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Brian,
The process is called "drawn over mandrill" or DOM.
Tubing is available in both welded and DOM forms. I didn't realize they
draw tubing in a size large enough to be used for a sub hull, but maybe.
Out of roundness isn't that much of an issue to warrant DOM
tubing. Tubing can be rolled from sheet and an acceptable tolerance held
for a sub hull with no big problems. What little out of roundness there is
can be worked out while welding in the frames.
As far as the welded seem, orient the seam so it's on top and
the conning tower cuts out some of it. On a two place K-350
there is only four feet of seam weld on the hull tube.
Dan H.
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