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 Post subject: Mast Corrosion Hobie18
PostPosted: Wed Mar 14, 2018 6:17 am 
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Hi all,

My name is Sam, I am from the Netherlands and I have a question with regards to some corrosion going on in the mast of my H18 (see pictures).

Should I consider this to be dangerous? If the answer is yes, what is the best way to fix this?

I hope someone has already encountered this problem and can help me.

Have a nice day!

SamImage

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PostPosted: Wed Mar 14, 2018 9:10 am 
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Location: Jersey Shore
Yes, it's definitely a problem. I've encountered similar corrosion before on my H18, but not as bad as that.

The only way I think you can effectively fix it would be to reinforce the area with a large aluminum plate that is contoured to fit the shape/curve of the mast to cover the corroded holes. Ideally the plate would be on the inside of the mast, but that would really just be for cosmetics, on the outside should work too (although it would require a longer rotator bolt). If you had someone that was really good with welding, you could have the plate welded on, but the easiest would be to just attach the plate(s) to the mast using stainless rivets. Then re-drill the holes for the rotator bar and backing plates and re-install that hardware. Be sure to coat everything in Tefgel or similar corrosion inhibiting paste.

The other option of course is to try to find a new mast that isn't corroded, but that isn't always easy to do.

sm


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PostPosted: Thu Mar 15, 2018 5:19 am 
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I agree with srm. Can you remove the anchor plates for the diamond wires to get a better look? It seems that the rivet holes have corroded to the point that the rivets are no longer securing the compression plates in place. The rotator bar runs through a sleeve in the center of the mast, and I'm wondering what the condition of that sleeve is, and/or the holes it slides through. I'd say at a minimum your mast is no longer watertight, which could be a big problem when capsized - it'll fill with water and turtle the boat.

I agree that a replacement mast would be the easiest solution, and a reinforcing plate/sleeve in that location would be a pretty good fix. I wonder though if you might be able to basically move the diamond wire anchor point and mast rotator up a little on the mast and drill new holes for it. Having that corrosion & enlarged holes in the mast isn't ideal (hence the sleeve idea to reinforce it), but I think if you're able to make sure no cracks are developing, arrest the corrosion (drill out/clean it out really well and paint/seal the area, then cover up and seal the holes so that salt water can't get to it), you could live with it. I think it mostly depends on how high up the mast you can/would have to move the plates. It would actually be better to move the anchor points down, so you're not placing undue stress on the corroded area, but that wouldn't work because the boom/gooseneck would be in the way. It does make me nervous though, as the corrosion is right in an area that is under considerable bending stress, and in an area that needs as much material as possible to resist the bending stress.

srm, what do you think about moving the anchor point? Also, is the mast not a tempered aluminum extrusion? I didn't think you could weld to it without significantly affecting the strength.

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'79 H18 standard 'Rocketman II' sail #14921


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PostPosted: Thu Mar 15, 2018 7:27 am 
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Moving the anchor points up would potentially cause the rotator to interfere with the sail when downhauling (my guess is you'd have to move everything up about 2 to 3 inches in order to get into fresh metal). The diamond wires would also have to be shortened.

My guess is that when he removes the plates, he's going to find very little base metal remaining under them. You can see in the pictures that the holes look so enlarged that they've allowed the plates to shift upwards.

I think welding is a bit of a toss up. Yes, it would remove the temper, but it would also be adding more material to replace what's missing. Rivets are the other option, but then you're drilling more holes in the mast and adding more corrosion potential due to dissimilar metals (stainless rivets). There's no perfect solution, but leaving it as is certainly doesn't seem like a good idea.

sm


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PostPosted: Thu Mar 15, 2018 2:35 pm 
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srm wrote:
Moving the anchor points up would potentially cause the rotator to interfere with the sail when downhauling (my guess is you'd have to move everything up about 2 to 3 inches in order to get into fresh metal). The diamond wires would also have to be shortened.

My guess is that when he removes the plates, he's going to find very little base metal remaining under them. You can see in the pictures that the holes look so enlarged that they've allowed the plates to shift upwards.

I think welding is a bit of a toss up. Yes, it would remove the temper, but it would also be adding more material to replace what's missing. Rivets are the other option, but then you're drilling more holes in the mast and adding more corrosion potential due to dissimilar metals (stainless rivets). There's no perfect solution, but leaving it as is certainly doesn't seem like a good idea.

I agree that the base metal underneath the plate is either gone, or the holes are so big that the little bit of material between the holes is no longer contributing to the bending strength of the mast. I'm a little concerned about the possibility of a crack developing from one of the rivet holes, especially now that they're enlarged and jagged.

I suppose you're right about the additional cross sectional area of a sleeve, but you could have a failure right at the toe of the weld on the mast itself. I'm not sure how wide of an area is affected by welding (though I suspect it's mostly just the HAZ), or how completely the temper is nullified, but I know that 6061-T6 has a tensile yield strength of 45,000 psi, while annealed 6061 has a tensile yield strength of only 12,000 psi, so if the temper is completely removed it's a pretty drastic reduction in strength.

The engineer in me is dying to get two pieces of 6061-T6, weld it together and test the strength after welding. If only I had access to some testing equipment! :lol:


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PostPosted: Wed Mar 21, 2018 1:31 pm 
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Big thank you for both of your answers!

I made some additional pictures, maybe it will clarify some more.

I think I am going to look for a good welder, who can weld a thick plate of aluminum to the mast.
Then I can make holes for the rivets in it and attach the shroud plates.

ImageImageImage

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PostPosted: Wed Mar 21, 2018 2:33 pm 
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Big thank you for both of your answers!

I added some pictures, maybe it clarifies some more.

I think I'm going to find a welder who can weld a thick plate of aluminum to the mast. Then I can drill holes through that plate and attach the shroud plates to it with rivets.

I also added some pictures from the rest of the mast. There you can see some corrosion on the rivets, but the base metal from the mast is still fine. Can I do something to prevent everything from corroding further?ImageImageImageImageImageImageImageImage

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PostPosted: Thu Mar 22, 2018 5:07 am 
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This picture concerns me. It looks like the mast is starting to crack from the rivet hole, which could quickly grow and result in a mast failure & de-masting. I would at a minimum drill stop the ends of the two or three visible cracks and weld a thicker aluminum plate on both sides of the mast, under the compression plates. You will need a longer sleeve to go through the mast once you do this.


Image
It looks like you could have some corrosion issues under the spreader bar anchor plate as well, which could be just as concerning as the compression plates at the base of the mast.

As far as preventing the corrosion goes, you need to isolate dissimilar metals, however you can. If you can get either a coating or a plastic between the metals, you should be able to prevent further corrosion. You could get a thin piece of plastic behind the compression plates, or bed them with an epoxy. The tricky part is isolating the rivets from the aluminum of the mast.

For more, you should read up on galvanic corrosion. In any application where you have two dissimilar metals in contact with eachother in water or a humid environment, you'll encounter galvanic corrosion. The problem is exacerbated by salt water and/or electrical current, but even in fresh water and in the absence of a regular electrical current, the corrosion occurs. The idea is that one metal acts as an anode, and the other as a cathode. The anode deteriorates, while the cathode experiences no corrosion whatsoever. This is why ships have aluminum, zinc or magnesium anodes attached to their hulls - they protect the hull of the ship from corrosion, by sacrificing the metal of the anodes (which are regularly replaced). It's also part of the reason why electrical leads are often made of gold, as gold acts as a strong cathode.

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Mike
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'79 H18 standard 'Rocketman II' sail #14921


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PostPosted: Thu Mar 22, 2018 9:03 am 
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Location: Jersey Shore
In the first and second close up pictures from the earlier post, it looks like there is salt residue actually caked on the fittings. I would suggest doing a better job of rinsing everything off with fresh water after you're done sailing. For preventing corrosion, Tef Gel is really the stuff you want to use. Put a coating of it on all of the stainless parts, including the rivets, before you reassemble everything.

sm


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