Here is the picture of the sail with the s-bend.
Pic of sail with s-bend 300kB

first I have to say that the sail is specially designed for light winds. it has not allot of luff curve and allot of shape build in. It really seemed to me, that the luff curve didn't match the bending characteristics of the mast.

We experimented with the prebend of the mast. we started with 7 cm prebend after the recommandations of the sail maker. the more prebend, the more the mast will bend at the spreaders.

the more we tightened our diamonds the worse the s-bend.
we finally found the right setting, with only 3-4 cm prebend, witch is not much for a mast with 10m lenght.

the hinge, bill was talking about, was there, but not to the top of the sail, it wend from the clew to the spreaders.
that is wy the upper part of the sail looked good.

thank you all, for the usefull tip's! It really helped us thinking!

Pic of sail without s-bend 300kB