Smallest practical size for a steel boat?

For mig welding I can envision gas control at the weld point could be a problem if welding out-of-doors. How big would that issue be? I assume one would need to wait for calm days to lay down the mig welds. Alternatively, would flux core work well for thin material, or would one face the same difficulties as using a stick welder, like blow through or distortion?

 

It would become part of the boat.

You can use a ceramic backing of course, and I have seen, but never had our guys use a type where small ceramic blocks are attached to a tape and act as a heat sink.
I had seen this at Fabtech a few years ago. The tape holds the ceramics against the back of the weld. Not really sure if you need them but is an option

 

Our guys have never used a flux core wire with mig. My experience with 14 guage welding with stick was from many years ago and the machine shop that I worked in for a couple of summers
did not have a mig machine. They mainly did heavier structural but as I needed a fuel tanks for a jeep that I was building, the welder showed me how to weld 14 gauge with a stick welder.

Certainly mig in windy conditions can be tricky, unevenness of the bead, BUT the advantages for mig over stick are too big to overlook.
Back around 1980 a company that I owned built over 3000 fuel tanks, from 25 gallons up to 150 gallon capacity. All out of 14 gauge hot rolled mild steel, Mainly gasoline but 1/4 diesel,
in the automotive sector. All Migged of course.
One of our welders built 8 120 gallons tanks which is over 400 lineal feet of weld, 16 scully flanges, 16 draw nipples without a leak. All the welds were from one side, due to accessibility.
This would be hard to duplicate with a stick welder for sure.
If you are welding outside, you can just set up some screens around the weld site to reduce windage issues

Just to make a previous point, you will have a lot of distortion with 14 guage.

 

Plate thickness depends on the design too, the spacing of framing being most significant.

The thinner the the more important it is to fully shape it before welding. Wherever the plate is sprung around a curve and pulled into shape the more it distorts in the weld zone especially welds across the plane of curvature.

If MIG is an option it's the quickest easiest and cleanest by far but I'd opt for gas shield rather than flux core and just work around the wind when you can (but never when it's wet with any method).

There are some very cheap perfectly adequate single phase hobby grade welders. But buy the wire on 15kg spools from a welding supplier and run them on a spool stand, rent or buy a gas bottle rather than using the disposables.

 

Looking at car bodies, they seem to be a marvel of thin steel construction. Can any of those processes be used for small boats? How about riveting, spot welding, preforming, gluing, lapped and rolled seams or whatever?

 

For a small boat it is easy to build a tent or temporary shed.

 

spot riveting- my take on this is that due to the flexing of a sheet metal boat that the spots could break as shown in this video. The new Ford superduty with the aluminum box makes use
of spot welding to hold things together,
Riveting- Valco, Lund and many more thin sheet metal boats are made with rivets. Because the rivet joint is not continuous, often a mastic, in the form of a tape is put between the
sheets to stop water ingress. At least at failure, ie movement of the sheets with respect to each other, the failure would be a leak first before the sheets peeled away from each other.
It works for airplanes, it works for boats, but it seems like most of even the high production boats in sizes above say 20 feet where thicker sheets are used for rigidity and strength, most manufacturers are welding the boat
Gluing- a possibility but with aluminum?
Lapping and rolling seams- a good spot for corrosion to take place

 

A Fay 32 is to be build from 3 mm steel plate for the whole hull skin, some info from the FAQ...

 

They write:
"The frames usually only need welding to the hull skin where they meet the chine bars. Longitudinal stringers which are always stitch welded along the length of the hull, join the skin to the frames."

If the framing has a section modulus calculation that includes the plate as a flange ( already factored into construction rules ) then the plate must be welded to the frame.

It's perfectly valid not to attach the plating but then the framing itself will need to be heavier and may require an inside flange instead.

 

Thanks for the info Mike . .

Here's some views at Paul Fay's framing: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12

Pic 1 on page 2 and pic 1 on page 10 is a Bruce Roberts Spray, the rest are Fay boats, pic 1 on page 8 shows a Fay 32, maybe there are some more of pics of a Fay 32 under construction shown, but it doesn't tell everywhere the kind of the shown boat.

Fay 32 - (pic 1 on page 8)​

 

From the thread: Small Steel Boat...