I think he said in one episode that this is his third boat? I am pretty sure they have left it as a mystery how he gets it out.
My wife says he tears them down and builds a new one. He does it for he release, now for having a boat. She saw it in an episode.
I have a theory. He's just a TV character on a set and doesn't really build boats in his basement. Nah, that would be too simple.