Main sail design seems a dark art with the sail makers using terms of seam shape and luff curve. Now some say they are luff curve fans and others seam shape ( we are talking crosscut sails here ).

Now I can get my head around seam shape which will give a series of slightly " bulged " panels which when seamed together will give a fixed amount of shape to the sail. Now this will obviously give some depth to the sail at the point where the designer thinks appropriate.

Now add in luff curve, and this is what I cannot get my head around. Anybody offer a good description of what the luff curve does and how depending on how the mast is set ( diamond tension ) will effect that crosscut sail.

Now add in downhaul ?