More fin theory

General Discussion
1

Okay, I have something I want feedback on.

Fin shape, principally fin trailing edge.

Now, I made this template, and made it into a fin, and have been riding it. The squares are 1 inch on a side.

To make it, I started with an elliptical wing. Why, you might ask an elliptical wing? Induced drag and lift. A long time ago, the theory was put forth that an elliptical lift profile minimizes induced drag, and all elliptical templates have elliptical lift profiles (from base to tip).

Note: induced drag is caused by the tip eddies - currents that run around the end of the fin from high pressure side to low pressure side - it is the rationale for having end plates of one sort or another, for increasing rake at the tip, or for reducing chord length at the tip.

Note: chord length is the distance water travels from leading to trailing fin edge - the length of a line parallel to the bottom of the board.

Anyway, I digressed. must be the beer (IPA). Anyway, I started with an elliptical wing. I adjusted the chord lengths. Then I raked it. Why rake? Well, I wanted to generate lift across a range of fore-aft positions to give the fin stability and reliability. I re-adjusted chord lengths to my liking. Then, I added a cutaway. The rationale for the cutaway - a diversion from the elliptical wing, is that the water near the board is being dragged along with the board, and cannot create lift efficiently like water further from the board. There is, of course, some worry about causing induced drag in which water about 2 inches up from the base will flow through the cutaway, so I kept the cutaway angle modest to minimize this effect. Cutaways that run nearly horizontal and are large would be bad for induced drag.

So, we are left with a raked elliptical wing with a cutaway. And, as I mentioned, I’ve been riding it.

So I wonder, what’s the big deal with the more typically concave trailing edge templates? You can clearly see my fin template has a convex trailing edge. But, nearly every other fin is concave, running towards the tip.

I also made a thruster prototype, which I have not ridden yet.

So, I ask these questions…

Why should the trailing edge be concave instead of convex?

How do you think my fin rides? (this is a trick question, I know the answer already)

Induced drag becomes an issue mid-turn. Does a fin with a concave or convex trailing edge have more induced drag in the middle of a hard turn?

Sorry if I seem like an ass in this post - after a few beers, it comes naturally…seeing if my experiments can stoke some debate…curious to see if anyone else has mucked with fin templates with convex trailing edges.

Note: humpback whale pectoral fins have convex trailing edges.

Note: so do Orcas

Note: so do Right Whales, and I surfed with one last year (he was a much better surfer than I)

Note: I know it looks funny. But I started with theory, modified it for two reasons well-known in surfing, and ended up with something funny looking. And rode it.

Time for more beer (ummmm, beer - HS)

2

Hi Blakestah

Good to we are back on the subject

A couple of questions fin (a) I gather is for a single fin setup dare I ask in a whisper what size/type of board you tried it on

Fin (b) the same questions

The convex trailing edge Im not sure that it makes a huge amount of difference

However where the tip turns into the trailing edge I think does

I have found that if it flows round in a smooth line the fins tend to cavitate

earlier than when there is a defined break line between tip and trailing edge

Mike

3

Both are for 6’6" and shorter boards. I’ve not ridden the second yet, so don’t know how it will perform. The first I’ve ridden in a rotating single fin setup.

4

I should say part of this was motivated by others successful use of elliptical foil fins (Horan and the Starfin), my experiments on rake, and the overly pronounced tip on most single (and thruster) templates.

If you plot chord length vs depth on many fins, you will find the standard tip shape keeps a long chord length out very close to the tip, before there is a pronounced drop in chord length. This profile occurs as a result of the concave template trailing edge. It is predictably horrible with induced drag (I think it more likely the tip shapes are for washout and flex-action).

The convex trailing edge lets chord length drop off more gradually in an elliptical profile, which is theoretically, and demonstrably, optimized for minimizing induced drag. I would encourage anyone interested to make a fin matched in chord lengths and depth with their current fins, but with a convex trailing edge, to see if you observe what I did.

5

Blakestah,

I want to attempt to answer your question “How do you think this fin rides?” just because I want to know the answer. I don’t claim to understand the science behind all this yet. Over the last few weeks I have been following the fin posts and I have a huge amount of respect for all the contributors. I am just learning so here is my best shot and please go easy on me…

I would imagine that the fin has large amounts of drive. It seems that the convex trailing edge keeps a lot more area through the tip of the fin (would i be correct in saying the chord length stays pretty consistent through the entire template?). Also like the star fin, I would think your fin might be more of a tendency to pivot than carve… which forces one to rely on the rail for full turns.

Am I close?

6

The pivoting, I am fairly certain, is a function of rake.

The rake over the first 5 inches of depth was carefully chosen to match the common rakes used in the more generic looking single fins offered by major manufacturers. So, no, it is not pivot-like compared to other fins.

The chord length, however, is much LESS consistent over the fin length than that offered by major manufacturers. I carefully plotted it to peak at 2 inches of depth, and then slowly decrease throughout the fin depth following the ellipse profile. In sharp contrast, the more generic singles (and thruster fins like those of FCS, Futures, et al) tend to drop from the base chord length to a different chord length at 2 inches of depth (usually 3.5 to 4.5 inches), and then stay VERY CLOSE to the same chord length throughout the rest of the depth until the chord length abruptly changes close to the end.

This is the crux of the issue. Are you better off with chord lengths that slowly decrease, following the elliptical curve, or are you better off with classic surf fins, that have a nearly constant chord length over much of the fin, before an abrupt drop-off. How does this change performance??

You can adjust the drive as you like by scaling all the chord lengths together. I would just post my answer, but I am an N of one, and biased. So I am curious whether anyone else has previously followed this theoretical/empirical approach and tried a raked elliptical template with a cutaway (or without a cutaway).

7

“So I wonder, what’s the big deal with the more typically concave trailing edge templates? You can clearly see my fin template has a convex trailing edge. But, nearly every other fin is concave, running towards the tip”

I’ve been wondering this myself for a while…

Are the mfgs simply mimicking nature (dolphin fins)?

Will the market accept radical departures from the current standards (business decision)?

Are the traditional long bases a function of the strong adherence requirement of glass-0n fins?

Why not make straight trailing edges like most airplane wings?

Im interested in your experiments on rake and curious to know why you chose a traditional single rake. Does a sloped rake reduce pressure drag?

Finally, what are your performance goals from this template?

Mysterious post…

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Quote:
Im interested in your experiments on rake and curious to know why you chose a traditional single rake. Does a sloped rake reduce pressure drag?

As I mentioned earlier in this thread, I think the long rearward tips of fins are there either for flex action or to provide downwash through flex.

If they are there for flex I want to get rid of them, because my system provides its own flex in toe-in, and it is far easier to control flex from the box than in the glass.

If it is there for downwash…I want to know if downwash is better with an elliptical foil or a raked tip. Theory says the ellipse wins, but ya gotta see it in the water to believe it.

I more or less think pronounced rearward fin tips at there to flex in toe-in, and are actually bad for induced drag. That means they cause more drag in the middle of the hard turns than a more elliptical tip profile (excluding flex considerations).

I somewhat decided that the rake claims by tomatdaum and halcyon were correct. More upright fins are slightly stronger, and a lot less consistent/pivoty. Of course I could have accepted this a priori, but I like to see it in my own hands so I have a more intuitive feel for it.

Quote:

Finally, what are your performance goals from this template?

The goal is simple - more turning power and less drag. I want to lean into a bottom turn and feel the power, feel the acceleration into the turn. When you get that, you know you are doing something.

Halcyon sorta kicked me in the ass a few months ago (I don’t know if he knew he did this). He said something to the effect that I was using very generic fins, and that it was possible that I could improve function substantially by playing with the fins. So I ordered some from him, and at the same time starting making my own. I wanted to use unusual and normal templates to see how changing the template impacted the ride - in noticeable steps. The fin template above is the clear winner. Better than my other templates, better than more standard looking fins. But the fin wars are not quite finished. So far I’ve just explored rake, chord lengths, elliptical foils, and cutaways. Camber remains a big variable, what would happen if I add another 4 mm in camber to the wide point at 1/3rd back…so far they’ve all been rounded front edges, smooth tapers to sharp rear edges (sanded just blunt at the end), and wide point 1/3rd back. Also, I haven’t fine-tuned rake yet and chord length yet, either. It is possible I could improve the above template by reducing rake 2-3 degress and losing 1/2 inch of chord length…but I am happy with “the big picture” in my head, the details I can take my time with.

9

Hey Blakestah,

I’m gonna keep this one short.

Firstly, What does this mean? “More upright fins are slightly stronger, a lot less consistent” I’m in complete agreement that they are more pivoty.

“More turning power and less drag” Yep, that’s what it’s about.

I think you’re going to find that the fin that works best with you rotating system isn’t going to look at all like what will work best with a stationary single fin. I have know idea what it’ll end up being but I’ll be thinking about it today when I’m foiling some pretty radical, though traditionally templated, thruster set that’s on its way to Costa Rica for trails. There going in FCS boxes. I hope to hell they don’t blow the board up.

You’re so right about some of the very fast cetations having concave trailing edges on the fins, but as I recall they are always the big bulky fellows. The smaller members of the clan have fins with more delicate lines – so go figure.

At present I’m working on my “Negative G” template series that will consist of three different templates that are close to each other but vary slightly in configuration and rake but are easily recognizable as a family of fins. I’ll be keeping them under my hat until I settle on the final outlines but just as a hint they are inspired by the game fish photos that my world class sport fisherman friend Jeff Thomas has been kind enough to send me. The board he’s riding something Doug Schroedel and I put together for him that I’ve posted here. He’s having a great time on. There’s no waves where he is right now so the fishing experience is the best alternative.

Keep up the great technical assessments my man you are a true blue R&D man!

Good Surfin’, Rich

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Quote:
Hey Blakestah,

I’m gonna keep this one short.

Firstly, What does this mean? “More upright fins are slightly stronger, a lot less consistent” I’m in complete agreement that they are more pivoty.

One thing we did in engineering classes was plot free-body diagrams, and that is how I conceptualize it. Imagine the rider, on the board with his center of mass, going into a turn.

Now take a completely upright fin. It generates turning power from 1 fore-aft position - just on its main axis, at a range of depths.

Shift the center of mass forward and backward, and you change the turning radius a lot.

Now, rake that sucker. It generates turning power from a range of fore-aft positions. Move the center of mass of the rider forward and backward, and you get much less change in turning radius. It is more predictable in its turning, and at its limit can turn less quickly,

11

yep blakestah , your last post was true to the mark , ive made fins just like what you described , ive seen halycon post similar pics to…

thats exactly what i found when riding the board .your feet had to be precisely in the right spot to get a flowing feel , if your feet were right(slightly further forward ,over the leading edge) then you got a leg snapping super tight turn, but if your weight and position was slightly wrong it was hard to get good timing and connect turns evenly…

thats where the rake allows the fin to work ,coz the rider can have his weight or centre of mass over a larger range of the board and it will still function,

going back to your original fin pics…

because you have removed the tip , there will be slightly less resistence through the turn, which will become more noticable the deeper and harder you go into a turn…

but increasing the chord where you have will almost offset that, making the fin feel tracky plus drivy together,

coz youve slightly decreased the overall depth of the fin , one noticable difference will be ,increased sensitivity rail to rail…

over all a looser more free fin rail to rail in general down the line pumps.with a slightly free’er feeling deeper in the turn ,but still with good drive especially off fater slower parts of the wave, but if you push it past the point where it lets go , it will take slightly longer to recover and drive forward again…

hows that for bench surfing???

regards

BERT

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Quote:

going back to your original fin pics…

because you have removed the tip , there will be slightly less resistence through the turn, which will become more noticable the deeper and harder you go into a turn…

but increasing the chord where you have will almost offset that, making the fin feel tracky plus drivy together,

coz youve slightly decreased the overall depth of the fin , one noticable difference will be ,increased sensitivity rail to rail…

over all a looser more free fin rail to rail in general down the line pumps.with a slightly free’er feeling deeper in the turn ,but still with good drive especially off fater slower parts of the wave, but if you push it past the point where it lets go , it will take slightly longer to recover and drive forward again…

hows that for bench surfing???

Not bad at all.

I cannot push it far enough so it lets go though - because it is in the rotating fin box, and I’ve set the toe-in forces to compensate. Last week I hit a bottom turn hard and was truly surprised at the velocity with which I was flung back into the face. The matched fins with more pronounced tips are not nearly as free deep in the turn, and slowed me down more as the board came around.

I’d still be quite curious if anyone else has hands on experience.

13

Hey Dave,

I still remember very vividly that day at Sharks Cove when we traded the B.K. and the 7’8" with the rotating system. Climbing the wave fast rapidly is what this system does like crazy because when you set the rail and ask the board to grab the fin rotates into the turn and you turning radius is decreased enormously. The system makes adjusting the radius of you turning arc a rather sensitive affair because the fin redirects the board so quickly. The result is that of a 7’8" board that turns like a 6’5". The thing that is most fascinating to me about it is that you can have less tail rocker on the board and still make tight turns on the wave face. This means that the board will trim faster in a straight line. In the end how much tension you put on the rotation is what determines how the board will surf as much or more than the difference than two fins of nearly the same surface area may have – But with that said a wide tipped fin will always be less sensitive and more tracky than a one with a narrowed tip.

This is of course just my intuition and I am often wrong.

Consider that because the fin turns rail to rail transition has a completely different feel to it, and tip area will, IMHO, play much less of a part than it would in a stationary system.

The one thing that I can envision happening with the system is fore and aft fin movement, which could be done, but would require more engineering on your part, Dave. The fin could also be made stationary for the sake of comparision just by loading it on either side with solid blocks instead of one that compress.

Just some thoughts that will dissappear in the lines we trace across the wave face –

Off to the fin shop, Rich

14

The tubercle thing has got me by the noggin, so I think I’ll try it…I only just made my first set of fins recently, but they came out really well, and I think I’m up to it, but I’m definitely no engineer, nor am I embarking on thorough R&D…

The first thing I’m going to do is put some on the fins I just finished, and see how they change…

But I want to design a fin from ground up that (at least conceptually) utilizes the tubercles’ advantages and integrates them into the design.

Just adding the tubercules to existing fins, then modifying the design slowly would be the way to go, but I haven’t the money, time, carbon, or attention span to go that way, so I wnat to do an educated guess and backyard up a pair…

It seems the tubercles’ most salient feature is to extend the angle of attack tolerable by a given foil, by creating vortices that run fore-aft along the lift side and “stick to” the surface better. In addition, I think the paper mentioned that tip vortices are reduced as well…

Improving the angle of attack attributes of a fin that already tolerates angles of attack used in surfing would be redundant, and the advantages would not be used to full potential…I want to start with a fin whose chief drawback is tolerance of high AOA’s, with all other aspects improved…I’m thinking narrower, more upright, straighter fins, but I wanted to get ye gods’ opinion…

What designs are considered “really good, except they don’t tolerate high AOA’s”?

what would be a good starting point for tubercles?

wells

15

Why don’t you go from small to big?

Instead of copying the “bulkier fellows” why don’t you try scaling up first?

Drill some holes in the leading edge and put some bristles from a hair brush in… then if that don’t work try some round headed push pins… and THEN if that don’t work go to the big nasty nob…

Heck, I can hear the naysayers now, “How come you have a hairy fin mister?”

  • Stand clear of those vortex generators - clear props.

daddio

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Excellent insight gents…learning all the time here.

So then, rake is a turn stabilizing and sensitivity control element…

…a couple of analogies:

If you moved the front wheels of your car closer to the rear wheels then you would have increased turning sensitivity similar to reducing rake.

I used to race bicycles and fork rake has a similar effect…more rake, easier to hold a straight line (triathlons & time trials), less rake, very sensitive to handle bar movement (criterium racing).

“This is the crux of the issue. Are you better off with chord lengths that slowly decrease, following the elliptical curve, or are you better off with classic surf fins, that have a nearly constant chord length over much of the fin, before an abrupt drop-off. How does this change performance??”

Still a mystery…don’t quite understand how convex and concave trailing edges would make such a dramatic difference other than changing holding area/power…I guess the jury is still out…

Regards, Dave

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Quote:

Still a mystery…don’t quite understand how convex and concave trailing edges would make such a dramatic difference other than changing holding area/power…I guess the jury is still out…

The issue with respect to foils is this.

Start with a rectangular foil, same chord length everywhere. At the tip. there is a chord length discontinuity at which the chord drops to zero. So, you have high pressure fluid right next to low pressure fluid, and that creates a current, and induced drag. The current will backpropagate up the foil, so that fluid on the high pressure side of the foil moves towards the tip, and fluid on the low pressure side of the foil moves towards the base.

As the fluid re-attaches on the trailing edge, the fluid from the high pressure side and low pressure side are not moving in the same plane. The high pressure side fluid is angled down (in a surfboard case), and the low pressure fluid is angled up. By conservation of momentum we have created a swirl in the wake of the foil, and it is worst at the tip, but exists throughout the foil length.

The elliptical lift distribution is the best case solution to this problem. It minimizes the “swirl” coming off the foil. As a result, the induced drag is less.

You can predict that induced drag will be really bad on a fin with a long chord length into the tip - if it is rigid - it should really drag in a hard turn.

Now, induced drag will depend heavily on lift, so this is something you may or may not notice mostly when you are turning hardest. How much drag comes with lift on a hard bottom turn? For me the answer was pretty clear. A few others have ridden the raked ellipse with cutaway, and been satisfied.

I think it is interesting enough, and demonstrated to be good enough, to try on a standard thruster set, too.

18

Wells,

YOU seem determined and on a mission. Hope this helps. Regarding testing I would suggest starting a bit farther back and working your way forward. That way you can, among other things, compare old and new theories. This may not be purely scientific, but I think Slater pushes his board off of the tip vortices on his fins. I have no idea how to test that though. Point here is testing will be subjective until we have better measuring of fins in use. Until then test pilots are the preferred method. You’re next by your choice.

Turbulators it is said (in the “old” way of thinking prior to tubercles) are reportedly best located at a point that is at 80% of the cord measured from the back to the front. That jives with where it appears Boeing puts it’s VGs on the 737 based on my observation from my wing seats.

Why not draw some cord lines on the fins you wish to test at 1/4" intervals from the base to the tip then measure the cord of each line and calculate where 80% of the cord is. Mark those intersections on the fin. Place a small bump of material (described below) perhaps 1/16" or 1/8" diameter and height at each intersection of cord and 80% point. Surf. Then remove half the bumps from alternate intersections. Surf again. Which was better? Then remove all bumps and surf. Now which was best? Record your results.

Follow this test placing the bumps on the leading edge on the foiled side of the fin on the cord lines. Repeat removal testing procedure outlined above till fin is clean. Record your results.

These are just several possible tests. For bumps perhaps a small dab of polyester and microballoons. Or bondo. You can probably flick them off with a razor when you’re done.

First you may want to test the poly/balloon/bondo material fin bump on an old but similar fin to see if it is in fact easily removed to your satisfaction.

19
Quote:

You can predict that induced drag will be really bad on a fin with a long chord length into the tip - if it is rigid - it should really drag in a hard turn.

Interesting. Many popular fins today (FCS AM’s as one example) have pronounced progressive rake and thus, long chords near the tip. Is much of the surfing world using poor fins? I find the difference between using AM’s and G5s in the front to be very significant. In the 2003 JBay contest, Slater used more raked front fins (AM’s?) and his blue signature fin in the center…best radical surfing I’ve ever seen…Kelly admitted that was his best contest surfing ever…thankfully its in the personal video collection. I would tackle the elliptical thruster fin test but again…no waves here on the “right” coast. Im thinking a std fin set may be easily modified from concave to convex. Making raw fins is something I just dont have time to do so I always look towards customizing existing products…meecrafty kicks in. Looking forward to more feedback on elliptical thruster sets.

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meecrafty - are you on FCS? If so, tell me what template you are using now. I may send you some.