Dive Cups - Submarine Legend

 Here's an interesting topic for our Physics friends: How much sea pressure is outside the ships' hull, and what crushing effect can it have? Well, it makes any sub sailor's SKIN CRAWL to even consider sea pressure outside the hull (in the unlikely, yet nightmarish event that we might come face to face with it one day), but let's attempt to answer the question. There's a neat conclusion here, so bear with me.

Given that the sea exerts pressure somewhere around 440 psi every 1000 feet, and since we know TUNNY's diving depth (or at least we think we know, given the characteristics table) - we can calculate what the sea pressure MIGHT be at design submerged depth. That's wasn't very interesting, was it?

And, how can we appreciate this amount of pressure? Well, we could swim out of the escape hatch, where we would be instantly squished or develop insect-like singing voices - or we could squash cups. This is the focus of this topic, "Dive Cups".

During July, 1992, TUNNY performed a deep dive (Dive number 716). This was the only dive (in my 4 years) onboard TUNNY where the crew squashed up some dive cups. I don't know how frequently this occurs on other boats. Here's how it works....

 

SHORT LESSON IN SUBMARINE CONSTRUCTION: A submarine is basically composed of two long steel tubes, one inside the other. The internal structure is called the pressure hull, and it separates the ocean from the crew's environment. It is constructed of thick steel that has a high yield strength to withstand the enormous force of the ocean when the boat is submerged. The outer hull creates the external shape and forms the ballast tanks

 

APPLIED PHYSICS TO CUPS: Look at the diagram, notice the part where the ocean and air is? Well, before we left Pearl Harbor, we tied a mesh bag right there. Inside the mesh bag were styrofoam coffee cups. What do you think happened when the boat dived? RIGHT! The air space filled with sea water. As we went deeper, sea pressure started working on the styrofoam cups, and they compressed - becoming little tiny cups. They eventually went from 8 ounce cups to about 1 ounce. It was a pretty cool physics lesson to consider 400 pounds of pressure per square inch crushing the cups. It's also pretty un-nerving when you thing about torpedoes or flooding or what could happen if the pressure ever found its way into the people space. *shudder*

Anyway, to really appreciate the experience, you've got to write your name and the date on your styrofoam cup. Then you've got an unofficial Navy souvenir! I made a cup for everyone in my family (me, wife and both kids), although they didn't seem very impressed.

 

And, for the conclusion to this thrilling submarine legend - here they are! I am showing a normal 8 ounce cup to serve as the "BEFORE" comparison, and my four dive cups as the AFTER.

 Some of you internet visitors may consider yourselves gifted physicists -- I invite you to compare the original and squished cups surface area, and use the difference to calculate applied pressure, and therefore determine test depth of a Sturgeon class submarine.

To learn the real answer, you must enlist, work your way through Nuke school and pray you don't get assigned to a surface ship! Ah ha ha ha ha! (evil laugh)

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