Hull surface smoothness

Discussion in 'Sailboats' started by CrunchyFrog, Mar 16, 2017.

  1. CrunchyFrog
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    CrunchyFrog Junior Member

    Question for the hydrodynamic engineers: How smooth to make a hull on a 1 meter long RC sailboat.

    I've heard mirror smooth, 2000 grit sandpaper then polish.

    I've also heard that a slight texture is better, to pull the boundary layer along with the hull. Some RC racing sailors burnish with toothpaste for this reason.

    At a guess, the boat will spend most of it's time sailing between 0.5 and 2 knots, sometimes faster, and on rare occasions it might break 6.

    What say you?
     
  2. gggGuest
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    gggGuest ...

    The textured finish that's been used full sized only works if each groove in the texture is the right depth and correctly aligned with the local waterflow. So polish. At full size there are also practicality issues: mirror finish polishing a 60ft boat is just a little arduous and expensive, but at 1m long you can do it sitting in front of the TV.
     
  3. valery gaulin
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    valery gaulin Junior Member

    Depends at what reynolds number!

    Look at the golf ball explanation!!! Many Variable to consider. Complicated answer if there is no data. The best would be to run some testing!
     
  4. Joakim
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    Joakim Senior Member

    For that low speed it doesn't have to be very smooth. Anything finer than 400 grit will have no effect.

    Golf ball has nothing to do with it. Golf ball has roughness, which increases friction, but at the same time it reduces form drag even more. A sail boat has very little form drag, thus it most certainly will not benefit from roughness.
     
  5. valery gaulin
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    valery gaulin Junior Member

    I was just giving golf ball as an example to show that it is very unexpected result when they did discover that it did not need to be really smooth!!! Go check old golf ball, they used to be smooth!!!
     
  6. upchurchmr
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    upchurchmr Senior Member

    There is progress everywhere.
    You can't use this to say there is no knowledge about smoothness effects.

    And golf balls have nothing to do with hull drag.
     
  7. valery gaulin
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    valery gaulin Junior Member

    The golf ball effect act on the boundary layer!!! Anyway that is not the point! The point is that only with testing can we discover suprising result, sometimes even unexpected. And the other way is true what is believe to be a very good design with testing shows that it was not!!!
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    The golf ball example doesn't work on a 1meter class RC which is a very interesting class where you learn a lot. The boats are cheap so you can try a lot of things. I've been more class M in the very far time when the hulls were in cedar. In 40 years, every thing has been tested. Systematic testing of the effect of the boundary layer (determined by the speed and quality of the surfaces. The good recipes are known.

    The golf ball is eventually spinning, flying in air, has aRe number higher and not creating waves on the surface of a liquid 800 times denser than air. So the dimples create lifting turbulences (Magnus effect) and lowers the global Cx.

    On a class 1 meter well designed the hull is ofter almost isometric, I mean that when heeling at the good angle, the underwater shape is not distorted and does not create parasitic effects. At these speeds the Reynolds number is very low, the very light displacement with small dissipation energy by the waves make that the most important drag is the thickness of the coat of water (boundary layer) which remains stuck on the surface of the hull. If too thick it will have tendency to add weight and to create eddies at the aft part f the hull instead of being laminar. That means that the too thick boundary layer feeds a a separation layer. Those who worked on losses in pipes know the importance of the Re and the surface state. It's also similar of the problems on laminar wings of slow thermal gliders where the turbulences created by the dead mosquitoes stuck on the surface, or the drops of rain water destroy the lift.

    You have all interest to get the most laminar flow with the thinnest boundary layer as possible on such a hull which have very smooth lines. The surface state with the respect of the profile are the keel and rudder are primordial.

    A good surface is around 3000, you can go further with a polishing compound for plastics. To give you an idea my first own class M (Cedar designed by Paul Lucas) was not good at all. The epoxy resin I used was not made for polishing so the surface was too rough. I remade the class M with an epoxy for high speed windsurfers thus I got a mirror surface, but not hydrophobic. Tooth paste is a bit gross, you can use a 4000-5000 grain. You can find pastes for thar purpose to get a mirror surface but dull enough to control the boundary layer.
    The result was my class M was at least 10% faster, so with my low level skipping skills I went from the bottom the the first third group in the Brest regattas.
     
  9. valery gaulin
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    valery gaulin Junior Member

    Not sure of the result but what about using a superhydrophobic coating once your hull is well polished. Maybe it will reduce the boundary layer to a minimum???
    https://youtu.be/BvTkefJHfC0
     
  10. upchurchmr
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    upchurchmr Senior Member

    I heard that mayonnaise works well, just not for very long.

    Perhaps you should research boundary layer control methods.

    https://en.wikipedia.org/wiki/Boundary_layer_suction
    perhaps you could come up with a way to use this on a sailboat hull.
    It has a remote connection to how a golf ball works.
     
  11. messabout
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    messabout Senior Member

    A superb polish is worth the effort. There is more to it than that however. On a IOM or similar model the continuity of the surface is at least as influential. That is to say that the slighest undulations or ripples will matter more that a little bit.

    For the ultimate polish, wetordry paper at 1200 or more is a starter. The finish can be done beautifully with a 3M product that they call Finessit. The stuff that cleans the stains from glass stove tops is almost as good. Also the McGuires and other brands of polish that is sold to restore headlight lens is pretty good for the job. Tooth paste? No telling what is in that stuff.

    In any case, the application of the polishing compound is a critical matter. Doing it by hand, even on a one meter model is laborious. If you are fortunate to have a Dynabrade buffer you are good to go. The Dynabrade can be slowed to rotate at a very modest rate. Craftsman or other garden variety buffers run too fast and that tends to burn the surface. We are talking perfection here so burned surface is a no no.
     
  12. Ilan Voyager
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    Ilan Voyager Senior Member

    That can be very deceiving. On very fast windsurfers the surfaces are farted and polished with special waxes (like the fart preparation on the skis). Teflon has been tried with mitigated results. On surfaces where the flow is steady in straight line on the surface that works. When the flow is not in straight line but with a shear perpendicular component (heeling, passage of waves) the results can be disastrous.
     
  13. Ilan Voyager
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    Ilan Voyager Senior Member

    Far too much complication for a very simple one meter RC sail boat wighting a few kilos. It's the first class in RC regattas. Cheap and simple for beginners. So a well done polishing is the best and simplest way.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    Thanks. It's the way to go. You forgot two ingredients, good cotton flannel and one gallon of elbow grease. The one meter is a small thing, polishing by hand a good resin is a breeze and no risk of burns. The sanding can be a pain if the mold was not perfect
    I have used also hand made cotton pads, or the finest 4 inches cotton flaps for polishing jewelry or optical glass on very slow able variable speed drill/screwdriver.
     

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  15. Joakim
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    Joakim Senior Member

    Have you actually calculated the Reynolds numbers?

    For a 1 m hull at speed of 1 m/s (2 knots) in water the Re is 1.0e6 (20 deg C fresh water). For a 42.7 mm golf ball at speed of 50 m/s in air the Re is only 1.4e5. So about the same as the appendages of the 1M.

    Then you can look at the flat plate resistance with different roughness e.g. from http://www.mne.psu.edu/cimbala/me320web_Spring_2015/pdf/Flat_plate_turbulent_BL.pdf

    So at 1.0e6 Re for totally smooth surface the average friction coefficient would be 0.00467. For a fully rough condition the same coefficient can be get with e/L=7.7e-5 thus 70 um surface roughness for 1 M full. 200 sanding paper has 75 um particle size. So even if the hull surface was as rough as 200 paper, it would have the same friction coefficient.

    In laminar region roughness has no effect on resistance, but can trip the boundary layer sooner to turbulent. The roughness needed for tripping is even higher than that.

    For the appendages Re is much lower and the surface can have even higher e/L.

    Here's the graph for pipe flow, which is a slightly different thing, since Re is defined with diameter and the friction coefficient is for a very long pipe while flat plate (and boat hull) has a leading edge and developing boundary laeyr. https://en.wikipedia.org/wiki/Moody_chart
     
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