Rough might be good/See! I tol' ya so...

General Discussion
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Discuss…

 

The American Institute of Physics reports that some rough surfaces can reduce drag. While the majority of objects that travel through the water are smooth, researchers from UCLA have discovered that bumpiness can sometimes be better.

“A properly designed rough surface, contrary to our intuition, can reduce skin-friction drag,” noted John Kim, a professor in the mechanical and aerospace engineering department at UCLA, in a statement.

The researchers modeled the fluid movement between two surfaces coated with small ridges. They discovered that even in bumpy conditions the rough surface decreased the drag generated by the friction of moving water.

This concept has previously been investigated, but resulted in minimal success. More recently researchers have started working with rough surfaces that are also very hard to wet, a property known as superhydrophobicity. In theory this indicates that the surfaces can trap air bubbles, producing a hydrodynamic cushion, but in practice they often surrender their air cushions in chaotic flows.

The researchers decided to model a superhydrophobic surface design that another team of researchers at UCLA had already observed could keep air pockets entrapped, even in bumpy conditions. The surface was coated with tiny ridges positioned in the direction of flow.

The researchers modeled both laminar and turbulent flows, and surprisingly discovered that the drag-reduction was greater in turbulent conditions. The variable changes and swirling vortices in turbulent flows on smooth surfaces generally increase drag, according to Kim. However, the air cushion produced by the superhydrophobic ridges changed the turbulent patterns near the surface, lowering their impact, he explained.

Eventually, “properly designed” rough surfaces may coat the undersides of ships, which “could lead to significant energy savings and reduction of greenhouse gas emissions,” Kim posited.

The study’s findings are published in the journal Physics of Fluids.

http://natmonitor.com/2014/01/17/properly-designed-rough-surfaces-can-reduce-drag-researchers-discover/

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I believe this was an idea that both Terry Hendricks and Tom Morey played around with a good 30 years ago. The idea being that a rough surface would create adhesion to a degree and produce a boundary layer between laminar and turbulent flow.

Other guys also did a similar thing by simply wet sanding the bottom of a board to make it dull instead of shiny.

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Looks like Coil is leaving a textured bottom and rails on their boards using those fancy fabrics. I think they have it figured out.

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The ULCA team chose to model a superhydrophobic surface design that another group of researchers at UCLA had already observed could keep air pockets entrapped, even in turbulent conditions. The surface was covered with small ridges aligned in the direction of flow.

The researchers modeled both laminar and turbulent flows, and unexpectedly found that the drag-reduction was larger "in turbulent conditions."

Kayaks & 3M ridges/riblets/grooves – World Championships 1986, Montreal:

http://www.epickayaks.com/article/article/to-wax-or-not-to-wax

“The size of the grooves is matched to the density of the fluid [(fresh)water in our case] **and **the **speed of travel **… The grooves need to be carefully aligned so they are parallel with the flow of water over the surface. 3M donated some of this to the US national team in 1986 and we tried it on a few boats …”

Racing sailboats discussion:

http://www.sailrc.com/science-of-smooth/

Velocity, viscosity, ridge or riblet size/shape/depth/spacing, water conditions (glassy/chop), laminar vs. turbulent flow conditions, shape/contour/area of surface(s) overall and appendages …

 

 

 

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The other day I was listening to NPR they had a story on new suits that will be worn by Team USA speed skaters. The suits have dots that will brake up the air. In test They did increase the times of The Speed skaters. The suits are so revolutionary That they are under lock and key guarded until they are warn by Athletes at the Winter games.  

Of course we can also look at the speed swim suits that were banned because they gave to much advantage to the teams from countries that could afford the suits. !0 records were broken by those who wore The swim suits. 

Ship designers are also working on textured surfaces to reduce drag. It is not just making a rough surface it is making the proper textured surface for the job. A Massive Freighter or Oil Taker That had less drag could save 100 of thousand gallons of fuel as well as increased speeds.
How does all this translate into surfboards? A textured surface so far has done little to catch the imagination of Surfers. I believe that there is a texture out there that would aid a Board in reducing drag. It is is a matter of fining the right texture design for surfboards. Ever notice how many of the tow in boards in massive slab waves get sucked up the face? The same happens with guns. The mass of water moving up the face and the surface friction of the board can’t be over come by gravity and the current board designs and bottom contours alone. That little something extra just might be a properly designed textured bottom.

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I’m going to make another XPS board and glass the bottom last. I’m not going to seal coat or finish it and see how it works. I have done more than a few boards with an uneven or textured bottom surface, but nothing with a symmetrical pattern. The closest has been the woven bamboo compsand we do and left it with the pattern of the weave. That has rather large grid compared fiberglass weave, but smaller than what the Willis brothers did with their golf ball bottom.

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A few of my commercial fishing freinds have added “Shotcrete” to their hulls and finished them with ablative paint. They tell me the extra weight makes them ride a little higher in the water and they get better fuel mileage. One says he has increased his range almost five percent. The finish is much rougher than a gel-coat even though it is troweled smooth.

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I saw a surfboard for sale at a shop in Gardena that had a rough surface akin to sharkskin, and was claimed to move faster in the water.  The tech in the aritcle refers to parallel ridges, aligned with the direction of flow,  on the surface. I’m not aware of anyone trying that texture on a surfboard.  Not sure either if there would be much practical effect at very low speed as when paddling.  Very interesting.

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Here’s what we’ve been doing for the past year. 

1280×960 391 KB

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Is there a thread where you explain this/give feedback? I’ve often wondered how a board would go with the exposed cloth and no smooth coat. 

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I heard about this when I was interning at Northrop Grumman about 5 years ago. They basically showed me two panels one with a completely smooth surface and another one with a rough “sandpaper” like surface that was fine tuned (height/pattern of the ridges). You could see the difference by just putting a drop of water on each panel. The drop on the rough surface ran off atleast 3 times faster than the drop on the smooth surface. I forget all of the details behind it but it has something to do with surface friction. Seemed to me that it was a basic concept back then.

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Mike is that finish all over or just on certain sections? Very cool looking.

I have a few boards with a rattle can spray finish. The latest one has a mix of 2 colors with a light uneven coat of the final color. It came out with a textured finish much more like sandpaper than sanding the surface with 400 grit. Same with a few boards that I used a flat finish clear coat. This texture is very minute compared to what the Coils show. Next step is a board with an unfilled lam. That should create larger voids.

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We’ve tuned our process to get the Kick surface on the whole board now. At first we were fill-coating the fin box/tail edge area, the rails, and the deck where trac pad is attached. The guys who’ve been riding them since early testing say they just keep getting faster as we eliminate smooth areas.

This isn’t a new thing for the Brasington brothers. About 15 years ago they made a bunch of straight-out-of-the-bag product, which are remembered locally as “the itchy boards”. The exposed fiber gave a very unique surface, but also would irritate some people’s skin.

There’s some discussion of Kick over on the ride reports thread, in exile in the surfshop section of sways. In short, we set out to eliminate the excess weight and process steps of fill coats and sanding - which obviously is going to produce a “different” surface. So Kirk and Eric set out to optimize that surface but still maintain certain physical criteria in the laminate structure. Kick was the result.

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Have you done hydrodynamic testing to look at boundary layers (etc.) as a follow-up to rider observations?

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I started a thread on not hotcoating last year. I seal my blanks with epoxy/micro balloons so I figured I didn’t need the hotcoat. A few of the experienced guys (read: older) said they’ve done it, and it would collect dirt easier.

I never felt the difference between different surfaces, having ridden coil kick, too. I think the % might be insignificant relative to fin setup, volume, etc.

If we try to apply a directional surface, then what happens when you actually turn? Most people tire of going straight in their first year. In a turn, water is not shooting straight back.

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No formal testing because it is easier to just put the damn things underneath rider’s feet. Simulating or modeling for real surf conditions is nigh unto impossible anyway. Steady-state plate dragging doesn’t do much for me.

I did read enough papers on the subject to find that the science is mixed. We knew we were getting away from a “smooth” surface and as I said there were other considerations besides the dimples at work. 

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The panels I got to see this test on didn’t have a dimple structure like coil’s surfboards. It was just a rough surface kinda like sandpaper. Since thinking more about it, I think they also used polymer to coat both panels. Idk if that makes sense at all but I think they mentioned something like that. 

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Exactly…

[The golf ball dimpling effect – a small, solid sphere traveling through air – has been flogged beyond death at Sways.  Hans, an engineer, has discussed/explained this phenomenon here at Sways.]

Ribbed/ridged/grooved surfaces only feel rough when you move your hand over them perpendicular to the axis of the ribs.  Run your fingers perpendicular to the grooves on the surface of an old vinyl LP record.

From the Epic Kayaks article about the 3M surface used in the 1986 Montreal qualifying trials:

http://www.epickayaks.com/article/article/to-wax-or-not-to-wax

“The other item that really works is a “riblet” material manufactured by 3M. It is an adhesive backed plastic film with very fine grooves machined into it (similar to the grooves in a phonograph record). The size of the grooves is matched to the density of the fluid (water in our case) and the speed of travel so that the grooves dampen the turbulence of the water as the flow detaches from the hull. The grooves need to be carefully aligned so they are parallel with the flow of water over the surface.”

… However, I was “outed” by a bunch of kids. After each race, the top finishers are required to go to boat control, where your kayak is checked to ensure it meets specifications, including minimum weight. In order to get a consistent weight, the boats are dried with a towel before weighing. The kids drying the boats noticed that my kayak had an odd feel to it as they rubbed the towels over the surface. The towels seemed to “track” lengthwise down the boat, and had a lot of friction (and made a “zip-zip” sound) if wiped quickly across the grooves.

… So where does this leave you? The best advice I can give is that you want the surface of your kayak to be as smooth and clean as possible."

America’s Cup and racing sailboat hulls:

http://www.sail.ie/SW/Bottom%20Finishes.htm

… What About Riblets?–Riblets were used on the bottom of the 12-Meter Stars & Stripes during the 1987 America’s Cup. They are tiny v-shaped grooves that were applied to the hull on a vinyl tape (from the 3-M company).

Soon after their televised debut, they were outlawed by the racing rules, which now prohibit “specially textured” surfaces that alter “the character of the flow of water inside the boundary layer.” Fine.

But what if you are painstakingly wet sanding your new bottom paint to achieve a “hydrodynamically smooth” surface.

You might wonder whether you could sand carefully in a fore-and-aft direction using 220-grit paper, and then “just launch the damn thing.”

Fortunately for protest committees everywhere, this does not appear to be a smart approach. Early papers on riblets show that their effectiveness is sensitive to the geometry of the tiny grooves, and that rounded grooves are likely to increase drag. They have also been shown to trip laminar boundary layers into turbulence sooner than smooth surfaces.

As Kirkman points out, the optimum height of the riblets changes with speed, so any riblet choice is a compromise. So it seems likely that large scratches left in a surface from sanding with 220-grit sandpaper will increase drag rather than reduce it.

The second goal is to minimize drag aft of the transition to turbulence, and this is a little easier to do. Most of the turbulent boundary layer consists of chaotic, swirling eddies, but there is a thin layer next to the hull known as the “laminar sub-layer.” Any surface roughness small enough to be immersed in this layer is “hydrodynamically smooth.” In other words, making it any smoother will have no benefit.

… A human hair is approximately 2 to 3 mils in diameter, and a bottom finished with 400-grit sandpaper should have a hydrodynamically smooth finish aft of the transition point for speeds up to 7 knots.

So, for most keelboats, a bottom which is finished with 400-grit sandpaper in the aft sections is adequate.

For planning dinghies, which sail faster, the aft sections of the bottom need to be smoother.

… Turbulence at the surface from waves, microorganisms, and contaminants can all be disruptive.

… The best chance for laminar flow is on the keel and rudder, both because of their convex shape and because they are immersed below much of the disturbance. Aerodynamicist and dinghy designer Frank Bethwaite questions “whether any surface is ‘smooth enough’ for a racing dinghy,” when it comes to foils.

It is my understanding from the refereed research papers that sharkskin riblets/ridges/grooves are  biological grooves and ridges formed by aligned scales with the proper dimension, geometry and spacing, improving shark peformance as a highly efficient predator.

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Attaining perfect laminar flow in a surfboard is not going to happen, just because of the complexity of the  water conditions encountered. So we want to go ahead and trip those boundary layers as soon as possible. Some people feel a difference, some don’t (see ghostshaper’s comment). But I haven’t had one person tell us that Kick made their board slower, and it’s mainly a process/product improvement anyway (higher fiber-to-resin, higher strength/weight, eliminating steps, etc.).

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My understanding of your product has been that it is a structural improvement rather than hydrodynamical.