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

[Adam Caldwell]

@ Eric, Viscosity is not irrelevant in deep lakes. The thermal stratification (85deg on the surface, and say 70F and below as you get 5ft down and deeper), and thus the viscosity profile from the bottom of the lake to the surface is going to be different then a small warm lake with much less, if any, stratification where the entire body is much warmer, with lower viscosity overall, allowing for a ski to ride deeper.

 

From Wikipedia:

 

vis·cos·i·ty.

NOUN

1.the state of being thick, sticky, and semifluid in consistency, due to internal friction.

 

synonyms: thickness · gooeyness · viscidity · consistency · texture

•a quantity expressing the magnitude of internal friction, as measured by the force per unit area resisting a flow in which parallel layers a unit distance apart have unit speed relative to one another.

 

Viscosity is not a one dimensional variable. Lot of good tutorials on youtube that will explain it a lot better then I can, and are well worth the look.

 

@JAS, Lift is not a constant over the entire bottom of the ski, nor is the drag. The dynamic force of lift will be where the stagnation point is on the ski interacting with the water. Its somewhere near the water break of the ski, and everywhere behind that point will have less pressure and therefore less lift. The velocity gradient of the water flowing along, and with the bottom of the ski,(the volume of water that the ski is actually displacing/accelerating and/or dragging with it), which is highly effected by viscosity, will influence the pressure on the bottom of the ski and change the lift/drag characteristics, governing how the ski is riding in the water. Have to consider that besides the glide phase, the ski is hardly ever flat and is constantly running at a compound bank angle on some angle of attack, with lots of slip.

[DanE]

@adamhcaldwell Your comments on different water temps at different depths are spot on!

I ski in a bay on a big lake and it's quite deep, 50 feet or so.

Extremely different conditions can occur within the same surface temperatures because of that phenomenon.

Let's say we had some nice temperatures early in the spring getting surface temp up to around 75 but down below it's still freezing, that's when this lake skis the best.

Late in the summer if it hasn't been that hot but alot of onshore wind causing the entire bay to heat up (higher temps throughout the dephts ) but still the same surface temp the lake skis terrible.

Just feels like skiing in whipped cream, getting nowhere in contrast to the first scenario where it's easy to get wide and early.

[Horton]

@adamhcaldwell

 

Have you spent any time thinking about the impact of adding aluminium sulfate to lake water to control the particulate content?

[Adam Caldwell]

@horton, The lake will go to 11. Next question.

[MattP]

@adamhcaldwell Science?

[eleeski]

@adamhcaldwell Are you seriously saying that the viscosity at the bottom of the lake, nowhere near touching the ski, has a measurable effect? I don't believe that the viscosity in contact with the ski is at all tied to anything at the bottom of the lake.

 

Pressure waves do travel to the bottom but the speed of sound in water increases with both temperature and salinity - unlike viscosity. So any bottom effects are completely unrelated to viscosity.

 

The viscosity of the oil on top of the vinegar when unmixed is the same as the viscosity of pure oil.

 

@Horton My lake skis the same when clear on the first run as when very milky with clay. All the aluminum sulfate flocculant will do is make the water so clear that you will get pondweeds growing up from the bottom. Come ski with me anytime to repeat the experiment.

 

The important experiment will be to add alcohol to the lake. This will lower the surface tension and confirm or bust my theory. A couple million gallons of pure alcohol should do it. At least swallowing some of that lake water might not be so bad.

 

@DanE Improve your pull position to get wide and early. Use the sticky water to turn tighter.

 

Eric

[Adam Caldwell]

11

[Horton]

What about air density?

[eleeski]

Like a headwind?

[ToddL]

I seem to suffer from cognitive density when I ski. Water temp doesn't seem to have any impact. Maybe it is the surface tension at play.

[DanE]

@eleeski Make that alcohol experiment happen and you will achieve my pulling position skills.

[SkiItUp]

I am not a science kind of guy and most of this stuff goes right over my head. But isn't a lot of this thrown out the window by the ski dynamics? How much is the ski changing as the temp changes? Is the ski stiffer? Less flex? Even the bindings change, at least for us rubber boot skiers. I have always wondered how much this plays into cold water skiing and you folks may be smart enough to answer this.

[Howa1500]

I would agree that the depth of the lake/pond in which you are skiing at DOES NOT affect how you ski. Skiing is a sport that really only requires 4-6 feet of water. Colder temperatures make the water more viscous therefore the water is more dense due to the condensed molecular structure on the surface of the water. (The molecules aren't moving as fast). More density means for buoyancy to an extent, if you're more buoyant you're not "plowing" through the water (less drag) you're not sinking as deep. The viscosity of the water doesn't actually change until a decent depth. Long story short, your ski will ride higher on "thicker" water than it will thinner water.

A factor could also be the ski itself. Carbon and or fiberglass may react differently to different temperatures. The width of the ski. @Horton makes a good point about air density. To my understanding the air pressure theoretically should make the water denser If it has nowhere to escape to if that makes sense. Unfortunately I believe the idea is a bust because water is an impressible liquid. Put water in a pan and fill it to the brim, press down on the water with your hand. Spoiler alert, it overflows.

However I'm gonna disprove my own theory (great idea right)

 

[Howa1500]

 

The dark line represents the lake, the water has nowhere to escape to. This means that.....

 

a) the water becomes for dense

Or

b) I'm a 15 year old high school chem student who doesn't know what he's talking about and everyone should disregard everything I just wrote

 

[eleeski]

@SkiItUp The changes to the ski from temperature might correlate well with how the water feels but it doesn't explain the saltwater difference. Plus, it is possible to make a ski that is stiff enough to minimize the effect (like mine).

 

@DanE We were going to start the experiment in a pool to make the water softer to learn flips. But drinking and diving don't mix.

 

Eric

[wawaskr]

Interesting discussion on the dynamics of water and skis, but as we move farther and farther from the thing that is actually attached to the ski (human), have we ever considered the effects of cold air/water on the human body, and thrown that into the equation? All I know is - I do not ski as good in the colder months because I'm simply cold! Hey, who has data on marathon running times from say, a 37 degree day vs. a 83 degree day?

 

@eleeski - completely agree with your statement on .001" fin adjustment feeling/measuring.

[A_B]

My friends and I (from Midwest) have all experienced running over buoys when we go to hot water in FL. So is this temperature increase causing more drag or is it the tail sinking more so less acceleration and width from less ski in the water? The difference is real.

[Horton]

@Howa1500 I was being sarcastic when I suggested air density was a factor

[Howa1500]

I know you were. But it sparked up an idea that I figured I'd consider. You can give me the panda now, I'm definitely a gullible person. @Horton

[AdamCord]

I think this thread illustrates why it's sometimes easier just to say "uh....it's science, trust me I'm an engineer"

[Horton]

Blinded me with Science!

[A_B]

sounds like trust me, I am an IT guy.... you'll love it.

[RazorRoss3]

I was told there was no math on BOS, I was clearly misinformed.

[Than_Bogan]

@RazorRoss3 Whoever told you THAT should be crossed off your trustworthy sources list!

 

Any forum in which I have almost 5000 posts WILL have math. (And I'm not even the source of any of it in this particular thread!)

[Horton]

@RazorRoss3 I seem to be able to stop the spam and porn but not the math.

[RazorRoss3]

@horton, are you sure your priorities are properly aligned?

[tap]

I'm not sure which thread to post in now, but since this one has way more information it seems like the better choice. @Horton any way to merge threads?

 

Since I seem to have inadvertently restarted this fantastic discussion it seems only appropriate to actually contribute something. The effect of temperature on the flex of the ski was brought up a couple times but I didn't see any definitive answers. So, I spend the evening out in the cold with the flex tester (cold for Florida anyway). Within the repeatability of my measurements (+/- 1 lbf.) I can say that from room temperature at 70 deg F to 45 deg F there was no measurable difference in ski flex. I did a sampling of three different skis (blank) at two different flex points per ski at 25" and 41".

The skis:

1.) lower end ski with polyurethane core and fiberglass/epoxy skins

2.) an older top end ski with a polyurethane core and carbon/fiberglass/epoxy skins

3.) a newer very top end ski with a PVC core and all carbon/epoxy skins

All three skis were "big brand" skis. I allowed the skis to acclimate to temperature for 1 hour prior to taking measurements and took all measurements at temperature. Testing flex numbers ranged from 110 - 210 lbf.

 

It'd be great if someone else wanted to validate (or contradict) my results, but as far as I can tell temperature has no measurable effect on ski flex (within normal use temperatures).

 

And for what it's worth, I'm in the cold water has more drag and viscosity is the cause camp. And I did take a fluid mechanics class at one point in time, but I have no functional memory of it.

[Horton]

@tap I can merge threads but I it becomes more confusing I think.

[thager]

@tap How long did you let the skis stabilize at the temperature to be measured?

[tap]

@thager The skis set for about an hour at temp. Plenty of time to make something happen. I checked surface temps with an IR thermometer.

[riplash]

Hey tap,

 

Have you found a victim, or volunteer to test the body temp of the water skier. I think skiers tend to get awfully stiff when the temp falls just a few degrees below 98 degrees F.

 

Seriously though. I think that there are way too many variables that cold water affects, much more than just the viscosity of the water. If people make changes for cold water, they should do it empirically finding the changes through trial and error, rather than making changes based on theory of viscosity.

 

Just my two cents.

 

-Rip

[Horton]

I really need go get an IR thermometer for beer temp. I like my beer just a little less cold but not warm.

[T_C]

@Horton Do you drink cold beer faster than warm beer?

[JohnCox]

@Horton ....obviously I drink cold beer faster than warm beer...

[ToddL]

This is what I imagine the Adams would be working on if they weren't involved in our sport (with a little help from Than I suppose.)

 

 

[Than_Bogan]

It would have taken me about 50 takes to spew all that babble with a straight face.

Hilarious!

[ski6jones]

Sinusoidal deplanaration seems to be a serious problem.