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Thread: Ability to swim to surface

 


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    Ability to swim to surface

    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.

    Peter

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    Newhampster's Avatar
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    Wrong mental model of what a BC's role is

    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?
    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...
    Live Free and Dive

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    Easy enough to find out. At depth, dump all ...

    Easy enough to find out. At depth, dump all the air from your BC and swim up.
    The Devil's in the details.

    Disclaimer: All discussion of value, by me or anyone else, is opinion.

    For a comprehensive approach to diving education, check out Scuba Educators International (SEI) Diving.

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    Iguana Don's Avatar
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    The Dali Llama couldn't have.................

    Walter once bubbled...
    Easy enough to find out. At depth, dump all the air from your BC and swim up.
    said it better himself!

    ID

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    You'll float!

    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?)
    When the going gets tough, the tough go diving.

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    [QUOTE]If you're in reasonable shape, you sho...

    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.
    If you can't swim up without inflating your BC, you shouldn't be diving.
    Live Free and Dive

  7. #7
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    Wet steel...

    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.
    Rick
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    "... they saw the deeds of the LORD, his wondrous works in the deep." (Ps107:24)

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    Easy enough to find out. At depth, dump al...

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

    And if I can't ........

    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.

    Peter

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    swimmable weight

    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.

    Jamie

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    The other side of the coin.

    jrtonkin once bubbled...
    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.
    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.
    Last edited by Spectre; August 5th, 2002 at 10:24 PM.
    -Jeff

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