LCF/LCB/LCG confusion

Users
Deering
2005-12-28
2013-04-19
  • Deering
    Deering
    2005-12-28

    Guess I need some clarification on this...

    According to the wireframe data display the LCF is located at 19.92 ft.  That makes sense for this design (slightly stern-fat skinny hull 45 ft long).

    When I run the design hydrostatics report it states that the LCB is located at 21.2 ft and the LCG is at 22.4 ft. 

    These don't make sense since I have added a face to simulate engine load at 1.3 ft and given it a layer density/thickness of about 1500 lbs.  The LCG should be much further aft.

    At the bottom of the hydrostatics report is a table.  It shows the overall COG X at 11.9 ft.  Isn't the COG X the same as the LCG?  Shouldn't those values be the same?

    I'll upload the file...

    Thanks! 

     
    • Deering
      Deering
      2005-12-29

      I think I figured it out... I was looking at the LCG for a theoretical longitudinal face.  Took me awhile to understand that, if indeed I do.

       
    • Marven
      Marven
      2005-12-29

      You're mixing things up here.

      First of all, the COG reported at the bottom is an indication for the designer of where the center of gravity of certain surfaces are located. This might come in handy if you want to do a weight calculation . COG of curved surfaces are normally tricky, if not tedious, to calculate. This however has NOTHING to do with the hydrostatics. The data shown there is calculated at a fixed trim (0.0 degrees) and draft (design draft).

      The hydrostatics report shows you LCF (center of gravity of the dwl section) and LCB (center of gravity of the submerged body). as stated previously this is for a fixed trim. At a later stage FREE!ship will calculate the appropriate trim (and possibly heel) to get the two at the same longitudinal position and also the correct draft for a gived displacement.

       
    • Deering
      Deering
      2005-12-29

      No, I think I understand all that...  It's a workaround with this system, but here's how I'm handling it:

      I'm setting my draft to correspond with the expected overall displacement of the boat. 

      I'm trying to represent all of the major weight items, such as engines, fuel, batteries, cabin, etc as transverse faces on their own layers (I have a layer called 'engine' for instance) with an appropriate weight (density x thickness) located at the correct longitudinal position.  Because this is a catamaran hull I'm not too worried about heel or VCG.

      If I do an accurate job of inventorying all of the weight components on appropriately located faces/layers then I should have a realistic LCG. 

      Adjust draft until displacement matches total weight budget.  This hull is fairly symmetrical so LCF doesn't shift too much with change in draft. 

      Compare LCG with LCF - if they aren't longitudinally aligned then I know trim won't be zero, and I can adjust COG weights or locations until it aligns, or alternatively, adjust hull lines until LCF is located where I need it.

      It's a clunky way to do this, and any improvement in the system would be very welcome.  Being able to place point and distributed loads would be helpful rather than having to mess with faces/layers.  And knowing what the change in trim would be for an offset between LCG and LCF would be nice too. 

      But the advantages to this method are that the various loads are accounted for, and can by accurately located on the hull.

      Did that all make sense, or am I babbling? (as usual)

       
      • Marven
        Marven
        2005-12-29

        Well, I suppose that what you're doing is correct. But in a way you're just using the program for something that could be MUCH easier done in a simple spreadsheet. You are using it to sum weights to calculate the total weight and COG if fictive items. But FREE!ship doesn't do anything else with the aquired data.

         
    • Deering
      Deering
      2005-12-29

      I think I would have arrived at the same conclusion on my own eventually.  Screwing around with faces and layers was going to rapidly get tedious.

      Maybe I'll take the hull COG info from Freeship and plug that into a spreadsheet along with all of my other loads.  Get a composite COG. Then play with location/LCF to adjust trim.

      Thank you for your wise advice.

       
    • tighkey
      tighkey
      2006-01-31

      I'm the one confused, too.
      And this may be a too primitive question.

      To keep the boat float horizontally to designated water surface line, I suppose either COB and COG of the boat have to be on the same vertical line.

      I have learned the LCB stands for COB.  But is there any indication of COG on freeship display?

       
    • Flyinglow
      Flyinglow
      2006-08-04

      I am not sure I have understood all of this so, here's a question: When calculating the hydrostatics, is Trim the offset between LCF and the LCG that we would calculate externally from the program?

      Thanks,

      Flyinglow