I have a singer20U machine which is a "professional" rather han" industrial " set up . to sew sails I use a size 36 polycotten as my thread and size 140 needles.
most sewing shops only have needles to size 90, butif you get onto a wholsaer the needles come in packs of 100 for around $20.
Ive been told sailmakers thread starts at size 46 for light spinakers.
I rarely have the stitching come apart . All the sails Ive cut down have been for landyachts, but they still come under a lot of strain. monofilm is trickier to work with, so I would practice on older dacron mylar first.
it does feel great when a recut sail comes out well , AND performs