MVGs & Superchargers

General Discussion
21

I’ve been thinking about lift and low aspect ratio wings. I wonder if you’ve noticed differences in MVG effects that depends on aspect ratio. As in, do they work better (even subjectively) in front of cutaway or wide base fins? In front of fins with lots of sweep, or those with little sweep? Have you tried them in front of a Starfin?

Swept low aspect ratio ‘wings’, like a surfboard fin whose aspect ratio is typically under 3, at Reynolds numbers comparable to those in surfing, generate lift by generating vortices that roll down the swept edge. These create a ‘suctioning’ effect by both separating flow and yanking with a vacuum on the low pressure side of the leading edge. If MVGs interact with this effect, you might expect them to work better on more swept and lower aspect ratio fins.

22

Please post the longer explanation here for all Swayaholics.

Private Messages are neat and useful, but I suspect that some very useful information is bypassing the normal channel. Swaylocks does feel/seem different after the “upgrade.”

As Keith said we need more of this sort of discussion…

23

I’m becoming more and more convinced the model for the fin in hydrodynamic terms is that of a low-aspect-ratio foil with a swept leading edge. As such, the lift is strongly contributed to by a leading edge vortex on the low pressure side. This vortex increases velocity on the low pressure side without separating flow by the trailing edge, and is a large factor in delta wing design.

In particular, thin low-aspect-ratio foils, in the range of Reynolds numbers used in surfing, work this way. The vortex is responsible for much of the lift.

There are leading edge extensions on some jets (some US military jets) to more reliably initiate the vortex. I find it entirely plausible your MVGs perform in this manner. They are just another form of a leading edge extension that interacts with vortex formation.

http://adg.stanford.edu/aa241/highlift/sstclmax.html

http://aerodyn.org/Wings/larw.html

With this model of foils, tip stalling for higher Reynolds numbers becomes really important, which may be why faster surfboards all have low tip areas and narrow chord length tips. It also explains why surfboard foils are thin relative to high-aspect-ratio foils - reliable generation of the leading edge vortex is aided by thin foils, often 5% or less thickness relative to chord length.

Just food for thought. The thought that a recent poster put in my head, that surfboard foils should be 12% thickness, and shaped like NACA foils (which were all designed for high aspect ratio wings) has been itching at my brain.

24

Mark,

Feel free to add as much detail as you would like. This is precisely the kind of discussion we love here. Welcome back!

Lee

25

I’ll need a little time to get caught up with today’s posts. And if I could may I have a little time to reread my stuff and check for relevance. I must have hammered out about 20 p. Maybe very little of it is worth reading.

26

Blakestah,

Interesting. I can see that vortex rolling down the leading edge: A jump start perhaps for the venturi effect right behind it on the venturi part of the wing? Another one of the things we considered was the fact that the new removeable fin systems (this was the mid 90’s pre C-5 for reference) didnt have fillets like the glass-ons did. The abrupt leading edge of the FCS fins had to be causing drag. The Navy was at that time spending million$ installing fillets in front of the conning towers of it’s subs for speed and stealth. I invented a small retrofit fillet but never bothered to make a prototype, because we hoped that the draft effect of the MVG would help in that regard. But, now that you mention it perhaps MVGs would work even better with glass-ons because the sweep is greatest where the glass-on fin meets the board.

And once the vortex gets rolling, there it is.

By the way what Reynolds number did you come up with for surfboard fins? I cant remember what we used and my notes are filed.

Many thanks,

Mark

27

5 inch chord length

20 MPH

about 1.4 million (borderline for transitional flow)

The vast majority of paddle-in surfing is below this, I believe, safely in the region in which high aspect ratio fins will have well-described flow. Especially if you consider 5 inch chord length is kinda long for most surfboard fins (except bigger singlefins, even then only at the base). It decreases linearly with chord length and speed.

With swept wings and low aspect ratios, though, leading edge vortex formation creates lift not described by normal linear hydrodynamic theory. For some combinations of sweep and Reynolds number, the effects are huge. I haven’t figured a good way to theoretically translate the results to fins yet, though, mainly b/c most of the aerodynamic theory revolves around how swept wings deal with crossing the sonic barrier - something not applicable in a much less compressible fluid like water.

My best guess is that the most common sweep angles are close to the best sweep angle for vortex formation/lift in surfing - figured out over decades of empirical study.

Good surfing to you.

-Dave

28

I seem to recall we came up with a range of 1.2 to 1.3 or roughly about the same as a model sail plane.

You know, at supersonic speeds, air may be about a compressable as water at 20 mph. Water is afterall 800 times denser than air. So, maybe you can use the theories you mention. I’m thinking of the old bumble bee contradiction. About 20 years ago physicists theorized that bumble bees defied the laws of physics and should not be able to fly. Well, somebody forgot to tell that to the bumble bees. Then, one day they realized that to a bee air is a thick viscous fluid, so they revised their theories. You may just be right on track.

29

No, rather, I meant that the wing sweep was investigated in context of dealing with sonic booms, which are a result of air compressibility. I know of no such phenomena at speeds relevant for surfing. I was not suggesting the vortex effects were irrelevant - just that the investigations usually plot lift coefficients (and lift/drag) in terms of the Mach numbers of the air, and not the Reynolds number.

However, in concept at least, the vortex theory suggests

  1. Lower sweep angle results in a more rapid rise in lift with changes in AOA,

    and a lower stall angle.

  2. airfoil geometry, in particular foil thickness, is largely irrelevant

  3. lift is highly concentrated at the leading edge, much more so than for high aspect ratio foils

Together, it suggests, that for low aspect ratio swept fins, you should start with a large enough fin. Then make it steeper and steeper in sweep until you start to lose it with stalls. Then, back off a little. Don’t sweat the thickness much (although there is some gain from a convex/concave combination for a wing that generates lift in one direction). Remember, the Concorde has a really thin wing, and it flies just fine. And position fins based on leading edge position relative to the hull.

These theories do not, obviously, apply to all fins. Flex fins work on different principles. High aspect ratio fins are also different. They apply to swept low aspect ratio stiff fins.

30

Ahh, the differences between designing wings for supersonic trans-oceanic passenger jet aircraft and surfboards.

Seems to me the ol’ tried and true ‘form follows function’ needs to be applied here. The wings and fins mentioned above are all meant for straight line high speed use. Surfboards are slower and need be able to turn on a dime.

Interestingly, though, I think you have theorecically described wake and kite board fins. It’s good to know, that a theory can be used to describe something that was born through repeated testing. I’ve querried my brother on the water equivalent of the sonic boom. I seem to recall we discussed that years ago, but I cant remember much more than that. He’ll probably come up wth something interesting, though. He always does.

Next, I’m not very good at this, but maybe, if it’s at all possible, apply the vortex theory to surfboard fins and see what you come up with. Do they stand up? Or, even harder still, can a surfboard fin be designed from scratch based on this theory? I’ve never been able to do that. The best I can do is observe something in nature and then try to figure out how and why it works.

To that end, I think that maybe the tuna caudal fin is possibly best able to utilize the vortices based on the observation that it is ribbed. And according to all the research I’ve seen on how they generate speed it’s all about the vortices they can create all over the place and how they roll them down their sides and then how they push off of them with their tails.

Here’s some good light reading. grin

http://www.biolbull.org/cgi/content/abstract/200/1/9

The wing vs fin design contradiction has recently come to a head for one manufacturer. One problem that Futures made for itself was taking wing/fin designs from NACA and applying them more or less directly to surfboards. The cambered foils were originally for either aircraft or for torpedo fins. The Vector was the result. The problem as I see it is too many steep and deep curves that in water produce too much of the the desired effects. The problem, they fight the turns and produce drag and tear right off the boards in some situations, usually when you are in surf big enough to hurt you. Not a good scenario. I think the reason the fin hit the market with this problem unresolved, was haste. They only had a year. Honestly, I dont think they came un undeer that either. I’l tell you later why that matters.

I will, however, defend the concept of the cambered fin, because I’m the one who introduced it to the industry. The reason Rusty is involved is because I sat down with him for nearly an hour in September 2000 and discussed fins with him. That was about six months after the AB3. First, we compared stories on how we each came up with our ideas; for the him the C5 and me the MVG. Here’s something most of you wont know, so please dont forget who you heard it from; Rusty’s original C5 fin, the half moon fin, is something for which he got the idea from the tuna, not unlike me. He told me that it was the small half moon shaped fin that inspired him. According to one of my team riders Greg Sakowicz who is best described as a fish design scientist it’s called the caudal peduncle and it’s mid point on each side of the caudal fin. But one of it’s jobs is to evenly split the flow as the caudal fin flips side to side. There may be other uses, too

Anyway, next, I freely gave him my websites and links and research, and I talked with Rusty about how my brother says there should not be flat sides on the thruster fins, that “water doesnt like flat surfaces” which I have often repeated as “water hates a flat surface”.

I hear other people saying that now, too.

I went on to describe how a slight convex curve would help attach the water flow and make the fin faster. I taught and he took notes. No kidding. I drew air diagrams etc. It was cool. Deep down, I wanted to see what he could do with my ideas.

Actually, I had already had this discussion with many other shapers, but it was R. who is also a marketing madman, who saw the potential. Credit to R. for that. (MY Grandmother always said the Lord helps those who help themselves.) So, I was not suprised to hear it was R. who was teamed up with Curtis. And I was not too shocked to see words quoted directly from my MVG packaging used on the Futures brochure technology section. No, it was no suprise to me that it was Curtis that he took the idea to. That too was the only course. YOU dont go anywhere in this world if you can not seize the opportunity. Carpe diem.

Anyway, they only had one year to file a patent on the idea from the minute I told Rusty, at least as far as they knew, but like I said I’d talked about this within the industry before, but for them that was the precise moment when they were introduced to the idea and believe me Rusty is the industry. Anyway, I see no reason anyone should be afraid to make cambered fins. If anyone starts trouble call me. I’ll testify. I have emails going back to 1997 with a govt stamp.

Best of all I have already designed the next generation and, theoreticaly at least, it solves the problems of the cambered fins. That one is still safely stored in my head.

(rant) Such is the nature of the dilemma Bob Simmons and many inventors face. Once, you see others who at first trash your ideas, suddenly copying your ideas and making money with them, you are forced either into a cycle of having to both defend their work and coming up with something new to top it, or self destructively fade away. Usually, it’s a combination of both (cyclically). Anyway, this usually, comes across as arrogance, but seeing others profit from ones research work hurts. I dont know if anyone can understand the tension unless they are actually in that position. I find I’m forced to live somewhere between elation and depression. I try as best as I can to take the middle road. Bob Simmons, on the other hand, I believe took the whole thing way more seriously and intensly and forced himself into pursueing his ideas singlemindedly at such a fevered pace that, one way or another no one could have sustained or survived. My connection to Simmons is Yater’s shop bought MVGs and Renny rode for Simmons. Technologically speaking if you believe Simmons to be the guru, and I do because he laid the foundation, well then that made me.

For now, as I see it, the better fin is the new FCS version, and the OAM is probably the best, based simply my looking at them. I dont think the market will decide which design is best unless they are all made to fit all systems. And even then the market for surf fins is a very strange place. But keep an eye on what Taylor Knox is riding.

Well that does it for my morning. I’ll catch up tonight after work. peace

31
Quote:

Interestingly, though, I think you have theorecically described wake and kite board fins. It’s good to know, that a theory can be used to describe something that was born through repeated testing. I’ve querried my brother on the water equivalent of the sonic boom. I seem to recall we discussed that years ago, but I cant remember much more than that. He’ll probably come up wth something interesting, though. He always does.

Next, I’m not very good at this, but maybe, if it’s at all possible, apply the vortex theory to surfboard fins and see what you come up with. Do they stand up? Or, even harder still, can a surfboard fin be designed from scratch based on this theory? I’ve never been able to do that. The best I can do is observe something in nature and then try to figure out how and why it works.

Well, that was my previous suggestion. The rake angle defines the increase in lift with changes in angle of attack. This is known already by many surfers - steeper rake fins have more drive. They also stall at lower angles of attack (again, known by most surfers).

But it further suggests foil thickness is mostly irrelevant. Delta wings that operate in a constant AOA work really well as flat plates, not much better as thin foiled sections, and worse as 12% thick fins.

This is why I really doubt using NACA foils is useful. They are all based on 2-D theory that is relevant when aspect ratios are above 5. For a surfboard fin, that aspect ratio corresponds to a 10 inch fin with a 2 inch chord. People have designed fins like that, but for the most part the vortexes cannot be avoided.

Also, the strong prediction from NACA foils is that max lift occurs at 12% foil thickness. But surfboard fins average a little less than half of that. And if you make them thicker, they do not feel like they generate more drive (except at very low AOAs).

The rake of the fin is critical to how much drive you get as a function of how far you turn. Perpendicular fins generate the steepest lift:AOA functions, but also stall more easily. Working through a rake series would allow optimization of drive in the fins to the surfboard, and the prediction is that you will see a lot more tuning with changes in rake than you will with changes in

  1. base root chord length

  2. thickness

  3. concave inner surfaces

Another prediction is that boards that reach less AOAs (like longboards) may benefit from steeper fins, because you do not reach the stall AOA like you would on a shorter board.

As far as using concave inner surfaces to side fins, you will find shapers who did this in the 1970s. I doubt there is protectable IP there. Certainly this works better when the concave surface is the high pressure side (like the bottom of the Concorde, which is concave, or the inside of a rail fin during a turn). But what happens when the concave surface is the low pressure side (as it is when you go straight on a thruster) ???

Quote:

Here’s some good light reading. grin

http://www.biolbull.org/cgi/content/abstract/200/1/9

Tail fin on a mackerel. Aspect ratio 1, sweep angle 45-50 degrees. It definitely fits in the regime of low-pressure-side leading edge vortex foils. Thickness as a percentage of chord length 1-3%.

I would have liked to see them cut the finlets off and measure the change in thrust (or use a dye to observe vortex flow from finlet past the tail).

Quote:
Anyway, they only had one year to file a patent on the idea from the minute I told Rusty, at least as far as they knew,

They are pretty much ripoff artists if they filed a patent on an idea you told them (not to mention copyright thieves). That is another reason Swaylocks is a good idea.

You have one year to file a patent on an invention YOU generate. You NEVER have a chance to file a patent on an invention someone discloses to you. Failure to properly disclose inventors is one of the principal reasons to revoke a patent entirely.

32

does anyone know where these fins can be purchased via the internet. None of the local shops in NY have them.

33

In reply to Blakestah, whose opinion I respect, I see all the technology in offshore sailboats focused on high aspect ratio blades. Rudders and keels. Why would you think low aspect ratio is where surfboards ought to be? I’m thinking they are throwing untold millions at the question and all ending up high aspect. Your thoughts? thx

34

I wasn’t so much thinking of where surfboard fins should be, as commenting on where they are. Shortboard fins have base chord lengths close to 4.5", and almost equivalent depths. Almost no matter how you calculate it, the aspect ratio is 2 or less. Longboard fins often have 5-6 inch bases, and 9-10 inch depth. Rake in fins runs 35 degrees on thruster fins, and 25-30 degrees on longboard singles.

All of these qualify as low aspect ratio, swept, fins. Now, why would you want that???

A low aspect ratio swept fin has incredible stall tolerance. High aspect ratio fins DO have better lift:drag characteristics, but they also stall at dramatically lower angles than swept, low aspect ratio fins. In flying bird terms, the acrobats have shorter, more swept wings, and the distance birds have much less sweep, and higher aspect ratios.

Surfboards fall in the acrobat range (although not as much as wake and kiteboards, as Mark noted).

As you increase the rake, you increase the stall angle, and the angle of maximum lift (just before stall). So lower aspect ratios allow higher angles of attack. The more acrobatic you are (in terms of turning the board hard), the more rake you need to stay stable.

The rake also decreases the slope of the AOA:lift curve. So, with more rake, you get less drive at any given angle. The tradeoff is the stall angle has been increased. For longer boards that do not turn as fast, less rake may be used (if you look at longboard fins and shortboard fins for rake, you can see a 5-10 degree change). Again, this is more commentary on the way things are, not necessarily the way they “should” be.

Back to the question: why not unswept high aspect ratio fins? I think the answer is that you cannot turn very hard before you stall, and that makes them unsuitable for surfboard applications (as stiff fins at least - giving the fin some flex changes things). There are some high aspect ratio, largely unswept fins, that have been used in surfing. But for the most part they didn’t stay in favor.

I also surveyed some lifetime longboarders. I can show them a fin, and they will tell me if it is raked enough, or not enough, or too much. Within a few degrees, they are highly consistent, which suggests they firmly recognize the amount of rake their surfing style requires.

Now, at least one fin company has thrown a LOT of money at surfboard fins using 2-D flow profiles. I wonder if they started by looking at sweep angle effects and stall angles?

35

Blakestah,

There it is, you said it all.

My brother found something interesting today. I’ve attached a link for the release of test results I’ve been waiting for. It was reported several years ago that there was a possibility that the bumps on the Humpback whale fins might have some purpose. This study is very interesting. (see link)

And finally I have to add something to one of my statements. The caudal peduncle is the place, but the actual half moon shaped appendage is called a keel. There are two, one on each side.

36

Gentlemen,

First, I do beleive the mini forward draft fin has its place.

However, I have some basic questions for you experts:

  1. Has anyone empirically determined and mapped water flow vectors under a surfboard under real conditions?

Seems to me that angle of attack is critical for these things to work as theorized.

Water flow is hardly moving parallel to the stringer and is constantly changing direction is rail to rail surfing. In longboard trimming in the face of the wave, the water is also moving at angles. (I digress…I find it amusing all this fuss about fancy bottom schemes…concave bottoms etc etc…they only work as theorized when water is flowing parallel to the stringer at higher speeds…a rare bird indeed).

  1. Regarding some of Rusty’s C5 board loosening marketing claims: the “cleaning up…laminar flow…smooting…overdrive” claims…why so much emphasis on the outside rail fin when it’s the inside fin that’s doing most of the work during a turn?

I just love that “overdrive” claim…WOW forward thrust from a little fin attached?

Hardly. Reduction in drag? Perhaps…but thats not what is being promoted.

Want to feel real speed? Take the forward fins off a thruster and go surf. In terms of speed, the best fin is no fin.

It is evident by most photos that water is flowing at a sharp angle (assume 20 degrees for now) from the inside rail to the outside rail. Thus a C5 or supercharged setup is merely creating a draft for the main inside rail fin and not smoothing out anything. Looking at the inside rail fin system and angle of attack, the net result is a rail fin that feels as if it has been moved forward and hence loosening the board.

Test for yourselves: Take an old water hose and cut off the end fitting…this creates smooth flow from the hose. Set up your test fins on a board. Assume a realistic angle of attack…this is most critical. Turn on the water at full flow and place the hose end 6 or so inches ahead of the fin system your testing and observe what happens. Repeat with and without MVGs and Superchargers. If your really resourceful, place the test setup in a shallow bath. I have had my own observations.

I also made and surfed on a superc set up…yes the board was looser and it was fun to ride but it just didnt feel 100% right to me…like Id lost forward drive. So I began to think…then ran the test above and and better understood whats going on. I would get the same effect if I used smaller front fins (which I use on a semi-regular basis) and move them forward.

Until question number 1 is addressed accurately, we are just having theoritical discussion…which is just fine for Swaylockians.

Your responses are appreciated, thanks.

Dave

37

As a member of the academic community, I’m going to jump in here and say that this would make for a GREAT dissertation topic for an aspiring PhD.

Laminar flow is often studied in situ in large plumes.

Why not put a board in some kind of engineered plume or some other controlled environment such as a wave tank/pool where all conditions could be set and sensory equipment could be hooked up to the board?

There are PLENTY of great surfers out there who surf in a controlled manner who would make great test pilots. You could perform several repetitions with the same surfer and you could randomize error by using several different test pilots.

The questions have already been posed.

Now all that’s needed is a willing hydrodynamics laboratory to sponsor the study!!

38

Josh,

There is a cat in a bag somewhere, but I’m not saying any more than that. (knowing grin)

I seem to recall someone did surf a test tank wave. Or did they?

No reason to stop there. How about a flowrider with a clear Lexan wave wall that you can stand behind and watch the way water flows around the flowrider board? Or how about a dozen or so cameras mounted on the ocean bottom all around under Teahoupo, pointed up, that can film the bottom of the board as it goes by. Then put (silver/or black) mylar streamers (tinsel) on the bottom of the board. With computer editing you can get a continuously centered image from a number of cameras to recreate a single ride. That would be sick.

I’m not being facetious. Count me in!

39

Hey you guys!!! This is the best. How about a brief simplified “glossary/definition” of some of these terms for those who may not know what your referring to - then we can all share in the love and knowledge, e.g., Chord length, Reynolds #, etc.

40

Blakestah,

   Recently, sailboat keels have been transitioning from lower aspect ratio foils to higher. The boat I currently race on has a root cord length of three feet and a draft of nine feet. This boat, for it's size can point closer to the wind at higher speeds than any older generation boats I've experienced. But, at lower speeds it looses it's ability to create lift much earlier than boats with lower aspect ratio keels. 



   Accelleration and decelleration are vital characteristics that can often be over looked when you are just talking about maximizing lift and minimizing drag. Sure a high aspect ratio fin maybe more efficient at higher velocities. But, it may fail miserably just when you attempt to pump out of a stall. 



   Flex also has a major affect on the efficiency of foils. I have been testing carbon RTM cored fins against hollow glass filled nylon fins of the same template. We were able to make hollow fins by injecting nitrogen into our molds simultaneously with the glass filled nylon. I found that both worked great until I started pushing into bigger overhead conditions. I just could not get the drive out of the hollow more flexible fins that I could out of the stiffer carbon fins. 



   I've also been testing 70/30 and 80/20 leading edge contours and found the closer you get to a symetrical foil less drag is created. But, at the same time less lift is generated. The result is less to push off at lower speeds. But, a smoother feel at higher speeds.