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In a message dated 11/3/2010 2:47:39 P.M. Eastern Daylight Time,
JimToddPsub@aol.com writes:
I'd love to see posts about what others have encountered during hull fabrication With Git Kraken I ordered rolled parts ( two 84 inch dia. heads ) from
a supplier and to get exact measurements I used the "Egyptian Method" of
assuring roundness.
I first measured the circumference of both pieces with a flexible
steel tape measure held flat against the edges with small magnets placed at
intervals. Luckily the two heads were within 5/32 of an inch which is good
considering the circumfrence is 22 feet 5/8 inches. I marked off increments
( match marks ) and important center and compass points around both
perimeters for future use as " match lines ".
Then to check roundness I cut an "arc section" out of 24 gage ( thin )
sheet metal that was about 50 or 60 degrees. Using this by hand I slid it around
the inside of the tubs ( tank heads ) and used a soap stone to mark
any areas that appeared to be out-of-round, judging by the presence of a
gap or high spot that showed as the arc section template was dragged around the
"lip" of the tub.
I was again pleased to find that the two heads were nice and round
with only a few areas that showed a tiny gap or flat spot. Never more than an
1/8th of an inch.
After making sure the lower tub was perfectly level, the next step was to
lift the other tub up and set it on the lower tub so the two
"raw" ends were lined up. I used wooden blocks between the two tubs to maintain
a space of 6-1/8 inches to allow room for the short 6 inch straight section
that would be installed.
Once the upper tub was positioned EXACTLY centered over the lower tub, I
welded 1/4 inch by 2 inch flat bar
spars to the inside of the two heads ( using a small mirror to see the arc
) spanning the 6-1/8 inch gap. Once the two tubs were welded to
the spars, the wooden blocks were removed and the three rolled sections of
6 inch hull plate were "laid" into the gap and tacked in place. This formed
the 6 inch straight section between the two tank heads.
I wrapped the 6 inch wide "band" with a chain and used a come-a-long to
squeeze it down so it fit flush with the upper and lower sections of the hull (
tubs ) and by using a small peanut grinder with cutter blade I adjusted
the length of the band to fit both tank heads. A straight edge spanning the
two edges was used to ensure a flush surface and small weld tacks were placed
all around the perimeter. The first series of tacks were on 12 inch
centers, both upper and lower seams. I used the soap stone "match marks" to keep
everything right where it needed to be as I worked my way back and forth
adjusting, hammering, prying, and coaxing the two seams into near perfect
alignment before tacking in place, making sure all the "match marks " lined
up.
Eventually I ended up with weld tacks at 2 inch spacing all around the two
perimeter seams.
At this point I had the basic hull sitting on the floor all tacked
together.
I next laid out the hole for the hatch and cut out the 24 inch hole in the
top tub.
I crawled inside and removed the weld spars that spanned across the 6 inch
middle "band" section, ground off any weld lumps or spatter, and checked the
inside surface to ensure it was as " flush" as the outside.
I took the sheet metal arc section and slid it around inside the hull again
on the two tank head edges and the newly installed center band. There were a
couple spots that had flattened just a bit on the perimeter so I used lengths of
wood 4X4, and wedge blocks knocked into place on the inside to "stretch" the
hull sections round again. This sounds rough but the damn thing was almost
perfect anyway so it didn't take much to push it where I wanted. I never had to
push more than an 1/8th of an inch out.
I cut a length of 2X2 wood to just under the inside diameter and slid it
around the inside, hammering the wedges in and out until the 2X2 just
touched all the way around.
At that point the hull was less than 1/8 inch out of round with an 84 inch
diameter.
I climbed out and tacked the two seams at 1 inch intervals, climbed back in
and checked the roundness again. It hadn't moved.
I went outside and tacked the two seams at 1/2 inch intervals, and checked
the roundness again. It was GOOD.
Now remember these two seams are 22 feet long each so although it sounds
simple in the telling, it took a long time. I used a TIG torch to set all the
tacks, and all the edges of the hull parts were pre-beveled to the correct
angles, and there was a uniform 1/16 inch air gap between the two tank head
edges and the center band. I used small pieces of 1/16 inch dia. weld rod
to maintain the gap.
I welded the root passes with TIG using an alternating pattern,
starting with 4 compass points, moving to 8, then 16, then 32, etc. until both
seams were completely welded closed.
Many times during this stage I was checking the inside for roundness. As I
added more points to the root passes, the diameter would shrink
slightly but flex back most of the deviation as subsequent root welds
were added around the perimeter.
After it was all welded I found that the inside diameter had shrunk by
almost 3/32 inch. I sanded the end of the 2X2 and checked it again. It was still
very close to round so I knocked out the wedges and blocks, checked it again and
found it was still within 1/8 inch across the 83-1/8 inside diameter.
Using the sheet metal arc section on the smooth inside, it didn't even
show as much as a credit card thickness of out-of-round gap and
the 2X2 rod "just touches" all the way round.
I used the same method of pattern welding as more weld beads were laid down
over the root welds.
I am confident that the out-of-round on my hull is well within the required
tolerances.
As more things are welded to the hull, if I expect any stress to be applied
from a particular weldment, I install blocks and wedges to hold the hull
dimensions true and tack everything into place and then remove the blocks to see
what's happened.
I had to re-stretch some areas of the tubs when I cut and welded the big
window in the front, but it wasn't too hard to knock the wedges in until
the hull was right back where it started once everything was tacked in
place, ready for final welding.
Once the big window assembly was welded in, I removed the blocks
and it was still within the 1/8th inch tolerance across the 84 inches.
You need to get a feel for the elasticity inherent in the steel and
use it to your advantage. By using the heat of the welds to pull the hull into
the shape you want, and hard blocks to prevent movement where you don't want it,
a very round vessel can be made.
My treatise on " The Egyptian Method "
Frank D.
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