aluminum transom repair-- electrolysis

[Sgdale]

I have a 1994 17' smokercraft fishing boat and electrolysis has eaten up to ten holes in the transum just at the water line. The wood inside the transum is in great shape so all I really want to do is replace the outside alluminum skin. the skin is currently rivited around the corner to the hull totalling about 100 rivits side to side and undernieth. Would it be ok to just cut the skin a couple of inches inside the bend all the way around, and use it as a templete, then weld or braze the new piece in place? Also if I did it this way,, should I coat the wood with some sort of marine epoxy before replacing? And If I decide to braze with hts-2000 or equivilent , will it work in a vertical position without running?? Any info would be great,, I really am undecided. Thanks

 

[jigngrub]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

How is the rest of your hull around the boat? Seems strange that this would only happen at the transom and nowhere else.

 

[Sgdale]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

How is the rest of your hull around the boat? Seems strange that this would only happen at the transom and nowhere else.

I'm don't 100% understand it but I just bought the boat from a friend and he said it sat in a field for 10 years before he got it. I believe it is only in the transum because the alluminum top cap for the transum is missing on one side and the wood would get continuesly wet while in the water and not dry out. Amazingly there is no dry rot in the transum though. He repaired the holes with some construction grade silicone or something. I just wouldn't feel comfortable in the water all day with it like that. I'm not sure how electrolysis works but I only found it in the transum. Also I believe it is only there because there isn't any other wood on the boat. Not sure though..

 

[kfa4303]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

Well, holes are bad, but it may be the transom causing it. If the transom is/was made out of pressure treated lumber the chemicals in it can react with aluminum and cause the electrolysis and pitting. Which may explain why the transom itself hasn't rotted and the electrolysis is only occurring at the transom. Even though the transom may be dry, I would remove it inspect the aluminum underneath and replace it with sealed, exterior grade plywood. You can seal it with polyester, then fiberglass mat, or in one step with epoxy. As for the holes themselves, there are lots of things you can do. If they're small (samller than a dime or so) you can fill them with JB weld then sand them down flush after hardening, if your not too concerned with the looks, or you could take it to a local aluminum welder and have them patch the holes. In any event I suspect that the transom, though dry may actually be the culprit.

 

[jigngrub]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

^^^^^^^^^ I believe kfa4303 nailed it.^^^^^^^^^^^^

 

[Sgdale]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

I have removed part of the wood in the transom,almost like there are 3 pieces of 1.5" plywood ( 1 running the hole length of the width of the transum, and the other two as the sides above where the motor mounts. ) All I found was white powder in exchange for the thickness of the alluminum. It's not pressure treated in any way I can see. The wood is solid, doesn't look like any resin or epoxy was applied, and I'm sure is the stock wood in the transum. I havn't found any information relating to problems with the transom with my make and model boat. I don't understand why the electrolysis is eating it up. The biggest hole is about a 3"oval so JB weld is out of the question. Most of the larger holes are right where the wood stops in the bottom of the transom. I think I am going to have a metal shop cut the outer skin off , make a template with it,and weld a newpiece of alluminum on. Is 6061 alluminum the right marine grade?

 

[MichaelP]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

I had the same situation on my project boat. In my case salt water was the cuplret. Since your corrosion is only in the transom it could be 2 things. First is the wood as the gents mentioned above. It MAY not be PT but some natural oils in whatever lumber they used could like aluminum a bit too much if you know what I mean. If you have it out I would just make a new one and epoxy it together as it sounds like a basket case anyway. Use it as a template but make it from 2 pieces of 3/4" plywood. It's not that hard to do and rather inexpensive. Second; It's possible the aluminum sheet used on that back piece may be an inferrior grade from the factory (a faux pas). I'd be interested to know how much a fab shop would charge to replace that piece, but if you can afford that route make sure they use a 5000 series aluminum. 6061 is fine for top side stuff. For mine I cleaned all the corrosion, treated it with a product called Prekote made by Pantheon industries (this is an experiment and supposed to be a sub for alodine) then primed with Rustoleum aluminum primer. Made a patch plate of .080 aluminum and screwed and glued it on with 3M 5200. Check out the last page of my signature thread for more details. One more thing...I'd check the electrical system in the boat and make sure that the hull is not part of the ground circuit.

Check the Starcraft parts thread for your hardware issues.
http://forums.iboats.com/showthread.php?t=463895

 

[Sgdale]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

I would love to change all the wood in the transum but it creates a lot more work. I really think the wood is OK. I'm beginning to think that it is an electrical problem because 1, all of the holes seem to be in a horizontal line right where the wood stops at the base of the transum. The other pin holes are aalmost right at the vertical seam where the corner is. I will try to up load some pics so you guys can get a better idea. Also the electricallooks like a birdsnest under the transum splash pan. Thanks for all the input!

 

[bouttime007]

Re: aluminum transum repair-- electrolysis

Re: aluminum transum repair-- electrolysis

I was thinking electrical issue too like MikeP mentioned while I was reading what you were describing.