Basic buoyancy question

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KMD

Contributor
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Location
Bay Area, CA
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I understand that buoyancy can change on depth and is not a static equation but I was wondering…

What is a realistic amount of weight an average diver can swim against when diving recreationally? For example if one were 15 pounds negative would one be able to swim to the surface?

Kevin
 
As in having 15 pounds too many beyond neutral?
 
KMD:
I understand that buoyancy can change on depth and is not a static equation but I was wondering…

What is a realistic amount of weight an average diver can swim against when diving recreationally? For example if one were 15 pounds negative would one be able to swim to the surface?

Kevin
It varies a lot (how strong are you?) but 10 pounds negative is a lot of weight to swim up - many people will find even 5 pounds negative to be a struggle.
 
That is going to vari from diver to diver.

Think of it this way.... while diving you somehow rip a hole in your BC and you have no available lift devices. You are on your own to swim to the surface. Can you swim your rig to the surface without dropping weight? How much weight do you need to drop before you can swim to the surface?

Some people have strong legs and and kick a heavier rig to the surface. Others can't.

TwoBit
 
When I did my lifeguard cert, I had to hold a 10-pound weight and tread water for five minutes. That was tiring, but not difficult.
 
TwoBitTxn:
That is going to vari from diver to diver.

Think of it this way.... while diving you somehow rip a hole in your BC and you have no available lift devices. You are on your own to swim to the surface. Can you swim your rig to the surface without dropping weight? How much weight do you need to drop before you can swim to the surface?

Some people have strong legs and and kick a heavier rig to the surface. Others can't.

TwoBit

Funny thing is a lot of people never think to drop their weights. A lot of dead divers are found with weights still in place. (Obviously dropping your weights isn't going to help if you're being eaten by a shark...unless you maybe use them as a weapon... :D )

Nauticalbutnice :fruit:
 
Firebrand:
When I did my lifeguard cert, I had to hold a 10-pound weight and tread water for five minutes. That was tiring, but not difficult.
We didn't have to hold weights when I became a lifeguard but we did have to hold our forearms out of the water for 10 minutes, so we couldn't use hands to skull with. Of course, I was 16, ran 5 miles a day, swam and skiied competitively and could press 1000 pounds on the sled pretty much all afternoon.

People come in all shapes, sizes and capacities and change with time. Most will struggle mightily with 10 pounds which is the point of the test you endured. Now that I'm 50, fat and spend way too much time on the internet, I'd rather not be reminded of my former glory... :D
 
KMD:
What is a realistic amount of weight an average diver can swim against when diving recreationally? For example if one were 15 pounds negative would one be able to swim to the surface?

Kevin


It is not the swimming to the surface that is the problem. It is the addition drag during the dive that increases air consumption that you want to worry about. :D
 
m3830431:
It is not the swimming to the surface that is the problem. It is the addition drag during the dive that increases air consumption that you want to worry about. :D
Huh?
  • The question KMD asked is one that every diver should be asking: if my suit compresses or my BC fails, am I too negative to swim to the surface? I can't hold my breath long enough for this not to be a problem. :wink:
  • There is a relationship between being overweighted and the coefficient of drag but it's indirect. Are you confusing mass/inertia with volume/drag?
 
Makes it interesting with steel tanks :) Going from an AL80 to an E7-100 is a 5.5 pound shift, right from the get-go, buoyancy empty, and I only carry 8 lbs of lead diving with only a core warmer. That means I'm basically carrying no weight if I dive the 100 and a core warmer, would actually be negative since I use a pony with 2 lb trim weight.

That's weight totally empty. The air fill in a 100 is 7.5 lbs, vs 5.8 lbs in an AL80, so I start roughly -7 or -8 pounds. Budget some water in the suit, more like -9 or -10.

If I can't swim up against -9 or -10:
1) More wetsuit -- a full 5mm gives me 4 lb more buoyancy.
2) Dump my 2 lb trim weight.
3) Sit and suck on the 100, burning gas to lighten it. OK to do if the bottom is within limits.
4) Dump the pony and trim weight -- about a 4 lb increase in buoyancy.
5) Dump the tank, swim up on the pony.
6) Dump the whole BC, go up on the pony.

(6) would be last choice, as I would then be slightly positively buoyant
 
https://www.shearwater.com/products/teric/

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