Fill 8mm bolt hole below waterline in 6mm aluminium

[Fuss]

I need to fill two 8mm bolt holes that supported the old depth transducer in the engine room. Must I weld or could I use something like JBWeld?

I would prefer to weld but somehow, I think that an easier solution should provide a long term seaworthy fix as well.

Has anyone any experience with this??

[coopec43]

Can't you use SS bolts/washers/5200 sealant?

[Greatest Lakes]

I'd be careful with dissimilar metals. A weld is obviously best. Temporary fixes have a bad tendency to become permanent. If it's out of the water, do it right and you won't have any nagging feelings.
Or you could slather a stainless lag bolt in 5200 or similar and tighten it up. It will probably be fine.

[Fuss]

Thanks for the replies so far, I don’t want to use stainless below the waterline especially with the dissimilar aluminium issue.
I know welding is best but I then would have to re-epoxy inside and also the hull outside.
Someone with an aluminium boat must have been faced with the same in the past.
I read that often 5200 is used to plug these small underwater bolt holes and lasting 10 years, but this is going a bit far for me. I would have thought that JBweld would be a seaworthy solution as the holes are so small.

[s/v Jedi]

Take a M8 aluminum bolt, coat it with JB Weld original formula and screw it in so that the inside is flush and let it cure. Then it’s simple cutting off the excess on the outside, sand it smooth, maybe a little more JB Weld to fair it out before painting it.

[Tin Tin]

8mm (5/16") Orlock structural alloy rivets - you may need to borrow appropriate riveter to install.

[Fuss]

Thanks very much for the solutions.
The Orlock structural alloy rivets look interesting, I looked them up but , I think for this application they need special tools and might in this case , be overkill. So I am going for the 8mm aluminium bolt solution with the original JBWeld.
Thanks again to you both for these suggestions!

[coopec43]

https://www.boatdesign.net/threads/s...luminum.48219/

[coopec43]

Here is a directive from the experts!

[s/v Jedi]

Originally Posted by coopec43

Here is a directive from the experts!

From what I understand he has an aluminum hull so an aluminum bolt is the way to go. Keeping materials as much similar as possible is always good.

This is also why the JB Weld. This is epoxy reinforced with aluminum powder so a very good choice here as well, even though regular epoxy would be just fine.

Edit: I have to add this: that story, “ideally keep salt water away” is ridiculous. You must keep salt water away when using stainless and aluminum or it will corrode away quickly. Ideally you don’t use dissimilar metals but when you do, you -must- keep salt (any) water away.

[coopec43]

OK! I accept that.

Would you consider nylon nuts/bolts? (I need to know for the next yacht I build), (Maybe vulnerable in collision with underwater object? worst case)

[stainless guy]

if you don't seal it with a bolt and 5200 or the cheaper sika 291. and decide to weld it you will have to deal with local shrinkage at the weld location.

If a customer brought this to us we would also throw a small plate on the inside and a plug welded random hole in alu is a great sport for a crack to start

[Franziska]

Fill holes with JB weld, add small fiberglass patch on the inside using Epoxy.

Obviously adequate surface prep is a must.

[Fuss]

Originally Posted by coopec43

OK! I accept that.

Would you consider nylon nuts/bolts? (I need to know for the next yacht I build), (Maybe vulnerable in collision with underwater object? worst case)

hi, nylon is not good is it goes brittle in uv… when i bought my boat it had 4 year old nylon through hulls underwater… from inside i could see the sun shining through them illuminating the bilges.
when i took the boat out of the water, i hit these with the wooden end of a chisel and they shattered like christal glass… unbelievable!!…. i replaced with trudesign marelon.
ok, don’t get too concerned about marelon plastic under water as your depth is marelon type plastic right??

[coopec43]

Yes Marelon is reinforced nylon

Marelon is an injection molding grade of glass reinforced nylon. Marelon is absolutely fine below the waterline.25 Feb 2018
Marelon Alternatives - Boating and Fishing Forum