Hi Dan,
It look's like you have something under the magnets (and some sort of 'bump-out' at each location), between them and the magnet rotor. just curious what that is?
DaveB, who's pretty regular here has had quite a bit of experience with water heating applications. It changes the design parameters quite a bit from battery charger. He might be a good source of information, and has posted some of his experiences here also.
good luck with it,
~kitestrings