I have had a good play with sail material and trying to stick vinyl on it or print on it. But as said before, it has been designed so that nothing adheres to is..
I got in touch with people at Sericol inks who did some samples for me.
The only way to get the ink to stick was by treating it with a solvent first (stripping the waterproof chemical). I did not feel comfortable with this so I have walked away from those kind of jobs. The sail maker I sometimes work for uses the product that Chris Wool suggested, but after sticking it he uses the sewing machine to keep it in place permanently. It goes trough the cutter no problem…..Good luck with it, and if you find a succesful solution.. let me know 😀