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Race foil tuning warm vs cold water

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Breze
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Re: Race foil tuning warm vs cold water

Postby Breze » Mon Jul 26, 2021 6:51 pm

Stan Keusch ( SK Shapes) interviewed by Gunnar at Gizzeria talks about tuning Foils. When he talks about diff parameters he mentiones temperature first, currents, sweet- saltwater. Of course this means only changes in the structure of the surface eg 1000-5000 grit, not choosing bigger or smaller foils


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Re: Race foil tuning warm vs cold water

Postby flying grandpa » Mon Jul 26, 2021 10:06 pm

Schietwedder wrote:
Mon Jul 26, 2021 9:01 am
But viscosity does as flying grandpa writes.
Ask someone at a towing tank for ships why they measure the temperature so accurately. Even if the density doesn't change much, the viscous effects change massively in summer and winter or even morning/afternoon between some test runs and therefore need to be corrected to be comparable in case of a towing tank.

That this effects foils and their development also, no question! At what rider level you are able to experience this, I don't know.

As Gunnar writes here it is probably more a setup issue.
Thank you, Schietwedder. I didn’t know that.
Viscosity is responsible for lift generation. Without viscosity, there would be no lift.
I suspect the relation between lift and viscosity may be proportional and if that is true, we could reduce our winter foils to half almost.

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Re: Race foil tuning warm vs cold water

Postby tkaraszewski » Mon Jul 26, 2021 10:16 pm

flying grandpa wrote:
Mon Jul 26, 2021 10:06 pm
Schietwedder wrote:
Mon Jul 26, 2021 9:01 am
But viscosity does as flying grandpa writes.
Ask someone at a towing tank for ships why they measure the temperature so accurately. Even if the density doesn't change much, the viscous effects change massively in summer and winter or even morning/afternoon between some test runs and therefore need to be corrected to be comparable in case of a towing tank.

That this effects foils and their development also, no question! At what rider level you are able to experience this, I don't know.

As Gunnar writes here it is probably more a setup issue.
Thank you, Schietwedder. I didn’t know that.
Viscosity is responsible for lift generation. Without viscosity, there would be no lift.
I suspect the relation between lift and viscosity may be proportional and if that is true, we could reduce our winter foils to half almost.
This is preposterously false. If this were true racers from cold water venues like San Francisco would use much smaller foils than racers from warm water venues (like the Mediterranean). They don't, the same foils are used in all venues regardless of water temperature, and all the top foils (Mike's Lab, Chubanga, Levitaz) are roughly the same area and hit roughly the same top speeds regardless of water temperature.

Anyone who has ridden the same foil in different seasons will be able to tell you it doesn't generate twice as much lift in the winter.

I don't know how you could even write that seriously.

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Re: Race foil tuning warm vs cold water

Postby Schietwedder » Mon Jul 26, 2021 11:41 pm

Hey Hey,
A lot of misconceptions and confusion here.
I hope I can clear up some, graduating in naval architecture with a lot of fluid dynamics.

First off: We have lift and drag on a hydrofoil. Hydrofoil works in water, for calculations we often need the viscosity and the density. Density changes little with temperature, viscosity a lot.

Now for lift, we can (almost) neglect viscosity if we are not near stall speed or stall angle (for stall viscous effects are really important) or have issues with the interaction of our foil with the free surface (ventilation for example).
Hence Lift can be approximated quite accurately with potential theory CFD codes which neglects viscosity, if we are sure that we don´t operate near stall, or with very thick profiles for example.

So lift will change veery little with temperature as it´s not dependent on viscosity (almost) and density does indeed not change much.

For drag, theres also different kind of drag for example:
-viscous drag (friction drag-> viscosity really big factor as the name says)
-induced drag (something with eddies and vortexes around our foil mainly driven by wing angle, twist and AR)
-form or pressure drag (foil pushing away the water kind of, pressures acting on our foil)
-interference drag, parasitic, cavitation, ventilation and so on
That´s why in engineering we need more complex and expensive CFD codes (RANSE etc.) or experimental testing to estimate viscous forms of drag accordingly if we can´t rely on empirical formulaes for specific fields.

Back to drag:
Here you also have to look at how big the different kinds of drag are in comparison to overall drag depending on what craft you look at. On a ship friction drag and wave drag are the most important. If you go slow friction drag is important, while when going faster wave drag increases a lot and becomes the main driver.
Even if a ship has very very very little resitance compared to the mass that you´re moving you want to know the drag quite accurately to know how big a motor you have to install to go the speed that you want, and small changes or mismeasurements have a huge impact on the fuel you are burning if you run 24/7 if you estimate the drag not accurately.
That´s where you need tank testing and also why I brought it up because to estimate the frictional drag you depend a lot on the temperature and viscosity that you´re going in to make a good result.
I maybe went too far off topic and did not explain properly.

On kite hydrofoil I can´t tell you the proportions of all the drag forms accurately, but friction plays of course a role if you want to be the fastest at high speed, and even more importantly probably ventilation on the mast is a killer which are both viscous effects and therefore depend a lot on temperature.
And that´s why people sand their boards with different grid depending on the viscosity and temperature, to help the flow a bit to stay attached before ventilating or be low frictional drag. Apparently different surface roughnesses have evolved to handle different temperatures better in practice.

The only circumstance I can think of where lift could change a lot with temperature is when you have a very poor flow almost or temporarily detaching on a foil due to a wrong surface roughness or bad finish and it seems to like a different viscosity better to stay attached and you realise a difference. That is though not a condition a racer would be happy to operate in :)
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Re: Race foil tuning warm vs cold water

Postby Oldman » Tue Jul 27, 2021 4:11 pm

So which 'roughness' is ideal for which temperature / viscosity?

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Re: Race foil tuning warm vs cold water

Postby Schietwedder » Tue Jul 27, 2021 7:24 pm

Oldman wrote: So which 'roughness' is ideal for which temperature / viscosity?
Breze wrote: Stan Keusch ( SK Shapes) interviewed by Gunnar at Gizzeria talks about tuning Foils. When he talks about diff parameters he mentiones temperature first, currents, sweet- saltwater. Of course this means only changes in the structure of the surface eg 1000-5000 grit, not choosing bigger or smaller foils
It´s said in the video...basically rider experience, no real science available yet Stan says. I´m quite sure some Americas cup engineers have some experience and numbers there though which they won´t tell as always ;)

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Re: Race foil tuning warm vs cold water

Postby bohme » Tue Jul 27, 2021 9:04 pm

Viscosity is affecting the thickness and the robustness of the boundary layer. This has an effect on drag and how the foil 'feels'. The traditional way of taking this into account is by using the "Reynolds number".
A typical race foil at 20kn has a Reynolds Number around 500 000 at 5°c, and 850 000 at 25°c water temperature. If you take a typical 2D hydrofoil cross section optimised for this range, is probably has around 10% lower theoretical profile drag at 25°c (3D induces drag is the same). This translates into only a little difference in 'theoretical' speed.
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Re: Race foil tuning warm vs cold water

Postby Matty V » Tue Jul 27, 2021 10:17 pm

tkaraszewski wrote:
Mon Jul 26, 2021 10:16 pm
flying grandpa wrote:
Mon Jul 26, 2021 10:06 pm
Schietwedder wrote:
Mon Jul 26, 2021 9:01 am
But viscosity does as flying grandpa writes.
Ask someone at a towing tank for ships why they measure the temperature so accurately. Even if the density doesn't change much, the viscous effects change massively in summer and winter or even morning/afternoon between some test runs and therefore need to be corrected to be comparable in case of a towing tank.

That this effects foils and their development also, no question! At what rider level you are able to experience this, I don't know.

As Gunnar writes here it is probably more a setup issue.
Thank you, Schietwedder. I didn’t know that.
Viscosity is responsible for lift generation. Without viscosity, there would be no lift.
I suspect the relation between lift and viscosity may be proportional and if that is true, we could reduce our winter foils to half almost.
This is preposterously false. If this were true racers from cold water venues like San Francisco would use much smaller foils than racers from warm water venues (like the Mediterranean). They don't, the same foils are used in all venues regardless of water temperature, and all the top foils (Mike's Lab, Chubanga, Levitaz) are roughly the same area and hit roughly the same top speeds regardless of water temperature.

Anyone who has ridden the same foil in different seasons will be able to tell you it doesn't generate twice as much lift in the winter.

I don't know how you could even write that seriously.
Well the pro windsurfers have fins for European cold /warm warmer pacific conditions and the surface area is way smaller so not so preposterous

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Re: Race foil tuning warm vs cold water

Postby GuyinTCI » Thu Jul 29, 2021 4:16 am

I noticed in second hand racing equipment for sale the seller posting that there kit come with both warm water and cold water mast. Is there a foil racer who possesses this gear who can tell me the difference?

Sure the discussion going on about if this or that makes a difference is fun but does not in the least bit answer this question regarding the difference between warm and cold water setup.

Gunnar: Thanks for the feedback on the stab angle, ill adjust now and test tomorrow.

tkaraszewski: I did not say its set up wrong, im saying it should be faster. Regarding setup this goes back to my question about warm water mast vs cold water mast. I do not know the difference in profile, i speculated perhaps the trailing edge was shaped for san fran water not Turks & Caicos in the summer weather.


For those still following along i polish my foil with 2k sand paper than carnauba wax. Im fly sonic 2 kites. Will post a follow up after stab angle change.

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Re: Race foil tuning warm vs cold water

Postby Floating around » Thu Jul 29, 2021 11:17 am

Wax…. I’m no expert but that will be part of the problem. Water needs to “stick” to your wings, I’m assuming the wax will make it bead?


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