In these engines with water lift mufflers extra pressure is required
of the raw water pump at high power to over come the exhaust pressure.
Motoring into rough water adds air bubbles to the system reducing
pressure and can stop all flow of water. A through hull water scoop
helps control this problem by forcing the air through the pump.
Have you cleaned out the outlet fitting on the heat exchanger and the
inlet water fitting on the exhaust elbow? Has the raw water pump cam
been replaced? In charter boats these cams only last about three years
in the older small Yanmar engine.
From the author Of books on Boat Refrigeration
http://www.Kollmann-marine.com


"Mr. Catfish" wrote in message ...
During adverse conditions, 45knt gusts, blinding rain a tide going out and a
current besides, (At one point I looked at the GPS and with the engine
running 2400rpm we were going 0 knts) shortly after this the engine
overheated so I shut it off, there was enough sail sticking up so I had
steerage but I did get blown off course. After about half-hour of being
blown around I started the engine, the temperature came down to normal
slowly and the engine ran fine for the next few hours it took to get back to
the slip. Is this engine getting too old (1979) ? Thermostat and impeller
about 20hrs., raw water cooling, strainer not clogged, no air leak.
Do the water passages get clogged after years of use and if the engine is
under strain it will show up as overheating?