Form factor [scale effects]

Discussion in 'Hydrodynamics and Aerodynamics' started by vignesh, Mar 14, 2012.

  1. vignesh
    Joined: Jan 2011
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    Location: chennai

    vignesh Junior Member

    First i would like to thank members who answered many of my post .Actually i am sorry about the passenger ship thread ,i never expected that was a simple one ..i imagined something else .


    anyway. I wanted to ask you a question relating form factor of ship.

    The literature says form factor is same for ship and model.but how is the scale effects looked upon here ,like since form factor is derived from Cf,in model testing we are extrapolating by froude law ,here cf which basically function of reynolds number ,how is the error compensated or corrected.

    secondly,
    My point is on the whole ,model testing at a very low froude number is not advisable because you will have scale effects because frictional resistance dominate ,which is basically a function of reynolds number. Is my point correct?

    atlast one more question.

    Bulbous bows are effective for transverse waves only ? if so ,beyond 0.45 froude number or so ,transverse waves becomes extinct ,it means bulbous bow is not effective beyond 0.45 fr.no?

    Thanks in advance.

    Regards,
    B.Vignesh
     
  2. Leo Lazauskas
    Joined: Jan 2002
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    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    At the 26th ITTC held last year, the Resistance Committee noted their
    concerns that there is a Reynolds number effect on form factors.
    See pages 29 and 30 of the attached report.

    The 25th ITTC Powering Performance Committee found a significant
    effect with the ITTC line, but less so with Grigson's line.
    This isn't surprising to me as Grigson's line (and my own work)
    are based on 2D boundary layer theory and constants derived from
    data over a reasonably wide range of Rn.

    The ITTC line was adopted in 1957 as an interim solution because
    delegates wanted a friction line that agreed with Schoenherr's line for
    Rn > 10^7, but that was steeper than that line for Rn < 10^7.
    Note, however, that Schoenherr's "law" is empirical, it is based on
    spurious data, and it is based on forms of equations that are incorrect.
    As Grigson has said: "This is not Science but guesswork".
    (But that is probably good enough for some in the ship design racket :p)

    In my understanding (and I am happy to be corrected) a bulbous
    bow acts to reduce the size of the bow wave, and to (slightly) reduce the
    effective Froude number.
     
    Last edited: Aug 12, 2015
  3. vignesh
    Joined: Jan 2011
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    Location: chennai

    vignesh Junior Member

    Thank you sir,
    Regarding bulbous bow ,yeah the reduce the bow wave by interference effect ,but does interference exist beyond fr.no 0.35 ?.

    it means bulbous bow are not effective right ?..I would like to summarise that Its not economical to place a bulbous bow for such a high froude number .Apart from not being useful,it is going to addup to the frictional drag due to addition of wetted surface area.

    correct me if i am wrong .
     
  4. Leo Lazauskas
    Joined: Jan 2002
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    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    From a purely hydrodyamic point of view, it depends on the shape and the
    location of the "bulb", and on the shape of the rest of the vessel. For some
    thin hulls, bulbs (or rather a thickening of the hull) a little aft of the bow can
    be marginally effective. Whether they are worthwhile from an overall
    economic aspect is debatable. For example, the hull might be more
    expensive to build, and it might only be effective over a very narrow range
    of Froude numbers.
    See, for example, the "optimum symmetric ships" in a previous thread.
    http://www.boatdesign.net/forums/at...al-displacement-hull-shape-min-drag-fig30.jpg

    Resistance, of course, is only one facet of a very complex problem. There
    are other advantages and disadvantages to bulbs that you should also
    consider if you aspire to be a good, thoughtful naval architect.
     
    Last edited: Mar 14, 2012

  5. vignesh
    Joined: Jan 2011
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    Location: chennai

    vignesh Junior Member

    Thank you sir.Whenever talking about hydrodynamics ,I want to share one of my views-:

    I already created few threads regarding this .but couldnt get a convincing reply.

    Is there by any means i can increase the pressure distribution at the stern region and use that increase in pressure to assist the ship's motion thereby i reduce the power required to propel the ship.

    to summarize ,does increase in pressure in the stern region helps me to reduce the resistance?

    I guess stern wedge ,apart from trim optimisation ,can help ships forward motion ,by above principle .correct me if i m wrong.
     
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