Rookie questions regarding LCB and LCF

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

  1. TANSL
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    TANSL Senior Member

    With all due respect, a program or does or does not. "It seems to..." means nothing. But it would be amazing that a simple program, like HullForm, could perform a so sophisticated (complicated) calculation.
     
  2. Rabah
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    Rabah Senior Member

    Hi boys,
    Do you have not deviated from the main problem of milo12? Hi interest primarily from trim! Not heel!
    Tansl again showed the dropouts of knowledge on the Ship theory. Instead of to get in awkward position as enough was to read sections from the Ship theory: Addition and removal of a Small and Large Weight. There all is clearly said! But what for it is necessary to him to unclose the eyes?
    In effect all these formulas have lain in the basis of acting computer programs. For example when it is necessary to calculate the trim of the ship at a ballasting with the purpose to improve trim of the ship in accident it is made on the method of a reception of a major liquid load.
    ____________________________
    NA Razmik Baharyan
     
  3. Rurudyne
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    Rurudyne Senior Member

    Again, what Hullform does is take the displacement data (derived when the model is level at the waterline) and input heel data and settles the hull as appropriate (IIRC: "balance hull" is the proper menu option). It draws waterlines etc as the boat is heeled.

    It is not automatic as I was using it: for the visual waterline representations for a given amount of heel.

    I never really tried to program the hydrostatistics routines.

    It showed, for example, the tendency of some of my sailboat models to trim down at the bow as they heeled. Without flare (these were early efforts) I'm guessing they would have been pretty wet boats.
     
  4. TANSL
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    TANSL Senior Member

    Yes, it seems clear, Hullform does not what I was asking for but it is important enough that he does. What a designer should do is to know the tools at his disposal to use them properly without thinking that he has been calculating what he has not calculated. From what I understand, with Hullform you can approximate the position of the boat heeled but not calculated exactly eqilibrium waterplane, which is another problem that you, at that time, do not worry. I guess for what you need it is sufficiently approximate.
     
  5. Rabah
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    Rabah Senior Member

    Hi All,
    As I have understood milo12 have put three concrete problems:
    1. What there is a connection between LCF, LCB and trim?
    2. If there is between them a variance what is supposed to be maximum?
    3. A technical publications to read about these problems.
    On what from them you have given the useful answer? First it is necessary to think and then to write!
    With what you have helped of milo12? With what has helped your absolutely excessive "academic" dispute on capabilities of program Hullform?
    ________________________
    NA Razmik Baharyan
     
  6. Heimfried
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    Heimfried Senior Member

    Why don't you give a useful answer to Milo, Razmik?

    Does your criticism of other postings help Milo?
     
  7. Heimfried
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    Heimfried Senior Member

    Hi Milo,

    just in case you are following still this thread, the calculation sheet shows now the LCF (green dot at waterline) in the graph (section at midships plane).
    The LCB is (as before) shown as a blue dot.

    http://www.bootsphysik.de/rechner/boot_engl.php

    Input in the right column the "trim angle" in degrees, click button "calculate heel / trim". Try some angles, also negative values.

    You are able to follow the different positions of the CB and the CF and the changing distance between them.

    At least for this hull (but I think, it will be applicable to all conventional boat hulls) the LCB is more forward than the LCF if the trim is positive and not to small. And LCB is more aft than the LCF if the trim is negative and not to small.

    (A quick ad hoc extension of a software may cause errors or contain bugs. I hope, not in this case.)
     
  8. milo12
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    milo12 Junior Member

    That's a nice tool you made for visualizing trim changes etc.

    I was looking for comments like AdHoc and Rurudyne made. The effect of LCF and LCB on seakeeping is exactly the kind of experience I was looking for, thanks.
     
  9. Stephen Ditmore
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    Stephen Ditmore Senior Member

    A few notes on this subject. Please forgive me if I jump around a little.

    LCB and LCG in the static condition are, of course, same. If you add additional weight, the CHANGE in trim will be relative to the LCF.

    On displacement & semidisplacement hulls, I like to see LCB at 8/15 of the waterline length (53.33%).

    On multihulls, esp. multihulls with plumb or reverse stems, I like to see LCF at or forward of LCB to avoid plunging of the bow.

    A designer who I understand has played with this relationship, and it's effect on the damping of pitching, is Brendan Dobroth.

    Shift in LCF as a sailboat heals can be related to change in trim, which can be related to change in helm balance as heel angle changes. This provides a simpler (than "metacentric shelf") way to understand a sailboat's tendancy to round-up, or not, as it heels. This method has been developed in the writings of Cyrus Hamlin.

    Concerning full-keel boats, I've heard Olin Stephens note that he likes to keep Center of Lateral Resistance (CLR) farther aft than some other designers. This has led me to suspect that keeping the CLR aft of the LCF, on any kind of boat (including the rudder area when calculating the CLR), might lead to better directional stability in quartering seas. My own experience indicates that this is true, but that the effect will be negated if the boat tends to round up as it heels (see previous paragraph). I suggest keeping an eye on this relationship when experimenting with a forward rudder or canard.
     
  10. Rurudyne
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    Rurudyne Senior Member

    I had this the other way around. I'd been thinking that LCB indicates where the hull better supports a load so while trim changes around LCF it's distance from LCB helps to qualify the amount of change.

    I suppose it's for the best then that I've been trying to get LCB and LCF to come close to coinciding.
     
  11. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I haven't read all the replies, so please forgive me if I'm being repetitive with something someone else said, or overexplaining. LCF is the center of the DWL waterplane. Think of each station as having a width at the waterline, and a fairbody draft below the waterline (or consider the square root of its area). If the forward stations are wide at the waterline and shallow, this will pull the LCF forward relative to the LCB. A boat with a fine, deep bow and a wide, shallow stern will have an LCF aft of the LCB.
     
  12. TANSL
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    TANSL Senior Member

    Totally agree.
     
  13. milo12
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    milo12 Junior Member

    Yes that is exactly what I see on Freeship. The unknown, to me, is how far aft of the LCB can the LCF be before you introduce problems with pitch/heave coupling and problems with seakeeping in a following sea.

    I'm just playing around with ideas as I try and learn the software but I want to learn good design habits. No point in coming up with my dream boat if it is a big cluster* of built in problems due to my poor design decisions.
     
  14. Rurudyne
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    Rurudyne Senior Member

    I wasn't being critical. Far from it. If I have formed the wrong idea how will I be corrected unless someone posts the right one?
     

  15. Rabah
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    Rabah Senior Member

    Hi milo12,
    In the first message you were very economical in words, for this purpose I have lost the way that to you problems Trim received interest only at a cargo motion.
    Now I fathom that the question is about allowable running trim at operation.
    Except for that you search for some advisories about optimum values LCB and LCF such that warranted a top speed at acceptable trim.
    And as I fathom to you interest semidisplacement hulls, i.e. such which Froude number on displacement have less 2,5. Often call them as "hybrid motorboats“.
    As you have not updated about what there is a speech - motor or a sailing vessel, I have decided that the information on a motorboat is necessary for you. For this purpose I publish two files which illustrate a hybrid motorboats:
    1. Linesplan from book of Baader Huan - Tourist and sports motorboats, 1977
    2. Image of such motorboat from the book “Sorensen`s Guide to Powerboats”, 2008
    You should know that at such hulls no special requests about optimum running trim as it is at Planing Hull.
    I have not found as well statistical data about optimum values of LCB and LCF as % from L or Ldwl.
    In my opinion at designing it is necessary to search for the minimal value of the water drag which warrants a top speed at full usage of the chosen power plant.
    It is preferred stationary WL to be on an even keel.
    Some is possible small running trim by the stern, all the same we have a semiplaning mode of movement. But at all condition of a loads it is not necessary to receive trim by the bow as it foils on work of propellers.
    If it is necessary to you any advice - first of all you must to pick one or several prototype motorboats existing in operation with the close sizes, velocity and number of passengers.
    For software I would recommend you Orca3D /plug-in of Rhino/. There is very useful function Orca3D Hull Assistant which will help you to model hull and to cope with hydrostatics. With Orca3D you can make and calculations on velocity and power.
    Then already if you make calculations Stability and Unsinkability better to have programs Maxsurf Modeler and Maxsurf Stability in which easily to pass with IGES file.
    For the construction the program Maxsurf Structure is necessary for you. But it can be made and on AutoCAD.
    I wish you success.
    _____________________________
    NA Razmik Baharyan
     

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