Re: [PSUBS-MAILIST] Hull Construction Questions

[Daniel Lance <lanceind@earthlink.net>]

Charles,
The question of "T" bar hull stiffeners being put on the inside or the
outside of the pressure hull comes up occaisonally. If the stiffeners are
put on the outside they are dramatically easier to fit and weld, offer
convenient attachment points for external components and equipment and
provide more internal space.
   There is a downside though.
If you have any pin holes in the fillet welds holding the "T" bars to the
pressure hull shell. Salt water will be forced in under pressure and remain
in the unwelded area at the base of the "T". Which will result in a
maintenance nightmare.  This is why most builders put the ribs on the
inside. 
Dan Lance 

> From: Charles Perkins <chuck@memetech.com>
> Reply-To: personal_submersibles@psubs.org
> Date: Mon, 10 Jan 2005 14:29:33 -0800
> To: personal_submersibles@psubs.org
> Subject: [PSUBS-MAILIST] Hull Construction Questions
> 
> 
> Hi guys. I have some more questions about submarine pressure hulls...
> 
> First of all, regarding stiffeners: I see that the single-hull designs
> (predominant on PSUBS for reasons of simplicity and economy I'm sure)
> place the stiffeners on the inside of the pressure hull. It appears to
> me that larger dual-hulled vessels such as the fleet submarine USS
> Pampanito have their stiffeners on the outside of the pressure hull...
> perhaps to conserve interior space, I don't know.
> 
> (check out http://www.maritime.org/fleetsub/appendix/pages/figa-01.htm
> ... very cool! Did the German U-Boats have their stiffeners on the
> inside or the outside?)
> 
> The question is this: does it matter whether the stiffeners are on the
> inside or the outside of the hull for pressure resistance calculations?
> 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?
> 
> 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?
> 
> 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? I am
> contemplating a situation with a double-hulled sub where in some places
> there is little inside clearance and in other spots there is little
> outside clearance. The diameter of the pressure hull itself would
> remain constant.
> 
> My next question has to do with the fabrication of the pressure rings
> themselves. I see them described as simple T structures with a ring
> thickness and height and a flange thickness and width. But Carsten's
> Euronaut has I-beams! Why? And how were they welded to the Euronaut's
> hull... Perhaps with a bead on either side of the I plate adjacent to
> the hull plate? Is the un-welded but adjacent portion of the I-plate
> unimportant? I presume so because I can't imagine how one could weld
> the face of one plate to the face of another plate... but then again,
> I'm no welder.
> 
> Another thing I've been wondering about is end-caps. Hemispheres and
> flattened domes are traditional... and inverted domes where the end
> bows in to the tube instead of out seem easy enough to figure out. I'm
> thinking of something different. What I'm wondering about is this:
> 
> What if I were to end the (horizontal) tube as though it were
> intersecting a vertical tube... without the vertical tube. Looking from
> the top you would see a tube ending in a round shape, looking from the
> side you'd see a tube ending in a square shape. My intuition tells me
> that it would be the same as doing a T intersection and I'd need the
> same sort of stiffening as if I were extending a tube upwards for a
> conning tower...
> 
> By the way, what kind of stiffening do I need if I'm going to extend a
> tube upwards of the same diameter as the pressure hull? (How do I
> calculate the pressure resistance of a T junction?)
> 
> And finally I need to calculate the volume of the intersection of two
> cylinders when one cylinder is smaller than the other. Imagine a
> conning tower here. It would be helpful to know how much it displaces.
> I know the formula for when the tubes are of the same diameter ...
> (16r^3)/3 ... but all of the methods for when the radii are different
> look horribly complicated. Specifically I'd like to know the
> intersection volume when r1 = 3.5 and r2 = 2.5.  Are there any math
> wizards out there that aren't frightened of the Elliptic Integral of
> the Third Kind? (shudder)
> 
> If I knew the intersection volume for 3.5 and 2.5 I'd know the
> displacement of the whole thing... that's the last bit that is missing
> as far as volume is concerned.
> 
> 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!
> 
> Thanks,
> 
> Chuck
> 
> 
> 
> 
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