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Where is the fulcrum?

  1. Dec 28, 2012 #1

    I'm making an animation of how a boat behaves when hit by a wave. This is how it looks like after the wave:

    (see attachment)

    At this point there should be a righting moment that tries to balance the boat by turning it clockwise.

    Which point does the boat rotate around?

    I'm not sure because the forces act on the C of B and the C of G and so I was thinking it would be the black dot, but shouldn't the C of B not move since it is a circular bottom?

    What do you guys think?



    Ok so I tried to animate it:


    If this motion is correct, then does that mean that the fulcrum moves? Why?

    Attached Files:

    • boat.jpg
      File size:
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    Last edited: Dec 28, 2012
  2. jcsd
  3. Dec 29, 2012 #2
    Yeah, the centre of buoyancy remains stationary. (its, basically just the centre of geometry of the part inside the liquid, and hence as shown in the animation remains unaffected as boat moves).

    So the motion of the boat can be considered as pure rotation about its C of B. (like rotation about a pivot)
  4. Dec 29, 2012 #3


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    Science Advisor
    Homework Helper
    Gold Member

    Some hull shapes are designed so that the CoB moves outboard when the boat heels over. Consider a catamaran. With the mast vertical the CoB is mid way between the two hulls. With one hull "flying" the CoB has moved to the hull still in the water.
  5. Dec 29, 2012 #4
  6. Dec 29, 2012 #5


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    Staff: Mentor

    Most boats, including this one, will have a center of buoyancy that moves. The circular hull would only enable a stationary center of buoyancy if it was at the center of the circle. The wiki link explains it well.
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