The question is WHY does the ONAN eat impellers ?
Is it something to do with the design of that pump housing?
I changed mine for a Sherwood single piece machined pump housing, made no difference.
My Yanmar engine (and I am sure Volvo) doesn't go through impellers the way the ONAN does.
When I have changed the Yanmar impeller after hundreds of hours running it looks like new.I have seen a number of installations of this MARCH pump and it seems to work fine.
The pump housing is left on the engine with no impeller.....there is no issue with that, the shaft is now driving nothing.
The saltwater supply comes from the manifold pipe just cut shorter and the outlet of the pump connected to the flow sensor and to the heat exchanger.
The ONAN raw water flow rate (50Hz) is 18.9 l/min (60Hz 22.7l/min). The MARCH LC-3CP-MD has a max flow of 31 lpm (0 head) which decreases to 22 l/min at a head of 2m, so we are in the right area.
The 230VAC comes from the generator output with a circuit breaker installed in line.
One Amel 54 which I am quite familiar with has an 11kVA ONAN with a MARCH pump for its cooling circuit. This boat has been halfway around the world in 5 years with no issues at all....certainly no ONAN impeller issues.
That's my twopennyworth