Discussing Wavegrinder scientific fin design

I’m wondering if anyone can find something wrong with the scientific justifications that the makers of the Wavegrinder fins provide for their fin design.

See: http://finsciences.com/surfboard-fin-science/

I searched Swaylocks and found two previous short ride reports and discussion of these fins. Other ride reports from the web also seem to be exclusively positive, at least for an amateur sort of internet search.

I have read through their pages and cannot find any obvious fault in their reasoning.

I have also lent a Wavegrinder original fin to a much better surfer than myself, and he found it to be his favourite fin immediately. I have only surfed it once myself, then went back to using McCoy Gullwing fins for no real good reason, other than that I know they work. So basically I have not tested these fins myself, one session in poor conditions does not mean much.

Having messed around with attempts to make a fin for almost a year, I decided a couple of days ago to buy 4 small, 4 medium, and one large Wavegrinder fin, all of the ‘semi-flexible’ variety.

I reckon they have nailed it, or are at least very close to ideal fin design, and I have no hope in hell to ever stop wondering if I’m missing out if I do not try the fins out myself.

And no, I’m not paid in any way for this and I just paid full price for the fins from their website.

Question:
For a surfer who is not a top athlete in this sport, could it be that a fin with optimal lift to drag ratio is harder to surf than a fin that is much more ‘draggy’, but stalls in a more gradual fashion?

Or would a poor or intermediate surfer rarely be able to turn so hard that s/he approaches the stall AOA of average fins in any case?

Some links to ride reports:
http://community.magicseaweed.com/viewtopic.php?f=13&t=22477
http://www.supsurfmag.com/equipment-stand-up-paddle/reviews/finsights%3A-stand-up-paddle-surfing-the-wavegrinder-20080818503/
http://www.swaylocks.com/forums/something-new-wavegrinder-fins
http://www.swaylocks.com/groups/wavegrinder-test-ride-results

I like them cuz they look safer than standard fins - think I’ll order me up some when I get some $$ and give 'em a try. Question: with the quads it looks like they are all 4 the same size - what’s up with that? BTW the “science” is just marketing hype to me, the proof is in the pudding!

I believe the smaller fins as used in quads or thrusters come in two sizes and can be stiff ( black only ) or flexible ( colored ) the science or hype is the same as that of all the other fin manufacturers .

I’d be willing to try them.

If I liked how they worked, perhaps I would eventually find them aesthetically pleasing too.

There are a lot of surfboards whose planeshape I find to be abborrent to my eye, but they obviously work.

So the design is valid but a few areas can be improved. The bulbous connection at the base of the fin will reduce drag or increase it based on speed. those are put on large container ships that go a very specific speed. It’s a function of the displacement wave created at the bow of the ship and the geometry of the bulb to minimize that wave. At variable speeds the displacement wave is changing shape too much for the bulb to make much of a difference. Its probably just interesting theory with a minimal effect. the wing tip is good for reducing drag. IMO I’d go with tubercles that would reduce the vertical flow and organize it around the foil. This would have an effect over the whole fin rather than the tip and probably make manufacture simpler. The plan shape is very efficient but drag=control so maybe something inbetween a traditional fin and the high aspect fin here. I want to draw out turns and carve rather than do quick turns so i guess its just personal preference.

Other considerations with the high aspect ratio , and the ‘winglets’, is kelp/seaweed catching, and more stress on finboxes should they hit an obstacle.

I would like to try them, but have little desire to spend money on them.

And is drag really an issue on most boards? Like ‘Gee if only my fins had 20% less drag in a straight line, I might have made it around that section?’

I like the science. But one video where he recommends all side fins have Zero toe in to eliminate the ‘snow plow’ effect, kind of lost me. Such a board would seem to dictate how the wave is to be ridden, but granted I’ve never tried it.

IDK perhaps it would be great to step outside the box, and try.

I have one of the original single fin Wavegrinder and I really like it there is a noticeable difference to the board , how it paddles and how it rides but it is 9in and in my opinion a 7.5 in fin would be a better choice for more high performance longboards used with side bites , but it’s not available , I have contemplated cutting down the 9in fin to 7.5 but never got around to it , I also have a set of the stiff quads but have not yet used them so cannot offer opinion . Several years ago I had a long conversation with the designer/manufacturer , smart guy .

Hang 20 wrote: " IMO I’d go with tubercles that would reduce the vertical flow and organize it around the foil. This would have an effect over the whole fin rather than the tip and probably make manufacture simpler."
Do you mean something like the bumpy leading edge foil? When I make a BLEF fin using finFoil, then the bumps continue along the horizontal foil, in a negative image or interference pattern of sorts. I have been wondering if that reduces the tip vortex and if that may be what’s behind the performance advantage claimed by some.

Yes they pitch their fins are made by ‘science’ but their science is for boats and airplanes which act in different ways than surfboards. Reducing drag may not necessarily give a better ride. Drag=control. Control lets you go in more critical areas of the wave. Being in control in more critical areas of the wave can give huge bursts of speed (and be extremely fun!) what they are trying to do with maximizing efficiency similar to freighters racing boats and airplane wings is probably best suited for riding straight on big boards in mushy waves. They don’t acknowledge that the scientific method was used in developing surfboard fins as they relate to the boards in use and how people wanted to surf the wave. Experiment, analyze results, adjust, experiment again - that is the scientific method. It doesn’t have to be designed using CAD and complicated predictive models to be science…

Also super strange that they make fun of biomimicry. Most scientists and engineers highly value the functionality and beauty of the natural world

Take our small shortboard fin surfing on a 3-meter wave—at about 20 knots. That fin will generate 19.23 pounds of side force—57.69 pounds of side force for a thruster setup with three fins.

I’m sure this guy knows a great deal about foil efficiency and fluid dynamics but he obviously does not understand how a thruster setup on a surfboard works…

Also missed the mark on material selection too. Trying to pitch a high performance ‘scientific’ solution with molded plastic…

Maybe the science is there but still needs some work on the engineering side…

Not sure I agree with this free body diagram…

grasshopper wrote:
“Yes they pitch their fins are made by ‘science’ but their science is for boats and airplanes which act in different ways than surfboards. Reducing drag may not necessarily give a better ride. Drag=control. Control lets you go in more critical areas of the wave. Being in control in more critical areas of the wave can give huge bursts of speed (and be extremely fun!) what they are trying to do with maximizing efficiency similar to freighters racing boats and airplane wings is probably best suited for riding straight on big boards in mushy waves. They don’t acknowledge that the scientific method was used in developing surfboard fins as they relate to the boards in use and how people wanted to surf the wave. Experiment, analyze results, adjust, experiment again - that is the scientific method. It doesn’t have to be designed using CAD and complicated predictive models to be science…”
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That’s a big call that drag equals control. I’m not sure that is correct. Too much drag would certainly cause loss of all control.
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I think maybe the drag/control has something to do with this (but I may be completely wrong):
Hypothesis: Some less efficient fins will gradually cause more and more drag while they approach their stalling AOA. This slows them down progressively so they practically never stall. Conversely, maybe a perfectly efficient fin causes increasing lift with increasing AOA, until it very suddenly and irretrievably stalls. If that’s true, then a more efficient fin would likely cause more wipeouts for average surfers, but if you are expert enough not to go too far, then the efficient fin will be an advantage.
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Your description of the scientific method is quite incomplete. As a minimum, I would want to add in that for a scientific experiment, you need to test otherwise identical equipment, in the same conditions, under ‘blind’ conditions (preferably double blind), while changing one controllable variable at a time.
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For truely scientific fin testing, a double blind test could look somewhat like this:

  • Surfers who are honest enough to not deliberately cheat, or who are monitored against cheating by drone video or similar methods, have the fins in their board swapped by “assistant 1” while remaining in the water. Jet ski or boat or both might be very useful to have all the gear at hand. The surfers do not know what fin is in their (preferably favorite) board when they paddle for the next wave.
    After riding a wave (or a few), the surfers talk to “assistant 2”, who is also ‘blind’, i.e. s/he does not know what fin is in the board, and records what the ‘blinded’ surfer thinks about that particular fin. It would likely involve some structured questions and maybe free commentary as well. So neither the person who tested the fin, nor the person who records the results of the test know which fin was being tested.
    While a surfer talks to “assistant 2”, the fin is being swapped by “assistant 1”, and so on.
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    Using a water-proof recording device, this experiment could even be done by three surfers taking turns. The waterproof phone is ‘blind’ to the fin used, and 2 of he surfers are if they don’t cheat, while the third surfer (who swaps the fins at random) has no input into the testing or result recording. Take turns every few waves, so three mates could take turns swapping 3 fins between three boards, surfing and recording the results ‘blindly’ in (as much as possible) identical wave conditions.
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Experiments need not only change one variable at a time. In fact it is often benefitial to test multiple variables together to obtain interaction effects (Check out the theory of ‘design of experiments’)
Interaction affects are huge in surfing that is why empirical input is going to give a higher value than designing by computer using parameters set to maximize performance in a different physics model. The boat keels are the most similar physics to surfing but more applicable to a non wave powered activity like a racing windsurfer or kiteboard. These look surprisingly similar to racing fins for sailboards and kite boards made through empirical input :slight_smile: …and why they are not best suited for performance surfing. Maybe for cruising on a longboard…but watch the video of the guy trying to ride them on a shortboard…yikes!

So you double blind your surfers that only accounts for those specifics surfers on those specific board designs in those specific wave conditions and you could very likely get each one preferring a different fin! Change boards, surfers, conditions, fin placement, how you want to surf that day, different results.
Subjective…but still empirical, I would argue still more valuable than a predictive model. Could get some slighly more objective data by measuring speed obtained or max angle of attack achieved or max acceleration etc if you are surfing a very repeatable wave. I think Solomon did some of these types of tests when they were building engineered hollow blanks. Those boards worked pretty good too!

grasshopper wrote:
“So you double blind your surfers that only accounts for those specifics surfers on those specific board designs in those specific wave conditions and you could very likely get each one preferring a different fin! Change boards, surfers, conditions, fin placement, how you want to surf that day, different results.”
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Well, my suggestion is to perform a double blind experiment, so that neither surfer nor experimenter can influence the outcome of the experiment counsciously or sub-consciously.
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Surfing, like many other human efforts, is very highly dependent on psychological state of the surfer.
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Hypothesis: It is practically impossible to determine the effect of a slightly different fin in a certain surfboard, due to the overwhelming effects of the surfers beliefs about that fin.
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If you have invested much time and effort, maybe a life-long effort, into designing a surfboard, a fin, or a fin-board combination, then you are extremely unsuited to objectively judge the performance of this design of yours, scientifically speaking. You may of course obtain the ultimate joy from this approach, bathing in the glory of your conviction that the pleasure of wave riding would not be near so great without your great invention, but that’s not science.

Funny… mrmik description of ‘not science’ sounds like the wave grinder webpage but wavegrinder is preaching that it IS science ha!

Very well stated Grasshopper, very well stated.