SWATH resistance predictions using Michlet

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Mar 13, 2012.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The attached short report presents comparisons of Michlet predictions
    of the total resistance with measured values of a small model SWATH.

    Reasonable agreement is shown, provided that a form factor is applied
    to the bulb, but not the strut (which is very thin and can be considered
    as a flat plate). It is very likely that CFD could give even better
    results, but execution times will be much longer.

    This short example illustrates how some naval architects use their
    experience in applying form factors to separate parts of the demihull,
    rather than blindly applying them to the submerged body as a whole.
     

    Attached Files:

  2. Ad Hoc
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    Ad Hoc Naval Architect

    Bit confused by the results.

    The dots and continuous curves, is one produced my Michlet and the other experimental from the model 6A? If so which is which?

    Can you also separate and plot the residuary as kN/tonne too...?
     
  3. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The secret is hidden in the text at the bottom of page 1.
    Open symbols are measured values, continuous curves are Michlet
    predictions.

    In time.
    I assume that your "residuary" resistance is
    Rr= Rt - Rittc57
    where Rt is measured total resistance and Rittc57 is the skin-friction
    calculated using the ITTC 1957 line?
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh...unless text or symbols stick out like dog's balls in papers, i tend to miss them. My brain doesn't register subtleties...ask my wife!! :eek::(

    Yes :)
     
  5. BMcF
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    BMcF Senior Member

    Perfect timing. Seriously. I just happen to be poking about at the moment with deriving some numbers for a small SWATH that 'might' see speeds in excess of 50 knots....

    Surely I can count on the predictions to be spot on...yes? I find actual SWATH test data to be a bit sparse at those Froude numbers.

    Ad Hoc?

    :D
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    John might have something of interest.
    The only other SWATH data I have that I can release is for an early T-Agos
    (19?,21?) that was used by E.O. Tuck in an esoteric rebuttal of Salvesen's
    methods.
    The attached is from a spreadsheet bundled with Michlet. (I mislabelled
    the y-axis as Rw/W in the spreadsheet. It is actually Cw).

    In general, Michlet predictions at low Fr might be fairly poor because the
    bulb nose violates the thin-ship assumption. On the other hand, the
    poor predictions at those low Fr are due to an over-estimate of diverging
    waves which are not important if the bulb is reasonably deeply submerged.
     

    Attached Files:

  7. DCockey
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    DCockey Senior Member

    Does Michlet include interaction between the hulls of a SWATH or does it calculate the waves from each hull independently, and then add the waves for calculation of wave drag?
     
  8. BMcF
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    BMcF Senior Member

    He might indeed. He often does.

    Leonard's folks at SOS pushed some of Higdon's SWATH designs to the upper end of 20s..using a brute-force powering approach for the most part. Then there was Cloud X and Stillwater River..GT-powered SWATH designs that were intended to go fast and did..briefly. When it comes to SWATH that met some high speed objectives reliably, I'll have to give that to John and Nigel....
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Yes I do.
    But 99% of it is confidential. We, at my previous company, and our own testing, produced a design that does 40 knots, based on our previous successful 31knot SWATH and then took it one stage further and made it waterjet powered. Spent a long time developing it for a very specific client. We tank tested the hull in Hs=3.0m at 40knots and the vertical accelerations gave roughly 0.1g at best and around 0.2g at worse in Hs=3.0m, if I recall correctly.

    But going 40knots in a SWATH meant surprise surprise lots of power. It was this aspect that really put the client off, since he was "assuming" the power to be less, as they run several sized catamarans. We had to use twin TF50 coupled into one box, per hull. This was around 2002-3, and then just the engine package alone was something like $12-13million! Total cost was around $19m, and with a constantly fluctuating $ rate to the € (as MTU took the GTs by then) at the time, the cost went up and down like a yo-yo, by millions, daily!!
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Michlet calculates the interference resistance as shown in the 1st figure.
    For the T-Agos model SWATH, there is beneficial (i.e. negative) interference
    at some low speeds for the particular demihull spacing.

    Michlet also calculates other wave resistance components. The 2nd figure
    shows that the transverse component (Rwtrans) is dominant at low
    Froude numbers, but that diverging waves (Rwdiv) contribute more at higher speeds.
    Skin-friction dominates at the highest speeds.

    The free wave spectrum (dRw/dtheta) can give more information about
    the wave resistance. At 4 m/s most energy is being shed at wave angles
    between -37 and 37 degrees (i..e transverse waves). In the final figure, it
    can be seen that most energy is being shed at the larger wave angles, i.e.
    for the diverging range of theta-angles.

    Of course, it is better to convert the dimensional speeds to Froude numbers
    to get a better idea of how it all works :)
     

    Attached Files:

  11. Ad Hoc
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    Ad Hoc Naval Architect

    Leo

    I'm curious how you, if you do, separate the differences between surface waves from a surface disturbance, like a floating hull, to that of a submerged disturbance, like that of a buoyant tube of a swath. Since there is, as I'm sure you're aware a difference in the wave formation depending upon the depth of the disturbance, and thus how Michlet accounts for this?..since you're calculating surfaces waves, if my memory serves me correct?

    The transverse v diverging wave is surely highly dependent upon the width of the hull or pressure disturbance, not necessary speed alone? (Hence the L/D ratio effect)

    Is all this simply because you're calculating far-field waves rather than near field waves, as a total?

    PS..the interference aspect is an interesting one. Since SWATHs by their very nature have much wider hull spacings. Thus is there actually much interference effect, compared to a standard catamaran hull, i suspect not?
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Michell's integral includes terms that account for the depth of submergence
    of sources and sinks. (Those terms are absent in slender body wave resistance
    theory.)

    To save you a trip to the library :)P), you can download the preprint:
    "Optimum spacing of a family of multihulls",
    E.O. Tuck and L. Lazauskas, Schiffstechnik, Vol. 45, No. 4, Oct 1998, pp. 180-195.
    http://www.cyberiad.net/library/pdf/tl98.pdf

    The way Michlet accounts for the (exponential) diminishing effects of
    submergence can be seen in equation 4 on page 19.

    The dependence of interference on the demihull separation is complicated.
    At some Froude numbers there is an optimal spacing, at others there is not,
    i.e. the interference resistance is always positive.

    You are correct that there is less opportunity to gain much by a judicious
    choice of spacing with SWATH, at least for normal pod submergence depths.
    Structural and other factors are far more important to consider.
     
    Last edited: Mar 15, 2012
  13. Ad Hoc
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    Ad Hoc Naval Architect

  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks for that one.
    The attached thesis might be helpful for trying to find appropriate form factors
    for SWATH and SLICE. Unfortunately it seems to be limited to speeds up to
    40 knots. But NA's should be able to handle the obvious trends :)
     
    Last edited: Aug 12, 2015

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Thanks Leo, I'll have a read of this later.
    Their conclusions are interesting (having a quick scan)...We actually did this very same comparison ourselves. But our results were different. The SLICE has more resistance than a conventional SWATH. The SLICE is an amazing beast though on sea trails :)
     
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