Calculate resistance using Michlet; Errror:Bad displacement

Discussion in 'Software' started by sps5749, Mar 31, 2014.

  1. sps5749
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    sps5749 Junior Member

    Hello all,

    I would appreciate if you could help me to calculate the resistance of my catamaran hull-form.

    I used Maxsurf-Resistance software and I came up with a resistance of 6000kN at 25 knots. I considered this resistance calculation inaccurate and one of my advisors suggests me to use Michlet software. I follow the instruction and when I’m trying to run Michlet crashing..and the error states; bad displacement.

    Any hint?

    ps:my hullform is L-250m, B-80m, T-15m.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Have you worked through all the examples as suggested in the manuals?
    You should use the example files as templates for your design.
     
  3. sps5749
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    sps5749 Junior Member

    Thanks I solved my problem. Do you know what Ntheta stands for?
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I hope so!

    It is the number of theta intervals used in integrations that calculate wave
    resistance and wave elevations.
    For most calculations, Ntheta = 500 should work.
    For catamarans, use more, say 1000.
    For shallow water, you might need to use even more, say 2000 or 4000.

    Good luck!
     
  5. daiquiri
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    daiquiri Engineering and Design

    ROTFL!!! :D :D :D
     
  6. sps5749
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    sps5749 Junior Member

    wave interference

    Dear Leo, seems to be a problem with the wave interference between the demihulls. For my maximum speed of 25 knots, I can't found any difference between a lateral separation of 23 and 28 metres.

    Furthermore, please find attached the file that I have the problem. I would appreciate if you could help me to sort out the isssue.
     

    Attached Files:

    • in.mlt
      in.mlt
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      5.4 KB
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  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    There is no separation between the two hulls in your file. Both are at 23.0
    metres. The middle panel in the right hand column shows the planview of the
    hulls. As you can see, there is only one hull visible (in green) when you run
    your file.

    Change the lateral separation of the first hull to 0.0
    For example:

    # Lateral Separation Distance (metres) (0.0 for a monohull)
    0.0

    When you run the file you should see two hulls in that panel.

    You should use more waterlines to represent the hulls if you want accurate
    estimates of wetted area.
     
  8. sps5749
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    sps5749 Junior Member

    problem now with geometry differing to other sources and hand calculations

    Thank you very much for the help. I have another issue, the basic geometry of our hullform within the program does not agree with the information we are getting from the program we created the initial model from. the Cb is 0.6 when other methods, and the hand-calculations show cb of 0.4, do you have any thoughts?
    a picture of the hydrostatic data from the original model data is attached.
     

    Attached Files:

  9. Leo Lazauskas
    Joined: Jan 2002
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    Leo Lazauskas Senior Member

    I don't know anything about Maxsurf.
    Maybe Cb in Maxsurf uses the total displacement volume (i.e. the sum of the 2 demihulls), and maybe it
    uses the overall beam of the vessel. How is it defined in Maxsurf?

    I would also check the hull separation distance in your input file. It seems very narrow to me.
    It is the distance between demihull centreplanes in Michlet. Should it be 46.0 metres and not 23.0?
     
  10. sps5749
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    sps5749 Junior Member

    Plotting a ROPAX hullform with bulbous bow in michlet?

    Hello again,

    I'm trying to calculate the resistance of a modern ROPAX hullform with bulbous bow and I'm getting the error; bad displacement. Any clue?

    I'm attaching also the ropax hullform, I'm working on.
     

    Attached Files:

    • in.mlt
      in.mlt
      File size:
      18.2 KB
      Views:
      174
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'll have a look when I get time.

    Incidentally, your table of offsets has non-zero entries in the first row. They must be all zeroes. (See the manual.)
     
  12. sps5749
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    sps5749 Junior Member

    Thank you. Anyway I changed the values of first line to zero and still getting the same error.
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I know. That's why I said "incidentally". :)

    Look at each entry in the table and you'll soon see your error.
     

  14. Andrea Kursetgjerde
    Joined: May 2017
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    Andrea Kursetgjerde New Member

    Dear Leo

    Im trying to calculate the resistance on a trimaran. Michlet gives a total resistance 2X of what is expected, do you have any idea what might be the problem`?
     

    Attached Files:

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