Ok there's no link this time. Those who are not interested can stay off the thread, those who participate please keep to the subject matter.

When water temperature decreases, the viscosity of the water increases. This increases drag, increases turbulence and decreases the extent of smooth laminar flow.
Shown below is the effect upon a fairly typical single fin foil of change in water temperature from 20 degrees celsius down to 1 degree celcius. The speed and chord length in this example are kept constant.
As can be seen the Reynolds number is much lower in cold water, this shows the increase in water viscosity.



RAF 27 AIRFOIL (raf27-il)

Details

(raf27-il) RAF 27 AIRFOIL
RAF-27 airfoil
Max thickness 9.8% at 30% chord.
Max camber 0% at 0% chord


Water temperature 20 degrees C
Velocity m/s26.843 mph43.2 kph
Chord width m0.19685 ft2.3622 in
Kinematic Viscositym2/s1.054e-5 ft2/s
Reynolds Number735,166


Water temperature 1 degree C
Velocity m/s26.843 mph43.2 kph
Chord width m0.19685 ft2.3622in
Kinematic Viscositym2/s1.769e-5 ft2/s
Reynolds Number438,009

The red line in the two graphs below represents the higher water temperature, and the purple line the lower temperature.

As can be seen here in the Cl/alpha graph ( which shows changes in lift vs angle of attack) at lower water temperature the fin will stall ealier (stall is seen where the lift suddenly drops as angle of attack increases). In cold water the fin will stall at about 11 degrees aoa and in warm water, at about 14 degrees. this is a significant difference.



Below we can see Cd/alpha or drag vs angle of attack. As the angle of attack exceeds 7 degrees the drag produced in cold water soon becomes double that of the drag in warm water, and the difference increases as the angle of attack goes higher.


Results vary widely depending upon the kind of fin foil used, and the chord length of the fin. Increasing the chord length of the fin raises the Reynolds number and is one way to overcome the angle of attack limitation seen in cold water. All else being equal however, increasing the chord length will also increase skin friction drag. Increasing speed also increases the Reynolds number.

So, increasing speed, water temperature or the chord length of the fin all have the same effect on the Reynolds number. Chord length and water temperature are fixed during any given session.Speed however, constantly changes and the surfboard fin is required to operate over a range of speeds during a ride. A fin which is optimised for low speed will be larger than is necessary at high speed, and vice versa, and a fin which is optimised for warm water might experience stalling issues in cold water, when used at the low end of the speed range.

Issues are most likely to become apparent in cold water when using high aspect ratio fins with short chord lengths.
Ideally a different fin or set of fins will be needed for different water temperatures. the foil section can be changed, but the most obvious way to alter the fins is to use a longer chord in cold water, and a lower aspect ratio fin.

Thanks to airfoiltools.com for the graphs, foil section and reynolds number calculator

I need a holiday,I swear this topic was posted earlier today if not I've finally lost it

Anyway I hope that there are a few readers who will find this interesting, I certainly do.

I've been working on it for a month and find it fascinating.

No I do not have an FCSesque fin buying app which lets you plug in water temperature as a variable, if I make and test fins on the basis of this analysis it will be a very long process, for my own satisfaction.

It's not like I'm in need of money so take it for what it is, a surfer thinking about stuff and sharing the stoke.

Yes admin this means you.

There's more to come on the topic, this is just a taste.

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Macaha said..
I need a holiday,I swear this topic was posted earlier today if not I've finally lost it

You are ok, the other thread got lost in translation.

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RoyStuart said..

Macaha said..
I need a holiday,I swear this topic was posted earlier today if not I've finally lost it

You are ok, the other thread got lost in translation.

Whoa now I'm lost

According to the latest Sherlock Holmes movie, Prickly Ash harvested from Hiroshima doesn't help.

It all looks a bit hard Roy but stick with it. Will you be designing new fins with all this info ?

I'm planning to.

The drill will be to find the shortest chord/base fin which will work well over the summer and then see if it needs a longer chord when the water cools down.

What about salinity? Now there is a can of worms for you !

Do do some calcs on those puppy's bro !!!!

That at will keep you of the streets for a while !!!

What about leggies?? I bet they stretch longer in warm water than cold

jokin

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DARTH said...
What about leggies?? I bet they stretch longer in warm water than cold

jokin

Like wax there are warm and cold water versions

Doggie in the greenroom.
Just like the old days.
Good to see

Cheers Roy, Interesting read,

One thing of note is in regards to wave power in differing temperatures, particularly in regards to the viscosity of the water, it is hard to calculate this in regards to drag.
If for instance you are surfing a two foot wave in 25 degree water and the speed the wave moves is exactly equivalent to the same wave in one degree water the colder wave which is more viscous will have more power behind it.

It is an odd thing however that should this experiment be able to carried out the fundamental flaw is that due to the more viscous nature of the colder water this résistance will also slow down the waves speed through the line up and result in a longer wave detonation time (ever so fractionally)

There is nothing quite as invigorating as a good working in solid waves in cold water. Obviously taking into account the higher stress that your body is under due to the cold is a relative factor. But.. the workings are in reality a lot worse in cold water than warm.

How does this relate to fins? In summary whilst the drag may increase in cold water it has to be factored in that the wave whether cold or warm will have different levels of speed and power which will change the outcome of a calculated drag even on the same wave which has been thawed or frozen. I know that its currently impossible to teleport whole lineups around the world but if you could it would be an epic experiment to initiate design in surfboards.

Warm water will also have a higher settling rate to allow sediments to settle to the sea floor, particles of sand and objects of low specific gravity will settle slower in cold water than warm. How does this help? The difference between a sand bottom point break and a kelp covered reef will create differing levels of water density in wave zones.

When taking fins into account length of board and volume will be attributable factors when selecting fins, however if utilizing the same set of fins on differing boards within the mystical teleported wave it may be possible to have two differing volumes and surface area boards ride the same in different conditions (cold water/warm water) by utilizing one common set of fins. Whilst this is obviously an impossible goal to achieve (personal opinion only no science required) the magic board and fin combo for most people would be found smack bang in the middle of these two parameters. (No haven't found this board yet and will notify all when I do)

do not know what it is doing to my fins...however I'm sure that cold water is tightening my nuts.

I love it when Roy posts.
Just read a whole lot of stuff that made as much sense to me as the financial pages of The Australian.
Edit.
Actually a lot more sense. Just not sure fin performance in changing water temperature is the limiting factor in my surfing.
Might explain why I have much more fun in Byron Bay and Merimbula than Victoria?