Monday, June 04, 2012

So What Really Happened?

Elizabeth City, NC
36 17.92 N 076 13.09 W

I promised to write more about our expensive heat exchanger problem.  Here goes.

The short non-technical answer is that it was my fault; my negligence.  The more technical answer follows the pictures.


The assembled heat exchanger.

Aft end with cap showing attached flange
End caps, note the pencil zinc sticking into the left cap.

Compare the two brass caps, What's the non-brass metal?

The aft end of the water box showing missing flange

See how the rubber O-ring can separate the brass from aluminum?
The damage to the water jacket was more extensive than I thought. The aft end cap actually has a flange that was part of the jacket stuck to it.  Looking at the metal in the flange, I see unmistakable signs of galvanic corrosion.

There is a pencil zinc that screws into the assembly.  It's function is to protect against galvanic corrosion.  I've replaced this zinc often in the past; several times per year.  However, no matter how many times I changed it, I never found more than a stub of zinc remaining.   I'm afraid that the answer is that I didn't change it often enough, and during parts of the the four years that we've owned that engine, the aluminum water jacket, not the zinc was being sacrificial.

The things I said before about the proximity of brass and aluminum, and of the difficulty of getting the O-rings exactly right are all true.  This is not the best design.   However, those things are secondary.  The primary cause of the failure was not keeping the zincs replaced sufficiently.

Oh well, it is always easier to find an explanation that puts the blame on someone else.   But as captain and owner, one simply can't hide from the truth.  It was my fault.

p.s. My friend Doug, posted a comment on the earlier blog saying that he made the same mistake on his Beta Marine engine and he also had to replace the whole expensive assembly.

4 comments:

  1. Thanks for the update and the honesty. Makes me realize I better check out the heat exchanger on my Yanmar 2GM20. I cruised in saltwater for about 2 years.
    Bets wishes making it to Boonville for graduation and sail safe. I live a few miles north of there near Lowville, NY.

    Loren

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  2. A quick search online shows that the 2GM20F does not have a zinc. Yanmar uses cupronickel heat exchangers in all the fresh-water cooled GM series and do NOT need zinc's.

    Loren

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  3. It sounded like a fairly easy task to build myself a little deck overlooking the pool and, at first it was.

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  4. I have a huge wood deck for my swimming pool. I would like to have a sliding gate that come together in the center. Any idea's?. thruflow interlocking panels

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