I took my kids (I still call them kids even though they are in their 30's) on as crew in a sailboat race. They had never sailed let alone raced, but it was a social race and low key (well at least for me). I am trying to explain a hundred things as we leave the dock and I forget to open the seacock to the engine. About 5 minutes later the overheat buzzer comes on and I immediately know what the issue is (funny how you remember things you did not do then). Opening the seacock did not help, so I shut down and began working thru where the problem was. Water was coming in, but not getting to the engine was what I found, so I replaced the intake pump. Fortunately, I long ago realized tryiing to change an impeller on a Yanmar was an upside down and back-wards affair, so I carry a spare entire pump, with impeller and bracket. I just need to swap out the new for the old. To make a short story long it is not quite that easy as I have an extra belt on my 30 GM Yanmar for my fridge compressor. I have to take apart the main pulley to remove that belt, but I was ready for that and had packed a bunch of tools on board. I dropped the hook, which allowed me to later use my newly installed wash down pump, but that is an even longer story. And in about 30 minutes we were back on our way with water nicely flowing out the exhaust and the engine running no worse for wear, at least I have not noticed any degradation of performance. I motored home, about 2 hours, after the race weekend and had no issues.
When I got home I took apart the 'old' pump and found 4 of the six impeller blades still in the pump but un-attached. I am assuming the other two are working their way up into the heat exchanger and will eventually plug the mixing elbow. So I plan to back flush the heat exchanger to see if I can clean it out. Any thoughts, comments or suggestions on this procedure. I had not planned to open the end face plate of the heat exchanger, just flush it using the output and input hose connections. But I could be convinced otherwise.