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RE: [PSUBS-MAILIST] Hull Construction Questions



Charles,
 
Although it is counter intuitive, it can be demonstrated there is no structural difference between having the stiffeners on the inside and the outside. On the outside would increase the usable space inside the sub, or what is equivalent, you might be able to use a lower displacement hull and fit the same crew inside. On the other hand, you might want to put a skin on the outside of those stiffeners just to fair things. Then again, the space between the two might serve for ballast tanks, etc. There are plenty of designs with stiffeners inside, outside, or a combination of inside and outside.
 
There is a fabrication issue I ran into, however, that I haven't seen referenced in any of the engineering books, which tend to concentrate on the calculations. Making the stiffeners internal actually helped in a big way to make the cylinder rounder. Without the stiffeners, the cylinder was definitely not circular. The stiffeners pushed it into a proper circle, even though my hull is pretty thick (3/8"). I suppose I could have figured out a way to stretch the cylinder out until I welded it to an outside stiffener, but it doesn't strike me as an easy thing to do.
 
rgds,
 
Alec

	-----Original Message----- 
	From: Charles Perkins [mailto:chuck@memetech.com] 
	Sent: Mon 1/10/2005 5:29 PM 
	To: personal_submersibles@psubs.org 
	Cc: 
	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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