Phil,
If I said I knew exactly what I was doing my nose would be a yard long. I'm posting these drawings to share what I'm doing, rather than encourage others to do as I do. If my explanations make sense to what you know of sailing and rig tune, then feel free to use or adapt anything I've done. Chances are that some of my plans are more ambitious than good sense, so I'm not the least offended if you think I'm nuts.
How I got to my rig design: First, I wanted typical sport boat tuning characteristics, as you get from your typical swept-spreader, I-point shrouds, and nearly balanced (round tube, or close to it) mast section. But I also wanted to see if I could improve the aero of the round tube a little, incorporating an updated version of what Bethwaite did with his Tasars with spruce masts and balsa fairings- the spruce was nearly square or round so when it rotated it didn't get all stiff in the wrong direction like a solid wing mast does, and the balsa was light enough to be useful enough to justify its weight. That class has moved on from spruce masts, and Bethwaite's ideas lost traction. They sorta struck a chord with me though, so I wanted to have a go with today's materials to see how it could be made to work.
So the hounds high/I-point shroud that goes over the lower spreader (in green in my pic) is the string that will be adjusted with turnbuckles at the dock (or with difficulty on the water)- it sets the base prebend and rake. The gnav stay is probably just going to be set "a little more than hand tight" once the hounds spreaders are set. Then, with an adjustable forestay, pulling the forestay on will pull a little extra bend in the lower mast flattening the middle/lower part of the main and hopefully if the mast doesn't go noodley it will also increase headstay tension.
The blue topmast shroud (is that what you are calling the backstay or runner?) is also on-the-go adjustable, so I can play the upper part of the main shape upwind, as well as setting the point where the top mast falls off in response to a gust. I lead it to the base of the mast instead of the chainplates so that I could more easily adjust both sides with one tackle, and I think I can reduce the rotation friction with the geometry setup this way. I can also vary the point along the lower spreader where the topmast shroud bends, making a further tuning opportunity to either pull or push on the mast at that point. I honestly don't know which I will need more of at this point! I'm learning (from others, I haven't sailed one myself) that fathead or square top mains like stiff masts. Having a topmast shroud greatly stiffens the topmast (duh!), but also allows you to adjust the pressure where the topmast begins to deflect. With no shroud, the topmast is a simple spring cantilevered from the hounds. It will deflect in direct proportion to the pressure applied. If you add a stay and tension it, then the mast is "pre-stiffened" so that it does not deflect (much) until a higher pressure threshold is reach, and then it does deflect, to aft and leeward along the arc of the stay that is in tension, opening the upper leech of the sail and helping the boat go forward instead of heeling or staggering. That's ye olde "gust response". Somehow, it just seems like it will be hard to find that correct tension with a turnbuckle versus having the string in your hand and pulling a little on, looking, easing a little off, looking, etc. And if you know you'll have newbs for crew, it means far less instruction and hassle to get it set up.
As for the DM-4, here's how they compare to a Forte's carbon mast (I don't have the I values for C-Tech, which also has a 70mm/2.74" diameter but has a .10" wall thickness so should be similar to slightly less I value):
DM4.png
FORTE 2740.png
The DM4 is likely to be very similar fore and aft to the carbon masts, but a little softer side-to-side. I suspect this means you'll need to use a little less tension on the shrouds, use longish spreaders to help your geometry (increase the staying angles reduces the needed tensions), and set your chainplates as wide as you can. And do install the 2nd, I-point spreaders!
Some online references:
For classic sportboat rig tuning:
http://www.tboat.com/Editorials/Edit-de ... t-rig.htmlSome other whacky options:
http://www.uk-cherub.org/doku.php/tech/rigging