Advice: Random plugs/holes in top side of aluminum hull. What are they/how to repair?

[mlt2819]

Hi,

I bought a late 70s aluminum yacht last autumn and I'm in the process of making a few improvements to it. Running along the top edge of the hull (just below the toe rail) was a 6 inch strip of epoxy paint. There was a bit of corrosion underneath a few sections of the paint (which had bubbled up), so I've removed the entire strip. Aside from applying a protective coating of Alumetron, I'm planning to keep it bare from the waterline up. However, when I removed the paint I saw that there are multiple plugs/holes about 5mm in diameter (see attached photo) running 1-2 inches below the toe rail/top edge of the boat (not in any particular order/spacing either). It's my first boat, so I've got a lot to learn... can anybody explain what they are? I guess it's something that was necessary for the assembly/welding process of the boat... or is it structural? There's a couple of dozen of these on both sides of the boat and maybe 1/3 - 1/2 are filled up with what looks like white oxide paste from corrosion (which I can scrape it out easily). The rest look fine.

How should I repair and fill these holes? I'm guessing the first thing is to scrape out the paste with a wooden toothpick, sand the holes with some light grit aluminum oxide, passivate the area (I'll use Marine Grade Alumabrite), clean/degrease the area and then fill it with a Durafix (or similar) welding rod?

Any info/input/wisdom would be greatly appreciated!

Thanks

[model 10]

Those look like drilled holes. What is behind them?

[Franziska]

Could it be that someone had a rubbing stake attached there?

[mlt2819]

Hey, I've only partly dug out one of the holes and I think they just go straight through into the interior of boat (although I could be wrong). I need to have a proper look this weekend. Is it possible its old pitting corrosion that was drilled out and then filled in (either with a weld or an aluminium round bar tapped in)?

[Cheechako]

It would be nice to plug and weld them up if they are just filled with a diller epoxy type of thing.. I wouldn't just fill with weld or you may end up with an oil canned hull in that area from shrinkage.
I have no idea why they are there though. It could have been to draw the aluminum pieces together under tension in fabrication or something.
The other option is just to flare the holes inside and out if possible and fill with a tough epoxy mix with some glass in it like "Kitty Hair". ?
Make a decorative name plaque from aluminum and bolt or weld it over them? :>)

[mlt2819]

It would be a mighty big name plaque to cover them all up Good point and suggestion about the possible shrinkage and that maybe they were there for drawing the aluminum together under tension. If I used a welding rod (like Durafix which has a 732°F melting point) and a handheld torch to fill in the globs, is that temperature low enough to avoid shrinkage? Or what about just using marine grade J-B Weld putty?

[Cheechako]

Originally Posted by mlt2819

It would be a mighty big name plaque to cover them all up Good point and suggestion about the possible shrinkage and that maybe they were there for drawing the aluminum together under tension. If I used a welding rod (like Durafix which has a 732°F melting point) and a handheld torch to fill in the globs, is that temperature low enough to avoid shrinkage? Or what about just using marine grade J-B Weld putty?

Yeah, I thought it was just a few holes at the bow.
I think it can be fill welded. Weld part of the hole up, let cool. weld a bit more, let cool etc.

Why not just buy an aluminum rod the size of the hole, tack it in, saw it off, fill weld and grind flush?

[mcarthur]

Would you have to ensure the alu rod/insert has the same nature as the original alu? ie. 5083, 5383, or 6000's?

[mlt2819]

Sounds like a good plan, thanks! @macarthur I'm a total layman when it comes to welding metal, so I could be wrong... but I recall reading that it's possible to use different grades for the base metals (mine is 5083), but it depends on what filler you're using.

[coopec43]

I bought a late 70s aluminum yacht last autumn
Do you know the yacht design or is it a "one-off? I'm wondering if you can work out what the holes were for?

[mlt2819]

@coopec43 It's a 1978 Robert Tucker Sarum 28, roughly 30 or so were made.

[hpeer]

If for some reason weld filling is not an option or must wait I think you could use a right nut. Insert the right nut and then put in a SS screw. I would view this as a temporary fix while working out a better solution.

[Cheechako]

Originally Posted by mcarthur

Would you have to ensure the alu rod/insert has the same nature as the original alu? ie. 5083, 5383, or 6000's?

It would be nice, but 6000 or 5000 series should be ok and 6000 series is very common. 3/8 or 1/2 round rod.
Stick it in the hole, tack it, saw it off and fill with weld.
I managed an aluminum boat building shop for 3 years. Commonly, certain components like channel, angle, Z shapes, stingers, etc were 6061-T4 or T6 welded to the 5052 or 5086 hull plate and pieces.
If the holes are fairly close together, fill one, move away to another area and ill that one. By moving around you reduce warping. Those holes should fill fine without a bunch of warping i you use rod.

[model 10]

I just read a little about your boat and I think they knew what they were doing at the factory. I would just fill them up with epoxy like they did. Put some AL powder in the mix so it looks ok.

[mlt2819]

Great input guys, thanks very much. One thing I forgot to ask... if I fill weld it using a blowtorch and Durafix rod, should I know what's on the inside of the hull around the hole (in case there's anything which could catch fire/melt) or do you think the temperature is low/localized enough that it won't be an issue? Obviously I don't want to find out the hard way!