Cold Vs Warm Water: and the effect on ski and settings

[Skislalom]

After reading the comments on the vapour 2016 fin settings, and the discussions regarding water temp and viscositiy and its effect on the ski, I would be interested to look at it further, down here in NZ our water them ranges from 10deg C in winter to 23-24deg C in summer, would be interested to hear from you and your thoughts/ experiments on ski and fin settings, and maybe theories on why these changes occur.

[GOODESkier]

Well, I am just a dumb skier with little to no engineering "science"....... BUT.....

 

I ski on a river that is between 44 degrees F and 64 degrees F. Then ski tournaments on lakes that are 70 degrees to 80+ degrees F. I know that in order to have a working ski, I have to move my front binding ahead in COLDER water and back in WARMER water. Somewhat the same in shallow and deep water for that matter. I call it the cheap lawn chair test. If I fold like a cheap lawn chair on my off side, I have to move front binding back. I tend to use that as my "breaking point."

 

On fin, I tend to find my sweet spot and use the bindings as my adjustment. However, I do add DFT as it gets really cold. Possibly even remove depth to free the ski up a hair, but that is only on a winter set on my river. We are in the upper 50's right now and I have not moved my fin yet, just ahead an 1/8th from the private warmer ski lakes.

[RazorRoss3]

I ski a wide range of water from near freezing to 80F, 4ft deep to 30ft deep, and crystal clear to really murkey. What I've found is warm, shallower (to a point), and murkier feel slower than colder, deeper, clearer. For depth, too shallow and the ski tries to over turn a little bit. Despite that, I never play with settings to account for water. I figure if I'm doing my job right bindings up that the conditions below the ski don't seem to matter very much. The slower water to a point can be more fun though.

[LeonL]

Well here we go again. The debate as to which is slower, cold or hot, and what to do to your ski to adjust for temp changes. My ski partner and I discussed today the seeming feel that the ski doesn't finish the same in cooler water. We wondered if denser water (again the debate) reduces the "schmeer" factor. If so, would shallower settings be in order, or another fin with a couple of extra holes in it.

[GOODESkier]

I still believe, if you are willing to tweak slightly on atleast binding placement between different waters, you have the potential to ski oh so well! Just think what you may have left on the table.........

[Skislalom]

I have given this a bit of thought this morning, I feel that in 50f water my ski feel "skitterish" like it skates over the water, rather than bites, and drives, I am definantly narrow and with a cold body and brain, it feels faster, most of out club does no adjustment to their ski or binding placement and everyone appears to ski better when the water warms up, maybe that tracker device @horton had would give us the variations, if someone could test it.

 

[OldboyII]

@Skislalom Good point.

It is virtually impossible to make precise experiment to compare ski behaviour on cold and warm water.

Our body and muscles and brain act differently when cold or warm.

We ski in shorts & west when warm and in relatively "heavy" wetsuit which put some limitation on movements when cold. Baggy drysuit make it even more noticeable.

Different skis are made from different materials and they react to temperature variations differently.

Etc..

So it may happen that what we think goes from ski in reality goes from our body.

I hope ski makers can do lab temperature tests for their skis and give recommendations.

 

What I personally noticed - in the cold water I feel more confident and agile when I have autumn jogging suit under wetsuit than when I ski in wetsuit only. Though both setups are pretty comfortable and warm.

[GOODESkier]

@Skislalom perfect opportunity for a test. In the colder water conditions try a slight movement forward with bindings. You might be the one at your club to kill it in all conditions/temps. Sounds like that is a GREAT control group of scientific proportions.........

[A_B]

You could ask 10 top skiers and half wouldn't adjust and the other half would tell you conflicting setup changes. You have to test and make sure you record dates and temps and see what works for you.

From my personal testing, I agree mostly with what Rossi wrote. I am somewhat differing on the dft, as a move back will flatten the ski, which is opposite of the other moves, so by itself, I have not found moving the fin back to help.

 

Of course, I found that real late in the season when I was just skiing for exercise, a drop in speed helped the tail settle in better and the cold and stiff muscles seemed to like it as well. So no ski change.

[SkiJay]

For interest sake, here is a chart that illustrates how much water viscosity changes with temperature compared to water density over the same range:

 

 

Density changes, but the change is so small that it might as well be zero. Viscosity, on the other hand, is a very big deal. Viscosity is the measure of a fluid's internal flow resistance, or its resistance to being deformed. The colder it gets the less it wants to get out of the ski's way, so it creates more more drag. Really cold water is about 2 times more viscous as really warm water.

 

It sometimes helps to exaggerate. Honey is about 10 times more viscous than 20°C water. When you are reasoning your way through what might make your skiing better in cold water, imagine what might help if you had to ski in honey.

 

I totally agree with @A_B that cold water advice is heavily divided, and there's a perfectly good reason for this. A well balanced setup will remain a well balanced setup throughout most, if not all, of the full skiing temperature range. Nothing we can do to the setup will return water's physical properties to what they were in summer (honey will remain honey no mater what we do to the ski). All we can do to a great summer setup is make trade-offs.

 

1) We can regain some of the width lost due to the colder, higher-drag water—but these adjustments will make it harder to turn the ski (Rossi's article focuses on regaining lost width).

 

2) We can make the ski turn easier in the firm cold water, but that will further compromise our width (basically the exact opposite of the adjustments listed in Rossi's excellent article).

 

Neither of these two compromises is right or wrong. It depends on the skier's goals, strengths, and weaknesses, and more importantly, on the setup the skier had all summer. We can get away with an imperfect setup easier when the water is warm. But what may be a great change for your buddy as the water gets cold, may just be making your goofy summer setup even worse.

 

The bottom line is that we need to forget about cold water tuning "rules." Either the ski already has a nicely balanced predictable setup on it or not. If it does, then don't change anything (except maybe the wing for drag) and just ski harder and more precisely to overcome the increased viscosity.

 

If your ski is fighting you, take advantage of the cold water's harsh setup feedback and tune your ski based on what you are experiencing, not based on someone else's unique set of circumstances. A ski with a great cold water setup will continue to work well next summer too.

[GOODESkier]

I'll remember that when I help my buddy on the cold river tomorrow. Just ski harder and more precisely would ya!

[Drago]

@SkiJay or... The colder it gets the less it wants to get out of the skis way, so the ski rides higher in the water and therefore has less surface area and resistance ;)

[Siouxcitysmitty]

"...The colder the water the higher the tail rides compared to the tip which puts more ski in the water and more resistance....."

 

Make sense, but, if this is true, it would seem that the "conventional wisdom" of moving the bindings forward, is the opposite remedy for a higher riding tail.

[Adam Caldwell]

Colder water has more drag despite having more or less wetted surface area on the ski. Not so simply, a function of Viscosity, Speed, area and resultant Reynolds number we are operating in.

 

The tail will ride higher, and if planing angle gets too flat because of that, you start engaging tip bevels too much or too soon into the first wake and off the second, killing ability to create effective speed, or maintain desired trajectory off the second wake.

 

Typically fin back moved are needed, as well as slightly less length, and/or sometimes more depth. Boots back is also an option in some situations. Adjustments are a little different for everyone, but concept is the same.

 

People's perspective of colder water feeling so fast is because they loose the ability to create space before the buoy, and more importantly, getting up on the boat and "free".

 

It's less about creating more speed or less speed, and more about giving the ski an opportunity to take the speed generated into the first wake on the right trajectory into the buoy.

 

Colder water forces the ski to turn downcourse earlier if no adjustments are made, thus making the turn radius longer and path narrower. Making it very challenging to make a tight/fast turn on the back of the buoy.

 

A couple small 2-3/1000s movements every 7-10degs can keep your scores high moving into cold water seasons.

 

My belief is that cold water is misunderstood because there's a lot more going on than changes in viscosity. It's also a function of speed, surface roughness, surface area, etc, and the compounding impact on drag, which is no where near a linear function as everyone seems to think/believe.

[ktm300]

@adamhcaldwell Please explain the fin back move. Seems that flattens the ski even more; more tip down? Does the attached PDF have any validity. Mind you, I'm not arguing; just asking. I have discussed this with top pros and get answers that are all over the place and contradictory to one another. Having you throw some science on it would be great. Thanks for helping educate us!

[gsm_peter]

Thanks all for explaining a few things for a intermediate skier (30@15 off).

 

Even though I have a lot of limitations that are not water related I recognize a few things.

I currently ski in colder water (44F) and have problems.

- Turns are even wider

- More falls when ski 'slip's' on surface

- A few OTF that I have not made in a long time

- Gate seems to be more narrow with my normal pull out

 

I felt great coming into the turn and then the ski just slipped away...

 

 

 

 

[Adam Caldwell]

@ktm300

Fin back adjustment will move the ski out in front of the skier more during gate roll in, and as well as in the turns, which subsequently takes pressure off the forebody bevels when loading into first wake.

 

This allows for higher speed to be reached before second wake, which frees up the ski further and promotes better ski trajectory into a wider path with more space before the ball.

 

Once moving out off the second wake, as the ski is rotating into the preturn, the increased moment arm with the fin further back will hold the tip up longer into the apex. (Delays dropping the nose as the ski is banking up, sliding and rotating)

 

However, once the tail of the ski breaks free and is washing/sliding through the apex, the turn may feel slightly more agressive as the tail slides a little higher, and lowers the nose slightly through the finish. Again, the ski should stay out in front of the skier during this process with the fin back, helping to pick up the line with more speed before loading into the first wake.

 

The way the colder water adds drag to the ski overall, and also raises the ride height of the tail, the ski will tend to get behind the skier through the wakes, and pitch skier forward off the second wake with ski tip down too low to early, creating a very narrow line into buoy.

 

In simpler terms, moving the fin back will move the ski in front of the skier entering the first wake. That change in dynamic will unweight the tip of the ski, and again allow for better direction off the second wake.

 

(For me length needs to come out sometimes as well, just depends on how cold the water is getting. Usually once below 70F I am taking length out as well as a rearward bump. And below 60F, might need to even add depth with less length and back again to pull the ski back down in the water in the preturn to help hold skis direction better to keep the space upcourse of the ball)

[gsm_peter]

What effect could be achieved by just adjust the wing instead of moving the fin?

 

I.E. could this be an advice to a intermediate skier like me that already fight inconsistency 99% of the time but still seem to experience cold water effect? :p

[ForrestGump]

I think a lot of people think backwards in comparison to @adamhcaldwell 's explanation. Which I think creates a lot of confusion. I think most people think that moving the fin back drops the tip because of the longer lever arm that it creates and don't consider that it pushes the ski out out in front.

[Dano]

Not the effect of cold water here, but maybe relevant. A few days ago I was having a really hard time with my ski. I was having a very difficult time rolling up on edge, Couldn't build any speed, couldn't get wide, and it simply did not want to turn offside. Because I'm only a 22@34mph skier I thought maybe just a bad day. I read this thread and decided to try a fin move because our water temp has fallen rather quickly here. So I measure my fins current position and determine my Fin length was 6.95". It had moved from where I originally set it at 6.845. I reset it to 6.845 and couldn't believe the difference. All is good again.

 

I realize that fin was out by a lot. But I wouldn't have thought the ski would behave so much differently, especially at my level. If your having trouble with cold water try a fin adjustment.

[lcarnes]

We hit 32 degrees in the morning for the first time Saturday morning, and it's been dropping that low every morning since. That just happened to coincide to me not being able to run even a decent opener. The ski was flying past the buoy and I couldn't get it slowed down to turn for the next buoy, although the skied turned fine once I had it slowed down enough.

 

I texted Adam Caldwell with my dilemma and after some back and forth on what the ski was doing, he suggested a 5/1000 move back. I couldn't imagine that such a tiny move could make a difference. It was truly noticeable from the beginning ...I skied some of my best passes of the month today.

 

What is interesting about this to me is that I ski 30/32 at 15 off. (I'm the low speed Denali guinea pig) While I've always tried to have my fin dialed in, I'm by no means a tweaker. However, this experience has taught me it's worth understanding the differences.

[JohnCox]

I don't move my fin... ;)

 

[Razorskier1]

funny thing is I think many of us have always thought of cold water as "fast", when in fact it is not. It feels fast because we generate less speed and less width, and therefore the buoys seem to be coming up really quick. I have not historically adjusted my ski for water temp. In years past I have run -38 in Florida in late September, then returned to MN and run it here in water that was 40 degrees different with no adjustment. Doesn't mean that adjustments don't make sense -- they probably do. I'm just not necessarily the best test case for temperature sensitive skiing.

[SkiJay]

I'm watching a cold water "rule" take form here and just want to caution against one-size-fits-all rules. A lot of skiers haven't had a proper setup on their skis all summer long. And blindly following a "rule" like this without some understanding can just make matters worse.

 

Moving the fin back will move the water break back on the ski in the pre-turn keeping it more out in front and less likely to turn narrow ... BUT ... it will also cause the tail of the ski to smear less around the ball, and generate higher loads during the cut (crushing and OTF type loads). Nothing is for free—everything is a trade-off.

 

If you have a perfect setup to start with and you're a turning machine, fin-back in the cold can be a worthwhile trade-off. But if your setup coming in is already a bit tight (the tail isn't smearing enough to complete crisp tight turns), then you move the fin back because "that's the rule," you will end up with a ski that turns like a school bus, making you even later in the course.

 

So maybe this notion can be slightly altered to say something like "small rearward fin movements can help create space before the ball—so long as the ski continues to fully complete both turns."

[eleeski]

Density and viscosity changes with temperature are small changes relative to the changes in surface tension. Observe how the droplet of water flattens out in the fry pan as the pan heats up. The droplet changes visibly across the temperature ranges we experience in our ski sites. Cold water is harder. Temperature deniers are wrong!

 

I think @adamhcaldwell has it backwards. Cold water has less drag because the cold increased surface tension prevents the ski from riding deeper in the water. This will make the ski less stable on edge and resistant to deceleration. Perhaps you can't get as much acceleration because it is harder to get the ski on edge. I don't challenge his adjustments, just the theory to justify them.

 

@GOODESkier 's adjustments make sense to me and my experience. But there are only a couple weeks where my water is cold enough to benefit from adjustments. So I'm lazy and just adjust my style. I do think that moving the bindings is easier and more repeatable than fin changes.

 

Eric

[Adam Caldwell]

@SkiJay,

 

I agree completely.

[Adam Caldwell]

@eleeski -

 

?

[Adam Caldwell]

@eleeski, I am far from an expert in fluid dynamics, however, this is my understanding....

 

Even from a molecular perspective, colder water with more tightly packed molecules will effectively be able to transfer or absorb more energy then warm water. Makes sense to me that just from a conservation of momentum perspective that there is more energy lost to cold water in forms of friction, (drag) then there would be in warmer water. However, taking a fluid dynamics approach, this is my limited understanding of what is really going on. (Anyone whos suffered through some fluids dynamics classes will appreciate this..)

 

Reynolds number (Re) will fall as the viscosity of water increases in colder water. Conversely, Re will rise as viscosity lowers as the water temp increases. For temperature between 50 and 100F, (a reasonable range here in Charleston, SC) the viscosity of water changes from 1.407(ft2/s) x 10-5 to 0.738(ft2/s) x 10-5, roughly a 52% change in kinematic viscosity, enough to make a significant impact on Re# and resulting drag coefficients.

 

This relates to Re# based on the following.

 

Re = (velocity * D ) / kinematic viscosity - where D = a linear dimension of the object ,

 

So as water temperature falls, and kinematic viscosity rises, we will see a DECREASE in Reynolds number. More simply, as;

 

Water temp Increases = Re# increases

Water temp decreases = Re# decreases.

 

Looking at a Moody chart (see attached) plotting the friction factor vs Reynolds number we will see that as Re# decreases, the friction factor will increase, or in other words, increase the friction drag of the fluid. Additionally, looking at the second chart of Friction Drag vs Reynolds number, it can be seen that "generally" as Re# decreases, friction drag will increase.

 

 

 

Some quick calculation and you can easily determine a ball park value of Re that we experiencing as skiers. The range is easily both below 1X10^5 and above 1X10^6, just on speed and surface area gemoertry alone, not including what is going on with changes in water temp. You can see in the chart that there is a very interesting swing in drag dynamics within that range (10^5 to 10^6) There are plenty of calculators out there for it, just make sure you use correct units when you use one. (I went through this issue about 10+ years ago, but was not able to find the excel sheet I made on my old laptop this morning...ill keep digging). Anyway, the thresholds are extreme enough to create some very interesting dynamics on the water. I do believe the "dip" seen in the Re# vs Drag plot is one reason why there is so much confusion about the "cold water" tuning. If I remember correctly, we are moving well through that region.

 

The unique part about this is that given that we are rapidly changing speed on the water, angle off attack, effective surface area acting in the water, etc, we are constantly changing the fluid dynamics configuration. IMO, its largely due to how much surface area of the ski is actually in the water, and further more, how the lift/drag effects of the fin and its geometry play into the resultant overall dynamics.

 

This issue is far more involved then most will ever imagine, and its no surprise there is a tremendous amount of confusion.

[Horton]

@adamhcaldwell so I need a Denali?

[eleeski]

@SkiJay 's chart is not clear to me on the dimensions for viscosity. Swimming in cold vs warm water does not affect times - actually serious swimmers prefer colder water - so viscosity can't vary that much in real life drag considerations.

 

When I swim, I do tend to be less buoyant in warm water. Maybe I can relax and sink in more. Maybe that's why swimmers prefer the cold - despite the viscosity change? And most pools are a bit salty for speed (buoyancy vs density).

 

My skiing experience with water temps that are near 100f in my lake is that the hot water feels slow. Very slow. Adjust the speed while tricking up slow. Get away with overpulling in slalom slow.

 

Salt water feels different from fresh for skiing. It's noticeably faster. Unless it is very warm - then it feels like cool fresh water. The bubbles from the wake are smaller and frothier in warm water.

 

@adamhcaldwell Fin adjustment is more art than science. Empirical testing results don't always correlate to theory. Warm water feels softer despite viscosity or density changes - both in skiing feel and just swooshing your hand through the water. We might be missing a higher order factor (surface tension?) or getting the feeling wrong.

 

So often in ski design, I have gotten the exact opposite effect that I expected from a change. I have to be very open minded to counter intuitive feels.

 

Eric

[Adam Caldwell]

@eleeski,

Swimmers are moving at such a slow speed the flow is defined as laminar, which exhibits different boundary layer effects, and is a completely separate conversation than high Reynolds number turbulent flow (like in waterskiing).

 

Accurately measurable Reynolds numbers that fall between 2000 and up to 10,000 are typically defined as laminar flows, above 10,000 are defined as turbulent flows. Youtube these terms and there are thousands of videos that get into the details, explain the theory/concept and show the dynamics of how it impacts lift/drag forces.

 

Fin tuning and ski design is not easy, and I think everyone whos tested an idea has run into confusion with things that are very counter-intuitive. Personally, I wouldn't investigate slalom ski dynamics and invest time/energy/$ in ski design if I thought it was only black magic and "art".

[Mateo_Vargas]

My winter skiing is mainly for conditioning so don't mess with the fin. I do move bindings back a little when the water cools down.

[eleeski]

I did pass my fluid dynamics class and learned enough to recognize when a problem is too complex for generalized modeling. A waterski bears little similarity to Reynolds' fully immersed flat plate.

 

Ignoring the huge variability in area in contact with the water and pressure on that area, the partial immersion of the ski represents a unique set of forces. (Model that transition accurately and we'll refer to the "Caldwell" number.) Perhaps that's why I feel more of an effect from surface tension changes than Reynolds number variations.

 

Most people agree that saltwater feels like cold water. Both feel fast during the portion of skiing when there is no energy being added (basically any time not in a stacked pull position). Perhaps there is more drag during that time when energy is being added to the skier through the rope load - but there's 300hp and lots of strength in the rope to dwarf that effect. Something more than just added viscosity or density is at work here.

 

The best skis are designed by technically minded and talented skiers (like @adamhcaldwell ). These designers understand enough theory to conceive advancements and enough skill to evaluate the practical value of any changes (including the failures). I call that art (not black magic). Scratching the bottom of the ski with 24 grit or taking a grinder to the ski and having that improve the ski might be black magic...

 

I am however quite insulted to hear that I'm not swimming fast enough to get out of laminar flow. Ha!

 

Eric

[Adam Caldwell]

@eleeski, I think the impact of running 24 grit to the bottom of a ski is actually supported/explained by fluids concepts and Reynolds# & Drag relationships. The two colors on the Re# vs Drag plot shown above in my other comment are actually representative of different surface roughness (blue = smooth, orange=rough). In general, smoother surfaces will experience a much greater change in drag as Reynolds number increases/decreases within that region (Re#=10^5 to 10^6 ), (SPEED is the driving factor at play here), where as a rougher surface operating in the same relative region would have ~ 30% less of a swing in "drag" over a similar range. Makes sense to me that a ski with a rough bottom would have a much more consistent "feel" and thus a larger sweet spot for fin tuning. I've never been a fan of skis with smooth bottoms finishes. In fact, I think my lifetime PBs are on skis with the bottom sanded with 80grit paper perpendicular to length of the ski.

 

The flat plate concept does have bearing in very specific instances, but is far from covering all the bases of what's going on. As much as I would like to, there's no way I am about to dive into that conversation on this forum and give away the farm!

[thager]

ZOOOOOOM!!!! Right over my head!!!!!

 

[auskier]

Well this is now the geekiest thread on BOS ever..

 

I am certainly no engineer, and dont know much difference between a moment arm and a forearm (im in the medical field), however one aspect I have thought about that may add to the complexity is water compression or density and its journey through a concave of a ski. More force is required to move a more dense liquid through a space (ie a ski concave), or has a higher resistance to deformity. Therefore, as a result of the higher water resistance and pressure, I see the result being that the ski will sit higher and reduce speed more quickly. This causes the common complaints of being narrow, fast and unstable.

Warmer water having a lower density will be able to be compressed much more easily underneath a ski and requires less force to do so - the ski will sit deeper, hold better direction, speed etc. This means less fin surface area required, fin can be further forward and more length to initiate the turn sooner.

 

Our water temp is rapidly warming up, however I found myself having to move back, deeper and reduce length to try and get width and maintain outbound direction. Nice to see im on the same page as a few of the brains trust!

[ToddL]

 

[SkiJay]

Because facts are important to proper understanding, here are the numbers and graphs for how temperature affects viscosity, density, surface tension, and compressibility. While the density of water is affected by temperature, the change is so minuscule that it's virtually insignificant to ski behavior. And while water actually does compress (1.8% at a depth of 4 kilometers!), compressibility too is irrelevant to ski discussions.

 

Surface tension only increases 7% between 95° and 40° water, and the change is linear. Viscosity, on the other hand, more than doubles over the same range, and the change is exponential with the greatest change occurring at the lowest temperatures. All of this suggests that viscosity, water's resistance to being deformed, is where most of our attention should be focused when discussing cooling water.

 

 

[JAS]

Question, does flattest riding ski have least drag, or ski with least wetted surface area. Seems that ski shape depends upon a particular riding attitude to provide designed performance characteristics. Why shouldn't changing binding position, and or ski size be the primary options, with fin adjustment left for fine tuning?

[Adam Caldwell]

@JAS, Shape/Rocker definitely play a role in the dynamics. If ski is riding too flat (tip too low), then you are plowing the forebody of the ski to much, and that is also bad. But, we don't want tip too high either.

 

IMO, Unless you have the capability to change the bindings 1/16 at a time, 1/4" moves are too much, unless your going directly from 95deg to 50deg water. Micro-Just was a tool developed for that. Fine tuning binding settings. 5/1000 moves every 10-15deg water change is usually more then enough to get the ski to come alive again without disrupting the overall dynamics of the ski.

[ForrestGump]

@Horton You don't need a Denali. You need an Eleeski. Come on. You know you want to.

[gsm_peter]

According to my limited understanding.

Sorry for the language and spelling.

 

Friction is not linear.

The needed effort to move a longer narrow wet area

is substansially larger compared to the same area twisted 90 degrees.

So if the ski has more resistens to glide the ski get more wet area and breaks even more.

 

 

[eleeski]

@SkiJay Salt water feels fast just like cold water. Surface tension increases with salt. Density as well. I don't know about viscosity and salt.

 

As a reasonably skilled snow skier, I do have a good understanding of fast vs slow skiing feel. Cold water feels fast. Tricking requires adjusting speeds down. Slalom requires more preturn. I'm not buying the drag theory causing the changes.

 

I've never had a problem with too much drag preventing me from getting out to the buoy. Nor have I seen any of the pros struggle with that either. They do weird double pumps to burn off excess speed. Increased drag should help - especially if you pull like those pictures of @FWinter . Adding extra wings, raptor edge fins, protruding screw heads, 24 grit wallboard texture and many crazy but performance enhancing draggy items suggest that drag is not a major problem. Perhaps the opposite - drag is needed to change direction.

 

Most skiers ski best with temperatures in the middle of the range. Weird temperatures change things enough to affect the buoy count.

 

In tricks, we adjust the speed for variables that change the feel like lake depth, salt water and temperature. Those changes are real but can easily be dealt with. Tuning a slalom ski is much more complex than slowing .2mph. Compensating with form might be more reliable.

 

The goal of ski design and tuning is to have as much edge in the water as possible. The properties of the water do influence this. Regardless of the science, the clever designer can make this happen. @adamhcaldwell may be giving us disinformation to keep me from giving @Horton an Eleeski that outperforms the Denali!

 

Eric

[CsSkis]

@adamcaldwell

Can you provide more info on how a fin back movement helps keep the ski out in front of the skier. You made this comment earlier in the thread, and I just need a little more explanation. Is it an adjustment that the skier is making at an unconscious level, or is it a change in how the ski responds? Thanks

"Fin back adjustment will move the ski out in front of the skier more during gate roll in, and as well as in the turns, which subsequently takes pressure off the forebody bevels when loading into first wake."

[Adam Caldwell]

Both viscosity and density are higher with salt water, therefore it again, makes sense with all stated above why it feels the way it does.

 

According to SkiJays plots, over the range of temps 50-100F (~10-35C);

 

Density changes less then 1.0 %

Surface tensions changes 5.4%

Viscosity changes : 55.4%

Hard to dispute what underlying dynamic is when you look at the data from this perspective. I'm sure surface tension plays some role in how the ski behaves, however, the specific area of interest (ski surface to water interface) would be small compared to the massive volume of water being accelerated in all directions as the ski moves through the water.

 

Water temperature changes are far from being a one dimensional. There are so many dynamics going on within a lake itself (viscosity, thermal stratification, water depth, water chemistry, dissolved solids, etc). Changes in any of these parameters will impact lift/drag on the ski enough that "feel" and "performance" will mean something different for everyone as we all ski in different bodies of water.

 

What is important on a waterski is WHERE on the ski drag & suction are coming from, and how it is balanced relative to where the lift is being generated. This is ultimately where the fin changes will help. Changes to the lift/drag dynamics due to changes in viscosity will impact the forces at work and in some way influence the plaining angle, ride height etc. The skier will then be required to have more or less mechanical input to the system to overcome these changes. Options are being technically better on top of the ski (which will not always work very well), or move the fin to bring balance back to the system.

 

@CsSkis, Started writing an explanation for DFT move and it got long and slightly confusing, no surprise really. I would like to spend time making the concept more clear. Already enough confusion! Ill work on it more and repost in this forum.

[BG1]

@adamhcaldwell Do you have any data on how much the flex of a carbon fiber ski changes with temperature changes. Maybe this is yet another variable. Skis have always felt stiffer in cold water to me. Thanks for all of your insight into this very complex topic.

[eleeski]

@adamhcaldwell Water is a fluid with very low viscosity. A 50% increase in a small number is still a small number.

 

My lake is in bentonite clay soil. The stuff well drilling mud is made of. Adding mud to water can increase the viscosity so much that sand and rock are lifted from the bottom of the well. So you would expect that a very muddy lake (like mine at the end of a very busy ski day - even if its's a long way from drilling mud) would ski differently from a lake with clear water (like mine when we first get there after a few days of no activity). Yet I can feel no difference - either on a slalom or trick ski.

 

The ski passes over the water rapidly enough that the amount of displaced water in the vicinity of the ski is small. It makes sense that viscosity would affect the spray but maybe not affect the ski too much.

 

Water is a fluid with one of the highest surface tensions. Skiing involves interacting a lot with the surface. 5% of a strong effect can be noticeable.

 

Wax has a pronounced effect on surface tension. I've learned to always clean any mold release wax off my ski to get it to feel right. And never wax your ski!

 

Salt raises surface tension. My lake is noticeably faster in the fall when my water tastes a bit salty than in the spring when it tastes fairly sweet (I evaporate a lot during the summer). And salt water feels faster.

 

Now I don't really know exactly what causes cold water to feel faster. I just have a hard time accepting that increased viscosity, which should increase drag, yields a feel like the drag is reduced. At least surface tension moves the right direction with what I feel - but that's the only reason I suggest it has a noticeable effect. Perhaps there is some dynamic that I can't picture that affects slalom skis, flat trick skiing and edged trick skiing. I am confident in what I feel, that cold water feels faster, harder and slipperier.

 

Eric

 

PS @BG1 My skis are made as stiff as I can make them. While there is some change with temperature, mine don't change much. Other skis with engineered flex may change more.

[Adam Caldwell]

@eleeski

 

1/1000 of an inch is also very small number, however, I think everyone here would agree that a fin movement of 1/1000 of an inch is noticeable. The ski is acting on much more then just a thin layer of fluid at the boundary layer where surface tension is part of the discussion. Unless 'surface tension" is responsible for the difference in feel of the 1/1000" movement.

 

The resultant forces and pressure to generate lift, are a function of a rather larger volume of water underneath and around the ski. This is why a shallower lake, or shallow spots, cause the ski to ride higher (or at least the tail of the ski) The pressure gradient beneath us will change. Same is true for freshwater lakes where there is significant thermal stratification where the water 5-10 feet below the surface down is nearly 15F + cooler. That colder water well below us is acting to change the feel of the ski due to the waters "resistance to movement" (defined by viscosity), and the resultant pressure gradient that is developed. Remember, water is an incompressible fluid, its not like "air". Colder water will not displace as easily or quickly (due to higher viscosity) and will cause tail of the ski to ride higher. A shallow lake will impact the ride height of the tail of the ski in the same way, as there is much less water below the ski to displace.

 

I agree waxing a ski is bad. Or any thing water repellent is bad. They all impact boundary layer energy and adhesion forces. I agree that changing the surface of the ski in such a way will change how the ski feels in water significantly. The mechanism in this case is surface tension and related adhesive and cohesive forces acting on the boundary layer between the ski and the water. Salt water is also part of this conversation. But, when you are talking about everything else on the ski being a constant, and the only variable being temperature, I'm reluctant to agree that surface tension is the smoking gun.

 

If you have ever used epoxies/resins to wet out a laminate, using an epoxy with a viscosity of say 700cps @ 72F, its like spreading molasses compared to something that is near 350cp @ 72F.(50% change), which feels like spreading water in comparison. Per you statement above, 5% of a large number should be noticeable, then 5% of 700 is 35. An epoxy of 665cp (5% less) might be hard to tell a difference at all compared to something 50% less viscous.

[eleeski]

I do agree that the surface tension theory sounds far fetched. But that is the only change I have seen that matches what I feel.

 

Skipping rocks is more like skiing than dragging in a spinning lure. Do rocks skip better in the cold?

 

You do make a good point about deep lakes - I can feel that and surface tension is irrelevant there. But so is viscosity.

 

I'm missing something.

 

Eric

 

My sensitive kinesthetics can't feel .001. I doubt there are many people who could measure .001 repeatedly much less adjust that closely. And even fewer who could feel it in a double blind test.

[JAS]

Why wouldn't the resultant forces and lift/raise the entire ski at same attitude. Considering more surface area up front I would think if anything, the front of the ski would raise more than tail?