To share more info. Two stringers in foredeck, 3 in main deck. As previous 1 in rear deck. All 12 x 10 in the vertical. Foam b/heads have a 10mm topping, glued and filleted to the gunwale, glued only onto the foam, no fillet. Ply frames have 1 'topping' glued to the side of the ply. Study photos.






I shaped the deck curve into the toppings, then ran a 10mm router bit through them to cut the housings for the stringers. Set up a 'straight edge' for this operation, finished in 20 minutes, all sections, both hulls. Stringers dropped down into their housings to the correct level, no levelling required except for minimal sanding with a torture board.




The photos show breather holes at the top of the frames, particularly for the foredecks' foams. My experience is the moisture condenses under the decks and airflow is improved if gaps for flow is up where the moisture is. Bottom moisture flows out through the limber holes and can be sponged out, the condensation remains.
Another reason for moving the holes up to the deck, I discovered the polystyrene foam lost it side strength with holes in their centre and they flexed in when compressed; and when one considers these forward foam bulkheads are there to stop the hull sides flexing and pumping. I have found the forward flat sections of the hull much stiffer on 1827 than 1765.
On foam; closed cell polystyrene foam is better then open celled rigid polyurethane. A coat of epoxy (It is compatible) will seal the styrene and with it being closed cell and sealed, will not take up moisture. Urethane will only seal with a layer of glass and the resin as with the foam sandwich hulls.
I used 'medium density' polystyrene foam, 25mm thick, light being too soft (popular with sign writers), high being too heavy.
Always willing to share my thoughts and experiences.