Adjusting full length mainsail battens.

[Quandary]

Spent yesterday afternoon putting the sails back on the boat, the main has full length battens with about 10cm of adjustment at the cars. There is no taper in the battens so when you tension them the camber is the same for the full length and does not look very aerodynamic, I think a sail profile should resemble the top of an aircraft wing?.
Last year, I suspect I overtensioned them because in light airs I often needed to flick the boom to reset the camber after tacking, though the shape seemed ok and the light air camber did not look that deep. This time I just wound them in to take up the slack then screwed between 1/2cm. on the short ones up to nearly 1 cm. on the longest,(about 4m. long). At rest there is less than 10cm. of camber. To me the battens seem to be on the stiff side compared to tapered conventional battens but I have never had a main with full length battens before and I would be reluctant to try tapering them as they are already quite fine in cross section.
Is there a rule of thumb for a starting position for batten tension or camber, I have tried some sailmakers sites but while they tell you how to adjust them, they seem reluctant to suggest by how much.
I am aware that the more camber I apply, the harder it will be to depower the sail, if I ever need to stop.

 

[johnalison]

I've always assumed that the sailmaker who provided the battens with the sails knew how stiff the battens should be and had cut the sail accordingly. The instructions with my last sail were to start by tensioning the battens just sufficiently to stop the sail from showing creases and then add more tension according to taste. If this is your first fully-battened sail, you would expect it to be flatter than you are used to, but in my experience they seem to perform well, and my current battens are almost as stiff as a school ruler.

 

[simonfraser]

the deepest point of the sail should be 45% aft of the mast.
i had to taper one of my fully battened mains to make it pop from one tack to the other.

 

[Quandary]

Thanks, it is the 'according to taste' bit I am asking about. The batten screws allow for 4" of compression, and I am using less than 1/2" so far, which gives me about 4" of camber when at rest; the sail is by North Finland and is now four years old. I can not envisage the sail ever stretching that much, so there must be a purpose to all the scope for adjustment. I raced for years with conventional large mains on fractional rigged boats and we usually had the max. draught well forward of the centre of the sail so this shallow draught close to the middle of the sail just does not seem as if it would give the same lift.

 

[simonfraser]

measure the VMG with the current setting
thin the battens to change the camber
measure the VMG again
now you know if you've done the right thing
if you have, apply to the americas cup for a job as sail designer, if not get some more battens

i went through the same loop, there is no exact answer !

 

[Sans Bateau]

Mmm, know what you mean, I had the same problem, compress the batten too much and it will not 'pop' over when taking in light airs. I also found when set like this you cannot depower the sail enough.

So what I do now is take the screw up to the batten then no more than a couple more turns, that seems to work for me.

 

[Martin_J]

AS the others have said - it looks like they just need tensioning to remove the wrinkles. They are there to 'keep' the shape of the sail rather than to create the shape (as in a modern windsurfer sail)...


Interesting quote from the Ullman Sails website.

>>> These battens should be installed tight enough to just remove the vertical wrinkles or puckers around the batten pockets that appear while sailing. This tension will keep the sail smooth and not alter the designed sail shape. One note here is that the sail can stretch and the batten pocket adjuster can loosen while sailing. It is a good idea to tighten the battens each day before sailing in a multi-day regatta. You may even need to tighten the battens in between races on heavy air days. You can usually tell if a batten is too tight by several clues. The clues for full battens are as follows: a sail shape that is too full, battens that will not invert in jibes and tacks for the wind strength, and for a draft that is pushed too far forward in the sail.

It's only one view but it's from a sailmaker...
http://www.ullmansailssandiego.com/mainsail-batten-adjustment-techniques-tips.php
then click on the 'Batten shapes and tuning' tab.