I've now deleted the backward facing polygons, from the main rectangular sails, using Hexagon and re-exported the obj back to the geometry folder in my runtime.

This has worked perfectly for the sails that I doctored, when rendering with IDL... the artifacts are gone completely, no trace...

I notice that, if I turn the model round, the sails I altered are now invisible in OpenGL preview, from the back.

However they are perfectly visible from the back in an actual render.

Anyway, many thanks for all your help everyone... and to Laurie for nailing it ultimately :-)