The Counterintuitive Concept of...................

General Discussion
101

Right on RDM. I’ve had that same thought for years since this discussion back in 2005:

http://www.swaylocks.com/groups/thick-fins-vs-thin-fins

Something poignant that stuck with me was Greg Loehr’s comment:

“And yes, we could reduce the size of the plan shape with a thicker foil.”

 

So in theory by going with a thicker foil you increase lift while decreasing drag, pull the max chord forward to make a more responsive/sensitive fin and then you can also decrease the overall area of the fin.

I’ve got 3 sets of side fins I made to test the concept off a known template. I did a 100%, 95% and 90% area to test the theory but have never glassed them.

102

Lawless, Thanks for posting that link I’ve only been able to read the first two pages so far but learned a lot already…

It is also a testament to what resource the Swaylocks  archives are for those who know how to find them…

 I have designed my foils as thin as I could make it  to minimize drag while  still maintaining desired stiffness  so this subject is important to me.

 You summarized that “going with a thicker foil you increase lift while decreasing drag”.

It was my understanding that while the thicker foil does increase lift it also increases drag, because more water has to move out of the way, ( there was a comparison of the pencil and a ball)…

although the thicker foil does have a better lift to drag ratio and is generally better when talking normal surf board fins…

I’d like to clarify to make sure I understand this correctly because my fin is currently 32 inches wide and drag while paddling and launching is a huge deal… 

103

If this is not true, I woud like to see the physics that explains why.

 

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105

Streamlining. Keep in mind I’m making a lot of assumptions, but surfing being what it is, the best way to test those assumptions is to just build the fin and try it. Theory doesn’t usually work too well in surfing because we can’t really quantify all the variables.

My basic assumption is one that current fins are acting more like plates than foils, and by increasing the thickness you’re also improving the flow over the foil. A cleaner foil has less drag even if it is MUCH thicker.

Again, going with the thread title, it’s counterintuitive. I’m not sure how that plays out with the Reynolds numbers and air vs. water. But my thought was make a thicker foil, test it to find the happy medium where it’s thick enough that you can reduce overall area and get the same lift, but by decreasing area and decreasing drag you may get a better performing fin.

Keep in mind, fins used to be much thicker when they were glass-ons. Thinner fins really came about with fin systems and the functional need for thinner fins because molding the plastic fins they originally made, didn’t do well with thicker fins.

It’s just something to try, a cool experiment to play around with possibilities. I wouldn’t argue for it too passionately, but it’s an interesting enough concept to make a few fins and try it out. Come up with a hypothesis and test it. With surfing, really the only way to do that is build the fins yourself and try them. So that’s what I’m doing.

106

Wooddave, there’s a goldmine of info in the archives.

Just go to Google and type in what you’re searching for and put this after the search terms:

site:swaylocks.com

That will limit that search to Swaylocks and give you a ton of good stuff.

107

Stoneburner, read the second paragraph on page 308:

http://books.google.com/books?id=1OeCJFJY3ZYC&pg=PA308&lpg=PA308&dq=A+thin+airfoil+at+low+angles+is+similar+to+a+flat+plate&source=bl&ots=WhBt4_-AEu&sig=Wv9-uEvRxV5_5Ipnp0wkuvixFaI&hl=en&sa=X&ei=DXslVImBM4K_ggTm5IH4DQ&ved=0CC8Q6AEwAg#v=onepage&q=A%20thin%20airfoil%20at%20low%20angles%20is%20similar%20to%20a%20flat%20plate&f=false

Again, the Reynolds numbers have a huge effect, and I’m not sure how that comes into play with water vs. air. It’s much harder to find quantitative info on foils in water.

I guess something like this could come in really handy - http://vacantisw.com/foil40-1.aspx

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109

That is what it is all about**…**

When things slow down, I will try to read p. 308 and check out the other link.

Thanks for the references.

110

This site is a treasure trove of data as well -

http://airfoiltools.com/

Calculating the Reynolds numbers then applying that to a specific foil can give some realistic data. I won’t pretend I understand it all, but it’s not super complex if you wanted to quantify some very specific data.

Hand-foiling a fin to match a NACA profile is a whole other issue. That’s why the 3D printing stuff has me pretty excited. It’s fairly easy to generate CAD models with NACA profiles. If you can then print them out to try, things may get interesting.

Maybe my assumption is back-assward, and that’s the point Thrailkill is trying to make. Maybe a thicker fin works better BECAUSE it produces more drag. It’s something fun to experiment with. It’s just surfing after all…

111

Lawless, 

WELL DONE.        VERY WELL DONE. 

 

112

It would make more sense if people stopped regarding “drag” as a problem . It leads them into some mythical crusade to reduce the evil drag…(lol) …I always do my glass-ons at 15mm thick , and the difference is quite noticeable when compared to the same board with a 10mm thick fin , fitted into a slide box .

113

Dolphin fin half chord ratios. I suspect that these might be a good place to start. Reynolds numbers will be pretty much the same as surfboard fins. Millions of years of R&D, or design by the perfect Designer? Either way the end result is pretty impressive.

1382×983 226 KB

114

   The half chord of the Dall Porpoise dorsal fin, is almost exactly the foil I’ve used for years.      Thickest @ 40% of chord length, and thickest part in the 10% to 12.5% range, of chord length.   

115

Thank you Lawless for the search tip, I’ve used it several times already with good results.

My understanding of Mr. Thrailkill’s assertion is that the increased (surface) drag enables the surfer to remain higher in the wave face where it is steeper and the angle of attack is more favorable to convert gravitational force into (forward) momentum…

by using the surface (texture) to create the drag he is still free to have nice quiet  flow around his fin(s).

 

Kayu Wrote.

“It would make more sense if people stopped regarding “drag” as a problem .”

I understand your point, in reality surfing wouldn’t be possible at all without SOME drag. 

 But I think there is a difference between the articulate drag that we employ to convert our stored gravitational energy into directional motion and inefficient drag that can bleed off that energy without benefit.

It is the second type of drag that is (almost) always undesireable.  In many instances you may have enough energy to overcome the inefficiency which minimizes the need to obsess about it .

For my project,  minimum drag is the biggest factor,  an efficient foil is useless if I can’t paddle into a wave to use it…

It would be a great thing for me to learn that I could make my foil thicker without increasing minimum drag at lower (paddling) speeds, because it would allow for the use of a lot of different (cheaper) materials and options…

Though the concept is also counterintuitive, there are a few real life examples of  “thick” things moving through the water easily…

I’ve noiced that seals and penguins etc.seem to cruise through the water with very little effort although they are essentially hydrodynamically efficient tubes and could maybe be described as  “radial foils”.

Another is that torpedo pool toy many of you might have seen ,that thing hits the water and just keeps going.

 But if you look at the video posted by Lawless at around 37 seconds it shows the flat plate at zero angle of attack versus a NACA foil at appx. 1:59  and to me it is pretty clear that the flat plate has less drag at that angle.

 

116

While gravity plays an important role in surfing performance, do not overlook the major impact of the kinetic energy in the wave itself.    The ability of a surfboard fin to tap that wave energy, is the most important aspect in propelling the board and rider across a wave.

117

My thinking is that the fin(s) only help propel the board during a hard turn. When the board is angling across the wave or in a minor turn, the fin is adding control, and adding drag. Reducing the drag is a good thing. Finding the best way to avoid cavitation is probably the direction we should be heading. I’m not sure if finding the right foil for maximum the AOA is as important as reducing cavitation through other means. In today’s surfing with short boards and multiple fins out along the rail, turns are typically very quick and intense. The working fins are almost always the single or assymetrically foiled outer fins. I think a combination of changing the layout (tow in, cant/camber), adding “supercharger” type devices that decrease cavitation will help make a board work better. My goal is to find the least amount of drag when the board is angling across when you are not turning, and the best “bite” or hold when you want to turn.

With respect to fin width, the fin needs to have enough thickness to achieve the proper NACA foil. I think that as the fin’s base increases the width needs to increase to get the proper foil. There are so many other variables like the shape, rake, and height that have to be factored in too.

We should all know by now that finless board can be propelled across a wave with great velocity. They just don’t have the same control that boards with fins have.

118

Six zillion words about variations of a fin that looks like it came from a fish. Been seeing these fishy fins for 50 years.

No new fin designs being seen…just more tweaks to what Greenough did waaaaay back when.

I want to see someone with the balls to do something new and unique. I told the story about epoxying a 16 ounce budeweiser can to a board and it worked. Don’t make me tell it again.

Swaylocks still rules…it’s just stalled in a yada yada yada mode.

119

  I think my Twin/Single fin setup qualifies.     Also my ‘‘secret foil’’  is quite unique.    It just isn’t  out in the public yet.     I’m still looking for the right relationship, to bring it to market.          .  

120

Mr. Thrailkill it is unfortunately waaay to easy to overlook the Kinetic enery of the wave here in Florida haha…

Cleanlines,It should be no surprise that the best performing fins are those that resemble fish fins…

It’s not a question of whether or not we should emulate fish, It’s a question of WHICH fish we should emulate…

 While no beer cans were used to make this (although more than one beer was consumed while building it):

I hope it might qualify as new and unique.