Location of VCG/KG

Discussion in 'Stability' started by colinstone, Apr 13, 2007.

  1. colinstone
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    colinstone Junior Member

    I was advised by a Naval Architect that a good place to start with VCG/KG is on the proposed waterline.

    Any comments???
     
  2. sailaweigh
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    sailaweigh Junior Member

    How about where you determine it to be by a weights and moments spreadsheet? Or is that not possible?
     
  3. colinstone
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    colinstone Junior Member

    Yes, it can be done that way, but the architects advice when starting with a design was to put VCG at the waterline to get an initial feel for the stability.
     
  4. nautical
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    nautical Junior Member

    thats what someone told me too. Thats why I posted here and I havent got a reply for my post from any of the experienced people here. I am not satified with that answer because the hull envelope (with its thickness) it self has a VCG higher than the waterline in my case. what if we take a higher value for VCG and start doing the initial stability. wouldnt be better

    Thanks
     
  5. smartbight
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    smartbight Naval Architect

    Preliminary KG at design W.L. is too low for many work boats! Moving it to the Main Deck height would be a better approximation; Followed by a weight take off if you need to meet certain stability criteria.
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    That's for sailboats. I agree with smartbight: for work boats, deck height is a better approximation.
     
  7. nautical
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    nautical Junior Member

    well mine is a motor yacht, wich one is the better aproximation
     
  8. robsime
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    robsime Consultant Naval Architect

    For a motor yacht I would still say that main deck is the better approximation.
     
  9. french44
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    french44 Junior Member

    Hi all ,

    estimate the KG position when you don't have nothing is the challenge of many designer , there are lot of parameters ,but for the first estimation I use this formulation :
    -for ship with short superstructure (fishing vessel , tug ,patrol boat ...) the Kg is 0.75*depth .
    -for big ship like cruiser or other ship with hight superstructure I use 0.82*depht.

    Those formulations are based on my background and publications , in lot of case when the maximun of the real KG was nearly these first esimation the stability was good with regard to IMO .

    the best way is to make your statistic with your data base .
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    That may be very variable, depending on the type of motor boat. I feel we are here somewhat stupidily trying to guess and be nice guys, with no information on the side of askers. Why don't you post images or sqetchs of the boats in question?
     
  11. Tim B
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    Tim B Senior Member

    If you're clever, you use the design waterline and VCG both at Z=0. The hydrostatic results are then the same as the geometric properties of the hull. Of course, the water-line won't be where you thought, and niether will the VCG, but it's a good place to start, as most parameters are defined from the DWL.

    Tim B.
     
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  12. CDBarry
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    CDBarry Senior Member

    For typical powered craft 80% of hull depth up from keel (just a bit under the deck) is a good guess.
     
  13. Dingo
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    Dingo Junior Member

    Estimated VCG

    I have just performed a basic inclining test on a steel monohull luxury motor yacht (92 ft long). The VCG came out to be on the main deck (light ship). If you design to this height, it will be conservative, unless you have many cranes, davits etc. on the upper decks.

    Note also regarding catamarans built to Subchapter T in the U.S.A., the U.S.Coast Guard assumes the VCG to be on the main deck unless otherwise notified.
     
  14. nautical
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    nautical Junior Member

    Thank you Dingo for sharing.
     

  15. Willallison
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    Willallison Senior Member

    The problem with any "rule of thumb" method of preliminary VCG location is that it can vary so much from one boat to another - particularly with motorboats.
    I've found that a remarkably accurate method is to look at your preliminary inboard profile and simply plant your finger where you (realistically) think it might be..
    As others have pointed out - you won't be right, but with a bit of practise, it's surprising how close you can be;)
     
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