: Don't know how you would ever get the mandrel out with a cigar shaped kayak.

Cut along the sheer line, or along the keel line and center line of deck. You get two perfectly matched pieces which you have to join back together -- but that is no worse than the "conventional" process of casting two separate pieces and later joining them. In fact it might be easier as you could put on as many (temporary) alignment marks as you wanted before cutting the thing open, and you would have certain knowledge that the parts DID fit properly before you cut into it.

Just a thought

PGJ