Holes in your fin

[schroed]

What do the holes in your fin do anyways? What would happen if you didn't have any holes? What would happen if you drill more holes or move them around? I've been riding a D3s for a long time and it seems like they've been using the same fin with the same holes for a long time. Why haven't the holes changed?

[BraceMaker]

There are different fins.

 

http://www.schnitzskis.com/finswings.html

 

http://www.jagersport.com/index.php?cPath=77&osCsid=e225cb0c73e217391c0c04143e62f522

 

All your fin measuring does is set distance from tail, maximum depth and length. Length really being a comparison of the depth of the front of the fin. Between max. depth and fin length you essentially are setting the amount of fin surface area.

 

I believe the holes simply reduce surface area, while allowing a deeper fin. If you remove the holes you have way more surface on the fin. Meaning you probably would need to reduce length and depth to reduce surface area, which then would be a shallow short fin.

 

Variously you can fill the holes and cut notches out of them, which means you maintain area but relocate the surface area to different parts of the fin.

 

There is probably a more specific science here. I remember Obrien had a pressure vent with a flapper on it, that let the fin act larger on one side than the other. Unknown if that lasted long.

[norcalibu]

I have an old HO Extreme with the fin that has a big notch cut out of the back edge. Been thinking about putting it on my S2 just to see how it skis. Thoughts?

[Horton]

@schroed

Holes do three things

 

They make it easy to hang your ski on a hook

 

They allow pressure relief from the high pressure side of the fin to the low pressure side of the fin.

 

They create drag & drag = stability

 

 

I have never tried to ski with a fin without holes but I understand that will make the ski faster and unstable. I have skied a lot with fins that have 7 holes instead of the standard 5. Seems like there have been holes in fins since the early 60s

 

[Horton]

@norcalibu We called that the Pacman shape. It helps the ski turn in the same way that my old CarbonFins did. If you press on that old fin you will see it flexes a lot more than a newer fin.

 

Lord help me.... lets not go down the flexible fin route again. I still have not paid BofA back for all of the CarbonFins loss.

[norcalibu]

@Horton LOL Thanks for the heads up.

 

By the way - Nice meeting you at Diablo Shores this year. My buddies and I were sitting in front of the announcers booth on the shoreline. We were discussing Chris Parish's AM ski. Cheers.

 

Michael

[SkiJay]

Deep fins makes skis more stable, accelerate faster, hold more cross-course angle, and they help to create width and space before the ball. But a deep fin also makes it hard to smear the tail of the ski around a tight turn at the ball. The holes allow us to enjoy most of the benefits of a deep fin without adding so much surface area that the ski becomes too hard to turn. Playing with where the holes are is a bit like playing with fin length and distance-from-tail characteristics.

[Wish]

I think I just saw a new AM fin. Only 4 holes from what I saw. Hmmmm?

[Brady]

@Skijay, could I go with a much deeper fin then and put more holes in it?

[Horton]

@Brady I do not think so. See thread about who should tweak....

[Brady]

@Horton. I know. Just curious. I am impatient and am looking for every potential advantage possible.

 

Part of what makes short line skiing fantastic is also the biggest frustration---taking the proper time and having patience and getting enough time on the water!!

 

I look at the ballers here and I need to understand that being behind many does not mean it is a bad thing. I am on the same growth curve each of the better skiers have been on and I need to stay focused and continue on the path!

[SkiJay]

@Brady Everything has a trade-off. If more holes were clearly better, we'd see more holes in the pros' fins. Chet Raley cuts up fins quite a bit, and the Schnitz slot fin is specifically designed for more depth with fin area reduced by the slot. On the other hand, @Wish just reported seeing Andy Mapple's latest ski with only four holes in the fin, and Andy's previous ski, the Elite, only had four holes too. No shortcut to glory here unfortunately.

[MattP]

Andy's previous 5 skis at least only had 4 hole fins, it's nothing new

[eleeski]

@Brady Fins are reasonably common. DO NOT lose the factory fin! But if you can scrounge up a Horton Carbon fin, an old HO pacman fin, a Schnitz fin or any old fin to drill out, you can waste a few rides having some equipment fun. Who knows, you might find a fin you enjoy. Every ski I seriously ride gets evaluated with a pacman fin - it became a "stock" setup for me (as did the soft Horton fin). Note that the scribe line (scratched into any fin when it has felt good) allows me to get known fin settings and not have to chase too many variables at once.

 

The ski I rode all summer worked best with a homemade ultra flexible fin. What started as a crazy experiment ended up as a couple buoy advantage on that ski. Rest assured that the new ski I build will get tested with that soft fin - in addition to my "stock" fins.

 

@Horton will ban me but we need his fins back on the market!

 

Eric

[Brady]

@eleeski. Thanks for that. Now the question is where can i buy each of these fins? I will have my spankin brand new ski in about a week and I love to tinker. And don't kid yourself, @Horton needs more projects. This site is self sustaining and I am sure he is going nuts with boredom!

[Horton]

Self sustaining my ass!

[BraceMaker]

@eleeski - what was the ultraflexible fin? Thin aluminum? Carbon?

 

 

[eleeski]

6 plies of carbon cloth in a wet layup weighted by a bag of cement to pressurize the cure. Traced the Pacman fin for shape. Decidedly low tech. Skied well.

 

Today I'm using an old Carbonfin.

 

Eric

[Brady]

@eleeski, do you sell any of the fins you have mentioned in this thread. if so, I would like to buy some. Does @Horton sell his anymore?

[Horton]

@brady play with this stuff all you want until you want to ski better

[Brady]

@Horton, you must have forgot. I am out of commission til March cause of my injuries. The best thing I can do to ski better next spring is to lose 30 pounds over the winter! No amount of tweaking will have that kind of impact. I am just learning and trying to understand, and am doing exactly what most of you said here, which is to take the damn wing off til i hit 28off.

[eleeski]

@Brady No! I will take personal chances with crazy equipment. I'm not sure I should inflict it on you. Unless you really want to set your skiing back.

Besides, I'm not ready to make these things now.

Eric

[tfriess]

What if since I dont have my wing on, I have 6 normal sized holes and 1 small hole. How might that affect the ride compared to a 5 hole

[Horton]

I do not think you could notice. We are talking about very small differences.

[wilecoyote]

Coming from a windsurfing background and as a (very) casual hydrodynamics student, I find ski fins quite perplexing. The two things that make no sense to me are the lack of foil shape and the holes. I'm quite certain that this is something that has evolved over decades of trial and error. I'm still struggling from a hydrodynamic standpoint to understand exactly what we want a slalom ski to do.

 

My initial suspicion as far as the fin goes is that it is a variation of a delta wing, which never stalls, it just keeps loosing lift as the angle of attack is increased. I'm no where near a good enough skier to have any kind of opinion on whether we actually want the ski tail to slide or not, due to diminishing fin lift but, it is certainly better than having it suddenly loose all it's lift, at which point we will most definitely fall. (in windsurfing this is known as spin out) I'm also very surprised that the very small adjustments to the fin can make such a big difference to how the ski performs. If we're using callipers to make fore and aft adjustments, adding or moving a hole should have massive consequences, yet you see all kinds of funky hole patterns. I dunno, my head hurts.

[lpskier]

My understanding is that the first hole in the fin came from a need at Maharaja to put a delivery of fins somewhere, so Bob drilled a hole in them so he could hang them on a nail. He discovered those fins performed better than those with no holes. Next thing you knew, fins looked like Swiss cheese.

[MattP]

@lpskier now that sounds like a tall tale.

[Brady]

@wilecoyote. My head hurts that in your spare time you study, "casual hydrodynamics student" I think there are some very smart people on this site!!!

[wilecoyote]

LOL @Brady, note the 'very' in brackets. Lots of smart folk here, not sure I qualify though. When I was dating my wife at her University in the '80s I looked up and photo copied everything I could find on hydrodynamics pertaining to planing surfaces in the engineering library. I was shaping windsurfers (still am) and wanted to be a bit more methodical than the trial and error method. Unfortunately for me, I never went further than high school math, so i never got the full picture. It's pretty dry stuff but I'm re reading a lot of it with an eye to water skis now. I have some ideas that I'd like to try, shaping a ski or two should be a lot of fun. But back on topic, I'm a huge fan of the guys at Maui Ultra Fins, and I've been threatening to talk to them about ski fins.

 

BTW, love the hole story at Maharaja, doesn't matter if it's true or not, it's cool.

[DanE]

@wileycoyote

Back in the days when Dave Goode started marketing his waterskis he was pushing the idea about having a fin with a foil shape,never saw one though. On my first 9100 there wad a standard alu fin.

[DanE]

Btw, isn 't a foil shape supposed to create lift?

That would not go well at the tail end of a slalom stick.

[liquid d]

A foil is like a wing, it just depends on the angle

[Brady]

@DanE while you and I are going to be watching football today, this is what @wilecoyote is going to be watching.....remember, this is his hobby. "Honey, there is a great video on dynamics....can we eat our Turkey by the TV?"

http://www.youtube.com/watch?v=xK36rinGxIk

[wilecoyote]

@ DanE,The fin, whether foiled or not, will create lift. It's job is to stop the tail of the ski from sliding sideways. As soon as the ski is pointed in a different direction than it's direction of travel, the fin starts "lifting" in the opposite direction.

 

The difference between a foiled shape and an unfoiled one is that of efficiency. The ratio of lift to drag improves as foil shape improves. If we were to foil our fins, they could become smaller, and have less drag. The burning question is though, is this desirable?

 

I'd love to know what happened with the Goode experiments.

 

[DanE]

Maybe if foil shapes are going to be used the fin dimensions and shapes needs to be totally reinvented, with the R&D allready going on with slalom skis it might not be economically viable to get into that just yet.

And those fins would surely add to the cost of an allready expensive piece of product.

[wilecoyote]

@ Brady, that stings. But the funny thing is I started watching it and now I can't stop. he he.

 

I won't be watching football or eating turkey today, all there is is NFL and I'm working anyways, thanksgiving was almost a month ago. The proper game is on Sunday when the Argos meet the Stampeders in the Grey Cup.

 

@ DanE based on the fact that a brand new D3 blade is advertised for 12 bucks and a decent windsufing fin is over $100 you're right about extra cost.

[Horton]

@wilecoyote I think lift is the wrong term in this case. There is always a pressure differential and that creates a side force. One of the biggest factors in drag is frontal area. If your fin is .090 of an inch think, that is the amount to thickness you have to shape. Good luck getting much effect from the cross section shape of a wing that is .090" x 7”. At that aspect ratio the impact of a better shape is going to be very hard to measure and in order to taper the trailing edge you will be impacting flex.

 

If you want less drag you simply need to go thinner. The thinner you get the less the shape of the fin matters. As you get thin flex comes into play....

 

If you want to get really freaky about it look that the shape of your wing.

 

[DanE]

Yea, but what if the fin was only 2" long?

A foil shaped fin could potentially have a totally different shape from the standard fin most skis have today.

[Horton]

@DanE "One of the biggest factors in drag is frontal area." To be only 2" long it would need to be roughly 8" deep to get same surface area. Now you want to make it think enough to shape it back down? My truck could run on 3 wheels but that does not mean it is logical.

[eleeski]

At first I thought the lecturer in the video was @Than_Bogan before his goatee. But after @Horton's post, I think it was Horton with short hair!

 

Next I will have to try golf ball dimples in a fin...

 

Eric

[DanE]

@Horton -Never said the 2" was logical nor the final exact lenght either, but if a foil shaped fin creates more lift than a flat one as @wileycoyote was saying maybe you don´t need the same surface area ?

Making the bottom edge of the fin longer before it starts to taper up in the front is also a way of keeping the surface area without making the fin deeper, bottom line is I was thinking out of the box.

"Now you want to make it think enough to shape it back down?"- Do not know the answer to that one?

[Brady]

@Horton

 

http://www.youtube.com/watch?v=M7cyDac_tKI

[Horton]

@DanE I love creative thinking. I just think you are barking up the wrong tree. I mean if you have the time and means- knock your self out. Prove me wrong and I will have to buy a DanFin. Lord knows I have hatched some half baked ideas over the years.

 

There is something to the shape of the fins edges. This is something that has not been fully explored. A few pounds of pressure in one direction or the other could be a big deal if you could work through all the variables.

 

 

[DanE]

@Horton -See my post @ 4:01 PM

[Horton]

@DanE the effect of becoming thinner (less frontal area) is greater than change in overall shape. In order to get much cross section shape into a fin you would need to add a lot of thickness. If fins were .500" thick shape would be critical but they are .080" to .095" depending on brand.

 

A fin that is .005 thinner noticeably faster.

 

@DW can you please chime in.

(I tweaked your settings so you would get an email from this post - hope you do not mind- Go Kimi!)

[wilecoyote]

With all due respect, I think you guys are missing the point here. There are two basic types of drag, parasitic and induced drag. The induced drag is a result of the fin producing lift, and the parasitic drag is the friction of the fluid moving across the surface. You're focusing on the parasitic drag but the big number in drag for us is the induced drag. The only point in your pass where the fin is not producing considerable lift (and thus considerable induced drag) is at the neutral point in your edge change. Therefore, having a more efficient foil will have a pretty large effect on the overall drag being produced by the fin.

[Horton]

@wilecoyote Hmmm ok I hear you. When you say "lift" you mean "pressure differential"? Lift to me is what an airplane wing does to keep the plane up in the air.

I need the right amount of low pressure on the left side of the fin to keep the tail from sliding right as I approach 1/3/5

[DanE]

When the ski is on edge lift created from the fin will impact the skis planing attitude .

I believe that is one of the largest factors a minor change in fin setting can easily be felt.

When the skis planing attitude changes so does its behaviour.

You would think an overly deep fin would hold in the turn no matter what? Guess again, if the fin is too deep you will experince blowing out the tail on offside - because the fin creates too much lift. When @wileycoyote said you could ( potentially)get away with a smaller fin I threw out the idea of a totally reinvented fin shape and in the same sentence made sure to y 'all I am fully aware of the financial challenge to make this happen.

There are a few designs in this world that cannot be improved upon, the wheel being the first that pops up in my head, is the 5- hole standard rounded fin right up there? I don 't know but I do know that back in the day when Kjellander was a pro they tested all kinds of crazy fin shapes thrown at them and always went back to the standard (even the standard roumded fin does not have the exact same shape or number of holes in it from different manufacturers)

[wilecoyote]

@Horton, lift is the generic term used in aerodynamics and by extension, hydrodynamics to describe the pressure differential. In this case lift can be in any direction, not just up. The fin has 2 main functions on a slalom ski: keeping the nose of the ski pointed forward, and keeping the tail from sliding out in turns. For the first, every time the fin is at any angle to the direction of travel it will begin to produce lift which naturally pulls the tail back in line. The second case is really the same as the first, except the turn is exerting a lot more force.

 

I completely agree with DanE when he says

When the ski is on edge lift created from the fin will impact the skis planing attitude .

 

This is, however a by-product of the main function of the fin, yet I think it may be the most important aspect of the fins function to study. I don't agree however, that an overly big fin will blow out because it creates too much lift during a turn. As I said early on in this thread, I suspect the shape of the fin works similar to a delta wing in that it never really stalls (total loss of lift due to overloading the fin) A delta wing stalls progressively, whereas a normal wing (windsurfing or surfing fin) stalls all at once. This may in fact be desirable in order to allow the tail of the ski to slide a bit, but I'm not so sure.

 

Back to the question that started all this. I'm pretty sure that holes effectively make the fin act like a smaller thicker fin with no holes. The holes will "vent" pressure from the high (pressure) side to the low side, which will create turbulence and kill lift. What may be going on here is a kind of "stall proofing" where the turbulence created by the holes are creating some kind of flow pattern that keeps the flow attached. The fact that you guys , (I'm way too inexperienced to feel any of these tiny tweeks) can feel a .005" difference in thickness may have to do more with flex than with drag, but this so complicated that without someone who really understands the fluid dynamics and can do the math it's all just supposin'.

 

I am 100% confident though that a far better fin shape awaits, starting with a foil.

 

Food for thought! I think I'm full!

 

Chris

[Jody_Seal]

just like a rudder on a ski boat i have been known to chamfer one side of the trailing edge of a fin setting up a high pressure side and a low pressure side. For me this accomplishes two things. first I do it to help with my off side turn, second it helps the ski stay more stable on my gate pull out and helps decrease the skis tendency to edge search while riding a little flat. Another little trick that Andy showed me a number of years ago is to bore an extra hole in the fin just below and in between the the two far rear holes. This allows for a little tail slide (if you like that feel) again more for my off side turn.