5500rpm and 12 knots? Doesn't sound right at all. Usually, if you are unable to get up on plane, the motor will bog down well into the 4000's, probably even lower (don't ask how I know this.)
Does the speed (as low as it is) seem to be linear with the rise in rpms, or does the thing accellerate to 12 kts and stay there while revs rise?
I'm thinking possibly a spun prop hub. If you have a buddy with the same engine, see if you can borrow his prop for a bit.
A waterlogged hull is a possibility, but I would think that if the overall weight was the problem, the revs would be much lower.