ITTC Dictionary of Ship Hydrodynamics

Discussion in 'All Things Boats & Boating' started by daiquiri, Jul 14, 2011.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    You mean say higher than Fn 1.0..?

    Like these : http://www.turbojet.com.hk/eng/vessel/vessel_layout.asp

    Typical fast ferries over Fn 1.0, these aren't planing! Bummer!!

    Me, pedantic...nah :p
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Is the draft much smaller than the (demihull) beam at Fn=1 for those vessels?

    My personal definition of planing is that it begins when at least half of the
    weight is being supported by hydrodynamic forces. Someone had a criticism
    of that, but I can't remember why. :eek:)
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Not on the Cat's...but on the larger car/passenger ferries like these typ. monohulls it is:
    http://en.wikipedia.org/wiki/Super_SeaCat

    There are a bunch of them made in France and Italy at varies companies such as: Fincantieri/Rodriquez etc.

    Gets tricky doesn't it....!! :eek:
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    There's no doubt it's tricky if the definition is based on the principal hull
    dimensions.

    There might be less doubt if we based it on hydrodynamic features, such as
    the Froude number based on the distance from the centre of pressure to
    the stern, or lift/weight. But that would also make the definition very
    technical and opaque to most people.

    Apologies for that earlier crack about NA pedantry.
    As of today I'm now a fellow traveller!
    Leo (AssocRINA).
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Apologise for what??....it was a fair comment :D

    Words have clearly defined and universally (well to most) meaning, if we don't say what we mean we don't mean what we say ;)

    This was the same difficulty when the HSC code about in 1994. The debate in the early 90s, prior to being published was very similar indeed. It is one of those "definitions" that is visual, rather than maths based.

    Such as, if we were to look at a hull, which has chines, hard chines, and saw the flow of water was all below the chines and being "over turned" with no cross flow back down to the free surface as such and not creeping up around the sides etc....and looked at the transom and saw it was ventilated...we would all say...hey "we" are planing!

    Trying to make some kind of "measure" of this condition for a definition becomes tricky. Perhaps a different approach is required. Like the related to the virtual change in VCG..but that gets more tricky.

    I don't think there is a simple answer, despite it being obvious visually. Other than more than half the weight being supported by forces other than hydrostatic.
     
  6. medaca

    medaca Previous Member

  7. masrapido
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    masrapido Junior forever

    Or, just remind the NA of the crucial difference: the speed.

    Can't see a barge plugging along at 25 knots...
     
  8. Stephen Ditmore
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    Stephen Ditmore Senior Member

    The Daniel Savitsky paper that used to be here:
    http://www.sname.org/newsletter/Savitskyreport.pdf
    and was referenced at post #2 here:
    http://www.boatdesign.net/forums/powerboats/displacement-glider-powerkeel-etc-12512.html
    offered rather persuasive evidence that the planing regime generally starts at a speed/length ratio of about 3 (Fn = 0.9), or perhaps more accurately that Fn=2.9 is where relatively narrow (ship-style) hulls in the semi-displacement regime typically commence problematic behavior. It seems to me one hallmark of planing is running trim shifting down by the bow and/or optimum CG moving aft.

    I've heard it said planing is where 50% of displacement is supported hydrodynamically, but I don't personally think that's a good way to define it. I've seen it defined using volumetric Froude number; perhaps someone can offer a candidate value?
     
  9. daiquiri
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    daiquiri Engineering and Design

    I think you have missed the thread. There is a very active thread about the definition of planing (started by Leo), where imo your post will be much more appropriate and visible.

    Cheers
     
  10. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Thanks. I might have the Savitsky paper saved to an old external hard drive. Perhaps tomorrow I'll have a chance to look for it, or if anyone else locates that or the thread daiquiri's referring to perhaps we can post the Savitsky paper there. I think the point of the paper is that if we want to build high speed freighters we should build them reeeeaaalllllyyy long, but it has good graphs and data up to about Fn=1.0 (displ/length 3.3).
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Canttttttttttttttttttttttttt argue with that :D
     
  12. daiquiri
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    daiquiri Engineering and Design

    I remember the paper you're talking about, but since I'm writing this from my smartphone, cannot upload the version I have on my PC. However, I've located one copy in internet, it's here: http://www.staatsgeheim.com/wp-content/uploads/Savitskyreport.pdf

    As about the said Leo's thread, you can find it here: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/definition-planing-45248.html
    Except for the last 2-3 pages, where Tunnels had to show his usual contempt towards those who have a scientific approach to boat design, it is a very good and inspiring reading.

    Cheers
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

  14. kach22i
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    kach22i Architect


  15. gonzo
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    gonzo Senior Member

    NIce link
     
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