Long Shallow vs Short Deep Fin

[Horton]

[ScottScott]

Who's in charge of blowing the leaves off that nice new prostar?

[Horton]

@ScottScott believe it or not I intentionally left those leaves on the cover and the swim step as decorations for this video. They will be gone tomorrow. Now back on topic please

[gregy]

There is also a forward movement, not all side slide. Not sure how that effect things.

[Bruce_Butterfield]

@horton, wow, you are actually learning something:)

[Ed_Johnson]

This is precisely the reason I used differential depth on my NRG of .020 to improve the offside turn. Really made it smoother with a little bit of smear to help set angle.

 

 

[gregy]

From a physics standpoint. If fin area and DFT stay the same then the force exerted on the fin by the water should remain approximately constant. Then what changes is the center of force on the fin. If you have a deeper short setting then the center of force will be moved back (further back from the fulcrum) so it should increase torque in a plane perpendicular to the water surface pulling the ski downward in the rear. But also there will be a increase in the torque resisting the rotation of the ski around the buoy for the same reason.

[Horton]

@Gregy

 

In terms of "center of pressure" I do think you are correct. The center of pressure has to move as we swap length for depth.

 

I have zero science to back this up but I think that the interaction between the pressure on the bottom of the ski and the pressure on the face of the fin has an impact on how much downward pressure there is. A deeper the fin is the less the bottom of the ski interferes with it. If my theory is not right, I assume there is some other phenomena to the same effect.

 

I do not think exchanging length for depth has the exactly same impact as changing DFT but I do agree that they might be related. ... Interesting.

[gregy]

There's definitely a lot going on. Any ME students out there need a fluid dynamics project? This would be interesting. Mount on ski on some mechanism you can rotate to simulate a slalom turn. There's probably simulation software out that you could use to predict it.

[AdamCord]

@Horton good video. My favorite example of this is Mapple, especially when he competed. He always ran short/deep settings. It's not a coincidence that he also had the best offside turn in the business. But his onside turn suffered as a result, and he was known for slam dunking that side, because that was what he had to do to get the tail to wash enough. When I skied with him he never even tried long/shallow settings, because he hated how it felt on offside.

[skibug]

I hate offside tail blow outs if/when I am late and have to push on the ski. That is why I always wide up short/deep.

[Ed_Johnson]

@AdamChord ... Could you please go into the benefits of differential fin depth and how it can improve the onside turn. After all, I believe this was your idea with the original washer trick, and developing that into the Denali.

 

Also what do you consider an ideal range to strive for. One washer took away .020 for me and I noticed immediate improvement. THX

 

 

 

[fu_man]

He said doo-doo.

[AdamCord]

@Ed_Johnson I could probably write a novel on this so I will keep it to the elevator pitch version:

 

The shallower fin will start to wash (smear?) the tail earlier and more easily. As @Horton stated in his video, pretty much everybody is more on the tail into and through their onside turn. The downside of this is that we are not engaging the tip of the ski as much, which would use the bevels and concave to slow the ski down and "steer" the ski back toward center. Because there is less of this effect on onside, we need the tail to have less resistance to rotation.

 

If you are falling over on your onside or getting "dumped", where you finish with too much angle and not enough speed (too much load), it's not because the ski turned too much, it's because it turned too little. Basically the ski kept going straight when your body was moving to the inside of the turn, and you fell over. The ski then is forced to rotate all at once at the finish of the turn, hockey stop style. If the tail had started washing earlier and more easily, you would have had a more progressive turn and you'd end up with a tighter line, in a balanced position, and without dropping into a hole.

 

In a perfect world we would run a fin that is shallower, longer, and further forward for onside, and back, deeper, and shorter for offside. If only someone could come up with and patent something that does this... ;)

 

As far as how much "offset" to put in with a washers? As much as you can.

 

[Ed_Johnson]

@AdamCord ... Thank you for the response and shedding light on this. When I tried a thin washer, it reduced the differential .010 and I felt no difference. With a standard washer it was .020 and I felt a smoother, earlier entry off the apex. I thought I had read somewhere the limit was .030. Guess I will just have to try it and find out...Thanks again for the insight.

 

[JAS]

What would be downside of a fin that was adjustable from 90 degree axis

[santangelo]

@JAS nothing, but can't be used in AWSA tournament skiing per rule 8.03 C. Fins must be fixed.

[Horton]

@santangelo I think you misunderstand what @jas said.

 

I think what he is asking is "Why does a fin have to be 90 degrees from the top of the ski?" Interesting. If anyone tests this please take video.

 

Seriously, I think Schnitz played with this. I think he told me once that he was working more adjustment parameters. Rudder right or left and tilt right or left is what I think he was testing.

[JAS]

My thought was that if a ski was adjusted from 90 degrees relative to the top, there could be some sort of asymmetric resistance to tail slide as discussed in hortons video. Would it be a useful difference, I don’t know.

[skibug]

widen the fin slot and shim all the way down one side if the fin under the block, test it out and tell us how it works.

[Adam Caldwell]

@Ed_Johnson

As far as fin surface area offset is concerned, I just start to feel the synergy at about 30/1000s offset/differential.

 

Once I have more speed&rotation through the onside turn, I typically start getting faster/earlier/wider into the toeside. This is good, but only if I can keep it from overshooting the targeted apex.

 

Next step is usually to bump the fin longer/back/deeper (by a few thou. each) to get the speed into apex of the toe-side back under control.

 

BUT...in order to not loose the on-side magic....each time I add had to tweak the fin deeper/back/longer to dial in the toeside, I would also have to put in more offset to sustain what magic was going on with the HS. If there was a combination move of deeper and back for the off-side, I would make the differential significantly more to correct & re balance it.

 

I maxed out at 95/1000s of offset last summer. Had slowly ratcheted the settings up starting from 35/1000offset. It all adds up to more and more space before each buoy with far less effort!

 

My lifetime practice PB this summer at 41 was run with depths of 2.520 (offside) and 2.425 (on-side).

[Ed_Johnson]

@adamhcaldwell ... Thanks so much for that info. In my wildest dreams I never would have thought of that much offset. This really, really, helps a lot. I had thought 30 was probably the limit and you were starting at 35. Looks like I'll be buying more washers.

Thanks again !!!

 

 

[AdamCord]

@Ed_Johnson @adamhcaldwell the reason I said to max out the offset is because on a normal fin block using washers you can’t really get much more than .040” offset.

 

The Denali fin block has the mounting screws spread further apart and it has an offset jacking screw that lets you get a lot more.

[Ed_Johnson]

@AdamCord ... Thanks Adam, that Fin Block was a BRILLIANT move !!!

 

 

[Horton]

Hey guys I'd like to put this thread back on topic. Denali maybe growing and innovating but the vast majority of the readership here do not have a differential set screw in their fin block (yet).

 

If somebody wants to start a new thread about the Denali differential setting go for it. On the other hand I have seen the 2018 Denali stuff and offset is much much less emphasized.

[Bruce_Butterfield]

@horton understanding the interaction of balance points, fin depth on each side and how each skier controls the ski is very much on topic to your original post. Having different fin depths on either side - via a washer, unique fin block, or hacksaw - is definitely part of the short/deep vs long/shallow understanding.

[Horton]

@Bruce_Butterfield understood but 99.43% of the readers live in a world without different depths on each side of the fin.

[Bruce_Butterfield]

@Horton yes but if a baller is trying to understand/ experiment with short/deep vs long/shallow, asymmetric depth is definitely a factor and can easily make improvements in either configuration.

 

If a baller has a set of calipers and an Allen wrench, they are certainly capable of putting a washer under the block.

[Adam Caldwell]

Perhaps the 99.43% needs to read this post then go home to jam a washer under their fin block. At the end of the day this adjustment is massively effective at increasing the performance of any ski.

 

The new Denali fin does not rely so heavily on 'offset' to get the job done, but thats not to say it cannot still be used in tandem.

[customski]

That video made a lot of sense to me, I was having too many hair on fire offside turns and went what i thought was "safer" with some more depth as well as slightly longer and less dft. Have had 6-7 sets since then and feeling and skiing better.

Thanks for your explanation @Horton

 

[Ed_Johnson]

I can totally testify what the Adam's are saying about differential depth adjustment. Takes less than 5 minutes to do it, costs about .15 cents, and the results are amazing. Adding one standard and one thin washer resulted in a depth change of .030, and completely solved my overturning issues with the D3 NRG. I cannot thank both of them enough for this advice.

 

 

[Horton]

Reading the thread by @Tap about fin surface reminded me that my original thesis was lost in this discussion. I happen to be a firm believer that a shallower fin or less total fin for On Side is almost always better. So for the "fan boys", yes, I think Adam and Adam really found something there.

 

The point of the video and this thread is that depth impacts tail depth and ski attitude. My question for the brainiacs is if surface area and Center of Pressure are kept static why would a deeper fin pull the tail down more than a shallower fin?

[Adam Caldwell]

@Horton,

 

While you can keep surface area constant, I'm not sure if its possible to keep the center of pressure the same for two identical fins of the same geometry when going from long/shallow to short deep.

 

A deeper fin is going to plow a deeper or taller column of water. That column will act to drive the fin deeper in the water. Think of it in an extremes. Imagine pushing a 1" deep 10" wide shovel through the sand versus a 10" deep 1" wide shovel. The deeper shovel will want to dive more as its pushed forward, where as the shallow long will just plow the sand up over it.

 

I think of it like this... Changing depth allows us to play with the delicate balance of lift versus drag ratio, L/D in the tail of the ski. Changes in L/D ratio in the tail of the ski will impact YAW,PITCH and ROLL under all conditions in the course.

 

Anything that causes L/D ratio to drop will cause the tail to sit deeper. Increasing fin depth will increase drag and L/D ratio will go down. Decreasing fin depth would act to increase the ratio and subsequently cause the tail to ride higher.

 

Drag is a function of the effective cross-sectional area of the fin as it slides through the water sideways. As the relative flow of water reaches higher angles of attack, the fin drag will increase.

 

The lift force is being produce by the bottom of the ski. A significant source of lift being generated is in the area of the ski directly adjacent to the fin. A portion of the water flow interacting with the fin is diverted upward into the bottom of the ski and creating lift there. If you have ever heard of someone (like @AdamCord ) drilling holes through the tail and venting the bottom of the ski, this is why - to get it to sit deeper on a particular side.

 

The ski's width in the fin block area significantly impacts overall tail depth in a similar way a deeper or shallower fin would. If we keep the same fin depth but make the tail narrower, then we reduce the lift in the tail while keeping fin drag the same. The net effect would be a deeper riding tail and a higher riding tip. Conversely, make the tail wider with the same fin depth, the tail will create more lift with the same amount of fin drag. The net effect is a higher riding tail that keeps the nose of the ski lower.

 

Its worth noting that increasing the depth can make a ski go faster, or slower. It just depends on the overall final balance of the setup. Our objective is to get the lift and drag to act in harmony with the position of the COM so the ski does what we need it to everywhere in the course. Proper planing angles, ride height and rates of yaw, pitch and roll.

 

This is why asymmetric fin tuning is so powerful. You can have completely different L/D ratios on each side of the course. If making asymmetric adjustments to ski's tail-width was easy, we would have done that a long time ago....but we figured out a better way to do it instead.

 

Going further, the bevels on the tail work in a similar way. A smaller/sharper bevel creates less drag and allows tail to slide through higher. Bigger and rounder bevels will make the tail sit deeper.

 

[Stevie Boy]

So I have been skiing on long shallow fin for a while, there are a few things I am not comfortable with, tried moving bindings around etc.

 

History is I used to mess with my ski a lot and managed to ween myself off of ot now I am not a fan of messing with the ski too much

 

Somebody has given me some numbers for Short and Deep and Bindings back, which they claim are really good.

 

So is there a difference in style required between long shallow or short deep fin.

 

I know you can try it and then put it back if you do not like it, but am very aware it does not take a lot to screw your skiing up for a while with only a few bad sets.

 

67" Radar Pro Build 2017 running pretty close to stock at the moment

 

I am also aware that numbers that work for one person do not neccessary work for somebody else.

 

Any Thoughts ?

[DanE]

From a very reputable source: Long shallow promotes longer preturns and require a bit more patience from apex to hookup.

Short deep results in a more hooked style turn.

[Joeprunc]

Interested in this discussion.

I just picked up a 2018 Vapor Probuild (coming off a Strada), and set it up with a long and shallow. I only have two sets on it so my thoughts are very preliminary.....but ski seems to be very predictable and stable (not as fast as the strada), but from the post above "the longer preturn and patience hooking up" makes sense with what I'm feeling. Maybe I'll play with the fin this weekend.

[Horton]

@Stevie Boy hope you do not mind that I merged out threads.

[Stevie Boy]

@Horton np people have suggested the washer trick to me and I was a bit sceptical but it seems like there numbers of people that do it,

Would still like to know if either of the setups would require a slightly different approach in style whilst skiing.

 

[Horton]

@Stevie Boy no need to change skiing style. Ski as technically well as you can and ski with gear that works w/ your skill set.

[DangerBoy]

I find this discussion fascinating and intriguing, especially the talk about differential fin depth settings. I've done some searching on this forum and the 'net in general and I haven't yet found anything that shows me how this differential find depth settings using thin washers is done and how I might use it to my advantage. Can somebody please be kind enough to point me to a video, thread and/or web page that will teach me all about this part of finlore? You can PM me if you prefer.

 

This finlore stuff is kind of like wandlore in Harry Potter; all steeped in legend, alchemy and mystery... ;)

[Ed_Johnson]

@DangerBoy .... As I said before it works great. I use the same settings Caldwell mentioned before with the CG Fin on my NRG, it's a totally different ski than when it was stock. Plus I use long/shallow, and it turns really quick off the apex, setting a lot of angle.

Here's the info on differential tuning.

https://www.denaliskis.com/single-post/2016/01/31/Fin-Area-Tuning-System

 

 

[Glydon]

@DangerBoy you want to get into "finlore" pickup a copy of @SkiJay 's The Fin Whisperer !

[BraceMaker]

@DangerBoy you literally just loosen off the clamp remove one middle screw slip a washer between the box and the fin and snug all 6 screws back up, the one with the washer tighten last, where to put it and how much it should be is a good read on their site.

 

A slightly tougher method is to remove the box, tap the hole for a set screw and replace the screw with a flat faced set screw.

[DangerBoy]

The diagrams on the Denali skis page aren't that clear to me. What are we trying to do here? Tilt the bottom of the fin slightly to towards the onside turn (i.e. tilt bottom to right for LFF/to left for RFF)? If that's the case, why couldn't you just put a strip of very thin shim metal under one edge of the fin block so the fin is still 90 degrees to the finblock but the finblock is slightly tilted relative to the ski? Wouldn't that accomplish the same thing?

[Ed_Johnson]

@DangerBoy .... Your NOT trying to tilt the fin, your just very simply lowering one side of the tail relative to the fin so the depths are different on each side. That is as simple as it gets. A .15 cent, 5 minute mod.

Key is getting the thickness of the washer or washers to equal at least a .030 difference to have any effect.

 

 

[DangerBoy]

@Ed_Johnson whether you envision moving one side of the ski tail relative to a fin that you imagine is fixed or tilt the fin relative to a ski that you imagine is fixed is the same thing. When all is said and done, if you look at the setup from the rear of the ski and the ski base is on the horizontal, the fin will not be perpendicular to the bottom of the ski it will be tilted or slanted slightly to one side, correct?

[Ed_Johnson]

NO...NO...NO...The fin is not tilted !!! Only the bottom of the ski is lowered on one side, making that side shallower.

 

 

[Horton]

There is no tilt. You are deforming the bottom of the ski so the fin is effectively smaller on one side.

[BraceMaker]

@DangerBoy

 

 

[BraceMaker]