Rhino3d Script to get trim angles for different heels

Discussion in 'Stability' started by Tapsa84, May 22, 2016.

  1. Tapsa84
    Joined: May 2016
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    Tapsa84 Junior Member

    I did some experiments with a dynamic heel axis, but the end result was still the same. The problem I had, was that after 180 degrees of heel, the rotation of the model was not exactly this. It was always about 2 degrees shy of this. Also, I had to attach a reference plane that stayed aligned with the transom, because it would distort the GZ-arm measurement otherwise. The total amount of transverse movement was about 3 degrees at around 178 degrees of heel. Interesting idea thou (for this case), and worth more investigating.

    I have a bad case of flu (man flu) at the moment, so my brain is not working at optimum efficiency. I will investigate more when I get my bearings back and I think I have to dig up my geometry books and brush up on things. :D

    Thanks TANSL for the data.
  2. vv3k4i
    Joined: Mar 2008
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    vv3k4i Junior Member


    You are right. Most of the software is computing stability in a bit simplified way and the jack up is special case. Breuer and Sjolund has inncluded a GZ curve in their publicaion, but it is not calculated basing on the minimum energy approach so not possible to compare data with. At least, I can say, that I have emplyed the minimum energy approach in my calculations and I get different results, which still could mean I'm wrong. But honestly I trust my calculations very much. As far as I can see there is not much reliable data to compare with.

    Tapsa, I tihnk you should just leave this case and just limit your software in case of bigger trim angles. I made the calculations of the model you provided with standard approach and got more or less the same results:

    Heel [deg] GZ [m]
    0 8,64071E-12
    10 191,4254453
    20 377,930304
    30 555,0142918
    40 718,8104347
    50 841,6963949
    60 873,5135934
    70 823,7130523
    80 712,6977937
    90 557,8129036
    100 371,9268566
    110 167,96202
    120 -42,97111675
    130 -249,1658512
    140 -437,4232239
    150 -588,9796039
    160 -674,4314638
    170 -588,2614609
    180 -2,03616E-09
  3. Tapsa84
    Joined: May 2016
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    Tapsa84 Junior Member

    The software works fine with a fixed heel axis, the model just pitch-poles, as it should. It continues from this inverted position from 164 until 180 degrees of heel. So no need to limit for bigger trim angles, because it seems to work just fine.

    The dynamic heel axis approach is still a bit buggy thou :rolleyes:

  4. Joost vs
    Joined: Jan 2017
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    Joost vs New Member

    stability calculations

    To my surprise, I saw reference to a jack-up analysed by Andy Breuer (ABS) and myself.

    I did not read the thread in detail, but just a remark in general.
    When applying only heel around "a" axis, there is no problem in calculating the rightinga arm. BUT.. when allowing the structure to trim freely, there are heel angle and axis direction combinations for which the free trim position (i.e. zero trim moment) can not be found. It just does not exist. In general this can happen for larger heel angles.

    Just another note: in the calculations, one can solve for the displacement being equal to weight and zero trimming moment in one go. This is done by using coupled equations and iterate towards the required solution. So one does not have to search first for the proper draft and perform another step for zero trimming moment. My CADMO paper discusses this.

    Joost (van Santen)
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