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Instead of using wood blocks to keep the deck from sagging after the foam has been removed use PVC pipe. get a piece of 1" sched 40 and just cut them to whatever length needed. You can just leave them there, not a good idea to leave wood blocks in place. Now you need to think about how to keep the deck from growing a belly when it expands. No way to pour the exact amount needed. Either too much or too little. Some expansion holes need to be drilled in the deck before foaming. I don't think you are ready to start. Your plan is too sketchy. If you have any doubts you won't be successful. People have removed sections of the bottom to remove bad foam. Its another way to do it, but you already have holes in your deck, so why change direction in mid stream., and the deck which is thin in a 13 footer would still have to be suported if you flipped the boat and went through the bottom.