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Old 9th October 2009, 11:30 AM   #6
Roger
Dream Team - School Guru
 
Join Date: Aug 2006
Posts: 2,106
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Hi again PH68,
I'm not sure what brand your 11.0 m2 CR R2 sail is.
I'd like to see the specs and see how it rigs on the sailmakers website.
As far as the "seam" I'm referring to, down the front of many 2 piece luff sleeves the material is sewn together right down the front of the sail, making it stronger and helping
the designer get just the right luff curve.
On sails with a one piece luff sleeve that is only sewn at the back where it connects to the other panels in the sail, a crease develops when the sail is rolled up for shipping and when you roll it up after a session for storage in the sail bag.
Regardless of whether you sail has a one piece luff sleeve with a crease, or a 2 piece luff sleeve with a seam, there is almost always some indication (seam or crease) as to where the exact front of the sail is when it's laying on a flat surface.
When you push your mast in the crease/seam remain flat except at the very top and bottom of the luff sleeve, until you begin to add some downhaul.
Then the crease/seam will pull back against the mast curve as the mast bends to the luff curve.
Sailmakers use "broad seaming" to put lots of shape into the luff curve to make it compatible with the mast bend.
As you add more and more downhaul, the crease/seam at the front of the sail will pull back, and then begin to rotate around the front of the mast.
If you don't use enough downhaul, the perimeter tension (around the outside edges of the sail) and the panel tension will be too low and the front of your battens will tend to kink or knuckle due to the back end of the cam being rotated too far out to the side on the leeward (convex) side of your sail.
As you add downhaul, the cams will rotate futher until the luff sleeve profile is exactly what the designer intended.
This will give you the optimum draft for power and at the same time give you the correct amount of twist up on the higher parts of the leech so your sail handles well and is responsive to gusts by having the upper leech "open up" slightly.
So, by looking at the seam/crease, you can tell pretty well whether you have the correct amount of downhaul to optimize the power and handling.
Not enough downhaul lowers the tension throughout the sail and results in cambers that are "under rotated".
Too much (well beyound the additional 1.5 cm you are adding I would think) downhaul will cause the cams to rotate further around the mast (rear of the cam moves in toward the centerline of the sail) due to too much tension throughout the sail.
This can also occur if you use too much outhaul tension as both too much downhaul and too much outhaul can draw your sail out too flat, reduce it's draft and power and make it feel heavy and gutless.
Since even the mfg's "best" suggested mast can vary slightly due to manufacturing tolerances, you need to really pay attention to when the sail is loaded uniformly throughout and the cams are rotated to the designers specifications.
This means on some sails you have to stay under the designers recommendations for downhaul as your mast is a little too soft, and conversely if your mast is a little too stiff you will need to add more downhaul to "spread" the tension and get the battens and cams rotated optimally.
Hope this helps,
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