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saltwater/freshwater differences in buoyancy

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DiverDown

New Member
Mar 1, 2003
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Does buoyancy change from saltwater to freshwater? I've just moved from CA to CO and was wondering if I need to adjust the amount of weight I'll be wearing. If so, is there a magic formula for figuring how much weight I should have based on what I used in CA? I dove in CA with a 5mm suit and 24 lbs of weight.

Thanks!
 

Jon

Dairyland diver
Supporter
Apr 7, 2001
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It sure does.

Freshwater is less bouyant than salt water. You will use less weight for sure. How much less is up to you. I change my weights around all of the time depending upon what suit I am using and what kind of diving I am doing- spearfishing or wreck diving.

With my 3mm Picasso two-piece suit I wear 6 pounds for wreck diving, in the deeper depths, and 12-13 pounds for spearfishing, in shallow water. Same suit, different types of diving.

I suggest that you take some time to play in the water and see what you need. Taking 2-4 pounds off of your belt is a start and you can work from there. Some people say that you can take 10% off of your belt when you go to freshwater- a rough estimate at best.

Hope that helps.

Jon
 

Bill

Baron of Breathold
Oct 17, 2001
1,805
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If you remove about 5# of lead (2.5% of your total weight, body, suit and belt) the depth for neutral buoyancy should be the same. The 'book' number is 2.5% from normal sea water to distilled water. Pool water is about midway between but I'm not sure about a lake or reservoir. Temperature can change things too. I've been playing with these calculations for a while and it gets complicated. Nothing beats trial and error.
Aloha
Bill
 

DiverDown

New Member
Mar 1, 2003
11
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0
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Jon,

Thanks, that does help a lot. I figured I'd have to experiment a bit with weight but this gives me something to go on. I miss CA but it'll be nice to shed some weight.

Cheers!
 

efattah

Well-Known Member
Mar 2, 2001
3,294
487
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Although the difference in buoyancy is about 2.5% of your body mass, there is a 20-30% decrease in viscosity in fresh water. This means there is far less drag. What that converts to is that when you are 5 pounds negative in fresh water, you sink a hell of a lot faster than when you are 5 pounds negative in salt water. People then erroneously conclude that they were way 'more negative' in the fresh water, when in fact they were just as negative, but sinking faster. So, if you want to hit the same 'terminal' sinking velocity in fresh water, you will need to drop your weight way more than 2.5% of your body mass.

Think of the analogy with air. An inflatable human mannequin, weight two pounds, is two pounds negative in air. That same mannequin will fall much faster in air, than a diver who is two pounds negative in water. All because of drag & viscosity.

Likewise, each stroke of your fin or monofin will produce less thrust in fresh water. Again, think of the analogy of air. How much thrust does a fin thrust produce in air? Near zero.


Eric Fattah
BC, Canada
 

Jon

Dairyland diver
Supporter
Apr 7, 2001
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473
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Eric,

I remember you talking about this in Miami- that fins are less efficent in freshwater.

It made me wonder if doing free immersion in freshwater would be the way to go. You can still pull yourself along at the same speed, but go further because of less resistance.

I go out and just use my depth guage and see where my bouyancy is at 33'. For spearfishing I go a little bit heavy and clip my extra weights off to my float as I work towards deeper water. The 10% rule was an old carry over from scuba diving. No a great answer, but a place to start from.

Jon
 

efattah

Well-Known Member
Mar 2, 2001
3,294
487
173
Jon,

Good point about the free immersion. Makes sense; after all, Herbert's pb of 97m in 4'15 in free immersion was done in a lake!


Eric Fattah
BC, Canada
 

Frank O'Donnell

Apneic shutterbug
Apr 23, 2003
132
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Clipping off weights

Jon, just out of curiosity, what style of weights are you using that allows you to clip them off to your float depending on the depth you're working?

So far my pretty much one and only freediving weight belt is a Rob Allen Marseillaise rubber belt with low-profile rectangular metal weights that slide onto the belt. Kind of hard to change out on the fly.
 

Jon

Dairyland diver
Supporter
Apr 7, 2001
4,080
473
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I use the same kind of belt with square 2 pound scuba weights.

I buy the kind of weights with the larger slots in them so I can just take them off and slide them onto some cave-line loops that I have tied off to my float.

I lay forward in the water, unbuckle the belt, slip off one weight, buckle-up, and then slip the wieght onto the line on my float. It takes less time to do it than it does for me to type it.:duh

Jon
 
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