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Boats over canoe size need ribs to keep the water outside from pushing the sides in. While i am building a round hull, the 1/4 plate itself isn't enough to guarantee a round shape, or deal with point contact loads, like being pushed against a dock or piling. So here's how i built 4ft diameter perfectly round ribs.
I had to start with building the tools to build ribs. Here's the jig to hold all the bits in place for welding. It's sitting in the brake infeed table there. I'd never built a boat rib form before, but here it is. Now i need it off the brake table, down lower, and be able to rotate the work.
Here's the power roller to curl the 2 inch wide, 1/4 inch thick, "ribbon" steel into a round rim or flange shape. I roll thru about 2ft more than needed, then cut off what i need with the torch, place it into the form, mark it exactly, and cut with the bandsaw.
The bandsaw cutting. I clamp the flange while in the form, so the cut is thru two thicknesses, and the cut is identical on both ends of the flange when done. Once back into the form, it's often a hammer fitting, and no space for the thickness of paper.
Outside flange fitted, clamped outwards and down. I tack weld the butt joint so it won't offset, because i remove some of the clamps for the following step.
First, make a jig to swing the torch in a set of arcs, as the ID and the OD are different enough. I ran thru 4 bottles of oxygen one weekend, and the cuts weren't perfect, and the heat tended to distort the arcs, and it was sunstroke hot doing this in the sun, and cleanup was a royal dirty noisey pain. I had the next batch done by Besco Steel in Birmingham, they have a CNC torch, and can cut underwater if needed.
Here's the arcs after cleanup, clamped down on risers to center them on the flange. I tack them to the outer flange. Then the inner flange is laid in, marked, cut, reinserted, clamped, and tacked.
This is the tacked up rib, coming off the assy jib, to go inside to the turntable there for continuous welding of the bits together. I know, but it's not a Star Gate! It looks jagged on my browser, but it isn't.
The setup for bulkeads is pretty much the same, only no inner flange. Which i may rethink.
I liked the inside welding turntable so much, i decided this form needed to be rotateable also. After all, why should i move me and the tools to the steel, when i can have it move to me? This also makes it more portable (it's rounder now, and rolls). And gets it off the infeed table for the brake. This is it upside down, after being built. You just cannot buy these things in a store down the street, you know?
Here it is sitting upright, ready for business. It spins nicely.
It tilts 45° too, you wondered why the legs were tilted in like that, didn't you?
The turntable rotates on these bearings. It's quite the closeup, actually. That's a 1/4 inch dia bolt. The bearing is a R8-2RS. After all the welding was done, i replaced the common nuts with nylocks.
The bearing assy made this way. I wanted to use a smaller and cheaper bearing, which fit the 1/4 bolt, and i drilled the supports for that. Then due to the angle, the boltheads scrubbed some, and rather than grind them, possibly making putting a wrench on them problematic, i opted for a bigger bearing. Here's the sequence of making a 1/2 id bearing fit a 1/4 bolt. The drilled out nut is drop-in 1/2 od, so centers the bearing, and the bolt is grade 8.