[BigBand]

So - first post here at the forum, and I need some help with a welding project.

I'm building a boat ... actually a barge. 10' long, 6' wide, and 2' high. Material is 5052 .125 aluminum. Purpose is to use this as a duck hunting blind. It will be towed into position - won't have it's own propulsion.

What I'm worried about is warpage. When I weld those long outside joints (and maybe the inside joints too) I'd sure like to see nice straight lines in the aluminum, not looking like a river is running through it.

Right now I'm thinking I need to tack everything together at about 4" intervals before doing those long welds.

Questions: will I need to skip weld (weld 6 inches here, then skip a few feet and weld some more), or will it be safe to just go for it?

Any ideas about how to draw the two pieces together before tacking? The only thing I have come up with is to weld a stick of angle along each side near the edge, and use clamps to draw the two sheets together. Kind of like clamping cauls.

I'm also wondering if just welding the outside joint would be enough, or would I expect to need to weld the inside as well. Obviously waterproof is a must, but the barge won't be seeing any really tough duty.

I'm sure other questions will arise as some of these get answered.

Thanks!

[Rick_H]

First welcome... Swing in the new member section and introduce yourself when you can.

What are your plans for the inside support structure? Do you have a sketch of your plan you could post?

Tacking often and skipping around is a must... 5052 welds very easy, nice fluid puddle, fit up will be key. Depending on your design a rachchet strap could be wrapped around and used to pull it together.

[MosquitoMoto]

I've heard wiser ones than me recommending the use of stainless or copper chill blocks for this kind of work, too.



Kym

[big gear head]

I've found that sometimes TIG is not the best process for welding things like this. Just getting a puddle started for a tack can cause a huge bow in a sheet of aluminum. Sometimes MIG works better when welding sheets if you want to reduce warpage. It's a lot quicker than TIG, so making tacks and short welds doesn't put as much heat into it. I know that most people don't have the equipment to MIG weld aluminum, so many times it's not an option.

[BigBand]

What are your plans for the inside support structure? Do you have a sketch of your plan you could post?

Calling this a 'boat is a travisty, but it's the easiest to imagine. Think of this as more of a floating 'box'. vertical transom, almost vertical sides, and a slanted bow. It will have 2" x 2" x .125" extrusions around the top, to add stiffness. Right now, no other inside structure is planned, pending actual testing. The load it will be under is 3 men and a dog, and that will be only when the barge is stationary.

Here is a link to a commercial version of what I'm making:
http://www.gatortraxboats.com/boat-mode ... model.html

Ratchet straps sound like an interesting approach to insuring good fit up while I tack.

I've heard wiser ones than me recommending the use of stainless or copper chill blocks for this kind of work, too.

Are these chill blocks functing as heat sinks? Would I place them very close to each weld to keep heat from traveling down the aluminum? Any other materials I could use? Stainless anbd Copper are difficult to get, and expensive.

... consider MIG instead.

While I have aluminum MIG in the form of a spool gun, I've been shying away from it as I've learned to do TIG, but maybe I need to re-activate those MIG brain cells for this project.

Excellent ideas folks - keep 'em coming!

[Braehill]

Big Band,
Welcome to the forum,
The ducks don't care if it's straight if your aim is straight.

I agree that mig is probably best for this and I don't say that often.

If you can do it without full penetration, you can use a block of Aluminum as a backer, but if you're not getting full penetration I would weld both inside and out.

Len

[Coldman]

As a general rule for long joints like this its best to start in the middle and work your way to the ends skipping as you go. This reduces restrained stresses and reduces the risk of cracking later particularly with aluminium.

[Coldman]

By skipping I meant to say back stepping.

[MosquitoMoto]

BB -

Yes, chill blocks as heat sinks, placed near the weld. If you are careful (and not achieving full penetration) aluminium might just do it.



Kym

[BigBand]

By skipping I meant to say back stepping.

Hmmmm ...... Back stepping, not sure I know what that is.

I can do MIG, and I understand the idea about starting in the center of a long welded joint to reduce stresses.

[BigBand]

By skipping I meant to say back stepping.

Hmmmm ...... Back stepping, not sure I know what that is.

I can do MIG, and I understand the idea about starting in the center of a long welded joint to reduce stresses.

Ok, I did some research on what back stepping is. As I understand it, I start the weld at inch number 1 and weld towards 0, then start at 2 and weld towards 1, then start at 3 and weld to 2, etc.

Any feel for about how long each weld should be? Still start in the middle, and back step out toward the ends?

Also, can I use a wet towel as a chill block? I don't have access to any blocks of metal, and I'm not sure how I could clamp one to the middle of a 10' piece of sheet.

[Coldman]

Step about 4" should be ok. Don't use a wet rag you certainty don't want to introduce moisture into your weld pool. If you don't have chill blocks you can always use compressed air to cool it between runs if its getting out of hand. Mig is ideal for thus kind of work.
Also nobody would accuse you of overwelding if you welded both sides of the joint particularly with aluminium.

[BigBand]

OK, 4" welds. Should I start in the middle and back step towards each end, or just start at one end and back step along the whole length?

[Coldman]

Start in the middle and back step toward the ends.

[BigBand]

Thanks for all the advice guys!

[Coldman]

No probs. We'll all be interested in how you got on so please let us know and maybe some pics as well. Cheers.