Cross Curve for Small Displacements (4 tonne marine buoy)

Discussion in 'Stability' started by henrynel, Mar 5, 2013.

  1. henrynel
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    henrynel New Member

    Hi I'm new to this forum. I was wondering if anyone could briefly explain how to calculate the stability on a marine buoy which i'm designing as part of a 2nd year project.

    From various articles I've read it seems possible to calculate the initial stability using basic geometry which is valid for an inclining angle up to 10 degrees. I've done this shown that the metacentre lies above the centre of gravity. Hence stable.

    However for further angles (>15 degrees) it appears you need to either use software or cross curves to obtain a stability curve. Ideally i'd like to use cross curves however I can't find any which have a displacement <15000 tonnes? (The marine buoy i've designed is 4 tonnes) Can anyone advise me on this topic?
     
  2. DCockey
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    DCockey Senior Member

    How do you think that the displacement affects the process of calculating the curves?
     
  3. gonzo
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    gonzo Senior Member

    The horizontal distance between the Center of Flotation and the Center of Gravity will give you the righting arm. The math is the same for 10 degrees or 90. You can do it for several angles, make a graph and then interpolate if you need intermediate values.
     
  4. jehardiman
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    jehardiman Senior Member

    And unless it is a drifting buoy, the effect of the mooring makes everything different. Go get a copy of Buoy Engineering by Henri Berteaux of WHOI, John Wiley & sons 1976 or his 1991 re-write. Should be in any good technical library
     
  5. gonzo
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    gonzo Senior Member

    The mooring acts to effectively lower the CG. It can be viewed as a ballast keel.
     
  6. henrynel
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    henrynel New Member

    I tried doing that:

    Angle Centre of Buoyancy x-direction/m
    0 0
    2 0.0329
    4 0.06587
    6 0.09901
    8 0.132
    10 0.16609
    12 0.20021
    15 0.25185
    20 0.33181
    25 0.39577
    30 0.44131
    35 0.47482
    40 0.50089
    45 0.52213
    50 0.53932
    60 0.56325
    70 0.57817
    80 0.58618
    90 0.58852
    100 0.58521
    110 0.57516
    120 0.55832
    130 0.53448
    140 0.49949
    150 0.44179
    160 0.3235
    170 0.16001
    175 0.07939
    180 0

    Is that correct with maximum righting arm at 90 degrees, and zero at 180? Looks a bit too trivial
     
  7. henrynel
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    henrynel New Member

    i have had a look for this book in my university library however no copy exists. at present another design team member is looking the mooring. he did mention that there could be a lot of slack and that the buoy could invert completely.
     
  8. henrynel
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    henrynel New Member

    i don't think it should matter? so why is there no cross curves for 4 tonne displacements?
     

  9. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Could be correct. It depends on the shape of the buoy. How did you determine the center of buoyancy position?

    What are you studying?
     
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