I installed bulkheads in the Ganymede that my daughter built from plans and highly recommend them for safety reasons. I did not add hatches, however, not wanting to mess those gorgeous decks. Instead I put access ports in both bulkheads. Photos and description at http://twofootartist.com/ganymede-construction-notes/

As for the fiberglass, I just pulled it off the rolls I had on hand and didn't measure, so I can't help you there. As I recall, we used 4 oz. on the deck inside and out, and 6 oz. on the outside of the hull, but only on the seams inside.

-Wes

Looking back over my notes, it appears that we fully glassed the inside of the hull too, but I don't remember what weight we used.

This is a terrific design, by the way, and performs far better than you might expect from such a simple hull.

-Wes