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Question about Center of Gravity

  1. Jan 28, 2010 #1
    Hi!
    I am new to the forum.
    I have a short question:
    I have uploaded drawings of two "speed boats" with fuel systems.
    Which boat got the best CG?
    The one with one, or the one with two fuel tanks?

    What do you think.
     

    Attached Files:

  2. jcsd
  3. Jan 28, 2010 #2

    Lok

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    Best meaning lowest?

    Well for the CoG weight is very important and missing in the pictures. Still i would say the first ( personal opinion, that has to come out).
     
  4. Jan 28, 2010 #3
    Yes, I mean lowest! (Best stability.)

    Everything is off course theoretical.

    The weight of the boat is the same in both examples.
    The weight of the fuel is also the same, 2 lb. (Small RC boat)
    The area of one big tank is the same as the area of the two small combined, meaning lenght of all tanks will be the same.

    If you ballance on a thin rope, why do you lift arms up and out if best CoG is keeping arms down close to your boudy?

    What do you think?
     
  5. Jan 29, 2010 #4

    Lok

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    In this case the first boat layout is best ( no more personals)
    Cause the lower the fuel is the lower the CoG is.
    Btw a 2 lb fuel tank is insignifiant considering that the boat is probably 100 times heavier.
    Best to just make the fuel tank as you wish ( comfort and overall design where the second layout might be better) and sit in the boat in the lowest position as your 160 lb really make a difference.
     
  6. Jan 29, 2010 #5
    The total boat is 25 lb included 2 lb fuel,

    but why do you take arms up and out to gain ballance and stability?
     
  7. Jan 29, 2010 #6
    Can anyone please help me compare them by doing a simplyfied calculation?
     
  8. Jan 31, 2010 #7

    Lok

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    I don't really understand this question.
     
  9. Jan 31, 2010 #8
    Theoretically speaking, the first one(fuel being in the center) will be more stable than the second.

    However, my answer could be wrong due to the fact that condition may be different on sea. Many other factors must also be take into consideration, one ought to study 'maritime engineering' to have a total grasp on it.

    But then again, is there such things as an absolute? I believe not.
     
  10. Jan 31, 2010 #9
    If you're walking on a rope your center of gravity will be above your feet, so you
    will be unstable anyway. If you stand still you'll fall. To keep upright you have to
    move your arms.

    If you start falling to the right, you can stick an arm out to the right and move the
    rest of your body somewhat to the left. This will move your center of gravity to the
    left, so you can stop falling.
     
  11. Jan 31, 2010 #10
    But does the theory apply on water? That explain why I choose the center one instead
     
  12. Jan 31, 2010 #11
    Consider also the resistance towards a rolling motion of the boat, caused e.g. by a wave pushing it up only on one side. The divided tank will provide more rotational inertia and the boat will be less likely to roll around its axis of motion.

    A speed boat experiences very complicated/chaotic forces on an uneven sea. It's probably best to go for a tried and true design, so as not to cause any uneccesary risk. If innovation is not an important design goal, that is.

    Torquil
     
  13. Feb 1, 2010 #12
    The article below article discusses stability of ships (and boats). The center of gravity may be above the center of buoyancy, but must be below the metacenter. The distance between the center of gravity and the metacenter is related to the stability of the ship.

    http://en.wikipedia.org/wiki/Metacentric_height

    Bob S
     
    Last edited: Feb 1, 2010
  14. Feb 2, 2010 #13

    Redbelly98

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    Why do you need a calculation? Is this a homework problem?
     
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