Chainplate Replacement Reveals Something Interesting - SailNet Community

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  #1  
Old 05-15-2013
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Chainplate Replacement Reveals Something Interesting

These chainplates are around 13 years old. I noticed very soon after I installed them that they were rusting very quickly so I've kept a good eye on them over the years.
This picture of the Port Aft Lower chainplate shows the reason that I decided that now was the time to replace them.



The material is slightly magnetic so I don't think they were 316 stainless. I have decided to replace them with bronze C61400 plate. I have up-sized them from 1/4" to 3/8" thick. Overkill I know but I don't ever want to do this again.

Anyway, the interesting thing is that when I removed the stainless steel backing plate I discovered that when I replaced them 13 years ago I, or whoever was inside the boat, inadvertently trapped and crushed a negative wire behind it.



You can see a small hole in the insulation.

Here is a pic of the top of the backing plate.



It's a little difficult to see because of the reflection of the flash, but you can barely see a little pit in the metal that lines right up with the hole in the wire.

Here's the thing; When I pulled the bolts from that chainplate, the port aft lower. There were three bolts that were completely corroded through. In the next forward chainplate, One bolt was completely corroded through. None of the bolts from the other chainplates were corroded anywhere near that much.



I spoke to Ron from Ever After Marine here in Saint Petersburg, an electronics specialist, and he said that he didn't think that the squished ground wire would have anything to do with that. Though he did say that it was an unusual coincidence.
The boat hasn't been sailed that much and for most of the last decade she has been kept on a lift out of the water and I almost always turn the battery selector switch to Off when I'm not sailing.

I have a hard time believing that the squished wire didn't have something to do with the condition of that chainplate and the bolts.
Any thought?
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Old 05-15-2013
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Chainplate Replacement Reveals Something Interesting

I don't that the wire was a contributor to the corrosion. All that you did by by trapping the wire was to ground your shroud. On our boat, each shroud is negatively ground. More likely is that some moisture was trapped.
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Old 05-15-2013
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Re: Chainplate Replacement Reveals Something Interesting

That's what Ron said. But I really didn't see any indication that the chainplate was leaking. The wood around the holes inside the boat looks fine and dry.
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Old 05-15-2013
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Re: Chainplate Replacement Reveals Something Interesting

If the negative wire was returning current it could cause problems a bonding wire that would normally NOT be passing current
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Old 05-15-2013
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Re: Chainplate Replacement Reveals Something Interesting

a bonded ground would not cause corrosion.. but a regular ground could. I am not sure how your boat is wired, but on mine the battery disconnect only affects the positive side of the battery, thus the negative is always connected. I doubt it floated any noticible voltage, but it might have kept a few milliamps flowing, especially if something around you was leaking voltage.

You were also looking at dissemilar metals, that can also cause some natural voltage
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Old 05-15-2013
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Re: Chainplate Replacement Reveals Something Interesting

Mad, that was my thought - a galvanic battery (dissimilar metals) and a potential discharge path.
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Re: Chainplate Replacement Reveals Something Interesting

Nothing on the boat is grounded, but I bet that the battery switch has only been on for maybe 300 hours since those were replaced. Plus as I mentioned, most of the time the boat is on a lift.
I guess it really doesn't matter much though anyway. It's probably not ever a good idea to squish one's electrical wires behind chainplate backing plates.

Now my next big issue is deciding what bolts to use for the new chainplates. I'm thinking that because they are bronze. I should use bronze bolts. But I'm finding that they aren't all that easy to obtain and they are reallllly expensive. I was thinking that perhaps I could look for bronze threaded rod and use acorn nuts inside and regular nuts and washer on the outside. The only down side to that is that there would be no unthreaded shoulder in the hole of the chainplate. Would that be a problem?
Would it really be a big no-no to use stainless bolts in bronze chainplates? Most turnbuckles are bronze and stainless.

New chainplate material.

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Re: Chainplate Replacement Reveals Something Interesting

Boy oh boy, these chainplates are going to bust my budget. Are they ever proud of bronze threaded rod. The least expensive I can find is McMaster Carr where they want $54.00 for a three foot piece. I will need three of them plus 72 nuts.
But after spending nearly $500.00 for the chainplates themselves, I don't want to cheap-out now.
Anyone know a good source for bronze all-thread or bronze bolts?
Also, all I can find is silicone bronze. Will that be a problem when used with the aluminum bronze?
If there is still going to be a problem with dissimilar metals, maybe I would be just as well off to use 316 stainless. Though that might look odd.

Just as an aside, why is bronze so darned expensive? I could buy a computer for less than what I paid for enough bronze flat stock for chainplates on a 27 footer.
How long ago was the bronze-age?
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Old 05-15-2013
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Re: Chainplate Replacement Reveals Something Interesting

Bronze is mostly copper. Look at copper prices these days.
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Re: Chainplate Replacement Reveals Something Interesting

Quote:
Originally Posted by Alex W View Post
Bronze is mostly copper. Look at copper prices these days.
Yeah, it just pi$$es me off that it's all contrived and manipulated.

Copper Stockpile Near 16-Month High on Storage Incentive, Supply - Bloomberg
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