[PSUBS-MAILIST] Lurker finally surfacing!

Wed Aug 29 22:32:00 EDT 2018

Cliff, agreed, but using worst case configuration would give a good conservative starting point.  With that in mind, I used 150 inch length seam to seam since I didn't have info about the tank heads.  Those would have to be calculated separately anyway to determine which was weaker, the cylinder or the heads.  I also assumed no stiffeners, just a hollow cylinder with two tank heads.
Using 150 for Line #12 (Ls) and Line #15 (Lc) in addition to .001 for stiffeners, the spreadsheet shows overall buckling (Line 63) is 842 psi with n=.5 which seems unreasonably high.  I think the inter-stiffener Pa result is still valid since ABS uses the same inter-stiffener strength calculations for unstiffened cylinders with the exception that F is not calculated but assigned a static value of "0".  However, I do defer to your expertise.

Jon

      From: Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Wednesday, August 29, 2018 7:25 PM
 Subject: Re: [PSUBS-MAILIST] Lurker finally surfacing!

Jon/Jim there is not enough info in the thread to do this ABS stress calculation rigorously.  Not sure if the 12.5' length is overall or SS (seam to seam).  Also we need details of stiffeners including dimensions of the inner stiffeners and spacing.  The ABS spreadsheet is for stiffened cylinders.  As you note, you can trick the spreadsheet by putting some stiffeners in with 0.001 web and flange thickness.  If you do this, then you need to ignore results (maximum allowable working pressures) associated with stiffeners and just look at the depth associated with overall buckling.  With this diameter of a hull, even with 0.625" wall it really should have stiffeners to address the concern of overall buckling.   This is what happens on an externally pressured unstiffened cylinder when overall buckling occurs.
Cliff

On Wed, Aug 29, 2018 at 10:09 AM Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

I'd like to see Cliff and Sean respond.  
The unstiffened cylinder calculator and Cliff's hull spreadsheet are pretty close, the former at 315 feet and latter at 308 feet.  Both are based upon ABS rules but Cliff's spreadsheet does consider some of the finer and nuanced points of failure.  There is one section of Cliff's spreadsheet calculator (inertia requirement) that shows a cause of concern but I think it is due to the fact that his spreadsheet requires stiffeners for this section to be calculated correctly and I assume Jim's cylinder has none.  The spreadsheet will not accept "0" for a stiffener dimension to simulate a cylinder with no stiffeners, so I usually enter ".01" as a value with the distance between stiffeners the overall length of the hull to represent an insignificant stiffener effect.  Using .01 for stiffener values, the "inertia requirement" section shows max depth of 5 feet.  Using K-boat stiffener values the same section shows max depth of 101 feet, and using more appropriate stiffener values for this hull of .625 thickness and 2 inch flange the same section shows max depth of 227 feet.  Now, I'm pretty sure...would bet money on it...that Jim's hull will easily survive 5 foot of depth without collapsing so I don't know how to really interpret the inertia requirement section of the spreadsheet in this case.
Sean's calculator doesn't seem to take into account cylinder length so I couldn't use it as a triple check on the other two calculators.

At this point Jim, I would tread cautiously with max depth until you (or we) are comfortable with a figure.  Personally, I think your original max depth of 100 feet is reasonably appropriate.  An unmanned test dive when the sub is ready would be highly encouraged.
Jon

      From: james hughes via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Alan via Personal_Submersibles <personal_submersibles at psubs.org> 
 Sent: Tuesday, August 28, 2018 9:24 PM
 Subject: Re: [PSUBS-MAILIST] Lurker finally surfacing!

Thanks Allen for that hull calculation!  I hope you're right!  I thought it might be shallower than that without reinforcement.  I've tried to build a good safety margin in all the parts on the boat (using thicker steel in the conning tower, for example, than what the K250 plans called for).  Still, this being my first boat I don't really want to go deeper than I could safely get up from on SCUBA (I've got a 2" flood valve if I ever need to get out while on the bottom).  Plus most of the sea life I'm hoping to see is usually in shallow water.  But you're right, little more investment now may make a difference if I decide to go deeper for some reason in the future.  
THANK YOU again and God bless, Jim   On Monday, August 27, 2018, 2:51:34 PM PDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Hi Jim,thanks for the introduction, and welcome along.Can't help you with the seals as my motors  are external.I have been making my own thrusters & have contacted seal reps &even emailed the manufacturer for advice on a non standard mountingmethod I was proposing. Normally seals are used on pumps to keepfluid that is under pressure in rather than out as in our case.I did a quick calculation on your pressure vessel & it came out as a 600ftcrush depth. Don't take that as gospel, but it may be worth looking at itin depth if you haven't already. If the 600ft is right you could dive to 300ftwith that hull! For all the work that goes in to building a submarine you mayfind that it is only a relatively small amount of extra effort & cost to giveyou an extra 200 ft of diving capability. I would think about this first beforechoosing a seal as the added pressure may have bearing on your choice.All the best,Alan (New Zealand)

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