Holy Grail of fin modification

[gator1]

I have been trying to figure out how to get a strong side/weak side differential in turn ease.

 

I want the weak side turn to be as tight, and I want to exit it in as good of a pulling position as the strong side.

 

So, I cut three "sea plane steps" in the left surface of my fin, running from surface of the ski down to the bottom edge of the fin. These steps are the same as the steps in the bottom of my Prophecy, except now they're on the side of my fin.

 

My theory was that if I sized the steps appropriately the water flow would not notice they were there at high speed, but at low speeds the flow would collapse into the air channel and create what felt like a smaller fin during the washout stage of my turn around 2/4/.

 

So, after pulling out the fluid mechanics books and doing lots of painful calculations, the steps are .025" deep, and the taper up to the surface of the fin is .725. I wanted the flow to collapse into the cavity below 18 mph, and go from laminar to turbulent from 18 up to about 30 mph.

 

I slapped it in the ski last night and took a ride. I thought I'd probably not notice anything, or at most a slight effect.

 

WOW!

 

I equaled my year's PB, walking down the line. And that was with all kinds of crushes and rodeos as I got used to the ski JUST F--KING CARVING a deep smooth tight progressive turn that is easily modulated by driving the washout. Holy Crap, this is cool. The guys could hear me laughing from the boat.

 

It doesn't seem to affect anything except the weak side turn and hook up. Which was theoretically the goal, but the last time a design experiment went as planned was... Well, I guess never. On my stuff anyway.

 

And, since my weak side turn is also my rotate into those g--damn gates, they got wonderfully better. Typically, if I don't get a good 38 gate in my first two tries I regress to the point of needing a training handle to do a deepwater start. With this mod they were easy, wide and repeatable. I can push the tail from the center of the ski during the turnin.

 

So, maybe in my next rides I'll regress to the mean, and find a way to defeat this. And, I'll be trying some different depths and placements and shapes of the steps. But the central theory of making a "softer" fin on one side seems to work.

 

If anybody has messed around with this I'd sure like to know what you've learned. Pics on the way as soon as the stupid email is up here.

 

Oh, its now officially called: Gator Tail.

 

[Than_Bogan]

I'm following about 25% of that, but my first thought is: Why not on both sides?

[mwetskier]

long time ago schnitz did a lot of experiments with the idea including angled holes -composite fins with different surfaces on each side -various shapes of the leading edge of fin and other stuff like that.

[Horton]

@gator1‌ pictures? Video?

[gator1]

 

[MattP]

@gator1‌ I fixed your photo.

[Horton]

@gator1‌ a lot of what you are feeling is the change in the flex of your fin

[Than_Bogan]

@Horton That doesn't sound right. (Apparently it's my Disagree with John Day.) The amount of material removed is a tiny fraction of the total. I would expect the flow effect to be far more significant. @gator1 Got any easy way to flex test that and see if Horton's theory may be right?

[skidawg]

FLEX matters! Didn't someone used to have fins based on this made in his garage?

[RazorRoss3]

I can't tell exactly what I'm looking at there. Are the steps hard to see or am I blind? It's been know to be the case in the past.

 

I think I understand the theory.

[MattP]

@skidawg‌ I think so. I think he still has nightmares about it though.

[Horton]

@Than_Bogan‌

That fin is .095 - .090 thick and the alloy Connelly uses is pretty stiff. When you make a vertical groove .025 deep that is a lot. I would guess the fin has not changed a lot vertically but horizontally bet it is a lot softer. Horizontal fin flex can be wacky. A little give like a rudder will make a HUGE difference. Think diagonal flex at the bottom trialing radius.

-

The turbulence from the cuts would certainly have impact. I would guess they create a little drag/stability. More than a few factories have experimented with just the trailing edge shape of fins. Softening the edges on the holes makes a difference. Adding a 6th and or 7th hole forward in the blade creates a lot more stability/ turbulence and can really feel good.

 

[TylerR]

@Horton‌ so is there any way to test this idea so that one could theoretically tell if it's the fin flexing? How would you as one who has made fins before suggest trying to isolate the variables.

@gator1‌ this is an interesting idea.

[Horton]

Fins flex. No question. Exactly how much? Google Probability Density Function

[TylerR]

@Horton‌ I understand the fluid dynamics I'm asking how much should a normal fin flex and is there any experimental way to eliminate that variable.

[Horton]

You want a little flex

[TylerR]

@Horton‌ as much as the ski? Half that? Less than a wakeboard fin? How much flex did your 34mph carbon fin have that everyone loved so much? Would it have degraded over the life of the fin or could I flex test the 3 I have and get a solid starting point?

[BraceMaker]

This is when you also have to introduce the scientific method.

 

Double blind fin study? Where in you have a trusted third party install a fin and a fin guard, you get in the water, they remove the fin guard and an un-informed driver pulls you for a pass. For some period of time the third party installs a fin, notes what is installed and you ski. Notes from the skier, driver and coach are taken as are results and then later you match it up to see if the fin correlates to actual improvement.

 

I can dig it - if it were purely a flex related improvement I would think leaving "webs" in the slots would reduce the flex component, having the fin bolted across one slot has to reinforce it.

 

Similary if you made one for the opposite foot forwards skier in the same manner and tested it yourself it should only impact your onside then and you'd see if that's the case.

[Horton]

When I had the fin business I tried to find a good want to measure static fin flex. (FAIL)

 

What happens on the water is totally different. All I know for sure it fins flex and it can have a huge impact on your skiing.

[TylerR]

@Horton‌ ok so what about a water tunnel test on fin's? I know it doesn't work ok skis, but would that let us visualize the movement of the fin as force is exerted on it by the water?

[SkiJay]

Interesting idea @gator1‌. Water isn't really "collapsing into the air channels" though. There won't be much if any air in these channels. They will be full of water that is travelling forward with the fin in eddies. These eddies and the water that piles up above and behind them create turbulence. This turbulance reduces pressure on that side of the fin causing lift allowing the tail to smear more easily on its 2/4/6 side.

 

If this is what's happening, it can be done a number of ways, like with a bunch of dimples or with simple narrow grooves on one side. Airplanes and race cars have been using turbulators for decades. We used the turblator tape shown below along the top length of the wings on sailplanes to generate extra lift with less drag this way. It works.

 

http://www.wingsandwheels.com/images/Zig%20Zag%20Tape_1.jpg

 

http://www.homebuiltairplanes.com/forums/attachments/light-stuff-area/20392d1351016663-ul-sailplane-footlaunchable-foldable-tubulizator.jpg

 

[liquid d]

don't worry about measuring it....just keep testing it! If you can help one side without the takeaway normally associated with fin adjustments, you're on to something. Get some people on them things!

[gator1]

@MattP‌ Thanks. Appreciate it.

 

I don't think flex explains the effect. If so, why has the weak side only changed? If I had weakened the fin to an appreciable degree it should be displacing equally in both directions.

 

I can FEA it, and it will show very little change in displacement vs load. But, nobody knows what the hell the load actually is.

 

So, the way to answer the question @Horton‌ raises is to bondo the steps back in, thus recreating the surface without adding any strength. But, right now, at the end of the season, I don't care enough to do that. And if it is flex, don't care. I'm pbing. And if it is flex, why didn't people do this years ago? And, you'd want to google modulus of elasticity,

 

@SkiJay‌ Ya, no, not so much. this is water, not air. Reynolds number is drastically different. At high speeds that slot is full of air, not water. They aint no eddies of water. Defintely agree there are eddies of air if were on a race care working aerodynamics.

 

My buddy skis rff. He was witness to the laughing and PBing on the first try. And he wants a RFF version bad. So we may end up with two mirror images we can swap. But we don't need a double blind study. This is no subtle effect. It is "holy shit, did I just turn like that?!"

 

So, I'm back to my question: Has anybody seen steps like this tried on weak sides of fins?

 

[gator1]

@Than_Bogan‌ I'm thinking about cutting one slot in the strong side. My turn on that side is plenty sharp already, though. But if it gets easier to modulate, then that might be muy bueno also.

[Steven_Haines]

That's bad ass @gator1! That'd be fun to try!

[SkiJay]

Google the Bernoulli Principle and fluid dynamis @gator1. They apply to both water and air. In fact a lot of aerodynamic testing is done in water flow tanks because water is slower, easier to observe, and because air behaves like a fluid...and vice versa.

[gator1]

No. Actually it doesn't. Air behaves as a compressible fluid while water is not and does not. Once the flow separates our buddy Bernoulli won't govern the behavior.

 

Seriously. If your are trying to tell me there are meaningful eddies in the water flow inside that step at 30 mph I'd ask you to check with your aerodynamics guy and let me know if I'm talking through my hat.

[eleeski]

Wallboard texture on a fin?!

 

Interesting idea of gator1's. Very cool!

 

Flex doesn't seem to be the major factor here. Certainly a thinner fin will flex more but there's an awful lot of unmodified fin left to carry the loads. Flex absolutely makes a performance difference but I bet the modified fin is still uber stiff compared to @Horton‌ 's carbon fin or a thin fin. Note, I made a very flexible fin which turned great but wasn't great at holding angle. Also, one of Horton's carbon fins was too stiff for my tastes. So there are limits. (My favorite fin was one of Horton's softer fins.)

 

Perhaps the drag is increased by the steps but that sounds a bit fishy. The drag from a wing should be much more significant. And when I've changed wings, wing thicknesses, too long wing screws or added ventrals, the changes were subtle despite significant changes in drag. To be fair, my skis have a lot of hydrodynamic drag to start with. Drag effects are real but there's so many other factors affecting laminar flow, turbulence and cavitation that I'd have trouble feeling any drag changes from small steps like that.

 

My guess would be that there's a pressure flow down the steps. In a turn, there's a lot of skidding loading the intersection of the base of the ski and the fin with pressure. Much like holes in the fin relieve some of this pressure, the steps could do the same thing. Except only on one side! With less pressure, the ski can ride deeper and more stable in the turn. (Note that I adjust my fin as shallow as I can without skipping out - I'd get the same feel by lowering the pressure in the fin area but get the stability of a deeper fin.)

 

I haven't tried the fin or anything like it. So I am just wildly speculating in a jetlag induced fog. Or gator1 could have just had a skills breakthrough and this is a phantom effect. But this is a fun thread with interesting speculation about a very creative experiment. Well done, gator1!

 

Eric

 

 

[Craig]

My initial reaction to this modification is the same as @SkiJay. Steps causing reduced drag on the modified side of the fin causing a bit more tail rise out of 2/4.

 

For all intents and purposes air can be modeled as incompressible at the speeds you are talking about. The only caveat would be if there is enough change in pressure on the modified side of the fin to cause the local pressure to fall below the vapor pressure. Then, things get interesting.

[sfriis]

Interesting that so little has happened to fin and spoiler development over the years - look at the boats and surfers - (with much less speed) they can change performance dramatically with fins and foils etc.

 

The Finn must be just as important as the ski itself - and I'm surprised that nobody invented an asymmetrical finn before - it should be obvious - because everybody feel a big difference between the good side vs. bad side.

 

Is it because everybody can copy a good fin ? (=no interest from manufactors)?

[MattP]

@sfriis‌ I have some friends using asymmetrical wings. Obrien had an fin with a flap that opened on one side of the turn but not the other.

 

Guys like @Mapple‌ KLP, Bob, Asher, @AdamCord‌, Schintz have tested many many variations of thickness, shape, hole pattern, ect of fins but keep coming back to the 4-5 hole we have today. Is there something out there that could be better? I would absolutely have to say yes. But finding it and having it work for everyone is a different story. If someone hands me a different fin that will fit the Vapor. I will try it. Making them myself is out of the question.

[DW]

@Gator1: nice work, great experiment and congrats on the success. I agree with your analysis, it appears to me that there is cavitation generated thus creating the asymmetrical load and desired slip. I would also bet the first groove is the key to the experiment and the intersection of the fin hole and slot really helps generate cavitation and asymmetrical water flow via the added pressure differential across that particular hole generated by the slot. Can you post your calculations? CFD analysis would provide a great view on what is happening.

 

@TylerR: the way to prove out the flex issue as noted above would be to bondo the slots back up and see if the performance returned to previous characteristics. I personally don't think the effect here is mostly flex related, I think its a hydrodynamic phenomenon. As for flex, you can attempt to measure it, I have done it for several fins and one can easily find differences across a variety of fins (based on thickness and material). I also feel that the rebound effect after the fin is loaded is a key to the performance characteristic of the fin.

[gator1]

@MattP‌ Send me a fin. I'll cut it for you.

[MattP]

@gator1‌ I need to pick up a few anyways. I jacked mine up a few weeks back hitting a rock... but I PB'd on it..

[BraceMaker]

So when you milled these slots the leading edge is a step down. The trailing edge is a full taper to the surface?

 

Any advantage to cutting this on the trailing edge and not tapering back up?

[DW]

@Gator1: keep your jig handy, you probably have several interested "victims", myself included!

[gator1]

@BraceMaker‌ yep. I don't know if something might be better. First try worked.

 

@DW‌ anybody with a mill in their garage can do this. I'll publish a sketch of what I did.

 

@MattP‌ "hitting a rock then pbed" is my fear. Maybe any change is good. For awhile. But send me a fin and we'll see!

 

I do want a bunch of opinions to convince myself this is real not a placebo. So far it seems nobody has tried this before so maybe we're on unexplored territory.

[mwetskier]

@sfriis -asymmetrical fins are nothing new steve schnitzer probably did more experimentation on this subject than all other tinkerers combined. he discusses some of these experiments in his ski tuning guide which i bought from his web site - http://www.schnitzskis.com/skituning.html

in there he explains different ways to make a asymmetrical fin and how it affects the ski on each side. he thinks the results are caused by different pressure and lift generated on each side of the fin. if you talk to him in person he can tell you a lot more about all the different things he and others worked on many years ago and i would be surprised to see anyone come up with something he hasnt already studied in great depth.

[webbdawg99]

@Mattp Dont ski so close to shore

[MattP]

@webbdawg99‌ I was following the boat! kinda..

[skiray]

@gator1‌ keep up the good work. It's always good to see innovation in our sport.

[gator1]

BTW, in case what I thought was obvious was not, the title to this thread was meant to be ironic. I have not quit my day job. And like gatormod, this is just therapy for my poor battered brain.

 

And, I'll tell you, it is really hilarious big time fun to change my physical performance, something I've been fighting for years, with 15 minutes work on a Bridgeport mill.

 

Its kind of like being a grandparent. Until you've actually thought of something, built it, tried it, seen it work, you'll never know the sheer joy when it works. Just like when that little bitty girl first calls you gampa.

 

 

On the other hand, the "people have been working on this type of stuff for years, I'm sure what you have done has been tried before" sentiment puts a lot of food on the table from my day job. So that type of thinking never stops me. But big thanks to all you guys who support wild ass ideas with advice and encouragement.

 

But I do look very earnestly for somebody who has ACTUALLY done IT before. Not just tried some stuff like it.

 

So, if anybody has seen THIS before, I'd sure like to know.

 

And, wait'll you guys get a look at my velcroless Velcro plate. ROTFLMFAO!!

[BraceMaker]

A velcroless velcro plate!!!!

 

Your project looks so fun I may make a Righty version for giggles.

 

[gator1]

@BraceMaker‌ go for it! Worst case it is fun. Best case it gets you some pbs.

[BraceMaker]

Worst case scenario is setting a .025 step tapering over .725" with a mill that doesn't have a pivoting head. This is going to require some thinking...

[gator1]

Get a nice thick chunk of Al, bigger than the fin. Clamp that in the vise at the proper angle. Use some c-clamps to pin the fin to the chunk. Grab a beer and fire up the mill. Works just peachy, up to about 4 beers. Then, things seem to go wrong.

[BraceMaker]

Wait... which comes first 4 beers or turning the dial knobs the wrong direction?

http://i107.photobucket.com/albums/m298/hawkskier/IMG_20140916_210659389.jpg

 

[gator1]

Haaaa! Can't wait to hear how it works!

[mwetskier]

as far as i know schnitzer skied in tournaments with all his developed innovations and did pretty good at it. he also mentioned that he and warren witherell both used some of his asymmetrical fins to ' set records back in the 1980s '. given the number of top name skiers who have sought his input and used his products i would bet just about every thing he put out over the decades got used by some one of note. none of this is only a guess that it's been tried before. schnitzer wrote about much of it and documented it with photos years ago and given his love of photography i bet he has hundreds of more unpublished photos of his innovations too.

[BG1]

I've been thinking of doing that to a fin ever since the HO S2 came out. I just never got around to it. Sounds like I'll be making one tomorrow.