Rhino3d Script to get trim angles for different heels

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

  1. vv3k4i
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    vv3k4i Junior Member

    Let me add something in the subject of platform model. With such a high center of gravity trim values reaches very big values. So the question is what are the limitations of your plugin. Maybe you just don't want to calculate stability when trim exceedes big values.
     
  2. TANSL
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    TANSL Senior Member

    The height of the center of gravity has nothing to do with the trim the boat adopts in its equilibrium position.
    Although any software should calculate the stability of a boat with much trimming, in reality should not accept a vessel sailing with a trim bigger than, let's say, 4% (this figure is debatable)
     
  3. vv3k4i
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    vv3k4i Junior Member

    The VCG has a lot to do with the trim boat adopts in it's equilibrium position. When the boat trims, also the Center of Gravity is being rotated by the trim value. So if the VCG is on the same level as VCB trim will not influence the center of gravity position much. But when the VCG is for example 20 meters above the center of rotation (often taken as center of floatation), than trimming the boat by 4 degrees will move the LCG by about 1,15 meter.
    This also means, that biggest LCG movement occures when trimming the boat at heel angle of 0 and 180 degrees, and the least on +- 90 degrees.
     
  4. TANSL
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    TANSL Senior Member

    Frankly, I do not know what to say to such detailed excplicación. Apparently I was totally wrong. I was convinced that the trim of the boat depends on the longitudinal center of gravity. And the worst of it is that in all the projects I've done in my life, and there have been many, I have always used the variation in the longitudinal center of gravity to achieve the appropriate trims.
    It is never too late to learn something new.
    By the way, the rotation axis is not taken often to pass through the center of gravity of the float, but must be taken on it (not an option, physics clearly states where is the axis of rotation)
     
  5. vv3k4i
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    vv3k4i Junior Member

    I don't think you are wrong, just meybe we are not on the same page. You are talking about rising center of gravity in case where the boat is in equilibrioum with 0 degrees heel. In this case it will not influence the equilibrioum trim. But if you will heel the boat and the trim will change due to heel, then also center of gravity will rotate by the trim angle in the trim plane. This means, that in most cases you will observe X and Z translation (assuming that trim is in OY axis. This means, that the higher the thCOG is the more LCG movement you will observe.
     
  6. TANSL
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    TANSL Senior Member

    No, what I'm talking about is that when I study the stability of a load condition, if the equilibrium achieved has a trim too big, what I have to do is to change the longitudinal center of gravity trying to get, if possible, a zero trim or as close as possible.
    Everything you say that when the heeling boat changes its trim, I already know. To reduce the trimming no other solution to act on the LCG. Physics is very stubborn.
    Nor does it seem very wise to change the VCG as the boat is heeling or changes her trim.
     
  7. vv3k4i
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    vv3k4i Junior Member

    I'm not tlaking about adjusting any values on the go. Only about performing calculations for known case.
    You can take the geometry I attached "Platform", perform the stability for two different cases, one is VCG on the level of waterline, another is VCG on the level I gave 24.37m. Then compare trim angles for consecutive heel angles. And just spot the difference. This is what I meant.
     
  8. TANSL
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    TANSL Senior Member

  9. Tapsa84
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    Tapsa84 Junior Member

    I will be home tomorrow to run the model, but I don't really see a problem with bigger trim values. Thou, this statement is based on code running in my head away from the computer, but we will find out tomorrow :D
     
  10. Gary Brown
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    Gary Brown Junior Member

    vv3k4i:

    I am in agreement with you in regards to the VCG and trim. The VCG will influence your trim calculation for the same reason that it will influence GZ.

    Also, TANSL's statement that the ship must be rotated about the center of flotation only applies if you must simulate the actual ship movement. If, on the other hand, you are just doing a static stability type calculation, it doesn't matter if you rotate about CF, or CG, or a point on the Moon. The only thing that will be different using different centers will be the resulting ship translation. For a flat water surface, this won't enter into the calculation. If the water surface is a wave, then consideration of rotation about a constantly changing CF might be required to get the ship translation correct relative to the wave.
     
  11. TANSL
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    TANSL Senior Member

    Gary Brown, I do not agree with anything you say but no problem. It is precisely disagree and contrasting opinions.
    You can turn the boat around the axis that you like the better but when the boat rotates, free floating in the sea, she always does around an axis passing through the center of gravity of the water plane (which it is not on the moon, or close to it).
    What interest can have to study the properties of the hull when rotating around an axis around which it is impossible for the ship to rotate, and forget the study around the real axis of rotation?
     
  12. Tapsa84
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    Tapsa84 Junior Member

    I got some interesting results to say the least :D

    So, the model vv3k4i supplied to me actually pitch-poled after 16 degrees of heel. The model was oriented so that the hole in the transom was on the left and rotation clockwise. And to be precise, the model did not have enough bouncy in the transom, so the bow lifted up and it pitch-poled over the transom.

    All I have to do, is edit the code to jump heeling from 16 degrees to continue from 164 degrees to 180. If trim exceeds 180 degrees, it will continue from 180 degrees - last heel value. That's it. And boy is that model stable inverted! :D

    But, only IF I am correct! Can anyone confirm the results I got? What did you vv3k4i get?

    I will try to post the results tomorrow (spend the whole day scratching my head on as to was there something wrong :D).

    And thanks vv3k4i! You were sneaky ;)
     
  13. vv3k4i
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    vv3k4i Junior Member

    Well, this case will make you proud of yourself :) But it is worth spending time on such cases to make the software bullet proof. Honestly I know some top shelve software which is just stopping the calcs at around 15 degrees for this model so frankly since I don't have any reliable data to compare with I will not dare to publish my results :)
     
  14. TANSL
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    TANSL Senior Member

    Here are some results for the very simple exercise proposed by v3k4i, "Platform".
    The coordinate system I have used can be seen in one of the images.
    I hope I have helped with these results.
     

    Attached Files:


  15. Gary Brown
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    Gary Brown Junior Member

    I recognize the sample platform you have provided as a jack-up that was originally proposed in a paper by Breuer and Sjolund. It has also been used by Joost van Santen. If you have their papers, you should have some reliable data for your comparison. I am sure you also realize that these structures are special case types where conventional ship stability assumptions are not valid and you shouldn't be using general ship stability software to analyze them. With a conventional ship hull, the heel axis is fixed in the longitudinal direction. This is considered reasonable due to the shape of the hull. Free trim is also applied to make it even more accurate. In the case of a platform, in general, I don't think a fixed longitudinal heel axis is reasonable. Your first indication of trouble is when you start seeing large trim after heeling. This indicates that you may be using the wrong heel axis. I think software would have to find the lowest potential energy axis in order to be realistic. In one of the papers by Joost van Santen, he analyzed your platform model and found the "optimal" heel axis. Even using this optimal axis, he never heeled beyond 15 degrees.
    Look at some of the GZ curves for these structures in the literature. Some of the curves will show discontinuities and bifurcations. Software to plot GZ must also be able to spot these bifurcations and analyze each branch.
     
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