Cold Seas VS Warmer Seas

Hey hey… Im wondering if anyone else has noticed the difference in density on the FOIL in cold seas VS warmer seas. Any of you guys in flat water conditions feel the FOIL releases easier in warmer temps? Maybe someone out there has noticed in surf - prone foiling that in colder temps that you would feel overfoilied VS the same kit in warmer temps. I thiiiink it would be. I mean I really feel the difference and thought I would just ask the question here. I cant rule out my conditioning being a part of the perceivable difference as I took some time off to rehab an old knee injury. But it got me thinking…

I asked ChatGPT and the AI overlords and it did say 5-10% denser but I didnt really talk about temperature deltas. I am in Vancouver BC. For example I was out last on Wednesday 4deg C air temp and 7 deg C sea temp. Fun session and really amazing training but it stuck with me … this idea…

Any thoughts guys?

a guy that doing dockstart and wake thieving in the leman lake ( Lake Geneva) yvonkite on youtube ,recently said in one of his video that he had to use a bigger foil to have more lift due to the water density being lower in winter, i didnt noticed beeing overfoiled in cold water but i really noticed it’s was harder for pumping .
It’s probably depend on the foil you are using , you probably need to find something with a higher ratio , but you have to choose wisely because you will probably loose your pumping ability if you take the wrong foil just to not be overfoiled.
For me ratio above 7 are way better compared to my old 5 in term of progressive lift during take off.

I had look up my foil stats:

Size Front Wing Design Aspect Ratio Wingspan Projected Surface Area
75 cm Front Wing Jet HA 2140 5.4 42.3"/107.5 cm 379.7 in²/2140 cm²

5.4 aspect ratio

I just think back to last season during the warmer months I really did feel like I could get the board and foil moving through the water with more pace earlier in my paddle sprint but I really can’t discount the idea that it’s not just me.

Lake Geneva is fresh water. I can vouch that freshwater lakes are much easier to paddle a foil through. It’s way easier. Very fun actually. But by that logic any water when it’s colder should be more dense regardless of salinity level. Hmmm.

I agree it feels harder to pump in the cold. It’s a Great training tool come to think of it!

I do buy the idea that colder water is 5-10% more dense than warmer. It sure feels that way!

It’s crazy to think that the velocity of foil to achieve sustained flight would be directly influenced by the density/temp of the water.

Hard not to over think this. :smirk:

I think its the wetsuit, 3mm vs 4mm is huge, and then 5mm+ with boots and gloves and you’ll feel like you can barely move. I notice this every time I have to change wetsuit, everything feels terrible.

As for density, 0.5% difference between lake geneva and the tropics, I doubt it:

Temperature Density
5°C 1000.00 kg/m3
30°C 995.71 kg/m3
0.5% difference

Saltwater 1025 kg/m3
Fresh 1000 kg/m3
2.5% difference

People claim to feel the salinity difference, I don’t - we flat water practice in fresh and salt often enough. Maybe a slight difference but not obvious.


Man you could be right…

I am in a 5/4 in the winter … 4/3 in spring and 3/2 suits in the summer. All though I have to say the newer suits are night and day better than older ones. Old school suits were straight jackets. Either way I appreciate you weighing in buds.

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Yeah the new tech is really insane. I got a new 4mm radiant rebound Xcel, I always thought this stuff was some kind of gimmick, but definitely warmer than their normal suits, I nearly made it to through December without needing to use the winter suit… issue is there is no fleece in the arms so they start feeling it. Putting on the 5.5mm with hood and then gloves in January and it feels as if you’ve gone back six months in progression and lost most of your fitness.

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Sorry if this topic is not fully related to foil feel on cold / hot water, but goes on the lines.

As said, density changes very little with water temperature.

Though, I did notice significant differences between foiling at sea level vs high altitude lakes (more than 1500 meters above sea level, or 5000 foot).

Main difference is that foil is more prone to ventilation.

There is some science behind this, especially due to the difference in air pressure and its effects on the boundary layer between water and air.
But I am interested in hearing if someone experienced this effect or others.

In the past when designing mast profiles we used to go test them on altitude. If it worked there it surely worked at sea level.

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Have to say I’ve never felt much difference in ease of pumping in different water temperatures. I think in winter there tends to be more water moving around which does affect it though.

On a side note Matt, I wear 4mm all winter now (south coast UK) Foiling keeps you so much warmer than surfing with all that pumping. Plus you’re out of the water for at least quarter of the sesh! Another reason to love it!

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Im down in Southern California. Ive noticed the biggest difference in foil efficiency is sediment/water clarity. The reefs hold much more sediment in the water and it feels much less efficient than a clear water day at the beachbreaks.


The density change is minor, but viscosity does change significantly. 5C water is twice as viscous as 30C water.


Seawing… BRO. That sent me down a rabbit hole…

Viscosity- is a measure of a fluid’s resistance to flow.

I found a pic of the curve of the viscosity of sea water from 5C to 25C and can concur what you said.

Viscosity is the measure of a fluid’s resistance to flow. The higher the viscosity of a fluid (liquid or gas), the slower it traverses across a surface.

Also found this…
This is from the UW School of Oceanography:
The viscosity of seawater is greater than pure water at all temperatures. This suggests that seawater is dominated by structure makers. Thus, at a given temperature, seawater is more viscous than freshwater .

I didnt even think about the viscosity. I was stuck on the salinity as the culprit but I needed to zoom out.

That feels exactly like what Im experiencing. What a godsend. Seriously, Im convinced this will make me a much more efficient and explosive paddler. I just couldnt get it out of my head that it was harder to get up to speed in colder conditions. I was thinking is it injuries, or just plain ol’ Im not 25 anymore!?

I think it IS both. I was out of paddle shape and that the step is harder to accomplish based on the increase of resistance. Im properly stoked on this… rad to know and it is yet another step of demystifying what I am training to do. The plot thickens!

I got out for a couple of sessions on the weekend and I was able to crack through a handful of times on my 2140. Up on FOIL and trying to carve the tidal swell but man it was euphoric to get up. Ive felt this ramp up before…

As y’all know being on FOIL is second to none. Having to work the conditions, swell & wind direction, tide movements, plus the deg C of the water makes it hugely rewarding. Im all in boys. Thanks for the help on this… def needed to reach out as this one was was bugging me!

I noticed this has an incredibly obvious effect, gritty water is pretty terrible. Backwash or shorebreak that churns up the water with plants and sand is not good at all.

Interesting! That is huge. I have no idea what to make of that, but certainly enough to notice. Here is an explanation of the concept @FOILstoke you might like the interactive parts.

I can’t quite grasp why the viscosity makes a difference but I guess … it quickly goes into Reynolds numbers where I think the science goes beyond me. I think 5 °C water has half the reynolds number of 25 °C water, and that indeed would make a huge difference.


If we hop passt the terminology and non dimensional stuff for a moment, at the end of day viscosity only matters when there is shear in the flow. Shear in the flow means that one bit of fluid is trying to go faster or slower than the bit right next to it. A couple of the most common areas where you get shear in a fluid flow are boundary layers and vortices, such as wing tip vortices. Viscosity is essentialy telling you the amount of energy required to support a given shear rate in a fluid flow. So, up the viscosity and you up the amount of energy required to maintain a given flow pattern. We generally have a pretty good intuitive understanding at this conservation of energy level - dragging our hand through a bowl of honey requires more effort (energy) than dragging that same hand at the same speed through a bowl of water.

At a more detailed level it definitely gets more complicated. If you change the viscosity alone (speed and characteristic length stay the same) the entire flow field around around a given body (foil, in this case) changes. This is because changing the viscosity has changed the ratio of significance of viscous (shear) forces to inertial forces (momentum), and this ratio is really what the Reynolds number is telling you - low reynolds number means viscous effects dominate, high means momentum dominates. Unsurprisingly then, it is low reynolds flows, where viscous effects dominate, that are most prone to things like flow seperation (stall). But, for all the complexity, it still takes more energy to pull your same hand at the same speed through a bowl of honey than a bowl of water.

Coming back to the foil, if you want to enjoy the same stall angle of attack on your favorite foil in cold water, you have to go faster than in hot water to get enough momentum into the flow to balance out the increased viscosity and get back to similar(ish) Reynolds number. Unfortunately, it was already going to take more energy just to go the same speed due to the increased viscosity, so going faster again would really take some doing.


This really helped things make a bit more sense. I wonder if this does explain why foiling in winter feels so terrible? 7C to 17C? Based on my puddle deep understanding seems like it could be something in it.

(Combine that with all the silt and grit that runs off from the farms in the winter rains?)

Well I’ll be darned! You learn something every day. Looking forward to that Costa Rica trip even more now! :call_me_hand:t2:

Matt, SILAS, thank you both.

Matt, you’re right, I did like the interactive parts. :slight_smile:

SILAS, I read what you wrote a bunch of times before it properly sank in. I believe you have well articulated what I am feeling in the water. Thank you so much for that. For me with my gear and conditions locally the difference is massive. Not only is it a relief to know that my strength and conditioning has not been fully lost in the last few months but I also am so stoked on what that means for me in terms of my development. I see that clearly now.

A couple of days ago I tried an experiment based on this chat. I put on my smaller front wing on my deck (an 1800cm FOIL VS the 2140 I usually use). The expected results were there, as in, the FOIL was faster through the water based on the reduced drag. But I was VERY curious to feel what a lower drag wing would move like now that I have a better understanding as to why the FOIL was behaving the way it was. Knowing now that the energy that I would put into the board & FOIL through the paddle stroke would “stickier” based on the increased viscosity and colder temps, I was able to focus on lengthening the FOIL - fulcrum arc. This change has massive effects for me. I was able to make sense of my back foot engagement in a way that I had not prior. I was able to be pulling the board across the paddle stroke with more stability. A massive win!

As expected, the FOIL felt more free and def quicker. However the best part is that on my sprints I was SUPER challenged to release the board up once it was on step. My CORE strength, flexibility, and reaction time were all challenged in a way that I had not felt for some time (or at all). Whereas before, not knowing the viscosity deltas I would think “wow I’m sluggish today”. I see now the full arc of FOIL movement that I was deploying before was super shallow and now I see so clearly the path of progress ahead of me on my way to more - onFOIL time. I believe in order to able to get up in colder temps I need to be more efficient with the FOIL, this means more accurate and more powerful on my paddle sprints. Now that I know where to key in on, it unlocked a range of motion that I had not seen or felt prior, my board-FOIL range was lengthened. Something I really needed to understand and implement to progress.

I did get video…

Needless to say it was a GREAT day. I came off the water feeling super excited. I see now that this key understanding will result in me be a better sprint paddler and FOILer. Im stoooooked up gents… thank you !!!

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We (all 2 of us) prone in 1 degree C seawater temp this time of year. Be it a combo of the viscosity and the 6mm suit we go up a foil size compared to summer because it’s much harder to pump and link waves than summer

Water density and viscosity didn’t change much.

At 25C and 4C (most dense temp) the difference is only 0.3%. You can’t feel the difference at this scale.

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I suggest you review and correct your assertion about viscosity change.

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I also agree with the assessment that there a a few relevant factors in the summer/winter delta. Getting more precise about weight, I see that the difference in gear and my own typical body weight is about 5 or 6% summer to winter. The viscosity difference is big enough to matter. Silt and particles in the water definitely messes with the flow, the boundary layer in particular. My movements are less coordinated because I fatigue faster in the cold, even if I don’t feel cold (and I am in worse shape in the winter). I had not, however, put it all together into a rule of thumb of taking a size bigger foil. Thank you @rossdrobinson for spelling that one out, it will help a lot on marginal condition days.