Re: 1973 Chrysler Fin & Fun Rebuild
To use sheet and expanding foam together:
Install the sheet foam pieces in the deepest part of the foam pour, allow 2" between the top of the foam and the bottom of the decking for coverage of the sheet and 3 or 4" between each piece of sheet foam.
Bond the sheet foam to the hull with a foam compatible adhesive like this:
PL300 10 fl. oz. Foamboard VOC Adhesive-1421941 at The Home Depot
... or mix up very small batches of expanding foam, pour and place sheet on top of expanding and hold in place until expansion stops and then move to the next piece.
To foam your boat without drilling holes in your nice new decking:
Use a sacrificial sheet of plywood (an old beat up piece you can get for cheap or free if you can). Drill pour holes 16" o.c., lay on top of stringers and fasten or weight down well. Pour foam and wait for expansion to stop and foam to cure a little, remove sheet and place on another section of the boat and pour again until all foam is poured.
You'll pour your boat in sections (probably 2-4), start at the stern and pour forward since this will require less cutting of the sheet of plywood to fit the hull at the beginning. Trim the sheet of plywood to fit the hull as you move forward.
You'll notice there will be voids (craters) in the foam like this:
These can be poured with expanding foam making sure the foam rises above the flat foam and cut off flush with the flat foam with a carpenters handsaw.
This will give you a nice flat void free foam job that you can cover with nice new decking without holes in it.
You can paint or seal the top of your foam with epoxy resin for additional waterproofing before installing your decking.
Safety tip:
Expanding foam can be dangerous:
The do's and don'ts of expanding foam - YouTube
Be careful!