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Ability to swim to surface

Discussion in 'Basic Scuba Discussions' started by Scuba, Aug 2, 2002.

  1. Scuba

    Scuba Solo Diver

    I dive with a 7/5 mm wetsuit, single PST 104 tank, and 16 lbs of weight. I am properly weighed at the suface. In the event of a BC failure, at what depth would I be unable to swim to the surface without ditching weights? Ditching weights?

    I have read other threads regarding this and the info has been helpful but I am unable to answer these questions with certainty.

    Is there a simple formula to make this calculation?

    I realize people will have different conditioning and strength, but a generalized ballpark figure would help.

  2. Newhampster

    Newhampster Nassau Grouper

    If you're in reasonable shape, you should be able to swim to the surface with all your weights and without the lift of a BC or the lift of your neoprene suit.

    The BC's primary purpose is to help you maintain constant boyancy, not provide lift to raise you up and down in the water column. You shouldn't need lift to do that. In fact, when you want to ascend, you should prematurely dump air from your BC so you don't get into an uncontrolled positive feedback - lift situation and emulate a Polaris Missile trying to launch itself towards the heavens.

    Your BC is not a lift, it's a stabilizer...
  3. Walter

    Walter Instructor, Scuba

    Easy enough to find out. At depth, dump all the air from your BC and swim up.
  4. Iguana Don

    Iguana Don Guest

    said it better himself!

  5. art.chick

    art.chick Manta Ray

    # of Dives: 100 - 199
    Location: Hollywood, USA
    With 7.5mm of neoprene & only 16lb of lead, you are in little danger of permanant residence in Davey Jones' locker. Besides, the weight belt is way cheaper than the BC if you ever have to make a choice. (Wait, did that make sense?)
  6. Newhampster

    Newhampster Nassau Grouper

    If you can't swim up without inflating your BC, you shouldn't be diving.
  7. Rick Murchison

    Rick Murchison Trusty Shellback Staff Member ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Gulf of Mexico
    In addition to being able to swim to the surface with a BC failure it'd be nice to know you have a backup. If you are diving a PST104 and a wetsuit, you should carry redundant buoyancy with you - a lift bag will do nicely. A drysuit is better.
  8. Scuba

    Scuba Solo Diver

    Easy enough to find out. At depth, dump all the air from your BC and swim up.

    And if I can't ........ :jester:

    Seriously now, I recall reading a post where someone estimated the buoyancy of a 7 mm wetsuit at aproximately 20 lbs., most of it being lost at approximately 90 ft. So if this is the case I am no longer neutral at depth, and the only way to become so is to inflate the BC or ditch weight. Lets say I am now 20 lbs, negative at 90 ft, if my BC fails and I ditch my 16 lbs. of weight. Am I now 4 lbs. negative? or am I now 4 lbs. plus the negative weight of a full tank?

    How much negative buoyancy can the avg. fit diver overcome?

    Having a lift bag as redundancy until I get a dry suit makes sense to me.

    Thank you all for your response.

  9. jrtonkin

    jrtonkin Nassau Grouper

    I'm a fairly strong swimmer (ex-lifeguard). I know that I can comfortably swim about 20 lbs to the surface (on a good day, I can get 30 though that's really pushing it), but that's using a legs-only stroke, and no fins... However, I don't think I could do that stroke (eggbeater kick) with fins; then again, I'd have the use of my hands...

    So it's probably reasonable to say that you can lift 10-20 lbs, depending on how good a swimmer you are.

    As for buoyancy calculations...

    Discounting air in the BC, things that change your buoyancy are wetsuit compression and extra air in your tank.

    My two-piece 7mm suit has about 20 lbs of buoyancy. Air weighs about 1.5 lbs/ 20 cu. ft.

    So if you're neutral at the surface, at the end of a dive, with no air in your bc, then:

    At depth sufficient to completely compress your wetsuit, you'll be 20lbs negative buoyant, that your BC has to make up for.

    At the start of the dive, the 100 cu.ft. of air in your tank weighs 8 lb that your BC has to make up for.

    So at the start of the dive, when you reach full-wetsuit-compression-depth, your BC will have to provide about 28 lbs of lift for you to be neutral.

    If your BC was to fail at that point, you're looking at more weight (28 lb) than you can probably (depending on your physique) swim up. But you said you've got a 16 lb. weight belt... So if you drop that as well, then you're looking at "only" 12 lb of negative buoyancy... probably within the realm of what you can swim; again, that's a "probably", it's really going to depend on your swimming abilities.

    In answer to your question about what's the limiting depth for what you can swim up from, that's going to depend on how much you can lift, and how much your suit compresses at depth.

    The tricky part of the equation is how much wetsuit compression changes the buoyancy of the suit... I don't know of any simple (or even any complex) formulas to get that number.

  10. Spectre

    Spectre Solo Diver

    # of Dives: 500 - 999
    Location: Wicked farther south of familiar
    Excellent explination jrtonkin.

    One other part to remember is the other side of the coin. You don't want the only method of swimming to the surface to put you into an uncontrolled ascent.

    Continuing with the model provided... in the situation, you'll want to be able to control 8 lbs of positive bouyancy as your wetsuit buoyancy returns on your ascent. Now if your nearing the end of your dive, you'll have to add back in the added buoyancy of the tank... lets say another 4 lbs. So that means you'll need to try and control an ascent 12 lbs positive at the end of your dive.

    If you play around with a spare weight belt, you can get a decent idea of what you can swim with. If you find you can swim with 20 lbs, but not 28 you would still need to ditch weight to be able to swim to the surface, but not 16 lbs. You'd need to ditch 8 lbs if you have a full tank, 4 lbs if you have a 1/2 tank.

    So if you made 8 lbs ditchable and 8 lbs fixed, you can now ditch the 8 lbs to be able to swim to the surface, and have a little less positive buoyancy to fight on the ascent. Divide that 8 lbs into two ditchable 4 lb pockets, and the most you have to fight is 4 lbs positive. Since the average adult can displace about 9 lbs with their lungs, that should be controllable to ascend safely.

    I personally don't like the idea of ditching a weight belt for anything other than as an absolute last resort (I came close once, but that's a whole other story, which started me thinking about this stuff for the last few months). A better option, as mentioned, is to have buoyancy redundancy... such as a dual bladder BC, a drysuit, a lift bag, etc...

    Disclaimer to make those from the 'other' thread happy, both the vocal one and his silent supporters:

    I'm only a novice diver and I don't know what I'm talking about, so ignore me.

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