Water Temperature and Drag Force

taptap Posts: 55 Baller
I was skiing this last weekend and got into a conversation regarding why it felt like such a struggle (probably just an off day). Somehow we ended up on water temperature and viscosity. So... I looked it up. At 90F the dynamic viscosity of water is roughly 0.8*10^-3 Pa*s. At 60F it is roughly 1.15*10^-3 Pa*s. That's a 44% increase in dynamic viscosity! I am in no way an aerospace engineer, so I can make really bold statements and clam full ignorance. What does this increase in viscosity do for the force of drag???

The easiest theory I can find (and here is my ignorance) is the Stokes' equation for spheres traveling in a fluid where Drag Force = 6 * pi * mu * R * V. Where, mu is the fluids dynamic viscosity, R is the radius of the sphere, and V is the velocity. Using the Stokes' equation, a 44% increase in dynamic viscosity equates to a 44% increase in the Drag Force!! That all assumes pure laminar flow, small Reynolds number, and so forth (which I'm sure is a gross oversimplification).

Any aero/hydro guys/girls out there that can provide some better insight? For now I'm thinking of changing my wing from an 8 deg to a 5 deg until Spring gets here.
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Comments

  • eleeskieleeski Posts: 3,040 MM Trick Skier / Eccentric Person
    This issue again? Personally I think cold water has less drag and feels much faster. Viscosity vs surface tension or maybe something completely different changes the feel.

    Most people add angle to the wing when it gets cold.

    The Reynolds number and theoretical physics misses some real higher order impact. If you get to 41 off, maybe there's a different feel than what I feel at 35 off.

    Add wing, scratch that base and add some drag for the winter. Hit the gym to add your strength to overwhelm any tiny imaginary water effects.

    Eric
    DragoHorton
  • Steven HainesSteven Haines Posts: 914 Solid Baller
    Ummmm scratch the bottom? No thanks! Lol. I'll add sheetrock mud before that!
  • MISkierMISkier Posts: 1,860 Mega Baller
    Chevy Chase (as Gerald Ford answering a Gross Domestic Product calculation question in a presidential debate skit on SNL): "I was told there would be no math today".
    The worst slalom equipment I own is between my ears.
  • jcampjcamp Posts: 609 Crazy Baller
    When it's cold I pop my ski in the microwave for 18 seconds to warm it up and decrease viscosity. Adds at least 3 buoys to my ball count.
    Than_BoganB_Sjerrym
  • MSMS Posts: 4,290 Mega Baller
    I am not sure if a definitive answer has ever been agreed upon.
    Shut up and ski
  • AdamCordAdamCord Posts: 462 Open or 55K Rated Skier
    edited January 2016
  • HortonHorton Posts: 23,547 Administrator

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  • AdamCordAdamCord Posts: 462 Open or 55K Rated Skier
    On mobile plus I figured you needed something to do
    Wish
  • taptap Posts: 55 Baller
    @AdamCord thanks. somehow I completely missed that thread.
    adamhcaldwell
  • thagerthager Posts: 3,790 Mega Baller
    And then there's the effect of thin air and thick air!
    Stir vigorously then leave!
    AdamCord
  • MISkierMISkier Posts: 1,860 Mega Baller
    @SkiJay, here is the one question I have trouble reconciling. While I won't disagree with the discussion on viscosity calculations, I do wonder if there are temperature effects on buoyancy. If molecules tend to have a lower range of motion at colder temperatures, can one infer that they produce more buoyancy support? After all, really cold water (solid ice) supplies a skier with maximum buoyancy.

    Assuming that colder water may supply more buoyancy support than warmer water, could temperature affect how much a skier sinks down and, in doing so, reduce drag more than the change in viscosity increased drag?

    Cold water, more buoyancy, more speed due to less drag created by sinking, narrow skiing due to slippage as less ski edge in the water to hold direction.

    Just throwing it out there for consideration.
    The worst slalom equipment I own is between my ears.
  • DWDW Posts: 1,544 Crazy Baller
    Adding to @SkiJay and @adamhcaldwell and perhaps why there is some confusion on fast v slow water feel. The feeling of fast v slow water also has to do with the fact the handle is pulling you through the water, you are not sliding downhill with the help of gravity mor having your own method of propulsion. The sensation of fast might be coming from the fact that there is a higher effort (core strength) to keep your ski under you under the same speed or pull from the handle itself and also riding on slightly denser water.

    I am willing to bet there is both laminar and turbulent flows along the ski and fin surface, which would also mean differing Re numbers.
  • eleeskieleeski Posts: 3,040 MM Trick Skier / Eccentric Person
    Waterskiing is not pulling a teardrop shaped object underwater. It's a lot more complicated than that. Cold water feels faster, period. Ripping my technique does not explain the feeling (I scramble a lot in the warm water of summer too!).

    Now I'm not sure that my maximum pull force doesn't increase by a few percent in cold water (or maybe decreases) because any drag changes are much smaller than normal variations in my pulling force. That doesn't mean that the effect isn't real, just that I can't feel it. I can feel a difference in the water and it feels like the boat is going faster. And when the boat goes faster, there's less drag feeling (cruising behind the boat is a lot easier than plowing slowly). Adding wing helps my buoy count.

    There's a lot more science than just viscosity going on when skiing (like surface tension which might be a much higher order effect).

    Eric
  • AdamCordAdamCord Posts: 462 Open or 55K Rated Skier
    @eleeski I'm sure surface tension does make a difference, just not as much. The change in surface tension from say 50-90 F is about 4%, while the change in dynamic viscosity is about 40% over the same range.
    SkiJayFLeboeufscotchipman
  • eleeskieleeski Posts: 3,040 MM Trick Skier / Eccentric Person
    A couple simple tests : first, swish your hand through a cold pool. Then do the same in the hot tub. Can you honestly feel a difference?

    Next, drip some water on a cold skillet. Notice how it beads up. Turn on the heat and watch that bead flatten out!

    Maybe there's a hand worth of flat plate area in a ski's drag but there's a LOT of droplets of surface area on a ski.

    I'm not sure exactly what causes the fast feel of cold water but I'm confident that my body can recognize and identify it.

    Eric
  • SkiJaySkiJay Posts: 2,152 Mega Baller
    edited January 2016
    Great question @MISkier.

    Buoyancy is largely a function of density, and the density of water changes so little throughout the water skiing temperature range that it is virtually irrelevant. Ice floats because as it freezes, it becomes less dense than water.

    The reason colder water is more supportive, causing the ski to ride higher in the water isn't about buoyancy, it's that the water has a higher resistance to flowing around the ski, or getting out of the ski's way—and the term for this phenomenon is viscosity. So as water gets colder, it gets more viscous, and the ski rides higher and has to overcome a higher level of drag as a result. Buoyancy, on the other hand, remains virtually unchanged.
    www.FinWhispering.com ... because understanding is better than memorizing
    scotchipman
  • Than_BoganThan_Bogan Posts: 5,620 Mega Baller
    edited January 2016
    @eleeski I'm not sure you're right -- in fact I doubt it. But I really like how you're not bowing to the majority. You're certainly making me think with interesting examples and reasonable questions.
    Nathaniel Bogan -- GUT Padawan
    skialexHorton
  • ChetChet Posts: 39 Open or 55K Rated Skier
    Went from Florida To Vancouver one year. 92 degree water to 52 degree water. BC water felt good but was a lot of work and the ski did ride high even in a deep lake. Came home and over shot my first two gates dramatically. My take is cold water high drag and ski rides high. This in turn makes me work harder to achieve the same effect as a result the boat works harder and with out the ability to get the ski back deep into the water (density) I slow down less. As I progress down course the exaggerated effort on my part and the increased swing of the throttle make me feel fast.
    Did I mention I love warm water.
    Horton
  • thagerthager Posts: 3,790 Mega Baller
    Is there a posibility that the cold water also makes the prop more efficient and less forgiving?
    Stir vigorously then leave!
    SkiJaydchristman
  • SkiJaySkiJay Posts: 2,152 Mega Baller
    Interesting thought @thanger! Higher viscosity would also make the hull and its fins more effective. So in cold water, both the boat speed and tracking will be less forgiving to the skier. The wakes and prop roost will also be firmer.
    www.FinWhispering.com ... because understanding is better than memorizing
  • DWDW Posts: 1,544 Crazy Baller
    Along with the engine having more power and crisper response to throttle inputs due to cooler / denser intake air.
  • SkiJaySkiJay Posts: 2,152 Mega Baller
    edited January 2016
    So to bring all this back to the thread's title, the skier now has to contend with a draggier ski while trying to keep up with a less forgiving, more powerful boat. Adding even more drag to the ski by increasing wing angle is just going to jack up the workload or make us run even later in the course.

    The reason some skiers like what they find when they add wing for cold water, is it helps the ski to turn better at the ball ... a little bit, and only temporarily (a long story for this post). But an LE-neutral DFT increase (increasing DFT without moving the fin's leading edge forward) is a much better way to improve cold water turn performance, i.e. it's usually best to use the right tool for any job.
    www.FinWhispering.com ... because understanding is better than memorizing
    Horton
  • Than_BoganThan_Bogan Posts: 5,620 Mega Baller
    edited January 2016
    I feel seriously cool that I knew what an LE-neutral DFT increase was! :)

    That's actually one of my favorite parts of your book, @SkiJay -- setting some terminology that makes sense with how you want to approach the change, as opposed to what is easiest to measure.

    The whole idea that the leading-edge-from-tail is the more important consideration (as opposed to fin length) make total sense and really helps me to begin to understand.
    Nathaniel Bogan -- GUT Padawan
    SkiJay
  • tctc Posts: 90 Baller
    SOOOO Which will feel "draggier" being pulled across a frozen lake by a snow mobile or being pulled across a 90 degree lake by a ski boat?
  • GolfguyGolfguy Posts: 75 Baller
    Try this. On snow skies in the spring ski from the snow (cold water) through a pond of open water. What happens is that when entering the liquid water one fights to keep from going out the front. The snow (colder water) is much faster than the liquid (warmer) water. Just saying...
  • Than_BoganThan_Bogan Posts: 5,620 Mega Baller
    After a state change, material properties are radically altered. Comparing cold water to ice or snow is not going to get us to a deeper understanding.
    Nathaniel Bogan -- GUT Padawan
    dchristmanscotchipmanadamhcaldwell
  • ToddLToddL Posts: 2,701 Mega Baller
    edited January 2016
    @SkiJay -

    "LE-neutral DFT increase (increasing DFT without moving the fin's leading edge forward)"

    So, how does one go about this? Do you need to use a really long caliper to determine the LE's "distance from tail" and maintain this while moving the fin forward? Or just grab a pencil, mark the LE point, increase DFT, and reset LE location?

    (and, no... I have not finished the book, or gotten to the really good stuff, yet.)
    -- The future of skiing depends upon welcoming novice skiers regardless of age to our sport.
  • eleeskieleeski Posts: 3,040 MM Trick Skier / Eccentric Person
    Are you guys serious? I've replaced several props on my boats. Never made warm water feel like cold. Solid tracking boats have never made warm water feel like cold. I might lose some buoys but I know what the water feels like and that doesn't change with boats or even ZO settings.

    I don't know exactly why it happens but I am confident that the feel of cold water is similar to the feel of a faster speed. I evaluate a lot of skis so my ability to be in touch with the feel is reasonably valid.

    Eric
    Drago
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