Fin tuning - how can it be so critical?

[Horton]

Most short line skiers accept that a fin move of .003” or .005” is enough to make a real performance difference. In my mind, this is especially true of DFT. Most of us accept this to be true but if you step back a second and ask how can this be true it is really seems impossible. I mean how can this be real? Any theories?

[PatM]

Higher the speed the more it is affected by small increments? Magnification. At a slower speed the movements are not effective. And by slower speeds I don't mean 30 vs. 34 or 36 mph. I mean 10 - 15 mph. I don't know I bury people for a living.

[Than_Bogan]

Fun topic. I'm certainly not going to provide a solid answer, but lemme throw around some incredibly rough ballpark analysis and see if something interesting emerges.

 

Several measurements, as well as some theoretical analysis, suggest that a short-line skier experiences at least 2g's of acceleration in the turn. This vector is roughly perpendicular to the fin, so it lets us make a ballpark estimate of the force from the water against the fin. F=ma so force here is m*2g or 2mg. (That's not milligrams! Skier's mass times the acceleration of gravity.)

 

Conveniently, what we experience as our own weight is simply our mass times gravity, so the force acting at the fin is somewhere in the very rough neighborhood of twice the skier's weight. So in my case about 350lbs.

 

This acts as a torque that prevents the tail from sliding right around, and balances the torques from the front of the ski which are acting to carve the turn, such that the net torque on the skier is just right to perform a smooth turn. As kind of a fun sidenote, the magnitude of the torque from the fin end looks to be around 900 ft-lbs.

 

Ok, so now we change the length of the lever arm via a DFT adjustment of, just to pick a super-convenient number, 1/1400th of a foot, or about 0.0085". The 350 lbs acting there, multiplied by 1/1400th of a foot, is an incremental change of 1/4 of a ft-lb.

 

As a percentage of 900, of course, this is negligible, but remember that this torque is maintaining close to an equilibrium, so what we can "feel" as a skier is just relative to that equilibrium of low net torque.

 

SO, can a person feel 1/4 ft-lbs of torque? Yes. It's not a lot, but certainly not below perception.

[Dano]

I’m just a hack of a skier spend most of my time 28@34, but here is my experience. I spent last year adjusting my fin. Tried large moves like .1”. And small of .0005 the bigger moves could definitely feel the difference in the way the ski felt, but really had no impact on the buoy count. Front binding adjustments were the single most noticeable and effective way to change the ski’s behaviour. This year I have my boots where I like them already, the fin will be at factory recommended settings and I’m leaving the tools at home so I can focus on just having some fun.

[kurtis500]

@horton "Most short line skiers accept that a fin move of .003” or .005” is enough to make a real performance difference. "

 

A human hair is .004 and people are saying they can feel that?

[lpskier]

I’m not agreeing with the width of a hair, but you can definitely feel any .005 fin adjustment.

[Horton]

@kurtis500 not only can we feel it, it could be the difference between a well-behaving and misbehaving ski.

[kurtis500]

@Horton what measurement or alteration can be moved .004 that goes from well behaving to misbehaving? Is the ultimate tool of measurement ‘feel’?

[JAS]

Would be interesting to see what the Adams think about Than’s assumptions and calculations? Would be easy to setup a blind test to evaluate an elite skiers ability to identify fin changes. I think everyone would be interested in results

[Bruce_Butterfield]

@Horton if 0.005” makes the difference between “well-behaving” and “misbehaving” you are on the wrong ski.

[kurtis500]

@lpskier what .005 adjustment can you feel? And what other factors do you calculate when making this adjustment.

[Horton]

@kurtis500 I guess "feel" is all encompassing so yes but it is about performance that can be measured by ball count.

[wettek69]

Just to throw in a hand grenade, how much is fin adjustment a placebo effect? "The experts say if I move my fin .005 this way, my ski should feel different. Wow, yeah, I can really feel the difference" Sorry guys, and this is just MY OPINION, not being good enough of a skier to need to adjust my fin, I find it hard to believe moving a fin the distance of a human hair can make that much of a difference. Again, this is just MY OPION, so don't flame me for questioning it.

What next, do we start putting index marks on handles to adjust hand position, or visual spirit levels (preferably filled with gin) on the decks of our skis to adjust hip position 1 degree forward or back?

[Horton]

@wettek69 learn to run 35 off or shorter and then tell me it does not matter

[Than_Bogan]

@kurtis500 @wetteck69 We've all been there. It sounds ludicrous. Then one day you find yourself starting to plateau and banging your head against a very short line length, and so you decide you should try it. The expectation is "of course this will do nothing -- I can't even see the difference."

 

And then you ski.

 

And the next thing you ask yourself is "WOW. How can it be that altering that fin by 0.01 inch has such obvious changes in feel?"

 

I don't think I've ever talked to a deep shortline skier who claimed 0.01" didn't matter. Thousandths are much more debated, but just about everybody trying to figure out -38 or deeper can feel a hundredth.

 

Do other things matter more? Of course! Learning to actually ski better will beat fin adjustments every time.

[kurtis500]

If a ski adjustment is made at one (air) temperature then thrown into the cooler water at a change of say 25F, what does that do to the fin? Plug in the fin length and area into a CTE calculator. Find both linear and volumetric expansion rates....it’s a set of calculations....scientific calculations.....then see how much of a .002-.004 tolerance is left after just the temperature change. Making an adjustment in the morning versus one made in afternoon heat and thrown in the same water temp will yield another number.

 

If a tight tolerance like that is required then why are theses made of aluminum. Aluminum will warp more than most other metals. These factors must have been calculated in somewhere if aluminum is the choice

 

@horton maybe rethink the carbon fin just on CTE versus aluminum alone.

 

Hmmm

 

[kurtis500]

@the_bogan

 

Your number of .01 is 2-3+ times the size of what was brought up. Just mentioning so nobody is confused

[wettek69]

@horton I did make it quite clear it was MY OPINION only. I was not debating it it, if you reckon you can feel a massive difference with a .005" adjustment, go for your life. You may have a slightly different body position on the day or just be skiing better (or worse) or the .005" might be making the difference.

I don't need a bad attitude for making a comment.

[Horton]

@wettek69 ok sorry for being grumpy. At shorter ropes, if very small fin movements make a big difference or not is not up for debate. That is the point of the original post. Those of us who have been doing this for years generally accept it to be true but frankly it's bizarre how sensitive fin adjustments are.

 

Sensitivity can be different depending on other factors but generally speaking if you take a good setup and you change any dimension by .010" the change is far beyond placebo.

[JAS]

@horton I took @wettek69 comment to allude that with every fin change there is a mind set. Do you think there is a great enough disconnect between these to differentiate between fin change and change in skier impute?

[wettek69]

@horton No worries mate. As I said, I am far away from needing to adjust my fin, I was merely commenting as an "outside viewer". With the technology in skis these days, I would not have thought .005" adjustment on a fin would be enough to offset the effects of tiny changes to body position, slight change in foot position in a boot due to a sore toe, or even a full belly from breakfast. .005" is .125 of a millimetre, .01", .25 of a millimetre. It really is literally a hair breadth difference. Just the difference in feel different people have on a vernier can be worth a couple of thou either way.

[scoke]

@wettek69

 

Your opinion and position is very curious to me. Could you share more details about your experience?

 

What starting speed and line length? What’s your height and weight?

 

Thanks mate.

[The_MS]

Fin adjustments don’t work. If anyone had a new Pineapple ? don’t bother tuning it, just sell it to me

[Wayne]

A few thoughts and questions that this tread provoked. @Horton how do ski manufacturers quantify ski performance? We look at bouy count but some time last year I started to think that bouy count is really an assessment of the skiers ability to use a particular ski.

 

Recalling threads I’ve read on this forum I believe skis are more subjectively evaluated for performance that objectively. Bouy count is a byproduct of ski performance, skier ability and environmental conditions. Skier ability and environment conditions are really noise factors that will have an influence on the skis assessment.

 

Here me out now, as I’ve been trying to develop something for a while and I’m kind of stuck so it just sits in the back of my head. In the automotive industry we’ve been able to objectively assess vehicle performance since the car was invented. 0-60 MPH, cornering acceleration, fuel economy and so on are objective measures. However it been only over the last 10-15 years we’ve had methods to assess driveability objectively. For years engineers provided an “expert opinion” on a 0-10 scale of how certain driving events felt. Across the industry most experienced engineers would have subjective ratings for the same event that were with in a half a point of each other if they evaluated things independently. I think the same holds true for skis but we’re stuck in assessing things subjectively and using a byproduct of the actual ski performance (bouy count) as the measure.

 

So what objective things can we measure to assess the performance of a ski? I really feel this would give evidence on how small, or large, adjustment increments should be and it would tell us if a ski is overly sensitive to a particular adjustment.

 

PS for all I know ski manufacturers are already doing this and just keeping it a secret. I’ve really been interested in being able to put measurement devices on a ski and capture real time data. I’m just not sure what all would be useful or practical for the budgets of a ski company let alone the average skier.

[skialex]

Maybe a 005” is not noticeable by all levels, but skis come out of the factories with fins at way off the stock settings and people just put their binders at the middle hole and expect a better feeling and an increase in buoy count.

This is why when spending close to 2 Grand for a new ski, it worth buying it from a shop or a ski school that can set it up for you.

If you are knowledgeable with fin measurements and adjustments, or know someone to help with, then it’s all good, but riding a ski out of the box and judging it’s performance or the brand is very wrong.

[dbski]

An interesting test of the placebo effect would be a blind test. Take a top level skier and without telling him (or her) either change or not change a fin adjustment. Have them take a set and see if they can correctly say what was done. Repeating this several times would be interesting. I know at my level 99.9% of my problems are above the ankles not below!

[Horton]

saying fin settings are not critical is like saying...

 

 

[wawaskr]

And of course all this discussion is assuming the bio-mass on top of the ski repeats all the physical positioning and movements identically the same each ride - correct?

[Than_Bogan]

@wawaskr Not sure if that was sarcasm, but of course not. Humans have terrible repeatability. That is independent of whether they can feel small forces.

[thager]

I have blind tested friends fin settings many times. It does make a difference. So fun to move their fin settings small increments without their knowledge, then enjoy the chaos. So entertaining to watch their frustration for that day! None of them have ever skied better with the new settings. Every one of them goes home and resets their fin to their numbers without me ever telling them. Most devious thing I ever did was to wax a friends ski bottom touching nothing else. Surprisingly it only cost my bud a pass that day but was enough to win our weekly beer contest. Next day he skied a PB on it. I've never told him.

[Horton]

@wawaskr No the skier is always a factor.

 

For any ski, fin & bindings combination the ideal setup compensates for some amount of skier inconsistencies and inadequacies. Every skiers is going to have a different mix of inconsistencies and inadequacies. That is why two skiers on the same ski with similar high scores may run different settings. That is also why the settings a Pro skier uses is often far from ideal for a lesser skier.

 

On the other hand an experienced skier knows or should know when they have executed "good enough" to expect the ski to do certain things. Experience means you know your level of inconsistencies and inadequacies. If you execute the way you think you should and the ski does something unexpected then either your expectations are misplaced or the ski is set up wrong.

 

As an example: In my case I expect to be able to consistently turn off side and be in a strong position to the wakes. If ski is does bad things ( loose tail, tip rise, slow finish, whatever ) at apex of off side I am getting out the tools. On the other hand if I am struggling at On Side the spot light is on my skills first because that is where my I need the most technical work.

 

To put it more simply - if I can not run my opening pass ( 32 off ) early and easy almost every time then something is wrong. That is the first place I will judge a ski and set up. It is not pass where the more minuscule issue arise but is a good starting place. When 32s are perfect I will work at 35 until it feels the way I want.

 

So is the skier an inconsistent factor? Yes but with experience that accounted for.

 

[RGilmore]

@Wayne You should go back in time on this forum and read the stories about the early development stages of the Denali ski. Tales of multiple pressure gauges mounted all over the ski, providing varying real-time values throughout the course, all being recorded by a go-pro camera for later analysis - that's the is the one I loved. 'Bout as far away from merely relying on a buoy count as you can get.

[Horton]

@Wayne that is a super interesting question. The bad news is objective data simply does not exist in this sport.

 

Imagine the auto industry did not have 0 -60 test or such. If all they had was Nürburgring times that is still a pretty interesting metric. It leaves a LOT of questions unanswered but it also says a lot. Ball count is the only NUMBER we have.

 

Besides ball count the other measure is how it felt ( in so many different ways ). I would love to be able to get any sort of data telemetry but the gear does not seem to exist / be practical.

 

[kurtis500]

Theres way too much data missing. To know what the fin is doing we first should know what the structure is doing, in this case the structure is the ski. Just for starters there should be flex, torsion, CTE and other numbers from the ski makers. These numbers really should be taken from NIST equipment so consistency is known. ANSI tests that all the skis are subjected to would show which is stiffer, softer or etc. Without these numbers theres no way to tell exactly what the ski is doing under a load. Just getting back in the boat and downing a sandwich and a couple bottles of water then jumping back on the ski changes the load on the ski and the amount of flex. And theres a lot of ways to test flex..

 

NIST equipment and ANSI tests..... Anyone know if the skis have undergone this testing and is the data out there? Without it the case made that .002 can drastically alter a ski seems too incomplete.

[DW]

With water being 25x the density of air and the lever arm ratio of fin to ski CG, small adjustments can certainly impact how the ski performs.

 

Probably more significant, depends on who is riding the ski as to whether small adjustments make a difference. I recall reading how finicky Aryton Senna was on everything within his office (the cockpit of an F1 car), down to 1/16th's of an inch on component placements such as shifter / pedals / steering wheel, etc. Crew thought he was full of it so once tried to trick him. Never happened again... (FYI - Senna was one of the all time greats).

[Horton]

@dw Is like how can a NASCAR driver feel 1 psi of tire pressure.

[MISkier]

@Horton, in your solar system graphic, what is your contention: that the Earth isn't flat or that Pluto is still a planet?

[mbabiash]

I do think DFT is the most touchy adjustment as well.

Quick question though. Can more DFT help the offside finish of the turn too or is it just felt in on side.

[Joeprunc]

Great, so you are telling me the most difficult fin measurement is the most critical? I feel like you can give the same pair of calipers to three different skiers, and they will all pull different DFT measurements from a ski. The change in ski tail concavity, placement of caliper, and tail bevel I feel all play into this inconsistency. I'm not an expert by any means at adjusting fin dimensions, but it just seems like this is always the most difficult to get consistent dimensions.

 

I guess that's why some folks swear by the GOODE Caliper with the fin slot.

 

 

[AdamCord]

Think about it like a very unstable airplane:

 

You are the fuselage, the ski is the airfoil, and the fin/wing is the tail stabilizer (rudder and elevator). The entire system on the water (skier, boots, ski, fin, wing) is one dynamic system that is constantly changing its center of mass, ski center of pressure, yaw, pitch, and roll.

 

As @Than_Bogan points out you can try and calculate the pressure on the fin based on the moment the skier imparts on the ski, but that is only part of the story. The fin shape and position is changing the yaw, pitch, and roll characteristics of the ski itself. Those changes will then change the center of pressure, center of rotation, pitch rotation, etc, which will cause the unstable dynamic system on top of the ski (you) to make changes in weight distribution in order to find a balance. Tiny changes in the yaw/pitch/roll characteristics to an adaptive dynamic system will have cascading effects moving up on a system with the Center of Mass so far above the Center of Lift on the ski.

 

Very small fin changes <.005" can sometimes be felt between the turns (when the skier has two hands on the handle), but they are much more noticeable in the turn. Why is that? When you are pulling behind the boat you are reliant on an additional anchor point (the rope) which helps to stabilize the unstable dynamic system. But when you lose that connection (reach into the turn) you are a dynamic mass riding a ski that has lift, drag, yaw, pitch, and roll characteristics, which can be adjusted based on the fin shape and position. You are then asking the ski to make a smooth arc-ing turn as it rolls over and comes back between you and the boat. Tiny changes in how that ski behaves at that point can be the difference between running or missing a pass when you are at your limit and right on the edge of losing your balance.

 

Most people will probably not notice a <.005" fin move on their opening pass. They might feel it on their 2nd pass. But as you approach your personal limit you are relying more and more on the ski to do it's job correctly, especially in the turn. Small changes to the yaw/pitch/roll characteristics of a ski can be the difference between the unstable dynamic system on top of the ski (you) maintaining balance (or losing it and then finding it again) through the turn, or swimming/eating a pile of slack/etc.

[Skoot1123]

And THAT @AdamCord is one well thought out description and explanation as to why a fin change can affect us so much and why fin settings are so user specific. Excellent description.

[Horton]

@MISkier I believe in Pluto!

[lpskier]

Is dft a critical measurement (the location of the trailing edge of the fin relative to the tail of the ski) or is there dft measurement really just a way of determining where the leading edge of the fin is relative to the tail of the ski, i.e., dft plus length= location of leading edge of fin?

[Adam Caldwell]

@lp skier

 

Its neither the location of the trailing edge - nor - the leading edge of the fin that is critical.

 

However, we can and do use those positions as a reference for overall performance/feel.

 

What matters is the fins geometry, area, drag and its ability to influence ski stability and speed. There is nothing magical about leading edge or trailing edge position. Modify the fins geometry enough, and those locations will likely dramatically change.

[skialex]

@Joeprunc Best way to get repeatable measurements of the dft, is to hold the ski vertical upside down on your foot. Press and hold the head of the caliper with your thumb against the bottom of the ski and then take your measurements. If done right only challenge is to hold the caliper straight, but gets easier after some practice.

[skialex]

I read a lot of people here saying... it’s not the arrow... it’s the Indian. Well yes as long as your arrow is straight and balanced... so is it? Adapting your technique to make your ski work is not always the best solution.

Ski tuning in my opinion is very important at all levels. It’s a great feeling when you fully trust that your ski is set up right for you and you can work your technique instead of playing with fin and boots..

[Horton]

@skialex i do generally discourage much tweaking for skiers not running 28 off. Set to stock and ski.

[kurtis500]

Rudder/Elevators on an airplane are design dependant on the airfoil/fuselage. The properties of an airframe are known. Do we know what the properties are for the ski before the fin is added?

 

Do skis undergo any standardized ANSI tests? If not, why not?

[Horton]

@kurtis500 water skiing is a tiny sport. No, there are no standardized tests. There are less than 10 people on the planet who truly understand high end ski design. It is basically a "black art".

[kurtis500]

@horton some simple ASTM tests are cheap and easy... But its up to the builders if they want that information