Fin theory,tom,halycon and others

General Discussion
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From my understanding of theory, a forward sweep is inherently unstable and will try to rotate itself 180 degrees. Interesting concept for a fin. It would be interesting to see what kind of twisting forces it is putting on the base as compared to a similar fin that is swept back.

Maybe a compromise between the two.

Here’s another take on fins.

I do not think the curve is essential for drive. I make fins with a straight leading edge and they work just fine.

The rake angle, however, is a very sensitive component of fin design. Tilt a fin up 5 degrees, and it changes a lot.

Forward swept fins are not inherently unstable if very stiff and mounted as part of a system. The tip of a forward swept fin is incredibly prone to stalling, which allows the boomerang concept in which the tip turns rearward.

The boomerang moves most of the fin area away from the board, where the difference in speed between the board and the water is greatest. So, aspects of the fin action are maximized because the foil acts on water with a greater velocity. This, I believe, is the key to the Mental template (and the boomerang), and not relieving oneself of the 3/4" turbulence layer. This point is also achieved in the starfin, which has a long chord length near the tip - and also an end-plate.

The fin construction is thus a balance. More fin area further from the board gives better action (for fixed fin area and leading edge rake). It does this because water flows with the board, both in the 3/4" turbulent layer, and also in the next inch or two in which laminar flow moves with the board. So the relative speed difference between board and water is greater further from the board. However, fin area closer to the tip promotes tip stalling. One solution to this is to have a wide tip with an end-plate (starfin). Another is the Mental template in which the fin chord length is longest in the middle, and tapers at the tip to reduce stalling.

99% of all thruster fins do not attempt to optimize in this respect. Chord length is longest at the base and tapered at the tip. I’d be willing to be money this will have a larger impact on fin performance than concaving the inner surfaces of rail fins (instead of keeping them flat).

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Hi Guys

Its good to see that the thread is coming back to life again

Blakestah I agree with most of what you say with regard to rake

Here is my additional two cents worth

The less rake,the more stalling will progress from the tip to the base

In aircraft such as gliders and hanggliders including ridged wing hanggliders the way around this is washout

Washout is a twist in the wing, this increases to a max at the tip

The function of wash out is to give the wing an increasing angle of attack at the tip,this means that when approaching stall the root will stall before the tip

This very simple thing has saved many aviators lives as it reduces tip stalls which lead to incipant and full spins

So if you decrease the rake of a fin to ensure a progressive stall, rather than a complete instant fin stall, you must either introduce washout or change the foil camber as it progresses up to the tip

The down side is that both of these increase drag when the foil is at zero ange of attack

This is where flex can become usefull, but is very difficult to build it to work well at changing speeds and loads

The next area of interest is the leading edge radius

Most assemetric thruster setups that I have seen are far too sharp on the leading edge

If it is a sharp edge it will work well at one angle of attack and will stall very quickly with moderate changes of attack,of course having a large rake angle will tend to help hide this prformance defect.

I have modified at the beach more than one of my friends FCS fins when they have complained that it does not feel right by simply running a key down that sharp edge

I would suggest that the vector fins are achieving better performance by simply having a better leading edge radius and the concave is not responsible for improving performance that much.

Apart from the turning role fins play, they are working quite hard when you are tucked up high on the wave going for the barrel.as at this stage the angle of attack is fairly constant.

The constant pumping that most surfers do going down the line is in part helping a poor leading edge radius by constantly changing the angle of attack,so that at least some of the time by pure good luck the fin is providing perfect lift,which of course is after all thrust ie speed!

Mike

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The constant pumping that most surfers do going down the line is in part helping a poor leading edge radius by constantly changing the angle of attack,so that at least some of the time by pure good luck the fin is providing perfect lift,which of course is after all thrust ie speed!

huh? 1. pumping is simply taking advantage of the potential energies that are avaliable to a skilled surfer to generate speed 2. Pumping is an active, high performace style of surfing 3. Pumping is fun Take your standard 3 fin board out to a flat body of water get on it and start pumping it…how fast are you gonna go? Then repeat while being towed rapidly behind a boat…let the tow rope go and pump…how far are you gonna go? How does fin lift translate to forward thrust?

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How does fin lift translate to forward thrust?

On my system, it works pretty much the same way the tail of a tuna works. The tail bends to establish an angle of attack, and pushes to the side. There is a reactive component of force perpendicular to the direction of travel, and a component IN the direction of travel. You can pump your way through flat spots this way with the side forces on the fin set appropriately.

Thruster rail fins work the same way, although drag between thrusts gets in the way of being efficient enough to pump through flat spots. Therefore the optimal way to ride it is to ALWAYS TURN. You can really juice it off the bottom turn on a well set-up board by pushing the inside rail fin towards the outside of the turn. There is a force component perpendicular to the stringer (hold), and a force component along the stringer (drive, or thrust). On the rail fin, there thrust is roughly 9 percent of the hold. The rear fin is all hold and no drive (makes jack a dull boy). Although much easier to do on a shortboard, I’ve seen longboarders quite skilled at thrusting.

This beats the snot out of a rigid single fin, which generates no thrust at all from the fin because it is aligned with the stringer. It has a drag advantage, but that is usually not enough to beat the thruster, which gives the rider some control over the speed - some acceleration.

For my system, with a slightly different angle from a thruster, the thrust is about 14% of the hold.

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Sabs says, “Most assemetric thruster setups that I have seen are far too sharp on the leading edge”

That’s absolutely right. Easing the leading edge (double foiling it) is clear and way to step up fin performance.

Sabs says, “I would suggest that the vector fins are achieving better performance by simply having a better leading edge radius and the concave is not responsible for improving performance that much.”

I differ with you here big time. The difference between the lift off on of my concaved a rail fins and a flat one is like the differnce between paddling and flying.

I’m not real clear on you’re description of washout, Sabs but the more I look around the more the idea is reinforced that there’s definitely a place for twist in fin design. That’s why I make mine the way I do.

When it comes to rake and holding power, they are inseparable. Fins that are very high aspect may be fast and there certainly is a place for them in big wave surfing but for small waves the have to be pretty big to get anything done unless there are winglets on the tip like one of Cheyne Horan’s Starfins.

Mahalo, Rich

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Hi Meecraft

When you are high in the wave going down the line the water you are riding on is not still,it is in fact going up the face as the wave moves forward

This upward movement is what creates the positive angle of attack to the fins on the wave side of the board

This is much like the keel on a yacht,if a yacht had no keel it would go sideways it is this side ways movment that sets up the correct angle of attack for the keel which now provides lift

So in a yacht the sails and the keel both provide lift,one away from the wind and one towards the wind the result is that the yacht moves forward

So as you say what happens when you pump your board in flat water,answer very little as with a yacht or plane or indeed surfboard what is needed is an additional energy scource,this is what the wave is providing.

I hope this explains it to you if not please tell me and i will try to draw a couple of diagrams

This is the reason why I get so excited about fin development as this is the real accelerator of your board and very few people understand why.

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When it comes to rake and holding power, they are inseparable. Fins that are very high aspect may be fast and there certainly is a place for them in big wave surfing but for small waves the have to be pretty big to get anything done unless there are winglets on the tip like one of Cheyne Horan's Starfins.

Is it the rake or the chord lengths or the fin area that gives hold?

If you change the rake only, and keep the chord lengths and fin area fixed,

hold is affected hardly at all, but the onset of drive with changes in angle of attack does change. More simply, you get more hold with smaller turns if the fin is steeper, but the maximum hold is essentially unchanged - a function of the fin depth and chord lengths.

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sabs, just wanted to point out your description of washout was reversed:

Quote:

Washout is a twist in the wing, this increases to a max at the tip

The function of wash out is to give the wing an increasing angle of attack at the tip,this means that when approaching stall the root will stall before the tip

The washout decreases the angle of attack at the wing tips, thereby allowing the tip (and ailerons) to still be effective as the wing root stalls. For example, if you decrease the tip AOA by 3 degress and the wing root stalled at 20 degrees, the tips would still be creating lift (and aileron control) because they would only have a 17 degree AOA.

In surfboard terms, if you had a fin with washout designed into it, if you were looking at the board from the bottom, the base of the fins would have, say, 5 degrees toe to them and would increase as you reached the tips of the fin, say 7-8 degrees, just as an example. (tips would be more toed-in relative to the stringer, edit: thanks sabs).

Here’s an interesting tidbit I found in a aviation newsgroup:

"Back in the thirties we had learned that the lift distribution on an ideal wing was elliptical.The easiest way to be sure of attaining an elliptical wing without

having to use washout to reduce the lift near the tips, is to use

an elliptical planform. Of course, if you use an elliptical planform,

you do not need to use any washout. In fact, if you do, you merely

louse up that beautiful elliptic lift distribution you obtained by

spending the time and effort to build a wing where every curve is

compound and every rig is different.

Unfortunatly you get eaten by Reynolds number. The key input in the

Reynolds number, that brings in the scale of what you are working

with so that it compensates for the scale effect aerodynamically

and defines the flow you see is a length. For a wing, the critical

length is the wing chord at the point under consideration. The

smaller the chord, the smaller the Reynolds number. The smaller the

Reynolds number, the smaller the angle of attack where the stall

occurs. As a result, with any tapered wing, whether that taper is

linear or elliptical, the tips of the wing stall at a lower angle

of attack than the root."

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Halcyon

interesting observation I just experienced regarding both Cheyne’s comments on stiff fins and your’s on the starfin.

Today for some unknown reason I decided to try the starfin on my 7’11" singlefin McCoy Nugget. I’ve been sticking to the McCoy gullwing fin that came with it trying not to upset the balance but this starfin really caught me off guard…

I’ve been a proponent of flex fins and fins with a larger tip like the FCS Rusty’s or the Paddlefin or SquirrelCutaway and would have never believed that such an ugly fin (straight up and wide profile) like the starfin would work well. (I guess it kind of fits in with the ugly lines of the nugget).

Anyway, with the starfin, I seem to have alot more power off the turn especially standing a bit forward and really surprising was that the board jetted out from my feet every time I cross stepped back from the nose to the tail. In fact it accelerated fast enough to flyout from my feet at least half the time. Definitely a strange sensation but the feeling of speed, control in the white water and on the nose will keep it on the nugget for now. There was some humming at speed though.

Just afraid that it’s going to snap in a bigger day say 4-6’ hawaiian scale.

Anyway it was an eye opening experience and makes me wonder if Cheyne not correct about having really stiff fins with a wide profile versus the stage 4 stuff everyone’s using. But it’s probably a combination of the board design and fin combination that really makes the difference… Geez I don’t know anymore after riding this wierd fin today.

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I recently asked George about this (again)…

His Stage 4 fins were never intended for use with longboards/noseriders. He thinks such an end use is amusing. Instead, they work best with shorter transitional displacement hulls, designs which are ridden from a balanced, central position, and derive much of their holding power from the board`s rails and bottom shape.

Creator of prime examples, Greg Liddle:

"…Greg first learned of the displacement hull designs when he watched George Greenough at Rincon on his Spoon. It was George who was in the process of changing surfing forever with two of his own ideas. His fin designs were modeled after a blue fin tuna, and his revolutionary, displacement hull spoon designed kneeboards, the ones with the flexible tails.

George’s M.O. was simple: make a shorter board to stand or kneel on that turned faster, tighter, went to trim easier, and maneuvered better. Something that was under your feet and you didn’t have to move around on…"

http://www.allaboutsurf.com/0305/articles/sixfeet/

http://www.liddlesurfboards.com/

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What a great thread; it’s scientific, historic, folkloric and funny (at times).

It’s the conversation that could easily take place in the lineup or sitting around a campfire. Half-drunk and stoned.

Somewhat ironic that it’s happening in the aseptic environment of keyboards and computer screens.

Question: do fins need to be somewhat flat or have a broad side?

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Is that an engineering freebody diagram you are proposing? Let’s see it…it will bring back brutal memories from college.

Gents, one cannot isolate particular micro-events (thrust from fins) without looking at the big physical picture - ideally a properly developed and analyzed freebody diagram.

The reality of wave riding: forward velocity/speed comes from the surfer’s ability to harness the potential energies that are available…namely wave energy and gravitational energy. These are BY FAR the dominant sources of energy that help generate speed. Forward projection/speed from fin foil/placement is virtually insignificant as compared to the real potential energies involved.

Blakestah is mostly right…but I have to dispute the degree of benefit from potential forward force vectors of a thruster setup. The reality of it is that a thruster simply has more holding/lateral-leverage and a skilled surfer redirects the downward motion (created from harnessing wave/gravitational energy near the top of the wave) into forward projection along the lowerface or bottom of the wave.

While Im at it…as to the many analogies towards nature (fish tails, bird wings, etc) again one must look at the big picture. Fish tail and bird wings are ACTIVE DEVICES…the action/performance comes from many different elements including muscle and bone structure just to name a couple… fin systems are completely passive devices (except for Blakestahs system).

Thats it for me…Im getting a headache

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sabs,halycon,blakestah,i agree with all of you on certain aspects of what you said,

also thanks to lawless for clearing up the washout …

sabs , halycon!!! i fully agree with softening the leading edge ,plus i also bring the camber as far forward as practical,that way your fin handles a way bigger range in AOA at higher speeds…im sure youll both agree with that especially you sabs …

im gonna post a pic of a fin it shows engineers calipers over the leading edge ,

these are the fastest fins ive ever owned , with the most hold and sensitivity but all with a smoothe flowing feel…

the reason there so smoothe is the softer leading edge , they have heaps of hold , coz they tolerate massive angles of attack before stalling happens…their extremely fast coz of the range of AOA they handle ,their working efficiently even at high AOA so there still driving you forward ,rather than going into stall mode and starting to create drag…

im going to make a bold statement here and i hope sabs will agree , coz i think i discerned a similar thought from your post sabs…

if your leading edge is to thin or sharp it causes stalling to happen at very low angles of attack,but your fin doesnt fail completly because of the template of the fin, the rake resists a complete stall,(like delta wing aircraft)

so coz the leading edge is failing and the fin isnt working efficiently its now starting to cause drag , even tho the fin hasnt completly failed its still creating drag…

its the biggest reason thin fins dont drive you out of turns …and also why so many surfers lose speed through turns…coz their fins are causing drag at AOA …

im also gonna agree with sabs with his comment on the vector foils…

halycon what your noticing is correct as far as freeness and speed …

the folied inside edge makes the outside fin more efficient through a turn ,coz the water can wrap the edge cleaner and stay attached to the flat side better , rather than creating drag if a stalling was happening on the flat side of your outside fin…

plus the concave also reduces the overall lift of the fin (the hold,pulling power)

coz there isnt as much of a pressure difference…

so while your outside fin is now working more efficientlywith less drag …

your inside fin has become less efficient doesnt hook you into a turn as hard and doesnt have as much hold…you might as well run a smaller flat sided fin…

so while the concave gives reduced drag theres a corresponding reduction in lift…

i agree more with sabs on the overall benefits of softening the inside edge of your side fins, gaining a more efficient outside fin without losing the bite of the inside fin…

my overall impression of the vectors is a fin that feels fast and free but doesnt handle high loads and doesnt respond well to pumping or trying to work them for speed…

definatly suited to a less aggresive flowing surfer…who isnt very heavy…

there claim is an increased lift to drag ratio???

i think you could word it as a decreased drag to lift ratio…

theyve decreased the drag and decreased the lift correspondingly…

halycon when i finally post all the results of my recent concave foil experiment and explain all the different foils and combination of foils i used on the same template it all gets pretty clear…

only bummer at the moment is i cant take any pictures coz all my team guys have poached all those fins …but im getting impressive feedback from 3 of them and even tho none of them have tried all the fins , im still getting the right information about the ones they have each used…

1 team guy had a heap of real different fins i made him , with crazy looking templates ,i made him copies of some of my favourite fins ,plus remoulded some fcs to fit futures , he had seven sets in total …

he came back 6 weeks later and even tho he had no idea how things work ,he gave a remarkably complete description of what each combination did ,where it worked best and where it didnt work,it really impressed me it was some of the best feedback ive ever had …totally unbiased…

alot of the time people think a fin will respond in a certain way , so there not looking at the fin objectively , almost wishing it to do something it wont…

time has caught me again…sorry blakestah i was gonna address the vertical leading edge thing versus rake ,i agree with halycon on that one …

ok im out

regards

BERT

ps the reading on the calipers is just over 14mm

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First a comment on Future’s treatment of Bert:

If you pick up a starving dog and make him prosperous, he will not bite you; that is the principal difference between a dog and a man

–Mark Twain

There’s a lot of theory flying around right now on this thread.

As a result we can go off on this tangent and get out of the circle of performance very easily. I.E. Trying to isolate on part of fin geometry and making statements about it’s overall effect on performance. That is cord length. The only way to really determine anything is to take a fin with exactly the same profile and outline and just change the rake. unfortunately this can’t be done because as you change the rake the root cord will change so the experiment looses it’s objectivity immediately. Theory is fine up to a point but when it comes to surfboard it is never cut & dry.

I sent a spinner/mental set-up out on 9’5"x22.5" Junod and got some sterling reports on it yesterday. The words were simply “control, smooth transition and speed.” The board can do more things ~ slip through the water more easily, bash the lip and I can still run to nose and holds in fine even with the realative short depth of the center fin. It all works better than anything else I’ve ever surfed on this type of board. – Photos to follow later today –

So next I’m going to scale the set-up down for a short board and see what happens.

Bert: try this:. Take a piece of the ply about 24"x8" that you’re making your rail fins from and pull it into an arc. take a photo and send me a photo in an attachment on an email to .

I want to discuss something about fin construction with you.

Off to work, Rich

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…so while the concave gives reduced drag theres a corresponding reduction in lift…

i agree more with sabs on the overall benefits of softening the inside edge of your side fins, gaining a more efficient outside fin without losing the bite of the inside fin…

there claim is an increased lift to drag ratio???

There is empirical data on airfoils to show that an increase in camber (what’s been called inside foil, concave) corresponds to an increase in lift. Their claims of an increased lift/drag ratio could be correct although the Future “Vector” fins I’ve looked at had a less than optimal shape to them. The FCS “F” series fins have a much better cross section which looks like it is actually based on a NACA airfoil. I was intruged when I heard of the “inside foil” fins coming out because everything I’ve read regarding wing theory would point to them having more lift and less drag. But, (a BIG but) as we all know some things just don’t correlate well when you apply them to surfboard fins.

As for what sabs said regarding the leading edge radius, you are absolutely correct. By spliting the leading edge 80/20 you are allowing much better laminar flow across the inside of the fin, making the fin that much more effecient over a wider range (think full rail cutbacks).

http://adg.stanford.edu/aa241/airfoils/airfoilhistory.html

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Jim Phillips propeller fins; pictures posted by Lokbox elsewhere.



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blakestah,

In would be extremely difficult to defend a patent that claimed concavity in injection molded fins. The physical properties of thermal set resins is such that they have a tendency to shrink as they cool . Thicker wall sections sink more than thineer wall sections. Consequently, all injection molded fins have some degree on concavity and twist to them. We’ve just worked really hard at minimzing it. So, prior art will make defending such a broad claim impossible. And, making a claim such as the application of any NACA series foil to any traditional template won’t fly either. So, at best they may have a very narrow design patent. But, since Future’s haven’t even supplied they patent information I requested 6 months ago, who knows what they are attempting to claim.

As far as the 80/20 leading edge is concerned. Red X had started offering the majority of it’s templates with a 80/20 leading edge for those that don’t want to fine tune their own fins. See the attached picture of our 80 gram laminted 80/20 fins. But, we will continue to offer our original composite templates with sufficient leading edge material that you can foil up to 95/05 leading edge for more to push off of and quicker direction changes.

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Well, they could easily specify that all such resins tend to shrink, and specify a limit on the bounds of the shrinking, and patent concavity above that limit. Or the like.

More to the point I was trying to illustrate the aggressive patenter’s approach than anything else. There are a lot of details involved in making such a fin, I would presume they are trying to get as broad patent coverage as possible for leverage to increase market share. In their shoes, I would specify all the details involved. Then, figure out which ones had prior patent art, name the patents, discuss them, and which details are unique. The patent covers the unique details without prior art, provided they are non-obvious (and at the patent office, virtually everything without prior art is “non-obvious”)

That doesn’t mean or imply they are knowingly patenting things that have prior art - part of the inventor’s job is ensuring the patent specifies the relevant prior art, and how the current invention is non-obvious in how it differs. The coverage only applies to the new aspects of the invention.

I’m also pretty sure no patenting agent would send anyone a copy of a patent that hadn’t been granted yet. The backup at the patent office is several years right now, so I think everyone will just have to guess for a while.

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blakestah,

    I've seen some pretty sunken and warped fins in my time. But, one of the most interesting shapes I've seen recently was a rotomolded wind surfer that was left with it's tail hanging out of a sabot on a shore morning for who knows how long. But, you want to talk about reverse tail rocker. This thing sagged like 8" plus.  



     I know what you mean about patent back log. I've got one pending now. The last two I had granted only took about a year. I've got to believe that their patent claim is very design specific. And, I still believe that concavity is not the end all cure all for what ailes you. 



     Optimization has a tendency to improve specific tendencies for ever narrower ranges of variables. We can design something that works great for ideal scenarios and that sucks every where else. There is something to be said for general purpose or having a broad performance band. 



     More than anything I think Future's market surge has been from really good hype.
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More than anything I think Future's market surge has been from really good hype.

Marker surge comes from a lot of things. I regularly watch shapers push Futures on surfers that otherwise requested FCS, the shapers I know do this because the reps treat them well AND because they take less time to install. They tell the surfers it is stronger/stiffer than FCS, and otherwise equivalent in function. But that isn’t why the shapers choose them - that is a function of the sales rep and ease of install.

I’ve even told them (as JJR mentioned) that Lokbox is supposed to be easier to install yet, but they seem too happy with Futures to care.

Another part is the FCS reps aren’t quite so good.

But from my vantage the change in selection for local boards is driven by the shapers choice.