I was out on a clients yacht recently, and because we had little time to cover the items that I was on the boat for, I did not do my normal full range of checks before casting off. Sure enough as we were motoring home at the end of the day, the over heat alarm activated on the engine.
Once he had the headsail rolled out and the boat sailing, I opened the water filter which was the obvious place to start. It was one of those modern black plastic ones with the large screw top and cup shaped nylon mesh filter. On first inspection the filter was empty and clear, but when it was removed, there was a thick layer of algae on the outside of the filter. This algae was thick enough to stop the flow of cooling water to the engine.
Apparently there is a history of this engine doing this occasionally and instead of the owner checking the filter regularly and cleaning it prior to every trip, they only cleaned it when the alarm activated. Fortunately the engine had not over heated when they were in a confined space or hazardous situation. An engine never overheats when you do not need it!
I have never seen this happen before as on most engines the water in the filter slowly drains out when the engine is stopped so that any growth would die between uses of the vessel. However, this vessel has a saildrive unit and I think that this is what made the difference, the seal on the system must be good enough that the water filter was remaining full all the time, thus creating an ideal environment for the algae to grow.
Hopefully this cooling system will get checked before use every time in future!