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

Two passes...with cooling...of the same volume on each side? Not likely...
Two passes...without cooling doubling the weld volume on one side? Way beyond my metallurgical knowledge...so I asked my wife. And she said she will need much more info or "you should go consult on a welding forum"....general topic of conversation was based in length of each pass.

 

Again, it depends upon the thickness and your skill level.
Often a single pass weld may be enough, depends upon the above and the set up.

But if you were to do 2 runs, the first would be the main root weld, turn the piece over and grind/gauge out the excess and do the final pass for a full-pen weld.

If you dont wonder/weave with your torch, the HAZ remains as the first pass, basically.

It only becomes an issue of you reweld over those welds endlessly...as the continual heat input increases the grain size which leads to weaker joint.

 

Thank you for the help! As a garage hobbiest I'm trying to decide the best route to go as I'm building my skills. Currently I'm leaning towards a pass on both sides for the hull seams. As for the inside console, seat bases, etc... one pass should be plenty taking time and materials cost into consideration.

 

Take your time, think things out, ask questions like you are doing and bear in mind that excessive weight can negatively impact safety more so than enhance it. Practise on some scrap with various welding techniques then destroy them to gain some sort of idea of relative qualities. I have noticed that foam "boat collar" thingies are cropping up around here, primarily marketed towards safety, but they make good fenders too. I suspect that a person with the skills to build a boat could sort out something like these, they are comprised of layers of about 1" thick sheets of high density closed cell foam, hopefully with good UV resistance. Boat Stabiliser for Small Boats under 5m | Kapten Boat Collars https://boatcollar.com.au/ Make sure whatever you do cannot trap water.

 

You had originally mentioned that you were considering Mig on the initial inside welds and tig on the outside.

Mig and Tig welding speed in inches per minute were difficult to find for .100 thick aluminum. So I am referencing .125 inches
Mig 250 ipm to 300 ipm (Miller weld)
Tig 12 ipm

Mig at 250 inches per minute, it is difficult to ensure a 100 percent EQUAL penetration butt weld. Try it and then look at the back side. A discontinuous weld can/ more than likely cause
issues of stress concentrations and susceptibility of cracking. Tig at 12 ipm means that you can much better control and create a more uniform bead but can create issues with distortion.

On thin aluminum .125 in, Mig is the best way to go if you can control the distortion, BUT the length of time that you are heating the weld area with Tig will increase the HAZ zone.
The speed difference between Mig and Tig is about 10 times.

FYI. you need know that .100 is not recommended for Short Circuit welding only spray transfer. For .100, you should consider an aluminum pulse machine if you are
going to use Mig.

While we never used .100 on a boat hull, we always used Mig on .125 and above and TIG only when we had access to one side only.

As far as accessibility is concerned, if you are using a spool gun, spend the extra 80 to 100 bucks and get a flexible nozzle.

Always if you can, weld both sides. At every start and stop, take a grinder or skil saw and cut a bit back into the previous weld to eliminate crater cracks.
When one side is done, back saw the weld until the black line shows up, and then weld that side as well.

It is unlikely that say a 2 inch flat bar will follow the picture of the boat that you provided. I would imagine that it would look like more of a curve to lie flat on the stem.

 

Based on nothing more than owning an aluminum boat and observations (@Barry ) here; why not make the bottom a bit thicker than 0.100"?

my old 16' Lund is 0.125", i.e.

dumb guy asking questions

 

A good point
On our first 21 foot jet boat build, we used 3/16 on the bottom. Due to distortion in welding, we had to add additional stringers to "straighten" the bottom. Ie additional stringers added
more weight. We then moved to 1/4 inch and reduced the number of stringers and the weight was pretty much the same except 1/4 bottom was not prone to warping with welding

.125 is a much better idea, particularly on the bottom where most of the loading and stiffening stringers will be required. For little if any weight difference meaning cost difference

 

In my neck of the woods all the boats are trending thicker again. I've pieced together work boats that hit bottom in a commercial fishery that are in the 16 to 18 foot range. When building my own I'll err on the side of thicker, and prefer heftier plate as opposed to doublers. We do inner and outer but we've all bought 255 and air cooled push pull guns. The pulse mig manages heat so dang well.