Aerodynamic drag coefficients

Discussion in 'Hydrodynamics and Aerodynamics' started by rww76, Jun 15, 2011.

  1. rww76
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    rww76 Technician

    Hello everyone,

    I am currently searching for aerodynamic drag coefficients for high speed craft. Actually for anything that looks like a small boat (13m).
    I ve found some for fishing trawlers and ships but nothing for this type of boat.
    I know that the ITTC method uses cd=0.73 for extrapolating resistances. Do they assume this number for any type of craft??
    Do you know where can I get some figures?

    Thank you very much

    Christopher
     
  2. daiquiri
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    daiquiri Engineering and Design

    The figure of Cd = 0.30 is claimed in this paper: http://www.hiper08.unina.it/cd hiper 08/HTML/Papers/28- Almeter.pdf

    On the other side, Faltinsen in his "Hydrodynamics of High-Speed Marine Vessels" says this:
    "The air resistance with no wind present may be expressed as
    RAA = 0.5 ρa Cd A V^2,​
    where ρa is the mass density of the air and A is the area of the above-water hull form projected onto a transverse plane of the vessel.
    ...
    Efforts are made to design streamlined superstructures for high-speed vessels in order to minimize CD. Wind tunnel tests are commonly used to determine Cd. Typical values of CD are between 0.5 and 0.7. Because ρa is only 1.25 kg/m3 for dry air at 10°C whereas ρ for salt water at 10°C is 1026.9 kg/m3, the air resistance makes a small contribution; however, it should not be neglected a priori."​

    So, one might infere that the aerodynamic resistance coefficient varies between 0.3 and 0.7, depending on the shape of the superstructure.

    Cheers
     
  3. rww76
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    rww76 Technician

    Thank you for the quick answer.
    This paper shows indeed a Cd of 0.3 but it also claims to be careful of an underestimated aero drag...
    For example, when extrapolating resistances, Ittc adds Cd=0.73 to the full scale resistance. However it doesn t substract any aero drag from model size resistance.
    Does it mean that the figure 0.73 is lower than a true Cd to take into consideration the model resistance?
    It can be said that this is negligible but I still get differences in total resistance of:
    1.2% at Fn=0.7
    2.6% at Fn=1.2 when adding a aero drag (according to ITTC)

    There is no way of evaluating my answer.
    As you said, I may use a value from 0.3 to 0.7 but this is a little bit random... :s

    Thankss
     
  4. daiquiri
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    daiquiri Engineering and Design

    If you want to use just Faltinsen as reference, then the range of Cd values shrinks to 0.5-0.7. The shapes of boat/ship superstructures also differ, and the Cd depends on the shape (and frontal area), hence the randomness.
     
  5. Olav
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    Olav naval architect

    From "Design of Propulsion Systems for High-Speed Craft" by Donald L. Blount and Robert J. Bartee, 1997:

     
  6. DCockey
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    DCockey Senior Member

    The paper referenced above has the statement
    the author has observed air drag coefficients of 0.30 and lower for non-streamlined shapes.
    He doesn't state what definition he's using for air drag coefficients, but if it's the usual one based on fronatal area then I suspect there is a mis-print. 0.30 would be low for a non-streamlined shape, and I'd be surprised at lower coefficients. I wonder if it should have read "0.30 and higher" instead?
    My guess is air drag coefficients for the portion of boat hulls and superstructures above the water for some boats may be higher than 0.7. I include the portion of hull above the water because there may be separation or trailing vortices shed at the sheer near the bow.

    Also, for "bluff bodies" if the flow is attached over the forward portion of the body with no shed vorticies there is little or no further drag reduction possible by altering the shape of the forward portion of the body. For a smooth bluff body with attached flow over the forward portion of the body, drag is dominated by the flow over the aft portion of the body, including separation and shed vortices over the aft portion.
     
    Last edited: Jun 15, 2011
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  7. yipster
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    yipster designer

    Rite' better analise and optimise aerodynamics than find constants in variables
     
  8. Alik
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    Alik Senior Member

    I use CAA=0.6...0.9 for high speed boats; this coefficient is related to frontal area of craft. Those numbers might look high but they work well for small boats that are likely to trim up, have exposed people or a lot of small details like handrails, lights, radars etc.

    In CAA general depends on slope of cabin front, and L/B, L/H ratio. 'Shorter' boats have higher CAA.

    You can search for report of Wolfson Unit, they tested number of HSC in wind tunnel.
     
  9. yipster
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    yipster designer

    hi all, i did not know there was so much existing on this subject
    still think every boat differs but this info can save lots of work
    not that i'm designing a HSC at the moment but thanks :idea:
     
  10. rww76
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    rww76 Technician

    Thank you very much for these quick answers !!
     
  11. PetterM
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    PetterM Senior Member

    If you are traveling fast then the lift coeficient also needs to be taken into account.
     
  12. otseg
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    otseg Junior Member

    So Aerodynamics for dummies.

    Disregarding a drag reduction from lift, If my 39' center console catamaran goes 75 mph with (2) x 350 hp, how much will aero drag increase if I put (4) 300's on it and go faster. Any idea of the total drag?

    The SKA guys towing 39' V bottom center console fishing boats all over the country will often remove the center console windshield while towing to improve their truck fuel economy.
     
  13. messabout
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    messabout Senior Member

    Otseg, Aero drag varies directly with velocity squared. SO just a little bit more speed will cause a disproportionate increase in drag. If you increase speed from 75 to 85, aero drag would increase something like 30%. That would be a questionable bargain for only a 13% increase in speed. If you are compelled to go faster you can do so but you will have to pay dearly for it.
     

  14. Ad Hoc
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    Ad Hoc Naval Architect

    Some good source data here too:

    Molland, A.F. and Barbeau, T.-E. (2003) "An investigation into the aerodynamic drag on the superstructures of fast catamarans". The Transactions of the Royal Institution of Naval Architects Part A: International Journal of Maritime Engineering
     
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