FreeShip Cross curves

Discussion in 'Boat Design' started by terrnz, Nov 24, 2016.

  1. terrnz
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    terrnz Junior Member

    is any one else using Freeship ? The Hydrosatics in particular. The cross curves seem to my simple mind to relate only to form stability. The earlier versions have no recognition of VCG ! The later versions have an input but it makes no difference to the output.

    The KNsin figure in the output this is the heeling arm or GZ ?

    would really appreciate some input from those wiser and more experienced than I. I have been playing around with the figures for example dividing the KNsin figure by the relevent sin of heel then correcting for VCG

    if KNsin= GZ
    then KN/Sin= GM = BM-BG
    BG=VCG-VCB


    Is the math correct?
    is the model correct?
     
  2. TANSL
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    TANSL Senior Member

  3. terrnz
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    terrnz Junior Member

    I appreciate the diagram.
    I am considering angles of heel greater than 6 degrees and trying to utilise the figures from the FreeShip hydrostatics which I hope takes into account the displaced centre of buoyancy heeled. As I said the version 2.6 takes no account of VCG and then the later versions after the Russian guy took up the baton after Maarten have an input for VCG but it seems to make no difference to the result which is nonsense.
    Because of the bulb keel my VCG is around -0.4m.
     
  4. TANSL
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    TANSL Senior Member

    Each transverse stability curve represents how the value of the KN varies, for a particular heel, as the displacement changes. It has therefore, as you can see in the figure, nothing to do with the VCG.
    I think I'm not mistaken if I recommend that, before using Freeship or any other naval CAD program, you try to collect some knowledge of ship theory.
    Cheers.
     
  5. terrnz
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    terrnz Junior Member

    I take note of your comments and came to that conclusion myself some time ago, which is why I got myself a copy of Elements of Yacht Design. I feel it is not specific enough with regard to stability calculation.
    Should it be possible for me to calculate the righting moment at 30 degrees?
    I would be grateful if you can recommend reading material to achieve this.
     
  6. TANSL
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    TANSL Senior Member

    I do not want to disturb you, but to know where we are, I have to ask you the following questions: do you know how to calculate the center of buoyancy? Do you know what that is? Do you know how to calculate the center of gravity of a boat ?.
    If you have answered NO to any of these questions I have to tell you that you will not be able to calculate anything relative to the stability of a floating object. But these are just a few of the many things that need to be calculated.
    There is a stupendous book "Principles of Naval Architecture" by SNAME.
    I have designed some ships and I have not read "Elements of Yacht Design" but I did study, long ago, very thoroughly the "Principles ...
     
  7. terrnz
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    terrnz Junior Member

    Yes I have a copy of Larsson and Eliassons book as well as Skenes furthermore I can calculate the centre of bouyancy and centre of gravity.
    Whilst my degree , many years ago, was not in Naval Architecture it was in Natural Sciences which had some math and physic content.
    I confess to seeking short cuts using software rather than laborious calculation.
    I shall look for the book you recommend.
     
  8. Heimfried
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    Heimfried Senior Member

    Hi terrnz,

    the loading condition of a ship often changes. Because the VCG is depending on loading conditions, the cross curves (or tables) are showing KN.

    To calculate GZ the actual VCG (and the actual TCG) must be involved.

    GZ = KN - VCG * sin(phi) - TCG * cos(phi)

    (phi = heel angle)


    Edit: may be have a look here http://www.bootsphysik.de/rechner/bootx.php

    Button top right: "English". Buttons " i " for information
    you can change VCG, TCG and so on, shift loads, calculate heeling arms in respect of VCG
     
  9. terrnz
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    terrnz Junior Member

    for my purpose I assume the loading condition is constant.

    Am I right in my assumption that VCG is the distance of the centre of gravity from the vertical centre of bouyancy?

    One of my problems as an untrained amateur is nomenclature between different publications varies. Can you clarify what TCG stands for.
    the link you sent me was interesting, thanks.
     
  10. terrnz
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    terrnz Junior Member

    TCG transverse centre of gravity ?
     
  11. TANSL
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    TANSL Senior Member

    In each boat you have to define a coordinate system that usually is :
    Base line (X axis), horizontal line passing through the low point of the keel.
    Y-axis, transverse, positive towards port side.
    Z axis, vertical.
    Origin: intersection of the baseline with the after perpendicular (use to be a line pasing through the rudder shaft). Therefore the Z axis is in the after perpendicular.
    With this coordinate system, VCG is the height of the center of gravity above the baseline (coordinate "z"). TCG is the "y" coordinate of c. of G.
    LCG : coordinate "x".

    Many calculations of naval architecture consist of calculating areas, volumes and their centers of gravity. So, for that, you do not need huge knowledge of math. Hydrodynamics is another matter.
     
  12. Heimfried
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    Heimfried Senior Member

    No. It is the vertical distance between lowest point of keel (K) and Center of Gravity (G).

    Therefor it is necessary, to gain a basic understanding of the hydrostatic parameters. After that you will be able to recognize the same point although it is marked by different letters in different publications.

    As you find out, it is the transversal CG. Not in all cases it is 0 or near 0.
     
  13. terrnz
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    terrnz Junior Member

    my baseline is lowest point of canoe body. the keel is a fin with a bulb so the actual cog is .45m below the base line. so I gave my vcg a negative value
     
  14. Heimfried
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    Heimfried Senior Member

    At the pic below is shown, that the locus of the metacentre (violet line) depends on the heel angle.

    Within only a few degrees from zero of the heel angle the metacentre is nearly not moving. At an angle of 30 degree you must use different calculations.
    The simple formulas are not longer valid.
     

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  15. Heimfried
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    Heimfried Senior Member

    If you use firefox as browser, you are able to see the apparent metacentre "walking" on the midshipsplane.

    (The site is different to that one mentioned above)
    http://www.bootsphysik.de/rechner/bootxp4.php

    click "English" top right and then "calculate data" bottom right

    Other browsers do not show moving svg-graphics.
     
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