The Physics of Fin Adjustments (Questions, not answers)

[skispray]

I would greatly appreciate it if any very knowledgable skiers out there that can help me understand the mechanics of how the fin (and adjustments to it) affect ski performance.  I'm looking for a deeper understanding than, "If you increase the length, it will lower the tip on the offside."  That's good to know, but why is that true?  I have a desire to become a bit of an expert on fin adjustments and I don't know where to start, so here are some questions:

DFT:

From the D3 website, http://store.d3skis.com/Articles.asp?ID=142:

"An adjustment forward (toward the tip of the ski) lifts the front and drops the tail during an on-side
turn. An adjustment backward drives the front into the water and raises the tail." 

Since it is implied that all other variables are constant, I can rationalize the above statement if moving the fin forward makes the ski "shorter" (In the sense that the distance from the tip of the ski to the back of the fin is less as the fin is moved forward).  Since binding placement is assumed to be constant this would shift the skiers weight distribution to a point further towards the "back" of the ski.  Hence, "lifts the front and drops the tail."  

However, the statement only mentions the on-side turn, why wouldn't this be true at all points in the course?  Is it really true that a DFT adjustment will not in any way affect off-side turns, or perhaps the degree of angle you can achieve across course, or speed through the course, ect?  One of the Adjustment Options on the D3 page I linked to above says, "Difficult to initiate angle across wakes Move fin forward."  So DFT has to affect how the ski acts across course as well right...

My intuition tells me that a DFT adjustment either makes the ski longer (by decreasing DFT) or shorter (by increasing DFT).  A shorter ski seems like it'd be more responsive but less stable at all points in the course than a longer ski.  Is this correct?  And, If this is correct, does anyone know why it's true?  It kinda makes sense to me that the further forward the fin, the shorter the ski would feel, but why?  To explain my confusion, consider if the fin was directly beneath the bindings.  The tail of the ski is still back there, and presumably still affecting the skis performance, so although I don't have trouble believeing this to be true, how is it that fin placement fore or aft affects how long the ski feels?

 

Depth:

From the D3 website, http://store.d3skis.com/Articles.asp?ID=142:

"More depth improves stability and holding power, while less depth makes it easier to turn."

I assume the depth of a fin would stablize a ski in the same way that a deep fin would stablize a sail boat, and so this movement makes sense to me. 

However, I know there has to be more to this fin adjustment since one of the Adjustment Options on the D3 page I linked to above says, "Too much ski tip in water on both left and right turns Increase fin depth." 

What is the relationship between fin depth and ski tip?

Finally, given that you don't feel very unstable, wouldn't it make sense to run as little depth as possible while ensuring the tail of the ski doesn't blow out at the end of most turns?

 

Length:

From the D3 website, http://store.d3skis.com/Articles.asp?ID=142:

"An increased fin length drives the tip of the ski into the water during the off-side turn. A fin with shorter length raises the tip of the ski. [...]  The less the leading edge of the fin is out of the ski, the more the fin will keep the front of the ski up. The longer the leading edge, the more it drives the front into the water."

My intuition is that the longer the leading edge, the more water is "hitting" that leading edge and putting upward pressure on the tail of the ski, thereby driving the tip of the ski into the water.  Vice Versa for a shorter length.  However, this leads to some questions:

1) Why does the description only say this is true on the off-side turn?

2) Wouldn't the Depth adjustment affect the ski in a similar way?  I guess is the fin is deep but the length provides for a shallow angle of protrusion from the bottom of the ski, then the pressure caused by water passing over the fin may be less...but I'm just guessing here.

3) Is my intuition actually correct?

 

Wing:

From the D3 website, http://store.d3skis.com/Articles.asp?ID=142:

"The wing is designed to help slow the ski down as you approach the turns. The more angle you set
on the wing, the faster your ski will decelerate into the turns. The tolerances for the wing angle should
be kept between 6-10 degrees."

I just think that there has got to be more to how the wing affects the ski than this.  For example, it seems that a greater degree of wing would drive the tip out of the water while less wing would drive the tip down.

 

So, is there a better way to understanding how the fin works?  Perhaps understanding what the fin does on the onside/offside?  Would understanding the fin in a more holistic manner be more useful than the "DFT does this. Depth does this. Length does this." manner?

Also, how can I learn what adjustments need to be made to a fin by watching another skier fromt the boat? 

Thank you to anyone that takes the time to respond to my post.

[Horton]

Wow good stuff. This is the subject matter that got me to start my first forum. Please Read this http://www.ballofspray.com/tech-articles/53-2004-lever-article

 

[Horton]

If anyone smarter then me wants to take a swing at a new version of the Lever Artical, I am totally open to edits.

[Than_Bogan]

Super-interesting article.

One of the many thing I've never grasped is why certain adjustments are more relevant to one side or the other.

While obviously (as you state) your article is more of a hypothesis than a fact, it does make a lot of sense, and hopefully can inform future testing and new hypotheses.

GREAT contribution to the community.  Thanks.  I wonder how I missed this for 6 years...

[Horton]

Do not make fun of my spelling! We all have strenghts and spelling is not mine! (Freak'n Papa Smurf keeps trying to teach my to spell. I am over 40. Not going to happen)

[Thomas Wayne]

Additionally, the following information was [ reportedly] written by Kris LaPoint in the late 70's"

"Where you place the wing on the fin makes a big difference–no, make that a huge difference. Even on the same ski, you may position it differently than another skier would place it. It takes plenty of trial and error to find the optimum location, but it’s worth the effort. Here are some general guidelines for adjusting your wing position:

"Moving the west wing toward the rear (putting the trailing edge of the wing about 0.5 in. From the rear of the fin blade) pushes the ski out in front of you in the turn and makes the ski finish the arc more quickly. This is especially good if your ski tends to finish behind you at the end of the turn.

"Moving the wing forward, to about middle of the fin blade makes the ski stay in the turn longer and helps keep it underneath you while finishing the turn.

"Moving the wing up the blade, toward the bottom of the ski, lets the ski roll up on edge easier in the pre-turn and may slow the ski slightly more than a lower placement.

"Lowering the wing placement provides more stability. The ski feels more locked in and is also harder to roll up on edge during the preturn.

"Wing angle is a totally separate issue. If you are using a wing, it has got to have some downward angle. The minimum is about 5 degrees and the maximum about 12 degrees. Common knowledge says the more angle, the better the ski slows down and the more drag the ski encounters while traveling across wakes. I don’t disagree with this, but wing angle plays another important role–it also affects how the ski turns. More wing angle promotes rounded constant-radius turns, while less angle yields a turn that’s slower to start with but very hard to finish. Most skiers like the characteristics of more angle on their off-side turns, and less angle on their on-side turns.

"Like a lot of things, with fine tuning your ski you must find the best compromise. Spend a lot of time trying new locations and angles you think might not work. You may be surprised how much difference it will make." [emphasis added]

TW

 

[SM]

I can't answer most of those questions but I can probably confuse the issue by saying that each fin dimension change has an impact on the others. For instance, with my bindings at 30" (stock) I have run my DFT as far back as .60 and as far forward as .95 and skied the same amount of balls. When I have run the fin far forward, the length is very long. When I run the fin back (as far as possible), the length is very short.

 

In both scenario's the ski skied pretty much the same. The difference between these two groups of settings is that the far forward setup is way more sensitive to temperature change and different ski sites than the far back setup, and more recently, I have opted for the far back setup. (Lucky Lowe setup my ski like this)

 

Getting back to my first point, It's my experience that DFT and length seem to be somewhat interchangeable. (My depth setting is within a couple thou with either setup.) So within reason, you could choose a number for one of the two dimensions and adjust the other dimension to accommodate it.

 

More useful than this, Schnitz describes the process by which he sets up a ski - which is very informative.

 

http://www.schnitzskis.com/skitips/skituning.html

[Than_Bogan]

I would never make fun of anyone's spelling!  (go-go-gadget edit-button!)

[Horton]

I think ski design has changed enough since that was written that you may want to take some of it with a gain of salt.

[Jim Neely]

If someone would create a comprehensive document on ski tuning, I would be willing to pay for a copy.

What I find is a lot of information that's stops short of being complete. For instance take the symptom of inability to get wide. Some people will say you have too much tip, or not enough depth or too much depth. The complete answer depends on what is causing you to not get enough width. Are you not getting enough width because the ski will not roll up on edge or because the ski will not hold angle cross course or is something else going on?

Here's another example:

Tip rise out of the off-side turn. The standard answer is add tip but I find that I can get tip rise if I'm running too much tip because I'm trying to compensate for too much tip by standing on the rear of the ski.

These are just an examples, not meant to start a discussion about these symptoms.

[bmiller3536]

Find something that works and leave it, you will get more results working on your form than chasing a magical fin setting.

[ktm300]

It took me a decade longer to learn than it should have but, if a ski will not turn one side or the other after a few rides on it and some adjustments that seem like the right correction, it probably never will. Get off of it.

[Thomas Wayne]

 

Steve Schnitzer pretty much wrote the book on fin tuning, and his original checklist - since modified (for the worse, IMO) - included caveats about addressing proper binding placement and correct body position.  It should go without saying that once a ski is set up correctly (i.e., to factory numbers), establishing a consistent technique and style should either precede or closely accompany any additional tuning.

If you're "all over the place" on your ski (as I often am) there may be no amount of tuning advice that can do much good.  That said,if you can get your hands on a copy of Schnitz's original tuning list - which at my lake is actually referred to out loud as "The Bible" - you will have the very best guideline I've seen in 20+ years of ski tinkering.

TW

[Drago]

on side=weight on back foot

off-side=weight on front foot

one slides more than carves

I think it's science mixed with art and some voodoo. Can't write a definitive. one of the most knowledgeable told a friend of mine once, I would add a little depth...or take a little out.

[ForrestGump]

I'm convinced that my fin works better with one ski vest vs the other. :)

[Than_Bogan]

"I think it's science mixed with art and some voodoo. Can't write a definitive. one of the most knowledgeable told a friend of mine once, I would add a little depth...or take a little out. "

This is so true.  And I even think there' s a reason for it, that somebody alluded to earlier (maybe in a different thread).  When something is way off, the skier is trying to compensate for it, and may over-compensate, making it look like the problem is the opposite of what it really is.

Earlier this year I was trying to help a local skier get his ski set up, and I was pretty darn sure that his boots were way off, because he was rocking back and forth -- sometimes on the tail, sometimes burying the ski.  HIs symptoms had me so confused that I actually took a set on it and I found myself doing the exact same thing -- sometimes way back, sometimes way forward.  After all this, my first guess was the wrong way!

[kona]

"If you're "all over the place" on your ski (as I often am) there may be no amount of tuning advice that can do much good. That said,if you can get your hands on a copy of Schnitz's original tuning list - which at my lake is actually referred to out loud as "The Bible" - you will have the very best guideline I've seen in 20+ years of ski tinkering."

 

Since my water skiing exp is limited let me draw from my vast knowledge of snow skiing as I have been doing it for 40 + years now and racing in my younger years.

I can pretty much take any ski in half way descent condition and tune it to work for me but I can a take a good ski set up to rip for me. I can put a someone of less experience on that same ski and they hate it. As TW stated above all the tunning in the world will not compensate for bad form. That is why I am at factory numbers on my senate and working on my turns right now. I want to have the best equipment I can use right now but I think continually tweaking my boot and fin position to get the ski over my body position is a bad idea right now.

My 2 cents

 

That being sad where can I get a copy of the Schnitzer's org tunning guide. :)

[Thomas Wayne]

It's not nearly as good as the original version (IMO), but it does contain most of the basic info:

http://www.schnitzskis.com/skitips/fintuning.html

TW

[Roger]

Clicky:

http://www.schnitzskis.com/skitips/fintuning.html

 

[Than_Bogan]

Schnitz's page also contains a really interesting idea for measuring DFT with much finer repeatability.

At first I was pretty confused about measuring the thickness of the fin, but I finally realized that's just a calibration stage that is used to figure out something about your caliper.  Measuring the thickness of anything by those two different methods can give the desired information (or else I still completely misunderstand!).

I definitely gotta try that -- I've been way too cheap to get a slot caliper, but my repeatability on DFT is probably not even 0.01", much less the desired 0.001".

[kona]

I picked up the fin tool from BOS and its a great tool DFT

[h2odawg79]

Factory specs are certainly the Baseline and should be very close for the masses. But, when you factor in that these "Factory settings" where possibly derived from the ski's R&D using a 170 lb. Pro skier skiing on a 66" ski @ 36 mph, in the warm waters of FL. I can't help but wonder what the optimum #'s would be for Joe Average skiing Longer line, possibly slower speeds, wieghing 195+lbs. on a 68" ski in much cooler waters and/or higher or lower viscosity, etc...  The differences in techinique, ability, speed, body wgt. water temps can be like Night and Day. 

[skidawg]

You would be surprised dawg#2 how many companies actually get input from non 170lb pros riding a 66" ski at 36 mph!

[h2odawg79]

That may be true to some degree across the board. But, how well does that serve Joe Ave. who wgh's 200 lbs. skiing 34 mph or under, @ longer line and in colder waters, etc...??? From what I gather, everything for the Bigger skis are all derived from the proto type Model, which is generally the 66" and then the rest is kinda Guesstimated from the Base line of the Proto type.

I guess my point is, that if I were 170-180 lbs. skiing simular speeds and line lengths and water temps. as the Pro's who helped develope a particular ski, then I would have a lot more confidence in the factory #'s being VERY reliable. Wouldn't it be Awesome if current data was available to better match your Skiing style, speed, line lengths, water temps to a particular ski? -Good bye to the days of the "Crap shoot" or the "Shot Gun" approach to buying ski's and adjusting them!  

[Horton]

Not 100% true. From what I have seem most skis start out as 66 inch and then the larger sizes are developed. Each size then is refined. It is true that when skis first come out sometimes then numbers are funky but that is true across the sizes.

[skidawg]

Not much of a crap shoot anymore w jth's ski test

fin data base Internet info. Anyone w a little time can research and determine what works and most companies have a few wks money back policy or u could commit to buyin a ski from TADD and ride till u get what u want!!!

[Horton]

Fin dbase went away thanks to the hackers

[aguylikeshark]

A skier jets through many phases while skiing the course, all rapidly linked together. For example the turn entry, apex and exit or finish etc. So fin adjustments affect each phase and each side a bit differently. It's a very dynamic force balance, nothing more and much easier to talk about than write about. If you write it all down it'll be a thick tech manual and probably take a couple of months to draft.

 

Lakes around the world ski very differently. At FM we created the skier's workbook about 5 or 6 years ago using a few dots on the graph. We have written about viscosity changes and the affect it has on skis. The Skier Workbook & tools work pretty well and has been downloaded by 1000's of skiers. So have a look at that for a primer.

 

How the ski rides for you is very important after that the fin is (to me) fine tuning. Many of the top pros are on custom built skis.

 

Settings start make real difference at shorter lines and higher speeds. 15 off 30 or 32 mph getting boots and DFT right is about all you need to do off stock. It is much better to share settings with skiers in your local environment than across the globe.

[kona]

Good words of wisdom from above ^^^^ Makes sense to me.

[A_B]

The Goode "slotted" caliper makes DFT easy to repeat. Although, the jaws are different from the standard Miutoyo that their base skis used to be setup from. I have a Goode calibrated caliper and compared length on my caliper to the length measured by the new Goode slotted caliper and they was quite a difference. It is due to different sized jaws.

[ktm300]

Paul Jager has written a lot about this subject as he noted above. Because we are dealing with something physical not mystical, I know that there have to be some scientific principles to explain not only what adjustments to make for various skiing issues but WHY to make them. I suggest Paul's stuff as a starting point for any discussion about fin adjustments. His water viscosity article is either correct or not from a hydrodynamics perspective. At least his articles make arguments that purport to be based upon scientific physical principles. Really good apparently under-appreciated stuff. If he is wrong about some of it, at least it creates a great foundation for discussion.

[ForrestGump]

Jager's fin program works. Crazy, wacked out numbers on my Strada(using his rs1 tab as a base), but it works in this 95-98 degree water. 

[Steven]

I've been saying it for a while, Jager's fin stuff worx!

[Horton]

The part of my Lever article that I now think is lacking is DFT.

I have seen two instances where DFT has had a big affect on outbound direction at edge change. In both instances the result was counter intuitive. I moved my fin FORWARD to achieve more space/width. My assumption is that with the blade further forward (shorter lever) the ski is less out in front and is prevented from rolling over as fast. Slower Roll at edge change logically means more time spent traveling outbound.

Further back also seems to impact how the ski flows out and back on Short Gate. Further back is better but I have not clear understanding. All I know is that on some skis moving the blade back lets me draw a smooth arc out and back.

[Thomas Wayne]

"...the result was counter intuitive. I moved my fin FORWARD to achievemore space/width. My assumption is that with the blade further forward(shorter lever) the ski is less out in front and is prevented fromrolling over as fast."

Another possibility is that this result fits in with perfectly your Ski-as-Lever hypothesis.

If the fin is back then the tail should have more leverage, thereby reducing the maximum amount of angle achieved at the finish of the turn.  Move the fin farther forward - all other things being equal - and the front of the ski should have greater [potential] leverage, allowing the ski to gain more angle before hook-up.  More angle out of the turn ought to equal more width & space... yes?

TW

 

[Horton]

I am really talking about what happens right off the second wake.

[Steven]

More angle off (directly) the ball would cause you to have an earlier edge change sending you directly at the ball wouldn't it?

[Horton]

Yea. True True. In my case extra angle is not really an issue. I do a lot wrong but angle off the ball is not my problem.

http://lh6.ggpht.com/_4-6ay9R-bLE/TDFoTQ5DhVI/AAAAAAAADFA/lJuy6iZJL1Q/s800/DSC_3436.JPG

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Thomas Wayne]

"More angle off (directly) the ball would cause you to have an earlier edge change sending you directly at the ball wouldn't it?"

Sure... if directly off the ball is where you stop turning and start pulling.

http://www.pbtcenter.com/PBTC/Chets_thoughts/Entries/2010/4/29_Spray_to_Spray.html

TW

[Horton]

My point is lost as usual.

DFT has an effect on how the skis acts at edge change.

[Steven]

It's funny, the first time I tried the Jager fin set up my depth was at 2.525, length was 6.79 (ish) dft was around .698 and the boots were 29.625 on a 68 MPD.(idon't remember for sure right now the exact numbers except the depth) but you'd think with that set up I'd be riding a school bus but my 1st set on it I got around 2 @ 38 which is real respectable for me. I knew it wasn't going to work! I guess I was wrong

[Donski]

Maybe i hve not been lurking enough, what is Jager's fin program?

 

[MattP]

It is a spread sheet based program that takes in account your ski size, your weight , hight, water temp, viscosity ect. to produce fin numbers http://jagersport.com/FM_FinChart_2009a.xls

[aguylikeshark]

Thanks here's a 2010 update - corrected some calc errors and removed images for quicker download.

 

 

http://www.jagersport.com/product_info.php?cPath=87&products_id=243

 

I have not skied any new skis this year. riding the 66.25 A1. The A1 ripped down in Mexico on the salt water, temp was like high 80's so it was much softer than our lake has ever been. It was so much fun. I ran stock settings for a set and then had to use the slide rule from there since I could only view xls files from the Club Med computers!

[Tuney]

What do you guys use to measure wing angle, just a compass from a geometry set?

[Steven]

wing angle gauges. you can get them from Goode or I've seen them on ski it again. 

[MattP]

h2oproshop.com has some good ones for a good price

[Deke]

I have these http://www.boothelectro.com/waterski.htm

[Ed_Obermeier]

Does HO still put out their wing angle gauges from 3 - 4 years back?  Compact and you could adjust in 1/2 degrees if you wanted to with those.  Love mine.

Ed

[Ali]

 I "won" a set of the HO wing angle gauges on a waterski forum. Since then I know that the internet is good for something..............

 

cheers

Ali