Thanks very much Danny, that's more like what wanted.

How did you get the two curves to align so perfectly. Try as I might I couldn't do it. There was always some sort of gap, but pretty hard to tell just doing it by eye.

The reason I ask is that now that the surface has been created, I'm not happy with it so I need to tweak the curves to get my fin more like the shape I want but they will come out of alignment again.

I also tried you curve setup in Viacad 9 out of interest and network surface couldn't be created.

So the current score is MoI 1, Viacad 0.

I think I better stick with Andrei's loft loose exact technique for these shapes, its less picky than network and tweakable.