I'd opt for #2 ... add FWC to extend service life.
there are some fairly inexpensive FWC heat exchangers and add-on pumps
that can work ... (similar to Universal Atomic-4 conversions sucha as
from www.indigoelectronics.com or www.moyermarine.com). Such marine
engines usually dont wear out but corrode through the block ... .

The simple fact of NOT letting the engine 'dry-out' by keeping the
cooling loop as a totally closed system HAS to be one of the greatest
'life-extenders' you can do to a marine engine. With cast Iron, when
you let the engine drain back or dry-out the internal protective
ferrous oxide (black rust) quickly reconverts to destructive ferric
oxide (red rust). Keeping cast iron WET (and HOT) will promote the
good ferrous rust. Corrosion is the ultimate destroyer of marine
engines; stop or lessen the corrosion and you will have the engine for
a LONG time. For a 'real' life extender consider to size the heat
exchanger for the engine AND the exhaust manifold loads; using raw
water only for the heat exchanger, water injection into the water lift
muffler (most system run the exhaust header with raw water --- a
salting issue).

With FWC cooling you can also run the engine much hotter (with a 190
degree T'stat instead of a 135 degree T'stat) for better combustion
efficiency, etc.

hope this helps.

In article , Don W
wrote:

Hi everyone,

I need some advice from those of you who have run marine diesels for
several decades. We recently bought a Irwin Citation 38 which came with
a newly rebuilt Yanmar diesel engine. The engine starts and runs very
well as you would expect for an engine that has just been overhauled.
It also has a great paint job and looks pretty much like a new engine
with no rust showing anywhere, however...

In researching this engine--a Yanmar 3HM 27HP--it became apparent that
this series was only produced from 1980 to 1983 when it was replaced by
the 3GM30. In a car engine this would not cause me any angst, because
the block would have been being cooled by antifreeze and/or fresh water,
and the parts would still be readily available. In this case, it is a
23-26 year old marine diesel that was probably produced in limited
quantities, and--since it is raw water cooled--probably had salt water
sitting in the block for much of its life.

My options as I see them a

1) Do nothing and hope for the best. The engine will likely run well
for several years, and I may not need parts for it. Many parts (h20
pump, injector pump, injectors, etc) may be the same as on the 3GM30 and
quite availabe. Of course, that old block is getting thinner all the
time, and may not have nearly as much life left in it as it appears.

2) Convert the 3HM to fresh water cooling. This would stop the block
from deteriorating any further, and could stretch the life of this
engine into a decade or more.

3) Pull the 3HM and sell it. Replace it with either a rebuilt 3GM30, or
a new 3YM30. This is the most costly option both in terms of dollars,
and my time, but should result in the best long term reliability.

What do you think?

Don W.