Thanks, it's an interesting response considering that five pressure hulls of this design (all rated to 1000 or 1100 feet) have been ABS certified and performed something on the order of 10,000 dives commercially. I understand your reservations, and the structural issues with the pressure hull. The Nektons/Delta use the box itself as a structural member, and the designer builders did some extensive testing, both with models and the real thing. Whatever it seems like, this design works, and works well--at least to the 1000 foot mark. I'm assured that the same structure would work just fine at double that depth, using Mil-Spec metal (only nobody wants to pay for it). At any rate, I appreciate the response. It would be interesting to run the numbers and see how it works with modernus digitalis.
From: Cliff Redus <email@example.com>
Sent: Mon, 10 Apr 2006 09:14:08 -0500
Subject: Re: [PSUBS-MAILIST] structural question
To calculate the structural strength of this box-keel, you would need access to a FEA code such as ALGOR. A simple static linear version would be adequate. I used ALGOR for all the FEA work on my hull. It would be important to model the stiffened cylindrical hull and the stiffened box-keel stimulatingly subject to an external pressure say 1.5-2.0 times the max operating pressure. You could then examine the stresses in every area of the combined hull and box-keel to see if you have reached the maximum allowable stress for the A516 Grade 70. As mentioned by Dan, the slot that is cut out of the pressure hull will seriously compromise the strength of the pressure hull. For 300' ft of fresh water (130 psig), for a 12"x72" box dimension, there would be an equivalent of a 112,000 lbs force on each side of the box trying to collapse the box/hull. By cutting out a section of the "T" stiffeners from the pressure hull, you significantly increase the likelihood of overall buckling of the pressure hull. The other problem with this design is that flat plates, even with stiffeners, don't take external pressure very well at all. At the mid bay between stiffeners, the stresses would be high. My guess is that if you ran an FEA on this design, to keep the max stresses in the elastic range you would have to limit yourself to a very shallow depth. I would be very careful in adopting this concept for a 1 atm boat even with using 1/2" plate and 1/4" stiffeners for the box. If left the "T" stiffeners in tack for the cylindrical section, your maximum operating depth would improve but would still be significantly below that of the K-boat.
----- Original Message -----
Sent: Sunday, April 09, 2006 12:22 PM
Subject: [PSUBS-MAILIST] structural question
I'm planning on another K-sub, but thinking about a box keel like the Nektons and Delta had, rather than pods. Has anyone got an idea about how to figure structure on something like that? The Nekton's box was 9/16" wall, flat side and bottom, and reinforced with flat plates running athwartships on 18" centers (more or less). The hulls were the same thickness and the same material (A516 Grade 70).
I'm wondering if there would be any penalty to building a 1/2" thick box with 1/4" dividers matched to the ring stiffeners in the hull (ie., 12" centers). The box would be a foot square in cross section, and if all seems well, would be welded through a slot in the hull and covered inside with a reinforced aluminum deck plate gasketed and vented for charging. If the pressure hull is six feet long in the cylinder, that would give me room for a grunch of 85 amp deep cycle batteries (more than called for) and still not kill my payload.
Anyone not familiar with K-subs might think all this sounds pretty heavy. Have a look at the lead load in Dan's boat, if you think we don't have it to spare.