Agreed on the engine pump, this is the most typical cause of low RPM over
heating and OK at higher. It isn't a displacement pump like the one on the
drive, it is a centrifugal pump and as it ages, the vanes corrode off, they
simply don't sling any water at low speeds then. End result is lots of
water from driver pump, however it isn't circulated through the block, it
just get poured out the exhaust. The engine pump is responsible to move it
through the block and heads.

Common issue on cars with high mileage also, overheat at idle, OK if revved
up a bit..

Greg

"Bowgus" wrote in message
...
Oops ... I read "water pump" as the engine water pump, not impeller. If
it
was me, I'd pull the boat out and hook up the muffs and garden hose. Could
be someone started the engine up with no water supply ... that burns out
the
impeller. Or could be the engine water pump, or risers blocked, or
thermostat, or .... could even be a faulty temperature gauge. Anyway,
running it on the garden hose should give some insight into the problem.