Quote -      Glad you got it working.  ^^

     If the sails were made as a thin box which was then curved to shape, that would explain the interference between front and back.  I now realize that MarianneR had an excellent idea, using displacement.   Brava, MarianneR.

It may just be polygons that are simply copied and normal flipped, I'm fairly new to modelling so not certain of course...but I can't see any sign of polygons forming any depth or thickness to the sails... even minimal ones.

Selecting the faces on the backs of the sails (with back oriented normals, apparently) and deleting them, still left an intact layer of faces, with front oriented normals.

Seachnasaigh, how did you model the sails on your skiff, out of interest? What's the correct / best way to do it for Poser? e.g. one-sided, or with some thickness?

I will try out that displacement trick too... it would be interesting to see if that did work with the model in its original state.

But removing the polygons is a definite fix for rendering with IDL lighting turned on ;-)