I do a fair job with my little MIG welder on any kind of steel. I've got a
gas bottle and flow regulator for stainless. That takes a little more work
to get jsut right. I do heavy steel stock with my cracker box AC welder,
but the best welder I ever used for heavy stock was a Hobart DC stick
welder. Wow talk about smooth.

Hi there,
Ive been a full time welder for about 5 years now, and a tinkerer most
of my life before that. I spent a year doing contract work stick
welding trawlers but in the last year and a half most of my work has
been aluminum; big tip trailers, and aluminum plate boats.

Here's my 2 cents worth... I dont think it really makes too much of a
difference what alloy you go with, particularly for fresh water. Just
go with something that isnt too hard. We use 5083 marine alloy, but a
lot of the 5000 series alloys are suitable. You'd do well to decide
early on if you want to go for a 'plate' boat or a 'pressed' boat.
Plate boats use relatively thick sections (1/4" or more) of aluminum
and hold their shape well. The boats my company makes are of this
variety, take a look at the web page if you like (note, this is not a
plug)

www.barcrusher.com.au

Typical dinghys and runabouts use much thinner sections, sometimes 1/8"
or smaller. The smaller sections wont keep shape without being formed
a bit, and the most common way of doing it is by pressing the sheets to
give them a sort of a clinker look. You can get the same sort of
effect by buying a rolling wheel, marking the lines on the sheet then
rolling them on the wheel.

The heavier boats are much more solid and easier to weld, but you're
probably looking at around $6,000 to build a 20" with a cuddy cabin -
not including fitting it out or an engine. Aluminum prices are going
up and up. If you're going with plate, you're pretty ok, you should
get away with yusing your small mig. You'll need a plastic liner for
your gun, and different tips. If you're using flux core, you'll need
to swap to a non-grooved roller set.

If you're going to go with pressed, you'll probably need a lot of
skill, or a better welder - preferrably both. The welders we use at
work are about $20,000 Fronius units with all the options (push pull
gun, water cooled leads) that do a synergic pulse mig, and have
adjustable current *on the gun* in the form of a dial or buttons in one
amp steps. This gives extremely fine control over heat and its
adjustable while you're welding, so you can turn it up as you get to a
corner, or down if you start to burn through. They also have 4 step
options (hot start, run, crater fill) to get a tig-like experience in a
fraction of the time. The synergic pulse is truly excellent for
overhead and vertical up welding and gives a minimum of spatter.

In truth, like me, you will probably never be able to justify buying a
unit like this, but thats what it takes to compete at that sort of a
level when it comes to aluminum. All the players have them. If you're
going to go ahead with your (im guessing single phase, off brand)
welder, start by getting some reasonable size sheets 1/4" or so. If
there's a boat maker in your area hit them up for scrap. You'll
probably get a box full for a six pack.

Aluminum is great in some ways - unlike steel you'll never get a pretty
looking weld that doesnt do anything. For this reason alone, i think
the standard of aluminum welding is generally higher than steel. It
has to be spotlessly clean, or you're wasting your time. use a
stainless wire wheel, or a flap disk. Temperature and root gap are
critical. Filling gaps is not easy. Unlike steel, it doesnt give any
signs the weld pool is too hot, so you'll be quite surprised to see the
puddle drop right through the plate every time it happens. Dont sweat
it too much; it still happens to me ;-)

you may find that setting the welder is hard, partly because it doesnt
make that good sound you get with steel, and partly because you may
only have 5 voltage settings on your unit. Hotter is better, so start
by just 'tack' welding until you get the hang of it. You'll see that
just before it drops out, the pool goes from greyish to a bit shiny,
then it sinks a little, then very quickly disappears. Once you can
control the puddle to the point you can 'sink' it level with the plate
and not drop it out, you're doing well. butt welds are tricky, so
progress to a fillet. a 'step' in your gun movement of around 1/4"
will give a pretty looking weld.

The most critical things are probably cleanliness of material, correct
heat, consistent gun angle and consistent stick out, in that order. If
you get a lot of grey/black discoloration around your bead it means
you're not getting enough gas coverage - move your gun closer.

If i were in your position, Id see if there is a local boat builder who
would take me on part time on weekends if i worked for free. In return
you'll learn how to do a professional looking weld, probably pressure
testing, have access to heaps of scrap, and they'll usually let you use
the equipment after hours.

good luck ;-)