Saturday, March 3, 2012

Disappointment redux

After last weekend's disappointment, I have made some progress.  Miller & Miller indeed made me a new endplate.  Here it is, sitting on my painter's points, so that both sides could be painted at once.   I will install it as soon as the paint is dry.

And I have started investigating (hi Chuck & Jackie!) a fabricated stainless steel replacement exhaust manifold.  Amazingly, it is apparently available for 5/8 the cost of a cast iron manifold from Perkins.  We shall see.
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6 comments:

Anonymous said...

You might be disappointed in stainless. Our boat has a stainless water muffler, and it repeatedly gets pinhole leaks. Apparently hot saltwater and stainless don't go well together.

How is the autopilot project going?

Cheers

Simon

bob said...

Hi Simon -

Do you know what alloy your muffler is? Not all stainless is equivalent when it comes to hot saltwater...

The autopilot is alive! Post coming Monday.

bob

bob said...

Simon -

I did some research and found that hot saltwater is bad, but *low pH* hot saltwater is deadly and causes just the type of pinhole corrosion you report. The combination of the exhaust gases with the saltwater would definitely drop it's pH, due mostly to the sulphur oxides in the combustion products.

I also found that 316L is the best alloy choice for corrosion resistance. The manifold is made from 316L, and should, as long as it is operating correctly, keep the hot saltwater and the exhaust gases separated.

Thanks for the tip! I always enjoy research.

bob

Anonymous said...

Interesting.

Thinking about the SS and salt water, we have a couple of other examples of pitting corrosion. The centreboard is stainless, and that is badly pitted presumably because it sits in the centreboard case in stale, non moving water. Our prop (kiwiprop) is an refurbished older one, and the stainless parts on that had some pitting corrosion. I suspect that any stainless in salt water that sits will get crevice/pitting corrosion because of localised/micro level conditions. I had understood that crevice corrosion can be an issue even with 316L stainless ( and did a quick google search which seemed to confirm it eg http://www.electrochemsci.org/papers/vol6/6115521.pdf).

On our last boat we had a freshwater flush set up, that we used to use whenever we had the engine off for a long period. I was thinking of doing that for the current one, and it would probably be even more worthwhile if you had a stainless manifold because then only freshwater would sit in it.

Look forward to the autopilot post!

Cheers

Simon

bob said...

Simon -

I sure hope that my new manifold (yes, I have made the substantial financial commitment) is going to outlast the original cast iron one. I like the idea of the fresh water purge - how did you plumb for that? A wye valve at the raw water inlet?

But is sounds like the worst thing is the lack of oxygen inside the manifold. How to solve this problem?

bob

Anonymous said...

I used the sink as the freshwater source. A T off that with a shutoff, then just T'd directly into the saltwater system - can't remember whether it was before or after the raw water filter. To run flush the sink system, turn the sink stopcock off, fill the sink, then close the raw water inlet and open the valve to let the sink freshwater in. I always turned the engine on immediately as I was paranoid about flooding the manifold. I left the tap running, then turned the engine off when the sink ran down.

I am sure the deoxygenated water is only a problem for saltwater. I don't think you get the same problems with freshwater.

Cheers

Simon

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