"Capt. JG" wrote in message
...
Just got an email from the maintenance crew who looked at the engine
situation.... interestingly, we did check the coolant level, which was
fine. So, whomever said the impeller, was dead on...

(edited slightly) The engine lost coolant (which was the consequence of an
overheat situation earlier in the month) and was running hot (in the
185-190+ deg range), which was enough to activate the over-heat alarm and
to still run "as if the alarm was a false negative." The raw-water cooling
was working, but the blades of the impeller had been deformed causing the
quantity of cooling water running through the heat-exchanger to be
insufficient to cool the engine to within it's operating temperature
range, thus causing the alarm.


We will routinely replace raw water impellers in all overheating situation
in the future, as well as positively check the flow of raw water cooling
throughout the complete system: through-hull to mixing elbow.

--
"j" ganz @@
www.sailnow.com

Been there, seen that.
The problem is that as the impellers age the lobes get stiff and do not
spring back into position quickly enough after passing the hump that
squeezes the water along its way.
It is during this 'spring-back' period that the impeller lobe is supposed to
seal itself against the casing and draw water into the gap between the
impellers. When spring-back is too slow the pump throughput gradually
reduces due to poor sealing and/or insufficient water drawn into the gaps
between lobes..