The stability of damaged ship

Discussion in 'Stability' started by xichyu, Apr 12, 2016.

  1. xichyu
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    xichyu Junior Member

    If a compartment damaged, the stability of ship is defined by righting moment,why is not it defined by righting arm?
     
  2. TANSL
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    TANSL Senior Member

    Suppose you know that the righting moment is equal to the displacement multiplied by the righting arm. Suppose you know the displacement of the ship in a load condition. Don't you think it would be indifferent to talk about one thing or the other ?.
    Incidentally, although there is flooded compartments, the boat's stability is defined in the same way.
     
  3. xichyu
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    xichyu Junior Member

    But a book quote “If a compartment damaged, the stability of ship is defined by righting moment”
     
  4. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Don't believe everything you read in a book.

    Further, how is righting moment defined for a particular loading condition? Does the righting moment change with, oh say, displacement or angle of heel?
     
  5. gonzo
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    gonzo Senior Member

    The complicated problem is to calculate how the free surface (water sloshing around) affects stability. It is a dynamic problem.
     
  6. TANSL
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    TANSL Senior Member

    When a compartment is flooded, the calculation of the free surfaces in it has no sense. That's an entirely different problem. You need to have very clear ideas about what happens on the boat when a compartment is flooded.
    The study of damage cases is a complicated issue and should not comment if you do not know anything about it.
    Indeed, the study of the effect of free surfaces is very simple, but very laborious. In the msl method, I'm sure you know, there's nothing complicated.
    Nor can mix the phenomenon of sloshing with free surface. Further study is needed before giving opinions that can be pretentious and ignorant and, of course, completely wrong.
     
  7. Rabah
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    Rabah Senior Member

    Hello xichyu,
    It is an elementary problem from "Ship theory". Separately about what there is a speech - injured or unimpaired vessel, the sense of calculation of stability will be that designer, classification organization and owner have been convinced that the ad-hoc loads restoring /righting/ moment of the vessel was more than the heeling moment arisen from outboard effect.
    Arms of the moments are only for calculation of these moments that is they indirectly participate in calculation of stability, but never direct are compared.
    ______________________
    NA Razmik Baharyan
     
  8. gonzo
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    gonzo Senior Member

    Sorry TANSL, in my experience ships float in water. Water enters the ship through the damage in the compartment. If there is no water intrusion, the stability is not changed. Take a Midol.
     
  9. TANSL
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    TANSL Senior Member

    Thank you, Gonzo, I will. And now, can you explain what does the entry of water into a compartment with what you say in your post # 4. What does the entry of water due to a damage with the free surfaces and, above all, with the sloshing?. Thank you for an answer that, I have no doubt, will be very clear and intelligent.
     
  10. RAraujo
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    RAraujo Senior Member - Naval Architect

    Before the advent of the computer, or before the computer made so easy to make all these stability related calculations, there were basically 2 methods to calculate the stability condition after damage:

    1. the added weight method in which the damaged compartment(s) were assumed to be filled with water until the level corresponding to the damaged draft; this method had the advantage of allowing to use the hydrostatics already calculated for the intact ship thus being much easier to use;

    2. lost flotation method which assumed that the hull, after damage, had lost the contribution of the damaged compartment(s); all the hydrostatics had to be recalculated with the eventual increased difficulty of dealing with an assymetrical floater.

    Theoretically the results would be the same (regarding the reserve stability). In method 1 the final displacement would be bigger (added weight) than in method 2 (where it would remain constant and only floatation would be lost) but the righting moment would be the same - thus being used for the definition of the stability after damage.

    I suggest you try to use a very simple model, rectangular prism, with a very simple tank (another rectangular prism) and try to compare the results from both methods.

    Today method 2 is universally used...
     
  11. xichyu
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    xichyu Junior Member

    I think “flooding”
     
  12. xichyu
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    xichyu Junior Member

    What is the meaning of msl method?
     
  13. xichyu
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    xichyu Junior Member

    I think it may be the problem of the book
    Flooding of a ship is a serious problem
    It not just influence the stability, but also the strength
     
  14. TANSL
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    TANSL Senior Member

    Araujo, good explanation. In either methods are asked to consider the free surfaces or sloshing phenomenon caused by the damage.
    Currently a third method, probabilistic, is being widely used.
    In the attached file it shows how to calculate the corrections to the initial GM and GZ due to the effect of free surfaces. This method is called mfs (free surfaces method), sorry msl is in Spanish.
     

    Attached Files:

    Last edited: Apr 14, 2016

  15. xichyu
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    xichyu Junior Member

    The reason is that if displacement is not constant, the righting arms alone do not give the complete assessment of the vessel's stability.
    Is not it?
     
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