CoG/breadth ratio

Discussion in 'Multihulls' started by ImaginaryNumber, Jan 28, 2010.

  1. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    In the design of blue-water sailing cruising catamarans, is there a generally accepted maximum value for the Height of Center of Gravity to Hull Breadth ratio? I am thinking of stability in storm conditions, being hunkered down, with all sails reefed.

    I understand that less is best, but at what value does more become too much? I realize when considering hull stability there are many more variables to consider than merely hull breadth, but I'm wondering it there is a general rule of thumb for the above?

    Thanks,
    John
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    No and yes.. Not just like that you mention, AVS (angle of vanishing stability) is closest to your ratio, actually about the same thing in degrees..
     
  3. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Is there then a particular Angle of Vanishing Stability which is considered a prudent maximum for cruising catamarans? Hummm, I think it might actually be a MINIMUM value that I'm looking for. Is it correct that the higher the value the better?
     
    Last edited: Feb 1, 2010
  4. oldsailor7
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    oldsailor7 Senior Member

    I am not sure I understand your question.
    Do you mean "Hunkered down with all sails reefed" whilst still sailing.?
    Or by "Hunkered down" you mean "Hove to" with all sails furled and lashed down.?
    There is a big difference in these two situations and we can't answer your question until you define it a little better.

    Just trying to help. :)
     
  5. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    oldsailor7,
    I mean 'hove to'. Just trying to survive in a bad situation. I'm wanting to get a better understanding of how just the catamaran itself will react in strong winds and breaking waves. How high can the superstructure get (or the center of gravity) before an experienced designer would say that it is too high? I realize that safe catamaran design must also include many other factors, like hull shape and buoyancy and aerodynamic shape, so perhaps my question is too broad to generalize on?
    Thank you,
    John
     
  6. oldsailor7
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    oldsailor7 Senior Member

    I am no expert on the subject, but the narrow beam, tall superstructure catamarans of the past are not seen today.

    From my own experience in the open ocean, where the shallowness of the sea bed is not a factor, big waves (40') are about 400' long so the Cat just rides up them sideways and takes the crest abeam.

    If the crest is breaking, it tends to throw the hull up---but immediatly passes under the craft and lifts the other hull up before the boat can even think about capsizing.

    It's a very uncomfortable motion, but the Cat can take it better than the crew can. (Ask Richard Woods).

    No I don't think the CG of the structure of a modern Cat is as big a worry as the overturning moment of the sails.

    Cats of 45' and over have so much beam these days that they are more likely to lose their rig than capsize.

    Just MHO. :D
     
  7. ImaginaryNumber
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    ImaginaryNumber Imaginary Member

    Exactly so. By spreading the hulls further apart and/or lowering the superstructure the effect was to lower the Height of Center of Gravity to Hull Breadth ratio. So I was wondering if there was a general rule of thumb that described current best practice for designing cruising catamarans?

    Many power catamarans appear to have a much higher superstructure than sailing catamarans do. Because they are powered I suspect they can afford to be less concerned with aerodynamic shape. But I also suspect that in spite of their relatively high superstructure they have a low center of gravity by having their heavy engines and fuel tanks low in the hulls.
     

  8. TeddyDiver
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    TeddyDiver Gollywobbler

    ISO standards and other classification standards have quite much stuff you are questing for.. There's no simple answer bcs much depends about the designed category, length etc..
     
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