MVGs & Superchargers

General Discussion
61
Quote:

At this point there does not appear to be any better way to analyze surf board fin perfomance than to just get out there and ride them.

I’ve been lurking on this thread for a while… Mark has done his research, we have committed many hours of discussion to this topic, and he has a pretty solid comprehension of the science, and has come up with some reasonable hypotheses and theories. However, perhaps his strongest argument is (paraphrased/my interpretation)" “millions of years of evolution cannot be wrong; lets try to mimic nature and see what we get”. That brings us back to Tom’s comment (quoted above)… surfing is partly dictated by the board’s interaction with the water; the other parts is the rider’s interaction with the board and the lovely/horrifying “X factor”. I am sure that given the infinite variables that come together to make a wave, the variables associated with rider’s technique, ability, and approach, and finally the chaotic nature of molecules of all types and forms… well, the penultimate “equation” has too many variables to solve. Hence the importance of the old fashioned approach of getting out there, trying it out, and coming back with descriptions, comments and suggestions. Mark and I differed on this at first…I wanted him to solve the equation whereas he wanted to flood the scene with MVG’s and have people test them out. In theory, my approach is “better”, but in reality it is impossible to satisfy for the stipulations mentioned above. So that leaves us with the “try her and see how it goes”. I have come pretty much 180 in my opinion- if anyone claims to me to have “proven” anything regarding boards and fins in a controlled test tank, I’m even less likely to believe it (not in a biased way, just in a sceptical nature)… surfing, in reality, does not occurr in a bottle with controlled flow, linear vectors, and robot riders. It happens in a chaotic environment, with constantly altering motion (sometimes gradual and deliberate, sometimes abrupt and unanticipated), with bags of bones, meat, and nerves at the helm.

62

We actually started with foils and templates that were provided to us by guys like Al Merrick, Matt Biolas, Malcomb Cambell, Maurice Cole, Casey McCrystal, Timmy Patterson. Then we had guys like Taylor Knox and Timmy Curren ride them. Then we tweaked them and now I’m listening to you guys and thinking of new ways to tweak them. But no, I did not start with a constant cord length foil and tweak depth camber and rake.

63
Quote:
I don't follow some of your post. For example,

"NACA foils are the results of wind tunnels, 2-D modelling, and computer simulation. "

NACA foils were developed in the early-mid 1930s. They weren’t the product of any computer simulations.

They’ve been extensively studied using wind tunnels, 2-D modelling, and computer simulation. That is how we know that 12% thick NACA foils are near optimal for lift-drag.

Quote:

I also don’t understand your argument that NACA foils are useful only for high (>5) aspect ratios. Small boat rudders, for example, with aspect ratios of between 1 and 2 are often designed using NACA symmetrical foils.

The amount of lift generated, and stall angles predicted, using NACA foils do not apply when using low aspect ratios at higher AOAs. Separation of flow occurs on the low pressure side of the foil and forms a vortex that re-attaches before the trailing edge. This vortex causes an above predicted lift coefficient in such foils. BECAUSE this vortex dominates lift determination, and flow isn’t attached to the low pressure side of the foil, thickness becomes irrelevant wrt lift in this range of AOAs. The foil still matters at AOAs too low for vortex development.

It has always bothered me that 2-D theory predicted surfboard fins would be optimized at MUCH thicker foils than they have, but I find this explanation reasonable. They only need to be optimized in the first few degrees of AOA.

Quote:

and as for “the Concorde wing not needing to work at high AOA”, the Concorde was in fact designed to generate lift at low speeds precisely by flying at very high AOA (and using tip washout) to create vortex lift (and ground effect lift); while at the same time using a very thin foiled wing to allow supersonic flight at low AOA…

That, I agree, is a little unclear. The Concorde goes down to an AOA of about 6 degrees when cruising. All surfboard fins go from +15 to 0 to -15, at least.

64

Tomatdaum,

I like your approach because it makes sense. For example; pharmaceutical companies send teams or researchers into the field all over the world to collect samples of all kinds of plants, bugs etc. They bring this collected stuff back to the lab and with hundreds of diseases growing in different petri dishes they grind up the plants or bugs and drip them on the diseases. IF they Kill a particular disease then the scientists work backwards to find the long chain molecule that is responsible, isolate it and then figure out how to manufacture it. To do otherwise, to just work in a lab, formulating, fabricating and testing long chain molecules would be totally cost prohibitive. Nature has already supplied us with billions of long chain molecules. Why not use them? The same holds true for your templates by studied shapers. It’s just good solid science to start with them. And good business.

I’m psyched to see how much thought and discussion is going into this subject by so many really smart surfers. A lot of what I’ve read is review for me, which cant hurt (grin), yet I’m also hearing new things, which really stokes me. Also, there are some ideas that some of you have proposed testing that interest me. If I can help out with product let me know. Stocks are currently low so I cant do too much, but if I can I will. As Hackeysakey pointed out I made a decision to test by swarm method, so basically it’s only rider feedback and very subjective as independent data. But cumulatively it’s hard to ignore. So at this level of testing I’m just listening.

It’s common knowledge that FCS has a govt grant in the million$ of AU dollars to test their fins. The only results you will see are the products they sell. Rainbow I’m told tank tests too. But since it’s private money dont expect public results. That’s about all I know about that.

As for aircraft wings: Aircraft wing formulas are well protected corporate secrets. The best anyone can do is look from a distance. I’ve narrowed the distance. I’ve stood next to the SR-71 and under the Concord’s wings. There is no substitute for that level of experience.

The ammount of money our govt has poured into aircraft research is way beyond anyone’s comprehension. Surfboard fins are stoneage by comparison. That means there is plenty of room to learn.

65

“The leading edge vortex effects are the reason foiling has minimal effects at larger angles of attack”

Yes! there is sanity in this posting.

“Unlike the Concorde’s wings, though, surfboard fins need to work at low AND high angles of attack. It is an interesting problem, be cool to have a budget and a flow tank to get down to brass tacks. But I wouldn’t expect anything revolutionary from such work”

From my limited recollection of Fluid Dynamics courses, almost all calcualtions similar to the ones communicated in this post were made with zero angle of attack assumptions (parallel to the cord). One’s analysis is only as good as the assumptions that go into them…GIGO.

Thanks for the sanity check Mr.Blakestah.

66

Regarding sonic booms in water, several days ago I sent my brother the following question;

Is there an equivalent in water, to the sonic boom produced in air? m

I included a link to this discussion. He gave me permission to post his reply.

“Not sure. He’s right in that sonic booms and shockwaves depend on air compressibility, and water is not compressible. However, acoustic waves easily propagate in water, and there may be some sort of shock front formed for extraordinarily fast moving bodies. However, while the speed of sound in air is about 740 mph at sealevel or thereabouts, the speed of sound in water is something like 3,200 mph. Unlikely that anything could ever approach that speed underwater (cavitation effects and drag are huge obstacles). About the fastest underwater movement I’ve heard of is the Russian Skval (Squall) supercavitating rocket-powered torpedo, with a speed of somewhere between 200-300 mph and a range of 10 miles.”

Also, on a different note, on a website forum out of CFL someone posted a comment about their new Vector fins and how great they were. They went on to say they even made a “cool humming sound”.

Sometimes I just stare in amazement.

67

assuming for the sake of argument that the leading edge vortex theory applies to raked, low AR surfboard fins, even this argument admits that a) foil has less effect at large AOA and b) fins need to “work” at both large and small AOA. The inevitable conclusion is that foil matters.

what does it mean for a fin to “work”? Fins must provide control, and optimally balance lift, drag, and thrust. However, this must be achieved differently depending on the placement of the fin (rail, center, or other) and their relationship to the bottom of the board (e.g. bonzer rail fins and longboard single fins probably ought not to have the same shape). For some applications symmetrical fins appear optimal, for others asymmetrical, for others probably undercambered is best. A lot depends on the application.

While I am all in favor of looking to nature for inspiration, there is not really a direct analog in nature to surfboard fins, in my opinion. Nature provides sea creatures with propulsion (thrust) and steering (control) in a number of different forms – but they aren’t indirectly controlled by a pair of feet putting pressure on a deck, like surfboard fins.

68

I had seen the MVGs a while ago and asked around about them and found a local rider (CFL) who raved about them on his twin fins. I’ve heard nothing but good responses on their performance so the saturation test method can’t be all bad.

Some very technical speak going on in this thread, so here’s a link with illustrations that explains the basic principles for those not well versed in aeronautical engineering:

http://www.grc.nasa.gov/WWW/K-12/airplane/geom.html

While searching through that site I came across another really cool tool where you can try out some of these ideas about chord vs. angle (fin toe) vs. chord %

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

It doesn’t depict flow seperation but it gives a good visual representation of some of the ideas being thrown around here. I’ve had alot of interest in the foiled fins but haven’t actually tried any yet but it’s interesting to play around with the concept in that foil sim. Theoretically by putting more camber into the fin it increases the lift:

[Image]http://www.barrierislandgraphics.com/forumfiles/foilsim.gif[/img]

As stated previously though, that does nothing for negative AOA, but also from what I have read above a certian AOA number the foil doesn’t come into play.

Another option to look at would be symmetrical foils, as the kind used on aerobatic aircraft. They generate lift by the AOA relative to airstream, thereby being able to create lift at both positive and negative AOA. Much like a fin going through positive to negative AOA through a turn.

BTW, longtime lurker, first time poster.

69

Well put, Keith.

I was principally motivated by Halcyon, actually. He rode one of my boards and thought tweaking the fins would be an area of possible improvement.

That raised the issue - how does allowing the fin to rotate change the way the fin geometry interacts with its function?? What exactly would you change from a very middle-of-the-road fin (the FU SRO fins) to gain an improvement in function?

Quite honestly, I had some idea, but no reasonable mechanistic reason why. The ideas put forth in this thread are my best guess currently - but honest-to-goodness testing would be the kicker. I will be doing some empirical testing - under my feet.

Assuming we get something rideable sometime soon.

70

I haven’t thought that much about rotating or pivoting fins.

In the aircraft world one equivalent would be the full-flying stabilizer, but those tend to pivot around a forward-of-mid-chord point, not around the leading edge (I assume that’s what your system does).

Not sure either what exactly you’re trying to gain with the moving fin? Is it primarily reduced drag? Trying to add forward momentum when pumping the board? Changes in sideslip, better control? All of the above? It seems to me that the goal is important because changes that improve performance in one area may well compromise it in another – using a cut away fin shape to reduce drag might seem logical, given the sideways fin motion you’re adding. But it may take away from any attempt to add forward velocity if you’re going to pump the board…

I like to experiment with my boards by varying only one thing at a time, and making that change fairly extremely at first to exaggerate the effect. If I were testing new fins in a pivoting box, I’d start with a fairly normal one and ride it extensively to get a baseline, then make up a batch of fins, each altering only one variable – rake, thickness, length etc – and see which movements improve the performance criteria you’re most interested in. You might eliminate some dead ends pretty early, or figure out which combination of things to play with in finer increments.

That’s pretty vague, I’m sure – but as I said, I haven’t pondered moving fins all that much either. If you have a theory that would lead you to test one thing first, or more extensively, so much the better. (You can invent a lightbulb by testing 1000 filaments, but it’s a slow process…)

71

The goal was to see how well an optimized rotating fin would do wrt providing thrust, low drag, hold, and drive.

I started with the presumption that toe-in on thrusters made them more turnable, and gave them the “thrust”. So I figured, what if I used one fin, allowed it to rotate, could it then generate thrust? And would it be as good in turning as a thruster, with the drag of a singlefin?

It easily generates thrust. There are obvious drawbacks to using one fin, like front-end spin out if your wide point and rocker aren’t done appropriately, lack of a rail fin for hold on steeper waves. And if the side forces are too little the front end is wishy-washy through the middle range of AOA. But the single rotating fin, if positioned fore-aft properly, and with proper side-forces resisting rotation, goes most of the way, in performance in turning, from a single to a tri-fin.

And it blows the doors off a tri-fin for coasting across flat sections, and coming to a plane early. It catches waves like the board is 6 inches longer. I don’t think people think of those as being positive selling points for fin systems anyway though, although they make a difference to the rider.

But I am still using generic fins. I will be trying a few different rakes first, and some other ideas, to see what works. But I think that as the fin has to work under a more limited range of AOA than other fins (because it turns), I can probably get away with less rake, and that would give it a steeper lift:AOA function once the fin is rotated. Just mental exercise to know where to look for improvement. It’s been a lotta fun too, the boards and units continue to improve in performance.

72

Halcyon,

I had an additional thought yesterday. When I was a kid I rode skim boards on the lake out back. I was able to throw 10 foot sheets of spray with my favorite snap turn trick (for all the FIN FOIL FREAKS out there…isnt it impressive how those extreme skimboardists do the things they do WITHOUT FINS?)

A lexan skimboard can be used for the streamer test to estimate Angle of Attack. But instead of riding it, just use your arms in waist high water to turn the board and observe the streamers on the bottom (no need for boat). The angle of the board with respect to the water level is the controlling input - when turning a surfboard, whether modestly or aggressively, the board always rides at an angle and this likely is the primary controller for AOA. Home Depot sells lexan sheets so I may go ahead and make a test board. Stay Tuned.

Others…

of course foil matters…its the degree of emphasis on foils and the lack of understanding of AOA wrt surfboard physics that I have questioned in this posting.

Calculating Reynolds numbers in an analytical vacumm is a big no no (as an engineer, if I had a dollar for every time I witnessed the micro-analytical theorists lose site of the big picture I could buy me a new electric planer).

Another source of debate if the concept of THRUST…this again seems overrated. When compared to the other overwhelming forces involved, namely surface propulsion from the wave/water and gravity pulling you down from the top to the bottom, fin “thrust” is negligable. The primary function of the fin is CONTROL.

Three fin boards gave us more leverage to push the board around (driving control)…toe-in gave us a definitive turning advantage (turning control), both toe-in and cant gave us drag and thus lift (early wave catching control).

First three rules of Real Estate…location location location

Surfboard Fins…control control control

The real beauty of the thruster is its SIMPLICITY…so many control benefits from only three fins…thanks Simon. The truster setup will be very difficult to improve upon. But its fun to explore other options.

73

Blakestah,

I went back and looked at our threads from last November. I pasted one below. From them I can see you have done a lot more thinking on the subject since then. As for whether or not tuna finlets reattach flow, all I can do is say that they may both reattach flow and contribute flow, but that’s based on assumption. In the research by Jennifer Nauen,

contributing flow was not tested, but only alluded to in the conclusion as a possible area for future testing. Now if there is contribution of flow and your leading edge vortex theory is functional, you may have hit on the real possibility of a function for the MVG as in Multi-Vortex Generator and not Micro Vortex Generator. Perhaps is a pre charger or supercharger (or turbocharger as my brother suggested and as I originally used in my first reports and packaging). Herb may have been right all along calling his finlet a Supercharger. That may be an accurate physical description and more than a name.

Regarding the reattachment of flow G. Loehr came up with a novel use for MVGs putting one of them on each of the tips of a fish. He said it cleaned up the flow coming out of the twins, and that may be what they did, but as they did that they may have also reattached the flow as it left the tails. As we know a pointed tail is a drag and if an MVG on the tip of the point reduces the drag similar to the finlets on a tuna reducing the drag by reattaching the flow as it comes off the back of it’s foiled body well then there may be validation for Greg Loehr’s theory, too. I have one and it was fast and even able to get through the flats where fish tend to lose speed. A lot of those boards were sold and one of them wound up at my beach last summer. I wasnt there, but some of my team were out and they talked to the guy who was visiting from FL and he said it was his favorite board and he would definitely order another.

At the time of our research in 97-98 my brother used this tag line on his emails which kept the whole thing in perspective:

"There are more things in heaven and earth, Horatio, than

are dreamt of in your philosophy." Hamlet, Act 1, Scene 5

The following is from Swaylocks November 2003:

"So, you’d presume the MVGs on a tuna are there to aid water flow heading into the tail…

and not

to promote reattachment of flow coming off the rapidly tapering body height in the rear half of the fish?

http://www.flmnh.ufl.edu/fish/Gallery/Descript/BluefinTuna/BluefinTuna.html

This is kinda important - the vast majority of MVGs are rearward of the leading edge of the foil. They act on flow re-attachment in the draft or rear of the section, where water needs to re-attach.

In airplanes, they disrupt flow at the leading edge, so that it is more disturbed coming off the rear of the foil, and separated flow is minimized at steeper angles of attack.

http://www.microaero.com/pages/v_howvgswrk.html

I kinda think the tuna MVG fins are there to promote re-attachment for water coming off the body, and have little to do with the actions of the rear tail."

74

I was bothered at the time by things not fitting. It just didn’t seem like the MVG served in a classic micro-vortex generating mode. Maybe it did improve function (I hadn’t ridden it then - even though I’ve ridden it now, I haven’t gone with and without on the same board). But it didn’t seem to fit the role of a micro-vortex generator.

But to initiate a leading edge vortex - that seems to fit - much more likely in my mind. The mackerel finlet is postulated in that role. The vortex translates into more lift - that should be noticeable to the rider.

but the role of all of this thinking and kicking ideas around is really to figure out what to try in empirical studies. Unless I come across a flow tank…

75

Kech rode one at Pipeline a few years ago. I think he had a bad experience because he said it created too much lift. I cnceived of it as a power assist for small wave surfing. Sort of a turbocharger for the Mulhern pump. At the time the main complaint aboutthe WCT was the small wave conditions at most of the contest venues. I thought it might help. Ther is one other more complex action that I think it may serve as part of contributing flow. As someone on this thread (I apoligize for not remembering who) mentioned earlier, Micro Vortex Generators are traditionally placed on both sides of the tail: Traditionally sounds odd since this is still fairly new technology, but so it is. Anyway that why this may help in precise function I’m trying to describe. That is as the pressure becomes unstable as the tail flips from side to side the destabilization created by he mvg may in fact help the tuna set a faster rhythm in it’s tail. They can flip that giant tail pretty darn fast whe they wan to and the faster they swim the faster they move that tail back and forth. That may be aided by the Multi Vortex Generator function of the micro vortex generating finlets. Keep in mind this is my own theory and I’ve not read or found anything to support this conjecture. But to me it makes sense.

Basicaly, I wonder what it would be like for the tail to pass through a vortex while in forward motion. That makes me wonder what would happen if an F18 or some supersonic capable fighter jet were to fly through the lower funnel vortex of a tornado. If an F18 can cut a 2 1/2" thick stailess steel ski lift cable with its’ tail feeling just a slight bump I’m sure it could withstand the debris. But would it kill the tornado?

76

OKAY,

Let me jump in here(or stick my feet in a bucket of ???).

…I have been privately re-working a couple of the MVGs that Mark had sent me.I’m always tweeking things around,and found that I prefer the stiff MVGs over the flexy ones.I also for the most part got good center fin placement with help from a wizard of general flow rates…rates of flow in any situation,be it water,air,space etc.

…The only problem I encountered was a increase in static drag on take-off,especially in slow waves,not necessarily small.SLOW.

…the new and improved MVG or Spitzerized…So ,I reduced areas and ht.,base,and overall thickness.The new Spitzer version in no greater that 1/2"(rear),(front vortex is 3/8"),is a 1/4" shorter.Here’s the winning edge…it’s only 3/16" wide at the bottom base.That means it is no wider than a stock basswood stringer on a clark blank.The shape has changed the most,dramatically.The shape,if you can invision in your mind is like that of a GREAT WHITE’S teeth,Identically…The results,(remember That I’m using these in a CrossXfire set-up) is positive,I get a closer set to my center fin,with less dead time or static drag.Plus it flows better = better performance.

…I took a load of 35mm pics yesterday…I’ll try and get them up within the week.I wasn’t going to say anything… just post the pics and watch heads spin… but what the heck! I’ll make them spin now…no pun intended Mr.Spindler.

…Mark,sorry we lost touch,I’m sure you’re as busy as I am,hope all is well and I like the Threshers,I’ll send you out a Spitzerized MVG to try-out/experiment/copy/market/whatever.And hey,tell your buddy in Jersy,I’m getting things rolling again after a long layoff due to illness…Boy, am I backed up with work!!!Sh*t ,whats going to happen when I die?

…By-the-way.The 35mm roll I completed yesterday has pics of my radioactive mater accident,from a couple months ago(face/skin burn),if they’re not to graphic,I’ll post em.Everyone should get a good look at Cali’s surfing toxic future…It’s been till now that I think I can take a look.Herb

77

HA! Finally, some real info for the less knowledgeable, like me.

Question: Has anyone knowledge of someone trying a thick fin? If not why?

78

Okay is right.

Glad to hear you’re bouncing back.

I’d be interested to see how you shaped the mvg down. I have a new shaped down version myself currently undergoing testing.

Looking forward to the pics.

Sorry, but I lost your email address in my computer hd trouble last winter. So if you would, please shoot me an email, so I can get it back in my book. I’m at

MVG97@optonline.net

later, m

79

Hey Lawless; Welcome and

thanks for the upbeat report.

If you see JH(?) say hey for me

and tell him I mentioned to ask him

for a sample hookup.

80

Yeah,I’m doing better…much better,

…Hold the molds,you might want to go with what I came up with,

…don’t worry it’s for free of course.Herb