Welding aluminum / sealed chambers?

[DougCim]

I am planning to (try to) build a 14' aluminum pram, for freshwater-only use. It won't be stored in the water, so I hadn't planned on coating it with anything.

I have seen elsewhere that the flux for welding aluminum should be cleaned off after welding. I have also seen where some boats have flotation chambers integral with the hull, that (in at least some cases) seem to be welded completely shut. My question is, did they just weld the flux up in there? Or did they just use flux sparingly on the filler rod, and hope that none gets in? Or is there an access method to clean the chamber out?

I had planned on using a BIG flotation chamber, since I wanted a flat floor raised a bit off the bottom of the hull.... The space beneath the raised floor will be made into one or possibly multiple air chambers. Because of how small my design is, I had planned on making a drain holes) in the deck. The cap for that drain would have a pin-hole (to protect against atmospheric changes, but keep water from entering very fast) and if the bottom of the boat did ever get a leak, I would at least have an easy way to pour out the water that got into the chamber. But I've seen 15'-20'+ boats done this way, and from what I read they appeared to weld the chambers entirely shut.


I don't have any boat-building books yet, so this might be a question they address......
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[Robbo]

what type of welding are you talking about?
With MIG and TIG there is no flux that I am aware of, just some soot which you brush off for appearance.
If you are talking of a type of welding I am not familiar with, if its a sealed chamber the weld will be external and so would the "flux"?

Why not just put a bung in the sealed chamber to drain any water out?

[EuroCanal]

All the boats that I have sunk in have had an access hatch to the floatation chamber. They are usually about 20cm circular plastic hatches - twist to close. Just don't forget to close them.

In an ally boat you could bolt the hatch shut for more security. Some boats have foam in the chambers for added security in case you hole the hull in that area.

[DougCim]

Originally Posted by Robbo

what type of welding are you talking about?
With MIG and TIG there is no flux that I am aware of, just some soot which you brush off for appearance.
If you are talking of a type of welding I am not familiar with, if its a sealed chamber the weld will be external and so would the "flux"?

The welding I will use is oxy-acetylene, that requires flux.
MIG would not work for the shin sheet I'll use (.50"-.080") and I can't afford a TIG, but the TIG welds in thin sheet seem to be a pain to get sealed anyway.

Most people will flux with parts and the filler rod. I haven't done very much aluminum welding. I'll have to see how it goes with just fluxing the rod.

Why not just put a bung in the sealed chamber to drain any water out?

That's what I meant--a bung in the floor, on one side of the boat.

Originally Posted by EuroCanal

...In an ally boat you could bolt the hatch shut for more security. Some boats have foam in the chambers for added security in case you hole the hull in that area.

Adding a hatch would mean a bit more materials and a bunch more work. It would be easier and lighter-weight to just install a bung and then weld the decking down.

The main reason for the chambers is not for safety, it's because I wanted most of the deck of the boat lifted up off the hull bottom.
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[alidesigner]

Seeing as though its a small boat have you thought of gluing the deck in? You could use Sikaflex to glue it to your deck stringers and then seal the edges. Will be strong, water tight and wont buckle due to heat (thin plate combined with oxy heat could end up with a lot of buckling).

[WestCoastFab]

Im not sure about your way of welding, I belive flux will leave little holes in your welds that water will get through. you need to weld with a mig machine
.030 or .035 wire is what your need with a argon mix or misson mix, Clean material or you will get peroxity in your weld causing it to be weak and leak.

With material that thin you are better off to rivit the material. As for floatation in your floor if you get water in there the foam will decay just like a dock in the water...... I would put your floatation under your seats and in the bow somehow...

I would leave your hull with some empty cambers for storage if anything and install a bilage pump in there incase you get water in there....dont forget aluminum sweats and will eventually fill up with water.....


Good luck

[DougCim]

Originally Posted by alidesigner

Seeing as though its a small boat have you thought of gluing the deck in? You could use Sikaflex to glue it to your deck stringers and then seal the edges. Will be strong, water tight and wont buckle due to heat (thin plate combined with oxy heat could end up with a lot of buckling).

I wanted the low-maintenance aspect of aluminum, though.
And the buckling isn't a problem, if the boat is tacked up as much as possible and then welded in very-short-seam runs.

Originally Posted by WestCoastFab

Im not sure about your way of welding, I believe flux will leave little holes in your welds that water will get through. ....

With material that thin you are better off to rivit the material......

The gas welding works fine, the flux doesn't leave holes.
There is definitely a learning curve though, and some special equipment helps a lot.

As far as riviting,,, it can sure work well, there's people with 30+ year old canoes that still are sealed--but I don't have the equipment to do it.

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I am left to just practice welding this year though, between learning to get decent welds and saving the money for the supplies, I won't have enough time to start any boat this year. I have no indoor shop space, and don't really want to be outside welding a boat while it's snowing (though it can be done) so I'll have to wait for next spring.

And I can't decide exactly what I want to build anyway.
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[alidesigner]

Originally Posted by DougCim

I wanted the low-maintenance aspect of aluminum, though.~

Sikaflex is fine for gluing ali so you can glue your ali deck in.