Inclining using U tube apparatus

Discussion in 'Stability' started by athvas, Jul 3, 2013.

  1. athvas
    Joined: Feb 2013
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    athvas Senior Member

    could anyone help me to solve this ....
    how to calculate arm required to achieve deflection of 75mm deflection per side as per IMO requirement. (For U tube)
    Lightship weight of vessel is 693 tons
    Inclining weight used is 25 tons
    Anticipated GM is 31.356 metres.
    What will be the distance between the U tube centres to achieve the above deflection while doing experiment. Plz help.
     
  2. Crowsnest
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    Crowsnest Junior Member

    Could you explain a little bit please ?
    My English is not good enough for getting what you want to mean.

    Thanks
     
  3. athvas
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    athvas Senior Member

    For inclining experiment we need to fix the u tubes in the vessel to take readings. we need to know how much arm (distance between u tube centres) required to achieve the deflection value (reading) taken during experiment. Min deflection required as per IMO is 75mm each side.
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    What you need to get is an heeling angle close to 3 degrees. Known that, with a deflection (I do not know what it is) of 75 mm, yourself can do the calculation.
    Why not use a pendulum or some other instrument that allows you to make measurements with more confort and therefore accurately?
     
  5. athvas
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    athvas Senior Member

    Class says two things,
    - the min deflection, to each side of the initial position corresponding to the total weight should be 150 mm.
    - The weights shall be sufficient to provide a min of 1 deg and a max of 4 deg heel to each side.
     
  6. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    If you are a student, then this should be a straight forward exercise. Because by now, one assumes you have learnt the following relationship:

    GM = w.d/W.tan(theta)

    The angle, tan (theta), is simply the distance apart of the U-tube centres and then the amount the water level has risen/fallen.

    Then the only other bit you don’t know, is the distance moved by the weight, the "w.d" part. But since you know "w", you can iterate with varying "d" or distance apart.
     
  7. athvas
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    athvas Senior Member

    Thanks for your replies.... i have used pendulums but in case of U tube i just had a bit of doubt with the distance between U tubes.
     
  8. TANSL
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    TANSL Senior Member

    Depending on the boat, her GM, and the measuring instrument, 150 mm may be sufficient or not. So the classification society can not impose this value.
    An angle of 1 degree is very little because the deviations that are achieved will be very small, which is not bad by itself, but errors that are committed to the measure may be relatively important.
    Moreover, if we use a pendulum, to obtain 150 mm offset at an angle of 1 degree would need a pendulum length of 8593 mm. That's not possible, is unmanageable. You should check what you say the classification society. Perhaps you misunderstood.

    The procedure is the same as with the pendulum: the height difference divided by the distance between the arms of the "U" gives the tangent of the angle. If you know the difference in height and heeling angle the ship is supposed to reach, I do not know why you can not calculate the distance between the arms of the "U"
     
  9. Crowsnest
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    Crowsnest Junior Member

    No clarification required once read all posts.
    Thats all about a stability test/experience.
    I agree with TANSL:
    A pendulum arrangement is a lot easier to fit.

    GM = w.d/D.tan(theta), as Ad Hoc pointed is the equation to use, inside the initial stability angles (GM approached as constant), either with a pendulum or a tube.

    The relation between the U tube lectures and the weight transversal distance .... better a pic than an explanation.
     

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  10. smartbight
    Joined: Dec 2006
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    smartbight Naval Architect

    8-10m pendulums

    GM = 31 m, that's a stiff 'mother' ... a barge or a wide catamaran type hull ? What is the total width on deck ?
    We use 8.0 m & 9.0 m (30'+) pendulums on drilling rigs and large vessels all the time in the GOM. As required by ABS/USCG. Wind can be a problem. This is when U-tube manometers come in handy (not influenced by wind) and easier to rig. We replace 2 pendulums with 2- 20 m U-tubes, or as long as the beam of the vessel allows. Less weights required. Excellent precise readings to 1/100"~ 1mm resolution. Tubing needs to be purged of all air bubbles.
     
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