I agree,
no doubt tom’s example is a great illustration for other functions of the fin.
the flow field is certainly non-steady.
yet the rules governing the behavior of the fin never change.
flapping a wing, spinning a propellor, directional stability or pumping your board.
it all works pretty much the same.
I dont mean to minimise the problem at all, but the ‘bits and pieces’ are pretty simple.
the rest is just magic.
-bill
Agreed … I was thinking of airplanes and boats. The natural world is much more applicable … even superior.
Although there may be some truth to this, i dont buy it for pumping as what creates propulsion.When you look at some one going into a turn or pumping, the bottom of the board is not parrallel with the surface of the water but heading more perpindicular and as such is loading the bottom of the board with pressure this pressure will come to a point of balance to equalise what the surfer is putting on.The surfer at this point is usually compressed and decompresses releasing force off the deck of the surfboard, at this point there is a pressure difference and the surf board has to move up until the pressures rebalance at which point the surfer and board are back up into a higher part of the wave where they have more potential energy(like picking up a stone and holding it in the air.) and are ready to race down the face.
This is probably over simple and i wouldnt say that fins dont play a part but i cant believe it is all fin when you compare the area on the bottom of the board with fin area.
But i would say efficient use of pumping is more to do with the surfers ability to incorporate his/her own energy into the equation through loading/unloading at the correct times etc.
I think you’re right and that most of what’s happening there is the fin is efficiently assisting the board’s bottom–by helping you hold the rail in
And again we are back to how much of a role does an asymmetrical foil is versus a symmetrical foil have in this equation?and again i beleive very little when you are taking into account the area of the bottom of the surfboard and overall fin area.
I don’t really agree here. I think the fins are huge just in themselves at producing drive. Water is 800 times the viscosity of air … They may look rather small but they are immersed in a rather viscous fluid. And high performance surfing is all about bottom turn and less about drop. Of course all aspects are used and important but bottom turn is where all the power is pretty much focused.
Who’s speaking in absolutes? I never said it is all fin. But, early on you discounted assymetrical fins and even said you might as well just use symetrical fins. I was just explaining where flat and concave fins have a clear performance benifit over symetrically foiled or convex 80/20 foiled fins.
This looks like a nice limb…
The bottom’s lifting surface area should be compared against two fins. The bottom is obviously the primary lifting/control surface–fins help the rider keep the bottom in control–pumping, turning, anything you do–the bottom is the overwhelming control interface with the water, based on surface area and/or work done. Fins keep it under you and you pivot off them secondarily. Matter of fact–I think you pivot and bank through turns off the bottom to a majority percentage–that is, the lift at the base/bottom juncture, and I think the bottom is doing the more work at that juncture. I think the rest of the fin has to just be efficient enough and stiff enough to not stall or bleed off all the rest of its lift. Highly foiled G10 with perfect single foils, say.
Double-foiled fins have to be oversized vs single-foiled fins to do the same work of controlling the bottom. Maybe better-stated: you can’t run single-foiled rail fins at the same sizes you can double-foileds–you’ll end up with “too much fin”–too much lift–result: tracking. Ergo: single-foiled fins do more work.
I used to agree with you (on both your points), but now I don’t, and I’ll tell you why.
You have to consider the direction of flow. Surfboard fins operate at a higher angle of attack, than just a fin/board moving through/over the water. It is more like a bird gliding along a cliff riding the updraft, than an airplane flying through the air.
Lift is created by turning a moving fluid.
BOTH sides of the fin turn the fluid!
For every fin you get two surface areas (the inside AND the outside) working to move the fluid. For the bottom, you only get one surface.
When you consider the high angle of attack that surfboard fins encounter you can see why a convex inside foil (by inside I mean the side that is facing more toward the flow) produces less lift than a flat or concave surface. It doesn’t turn the fluid as much.
Furthermore, the asymmetrical foil allows the outside foil (the one that is not facing the flow) to have a longer curve, for a given fin thickness, which will prolong attachment at higher angles of attack, allowing increased turning of the fluid, therefore increasing lift.
Hi obproud.Thanks for the reply, yes there is plenty of theory suggesting that the asymmetrical fin produces more lift than the symmetrical.But no numbers to back up how much.I calculated with my rough calculations and more than likely misapplied applications around 2 percent.Say its 10 percent, f*#k it lets go one step futher and say 20 percent.What would that be in the scheme of things when taking into account what lift and drive the board itself produces along with the fins?
If your fins are now producing 80 percent of what they were and they account for say 30 percent of overall lift when you include the surfboard you are starting to split hairs.And bear in mind this ratio probably does change depending on the waves you are riding as in the pictures(but how many of us ride waves such as in the second pic)?.
I have no doubt that alot of skilled ppl who are in the top percentage of surfers skill wise will notice the diffrence.But for the vast majority of us(like me in this pic) id be surprised.As i said earlier i dont even notice the difference in performance of plastic to glass(the price difference…now that is another story).IMO the difference in performance in alot of these types of things(not just fins) is not justifyable to the extra cost for the average surfer(who probably makes up a large chunk of the market).In other words its marketing imo.
You don’t believe NASA?
Play with this program and see how lift changes with shape. http://www.grc.nasa.gov/WWW/K-12/airplane/shape.html
And then play with this one to see how lift changes with shape and AOA. http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html
Do you play guitar or any musical instruments? If you do, then you will know that as you progress in your ability and spend more time with the instrument, that your senses will become more acute and you will be able to detect things that you couldn’t before. When you first start you are just trying to learn chords and scales, and how to physically navigate the instrument, and you are unable to detect differences (often subtle) between different instruments. As you become more familiar, and continue to progress, with the instrument, you are able to detect more and more subtle differences in how it plays and how it sounds. Tiny changes in the setup of an instrument can dramatically change the way it plays for an experienced player. There is a reason why top violinists pay huge bucks for a Stradivarius, or professional recording engineers are so picky about equipment, room acoustics, and mic placement.
To put it in percentages like you have, maybe at first you can only detect 10% changes, then as you can progress you can detect 1% changes, professionals can detect %0.1 changes, and the elite can detect %0.01 changes.
To bring it back to surfing, if you look at the two photos you posted you will notice that there is a big difference between how much of the board’s bottom is interacting with the water, and therefore a difference in the relative fin to board relevance. It looks like you are using over 60% of your board bottom surface area, where the guy bottom turning into the tube is using maybe 10%.
Heard a story relayed from Nike. Tiger Woods was blindly given three identical experimental clubs to try. He preferred the lighter one. It was less than one gram lighter.
The same clubs get sold to average golfers because it makes average golfers play better.
Same with boards and fins. Call it marketing if you want, but everyone deserves access to better equipment to allow them to improve and perform better, no matter their experience or ability, and no matter how small that improvement may actually be.
yeah thats fair enough, but imo ppl are often mislead wrt perfomance of items, and in no way is it just a surfing thing.I guess that is what marketing is all about.Nothing wrong with asking questions though imo.
Hey Hunty, I'm not saying marketing is not bullshit, sometimes it is........but sometimes not all of it is bullshit, and sometimes there's no bullshit at all.
Makes it pretty hard for the average consumer to make a choice though, unless they do some investigation. But I guess that then makes them an above average consumer.
[quote="$1"]
You don't believe NASA?
Play with this program and see how lift changes with shape. [url]http://www.grc.nasa.gov/WWW/K-12/airplane/shape.html[/url]
And then play with this one to see how lift changes with shape and AOA. [url]http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html[/url]
[/quote]
That is a pretty neat app.
the NASA site is really cool.
So in conclusion:
"Don't confuse me with facts, my mind is made up"
I've said the same thing to myself many times :)
hunty,
i gotta say, there are now multiple sources providing numbers to your question:
nasa, naca, your own math .
Its OK if you dont believe those numbers are big enough to influence a boards behavior.
(or even your board's behavior)
at this point, you'll just have to try it for yourself.
.....the best part about swalocks
no worries,
-bill
Yep that’s all there is to it Hunty–try both kinds yourself…but…if you can’t feel the difference, and you’re a “kook” as you say, it still won’t mean much. Whether or not it makes a difference is for people who can surf to say, innit? Math won’t make your surfboard make you a better surfer and it probably won’t explain why you’re not a better surfer, and Swaylocks members poo-pooing design elements used basically universally by the world’s best current pro surfers and shapers doesn’t mean they’re suckers.
David Carradine–look him up. One thinks one’s reinventing the wheel, really into the real shit, and then just as one reaches the proof and pinnacle of one’s cleverness, the wave swallows you up.
(barely resisted naming he-who-cannot-be-named here)
Hunty,
You can wax on with theoretical speculation all day long. A friend of mine did his doctorial thesis and wrote a fin design and evaluation program based upon lift/drag characteristics of the fins my company makes.
http://cetic.swan.ac.uk/surfs/pdf_files/Carswell_(2007)_Hydrodynamics_of_Surfboard_Fins.pdf
After all the numbers were crunched, the trends that we found empirically were indeed reflected in the plot lines of the data he found by tank testing and his program. What we felt by trying different fin foils on the same templates in more or less powerful conditions did show up in subtle trends in his data? The math in controlled parameters testing and prediction programs did not show huge differences only subtle trend differences. But, when we tested them in real world surfing conditions we definitely can feel a difference.
My company also manufactures the soft board urethane keel fins that are used in most of surf camps around the world. We also make a semi-soft thurster style and and a 2+1 setup for softboards. The Keels are fuller templates with symetrical foils on all fins. We describe them as point and guilde fins. They are stable, can take a beating and won't hurt you as much as hard thruster type fins. But, they make a softsurfboard ride like and ironing board. If you take the same basic board, put a surfer who has progressed beyond "point and gluide" type of surfing and switch to the semi-soft or even better stiff thruster fins the manueverability will improve dramatically.
That looks like some nice reading, but I’m having a problem contacting the server.
Are you sure the url is correct? If it is, perhaps it’s just a matter of waiting and trying to access it at another time, or could it be something else?
Also, you’ve indicated that you work for a fin company, are there any internal, but similar kinds of studies or data that your company would be willing to make available.
kc
...added afterwards
The following are two examples taken from Carswell's paper “Hydrodynamics of Surfboard Fins” see tomatdatum's post above. Thank you Dr. Carswell for access to your work and data.... great stuff.
kc
Daum Tooling, Inc. is a custom plastic injection molding company. We make fins for Red X, LokBox & 4-Way Fins. We also make leash components for Blackline, Block, Crow Haley, DaKine and Stay Covered as well as wax combs and wax boxes for Sticky Bumps and Famous Wax. We also make molds and mold parts for the medical and several consumer industries.
It's been a while since I posted that link last. Dave and Swansea University may not be supporting that link anymore. I have a pdf file that I can e-mail anyone who is interested.
[quote="$1"]
....., but I'm having a problem contacting the server.
.......kc
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same here.