On Starlight 1989 Beneteau Oceanis 350 has a Volvo Penta engine M2003 28hp anyway I was the second owner, so after I purchased the boat I was doing the shakedown cruise and ran the engine for about an hour and the overheated alarm went off , could not find a problem, cutting too the chase lots of growth and calcification of the tubes in the heat exchanger. Had too take the entire unit clean it and put it back, never wanted to do that again figured was not the use rather the extended, layup with saltwater, also the corrosion factor of the elbow, had to replace that. So as a solution I cam up with this manifold I constructed with brass and bronze fittings and ball valves for each connection, from the thru hull , to the manifold, ball on the thru hull and on the manifold did the same at the on end valve to the sea strainer to the engine, next thru hull, pressure fresh water, and at the other end again valve to a run to the bilge.
This allows me to turn on the fresh water, backwash the thru hull, then shutoff and run the engine for 2 minutes on fresh water before shutting down the engine for the night or an extended time. It also in the final configuration allows me to bring to bear as an emergency pump, on my main bilge the engine, if there is a major leak in the boat that bilge pumps cannot handle for whatever reason, (lightning strikes) etc. I often use the fresh water system when sh3 is on the hard, and I need to run the engine, like changing oil, etc. I have 100 gallons of water on Starlight, you should see the looks I get in the yard with no hose, on the hard, yet engine running and water from the exhaust and prop shaft running out. In 20 years have not had any overheating issues or had to deal with the heat exchanger or the elbow, which is notorious on Volvos..
Laying up the Engine on fresh water, makes all the difference..