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Hi Hugh. Although the conventional wisdom says no ribs are needed, I'm not
so sure. First off, I cut a big hole in the top for the hatch. To replace the
metal, I made the hatch flange 1-1/2 inches thick and put it directly in
line with the hull, plus the 3/8 inch thick hatch ring at 6 inches long. I
have no "tower" really. The hatch ring sticks up only 1-1/2 inches above
the hull, and protrudes 4 inches into the sub. I just had the feeling that with
that big hole in there, and the elastic nature of metal, it was a weak
spot.
I'm no engineer, but I've got 40 years of welding and general metal working
experience and it just seems like it's a good idea. Think of this....the hatch
at 24 inch O.D. has 452 square inches. If the pressure is say 18 atmospheres at
600 feet, how many pounds is actually pushing on that hatch?
Next, the actual shape of those steel tubs isn't
hemispherical. It's kinda flattened on top. So the pressure pushing down isn't
completely compensated by the pressure pushing in from the sides. Also, I've got
some big holes where the windows are cut in. Again, I've replaced the metal
removed with the window frames and flanges, plus the location is on a more
curved part of the hull, but it still creates a weak spot.
I take that whole "hull calculator" with a little
skepticism. I know it's based in sound engineering principles, but I'm not sure
It takes all the complexities of windows, hatches, metal irregularities and
induced stresses into account. I've built a lot of stuff over the years, and
I've got a pretty well developed "gut feeling" for what works. Besides, I need
more weight anyway, why not use the additional weight to add strength. I've got
to add almost a ton of lead to this thing. Steel is cheaper than lead.
Frank D. Get trade secrets for amazing burgers. Watch "Cooking with Tyler Florence" on AOL Food. |