Re: [PSUBS-MAILIST] Hull Construction Questions

[Ian Roxborough <irox@ix.netcom.com>]

On Mon, 10 Jan 2005 14:29:33 -0800
Charles Perkins <chuck@memetech.com> wrote:

> 
> Hi guys. I have some more questions about submarine pressure hulls...
> 
> The question is this: does it matter whether the stiffeners are on the 
> inside or the outside of the hull for pressure resistance calculations?

As far as I've been able to determine, with regard to cylinderical
pressure hull calculations the inside or outside placement of stiffeners
does not effect the end result.

The main consideration for outside vs inside stiffeners is the weld.
For outside stiffeners the weld is understrain as the cylinderical
pressure hull pulls away from the stiffener (due to compression of
the cylinder), if the weld fails the hull could calapse.  For inside
stiffeners, the weld is mainly to keep the stiffeners in place and
does not contribute to the overall strength.

> Obviously stiffeners of the same thickness outside the hull vs. inside 
> the hull will be heavier just because the diameter is larger. I am 
> wondering if they must be made thicker as well to compensate for the 
> reduced curvature... or does that not matter because the stiffeners are 
> just there to stiffen the hull, not to resist pressure?

Yes, the diameter of the stiffener will effect the strength and the
diameter, thinkness, etc. must be taken into account in the calculations.

> The very nifty spreadsheets on the PSUBS site don't specify whether the 
> rings are on the inside or the outside of the hull. Do they assume that 
> the rings are on the inside?

I'm not sure of this, but I think you can ignore the inside/outside
bit.  Providing you enter the correct dimentions for the stiffeners
the calculations should be accurate.  Note that the spreadsheet on
the website assumes all stiffeners are of the same dimentions.

> What I'm getting to is this: can a pressure hull have some of its 
> stiffeners on the inside the hull and others on the outside?

Yes.

I'm going to recommend that get a copy of Carl T. F. Ross's
"Pressure Vessels: external pressure technologies" (geez this
guy should be paying me to promote his book by now ;-).
Prof Carl Ross's book contains several examples of calculating
the collapse depth of cylinderical pressure vessels with different
sized stiffeners placed inside and outside of a pressure-hull.

Something else I've just noticed from looling over the book is
that the larger heavy stiffeners are outside, while the smaller
stiffeners are on the inside - i guess this is to save space
that would be taken up by the larger stiffeners.

> Once I learn the answers to some of these questions maybe I'll post a 
> design for your amusement. In the mean-time any insights on the above 
> matters would be greatly appreciated!

Please do.  I alway enjoy looking at other people's submarine
designs.

Also you might be interested in this:
  http://www.psubs.org/psub_pic/billv_pic.html

Although it's not out you described, the endcaps are similar.
I'm assuming you suggesting putting the conning tower on the
end and having it form the end cap as well.

Thanks,
  Ian.



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