self-righting life rafts

Discussion in 'Stability' started by anson_07, Nov 9, 2014.

  1. anson_07
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    anson_07 New Member

    i am working on this project of self righting life raft for my college.
    can anyone care to explain on how it works. initial research tells me that that when inverted it is in an highly unstable equilibrium such that center of bouyancy is above the center of gravity. but cant find any mathematical proof of it. any guidance would be much appreciated.
     
  2. gonzo
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    gonzo Senior Member

    Have you taken basic physics and/or statics yet? If you did they cover the stability conditions.
     
  3. anson_07
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    anson_07 New Member

    I am currently pursuing marine engineering so yeah I do have statics and naval architecture which includes ship stability.
     
  4. TANSL
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    TANSL Senior Member

    Gonzo, have you taken basic physics and/or statics yet? If you did you could you give a logical and reasoned answer to the question asked by anson_07
     
  5. anson_07
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    anson_07 New Member

    i did find what i was looking for if anyone else need it they can find relevant information to the topic here http://iris.lib.neu.edu/cgi/viewcontent.cgi?article=1038&context=mech_eng_capstone

    @gonzo working on a project requires one to have extensive knowledge on the topic not just basic stability conditions hence the reason i asked, maybe this is the wrong forum to do so.

    thanks anyways, if anyone can help me out further i would appreciate it.
     
  6. TANSL
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    TANSL Senior Member

    Although I have not read the file "Design of a self-righting kayak", I guess, it is always (at capsized position) to have the center of buoyancy below the center of gravity, but the fact that the kayak is perfectly closed and a man inside which can perform certain actions to break the equilibrium, makes it totally different from the problem of flip a capsized raft.
    If you think I can help you, do not hesitate to say so.
     
  7. DCockey
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    DCockey Senior Member

    The righting moment needs to be positive when inverted if the life raft is to be self righting in calm water. "Mathematical proof" is the same as ship stability.
     
  8. gonzo
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    gonzo Senior Member

    Hi anson-07: if you already took physics then there are a couple of ways of proving it mathematically. By integration, you can find the points of the curve where the equilibrium is stable and unstable. The stable points are the ones where the potential energy increases when the object is moved from the point of equilibrium. For example, if you hav a ball inside a bowl, the equilibrium is stable. Push the ball away from the bottom, and its potential energy increases as it goes up. Geometry can also give an answer by showing that a body is in stable equilibrium when the ballast is below the metacenter. However, calculus is more general.
     
  9. TANSL
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    TANSL Senior Member

    Sorry to disagree again, Gonzo, but I think some of the things you say are not entirely correct.
     
  10. gonzo
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    gonzo Senior Member

    Point one out
     
  11. TANSL
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    TANSL Senior Member

    • You may raise a book to a very high shelf. The book's potential energy has increased and its position on the shelf is completely stable, or unstable if you give it a push. When the book hits the ground, its potential energy has greatly decreased while its equilibrium is completely stable.
    • Wherever the metacenter is (always above the waterline), if the center of gravity is below the center of buoyancy equilibriume will be insetable.
    • I do not know how you can get the points of a curve by integration. By integrating what?. What curve are you talking about?

    Correcting my mistake : below rather than above (I wrote above).
    Center of gravity tends to be, and should be above the center of buoyancy .
     
    Last edited: Nov 12, 2014
  12. gonzo
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    gonzo Senior Member

    It would take for you to read a couple of books on calculus to understand how to get points of a curve by integration. Equilibrium points are maximae of the curve. It is usually taught in the first semester.
     
  13. TANSL
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    TANSL Senior Member

    That makes no sense, my friend.

    That may be true in some cases but the curves, I suppose, you're talking about (KN, GZ, DN) is not true. It is a blunder.

    Apparently you did not go to class that first semester, or if you were, you have not been able to understand.
    If you need help on the theory of the ship, I'm willing to give it to you.
     
  14. gonzo
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    gonzo Senior Member

    Could you stop hijacking threads? If you can answer the question of the OP, please do so and stop the personal attacks.
     

  15. TANSL
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    TANSL Senior Member

    Dear friend Gonzo, I already have offered my help to the OP for what he needs, read one of my previous posts.
    Now I'm just trying to get things that you say will not induce him to error. I believe, therefore, that in this way I'm also helping the OP.
    Again, if you need me to teach you something, on the theory of ship or naval architecture, I am at your disposal. I´m here to help.
    Cheers.
     
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