Re: [PSUBS-MAILIST] Hull Penetration and Reinforcement

[Jon Wallace <jonw@psubs.org>]

I've published the results from the latest run of my opening 
reinforcement program at http://www.psubs.org/design/pvho for discussion 
purposes only.  The values for nozzle thickness are a bit smaller than 
the previous results, but not by much, averaging about .05 inches.  
These values are based upon the formulas for Cylindrical Shells Under 
External Pressure in ABS Rules for Underwater Vehicles, sections 
6/19.5.1 and 6/19.7 which matches the rules used by the hull calculator 
spreadsheet on the web site.  I can now match various data points 
against the hull calculator.

There are a number of variables that can be manipulated to reach the 
final dimensions of a large penetration.  By going through the various 
ABS and ASME rules you could reach a unique configuration that might 
reduce the thickness of the nozzle.  For example, while a reinforcement 
"pad" is no longer acceptable by the standards for PVHO, a reinforcement 
"ring" such as was used in the PC-8 would be acceptable.  The difference 
between a "pad" and a "ring" is that instead of the reinforcement 
sitting on top of the hull as in the case of a "pad", the reinforcement 
ring is integral to the hull (becomes part of the hull with full 
penetration welds between hull-ring-nozzle) and therefore meets the 
standard.  I don't know how difficult fabrication of a ring would be for 
the home-builder however, and the setup/welding must be within very 
small tolerances.

As the diagram and text on http://www.psubs.org/design/pvho indicate, 
the results are based upon a nozzle that extends into the cabin.  This 
is not a requirement, however you get ALOT of reinforcement value by 
doing so.  For example, in the case of a design depth of 875 feet, a 23 
inch nozzle extending only .625 inches into the cabin of a 1/4 inch hull 
results in a 50 percent reduction of the nozzle thickness.  The 
extension into the cabin does not have to be much to get maximum 
reinforcement value, and by the standard is limited to 2.5 x 
Hull-Thickness.  So for a 1/4 inch hull, the nozzle only has to extend 
.625 inches into the cabin to get maximum reinforcement value.  For a 
1/2 inch hull the nozzle extension only has to be 1.25 inches.  You can 
of course extend further into the hull as is convenient, but any extra 
length beyond 2.5x hull-thickness can not be used for purposes of 
calculating reinforcement.

I am convinced as well, that by hiring a mechanical engineer and 
submitting plans with alternate configurations which could be justified 
appropriately, possibly with FEA attached, one could get ABS approval 
for a large penetration design incorporating a thinner nozzle.  However, 
I don't know what the costs involved are and whether it is financially 
feasible for us as home-builders.  I don't like the raw results the 
formulas pushed by the standards give us, and am still encouraging 
others to look into this issue to either confirm or challenge those 
results in case I've interpreted the standard incorrectly.  I would be 
more than willing to admit to misinterpretation of those standards, 
however I am pretty well convinced at this point that the posted results 
are accurate based upon the standards.

While I haven't made the calculator publicly available yet because there 
is still some minor work to complete, I will provide access to the 
program for purposes of verification, to anyone who wants to validate 
the process and formulas I've used.

Jon




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