Fresh water flush Yanmar diesel
I suspect your heat exchangers on perhaps both the fresh and raw water sides need to be de-scaled, especially if zincs have not been maintained. You can remove them and have them descaled in an acid bath. I had an overheating problem with my 25hp Westerbeke, and the mechanic pulled the hose off supplying water to the freshwater exchanger and blew air into it, and declared it almost plugged. He then grabbed a bottle of mild oxalic acid solution , held the hose straight up and poured the solution in, counted to 10, then flushed it out with water. Not something I would have the nerve to do myself! You may want to check your thermostat first to ensure it''s working properly.