Hullform Question

Discussion in 'Software' started by Don Case, May 18, 2011.

  1. Don Case
    Joined: Oct 2007
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    Don Case Junior Member

    I have been playing around with Hullform 9P trying to design an IOM (model yacht). I'm confused about the LCB. It never moves when I heel the hull. This is with a half dozen boats. It is different with different boats. Once I do the "evaluate statistics" to get the LCB it never changes. If I blank out the "X center of mass" and then do another "evaluate stats" with say 30 degrees of heel it changes the LCB but it also changes the displacement. Can anyone help?
    Thanks
    Don
     
    Last edited: May 18, 2011
  2. fastwave
    Joined: Jan 2007
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    fastwave Junior Member

    If you are leaving the boat free to heave and trim, then it shoudl always match your CG. Since your LCG is fixed during heeling etc, the software should find a trim that matched the LCG. Therefore your LCB is also fixed to your LCG
     
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  3. Don Case
    Joined: Oct 2007
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    Don Case Junior Member

    Thanks
    I think I follow your explanation(I'm new at this) but why do I see other people talking about a moving CB. As in "make sure your CB doesn't move much when the boat heels"
    Don
     
  4. fastwave
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    fastwave Junior Member

    what they mean is that if the hull shape is such that the submerged volume distribution with heel remains more of less constant then the boat will not change trim with heel.

    That is the boat will not go bow down when it heels, usually.
    Fix the trim in Hullform and run the heel sweep again. Then you can see taht the CB moves. The more the CB moves in fixed trim the more the trim angle will change in completely free conditons
     
  5. Don Case
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    Don Case Junior Member

    Could you explain this differently? By "fix the trim" do you mean change the trim and where do I do this? And "run the heel sweep"? I'm sorry I don't know what that means.
    Thanks
    Don
     
  6. Don Case
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    Don Case Junior Member

    Is this something that I should be using the "variables plot" for? I'm just getting into that now.
    Thanks
    Don
     

  7. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Hydrostatics is actually pretty simple. The problem that most people have is remembering this.

    All the software is doing is integrating the volume below the waterline and reporting the total volume and centroids thereof.

    Now, you can use that as the basis of a dynamics equation. From Newton's second law we know that a force or moment causes an acceleration, which causes the boat to move. So you can calculate the movement in a given time and re-evaluate the hydrostatics. If you do that enough (and include a damping term) then the result you get is the angle at which the boat will float, under your specified conditions.

    So, what this means is that you can now make a reasonable guess at the running heel and pitch of the model. The pitch angle will make a difference to the resistance, as will (more obviously) the heel angle. However, the pitch angle may cause you control issues as well, and this may be the reason for the advice.

    I'm afraid that this sort of misunderstanding is common in all areas of the model fraternity; You just have to "read past" the "modelling" language and interpret what they actually mean. As you have found out, the LCB must equal the LCG in free-floating situations.

    Tim B.
     
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