Calculating KG

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

  1. ed198
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    ed198 Junior 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
  2. rwatson
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    rwatson Senior 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
     
  3. TANSL
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    TANSL Senior 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.
     
  4. TANSL
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    TANSL Senior 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)
    Rmt = transversal metacentric radious
    GMc = GM0 - Free surfaces correction
    teta = heeling angle at test.​
     
  5. rwatson
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    rwatson Senior Member

    I understand. I saw Kilograms, not the variable. Thanks.
     
  6. TANSL
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    TANSL Senior Member

    Yes, you're right, it could be interpreted that way. Maybe I've gone too smart.
     
  7. Heimfried
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    Heimfried Senior 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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  8. Heimfried
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    Heimfried Senior 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
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    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.
     
  10. ed198
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    ed198 Junior 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...
     
  11. ed198
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    ed198 Junior Member

    Yes exactly...
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

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

    Do you therefore not have a set of hydrostatics for that hull?
     
  13. ed198
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    ed198 Junior Member

    yes the boat is in the water...we recently hauled it and know the weight..
     
  14. Heimfried
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    Heimfried Senior Member

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

  15. ed198
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    ed198 Junior Member

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