Rookie questions regarding LCB and LCF

Discussion in 'Stability' started by milo12, Sep 12, 2015.

  1. milo12
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    milo12 Junior Member

    I have done some searching and I know what LCB and LCF are and how they affect trim.

    What I can't find is some basic descriptions on how LCB and LCF may relate to each other.

    For example does LCF have to be behind LCB? If so why and what happens when it is not? Is there an acceptable range of the distance between LCB and LCF? Maybe a percentage of length at WL? What happens if LCF and LCB are separated by too much distance or not enough distance? I guess the answers may depend on the type of boat so lets assume a semi-displacement monohull.

    If my questions are so wrong as to be nonsense please point me to a book I can study.
     
  2. Rurudyne
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    Rurudyne Senior Member

    I've thought about it too and I may be of some small help if I've understood what I've read properly.

    As you know, LCF is the nominal center of flotation and as you add weight fore and aft it causes changes in trim accordingly. LCF can shift somewhat as the boat's trim changes but that's more than I'm ready to cope with here.

    That's when LCB and LCF roughly coincide.

    IIRC, if your LCF is behind your LCB then adding weight aft of the LCF should have a slightly greater affect on trim down at the stern and weight added forward of the LCB slightly less.

    The converse being true when LCB is behind LCF.

    As the distance between LCB and LCF increases the effect should grow larger.

    So a boat with the LCB relatively far forward of the LCF is going to not like following seas very much but it may at least be a bit drier plowing into the waves being the idea.

    Again: that's all "if I recall correctly".

    It's kinda like this old joke about over-steer vs under-steer where a NASCAR driver explains that with under-steer you plow your grill into the wall and with over-steer your right rear quarter panel takes the hit (they never turn right in NASCAR).
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Nope, it can be anywhere you like. It is totally dependent upon the hull shape. But then comes down to...what is the usual scenarios of a typical hull shape?
    Generally the LCF is abaft of the LCB. Generally.

    Again dependent upon hull shape. BUT, generally the further the distance apart the worse the motions. Since pitching occurs about the LCF and heaving about the LCB. Thus in waves you get what is called coupled pitch-heave. The vessel heaves and then pitches and the further apart the LCF from the LCB the more pronounced this effect is.
     
  4. Rabah
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    Rabah Senior Member

    Hi All,
    From the file connection between trim, Ta and Tf is visible. But main from which depends what will be trim-by the bow or by the stern, this is the positional relationship LCG and LCB. That is the stern or the bow will be more submerged in water.
    _____________________________
    NA Razmik Baharyan
     

    Attached Files:

  5. TANSL
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    TANSL Senior Member

    Rabah, I guess you know that the image you've shown us is not always valid, it is only for very small variations in trim. Can't be used to calculate the drafts fore and aft when the trim is not very small.
     
  6. Rabah
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    Rabah Senior Member

    Hi everybody,
    The method of the initial stability at small angles of inclination concerns to small angles of the trim - up to 2 deg. He well illustrates connection between LCF and LCB.
    All depends on that what cargoes is received or transferred on the length of the ship. Depending on it there are special formulas for definition Tf and Ta at the reception of a small or major cargo. Small it is considered cargo P =/0,10-0,15/∆.
    With the help of the method of the initial stability at small angles of inclination all industrial tasks bound with the trim of the ship are decided.
    This method can be applied and to final dip angles of trim /that is > 2 deg/. Thus it is possible to consider outcomes as the first approaching for fast orientation at a satisfactory exactitude.
    __________________________
    NA Razmik Baharyan
     
  7. TANSL
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    TANSL Senior Member

    There are no formulas that are valid for trim less than 2º and others valid for trimming> 2º. That is not right. There are of course procedures to calculate trimming but not formulas in the strict meaning of the word.
    The trim is usually expressed in % of the length between perpendiculars, not degrees.
    When the calculations are performed with a hydrostatic values that differ from the actual waterline by more than 2% of LBP, it is necessary to introduce corrections, very important in some cases, in the calculations. In many cases the Administration does not support even these corrections, they force you to do calculations with the ship in trim it really has.
    What do you mean by a small or large charge ?. "Considered small charge it is P = / 0.10-0.15 / Δ". You mean 10% or 15% of displacement?. Frankly, that's a heavy load.
     
  8. Heimfried
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    Heimfried Senior Member

    Hi milo,

    a given boat hull has not a fixed LCF and LCB. Both are moving - depending on heel and trim.

    The LCF is a parameter deriving from the times before computer calculations were made. In modern numerical methods of calculations of flotation it is no longer used.

    If you want to play a little with the static behavior of a boat:
    http://www.bootsphysik.de/rechner/boot_engl.php
    LCB = xB
    (Buttons marked with "i" provide information.)
     
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  9. TANSL
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    TANSL Senior Member

    Apparently the program does not take into account that changing the heeling, trimming should change automatically.
     
  10. Heimfried
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    Heimfried Senior Member

    That depends on the modus you operate it.

    If you just give a heel angle, the results show only the heeled boat. This ist to show that state (e. g. external forces like wind or current, boat is not at equilibrium, as is said).

    If you move loads and use the button "find equilibrium" it will heel and trim automatically to find the right position.
     
  11. TANSL
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    TANSL Senior Member

    Hi Heimfried,
    The problem I pose is this: when you change the weights transversely, without changing its longitudinal position, the ship heels an angle and at heeling, trimming also changes.
     
  12. Heimfried
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    Heimfried Senior Member

    Hi Tansl,
    starting at the initial state
    input: 172 kg (Mass to Relocate, 10 % of boat mass), 1600 mm (transversal, no longitudinal change)
    button: "apply relocation"
    button: "find equilibrium" (maybe the button must be pushed a second time, after calculations is finished, see info button "i")
    after that the sheet shows: heel angle -17 deg., trim 0.4 deg.
     
  13. TANSL
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    TANSL Senior Member

    Exactly !, that's what I meant. Congratulations because it is not easy to find a program (not extremely expensive) to make that calculation.
     
  14. Heimfried
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    Heimfried Senior Member

    Fine.
     

  15. Rurudyne
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    Rurudyne Senior Member

    HullForm seems to. Or rather it lets you enter trim, displacment and shows the waterlines a hull will have such values on a case by case basis.

    It's old and not pretty but it has some nice features.
     
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