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Yet another shark skin article

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Created by decrepit > 9 months ago, 8 Nov 2008
decrepit
WA, 11059 posts
8 Nov 2008 7:05PM
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Check out this article see what you think,
Are shark skin fins in this year???

www.newscientist.com/article/dn15151-why-a-speeding-shark-is-like-a-golf-ball/?ignored=irrelevant

Ian K
WA, 3960 posts
8 Nov 2008 7:30PM
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Sounds like they've rediscovered the turbulator.

en.wikipedia.org/wiki/Turbulator

Slotted fins were another way of energising the boundary layer to delay separation.

evets
WA, 685 posts
8 Nov 2008 7:41PM
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Any ideas on how we achieve this decrepit ?

elmo
WA, 8410 posts
8 Nov 2008 7:48PM
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evets said...

Any ideas on how we achieve this decrepit ?


I've heard cobbler skin works well, kill a few dozen of those feckers and use their skin to coat your board

evets
WA, 685 posts
8 Nov 2008 7:52PM
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I have heard you can make a cobbler less venomous by brushing its spines with a foot. Is that right Elmo

elmo
WA, 8410 posts
8 Nov 2008 8:06PM
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evets said...

I have heard you can make a cobbler less venomous by brushing its spines with a foot. Is that right Elmo


you can make them even less venomous with a 2lb ball pien hammer as well

NotWal
QLD, 7426 posts
8 Nov 2008 10:40PM
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decrepit said...

Check out this article see what you think,
Are shark skin fins in this year???

www.newscientist.com/article/dn15151-why-a-speeding-shark-is-like-a-golf-ball/?ignored=irrelevant


20 cm/sec is only 0.4 knots. It's not a significant test for windsurfers. However one suspects that those little low pressure vortices may well become minuscule gas extractors at high speed. The Mako shark mentioned is supposed to reach 50 mph - close enough to 50 knots.

I think its interesting. I have long thought the gas extractor idea is worth pursuing.
In my fevered imagination I saw a thin stick on sheet with a surface like that of a fine file. You can make and test different patterns that way.

The root idea of a gas extractor is a feature not unlike an automatic bailer on a dingy. The shape and size can vary down to the very fine scale. The key factors are that water can flow over the shape easily and a step on the back face creates a low pressure area. Instead of sucking water out of a boat it sucks air out of the water creating a temporary bubbly emulsion - much easier to tear apart than water

sandman
WA, 432 posts
10 Nov 2008 3:36PM
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I love stuff like this. turns out that the effect created on the golf ball will not be experienced on a fin, if that is what anyone was thinking. this is only efective on a blunt object (maybe baseballers should read this!) the teadrop shape of your fin is too streamlined already to benefit. something that would work though would be a 'leading edge extension fence' apparently.

www.aerospaceweb.org/question/planes/q0176.shtml

as Jim Drake to crunch some numbers and pop a couple out on their iSonics maybe

slowboat
WA, 540 posts
10 Nov 2008 4:11PM
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wet n dry basically does the same thing. Sandman, although a fin is a "streamlined" shape, it still has laminar flow transitioning to turbulent on the surface. just like on a golfball. The reason for using dimples on a golfball is to prevent laminar separation with no reattachment, which would make a thick, draggy, and unstable turbulent wake.
Same benefit with fins finished with wet n dry... as many who have tried polishing will agree.

sandman
WA, 432 posts
10 Nov 2008 5:23PM
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I daren't argue with your experiences slow boat, I have seen your speeds and trust your judgment! just passing on what I have read...



The reason we do not see dimples on other shapes, like wings, is that these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder. The most dominant form of drag on these kinds of shapes is caused by pressure, as we have seen throughout this discussion. More streamlined shapes like the airfoils used on wings are dominated by a different kind of drag called skin friction drag. These streamlined bodies, like that pictured above, have a teardrop shape that creates a much more gradual adverse pressure gradient. This less severe gradient promotes attached flow much further along the body that eliminates flow separation, or at least delays it until very near the trailing edge. The resulting wake is therefore very small and generates very little pressure drag.

However, there do exist other types of devices commonly used on wings that create a similar effect to the dimples used on golf balls. Though these wing devices also create turbulence in order to delay flow separation, the purpose is not to decrease drag but to increase lift. One of the most popular of these devices is the vortex generator.


...maybe this effect you experince is dependant on the hydrodynamic efficiency of your fin and chord thickness. either way, probably not worth busting out the drill onto your favourite slalom fin and just stick with the wet and dry.

C3
54 posts
10 Nov 2008 6:55PM
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www.aerospaceweb.org/question/aerodynamics/q0215.shtml

the link above is interesting, but this one is more to the point.

boogie

sandman
WA, 432 posts
11 Nov 2008 9:54AM
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yeah, thats the one I cut the image and text from. good stuff.

yoyo
WA, 1646 posts
11 Nov 2008 2:29PM
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"Vortilon" ..Underwing fence.

You learn something new every day.

Ta Boogie

NotWal
QLD, 7426 posts
11 Nov 2008 7:20PM
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sandman said...

yeah, thats the one I cut the image and text from. good stuff.


You get the Julie Bishop prize for this week.

slowboat
WA, 540 posts
14 Nov 2008 11:38AM
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sandman, feel free to argue... we cant learn if we never listen.

You dont see dimples on an aeroplane wing because there are more efficient and controlled ways to trigger early transition. Zigzag tape is popular on gliders and RC models. If the wing has vortex generators (needed for high AOA conditions) they also act as turbulators at lower angles. For windsurfing fins this is not particularly useful, since the fin will ventilate before those angles where vortex generators become useful are reached.

The ball has to be evenly covered (it spins when it flies), so turbulators have to be in the form of an even covering.

The boundary layer is very thin on our fins, so the roughness of wet'n'dry becomes effective as a trip. It has the same effect as dimples on a golfball. The boundary layer on a golfball is much thicker due to the much lower Re. Hence the size of the dimples.

The comment on the website "these particular forms of boundary layer trips only work well on a blunt body like a sphere or a cylinder" is wrong. Perhaps they should have said "these particular forms of boundary layer trips generally work well on a blunt body like a sphere or a cylinder"

club309
QLD, 66 posts
14 Nov 2008 2:34PM
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on fast dinghies rudder ventilation can be delayed by raking the rudder forwards and by installing fences at, or close to , the water surface.

However, according to bethwaite ( high performance sailing) aft raked leading edges establish "rollover vortices" that help maintain / establish flow attachment. This can be seen on the high angle of attack delta wing aircraft can maintain without stalling.

bethwaite also has conducted some interesting experiments on the drag of dinghy rudders. transom hung rudders operating in the turbulence created by the hull have significantly more drag than rudders not influenced by hull turbulence.

Slowboat I am really interested in the wet and dry finish being fastest. as you can tell i have just been revising frank bethwaite's book. they conducted experiments on the surface finishes of foils over a quite a a long time and came to the conclusion that as highly polished as possible was best. What grade wet and dry do you sand you fins with?

slowboat
WA, 540 posts
14 Nov 2008 4:05PM
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Hi Rory,

From a drag perspective, polish is best- assuming you can keep flow attached. This maximises the area of laminar flow. Its easier said than done on a windsurfer, which is forced to operate near the surface- and under turbulent and aerated water (from the board impacting the surface upstream).

I've done my fastest runs on fins with polished finish. But in rougher water, I use wet sanded fins (400 to 600 grit, and sometimes up to 1200 depending on the fin design) to deliberately trigger early transition to turbulent flow. This makes the fin less susceptible to ventilation in aerated water.

Forward raking produces an upward spanwise flow so it should significantly reduce ventilation events. Two problems with that- 1) seaweed, 2) twist. The thickness and surface area of a windsurfing fin is already so small that we would need to machine our fins from something very much harder than carbon to achieve stability in terms of twist. The positive twisting in the tip would be difficult to control in the geometries we require without an impossibly rigid material. The problem is that the angle of attack increases toward the tip with increasing load, meaning the foil generates a lot more torque for a given side load. This is the opposite of what we want for dynamic balancing with our torque production on the board. (as you load the fin harder, you are applying less downward force on the rail, thus reducing the torque on the baord, yet the fin is increasing its torque due to the positive twist). Foils with the maximum thickness aft also require this issue to be dealt with. I'm sure its a good solution for other applications where seaweed or structural rigidity is not an issue.

So the answer on surface finish is "it depends". If you dont need to induce early transition, then I cant see why anything but polished is desirable on any surface. For windsurfing fins the 400-600 finish is less susceptible to ventilation. I've tested this on several occasions by going out on a polished fin for an hour or so (having plenty of ventilation problems), back to the beach, wet sanding, and going out again to find the fin completely stable. As for drag, a fin that has partially separated flow a lot of the time is still going to have more drag than a turbulent flow section which is not separating. I havent noticed any difference in drag between polished and wet sanded.

Ian K
WA, 3960 posts
14 Nov 2008 9:27PM
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slowboat said...



torque production on the board. (as you load the fin harder, you are applying less downward force on the rail, thus reducing the torque on the baord, yet the fin is increasing its torque due to the positive twist).


Not sure that the decreased downward force on the rail as we hike out further is a limiting factor. Our ankles are pretty strong so the board legs and body can be considered rigid so the counter torque needed to keep the fin upright is part of the body righting moment.

I'd once estimated ( based on approximation of body stance and fin/sail moments) that the fin is generating about 40 kg of lift. If this was centred 10 cm below the board that's 40 newton metres of torque to be countered. I loaded up an old wavebord with a 3kg dumbell at 1 metre and a 2 kg dumbell at 500mm to see how the ankles handle this. I'm sitting on the floor so gravity effects on the rail are minimal. A bit of effort required but the ankles can lift the dumbells, ie provide 40 Newton metres of torque, so I think using board/legs body as a rigid body in calculations is justified. The breakdown of gravity vs ankle strength would be a bit different with wider boards and longer fins. Makes for a good exercise while watching TV


Ian K
WA, 3960 posts
14 Nov 2008 9:45PM
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Another photo of the experimental setup. Worried about using the fin as the fulcrum, potential for pulling on the footstraps to add torque about the length of the fin. So rested the board on a paint tin to better isolated the torque. Still pretty easily done.


slowboat
WA, 540 posts
16 Nov 2008 5:57PM
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Interesting experiment Ian. I thought a bit more about the ability to generate a torque with the back foot today while I was sailing. What I found was that in normal sailing conditions, most of the weight is just behind my toes. I noticed that its very hard to generate any torque at all when your back leg below the knee is fairly vertcal, which it is normally for me. In your pics you have your back leg very low and straight- you can make loads of torque with your foot pointed trying to curl up, but thats not a stance I encounter when windsurfing. Most of the corrective torque feels like its coming simply from weight on my front foot rather than any direct torque applied through the feet. I also noticed that the roll generated by chop impact is too sudden to compensate with foot movement and we are pretty much at the mercy of the dynamic balance of the board/fin. To me it reinforced the argument for fin twist in chop to increase dynamic stability. Thanks for that Ian!

yoyo
WA, 1646 posts
17 Nov 2008 2:43PM
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Came to much the same load facter via a different means. you should make your plank thicker so the pivot is about 10cm below the board.

This is Chris' rear leg posi on a broad speed run

Ian K
WA, 3960 posts
17 Nov 2008 9:20PM
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Took the hypothesis to the water tonight. Yep, we don't use that much upward torque- generating pressure under the footstraps. I use a little going upwind, but that may be because I curl both front and back toes to stay in the straps. So no don't use a rigid rider/ankles /board assumption when doing the torque equilibriums.

Agreed Yoyo, you can balance the moments about any chosen axis and come up with consistent results. I'd had the axis passing through the base of the fin and the uni joint in mind when I sat the board on the paint tin. If you chose an axis 10cm below the fin base, the torque generated by ~30cm fin would drop out of the equation, but you might come up with some conclusions about mast foot pressure in the roll equilibrium.

Thanks for the observation Slowboat. The 72 wide board I used tonight with its 44 cm flexi fin does a good job at self adjusting for roll inducing chop going off the wind. Gets a good oscillation going at times but it always feels comfortable to let it do its own thing. Sounds like having the confidence to let a board run through chop without too much input is a good thing. Not there yet with my smaller boards.




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"Yet another shark skin article" started by decrepit