Max KG and Heel

Discussion in 'Stability' started by Adarsh Edakkote, Jul 17, 2021.

  1. Adarsh Edakkote
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    Adarsh Edakkote Junior Member


    Initially my ship is in at Zero heel(TCG=0) with a displacement. But when I increase the value of VCG(KG) , at a certain point(Above allowable VCG i guess) Vessel heel and TCG also increasing. Why it is so?
  2. bajansailor
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    bajansailor Marine Surveyor

    If you increase the value of your KG, you will reduce your GM.
    If you get a slightly negative GM then she will want to try to capsize, and so she will heel over to her angle of lol, where she achieves slightly positive stability again (assuming that she does not capsize completely).
    Does this sound like a plausible reason?
  3. TANSL
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    TANSL Senior Member

    There has to be something else because when raising the CoG, without moving it transversely, the boat should not heel since there is no heeling moment due to this increase in height.
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Before answering fully, we need to know what you are trying to achieve and what you have as a means to get what you want to do.

    Since what Bajansailor has noted is 'correct'..but it also assumes "some" basics. It is these basics we need to understand before anyone can give a more full reply.

    So, you have a vessel in a hydrodynamics programme. But what is it you wish to do with this?
    Thus, before we go any further, what type of vessel , what are the dimensions of said vessel and what VCG has you assumed or it has been set to, in your hydrostatics programme.
  5. philSweet
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    philSweet Senior Member

  6. Heimfried
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    Heimfried Senior Member

    Because philSweet is linking to a thread, which in turn links to a former part of my website, which no longer exists, I will put a few pics regarding this point.
    Shown is not a ship nor a boat, just a prism (in a geometrical meaning) with some similarity to a hard chined boat with high deadrise, beam over all 2 meters, lenghts o a 6 meters, displacement 1,100 kilograms, KG = VCG = 0.527 m. The GZ curve shows this prism will float stable with no heel. (I put in 0.1 degree heel just to show the metacentre. picture Loll1)

    Then I increased the VCG (new = 0.800 m) without changing the displacement. The new GZ curve shows an angle of loll of about 17 degree (pic Loll2). So there is no positive stability around the upright position. Loll1.jpg Loll2.jpg

    Edit: Values of KG (= VCG) added.
    Last edited: Jul 19, 2021
    philSweet and bajansailor like this.

  7. Don92
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    Don92 Principal Naval Architect

    Hey Adarsh,

    As TANSL correctly stated, there must be something else wrong here. If the centre of gravity and centre of buoyancy are acting upon the same vertical line in the upright position, there is no overturning moment generated and, therefore, the vessel attitude would not be sensitive to changes in KG. As such, despite the fact that increasing KG reduces vessel GM, in the absence of any moment acting upon the vessel there should be no increase in vessel heel.

    This then leaves two possible explanations for your problem. Firstly, there could be some rounding error in the software you are using meaning that the TCG is not in fact 0, therefore giving rise to a moment acting upon the vessel which would be exacerbated by increased KG and therefore lead to a larger angle of heel. Otherwise, the hull model may not be entirely symmetrical which would have the effect of offsetting the centre of buoyancy, giving rise to an overturning moment that would increase with large KG values. Do you have any asymmetrical elements within the hull i.e. sea chests, appendages etc.?
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