Water Temperature and Drag Force

[tap]

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.

[eleeski]

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

 

[Steven_Haines]

Ummmm scratch the bottom? No thanks! Lol. I'll add sheetrock mud before that!

[MISkier]

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".

[jcamp]

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.

[The_MS]

I am not sure if a definitive answer has ever been agreed upon.

[AdamCord]

Link

[Horton]

@AdamCord This is why I gave you a panda

http://media.tumblr.com/fe497dd337d9af8479bb6398b9565d16/tumblr_inline_mg6n5ltl6X1rxe4lt.gif

 

[AdamCord]

On mobile plus I figured you needed something to do

[tap]

@AdamCord thanks. somehow I completely missed that thread.

[thager]

And then there's the effect of thin air and thick air!

[SkiJay]

Any time I hear the idea that cold water has less drag just because it's often referred to as cold "fast" water, I cringe. First, because it's factually incorrect, and second, because as soon as one embraces this fallacy, it leads to false ski-tuning assumptions and adjustments that do more harm than good.

 

Cold water seems faster to most people because it promotes skiing late in the course with narrow approaches toward each ball. Scrambling feels fast. And really good skiers actually do have slightly higher speeds behind the boat because it's needed for a proper shaped pre-turn in high-drag water. But this doesn't change the fact that as water gets colder, its increasing viscosity increases drag.

 

What effect does higher viscosity have on drag? Just imagine skiing in a liquid with a lot higher viscosity than water. Motor oil and honey are more viscous than water. Would skiing in honey feel like less drag or more drag? Skiing in oil would make the ski ride higher than it does in water, but would it make the ski ride so high that it would feel like less drag than warm water? If you had to ski in a tar pit, would you add wing or take it off?

 

A lot of math and science can be used to prove all of this, but a little exaggeration and imagination is often all that's really necessary to develop enough understanding to make good tuning decisions. There's nothing wrong with keeping it as simple as possible.

[MISkier]

@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.

[DW]

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.

[AdamCord]

To take what @SkiJay said a step further, the increased drag results in a slower skier speed. Slower skier speed will result in reaching the buoy line later and more norrow that what we're used to in warmer water. If you're late and narrow coming into the ball, it means you are still advancing on the boat, thus your down course speed is actually higher when you need to turn. So it feels like you're flying past the buoys, because you were slow!

 

There's definitely more to it than that, but so long as we're simplifying...

[eleeski]

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

[AdamCord]

@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.

[eleeski]

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

[SkiJay]

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.

[Than_Bogan]

@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.

[Chet]

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.

[thager]

Is there a posibility that the cold water also makes the prop more efficient and less forgiving?

[SkiJay]

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.

[DW]

Along with the engine having more power and crisper response to throttle inputs due to cooler / denser intake air.

[SkiJay]

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.

[Than_Bogan]

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.

[T_C]

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?

[Golfguy]

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_Bogan]

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.

[ToddL]

@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.)

[eleeski]

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

[SkiJay]

@ToddL

To move the fin's distance from tail DFT without changing the location of the fin's leading edge (LE), you just have to reduce fin length (FL) by the same amount you increased DFT. For example, if you make DFT .005" longer, reduce FL by the same .005". Keeping LE stationary will leave tip-sensitivity unchanged while increasing how much the tail smears.

 

No big calipers or marks are necessary. The location of the fin's leading edge is found by adding FL to DFT, or: LE = (FL + DFT).

[thager]

@eleeski What's cold to you? 100 degrees fahrenheit?

[dbutcher]

Perception, right or wrong, often becomes reality for some skiers. I am not scientific, but I know this. As water cools, I begin to struggle to get wide on the slalom course. As the water cools even more, I can't run my shorter line lengths. At that time I switch to my 69.5" ski from my 68.5" ski. Normal width on the slalom course returns. Thus, my perception (and reality) is that cold water has more drag. The bigger ski helps me enjoy water skiing all winter. Fun is what this sport is for.

[ToddL]

Maybe we are all just putting on more weight in the off season, resulting in more drag which we mistakenly associate with the drop in water temperature. ;-)

[dbutcher]

@ToddL My weight is unchanged.

[ToddL]

OK, guys. Where's the joke font? Jeez...

[Alberto Soares]

OK, so I guess most of us agree that:

 

1 - cold water feels fast

2 - on cold water the ski rides upper

3 - cold water has more drag

4 - drag is increased by surface area

 

My conclusion is:

 

The ski riding upper on the water has less surface area to be dragged, thus it rides faster, also less surface area implies less effectiveness of its bevels and design, making everything harder

 

[bkreis]

seems to me, we need to be more efficient, properly warmed up for mobility and get dry asap after in the colder temps, and train!

[MISkier]

@Alberto Soares, you arrived at the exact same conclusion I did. But, the hydrodynamic engineering guys here say the water is actually slower and it only feels fast because we are working harder and are late/narrow because the water is so slow getting us across course.

 

In the absence of an actual experiment that proves true speed/ski slippage in each condition, I can't argue with the math.

[DanE]

Imho if you feel fast you are late and narrow.

When you really hit it right and get wide and early, does anyone not feel slow and in control? To get wide and early you need to travel faster cross course right? Or as the Adams would put it "get as high up on the boat as possible as soon as possible"

If cold water really was fast you would get wider and earlier or as wide and early you are used to with less effort, I ski colld water alot, never feels easier or slower than warm water.

[MISkier]

@DanE, you overlooked the part where riding high in cold water lessens the effectiveness of the edge. Therefore, it is still difficult to get wide early, even if you were to be traveling faster. The slippage doesn't allow you to maintain as much cross course angle and you ski downcourse more than you should. But, this all assumes cold water is really faster, which does not appear to be the consensus here.

[liquid d]

You can come up with all the theory you want (right or 1/2way right or wrong), but if want to ski well in colder water, you better add some wing, or move the fin forward a bit, or a combo of both...simple as that.....

[AdamCord]

The effects of changing dynamic viscosity cannot be summed up in a simple "faster" or "slower", unfortunately. Dynamic Viscosity is a measure of the force required to slide the water molecules past one another. What that means is dragging something through cold water will take more force than in warmer water. What it also means, though, is that the water is more "supportive", allowing the ski to ride higher when it is colder.

 

If a ski were simply a planing hull, like a race boat, it may very well be faster in cold water. If a ski were a displacement hull, like a freight boat, it would certainly be slower. A water ski, however, lives somewhere right between the two. The ski is more of a planing hull when moving fast (coming into the buoy), and is more of a displacement hull when moving slow (coming out of the buoy).

 

As a result, the ski is often slower coming into the wakes (bad), and then also harder to slow down when it's time to turn (bad), as the water gets colder. If we all skied in 45 F water all the time, we ski designers would design the skis to work optimally there, and then they wouldn't be as good in warm water. Since most ski testing/designing is done in warmer climates, and most people ski in warmer water most of the time, skis tend to work better in warmer water.

 

This also means that different people will feel different things as the temperature changes. Also different skis, fin setups, etc. will effect how these changes are felt. That's why when you ask 5 different people what they feel as the water gets colder, you tend to get 5 different answers.

[Than_Bogan]

@AdamCord Now THAT is starting to match my experience. It's the "more supportive" part that is approximately 100% of what I feel when the water gets colder. The ski rides very high and doesn't "dig in."

 

This also jives with something that really surprised me at first: That while my Nano1 was far superior to my 9900 in warm water, the Nano1's performance declined much faster in cold water. On the third hand, I skied the Denali deep into October without noticing any substantial performance decline.

 

The Nano1 is a "carving" ski -- designed to turn by putting a lot of edge in the water. So if it can't GET into the water, it doesn't work properly.

 

The Denali lives at the other end -- it is designed to turn by lifting and rotating (intentionally sliding the tail). It makes sense that "extra support" doesn't cause it as much difficulty. And if it does cause trouble, it makes sense that it can be improved by making it harder for the tail to slide (e.g. by moving the fin back).

 

So the pieces of this theory are starting to fall into place for me.

 

And perhaps understanding what your ski is "trying" to accomplish is key to what adjustments (if any) to make for cold water.

[Than_Bogan]

Ok, so let me veer further into wild speculation. (Run and hide if you can't handle that.)

 

Suppose I increased my wing angle on my N1 for cold water. If our only understanding was that cold water increases drag, this would sound insane -- I'd be increasing it further. But it's also reasonable to expect that more wing angle will place more of the ski into the water. For a ski that requires that to turn, this steeper wing might give me something to turn with. That might net out to being quite helpful.

 

Now suppose I increased my wing angle on my Denali for cold water. As above, the drag effect is likely for the worse. And now, tipping the ski forward is also for the worse! With the Denali, the effect of cold water in the turn would be expected to be that the tail rotates too fast. Now I tip the ski forward, freeing it up to rotate even faster!? Not a good plan.

 

So it doesn't seem crazy (feel free to disagree!) that more wing would be helpful on the N1 and less wing would be helpful on the Denali...

 

However, based on my newly-acquired but still-limited knowledge of fin adjustments, I wonder if the first thing I'd try is shallower for the N1 (allowing it to roll over to put more edge in the water) and further back for the Denali (helping to slow the rotation at the buoy).

[ForrestGump]

Cold water=bike time. My BMX and road bikes don't care if viscocity, drag or the angle of the dangle increases.

[AdamCord]

@Than_Bogan now you're getting to the heart of the problem. Skier weight, technique, boat speed, line length, ski size, ski shape, ski setup, etc. will all play into how the ski acts as the temperature changes. There really can never be a one size fits all solution.

 

I personally tend to find that back/deeper is better as the water cools, but I can't say that other people will like that, especially on different skis.

[Than_Bogan]

@ShaneH I could have sworn I instructed you to run and hide!?

 

But your comment is too funny so I was forced to Like it.

[RazorRoss3]

are you sure you aren't all just a bunch of cold water winies? Come ski a season in Minne(snow)ta sometime and you'll see every water temp from 35-85.

 

In the spring my scores are typically low but in the fall I hit the same numbers in the cold as I had been in the warm.