How to find GG1 without the lenght of vessel

Discussion in 'Boat Design' started by Adeyele, Oct 7, 2012.

  1. Adeyele
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    Adeyele Junior Member

    A vessel with a displacement of 100000 tonne has two tanks, one 'x' metres aft of the ship centre of gravity, the other 'y' metres forward of the ships centre of gravity.

    When 120 tonne of fuel is moved from the aft tank to the forward tank, the centre of gravity moves 0.9m forward. 500 tonne of fuel are now consumed from the forward tank causing the ship centre of gravity to move 0.5m aft of the original centre of gravity.

    Calculate the distance 'x' and 'y'.

    Any help is welcomed.
     
  2. gonzo
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    gonzo Senior Member

    It depends on the shape of the hull. If it was a regular shape, that is a prism with square or rectangular section, it would be possible to calculate that somewhat easier. However, it would also depend on the shape of the tanks.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    This has nothing to do with shape of hull or shape of tanks. Nonsense, it is just a simple moment calculation. The moments of moving the weights must balance and thus be equal.

    For the first movement:

    The GG = (distance)/ weight

    so

    0.9 = (x + y)/100,000

    The second is

    0.5 = 500y/99,500

    So, you can now calculate y, and then using that value insert into the first equation.
     
  4. gonzo
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    gonzo Senior Member

    Irregular shaped tanks have their center of gravity change in a direction other than vertical. Also, the attitude of the hull changes the position of the center of gravity. Where is the nonsense in that?
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Read the question from the OP. What data is given and what data is required.
    It is simple for any NA, but not a student learning. Shape of hulls/tanks etc bears no relevance to the question.

    It is just moments, nothing else.

    If you wish to read more into it, that's your prerogative, but it does not alter the result nor the method or calculation. It is an unfortunate misdirection which shall only confuse the OP.
     
  6. gonzo
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    gonzo Senior Member

    It is the typical word question that doesn't have all the relevant information. I know it is possible to learn how to answer them to pass a test, but it is a method that leads to failures in real life.
     
  7. Adeyele
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    Adeyele Junior Member

    tnks to all for all the reply. The question has nothing to do with the shape of the tank, its just abt changes in the centre of gravity of a vessel due to shifting weight. tnks to Ad Hoc for some little advice, but in the first one, u didn't include the weight 120tonnes? can u explain this ommision?
     

  8. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    This can be explained by a simply analogy.

    If 2 people are sitting on a seesaw, the person on the left has a bag and the person on the right does not. If the person with the bag, passes it to the person on the right, the total weight of the 2 people and bag has not changed. All that has changed is the location of the CoG.

    In the second, the person with the bag then throws away the bag. Again there is now a change in CoG, but not just because the bag has moved, it is because it is now no longer a part of the total weight, that being the 2 people and the bag.

    So, in the first, the weight, or displacement used, is the total, 100,000 tonne. The fuel of 120 tonne is part of that total. Thus just a movement of weight.

    In the second, the CoG changes because the total weight changes. This is because fuel is being removed from the total, not just moved.
     
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