inclining test lightship vcg calculation

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

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naserrishehriSenior Member

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

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TANSLSenior 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.

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naserrishehriSenior 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?

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TANSLSenior 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

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NavalSArtichokeSenior Member

It's an inclining 'experiment', not an inclining 'experience'.

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TANSLSenior 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.

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athvasSenior 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.

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naserrishehriSenior Member

is it necessary for the ship to be even keel (without trim trim=0) during inclining experiment?

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TANSLSenior 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.

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athvasSenior Member

Trim of the vessel should be minimized. However the initial list of the ship not exceeding 0.5 deg is permissible.

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TANSLSenior 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

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NavalSArtichokeSenior 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.

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