How to derive KN value

Discussion in 'Stability' started by Adeyele, Aug 11, 2012.

  1. Adeyele
    Joined: Jun 2012
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    Location: Lagos

    Adeyele Junior Member

    Dear all, I want to calculate GZ prior to plotting a curve of statical stability. I know the formular as GZ= KN - KG sin tetha. but i dont know how to obtain the KN value from the KN curve plotted on the graph.

    I was given KN values for each angles of heel and displacement.

    Angle of heel are: 10, 20, 30, 40, 50, 60, 70, 80, 90.

    displacements are 130, 170, 210, 250, 290, 330, 370, 410, and 450 tons.

    I have ploted this values with the KN given to me; but i dont know how to derive the KN value on the graph from the KN curve plotted for each angle of heel.

    Please any idea on how to derive this average KN that will be used to find GZ.

    Regards
    Mike
     
  2. jehardiman
    Joined: Aug 2004
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    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Ok, you should have 81 KN points. 9 angles x 9 displacements. For each displacement draw a line through the 9 angle points, this should give you 9 KN curves, 1 for each displacement.

    What you need now is a displacement and a KG associated with that displacement.

    For a given displacement, you interpolate between the KN curves you have. So for a displacement 350 tons, you use the average between the 330 and 370 ton KN curves.

    To get KG you need the load plan. Lets say lightship is 130 tons with a KG 5m, then you bunker 60 tons of fuel with a KG of 1 m and load 160 tons of cargo with a KG of 4m. As loaded KG is then (130*5+60*1+160*4)/(130+60+160) = 1350/350 = 3.86m.

    From this you can calculate the GZ curve for that loading.

    You also have to do that for the last day of the voyage (i.e. burned out). If we burn the 60 ton bunker, the KG at the end of the voyage would 4.44 which is significantly higher and the displacement is only 290 tons meaning that a whole new GZ curve is required but is easy to find using the KN curves.
     
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  3. Adeyele
    Joined: Jun 2012
    Posts: 48
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    Location: Lagos

    Adeyele Junior Member

    Dear Jehardmab, sorry for late reply and many thanks for your contribution. I actly did it d way u said; bt am nt gettin ow to pik d KN value. shld i trace out d mean displacemnt for all heel angles and take dem as my KN value.

    Regards
    Mike
     
  4. fishpad
    Joined: Sep 2012
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    Location: Suzhou City China

    fishpad New Member

    which software do you use? many design software can export data easy.
     

  5. Adeyele
    Joined: Jun 2012
    Posts: 48
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    Location: Lagos

    Adeyele Junior Member

    Kn Curve

    Helo all, kindly check the attached file that contains the KN data with the curve.
    At full load, the vessel can carry 280 Tonnes of cargo. with the KN data and the curve attached, I was asked to prepare a GZ curve for the vessel.
    I need to find the GZ curve for this data. I was given many displacement; so i dont know whether am to find the GZ curve for each displacement; If am to do that, i will need to find different KG for each displacement before drawing the curve.
    Any idea of on how to find the GZ curve?
     

    Attached Files:

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