I had a very scary experience last Saturday with my 361 Volvo engine. A friend of mine suggested that I should rinse with fresh water the raw water system after using the boat at the end of the weekend. His rationale was that the fresh water would kill the potential growth of sea creatures and improve the heat exchanger performance. Not a bad thought. I had made a plastic coupling that connects the strainer exit tube, from the inside of the strainer, to an external tube that is connected to a fresh water reservoir before starting the engine. The method only requires to open the strainer cup and connecting the coupling for flushing the engine with fresh water without the need to disconnect any tubes. I had used this method last fall to winterize the engine and it had worked very well. This time the method worked well too. HOWEVER, apparently at the end I failed to screwed well enough the strainer cap and air was probably allowed into the system as the engine was turned on. I believe that initially the air leak may have been just enough to allow some suction of raw water since I could see water coming out of the pipe after starting the engine. Whatever the cause was, the raw water flow must have stopped soon after because the engine overheat alarm went off after less than 15 minutes of running the engine. After stopping the engine and sailing back to our mooring with smoke comming out of the engine compartment(Good practice for my wife and son) I found that the heat of the exhaust gases had melted the entrance tube and part of the Vetus waterlock, plus one of the vanes of the raw water pump impeller had developed a crack but was still complete. Fortunately, the engine was otherwise OK. As I was searching the Vetus.com web page, for a replacement waterlock ($130), I found that they sell a temperature sensor and alarm system that they recommend should be used with all their waterlock systems. (Nice that Beneteau never mounted one in my boat, or mentioned the need for one). Vetus claims, with good reason, that following a raw water system failure the increase in temperature from the exhaust gases occurs much faster than that of the engine fresh water due to the large thermal mass in the engine and water heater. Therefore, the temperature sensor on the exhaust gases can be expected to detect a failure of the raw water system much before the engine overheats. In summary, for those who still believe that watching the water discharge from the exhaust as one starts the engine is enough to prevent a disaster, I hope that my experience is sufficient to change their minds. Yes, one should make sure that every time one opens the strainer one should close it tight, but my point is that even if every thing is working fine at the start, a raw water pump can fail, or an obstruction can occur due to debris, resulting in the bad consequences mentioned above. I currently believe that installing some sort of a temperature sensor and alarm ($150 for a Vetus system) is the only way to prevent an engine from overheating and a muffler/waterlock from melting down should a raw water pumping failure occur while on the way. Comments or personal experience about this topic are welcome.