Hi Neil,
Hope this may be of interest, I have recently watched my cousin (a retired expert pattern/model maker
specialising in car prototypes for the likes of - Jaguar, Bentley, BMW, Volvo etc working all around the world)
produce his first model boat - a 28 inch scale round bilge model of his own full size 28 foot yacht,
by his preferred method of polyester glass over foam and I found it a revelation !
The ease of construction and speed the hull went together was amazing, bread & buttered from 2'' thick layers,
and I (traditional ply bulkheads & skins man) will be using his method wherever possible in the future,
I'm the one thinking about a huge 1/6 Atlantic 75/85 on FaceAcheBook,
and will be going down this route if I build the model.
I don't know the brand name but it's bright pink or bright blue builder's insulation foam, very dense 'closed cell'
foam that carves beautifully with a brand new Stanley Snap-Off knife blade protruding 4 inches out of its handle,
or easily sawn with a decent household bread knife ! (2 inch thick 8x4 sheets).
For the barrier layer he uses, what is industry standard in his profession - 2 layers of glossy 2'' brown parcel tape,
covering the entire up-turned hull and leaving an inch of excess tape overhanging to prevent any resin creeping up
attacking the foam.
Then he layers up in Polyester and matting, much sanding and P38, and on to paint - no need for moulds etc,
fastest boat building I've seen to date, saves all the hours of wood working/mould production for one off's.
When discussing how to build a large model tug hull, he suggested cutting bulkheads out of thin ply or
possibly 1/4'' Depron foam (as used by Aeromodellers for small profile R/C model wings etc),
and infilling between with pink/blue foam blocks.
When asked how to make the foam blocks the correct size to fit between the bulkheads,
he showed me his bench pillar drill with a lashed up with a horizontal sanding disc mounted in the chuck
used to machine the foam sheet to thickness, then laminated to form blocks etc,
or length ways bread and buttering with 2'' sheet laminated onto a full depth solid ply keel.
A different way of thinking ! and I think someones just posted a similar reply while I been typing this.
cheers Paul