Stuart,
I have no expereince with hyplon on a Folbot. However, on other boats Dyson Biadarka's I have been associated with (friend built) Its not the stretching that get you a smooth surface. That will work with other fabrics, but not hyp. Generally you have cut the Hyplon into panels that fit the shape of the boat and stitch them together. after you get a few stiched together you pull the other seams together and cut them back fold them over and stitch them good and tight. You'll get a few loose spots but you cut them back and pull the pieces together.and stitch them just as you would a dart in a pair of trousers you intend to take in.

Intersting project Good luck.

!RUSS