Should both sides of a hull seam be welded? (Alumnium)

It seems to be a common theme that on large plate boats both sides of the hull seams are welded. Which makes sense due to a lack of penetration on thick plate say .25" or thicker. When looking at small Jon boats with thin sheets they are only welded on the outside. When doing some research I found since 5xxx series aluminum is non heat treatable it is much more resistant to heat from welds. But I read a paper that did find that welding made the joint weaker from the annealing process that takes place around the heat affected zone. Would that affect be amplified by welding the same seam twice on both sides?
I am looking to work on an 18ft boat .100" 5052 alloy. Original I planned on MIG welding the inside and TIG welding the outside seams. Then for added protection and strength adding a piece of angle on the outside of the hull seam. Completely welding that making 4 total welds within a couple of inches of each other on the same plate. That amount of heat does concern me for base material strength.
Thoughts and opinions are greatly appreciated!

 

Are you building this boat from new, or are you repairing it? 0.1" thick plate is 2.5 mm thick, so that should be weldable ok with care.

How are you proposing to do this? And why? It seems to be a bit of an 'over kill' to me.

The ally cat in my avatar photo has 6 mm thick bottom plating, with 4 mm side plating, and we double continuous welded (inside and out) the welds below the waterline only.

 

Depends upon the thickness.
See below as your guide: -

Unfortunately, that is where a little knowledge is dangerous.
All aluminium has a high thermal conductivity, not matter its Series.
Being non-hear treatable, is merely its metallurgical composition of how the alloy gains its strength.

Just explain in more detail what it is you wish to do....and don't over read, it will take you down the wrong path, without context.

 

This will be a new boat build. I want to maximize possible strength over kill is the name of the game for me. Attached is a photo of one of the lakes I fish on, that is my reasoning for the angle protection over the seam. As well as the possibility that one piece of the weld might get compromised by contamination or user error. I would like the piece of mind that there are two welds holding. Unless that does the opposite! Would it be over kill or harm the strength of the joint by welding both the inside and outside of the same seam?
The boat is an 18ft semi-v

 

Did you incorrectly state the angle or butt plate would go on the outside??

Ad Hoc is an aluminum expert; you are very lucky he has engaged your query.

I think your concern is with the seam integrity and the butt plate is to help with that. But you cannot butt the bottom to sides; so what's the use in plating others?

Your question seems to be more about how to guarantee plate weld integrity.

check pic, please, too

I'm only asking, not going to answer against Ad Hoc.

 

This is a photo of the angle piece that I am referencing on the outside hull seam. In the photo it looks like the "angle piece" they used is two welded pieces. I am planning on that being one whole angle piece. In the photo to the right I made a diagram of the cross section in question. The red arrows are where I am planning on welding. Unless where the hull butts together (yellow) if welding that on both the top and bottom would compromise the integrity of the base material compared to not welding both sides.

 

Are you doing modifications to an existing boat (by adding this extra reinforcing angle in way of the stem), or are you building a new boat from scratch?
If the latter, is it your design, or can you provide a link to the design?
If you are building a completely new boat, I would look at the possibility of having a flat bar stem, rather than a shallow vee stem.
This flat bar stem could continue the whole way to the transom. If the hull bottom plating is say 3 mm thick, the flat bar stem could be 6 mm.
It would be relatively easy to bend, as it only has to bend in one plane - I think you will find it pretty difficult, if not impossible, to bend a 'one piece' angle bar around the stem shown in the photo.
If you wanted extra reinforcement, and not such a bluff / flat face on the stem, you could then weld a half round tube to the flat bar - again, this would be relatively easy to bend in one plane (assuming that you can cut a tube in half longitudinally fairly easily).
This is what we did to the stems of the cat shown in my avatar photo.

 

To make sure I understand what you are saying about a flat bar you are saying to do this below?
This is a new boat build (2.5mm thick). Even when doing a design like this would it compromising to the hull plate to weld both the inside and the outside of the butt joint?

 

Or are you referencing this, with the optional half tube?

 

I am proposing the arrangement as shown in your 2nd post above (ie post #9).
If the hull shell is 2.5 mm thick, then the flat bar stem and keel plate could perhaps be 4 or 5 mm thick.
And you would then have just one weld on each side of the flat bar, running the length of the stem and the keel.
We added a half round tube as shown in post #9, and while this doubled the amount of welding in way of the stem / shell plating joints, it has worked well for almost 24 years now.
Our keel / stem bars are 100 mm wide (ie just under 4") - the side shell plating is 4 mm, the bottom plating is 6 mm, and the keel / stem bars are 10 mm, on a cat that is 15 metres / 49' long.
The 10 mm thick keel / stem bar was a bit of an over-kill - I am sure that 6 mm would have been fine - but she has been beaching the bows on the sand 2 or 3 times a day for most of the past 24 years, and there is still a lot of thickness left where the keels usually make contact with the sand.

Whereas another ally power cat (which is used as a coastal water taxi, with multiple beach landings every day) here had an angle reinforcing on a shallow vee stem (like what you were initially proposing) - and a freelance welder had a nice job every 4 or 5 months renewing the 'sacrificial' angle locally in way of where the keels came into contact with the sand.
They finally changed their keels to a flat bar arrangement some years ago, and the welder has not been called back again since then.....

 

If you keep giving out advice you will put welders out of business! To make sure I understand what you are saying are you saying weld like the left or right of the photo. Where red arrows are the welds?

 

If you reference Ad Hoc's excellent table in post #3, you only need to do one pass for your 2.5 mm thick plate.
Hence the red arrow on the left would work - but here you would still have to weld the side shell plate to the stem bar first, and then weld the half round tube on afterwards.
So that would effectively result in two passes, both on the same side, in way of this joint.
In view of how only one pass is needed, then maybe you could do it like the two arrows on the right - do a continuous weld on the inside of the hull to join the stem bar to the side shell and bottom plating, for the length of the half round tube, and then weld the half round tube on.
You could taper out the half round tube just below the waterline perhaps.
And then do the single pass welds joining the flat bar keel to the bottom shell plating on the outside.
But I would welcome other thoughts and suggestions here.

The company that built the cat in my avatar photo currently uses US$ 2.50 / inch of welding when they are doing estimates for aluminium new construction and repairs, as the consumables are very expensive (especially the argon gas ), hence you really want to keep the amount of welding down to a minimum as the cost can easily add up.

 

The reason fro that is simple.

It stiffens up the stem/bow region and aids in impact resistance.

Most often in these small run about type of boats, there is not much access for a good weld up in the bow if you add a lot of structure internally, for stiffening.
So we tend to use a stem bar, imagine a flat bar that extends beyond the hull. This ensures a good weld between the 2 plates of the hull and at the same time makes it stiffer, as the joint is no longer just 2 plate butted together, but now it is shaped like a "T", which is stiffer.

However on small boats this can be difficult to do, especially as/if the Vee of the hull plate joints becomes more acute. So, simple solution add an angle bar, or fabricated angle bar to that region of the bow/stem on the outside of the hull, to stiffen it up. It is very common practice.

 

There are also things Ad Hoc and bajansailor have mentioned like joint configuration and accessibility...but I think those the two reasons above are enough. Just to add to the heat affected zone comments and costs for Al...the more filler material, the larger the heat affected zone and cost...so try to make the weld volume as small as possible.

 

Thanks for all the input. I am confident enough to weld the boat. BUT by no means am I a master tradesman that has been at it for a decade.
Therefore I am comfortable spending a little more on materials for the confidence knowing that if a small section of weld gets compromised it is reinforced on the other side.
I am saying that currently with the assumption that welding the same joint on both sides doesn't compromise the plate substantially more than one pass would.
Is that a correct assumption to make that the HAZ doesn't get substantially more compromised by two passes (2.5 mm).