that seems about right
Can someone convert all of this into % increased drag?
The density has ~ 0.0% impact because it changes so little. However the Viscosity could have a significant impact (given the honey explanation). Might explain why I can do loops during the summer in New England (Water 75 degrees), but feel like a dog all winter in Northern California (Water 50 degrees)
Links here:
https://www.princeton.edu/~maelabs/hpt/mechanics/mecha_54.htm
This is rough, and I’m no expert in fluid dynamics.
It’s depends on the shape, and what range of Reynolds number we’re in. I would estimate we’re in the 5x10^5 at 5c to 1x10^6 at 30c for reasonable speeds and chord lengths.
Eyeballing it on that airfoil in the graph (that doesn’t look much like any of my foils), it could be as much as a 50% drag increase moving from tropical to arctic water.
Can someone Freddy these answers?
(ie brief dumb down summary 
)
Cliff notes:
Cold water is denser. It may only be a 3% difference. So there is argument that you’ll never feel it. (Don’t tell Erik you can’t feel 3% difference)
Wetsuits make you heavier so it’s harder to pump the same gear.
Water quality makes a huge difference in ease of pumping. The clearer/cleaner the water, the easier it is to pump.
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Can’t figure out how to do this without equations, sorry in advance for any heads that may explode in trying to read this post. Converting viscosity change to drag change sounds like it should be pretty easy but it isn’t. Shear force at the wall (foil surface) has a pretty short formulation of
And suggests at first glance that shesr force is directly proportional to viscosity. But viscosity changes also change the other part of that equation, the velocity profile in the boundary layer as you move away from the wall perpendicularly. So, you get some simplified approximations, for a flat plates anyways.
For laminar flow
And for turbluent flow over a relevant Reynolds number range (I estimate the same values as you @seawing)
What does this mean? Last equation, promise
Putting it all together, skin friction is proptional to the square root of viscosity in laminar flows and the 5th root of viscosity in turbulent boundary layer flows. This means for a 50% increase in viscosity, skin friction increases by 22% in laminar flows and 8.4% in turbulent boundary layers flows.
So then the question is, how much of the flow over our foils is laminar and how much is turbulent? Boundary layers always start out laminar and then, at a high enough local Reynolds (take distance from the leading edge as length) number trip to turbulent. Exactly where this trips depends on Reynolds number (speed, viscosity, density, and distance from leading edge along chord), surface roughness, initial level of turbulence in the water (white wash, strong currents, etc) and contaminants like silt or oil. But in general, at low speed and especially for a light rider that can go slow on a small foil, flow over the foil may be entirely laminar. While at high speed the flow could be almost entirely turbulent, especially in a turbulent or silty chunk of water.
So up to 22% more skin friction at low speed and as low as 8% more at high speed if viscosity goes up by 50%.
But, have to remember skin friction isn’t the only cause of drag. Also have profile (pressure) drag and induced drag and their contributions will dilute the effect of viscosity changes. The relative significance changes with speed, for a given rider weight. At low speed/high angle of attack, pressure and induced drag dominate. At high speed/low angle of attack, skin friction dominates.
So, how much do viscosity changes matter at the end of the day? It very much depends, unfortunately. But given that many foilers notice (lose their mind over?) a few percent change in weight, I would say it is at least noticeable in all circumstances where the viscosity change is large, 25% or more say, and very significant in situations where skin friction is the dominant drag component.
If you are still reading at this point, cheers and I hope your head is intact.
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Thanks for the post SILAS. The formulas are baking my noodle. What I can gather from it really does confirm the difference in behavior of my FOIL. Its great news in terms of my training… as this sets up a super fun warm weather season. I think over time I will look at the cold weather sessions as a great way to improve my paddling and get more efficient with the FOIL. All great things. Thanks again.
I guess that salinity makes a big difference too. Salt water is denser than fresh water. Ships sit lower in fresh water vs salt for a given tonnage.
Anecdotally I was in San Diego last week and spent a bunch of time proneing my 150hax. Yesterday I was out behind a a jetski teaching some friends how to pump (I live in Buffalo NY and the water is ~40deg). Pumping in cold water felt sluggish compared to Cali.
100P. It was rad to get this confirmed this as I felt this too in BC Canada. Ive been trying to come up with the right metaphor… resistance training is the best way I can describe it. Tractor pull?? SILAS def baked my noodle with the equations. The viscosity model in general took me a while to get a working understanding of. The idea that molecules that are moving faster (higher temps) would allow for faster slippage- makes sense. What took me a bit was the idea that the paddle catch and power stroke would change based on the “weight” of the decreased viscosity in colder temps is what I needed to get my mind around. I think Ill get stronger every year that I paddle through the temperature fluctuations.
Either way training through the cold water season feels like the perfect way to increase on FOIL time. Its harder to get up and harder to stay there. I am grateful for the workout that flatwater/tidal SUPFOIL training gives me. The on FOIL time is the treat. 
You can see the salinity differences in the ITTC doc I linked above, Table 3 for saltwater at 3.5% salinity if I recall correctly, freshwater a bit further along. Typically a bit under 3% difference. Not negligible but 3% on a 15m vessel draught is almost half a meter deeper in the water, very visible. 3% less density in the lift equation is made up for by about 1.7% more speed, not sure you could notice that difference in a typical gps log.
On the other hand when it comes to stability and how deep the board is sitting in the water, James Casey talks about a 5L change on a 100L to 105L board as a big difference, so 3% density change would matter to him. I can’t notice a difference fresh to salt in that regard at all.
Yeah, sorry for the head cooking. Beasho asked if someone could try to quantify the impact of the viscosity change, challenge accepted but probably of limited practical use. I agree, time on foil is king and rossdrobinson summed up the practical conclusion nicely - whatever the cause, pumping is harder in the cold so go up a foil size.
Happy to have found this thread, as this has been a pet topic lately. We foil on Vancouver island, cold water year round, and rivermouths with a lot of fresh water. My theory has been salinity and buoyancy, but viscosity sounds good to me. I can feel the extra drag when foiling through the salt/fresh line, particularly noticable on efoil.
I was in Maui recently. On medium wind days, I was on 3m wing with ha120, the smallest rig I have. I would only use that 40kt+ at home, in Maui it was easy…a size bigger for both at home, any given day. Of course, I wasn’t wearing 20lbs of rubber, that probably contributes too.
No one locally has successfully dw paddled yet, though we have a few people getting started. I will be curious to see if this cold water issue affects realistic potential. A 5% reduction in capability in something with a really thin margin for error may be the difference between doable and not. Curious about cold water dw locales…are people doing it? Can’t recall seeing much, now that I think of it.
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I’ve been practicing paddleups in 35 deg fresh water (in waves and flatwater). I attributed it being harder than the summer months to the thick suit, but this makes me think there is a temp and salinity factor. Interesting discussion!
Thanks for the feedback. It’s so hard to control for the variables, as the winter vs summer winds are different too, making different wave conditions. So even if you had good stats on fail rates over a lot of sessions, it wouldn’t be directly comparable. Comes back to “feel” and that’s impossible to quantify. More input from cold water specialists would be great.
Personally I’ve been shy to pursue DW locally, as I haven’t really felt that endless glide thing that I see in all the videos. I get short moments when conditions are excellent, but it’s not sustained enough that I am confident I won’t end up paddling for many kms on a run. Because no one locally has done it, I remain unsure if our conditions are doable for regular folks on typical days – with this thread, I now wonder if the cold water might also contribute to this x factor. I was an early adopter of a bunch of foil stuff in our area, but for DW I am waiting til someone else achieves it, then I’ll follow their lead. 
Where do you live? From what I’ve felt (I am no expert), I don’t think that cold water adversely effects the ability to glide downwind. Its more about how much energy is in the water and how much wind you have to push you along. In instances when I am severely underpowered I do think pumping around in cold water requires slightly more effort, but with enough energy there is little need to pump
Winging downwind is probably the best way to prove that local conditions can work.
Hey fella,
I dont know anyone else that is DW’ing on the island or here in VanCity. Lots of us wing and kite/kitefoil OBV. I live in EastVan and need to as I run a business with my brother but if I didnt Id be in Sooke or Tof if I could swing it and be surfing daily. I looked to the flat water paddle ups as a way to try and surf the tidal waves we get at Locarno.
The colder water feels like 20% harder, at least. The FOIL just does not release like it does when the water is warmer. Id imagine your Sooke temp deltas are similar to ours. 6C winter & 17C -ish in the summer. I really noticed the difference when I took my rig to Sproat lake last year. Night and day difference. I really thought it was the salinity but thanks to this thread (SILAS, MATT and others) Ive clued in to the viscosity as the main culprit. All though I was out last night and saw more sandy debris in the water as there was a 15knot WNW blowing through. I couldnt help but think that the FOIL hitting debris wouldnt help the FOIL releasing. Its so hard to know which factor is in play. All I know is that in the colder months the paddle up feels like the FOIL is in porridge. It changes the timing of the FOIL releasing and I can feel myself getting stronger as Im able to paddle up earlier in the season but I am only 16 months in or so and have only had one season to compare to. This time last year I was juuust starting to get a feel for it.
Do you post online Id like to see what you guys are doing. Im assuming youre around the Jordan River break by your handle.
This is me yesterday at Locarno BTW:
I live in southern Vancouver Island along Juan de Fuca strait. The deep water upwelling means it stays cold year round…sadly we wear 5/4s in summer, but at least get to take the hood off, LOL.
I am fascinated by the potential explanations for why foil performance may be lessened in cold water…speed, glide, lift. Foilstoke: your video above, where you’re struggling to get up, and only getting on foil briefly…the question is, would a 5% warm water performance advantage mean you are able to get consistently up 10% more often and longer? Or 25%, or 50%? If you were in Maui that day, all conditions equal, would you have an advantage?
We do a lot of wing downwinders. I find in great swell patches I can go a couple minutes at a time, but in between the superb sections, I will glide for 20-30 seconds, then need to power up to get more speed, and repeat. I expect much of this is skill in reading the water, finding the power pockets, finding the seams…all that stuff I’ve heard so much about. It felt easier in Maui…but hey lots of things are easier when in foil paradise.
Half joking, some of this is seeking excuses for my own poor performance … as I’ve struggled more than most to unlock the code for pumping, which I am confident is 97% skills and fitness. But if conditions can account for even 3% of my failures, heck I’d take that.
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This thread makes me so happy. For years I’m watching everyone else pump around effortlessly on small foils and I can’t do that. I used to say it was my weight, which is on the heavier side at #205. But now I can blame the damn cold water instead. Its always good to have a scapegoat instead of looking at yourself.
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