Mainsail luff shrinkage---acceptable?

This all came into sharp focus yesterday when a club-mate said the stitching at the bottom of his luff had come undone, so it was a question of re-stitching it. We've all talked about unpicking the stitching when the bolt rope shrinks, but then what? How far/hard do you pull the sail down over it?

The question thus became how to get the relative tensions right in the sail and the bolt rope, e.g. should you tension the bolt rope to straighten it out a bit and then pull the sail less-so, equally or more-so before stitching?

Does anyone have a sailmaker-approved method of doing it?

 I cant comment on film sails as the only ones I made were for windsurfers and didnt have a bolt rope.

As for nomal (dacron ) sails the bolt rope should be just a little tighter than the cloth but not by much, Its one of those dark arts.

As for resewing its out with the palm and needle and some waxed thread, sewing through the cloth and rope then round the rope and throught the cloth to keep it tight to the bolt rope where it goes into the mast slot.

Hope this helps as  its a long time since i have made sails. 

I always thought it was to give something to grap to pull the sail down

I have to start with the caveat that my experience of sailmaking is limited, but I did it pretty solidly for two years.

With regard to manufacture in most cases the bolt rope is made up separately with its dacron casing and is then stretched out by pinning to the loft floor. It is then attached to the unstretched sail. My understanding is that this adds fullness to the luff as the boltrope returns to its original length which can then be taken out as required by halyard tension/ downhaul. It's the elasticity of the boltrope that allows for this, I don't think that using prestretched ropes as suggested earlier would be helpful!

I'm sure that they do shrink or at least lose their elasticity. If there's a tail it's a relatively easy job to make an adjustment, however most boltrope replacements I have done have been because of the worn casing jamming in the track and not for shrinkage...

If you're lucky you can pull a new boltrope up through the existing casing whilst it's still attached to the sail - it does depend how tightly the original has been made, though!

Like dacron laminate cloth is also susceptible to UV, particularly if it has a kevlar scrim. The mylar film becomes brittle with age (read UV damage and flogging) and I'm certain that this also involves some shrinkage.