Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    A while ago, I posted some early results from the ITTC's Facilty Bias World-Wide Campaign. The total drag measurements varied by about 18% at a Froude number of 0.4 for a 3m model. So, not only are there difficulties with small models, but there is also the uncertainty due to towing tank procedures.

    Trusting results extrapolated to full-size (using a dodgy friction line) at low Fn and Rn seems to be asking for trouble. It's no wonder savvy naval architects include large margins right at the start of their spiral. :)

    Leo.
     
  2. daiquiri
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    daiquiri Engineering and Design

    It looks like test results for Fn<0.3-0.4 are being particularily mistrusted.
     
  3. Alik
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    Alik Senior Member

    Powerboat designers - yes we do; sailboat designers - no we do not :)
    Delft sailing yacht series was studied on 1.6-2.0m models...
     
  4. Joakim
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    Joakim Senior Member

    Yes, but there is no reason why friction scale up should work better at higher Fn. Of course friction is less important at semi planing speeds due to high residuary drag, but it becomes again very important at higher speeds.
     
  5. Alik
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    Alik Senior Member

    No, no and no!

    At low speeds (low Reynolds numbers) there is uncertainty if there is laminar or turbulent flow, especially if no turbulence stimulators are installed. This does not allow to reliably deduct frictional component and make reliable re-calculation into fill scale (pls note that 62 series results as presented are re-calculated to 100,000 lbs boat).

    At high speeds this uncertainty does not exist as flow is turbulent.

    This is simple examination, sure Leo will have something to add.
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, there's no need for me to add anything. :)

    Except maybe to say that most of us are lucky we deal with hulls that are hydraulically fairly smooth. Roughness adds awful complications to an already difficult problem.
     
  7. Joakim
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    Joakim Senior Member

    Without stimulators this uncertanity goes to quite high Reynolds numbers, since you don't know where the transition happens (Re 3e5-3e6). Having part of the wetted surface laminar may change the measured resistance quite much. You have to consider the size of the model and also the length of the wetted surface, which may reduce very much at higher speeds, not just Fn. Certainly it just doesn't magically stop at Fn=0.4, just becomes less important.

    And then you get whisker sprays with their scaling problems as well at higher speeds.
     
  8. Alik
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    Alik Senior Member

    Sorry, but You don't have to explain this to me; I have enough hands-on experience of model testing with special expertise in acceleration tests (from zero speed) :)

    As to Fn=0.40 - yes I would use turbulence stimulators even at that speed, on small models. But coming back to series 62 - the series starts at FnV=1.0 that is FnV=0.85 for lighter models. So absence of stimulators is justified in 62 series range - FnV=1.0 to 6.0.

    I understand that Google helps, but series 62 does not have spray rails.
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    And you also have to consider the type of stimulator: small studs, trip wires, and sand grit might produce different types of transition and turbulence at different regions on small models.
     
  10. Alik
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    Alik Senior Member

    And their location also. For saiboat hulls with shallow forefoot we used wire stimulators at Stn1 and lower part of Stn3.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Ok, how about going deeper into this particular? :)
    Are you aware of any studies investigationg various types of turbulence stimulators for scale model ships? It would be interesting to know which one is closest, in terms of transition location, turbulence intensity and BL shape, to the investigated full-size ship/boat? I would expect that an entirely sand-gritted hull surface (with sufficiently big grains) would permit a turbulence distribution and scaling more similar to that of a full-size ship. But, is it really so?
     
  12. Joakim
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    Joakim Senior Member

    Even worse, then sprays have bigger effect on resistance, but probably not huge on series 62 at the measured range, but I was not talking specifically about 62.

    Yeah, Google helps, as well as writing your own VPP's for sailing boats and planing hulls with special weight on friction and surface roughness and having worked with turbulence modeling for almost 20 years.
     
  13. rxcomposite
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    rxcomposite Senior Member

    Where was it where it was said "Reynolds number starts to work as soon as the boat moves"? It was compared to a jet moving at high speed.

    Ah yes. PB magazine Aug Sep 2010 issue. article by Michael Peters. Daiquiri might be interested in this since he is an aerospace engineer.
     
  14. Alik
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    Alik Senior Member

    Why asking such basic questions then? Want to test me, or? :D
     

  15. rxcomposite
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    rxcomposite Senior Member

    Here is the excerpts.

    "The first thing he found
    was that the Reynolds Number of a
    sailboat at racing speed of 8 knots
    compared favorably with a P-51
    Mustang single-seat fighter, one of
    the war’s fastest prop-driven planes.
    He also discovered that the surface
    smoothness of the bottom made a difference
    almost as soon as the sailboat
    started moving.
    Peters On (Fast) Powerboats
     
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