I'm having a S-bend in my mainsail when I pull the mainsheet hard enough, so there is no twist in the mainsail. the S-bend goes from the bottom of the sail up and over the second batten. I d' like to get rid of the S-bend as I 'm loosing allot of power this way.

would stiffer battens help? the battens I use are from fiberfoam (Scott Anderson)

I'm having a older sail with the same battens, which dosn't show this effect at all.
What could be the reasons for the S-bend?

thanks for any help and tips!