If I was to guess I would say it is a high density Foam. Could be 12 pound foam perhaps. It is called a couple of different things Like Davinicell and some others that are not coming to my mind right now but lets just say for arguments sake it is foam. If you have not seen the price on this stuff you better sit down it is NOT cheap. I did my transom in Marine Mahogony. The old transom lasted 21 years so all things equall the new one will last much longer. Either way it is WAY longer then I plan on keeping the boat
