Center of Moment

Discussion in 'Stability' started by KetchKorner, Jun 1, 2007.

  1. KetchKorner
    Joined: Jun 2007
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    Location: Michigan

    KetchKorner Recreational cruiser

    For several years in my spirituality classes, I've been drawing correlations to life's experiences and the dynamics of a boat in a storm using hull design and sail trim to mirror one preparedness for the unexpected. I have used a design term Center of Moment (CoM) to describe the theoretical (and dynamic) point within a hull's interior where the three axes (X,Y,Z) meet. I was told some years ago that this theoretical point in space moves about somewhat as any hull is heeled or otherwise altered (fore and aft, port or starboard) by the actions of the wind and waves. It was also explained to me that the lower (-Z) and more centered (near 0 on the X axis) this point was in a particular design, the safer or more stable that particular design would be.

    The basic question is; am I using this term correctly?

    I am seeking some expansion (in layman's terms) and/or validation of this concept and (ideally) the movement of the CoM in storms or rouge waves. Are there factors that are particularly destabilizing to CoM; i.e. narrow beam?

    The only thing I have ever found remotely close in the literature are the references to the movement of the Center of Effort (CoE) in sails given various sail sets and/or hull speeds.

    Can anyone help me better understand Center of Moment or point me to a design reference that explains it?
     
  2. Guillermo
    Joined: Mar 2005
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    Guillermo Ingeniero Naval

    I'm afraid I don't know what that strange Center of Moment could be.....May it be the Center of Buoyancy or the Transverse Metacenter? (You say it moves with heeling, so...)
    To clarify concepts, I recommend you the reading of: http://web.nps.navy.mil/~me/tsse/NavArchWeb/1/toc.htm
    Cheers.
     
  3. KetchKorner
    Joined: Jun 2007
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    Location: Michigan

    KetchKorner Recreational cruiser

    Continuing confusion

    Hello Guillermo,
    Thank you fro the prompt response. I was given to believe that the Center of Moment was:

    A: Unique to each hull design.
    B: Was a dynamic theoretical point in space that was in constant motion when a boat was underway and moved when the hull reacted to the various forces that could influence its position, heel angle etc.
    C: Regardless of where the Center of Moment was located in physical space, it also represented the "safest" place to be for vessels caught in a storm.

    These "facts" led me to believe that the Center of Moment would likely be below deck most of the time. The simple example I was given was the reaction of the Center of Moment moving in the opposite direction as someone moved outboard toward the gunwale in a light displacement sailboat where the human represented the human ballast and the Center of Moment acted similar to the Righting Moment which sought to bring the boat back to 0 degree of heel. This example operates in only one plane (Y) while in reality the Center of Moment is effected by all three planes of motion.

    Am I describing some other term?

    Also, thank you for the referral to the Nav/Arch website.
    http://www.boatdesign.net/forums/images/smilies/smile.gif
     
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  4. MikeJohns
    Joined: Aug 2004
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    MikeJohns Senior Member

    I like your analogy but with some refinement You appear to be combining a number of separate factors for your “moment”, and as Guillermo says there is none by such a name.

    There is a righting moment that is the combination of a shifting centre of buoyancy relative to a fixed centre of mass. This moment dances around the centre of buoyancy fore and aft as well as side to side. The lower the centre of gravity the greater the resulting righting moment. Statically both the gradient of the water surface and the wind acting aloft apply the forces that are opposed by the righting moment.

    Beyond this I would recommend one of the basic books on naval architecture such as John Teale’s “How to design a boat” pub Adlard Coles.

    I hope this helps
     
  5. kach22i
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    kach22i Architect

  6. bhnautika
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    bhnautika Senior Member

    Could we be taking about the centre of rotation!
     
  7. MikeJohns
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    MikeJohns Senior Member

    Roll, Pitch or Yaw ?
     
  8. bhnautika
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    bhnautika Senior Member

    Pitch around centre of flotation
     

  9. Raggi_Thor
    Joined: Jan 2004
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    I think for comfort you want to sleep close to the center of rotation :)
     
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