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  1. #1
    Moderator RX951's Avatar
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    Understanding Drag Coefficient

    Understanding Drag Coefficient

    Below is some information to read over Drag Coefficient. some information pertains to the marine industry. Experts out there on our forum such as Tommy Jordan know exactly what this is all about as his career deals with this on a professional standpoint.
    I broke this down in simple forms to post the basic understanding of what and how complex this matter can be. Please refer to the reference links below for a more in depth view of the matter.

    Definition of Drag Coefficient:

    Drag coefficient [ Cd ] is proportional to { (gravitational force) / (inertial force; air or water) } and is used in momentum transfer in general and free settling velocities and resistance to flow calculations in particular.

    Friction often presents itself as drag, when an object is moving through a fluid medium. It is a force proportional to the object's velocity, but opposite to the direction of motion:

    Drag in motor sports
    Reducing drag is also a factor in sports car design, where fuel efficiency is less of a factor, but where low drag helps a car achieve a high top speed. However, there are other important aspects of aerodynamics that affect cars designed for high speed, including racing cars. Notably, it is important to minimize lift, hence increasing downforce, to avoid the car ever becoming airborne. Also it is important to maximize aerodynamic stability: some racing cars have tested well at particular "attack angles", yet performed catastrophically, i.e. flipping over, when hitting a bump or experiencing turbulence from other vehicles (most notably the Mercedes-Benz CLR). For best cornering and racing performance, as required in Formula 1 cars, downforce and stability are crucial and these cars have very high Cd values.

    Frictional Resistance
    Click on this link below for good descriptions over this topic. It uses Kayaks as an example
    http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm

    Now, a breif synosis over hull drag, as our hulls plow through the water, a thin layer of water trapped in the 'roughness' of the surface is dragged along with the hull. Since the 'trapped' water molecules are in contact with many other molecules further from the hull, their kinetic energy is transferred progressively through 'layers' further from the kayak skin. This viscous interaction of water molecules is confined in a space called the boundary layer. The boundary layer is where water causes 99% of Frictional resistance.

    Viscous drag (Rf) depends on the following:

    wetted surface area - S (m^2)
    speed of the hull - V (m/s)
    viscosity (mu)
    kinematic viscosity (nu)
    density of water (rho)
    coefficient of friction- Cf (assumes smooth hull) no units
    length of waterline- L(m)



    http://www.aeromarineresearch.com/tbdp6.html > an example calculation software used to help determine life



    Here is a chart over frictional viscous drag



    Stepped hulls to reduce drag by reducing the wetted area


    References:
    http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm

    http://www.rwc.uc.edu/koehler/biophys/2d.html

    http://www.olds.com.au/marine/nozzles/nozzles.html

    http://www.grc.nasa.gov/WWW/K-12/airplane/dragco.html

    http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

    Drag coefficient Calculator
    http://www.processassociates.com/Pro...men/dn_dra.htm


  2. #2
    ABBOTT's Avatar
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    daaaaaaaaammmmm!! RX THIS IS THE BEST DAMN POST THAT I HAVE EVER SEEN---THIS IS FREAKIN AWSOME MY BROTHA!!!! VERY INFORMATIVE!!

  3. #3
    Moderator RX951's Avatar
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    Thanks John

    Here is another chart showing hull drag of different factors. good one. the speed shown is low, but gives you an idea of how it increases with speed.


  4. #4
    Moderator RX951's Avatar
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    unfilled bolt holes in ride plate = drag
    unfilled bolt holes in trim tabs = drag
    rough rideplate surface = drag
    surface tension = drag

    and there are many other factors that you all know of.......

    you get the idea.....

  5. #5
    Hydrotoys's Avatar
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    What so amazing, is how many folks have been ROUGHING up their hulls over the years. If you spend any time with sailing craft, you would be AMAZED at the time they spend smoothing out every detail...every blemish.

    Lots of folks believe in sanding the hull front to back. That may have worked with skis that didn't handle properly in the past. I find it downright overkill, and will slow the heck out of things. I bought into this theory....once. That was an expensive mistake. Never again. SMOOTH is better. You want to handle better? Set your craft up better!

  6. #6
    ABBOTT's Avatar
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    Quote Originally Posted by hydrotoys
    What so amazing, is how many folks have been ROUGHING up their hulls over the years. If you spend any time with sailing craft, you would be AMAZED at the time they spend smoothing out every detail...every blemish.

    Lots of folks believe in sanding the hull front to back. That may have worked with skis that didn't handle properly in the past. I find it downright overkill, and will slow the heck out of things. I bought into this theory....once. That was an expensive mistake. Never again. SMOOTH is better. You want to handle better? Set your craft up better!
    Wow, now this is a topic worth polling.

  7. #7
    ABBOTT's Avatar
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    fixed the poll, had the options all screwed up. Sorry about that!

  8. #8
    MR. SCOVILLE UNIT SUCKMYWAKE's Avatar
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    thats why all the fountain boats and formula boats and such have the stepped hulls. look at the notches in the side of the boats. this alone equates for 5-6 miles per hour, over the previous type

  9. #9
    Moderator RX951's Avatar
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    ttt

  10. #10
    Old Ski Insanity w/rpm500's Avatar
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    great posting !!!

    You forgot the drag from the look on your face, when your ski attains a new faster gps speed reading , the mouth open in shock!!

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