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The Physics of Freediving

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cheese

a dairy product
Nov 1, 2002
34
2
0
There has been a lot of talk about things like pressure, density and viscosity in different threads all over deeperblue. Why not post your questions and thoughts regarding the physics that affect freediving all in one thread?


Here's something that occured to me today:
Eric Fattah among others has written a few times about the difference in viscosity between fresh water and salt water. This reminded me that water temperature also affects it's viscosity. The coefficient of viscosity for pure water at 20' C is about 0.001 Pa s but for pure water at 0' C it is 0.0018 Pa s. That's an 80% increase!
If you move at a relatively low velocity through water, the frictional force is directly proportional to the coefficient of viscosity.
So if you are moving slowly, you would experience 80% more friction in 0' C water as compared to 20' C water!
I don't know if a normal freediving velocity of around 1 m/s would be considered a low velocity in this case, but still it seems that there would be a pretty major difference between warm and cold water.

The density of water also decreases with increasing temperature after 4' C, but the change is nowhere near as dramatic as with viscosity. So from this point of view it would seem that the warmer the water, the better it is for freediving! Less friction and only a fairly minor change in density so there would still be plenty of power in your kicks.
I have never noticed the difference myself, and most likely there is not much of a difference in real life, but it is an interesting idea anyway.

In the summer I freedive in water that might be close to 20'C on the surface and 4'C at 30m. So as I sink deeper, the friction affecting me increases. But at the same time my buoyancy keeps decreasing. At depths less than 30m the change in buoyancy is the dominant factor, but still the situation is more complex than I thought before.

It would be interesting to read comments from others more learned in hydrodynamics.
 

fpernett

Well-Known Member
Nov 7, 2001
832
102
133
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Not an expert in hydrodynamics, but warmer water blunt the diving reflex, losing the oxygen conservation effect of it, and taking blood flow from brain to skin.
Without hydrodynamics concepts, I think cold water is better for freediving
 

cheese

a dairy product
Nov 1, 2002
34
2
0
Frank,
Yes, all things considered cooler water probably is better for good results but in my post I was deliberately talking about hydrodynamics only.
I suppose Eric's Hypothermic Diving System probably wouldn't work very well in 25 degree water either :)
 

efattah

Well-Known Member
Mar 2, 2001
3,294
487
173
The temperature-viscosity relationship is very easy to see if you try to swallow vitamin pills with water. With cold water it is easy to swallow pills. With warm or hot water, it is FAR more difficult, because the water has much less viscosity.


Eric Fattah
BC, Canada
 

cheese

a dairy product
Nov 1, 2002
34
2
0
Eric,
That's an interesting point. Gotta get me some vitamins and try it out! :D
Have you ever thought about how this affects freediving, and have you noticed the difference when diving in different water temperatures yourself?

If 1 m/s is not slow enough for fluid friction to be proportional to the coefficient of viscosity, what happens then? Friction must still be related to viscosity somehow, but I don't know how. Turbulence will make things a lot more complex, so maybe there is no simple relation.
 

Will

Freediver
Jun 20, 2003
556
151
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I am very sceptical as to there being a noticeable difference in viscosity with temperature. There are a myriad of confounding factors to the theory that pills are easier to swallow in cold water because of viscosity.
- Heat inhibits peristalsis, to prevent the stomach from being scalded.
- Because the water is hot you may not take as big a mouthful.
- If the water is closer to body temperature you won't notice the water going down, only the pill, whereas with cold water the pill is disguised by the sensation of the cold water.

Try swallowing pills in fresh and salt water, where the difference in viscosity is greater. (With all this pill testing mind you don't get scurvy from a Vitamin C overdose!)

The viscosities of fresh and saltwater are distinguishable in freediving however, and I am particularly interested in the affect on unassisted freediving, where high viscosity has benefits (propulsion) and detriments (inhibiting glide phase). My stroke period in fresh dynamic is about 6.5 seconds as opposed to 5.8 in salt, but my average speed may well be higher in salt (haven't managed to measure this yet).
 
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