Randy, the yard used coosa board specifically because it could bend a bit in one direction. They ran the boards laterally (athwartships? Is that the term?) so captured the curve in the deck nicely. I’m told that the coosa does not bend the other direction.
You might check out Sail Life on Youtube. Mads replaced the damaged core on his deck with foam (front half of the deck. He used plywood down the side decks). He explained his reasoning for doing so (cost was a concern, but weight and strength were big factors as well). He is pretty rigorous about sealing his thru-decks.
Coosa, being foam and fibreglass (“high density polyurethane foam with reinforced layers of fiberglass,” according to https://coosacomposites.com/) won’t rot or delaminate because of water infiltration, but obviously anything you can do to keep the water on the outside (such as butyl) will help, and sealing the holes is a pleasant time when it is warmer.
b.
Brian
SV Serenity
Nonsuch Nereus #003
Pax River, MD