self-righting life rafts

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

  1. PAR
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    PAR Yacht Designer/Builder

    I have to agree with Gonzo TANSL, you've intentionally looked to find fault with his posts and interpretation of his syntax, which frankly isn't a point you should make much of an issue over, considering your often quite broken English, which has to to be taken with considerably more interpretation than his. You've (assumingly tongue in cheek) offered him help in this regard, though your comments have mostly just found what you perceive as flaws, when in fact, most of the time you both are saying the same thing. This continued type of banter isn't productive and places you in a bad light, in terms of your attitude and the perception others, particularly that of forum newcomers, might have about you personally and this place in general. In other words, if you have a point you'd like to straighten up or clear the air about, skip the vague insults and barb throwing and state the points of discussion you have issue with, then everyone can learn, without weeding though this petty school yard stuff.
     
    Last edited: Nov 13, 2014
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  2. TANSL
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    TANSL Senior Member

    PAR, I greatly respect your opinions and, therefore, I will reflect on my attitude. I have not attempted to insult anyone, just say, when so I think, what is been said is not correct. Henceforth I will say that something is not right, without going into personal considerations. Here you are very right.
    You have better manners than me, when speaking english. So, can you prevent some people talking about what they do not know?. That can confuse those who make an inquiry.
    Thanks, really.
     
  3. anson_07
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    anson_07 New Member

    Ok, so i have some questions.
    1) for a life-raft to be self righting, when inverted Cb should be above Cg to create a righting moment? if i am right
    2) ideas on how this can done in terms of design of the life-rafts?
     
  4. TANSL
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    TANSL Senior Member

    I think that given the forms of a life raft, is not likely to be self-righting. You can try to facilitate the maneuver upright. In inverted position, the Cg must always be above the Cb. You must manipulate shapes to that in inverted position, the depth is as small as possible. This is achieved, for example, by a sealing ring around (similar to the RIB)
     
  5. gonzo
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    gonzo Senior Member

    One design that helps a raft to be self-righting is an inflatable arch over the top. It also doubles as support for the tent.
     
  6. PAR
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    PAR Yacht Designer/Builder

    If it's simply an exercise, then there are several things you can do to cause the boat to self right, while still retaining some level of operational effectiveness. A high mount, fixed chamber or inflatable bladder would seem a simple route to look toward. This is the rescue boat approach.
     
  7. RAraujo
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    RAraujo Senior Member - Naval Architect

    Sorry TANSL but I didn't understood your statement:

    QUOTE
    Wherever the metacenter is (always above the waterline), if the center of gravity is above the center of buoyancy equilibriume will be insetable.
    UNQUOTE

    This does not seem to be true for most ships...
     
  8. TANSL
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    TANSL Senior Member

    RAraujo, I'm glad you questions because maybe I have not explained well. That is not true when the ship is upright (submerged submarines are another matter). When the ship has shifted the center of gravity should be above the center of buoyancy, to help recover the boat.
    If I'm wrong, have no problem telling me.
     
  9. DCockey
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    DCockey Senior Member

    A sailboat with a heavy, deep keel can have the center of gravity below the center of buoyancy. It will be very stable.

    Righting moment increases when the center of gravity is lower.
     
  10. TANSL
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    TANSL Senior Member

    I think we are talking about life rafts. There is another active thread about self-righting boats.
    But what can we say about the sailing boat when it overturned ?. Is its center of gravity below the center of buoyancy?
     
  11. RAraujo
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    RAraujo Senior Member - Naval Architect

    TANSL

    I still think I do not understand your point...

    Read again what you wrote

    QUOTE
    Wherever the metacenter is (always above the waterline), if the center of gravity is above the center of buoyancy equilibriume will be insetable.
    UNQUOTE

    In most of ships the CG is above the centre of buoyancy (except submersibles when submerged and heavy ballasted sailboats)...

    Have a look at the sketches here for clarity...
    http://www.mathalino.com/reviewer/fluid-mechanics-and-hydraulics/stability-floating-bodies

    Rodrigo
     
  12. TANSL
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    TANSL Senior Member

    RAraujo, you are quite right. I'm saying big nonsense. Thanks for detecting my mistake and with good manners let me rectify.
    Many times I have tried to make the information given here is correct and now I am insisting on a serious error: the center of gravity tends to be, and should be above the center of buoyancy for a stable equilibrium. And, of course, below the metacentre.
    (I've corrected my post # 11)
     
  13. mchl
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    mchl MCHL Stabilitycalculation

    The only condition to have a self-righting boat is that the Gm must be <0 when inverted. When CG is below the CB when inverted it is not possible :GM will be always >0.
    To have a negative Gm CG must be above inverted CB and the metacenter must be below CG. That means that you must have a small waterplane inertia. You can have a part of the solution with a superstructure volume higher than displacement volume : The boat will float on his quite narrow superstructure and the deck will not be immersed.

    Have a look to this video :
    http://www.dailymotion.com/video/x16d1c7_lorient-un-bateau-auto-redressable-en-demonstration_news
     

  14. jimburden
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    jimburden Junior Member

    Life raft weight and water keel

    A self righting life raft needs an air tube hooped cage to support the canopy to not let it turn turtle. To keep it from flipping in the first place it needs a companion small weight with a self filling water bag with several shorud lines from the bottom going to this overturning resistant mass held below the bottom. Come to think about it, the shroud line could even be a water filled cone protruding down form the bottom deployed by the realativly small weight. This might also serve as a beaching water anchor to reduce rolling in surf and prevent wind overturning. I think this idea is already in use.
     
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