How LCB, LCF, LCG affecting Trim?

Discussion in 'Stability' started by anindrafarras, Jan 4, 2016.

  1. anindrafarras
    Joined: Jan 2016
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    anindrafarras New Member

    How LCB, LCF, LCG affecting Trim?
    Are there any relation about LCB LCF and trim in hydrostatic curve?
    thankyou :)
  2. jugal_dear
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    jugal_dear New Member

    LCB defined as longitudinal center of buoyancy. LCG defined as Longitudinal center of gravity. LCF defined as longitudinal center of flotation.

    Now, difference between LCG and LCB is your lever for trimming moment. Basically, if LCG = LCB, there is no trimming moment and so no trim.

    So, trimming moment = Displacement x (LCG - LCB).

    Now total trim = Trimming Moment / Moment to Change Trim 1"

    Now trim is defines as difference between Aft and fwd draft.

    To find change in Forward draft or forward trim = TRIM*(L/2-LCF)/L

    To find change in Aft draft or Aft trim = TRIM- Forward TRim
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  3. DCockey
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    DCockey Senior Member

    anindrafarras, are you a student? Is your question from an assignment or an exam?
  4. anindrafarras
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    anindrafarras New Member

    Thankyou jugal_dear for your answer :)

    Dcockey: yes i'm a student. My lecturer ask me about that but i'm still confused to read hydrostatic curve
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Then may i suggest you do what all students do, read a book.
    All the answers are in the books....that is the purpose of studying!
  6. anindrafarras
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    anindrafarras New Member

    Thank you Ad Hoc for your suggestion. I apologize for this very basic question, I've read the book but still confused and still need an explanation. This will be my evaluation to read more books :)
  7. gonzo
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    gonzo Senior Member

    LCF and LCG will always be vertically from each other unless there is a force applied to the vessel. It is the state of natural equilibrium.
  8. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    You meant LCB rather than LCF ?

  9. Heimfried
    Joined: Apr 2015
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    Heimfried Senior Member

    Hi anindrafarras,

    maybe it helps to understand, if you virtually "play" a little bit with a boat and look at the behaviour of LCG, LCF and LCB.

    This Website
    provides the numerically computed and graphically displayed CoB, CoG and CoF.

    (Click first the button top right "English" to convert the sheet in English.)

    Click "show legend" and the used graphic signs are explained (blue dot means CoB and so on).

    Input a small trim angle (different to 0) in the appropriate input box, click "calculate heel / trim" and watch the moving of CoB.
    The values of LCB in mm is given in the right column, called x (subscript B).
    So x (subscript G) means LCG.

    Use the buttons marked "i" for Information.

    Let the skipper enter the boat at stern (e. g. Mass to load 80 kg, longitudinal dist. -2500 mm, then "load"). Watch LCG.
    Is a momentum created?
    Click "find equilibrium" and look at the reaction of the boat.

    Move masses on board. Look at its behaviour.
    Is the LCF a reason for a reaction of the hull, or is it a result of a reaction?
    Compare CoB, CoF, CoG, which of them are representing a force and are able to create a momentum?
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