Great model, Ulco!
A tipp: some time ago I was modifying a sailing boat model, and I wanted to blow the sails like you're planning to do.
First I wanted to use a cloth modifier as well, but then I decided to make it easier. I selected the inner sail object's vertices and used soft selection, adjusted the falloff so that the outer parts were unaffected. Then I stacked a noise modifier on top of that. Works quite nicely for animations as well, and doesn't take as much time as a cloth simulation.
I don't know which 3d app you're using, but this should work in any program. Hopefully.