# Calculating KG

Discussion in 'Stability' started by ed198, Sep 24, 2018.

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### ed198Junior Member

Good day...an amateur here...I calculated the GM of my 25ft sailing boat by performing a inclining experiment ...i am now after calculating KG but not sure how to do this...can someone shed some light?
Ed

Last edited: Sep 25, 2018
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### rwatsonSenior Member

It will depend on the size.

Small, put it on a truck and go to a weighbridge
Big, Get a load cell on a crane and lift it
Tiny, Put it in a pool and measure how high the water goes up

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### TANSLSenior Member

@rwatson, I think he's talking about calculating the KG, not the weight of the boat. It is possible that to calculate the KG he needs to do something else.
Imo, to calculate the KG should make a relationship of the weights and their centers of gravity and calculate the resultant.

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### TANSLSenior Member

I think I have misinterpreted the question of the OP because, in saying that he had made the stability test, I assumed that it was already the ship's KG at the time of the test. In that case, he only had to add the missing weights and deduct the ones that were left over, with their centers of gravity, to calculate the KG in the final condition he wanted to study.
If what he asks is how to calculate the ship's KG in stab test time, he should use the following formula :
KG = KC + Rmt - GMc * Cos (teta)
GMc = GM0 - Free surfaces correction
teta = heeling angle at test.​

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### rwatsonSenior Member

I understand. I saw Kilograms, not the variable. Thanks.

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### TANSLSenior Member

Yes, you're right, it could be interpreted that way. Maybe I've gone too smart.

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### HeimfriedSenior Member

If this was the usual inclining experiment ("Betriebskrängungsversuch"), the displacement of the boat was necessary for the calculation of GM. Therefore I think, the TO is looking for the KG (Keel - CoG) to locate the VCG.

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### HeimfriedSenior Member

Hi Ed,

calculation by boat parts and their location is laborious but easy in case of a boat under construction but often not really possible in case of older boats with partly unknown structure, density of parts etc.

If your boat is not large, may be you can do an inclining experiment in air:
Document

Correction

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Which suggests he was either given the displacement, at inclining, or he must have a set of hydrostatics for the vessel.
If given, he can ask, or if he has the hydrostatics, he can just look at the hydrostatics as it will, or should be, tabulated in them.

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### ed198Junior Member

Hi i did the inclining experiment and therfore know the displacement of the boat since we recently hauled it...i am after KG to find the centre of gravity ...i have calculated GM...

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### ed198Junior Member

Yes exactly...

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So your displacement is via a physical weighing..not via hydrostatics?

Do you therefore not have a set of hydrostatics for that hull?

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### ed198Junior Member

yes the boat is in the water...we recently hauled it and know the weight..

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### HeimfriedSenior Member

Ed, did you find the paper about the determination of KG by inclining experiment in air, I linked above?

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### ed198Junior Member

Thanks yes but boat is in the sea and inclining experiment was done in the sea...

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