Is Matte Faster?

Discussion in 'Hydrodynamics and Aerodynamics' started by sabsfeigler, Aug 30, 2012.

  1. sabsfeigler
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    sabsfeigler Industrial Designer

    So even a good sand blasting wouldn't make a noticeable difference, especially over the short race distances racing canoes and kayaks race over (max 1000m)

    sabs
     
  2. Petros
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    Petros Senior Member

    too much roughness is not a good thing for sure. Do not confuse the rather large dimpling of the golf ball surface designed to increase surface turbulence, with a mat finish to reduce surface tension.

    One is to raise the Rn to keep the flow attached, the other is to reduce the small amount of "cling" at the surface interface due to surface tension.

    Sanding a dull finish will make perhaps a little difference over a polished surface, but having a rough surface will make the drag go up.

    A polished and shinny surface may slow the attachment and growth of sea life on the bottom, so it would be a faster boat than one with a mat surface that sea life finds easier to adhere and thrive.

    So, shinny is better.
     
  3. Erwan
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    Erwan Senior Member

    An alternative to random sandpapering with hazardous outcome, would be to stick RIBLETS on your hull.

    Cheers

    EK
     
  4. quequen
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    quequen Senior Member

    Very clear explanation, thanks Petros!.
    Now, questions: Golf balls must not exceed 72 m/s, at D = 0.04267m that gives a Rn = 3251454. In order to reach Rn = 6x10^6 , diameter must be about 0.08m. So, do little dimples make the ball "larger" as a consecuence of tourning boundary layer turbulent?
    And, can CFD transient analysis simulate and show that Cd dop?
     
  5. Erwan
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    Erwan Senior Member

  6. waikikin
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    waikikin Senior Member

  7. tspeer
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    tspeer Senior Member

    You need to be very aware of the Reynolds number and the character of the flow when playing around with surface texture to reduce the drag.

    Dimples on a golf ball work because the ball flies in a sub-critical Reynolds number range where there is massive laminar separation on the lee side of the ball. Surface roughness trips the laminar boundary layer to turbulent and allows the flow to remain attached much farther around the ball before undergoing turbulent separation. The drag is reduced because the separated wake is much smaller. If the ball flew faster so that it was in the supercritical regime where the flow transitions to a turbulent boundary layer before it reaches the separation point, then the dimples would only serve to increase the drag.

    A matte finish could reduce the profile drag if there was some laminar separation to begin with. It could turn a long laminar separation bubble into a short bubble.

    The research I've seen on the effects of surface finish for attached turbulent flows indicates smooth is fast. A good example is the testing of dinghy hulls Frank Bethwaite describes in "High Performance Sailing".
     
  8. tunnels

    tunnels Previous Member

    Thats for sure !! really nice looking !! and its where boat design should be headed !! if only the plauge would get rid of the brain dead designers that still dominate the boating scene !:eek:
     
  9. Erwan
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    Erwan Senior Member

    As explained by Mr Speer, the 15% drag improvment probably addresses the laminar bubble, in other words, I guess it should be read
    (15% gain in section drag), and section drag is only a little % of the total drag of the rudder blade.

    That's CFD marketing, but it testifies this A-Cat designer knows what is a bubble ramp. It is a guarantee, your boat designer is not mentally disabled.

    Cheers

    EK
     
  10. parkland
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    parkland Senior Member

    Mythbusters added golf ball dimples to a car, and boosted it's mileage.

    I would have to assume a similar dimple could do the same for water, but the size would probably have to be different to account for the density and weight of water over air.
     
  11. DCockey
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    DCockey Senior Member

  12. tom28571
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    tom28571 Senior Member

    You can achieve the same effect by peeing over the bow:D:D Actually much better since the warming takes place IN the boundary layer, rather than on the surface.

    The only thing that is logical to me is that a waxed surface should have more drag. That is because wax causes beading of the water on a surface, thereby potentially increasing the thickness of the water film that is carried along by the surface. That the water AT the surface has zero velocity relative to the surface is missed by many who think about this problem. That is why the international racing body allows any surface treatment without penalty but makes it illegal to treat the water away from the surface. This became an issue in the 1970s when Brittan Chance built a boat with tiny holes through which they injected long chain polymers (like fish slime) into the boundary layer. It did significantly reduce drag and was promptly outlawed. These materials are regularly used in industry to reduce turbulent drag in piping in power plants.
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Rowing regulations state that:
    "No substances or structures (including riblets) capable of modifying the
    natural properties of water or of the boundary layer of the hull/water
    interface shall be used"

    Are the kayak/canoe rules similar?
     
  14. tunnels

    tunnels Previous Member

    Get the hull clear of the water and drag is left behind . Build more tunnel boats :D
     

  15. Petros
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    Petros Senior Member

    than would putting a smooth surface also be against the rules? I do not understand this statement at all, ANY surface treatment modifies the boundary layer, one way or the other.

    Does that mean if the surface finish was rough without having to add another substance it would be legal? This is really bad rule making. they should just be specific to one surface roughness that is required for all the hulls. Than there is no way to cheat, all the surfaces would be the same roughness, and there is no need to hire lawyers to tell us what this stupid rule means.
     
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