Chesapeake - I don't know the extent of the damage in your floor, but if it's only a small area that's soft . . . you won't need to rip up the entire floor. You can remove the top skin on the area remove dig out the rot and glass the skin back down (make sure to bevel the edges and use fiberglass tape )
Here are some pics from another string off the classicseacraft site.
http://forum.classicseacraft.com/sho...o=&fpart=1
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For the hatch core -
Notice how the edges of the core are beveled and there is an area where the top and the bottom skin will meet and glue together. That's the right way to make a strong hatch. As far as what material to use . . . I think blocks of 1/2" marine ply would work great.
It's just when you use one piece . . . unless you drill lots of holes in it every 6 inches you're likely to trap a big air pocket underneath the core and that air pocket prevents you from making a bond to the core (also the more pressure you apply to the core the wider the airpocket spreads underneath the core)
