I have a difficult and interesting design project to work on and I am trying to get my hands on some ballpark figures from people who know engineering a little better. Help is appreciated!

I am trying design a lightweight pressure hull for a hybrid between a submarine and sailing vessel. The real question here is how thin does the hull have to be and of what possible materials. (All of the below figures are only approximate because their final form is affected so much by all of the other figures final forms.)

There are a large number of constraints in this problem.

  1. The pressure hull needs to be ~40' long with ~8' diameter.

  2. It needs to hopefully weigh under 20,000 pounds

  3. It needs to dive to 40' with a safety margin of 60' (built for 100' or ~44psi).

  4. It's only internal structure would ideally be shape keeping rings to keep a little of the force in tension.

  5. It needs to be manufacturable under 1,000,000 USD (foregoing labor costs)

  6. It needs to allow for a hatch, portholes, and other imperfections without completely eliminating the structural integrity.

Possible materials I have done minimal research in are steel, carbon fiber, fiber glass, and some impracticably expensive exotic alloys.

Another option I have looked into is taking an existing internal pressure steel tank, like those below, and converting it. The issue is I cannot figure out how to calculate the conversion from internal to external pressure. http://www.arcenergyequipment.com/index.php/inventory/h-separators-vessels-128.html

Light, cheap, big, strong... something has got to go.

If the constraints are impossible, any numbers and equations you could point me to would be great.

Have a great day! -Ben

  • $\begingroup$ Mightr find this useful : engineeringtoolbox.com/stress-thick-walled-tube-d_949.html as a start... $\endgroup$ – Solar Mike May 31 '19 at 5:19
  • $\begingroup$ None of the vessels shown in the reference are for external pressure . Vessels for service below atmospheric pressure ( eg vacuum pipe stills) have large external rings. $\endgroup$ – blacksmith37 May 31 '19 at 23:48
  • $\begingroup$ @ solar mike ; Actually API "wrote the book" on thick wall pipe collapse, Bul 5 C 3. Because these calculations are necessary for any well casing design and steel mills developed "high collapse " pipe for this application. $\endgroup$ – blacksmith37 Jun 1 '19 at 15:04

Let me first iterate that I’m not a professional engineer but if you can settle for a harder material, a composite like Kevlar or fiberglass could do. However, if you are willing to use a more flexible material, you’ll want more than rings inside the hull just to be safe.

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