Wingnut Sweep Fin Foil looks flat

General Discussion
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Hi all,

Interesting thread. I don’t surf but am into composite construction and find this site full of ideas and interesting techniques. I cannot explain the “feel” of differant fins but I can address why the flats on the fin are not good from a hydrodynamic perspective. Foil theory is very complicated and I don’t understand much of it but I do know enough to shed some light onto the subject of flat sides. Here it goes:

The basic ides of a symetric foil is to provide lateral resistance w/o paying any drag penalty. The best shape for this keeps the fluid in a laminar state where the foil just moves the fluid to the side and brings it back together (two bits of the fluid part at the leading edge and are rejoind at the trailing edge) and everything remains in alignment. If the bits of fluid change positions you have turbulance which results in drag. For this discussion there are two forms of drag: Parasitic and induced. P-drag is created by something sticking out into the fluid and the rate of drag goes up as the square of the speed. I-drag is a byproduct of lift and is greatest at slow speed. There is a cross over between the two curves and this is where the foil is most efficient. Slow down and I gets you, speed up and P is the culprit. Ultimately P drag is the largest contributer to overall drag. Lift is generated when the fluid has to speed up to remain in alignment and laminar as described above. When using a symetrical foil changing the angle of attack (AoA) (what angle the fluid strikes the foil in relation to the chord line(a straight line between the longest axis of the foil)) changes the distance the fluid must move to remain laminar. The point where the fluid seperates is known as the stagnation point(SP). The SP moves as side loads are are added to the foil. When the AoA exceeds about 15 degrees the fluid can no longer remain attached and starts to tumble losing lift and creating drag. The higher the AoA the higher the drag. The drag happens on the top side (side with the greatest distance from the SP to the TE of the foil) of the foil. In severe cases, the water will vaporise and the foil will cavitate. The void created by the cavitation is a very low vacuum and the fluid does not like this and wants to fill the hole. What happens is the foil is easier to move then the entire mass of the water and the lateral resistance is gone because a vapor is flexible and can get larger so the foil loses stability. This action of cavitation and refilling is extreme and the drag becomes very high so the entire craft slows. If there is no cavitation but turbulance the foil has stalled and is no more effective then a piece of plywood stuffed into the water. When the hole is filled or the stall recovered the foil hooks back up and does its job. A proper foil keeps generating lift and resists stalling to a higher AoA then the squared off plywood.

Take a break, have a beer, whatever you want. The reason for the bad flats is coming.

Fluids cannot make corners. They must move around curves. The transistion from any surface to another is a corner and this transition “trips” the fluid and it starts to tumble. As the AoA increases the abruptnes of the corner gets larger and the fluid detaches much sooner. This detachment is the stall or cavitation described above. The abrupt transition causes the stall to occur at a much slower speed and for it to be more severe and require more time to recover. The P drga created by the flats is much higher to. For example: let’s assume the initial drag of the smooth foil is 2 units and the flatted foil is 3 at speed X. Going to 2X makes 4 and 9 and 4X 16 and 81. Starting at low drag pays huge benefits at higher speeds.

There are symetric foil shapes that work better at higher AoA than others. I assume the back end of the board has more lateral movement then something near the CG (center of gravity) or CE (center of effort of the hydrodaynamics of the board) of the entire board nd thus the fins see more radical changes in AoA. The scimitar shape and eliptical tips to the fins are very efficiant regrdless of foiling or not.

I hope my distillation of basic foil theory helps a bit and was not too confusing. There is more to this subject but I wanted an explaination not a disertation.

Thanks for the ideas and help solving unique problems I’ve found the surfing community has discovered.

Take care, Kevin

Thanks…

That was so much more enlightening than making a fin both ways and going surfing to see if it makes a difference…

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So, in a nutshell, water can bend around a curve, but not a corner at higher speeds? Mike

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http://en.wikipedia.org/wiki/Coanda_effect

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Ah. Thanks Janklow. Another important use of the Coanda effect is in making a Black and Tan. Mmmmm. Mike

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Kevin,

That was a very clear and concise description of foil efficiency. Probably, the best I’ve read without going through twenty pages of technical papers.

The fin that 11ft started this thread with has very subtle flats with radiused transistions between. So, flow rate need to get pretty high before P gets noticable.

Flats or concaves on the inside surface of rail fins, actually preform a secondary function. That function has to do with kinetic pumping to gain momentum, when gravity and swell acceleration just aren’t enough. So, under certain conditions you can give up the fins secondary function by making it’s primary function too efficient.

But, I do agree water does not like to bend around sharp corners and leading edge radii are more efficient than a sharp edged corners.

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Questions: why do you say that about the leading edge? Because of AoA changes? Seems like more curve to the leading edge would make for bigger stagnation point(s), and relatedly, increased frontal surface area has increased resistance, doesn’t it?

Are surfboard fins primarily Newtonian, and the foiling just manages theoretical separations or separations that are mainly theoretical in their consequences, due to the low speeds?

If you have a foil that is symmetrically-foiled, wouldn’t the flows come to their confluence at a more opposed angle than if one side is flat? And engender turbulence right at the trailing edge?

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Questions: why do you say that about the leading edge? Because of AoA changes?

Sharp leading edge can cause flow separation at very small negative AOAs.

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Seems like more curve to the leading edge would make for bigger stagnation point(s), and relatedly, increased frontal surface area has increased resistance, doesn’t it?

You lose a tiny bit at small positive AOAs and lose an enormous loss at small negative AOAs by making the front edge radiused with half the effective diameter of the max camber.

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Are surfboard fins primarily Newtonian, and the foiling just manages theoretical separations or separations that are mainly theoretical in their consequences, due to the low speeds?

If you have a foil that is symmetrically-foiled, wouldn’t the flows come to their confluence at a more opposed angle than if one side is flat? And engender turbulence right at the trailing edge?

Surfboard fins are low aspect ratio foils that have a turbulent surface discontinuity at their base. There is almost no aerodynamic or hydrodynamic science or theory that applies specifically to this case, and a lot of what we know about surfboard fins says that taking a two dimensional foil theory approach will not be an efficient way to develop fins.

In this case, for example, the induced drag blows away the parasitic drag because the aspect ratio is below 2 and water is a lot more viscous than air.

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yep, that’s the process! bondo is a good, quick ding repair substance as well…or you can tack fins on with it, fill in a fin box once fin is tuned in, or apply to bottom of board and in same process as fin, tune in your rocker …