I am definitely no expert, but, I simply drain all the old coolant and add new, yearly as recommended by Yanmar in their service bulletin. Couldn't go wrong flushing it as well but just don't see the need.
We run our boat in the California delta, so the raw water side is generally fresh water, except for the occasional trip to the bay or offshore. Still we are due for an aftercooler service, thinking this winter.
Eric
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