It has been said that internal engine corrosion only happens in salt water boats or boats winterized without antifreeze (i.e. dry).  To the best of my knowledge, my boat has always been freshwater; but as the fourth (at least) owner, I can't verify the steps taken for winterization in the past.

First the disclaimer: while I do like the Bukh engine overall, especially for it's reliability in starting and ease of operation, I think it has one serious, potentially fatal, flaw, and that is the narrow passages at key points in the cooling system.  I feel this is a major issue not only because of the ease in which these passages can become plugged, but because of the potentially disastrous consequences without any outward signs of trouble.  Not a robust design IMHO.

One such point is the elbow at the fitting where the cooling water from the engine merges into the exhaust elbow.  The fitting, as previously discussed by Frank in another post, is only 1/4" ID and could easily be plugged by any debris or flakes of engine corrosion.  The solution is to install an inline water filter in the hose between the thermostat and the inlet port to the exhaust manifold on the port side.  A filter like the Jabsco 36400-0000 does the job nicely:  http://www.defender.com/product3.jsp?path=-1|51|2234255|2234258&id=121067

The other benefit is that the clear casing is another place where you can check to ensure water flow.  Be sure that the hose clamps are not too tight (crushing the barb and blocking water flow) and not too loose (allowing air in which would reduce water flow) and that the filter is oriented the right way for water flow!  I have had this filter in my engine for 4 years now and it has been very helpful in trapping debris.

The other weak spot in the system, described above, is the one I just dealt with.  On our first motor out of the boatyard this year after 20 minutes of running at 1700 rpm in smooth water, the engine overheat alarm went off.  There was no other indication of  any problem as cooling water was coming out of the boat exhaust normally.  

After checking the hoses and thermostat, I traced the problem to the water inlet port below the air intake on the starboard side.  To remove this inlet port housing you'll need to remove the starboard bulkhead (unless you are a contortionist).   

There are two bolts underneath you'll need to loosen with a 10mm box wrench to remove the inlet housing.  When you remove the housing, water (and maybe sediment) will gush out so have paper towel handy.  The pictures below show what came out of my engine.  I was astonished at the amount of flaked metal sediment!  The picture with the paper towel is showing only 15% or so of the total amount that came out.  The inlet pipe was so densely clogged that I had to chip the sediment out.

For $100, John's solution seems quite reasonable as an extra protection and I really like the ability to see the actual temp.

What would be best is some sort of water flow meter, or temp gauge in the exhaust pipe itself, in combination with an engine block temp gauge.  This combination will best cover instances of both engine block overheating  (due for example to blocked lower passages) and lack of water flow through the cooling system.

I believe it will cycle back and forth displaying the two temps alternately. 

So what I have is I am not sensing the water temp, I am sensing the block temp , the factory switch senses the water temp and if you have flow at the sensor but a blockage in the block it will not go off near as I can tell . Mine senses the temperature of the cast cylinder block . I will add the exhaust sensor this winter . 

I take it the factory alarm did not go off ?  I pulled that cooling pipe yesterday and mine was perfectly clean . I suppose the old owner must have done it . 

Motor has more now , starts with the flick of a key . Question , the key shut off can accidentally be left on burning it out I am told , was thinking of just moving it to a button beside the key so you can not  accidentally leave it on . Has anyone done this ? 

 As to the boat , the whole boat seems never used , but really really dirty . All cleaned now and largely restored - all painted and varnished down below  . Tons of maintenance done to it . Lots of stuff that would have broken if he used it -replaced prop , all the clutch handles , all the lights upgraded to LED  ,serviced all winches , new inboard tracks , glassed hull at chain plates , beam of destiny ,  the hull gasket for the sail drive, all new instruments and   new slides for companion way , new gasket for hatch , re bed everything (no more leaks) , pulled the fuel tank and dumped out the strange color fuel,  restored the trailer and moved the boat forward on it too and a few neat toys like a below deck auto-helm . That's the short version of the list Painting the engine and pulling the holding tank for cleaning this winter. 

Love this boat , as you probably know I got a smoking great deal on it and this one as you know was never used so its like new ,  but I put a ton of money in and even more time. Kind of like a barn find on a classic car , sure it was never used but you had better go over absolutely everything before you drive it and be prepared to replace lots of plastic bits  . 

I installed a 5 amp fuse holder and my worries are over. I put it behind the ignition panel, easy to do. It's the black wire to the fuel shut off solenoid so yeah a button is fine too, thats good.

I installed one lead directly to the starboard side engine block via a threaded plug that was already machined.  Easy.

The second lead was trickier. I wanted it directly underneath the plastic muffler where water and exhaust mix, since I previously had a water starvation situation that melted a hole in the plastic.  Not fun, especially once water flow is restored!  Anyway I had to use a large hose clamp and a fair bit of contortionist skills to get it in place, but it worked.

I wired it into the same line I have for my engine RPM gauge, so the display works only when engine running or powered up (turn key one click to right) as I see no need to waste battery power monitoring engine temp otherwise.

So far the highest number I've recored on the engine block is 156 degrees which occurred when throttling back after 45 minutes of motoring.  The muffler temp typically runs about 80 degrees.

https://goo.gl/photos/8wNXDmscuBsZmLMS9" rel="nofollow - https://goo.gl/photos/8wNXDmscuBsZmLMS9

I rebuilt my motor in 1990 due mostly to some excessive oil leaking from the bottom crankcase cover. So I did take apart the water cooling pipe where the sediment collects on the bottom of the head and it was nearly plugged... yet the engine was only 4 years old at that point. So last week I took the cooling hose off that pipe and attached a longer one so I could blow thru it and it was perfectly clear no obstructions at all. I didn't want to overheat my cylinder head and warp it so figured I needed to find out. 

In hindsight it becomes clearer. Original winterizing method from the manual was to drain the engine which most of us did and this caused the cylinder and head to remain dry for the layup period and this caused the cast iron surfaces to rust inside . Then the rust would collect at the lowest point on the engine (which is that cooling pipe attachment at the bottom of the head) eventually it would become blocked as mine nearly did at the age of 4 years old. So as time went on the head would overheat and warp and blow the head gasket and when rebuilding it we would find that clogged cooling pipe and clean it out like I did in 1990 on my engine.

Since then I've used antifreeze to winterize my engine so the inside cast iron never rusted. I started this in 1990 after my rebuild. Then around the year 2000 I started using plumbing antifreeze because it's better for the environment and it's less expensive. But then we wondered if it would prevent rust from forming because it's not automotive antifreeze. Well It's been 17 years winterizing with plumbing antifreeze and that cooling pipe blows thru like it's brand new so I believe the fact the cooling passages are filled with any liquid will stop it from rusting. It must be this way because during the summer for 6 months it's just fresh water that in there. 

So the the lesson learned is 1.) Test that cooling pipe by trying to blow thru it to see if it's blocked. 2.) once it's clean be sure to use plumbing antifreeze to winterize the engine. 3.)Never drain the cooling system.

The reason I ask is because Practical Sailor magazine has run tests and concluded that while propylene glycol is safer for the environment (and necessary in any potable water system as an antifreeze) it is more harmful to rubber, plastic and nylon.

See:   http://www.practical-sailor.com/blog/-11059-1.html" rel="nofollow - http://www.practical-sailor.com/blog/-11059-1.html

So for now I continue to use ethylene glycol in the engine, but am careful to dispose of it properly in the spring.  To be honest, even though they say that propylene glycol is "safe for the environment", I would still dispose of it in the hazardous waste area of our boatyard, and never dump it directly overboard, as I've seen many boat owners do.