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Old 09-06-2013, 18:15   #1
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Corroded out screw hole on my mizzen boom

Hi all,
Recently I noticed the block and cleat (it's one unit) for the mizzen main sheet was coming off the boom where it's attached. Turns out one of the screw holes has corroded out and the attachment point was bending when the sheet was under a load. I believe the corrosion happened because the screw is stainless and the boom is aluminum but.... My initial thought is to move the block aft a little, drill and tap new holes and fill the old ones. But what if this weren't an option (in this case it is)? I'd appreciate people's thoughts on (1) how to deal with corroded out holes when re-drilling in a new location is not an option, (2) whether just moving the block aft a little and filling the old holes is a good solution to my current issue, and (3) how to prevent this from happening in the future. I'll attach a picture of the holes.
Thanks in advance!

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Old 09-06-2013, 18:41   #2
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Re: Corroded out screw hole on my mizzen boom

You can clean the site and fill the old wounds with "Marine-Tex" or "JB
Weld" then drill new holes offset from the original position. New fasteners of different metals such as stainless in the aluminum can be coated with a compund to lessen the corrosion. I'd also look at the option of removing that end cap and then placing a back up plate inside the tube. Drilling you new fittings into a back-up plate would not rely on the weakened an damaged area for support.
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Old 09-06-2013, 18:49   #3
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Re: Corroded out screw hole on my mizzen boom

After you're done with your drilling, clean up the holes and "lubricate/brush" them with a couple of coats of Rustoleum. Prior to inserting any bolts, make sure that the bolts are heavily coated with Silicone Grease for marine usage. Finally, after everything is in place, another couple of coats of Rustoleum for the area you just worked on; use Rustoleum marine paint. Let's know how did it go!
Mauritz
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Old 09-06-2013, 18:49   #4
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Re: Corroded out screw hole on my mizzen boom

As said above if you could get the end cap off.. If not add a doubler on the outside and tap through both pieces. It really looks like the boom is not thick enough for that size fastner. You should have at least 3 threads to screw into. Next time do it properly unlike the last guy and use an isolator and thread sealant on the fatsners.
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Old 09-06-2013, 21:05   #5
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Re: Corroded out screw hole on my mizzen boom

Another way to go is with threaded inserts called rivit nuts or thread sorts. I find it a bit difficult to get online info.i. It's there, just takes a bit of work to ferret out. Below is a starter link.

Basically you drill out a hole, put in a river that has a threaded hole, set the rivit, remove the tool, put in your screw. Useful for putting screws into thin material where it is not thick enough to be taooed, and must be mounted blind, no access for a screw.

I have used this and find it works well. You need to drill an oversized hole for the rivit. So that may work out just right for you. As long as the corroded hole is not too big.

Tooling is a bit expensive, but it is a useful tool.


Not Ribbed Flat Head Rivet Nuts | Rivets In Stock - Rivets & Rivet Tools
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Old 10-06-2013, 15:57   #6
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Re: Corroded out screw hole on my mizzen boom

Thanks for the input. More questions:
CaptForce:
I like the idea of taking the end cap off, I'll look into that.
If I am able to put a backing plate on I imagine it should be stainless and that everything should be painted so that the different metals are isolated. Thoughts?
TekNav:
Does Rustoleum (marine grade) work as an isolator between the two metals?
What about a polyurethane based marine grade paint?
Does the silicone grease act as a sealant and isolator? Would Lanocoat be good thread sealant/isolator?
Guy:
I like the idea of a doubler. I'm imagining the double as another piece of aluminum(?) that is riveted (aluminum rivets?) onto the boom to increase the thickness. Am I imagining your thought correctly?
HPeer:
These rivets are great to know about! Thanks for the link too.

Cheer,
Jonah
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Old 10-06-2013, 16:13   #7
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Re: Corroded out screw hole on my mizzen boom

Just a note on riv-nuts. They should never be used for a structural item. They have very little strength. The are great for non-structural items but the amount of upset on the back side is pretty minimual and they will pull out or turn. A riv-nut that turns in the hole is a very big pain in the ass
Correct on the doubler idea.
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Old 10-06-2013, 16:31   #8
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Re: Corroded out screw hole on my mizzen boom

Hi! Rustoleum is not an isolator between two metals; it is an anti-corrosive agent. Polyurethane (marine) is an OK intermediate choice, but does not have anti corrosive properties; Rustoleum (marine) is preferred if you can afford it. Silicone grease is an isolator and is an overall protectant/sealant. You will need repeat applications if bolts are being removed, otherwise good for a long time; be generous when you apply it. You may substitute any name brand, for the silicone grease.

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Old 31-08-2013, 15:43   #9
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Thanks for the info and creative ideas. Thought I'd share the results of the fix. It was not feasible to open the boom so I ended up riviting an aluminum plate (with aluminum rivits) to the boom and tapping new holes. The result is strong and should last. Cheers!
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Old 31-08-2013, 15:57   #10
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Re: Corroded out screw hole on my mizzen boom

Looks good!
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Old 31-08-2013, 16:05   #11
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Re: Corroded out screw hole on my mizzen boom

Looks great.

Nice of you to come back.

Most don't and we don't know what the answer was.
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Old 03-09-2013, 09:07   #12
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Re: Corroded out screw hole on my mizzen boom

I do not believe aluminum touching aluminum is a good solution ! Does not sound correct but in most cases aluminum and aluminum are considered dissimilar metals ! Stainless is always used !
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Old 09-09-2013, 12:22   #13
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Re: Corroded out screw hole on my mizzen boom

Quote:
Originally Posted by pistarckle View Post
I do not believe aluminum touching aluminum is a good solution ! Does not sound correct but in most cases aluminum and aluminum are considered dissimilar metals ! Stainless is always used !

Interesting. Do you know why this is? On the anodic index the highest difference in volts between two types of aluminum is .20v. While .15v or less is best for marine enviroments, .20v seems better than the .25v difference between some types of aluminum and stainless steel. But perhaps I am missing something. I would love more information.
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Old 09-09-2013, 16:09   #14
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Re: Corroded out screw hole on my mizzen boom

Quote:
Originally Posted by pistarckle View Post
I do not believe aluminum touching aluminum is a good solution ! Does not sound correct but in most cases aluminum and aluminum are considered dissimilar metals ! Stainless is always used !
Quote:
Originally Posted by jonahmulski View Post
Interesting. Do you know why this is? On the anodic index the highest difference in volts between two types of aluminum is .20v. While .15v or less is best for marine enviroments, .20v seems better than the .25v difference between some types of aluminum and stainless steel. But perhaps I am missing something. I would love more information.
I'm guessing it is some sort of a typo.
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Old 09-09-2013, 16:28   #15
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Re: Corroded out screw hole on my mizzen boom

Why not just take the boom off and weld it.
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