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Physics question - possible mythbuster?

Discussion in 'Basic Scuba' started by raoulsttexas, Jan 12, 2011.

  1. raoulsttexas

    raoulsttexas Divemaster Candidate

    # of Dives: 50 - 99
    Location: New Jersey for now; formerly Pacific NW
    First off, pardon my ignorance on the technicalities of the below scenario. I'm not a physics guy, nor even an engineer.

    I was watching a documentary on the evolution of submarines. They mentioned that the props used to cause a huge problem in that they would create bubbles, and therefore sound, and be easier to detect. Evidently, this is called "cavitation". Modern subs have a proprietary blade which reduces cavitation.

    So, as I understand it, caviation is caused when the water on the "backside" of a blade creates a low pressure point and the water boils due to the decrease in pressure. O2 is extracted and produces a bubble.

    I'm sure there are man MANY details in the above that I am not mentioning nor aware of, but assuming I'm in the ballpark of the above being correct then how about this scenario:

    Cave diver is caught in an OOA situation and finds a cavity that is large enough for him/her to keep their head out of the water and is filled with air (big assumption...i know). I'm picturing something around 2'x2'x2'.

    The question is, how much oxygen could be generated by waving one's hand, fin, or any other object on a normal scuba rig, and would that be enough to sustain life?

    I'm guessing that enough air could be generated to prolong life, but only minimally. I've seen my hand make those bubbles when I wave it quickly in water, but they're small bubbles. The documentary mentioned that the average sailor on a sub needs 400cf of air per day to survive. That seems ridiculously low to me. They MIGHT have said 400cf of O2, and that would make more sense.

    Either way, any of you physics gurus want to chime in on this one?
  2. JKPAO

    JKPAO Captain

    # of Dives: I'm a Fish!
    Location: Southern New Jersey
    look up or gooogle Alon Izer Bodler Artifical Lung
    See you topside! John
  3. RJP

    RJP Scuba Media & Publications

    # of Dives: 1,000 - 2,499
    Location: New Jersey
    As I understand it, the "bubbles" created in cavitation are "vacuum" so technically, there's nothing there to breath.

    Take a bottle of water and bang it on the table to see this effect. The water molecules move away from the side of the bottle faster than other molecules can fill back in, creating a brief vacuum that looks like a "bubble" of air, but it's actually bubble of "nothing" hence a "cavity."

    Mythbusters actually looked at this in a episode on breaking beer bottles, and what you see is the rapid collapse of these vacuum cavities cause the bottle to break.
  4. raoulsttexas

    raoulsttexas Divemaster Candidate

    # of Dives: 50 - 99
    Location: New Jersey for now; formerly Pacific NW
    AHHH...found it.

    Alan-Izhar Bodner Artifical Gills

    Pretty interesting.
  5. raoulsttexas

    raoulsttexas Divemaster Candidate

    # of Dives: 50 - 99
    Location: New Jersey for now; formerly Pacific NW
    So, in other words, any exurtion used to wave your hand / fin around would be so inversely disproportionate that you'd just eliminate your air a little sooner.
  6. raoulsttexas

    raoulsttexas Divemaster Candidate

    # of Dives: 50 - 99
    Location: New Jersey for now; formerly Pacific NW
    So, to counter this...and mind you, I have NO idea of what I'm talking about... In the same documentary, they were explaining how modern subs create an "infinite" supply of O2 by reducing the pressure of seawater (to nearly a vacuum) and causing it to boil. It then releases the O2 out in the form of a bubble. That 02 is harnessed and used to supply air to the sub.

    Also, it would seem that if cavitation bubbles were actually vacuums, then they would immediately implode on themselves.
  7. Pearldiver07

    Pearldiver07 IDC Instructor

    A couple of things here - related to a small degree. RJP is right, but in a sense. And your last sentence about them imploding is dead on.

    The bubble formed is in fact a bubble, but can only exist in the lower pressure in the wake of the blade. As the bubble moves away from the blade it collapses because the higher water pressure of the surrounding water is greater than the bubble's surface tension can withstand, causing the sound that gives away the existence/location of said submarine. This is the same principle of "cracking your knuckles" although the actual mechanics of it vary.

    For the O2 creation for breathing air, the O2 is, if I remember correctly, trapped by crossing a membrane. You can then collect it via pumps and cylinders to augment breathing requirements as part of environmental management. This, combined with an ongoing process of adding pre-cursor material, keeps a flow of new O2 being released at the desired rate.
    Last edited: Jan 12, 2011
  8. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
    The system used in subs to 'extract' O2 from water requires an immense volume of water flow to extract a proportionally tiny amount of O2.

    You're talking millions of litres per hour, processed 24/7 to supply a crew.

    It's not feasible with current technology to minaturized this process to supply the needs of a single diver. It certainly isn't feasible that you could do it 'by hand'.
  9. emoreira

    emoreira Dive Resort

    # of Dives: 200 - 499
    Location: ARGENTINA
    Hi raoulsttexas :
    The principle of cavitation is fine, but the bubbles formed in the backside of the propeller blade (the side that faces bow) are boiled water, not Oxigen.
    As soon as the bubble moves away the propeller, as Pearldiver07 explained, it collapses (changes the physical status from gas to liquid) and produces the noise.
    There is no chemical reaction (oxigen separation from water molecule), only change in the physical status of water from liquid to gas.

    The oxigen generated in a submarine comes from the Carbon dioxide exhaled by the crew. The same method is used in the space shuttles and vehicles.

    The fresh water generated on board a submarine comes from boiling seawater (at reduced pressure) with the refrigerating water of the nuclear plant on board, plus a process to make it apt for human consumption.
    The same principle is applied in any modern ship, but the refrigerating water comes from the diesel engine.
  10. Garrobo

    Garrobo Contributor

    # of Dives: 200 - 499
    Location: Ohio
    I don't think that I would be breathing ANY air captured in a pocket trapped in a cave, thank you.

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