...it would run hot at full throttle when it was brand new! The cooling system is marginal at best on this design and if I do not clean and change the coolant the overheat condition returns. I use only Yanmar coolant (it is actually Dexcool but it comes pre mixed with de-ionized water...) and I watch it like a hawk. We are not over propped, we have great cooling water flow and the engine runs like a Swiss watch. The gages are reporting correct temperatures and the secondary buzzer is within spec too.It simply has a marginally sized heat exchanger. I have seen other examples of the engine that have this issue also. When I ask the people who own the boat what they do they usually say "we simply don't use full throttle..".

In your case I can't answer whether it would be beneficial or not to clean the exchanger. Yanmar calls for a coolant exchange annually even with the Dexcool but I don't believe they recommend anything else. I would probably do it because it is simply my nature. I have spent my entire life maintaining engines....

The other question you asked was if the contamination was visible. In fact you do not see much at all. It comes out looking slimy and it looks new when I put it back in. The overheat condition shows up after a season if I don't clean the exchanger. If I push the engine speed above 2500 of the 3000 available it will begin to overheat within a few minutes. Back off the throttle and it cools down again. After I clean the heat exchanger it will run as long as I want at full throttle.... cool as a cucumber. As I said, it was doing this when the boat was brand new. Mack Boring actually struggled with the issue and finally simply replaced the exchanger. That fixed the issue for about a season and a half. That is when I began cleaning the system annually.

Enjoy your sailing in Europe!
Bruce