inclining test lightship vcg calculation

Discussion in 'Stability' started by naserrishehri, Nov 19, 2013.

  1. naserrishehri
    Joined: Jun 2006
    Posts: 163
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: iran bushehr

    naserrishehri Senior Member

    dear friends
    please find the attached formula and tell me it's correct or not.
    kind regards
     

    Attached Files:

  2. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    The free surface correction is equivalent to an increase in the height of the center of gravity. The term FSM must therefore be additive. In fact, the right thing is subtract the term FSM from the GM value.
    Once the KG on experience, we have to discount certain weights, but it is also normal to have to add some weight existing at the time of the test. So the formula you indicate would be supplemented by an additive term.
    More complicated is the calculation of the deduction for free surfaces in tanks.
     
  3. naserrishehri
    Joined: Jun 2006
    Posts: 163
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: iran bushehr

    naserrishehri Senior Member

    dear TANSL
    what is the result of inclining test charts? VCG at test condition ok?
    finally we shall find LIGHTSHIP VCG by reducing or adding weights moment and free surface moment .
    is it correct?
     
  4. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Hi naserrishehri, nice to talk to you.
    What you get directly from the experience is the value of GM
    GM = heeling moment / (Displacement * tan (heeling angle))
    At that value you make the correction for free surface to get the GMc (corrected).
    Hydrostatics give you the metacentric radius (BM), the height of the center of buoyancy (KB) and thus the height, above baseline, of metacentre : KM
    Now KG = KM - GMc
     
  5. NavalSArtichoke
    Joined: Oct 2013
    Posts: 431
    Likes: 8, Points: 0, Legacy Rep: 83
    Location: GulfCoast

    NavalSArtichoke Senior Member

    It's an inclining 'experiment', not an inclining 'experience'.
     
  6. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    As we want to be retailers, we should speak of "inclining test". Thank you for your contribution, it responds undoubtedly OP's question and clarify his doubts.
     
  7. athvas
    Joined: Feb 2013
    Posts: 156
    Likes: 1, Points: 0, Legacy Rep: 16
    Location: india

    athvas Senior Member

    To avoid FSM (rise in KG) it is recommended to keep all the tanks empty while carrying out inclining test. so we can get better results.
     
  8. naserrishehri
    Joined: Jun 2006
    Posts: 163
    Likes: 0, Points: 16, Legacy Rep: 10
    Location: iran bushehr

    naserrishehri Senior Member

    is it necessary for the ship to be even keel (without trim trim=0) during inclining experiment?
     
  9. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is not necessary but if the ship has a trim <> 0, we must make a correction in GM due to trimming. You can also do the calculations based on the hydrostatic values ​​with the trim at the experience, in which case no correction is necessary. The latter is the correct choice.
     
  10. athvas
    Joined: Feb 2013
    Posts: 156
    Likes: 1, Points: 0, Legacy Rep: 16
    Location: india

    athvas Senior Member

    Trim of the vessel should be minimized. However the initial list of the ship not exceeding 0.5 deg is permissible.
     
  11. TANSL
    Joined: Sep 2011
    Posts: 6,059
    Likes: 257, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is necessary to clarify that an excessive trimming is not allowed (do not know the maximum value, but let's say 3% of Lbp. Would be wise to consult with local authorities). But when the trim is less, is allowed provided that :
    1. - GM trimming correction is made.
    or
    2. - Making calculations with hydrostatic values of trimmed ship ​​(no correction is necessary).
    When the trim is greater than 2% of Lbp, you must use the 2nd procedure

    P.S. : NavalSArtichoke (see next post), is correct.
     
    Last edited: Nov 21, 2013

  12. NavalSArtichoke
    Joined: Oct 2013
    Posts: 431
    Likes: 8, Points: 0, Legacy Rep: 83
    Location: GulfCoast

    NavalSArtichoke Senior Member

    In my experience (and I believe IMO resolution A.749(18) Para 7.3.2.7 will also bear this out), a trim deviation of more than 1% LBP from the design trim of the vessel during a stability test requires the use of trimmed hydrostatic calculations for the actual trim during the test in determining light ship weight and VCG.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.