"Don W" wrote

Several knowledgeable people have mentioned that marine engines exposed
to salt water corrode from the inside out, and it makes sense. Right
now, the engine seems to be in fine shape, but my nagging worry is about
how thin the iron around the water passages may be. Since it is cooled
by salt water, it seems that they are probably not only thin already,
but getting thinner--thin being a relative term of course. My thought
on adding FWC would be to run a 75%/25% mixture of antifreeze and fresh
water which would stop the block from getting any thinner.


Don,

At one time almost all engines were seawater cooled - Those older Yanmars
were much heavier than the later GM series and likely had much thicker wall
thicknesses. They are also equipped with anodes to reduce corrosion - Not
sure how many owners actually change these though - When we bought our boat,
I changed the anodes - they were partly used up, but still serviceable.
Might be worth checking to see if yours were replaced during rebuild. In the
early 80's, there was a drive to get lighter engines for sailboat use, and
the GM series provided that. And, most of the larger GM engines were
equipped with closed circuit cooling.

Our own boat has a 1978 Yanmar 2QM15 direct cooled. It spent most of it's
life in salt water - Never been overhauled and still runs perfectly.

Converting to fresh water cooling can be quite expensive - I would just run
existing engine and gain confidence in it. I would also contact rebuilder,
find out just what was done and get their opinion on expected life of
engine. You always have the sails, if all else fails

On the other hand, if you have $15k to spare, a new engine would be nice
- How much does a new Yanmar cost installed?

One thing a fresh water cooled engine would provide, is an ability to heat
water under way. The raw water cooled engine can't really do that.

GBM