOMs were junk. but i learned alot from them. you got a pic of this new IOM? what are the numbers? there are alot of books out here that can help you understand. I like principal of yatch design

this book helped me alot… i had to read it 3 or 4 times… but i helped

Hi Lloyd
I already have a multi position mast tube. My problem is that I can’t move my mast BACK far enough to correct the lee helm.

Hi Don,

There is nothing unusual about your hull. IOMs have a ‘one design’ rig, so the ‘lead’ should be very close to other IOMs. So you should not be getting lee helm with the rig further back than anyone else. I suspect the problem is not the mast position, but the sheeting position of the main. Think of the boat as having a pivot point. Sailing close hauled, the jib and main both contribute power. If the boat is out of balance with lee helm, then the jib is pushing more forward of the pivot point and the main is pushing less.

It could be that the main is too open and not contributing enough, but I suspect that the main is in too close and is stalled (also contributing less).

Let’s swap boats next time we sail for a second opinion.


Hi Don and all,

I am probably going to cover some old ground here but the alignment of centers as the boat heels determines the sort of behavior a boat exhibits. I have no experience with IOMs but it is an exhaustively pursued class of craft so some of the above advice I’m sure is helpful.

In general the center of bouyancy of a hull should stay in the same place in the waterline plane as the boat heels. If this location moves forward as the boat heels then lee helm can be expected if it moves aft weather helm is expected. My teacher, the late George Bursuch, preached never to have the C of B move forward and to limit the maximum shift aft to less than 1/8th of an inch. This was for Marbleheads and his experience was mainly with vane era boats but his guidelines are valid today. I would think that the maximum aft shift for IOMs would be less.

If you have such lee helm that can’t be cured with mast placement then go back to your computer model and check the heeled center of bouyancy at 10 degrees of heel and 20 degrees of heel and see where they are located. If they creep forward of their static location then the hull is unbalanced and has to be redesigned.

Since you are having trouble getting your boat going at all one option that may help but requires some surgery is to remove the aft part of the deck to section 4 and spread the hull slightly at sections 6 and 7. This can be done by cutting through any deck beams in the area of sections 6 and 7 with an xacto saw and inserting a couple of shims on each end to spread the sides a bit. This will shift the heeled bouyancy aft slightly and should help resist the lee helm tendency. Don’t go too radical with this adjustment, you don’t want the boat to go up into irons all the time instead. Make sure that the new shape is fair and smooth before re-decking the boat. This has worked for me in the past when I made an error with the math (yes some of us pre-date computer programs for yacht design and did the area measurements the old way with a planometer).

Good luck and I hope that you get it all sorted out.

BTW - Since you are in the Vancouver area get in touch with Bob Stern, he may be able to give you some expert advice since he has designed several IOMs of note.

Thanks Niel
The boat isn’t bad. It is keeping up with the local fleet quite nicely. As John suggested my tuning could be the problem. That said I always like to have a lttle more adjustment than what it appears to need. I like to be able to adjust past the problem and then back up a touch. I checked Hullform and my COB does move forward, 1mm at 10 degrees, 1.5 at 20 and 4 at 30 so a little tweak seems necessary. But something I discovered is that somewhere between pulling the mould off the plug and pulling the hull out of the mould, the mould had closed up like a clamshell. The beam was supposed to be 184mm and closed up to 180mm. I’m not sure how that happened as I put a strongback on the mould to stop that very thing. Anyway, that has been corrected.
Bob Sterne has retired and moved to the interior of the province. Last I heard he dosen’t play with boats anymore.
Thanks for the info

I thought Earl and Bill were of the opinion that the CB should move forward and that the waterline plane should move with it for good balance. BTW I found it very hard to get the CB to move forward with wide transoms. Those all move aft quite a bit which does make a boat round up badly.