Getting used to the "feel" of a mylar main takes a bit of time, a distinct difference coming from dacron.
IMO, luff curve of the mast matching the sail is the best you can do, this should allow the sail to move freely in the track. Several VG sailors I've quized about this don't "play" the downhaul much.
I stole the following quotes from catsailor. Again, IMO, I think they're right on for any boat with a pre-bend mast.
I would say set the spreader rake (with min down haul lots of wrinkles) to match the luff curve of your sail = max power from that particular sail. Hopefully, the sail is cut well so the shape is constant above and below the diamonds
Charlie Ogletree (US Silver medalist on Tornado last games) told me to think of downhaul as a course sail trim adjustment and the main sheet as the fine trim adjustment. Leave downhaul loose (no wrinkles in main) until both crew are trapezing and the hull is popping out too fast to handle with about 1 arm length of mainsheet easing. Then start putitng more and more downhaul on, inch by inch, until you reach a point where you can mange the gusts using an arm's length ease/re-sheet on the mainsheet (the crew should be handling the mainsheet from the trapeze...skipper's one arm is not strong enough).
Once you're at this point, crew will ease in puffs to control hull popping high...but must aim to keep the hull out of the water at all times...so re-sheet the sail again BEFORE the hull starts to fall back down. When you get this right, the boat should surge forward with each gust, not pop out the water. It takes practise, concentration and quick responses from the crew.
If the winds are so high that you're still needing too much mainsheet movement with each gust (downhaul at maximum...we use a 16:1 system!...rotation aft of shrouds), start feathering the helm up to weather as gusts approach. This is preferable to travelling out when doing a windward leg...as you don't loose so much ground windward.
One point to watch out for...as you start applying downhaul...if you go too far too soon, you'll find you need to steer further off wind to get the hull out the water. Then as gusts hit, you will still pop-up because the angle to the wind is quite large...so you'll put more downhaul on...this can continue until your boat is sailing flat and slow, far off the wind with your main sail looking like a piece of plywood! So, make sure you are always at the minimum amount of downhaul needed and no more!
Sheet In...Max Out
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