A conversation with my brother the other day got me thinking about how the IOM rig should work. We both used to sail on dinghies and yachts. Mostly we would sail on fractional rig yachts where the bottom of the mast was controlled by the shrouds, spreaders and runners. The bend was fixed up to the hounds and you increased or decreased that bend, using the runner, however once set the mast was fairly rigid. The trick was to have the mast work for you above the hounds on the un-supported section of the fractional rig. The ideal scenario went like this. If you hit a gust the top of the mast head would bend, flattening the upper part of the mainsail and opening the leech so the boat could accelerate into the gust. As soon as the wind eased the mast would straighten and power was restored. We once sailed on a boat which had the balance of the rig exactly right. It meant you could carry more sail in greater breeze and gave a massive competitive advantage. The boat won a lot of key offshore and inshore races.
However the IOM rig is more like a masthead rig on a yacht. The forestay and backstay meet at the top of the mast and the bend and therefore mainsail leech control is managed entirely through adjustment of the runners.
Of course you have other adjustments on yachts which are important, eg mainsail foot, cunningham, etc, all of which have to be adjusted through the wind ranges, but in this article I am just focusing on mast bend and impact on the mainsail.
The IOM rig is somewhere between a masthead and fractional rigged yacht. .Our forestay sits above the hounds with a backstay at the top of the mast. The geometry is set up so that with prebend build into the spar, straightened out by the backstay, tension is put into the luff and leech line of the headsail. The position of the jib swivel line to the deck ensures that most of that tension goes down the luff of the jib and not the leech line. Mast bend and therefore mainsail leech shape is controlled throughout the mast ram, spreader rake and tension on the backstay. Jib leech tension is controlled by the leech line.
We want a rig setup that will give a little in puffs so the boat accelerates and drives, rather than heels and stall. So how can that be achieved.
If you read all the key advice on rig setup, you tighten your shroud tension just enough to stop the leeward shroud going soft when upwind. This allows the mast to flex a little in puffs providing acceleration. If the mast is too rigid, airflow will stall and the boat will not accelerate in the puff.
Start your boats setup with the boat pointing as though on a run. Trim the back stay so the mast is straight fore and aft. Set the mainsail foot to a depth of 15mm or whatever your sailplane suggests. Set up the kicker tension so the mainsail leech is slightly twisted with the top batten just outside parallel to the main boom. Check both gybes to make sure the mast is straight vertically. The twist should be the same on each gybe. Then point the boat as though on a beat.
I assume you have set the rake as per the boat plan using a measuring stick or tape measure.
Our goal is to set the mainsail so the top batten is parallel to the centreline of the boom by adjusting the backstay and mast ram. Once set up there should be little need for change through the wind ranges other than 1mm tweaks on the backstay. In a recent zoom meeting with Brad Gibson for the Central Park MYG in the US, Brad talked about adjusting the backstay by plus or minus 2mm. I don't know about you, but in the past I was slightly more aggressive about the use of the backstay. Now I understand more about the precision of the setup and know what the top guys do, I am more careful.
With any luck if you have done all this you will have a perfect looking rig, and if you have not overdone the shroud tension, the rig will work for you in the gusts.
There is enough information available on rig setup up on the web, that you should be able to achieve the right setting first time and more important, recreate that setting every time you go sailing.
Here are some pictures to show the impact of 1 mm changes on the B rig backstay.
Start at the bottom and work up the set of pictures. They show the prebend built into the mast and then the gradual impact of up to 5mm of addition back stay. You can see the effect best on the softening of the leach. Try this on your own boat and see what your rig looks like. Ignore the setting of the jib as the leech is too tight and the boom could be eased out somewhat. Next time I try this I will set the camera up on a tripod so the angle doesn't change between pictures. I will get a similar set of shots for the A rig tomorrow.
Mast with additional 5mm adjustment on backstay with kicker adjusted to set top batten parallel to boom
Mast with additional 5mm adjustment on backstay
Mast with additional 4mm adjustment on backstay
Mast with additional 3mm adjustment on backstay
Mast with additional 2mm adjustment on backstay
Mast with addition 1mm adjustment on backstay
Mast set up straight with top batten parallel to boom.
Mast stays with no tension showing prebend