Is Matte Faster?

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

  1. sabsfeigler
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    sabsfeigler Industrial Designer

    Historically, racing boats have had a gloss finish coming out of their molds, but I know that while I was growing up racing sprint canoes, we were told to wet sand our wood boats to give a satin finish. Coaches claimed the boats should have the water stick to them so they'd be faster, they said water slides on water best. I guess it would improve laminar flow, like divits on a golf ball.

    But the question is, on a boat that is already so hydrodynamic like a sprint canoe or kayak, would a matte finish make it faster? I mean it works for the Mako shark.

    So I ask you Hydrodynamic gurus, will a matte finish make a faster boat than a glossy one?

    Sabs
     
  2. hoytedow
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    hoytedow Wood Butcher

    On a mako shark the dermal denticles are aligned in a particular direction. Any shallow surface sanding should be fore-aft not random. Super fine sandpaper.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Try it and see!

    Be sure to let us know the results.
     
  4. hoytedow
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    hoytedow Wood Butcher

    And post some micro-photography too!
     
  5. sabsfeigler
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    sabsfeigler Industrial Designer

    Using 000 steel wool would be ideal, uni directional, but could I buff that out if it didn't work? I mean on a wood boat you just revarnish, but you can't varnish a gelcoat.
     
  6. hoytedow
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    hoytedow Wood Butcher

    Don't ruin a good gel-coat for an experiment like this.
     
  7. sabsfeigler
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    sabsfeigler Industrial Designer

    No, no. Although there are many boats at my canoe club that would not be affected by an experiment like this, there wouldn't really be an effective way to test this. I doubt the speed difference would be measurable.
     
  8. Petros
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    Petros Senior Member

    you have incomplete information. the dimples on the golf balls to not maintain laminar flow, in fact they push the boundary layer past the laminar range (by adding turbulence on the surface) in to the turbulent range, which can lower drag if it operates in a vary narrow range of Reynold's numbers (Rn). The Rn is like a turbulence parameter, based of viscosity, free stream speed and length of the object moving through the fluid. A strange phenomenon occurs at about Rn=6x10^6, where transition occurs from laminar to turbulent flow, the Cd drops by about half or more. AS the Rn goes up (high speed for example) the drag continues to rise, but it does not rise as fast as it does before transition. Once outsite the transitions zone the dimples just add more drag. Golf balls, certain rigging parts on ultralite air craft, and some small low speed objects in water operate right in the 6x10^6 range, and can benefit by tripping the boundary layer.

    Everything else only adds drag, if you are fully laminar, than you want to stay that way (note there are not many things that operated in the laminar range, only the first few inches on small aircraft wings if they are very smooth).

    Do not confuse lamanr boundary layers and turbulant boundary layers with attached flow and detached flow. YOu can have attached and turbulant boundary layers, and they still operate very reliably and low drag (just not as low as an attached laminar layer). Any detached boundary layer will behave badly on any surface, lots of drag, loss of lift on a sail, wing or prop blade, etc. A detached boundry layer must be avoided to best performance. Trying to design around the tiny window of laminar boundary layers is usually a bad idea since they behave unreliably and unpredicatbly, only good for pure racing, and only than if the speeds and shapes lend themselves to creating reliable laminar flow. Many amateur designed homebuilt aircraft became unflyable when the ignorant designer selected laminar flow airfoils for the aircraft thinking it would make it go faster, but instead behaved very dangerously when the laminar flow would transition to turbulent and suddenly change lift and pitching moment very rapidly. Not good behavior for any aircraft, rain, dust, rough paint and smashed bugs on the leading edge were typical culprits.

    The interaction at the very surface of any large object flowing through a fluid (i.e. sea water or air for example) is not easy to models, and there have been claims reduced drag when someone sands the surface. The idea I think is that a fine mat surface prevents surface tension and therefore can reduce the "grip" the water has on it at the surface interface. But there are a lot of other things that can affect it, a rough water surface yields a different shapped hull for low drag than a smooth one for instance (this was determined experimentally in tow tanks, no obvious theory). So it might be valid, but for something moving very fast through the water, like more than about 10-15 knots, I am undecided if there is a noticable difference.

    Seldom are all boats identical, and seldom are the crews equally skilled, so there are lot of other factors.

    BEsides, shinny hulls look good for the sponsors, and the spectators.
     
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  9. sabsfeigler
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    sabsfeigler Industrial Designer

    So what you're saying Petros is that if there was a matte surface on a hull it would only be beneficial after the point of a detached boundary layer?
     
  10. PAR
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    PAR Yacht Designer/Builder

    Simply put, you'll not notice any difference assuming both the matte and gloss hulls had the same fairness and smoothness. Two identical hulls, one color sanded to 2,500 grit, then buffed to high gloss and the other wet sanded with 2,500 grit and left alone will make no significant difference in boat speed. On a technical level you might get some measurable differences in tank testing, but out on the water, absolutely nothing observable, that couldn't be accounted (or discounted) for to other variables.
     
  11. sabsfeigler
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    sabsfeigler Industrial Designer

    Stealth Boat could look cool and be just as fast!

    So if you hypothetically had a machine that could run gritted paper the length of the boat without making swirls or arcs the human hand would make, how deep would the sanding have to get before there was noticeable increase or decrease in speed?
     
  12. PAR
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    PAR Yacht Designer/Builder

    How fine would you like to divide up a 1/10th of a knot? You might be able to quantify a .003 knot difference at specific speeds, in a perfect fluid, but really?
     
  13. gonzo
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    gonzo Senior Member

    All the new testing show that anything polished past 320 grit only has a cosmetic difference.
     
  14. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Rub the hull really, really hard so it warms up and thereby reduces the viscosity of the
    water in the boundary layer. You should be able to achieve PAR's estimate of 0.003 knots.
     

  15. DCockey
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    DCockey Senior Member

    To expand on Petros' excellent post, the reason the dimples, etc decrease drag in that narrow range is because turbulent boundary layers are more resistant to separation, and causing the boundary layer to become turbulent at the lower Reynolds Number causes the separation on the golf ball to be delayed which in turn reduces overall drag.
     
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