Michlyt Demo

Discussion in 'Software' started by Leo Lazauskas, Apr 21, 2013.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    MICHLYT
    =======
    Michlyt is a very simple program that estimates the total calm-water
    hydrodynamic resistance of monohulls, with or without transom sterns.
    The program is called from inside an Excel spreadsheet.

    If you find Michlyt useful, please consider making a small donation at:
    www.cyberiad.net/michlet.htm
    (Donations go through my partner's PayPal account).

    INSTALLATION
    ============
    After downloading, unzip the file into a convenient sub-directory.
    There are two files; the DLL, "michlyt.dll", and a spreadsheet named
    "michlyt.xlsm". Instructions are contained in the "Manual" worksheet.

    To remove the program, simply delete the sub-directory.

    REQUIRED INPUT
    ==============
    The program requires three physical constants, namely gravitational
    acceleration, water density and water kinematic viscosity.

    Users must specify three principal hull dimensions, the length, beam
    and draft.

    An offset table is used to describe the underwater hull shape. The
    format of the table is the same as used in Michlet. Delftship and
    Freeship can also produce tables in the correct format.

    OUTPUT
    ======
    The program calculates several geometric and hydrostatic quantities
    of interest: LCB, LCF, KB, midship area, waterplane area, wetted area,
    displacement volume, longitudinal metacentric height and transverse
    metacentric height (assuming CG is at the waterline).

    Three resistance components are calculated:
    Rf: Skin-friction estimated using the ITTC 1957 line.

    Rh: Transom stern hollow resistance which includes the loss of
    hydrostatic pressure on the transom stern as well as wave-making induced
    by the transom.

    Rw: Wave pattern resistance.

    EXAMPLES
    ========
    The initial data in the spreadsheet is for a round bilge model hull
    with a transom stern. The NPL 4a hull has been studied by Molland and
    his co-workers at the University of Southampton over several years.
    See: Molland, A.F., Wellicome, J.F. and Couser, P.R.,
    "Resistance experiments on a systematic series of high speed
    displacement catamaran forms: variation of length-displacement ratio
    and breadth-draught ratio",
    University of Southampton, Ship Science Report 71, 1994.

    I have included a simple plot of the resistance, however, you are free
    to create your own output charts and summaries.

    LIMITATIONS
    ===========
    The main restriction in the demonstration version is that the hull is
    represented using only 11 stations and 11 waterlines. More accurate
    estimates are possible in commercial versions which allow more stations
    and waterlines. Other versions are also available that can handle
    multihulls.
     

    Attached Files:

  2. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, I'm missing something. What is the difference between this and the existing free version of Michlet?
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Many people have emailed me over the years saying they are daunted by Michlet and the enormous number of options and input requirements.
    Michlyt is much easier and it operates within Excel, which most people are familiar with.

    Also, it uses a different transom stern method which gives reasonable estimates for hulls with L/B as low as 4 or 5.
     
  4. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok. Makes sense.
     
  5. Remmlinger
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    Remmlinger engineer

    My understanding of the theoretical background of Michlet was, that the thin ship approximation does not allow such low L/B rations. I thought this is not only caused by a transom stern but generally by the magnitude of the angle between the water lines and the center-line. I understood that a large nose radius or an elliptical form of the rear part of the water plane (like sailing yachts with a bow down trim) would reduce the validity of the Michlet-results.

    Is this different in MICHLYT?
     
  6. daiquiri
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    daiquiri Engineering and Design

    At first I thought some Iranian student has misspelled the word Michlet, then I saw it was actually you, Leo. :p
    Thanks for that effort, sounds like a really good idea.
    Cheers!
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes, it is different in Michlyt. I have been trying out a new transom
    model I migrated from Flotilla. Michlyt doesn't do quite as well as
    Flotilla because it doesn't include trim and sinkage, or the hull wave
    profile, and it uses the ITTC line for skin-friction, but it isn't too
    bad.

    The reasons for poor predictions of wave resistance for low L/B hulls
    at low Froude numbers are well known. It seems to be much worse for
    canoe sterns where, as well as an over-prediction of wave cancellation
    and reinforcement, there are also issues with boundary layer separation
    and other viscous effects at the stern. Using a form factor would
    probably help. In fact, a small form factor in Michlyt might also be
    advisable for some hulls to compensate for the fact that squat is not
    accounted for.

    As you correctly noted, there are "theoretical issues" with elliptical
    bows and sterns where the small slope assumption is definitely
    violated. I haven't included "corrections" for those types of hulls yet,
    but I have some ideas that should work for submarines and SWATH-like
    hulls.

    You can check Michlyt predictions against the experimental data I have
    included in the attached Flotilla "validation" reports. These are first
    drafts so don't expect too much detail, however, there are comparisons
    with experiments for nearly 100 cases in the set, and there are tables
    containing the principal geometric particulars of the hulls.

    Michlyt can be easily validated against experimental results for the
    NPL hulls because the hull bundled with Michlyt is an NPL hull.
    All that you need to do is to change the length, beam and draft in the
    spreadsheet to the values in the tables given in the
    "npl_mono_230413.pdf" and "bignpl_230413.pdf" reports. (That cannot be
    done with the modified NPL hulls in npl5d5e_230413.pdf because they are
    not exact geosims of the standard NPL hulls.)

    Remember, though, that the number of stations and waterlines in Michlyt
    has been deliberately restricted, so the surface area will not be quite
    as accurate as when the hull is better represented. It's not too bad
    though. Of course, people can always pay for better accuracy and other
    features. ;)
     

    Attached Files:

  8. Remmlinger
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    Remmlinger engineer

    Flotilla reports

    Very impressive Leo!
    Especially if one keeps in mind the computing times compared to CFD-methods.
    Did you use any individual form factors or other individual factors to improve the agreement with the experimental values?

    Because of my interest in sailing yachts, I looked at the hullforms NPL 80C and at the NPL 100D. The L/B and B/T ratios are close to a yacht. Unfortunately, when beating to windward the interesting speeds are FN=0.3 to FN=0.4 and this is the area where your method shows the largest error (around 25%).

    You said, you "have some ideas". I wait till they will see the light of day.
    I will keep my fingers crossed.
    Uli
     
  9. MikeJohns
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    MikeJohns Senior Member

    Well done Leo, thanks for the effort.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No form factors have been applied to the skin-friction. I suspect that
    doing so would improve the correlation of the Series 64 hulls for Froude
    numbers > 1.0 because there is likely to be a lot of spray and splash.

    There are some factors applied to the transom stern "unwetting" based on
    2D theory, and the wave resistance induced by the transom is calculated
    using a very simple, but IMO remarkable, observation made by E.O. Tuck.
    It is much simpler than the transom hollow approach used by Doctors and
    by Molland and his colleagues. In fact, it is only about 10 lines of code!

    I appreciate that the Froude number range of interest to you is lower
    than mine. When I finish off some work on finite depth, I might look
    into improving the correlations at low Fr using some fudge factors. I'm
    sure you can appreciate that the flow behind a partly wet transom, or
    one that has just become fully dry, is a nightmare.
    And, yes, I have some ideas I want to test, but Tuck left me with a
    lifetime's worth of ideas to play with, so some will take longer than
    others to see the light of day.
     
  11. Remmlinger
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    Remmlinger engineer

    I know what you are talking about. I tried to calculate an estimate based on Doctors' theory. I works well for one hull-form and one gets all excited, but on the next hull-form it fails drastically. So yes, I can indeed appreciate your achievements.

    Is there a paper, that describes this observation?
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I like Doctors' work, and in particular the way he presents experimental
    data to compare with his theoretical approaches. I agree that some of
    transom hollow work is difficult to apply. Some of his papers use one
    type of empirical equation for the hollow length; others have an implied
    minimum hollow length that is not always clearly defined.

    I also like Doctors' work on the rate of unwetting of transoms, and the
    effect of the hull wave profile at the stern. Robards' thesis (cited
    in the bundle of papers I attached earlier) has a lot of experimental
    data for transom unwetting and measurements of the hollow length.
    The thesis is available online and has a wealth of experimental data that
    I have also used.

    One aspect I do not agree with is the wave-making of the hollow. As I
    have often said here, I cannot see how a hollow vented to atmospheric
    pressure can be a wave-maker. Tuck's idea came out of discussions I had
    with him on this very topic. Unfortunately, there is no paper; just
    hand-written notes and emails.
     
  13. daiquiri
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    daiquiri Engineering and Design

    This reminds me of the recent discussion about surface-piercing rudders, where I had expressed my doubts about modelling the water surface as a rigid boundary at low speeds - a method used by some cited authors (even cited by you)...

    Well, I guess mathematics doesn't always agree with the nature. :p ;)

    Cheers
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, we agree with Nature!
    The only argument is whether Nature can display more decimal places than
    we can. :)
     

  15. daiquiri
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    daiquiri Engineering and Design

    LOL!!! :D
    I guess she could... It's just that the numbers behind her comma sign keep running and swapping their positions, while yours are standing still. ;)
     
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