In my case with a different Yanmar engine
(3HM35F) I have several times found that the root problem was a lack of sufficient raw water
I hold a bucket under the exhaust
and catch the water
coming out over a 10 sec period. I compare it to the amount coming out at the same rpm
measured in the past when the engine
was not overheating
. I can also compare it to the amount shown in the Yanmar service
manual for a much faster engine rpm
ratioed down to the 2000 rpm that I use.
In one case I had broken a couple of impeller blades one of which was lodged in the discharge of the water
pump">raw water pump. In a second the pencil zinc that protects my refrigeration
condenser had broken off and plugged the raw water
hose. In a third case (not caught by this method) the mixing elbow
was nearly plugged off with carbon and rust.
I talked about it on "the other forum" Overheating - SailNet Community