Unless the sails are large (M-class, 10-Rater and beyond) sewing isn't the best way, usually only doublesided tape, as used by sailmakers, is sufficient.

I only zigzag the taped joints because I sail my boat in too much wind (or with too much sail) to keep the seams together.

The outside seams are just cut with a hot, sharpened soldering iron to stop it from fraying.

The more you sew, the more baggy your sails will end up...

Regards, Jan.