This looks to me like a halyard/down haul tension issue. It appears that you have a ‘Continental’ rig with a Stack Pc or Lazy Jack system. It should be easy to pull on some down haul to tighten the luff of the sail.
Ric
Dadventure.
HBNY

Sent from my iPhone

> On Jul 18, 2023, at 2:59 AM, Graham Stewart <gstewart.gm at gmail.com> wrote:
> 
> My mainsail hangs with a diagonal crease that I can't seem to get rid of.
> The sail was new a few years ago and was purchased while I was restoring
> the boat - which included replacing the inner stay chainplates. I also
> changed the furling gear and afterwards it seemed that the inner aft stays
> and the aft stays were shorter than before.
> Might the rake of the mast create this problem or might it be a
> poorly constructed sail? I have attached a picture in case one of the
> expert sailors on the list can make a suggestion.
> Thanks,
> 
> 
> Graham Stewart
> Agile 76
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