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Scuba
August 2nd, 2002, 08:42 PM
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

Newhampster
August 2nd, 2002, 08:59 PM
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...

Walter
August 2nd, 2002, 09:01 PM
Easy enough to find out. At depth, dump all the air from your BC and swim up.

Iguana Don
August 2nd, 2002, 11:29 PM
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

art.chick
August 3rd, 2002, 12:06 AM
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?)

Newhampster
August 3rd, 2002, 09:02 AM
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.

Rick Murchison
August 3rd, 2002, 09:18 AM
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

Scuba
August 3rd, 2002, 01:27 PM
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.

Peter

jrtonkin
August 5th, 2002, 06:30 PM
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

Spectre
August 5th, 2002, 10:01 PM
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.

WreckWriter
August 5th, 2002, 10:32 PM
art.chick once bubbled...
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?)

With a 104 and 16 lbs of lead he certainly won't float. 104s are heavy in case you never tried one. Luckily, unlike so many who post here (and most divers anywhere) he's not seriously overweighted, proving he has a clue as to how to dive.

Posts like this can be dangerous in that they promote overweighting. Anyone who wears more than 20 lbs of lead in a wetsuit with a steel tank should take a clue from some of the deep air divers and not answer questions related to that.

Just my opinion and feel free to flame but also don't be surprized if a boat crew refuses to let you in the water someday.

Tom

WreckWriter
August 5th, 2002, 10:39 PM
Scuba once bubbled...
Having a lift bag as redundancy until I get a dry suit makes sense to me.

Thank you all for your response.

Peter

Peter, Carry a bag just in case. If you have a BC failure, unlikely unless you have a "jerk to dump" hose, use your bag while keeping your belt in place (so you don't do the Polaris imitation when you hit about 40 feet). Be sure the bag has a dump valve with a pull cord so you can control it just like you would a BC. A drysuit would be better redundancy but a bag will do for now.

Practice, but be careful. Don't overweight, you're doing great now.

Tom

art.chick
August 5th, 2002, 11:13 PM
What I meant was that if u had to ditch either your belt or BC, it would make more sense for the belt to go. I know that a 104 ain't no AL63, but I guess I don't understand why ditching the belt is being avoided. Maybe I am not understanding the issue.

Spectre
August 5th, 2002, 11:41 PM
WreckWriter once bubbled...
If you have a BC failure, unlikely unless you have a "jerk to dump" hose

My one BC failure was due to a stuck deflator valve... Made for an extremely entertaining safety stop.

Scuba
August 6th, 2002, 12:47 AM
Thanks for the good explanations.

I thought I understood it well, but then I got to thinking.
( warning ). I have different figures.

I have a tank whose negative buoyancy when full increases by 8 lbs. ( PST 104 is actually -4.8 empty and -12.6 full accoriding to PST, including valve and boot. I will exclude this data from the following example and use only weight of air, assuming a neutraly empty tank), a wetsuit which is + 20 lbs positive buoyancy at surface, and - 16 lbs of negative buoyancy in weights. So I have - 8 lbs for tank, plus - 16 lbs for weights = - 24 lbs. total negative buoyancy. This is compensated by the + 20 lbs due to 7 mm wetsuit. The result is total positive buoyancy of + 20 lbs and total negative buoyancy of - 24 lbs., for a net negative buoyancy of - 4 lbs. Adding my lung capacity, about + 9 lbs. positive buoyancy to the equation, the result would be + 5 lbs at surface, turning to - 4 lbs negative on a full exhale.

The only tangible buoyancy variance affected by depth is the suit, which will loose its + 20 lbs. positive buoyancy until becoming neutral ( perhaps maybe even slightly negative ). Assuming the suit is now neutral and I still have an almost full tank I would now have - 8 lbs for tank and - 16 lbs for weights, for a total of - 24 lbs. If I now drop my 16 lbs. of weight I would be - 8 lbs. negative.

I am trying to thoroughly understand this subject. Are my examples correct or not? If not please explain in detail. Your examples assume the suit goes from positive + 20 lbs. to negative - 20 lbs. I say it goes to neutral, leaving you with the weight of the tank and weights as negative buoyancy. Jamie you account for the 20 lbs. loss of wetsuit buoyancy and you add the negative - 8 lbs tank buoyancy, but you fail to account for the - 16 lbs. in weights.

One more point on WreckWriter's comment:
Posts like this can be dangerous in that they promote overweighting. Anyone who wears more than 20 lbs of lead in a wetsuit with a steel tank should take a clue from some of the deep air divers and not answer questions related to that.

The only danger I see is ignorance and stubborness in the face of facts, which leads to Doing It Wrong. It is generalization like the last part which lead many too disregard well meant advised. With a maximun bottom depth below of 30 fsw. I would say its perfectly safe to dive with a wetsuit and a steel tank. Some of us enjoy long dives in the shallows.

Peter

Scuba
August 6th, 2002, 12:54 AM
art.chick I am not disputing what to dump in an emergency. I simply want to know how negatively buoyant I would be with total suit compression and BC failure.


Is a "jerk to dump" hose the BC inflator/deflator hose which when pulled also dumps air?

Peter

Spectre
August 6th, 2002, 01:33 AM
Scuba once bubbled...
The result is total positive buoyancy of + 20 lbs and total negative buoyancy of - 24 lbs., for a net negative buoyancy of - 4 lbs.


Well, it's a little different than that. We are focusing on the suit and the tank because those are the two things that will have a substantial change in buoyancy throughout a dive. Everything on your person, as well as your person, will have some effect to your buoyancy, both positive and negative... Your body will change buoyancy day to day, and throughout the day.

There really isn't any need to calculate out everything, as that is what you did by experimenting to figure out your proper weighting. Once you have your proper weighting, you have to then start thinking about what will change during a dive...

Leaving out the effects of the warhammer maneuver on your bouyancy ;) The things that will change are: Your wetsuit buoyancy, based on your depth, and your tank buoyancy; based on your time.

Your tank buoyancy changes 8 lbs, becoming more buoyant as time goes on. Your wetsuit buoyancy [using the numbers in our examples] changes 20 lbs, becoming less buoyant as time goes on.

Your weights counteract the bouyancy of your wetsuit at the end of your dive. Your BC counteracts the suit compression at depth, and the added ballast the air in the tank provides. In other words, if you have 8 lbs of air in your tank, you will be 8 lbs overweight at the beginning of your dive.

So if you loose 20 lbs of buoyancy in your exposure suit at depth, then your BC is compensating for that 20 lbs, plus the extra ballast you have to offset the air you will expend. This gives the 28 lbs.

So if you have 16 lbs on a belt, and 8 lbs of air, you have 24 lbs of ballast. If you are neutral with 16 lbs on your belt and no air in your tank, than the bouyancy of everything on your person, minus the air and the wetsuit is 4 lbs negative. Now your tank you know is -4.8 lbs negative. So that puts your net unexplained buoyancy at +.8, calculate your clips, your regs, yada yada yada... you get the point I think.


[b]
Assuming the suit is now neutral and I still have an almost full tank I would now have - 8 lbs for tank and - 16 lbs for weights, for a total of - 24 lbs. If I now drop my 16 lbs. of weight I would be - 8 lbs. negative.


The suit has lost 20 lbs of bouyancy... which will come back as you ascend. So it's a net loss from neutral of 20. Your 16 lbs of ballast was to counteract the suit bouyancy, plus everything else. The change in bouyancy was 20 lbs, as the weight doesn't change [until you drop it].

So everything less the weight of your air should be net 0. The air makes the net -8 [which is counteracted by your BC], at depth, your exposure suit is net -20, making your BC have to compensate for a net buoyancy of -28.

[And then we get back to you needing to compensate for the net -28 if your BC fails].

Does that help? Think of it as net changes in your buoyancy throughout the dive...



Is a "jerk to dump" hose the BC inflator/deflator hose which when pulled also dumps air?


Yes.. I believe that's what wreckwriter was referring to.

---
Disclaimer for the BS police: Same as before... don't believe me... I don't know what I'm talking about.

WreckWriter
August 6th, 2002, 09:29 AM
Scuba once bubbled...
The only danger I see is ignorance and stubborness in the face of facts, which leads to Doing It Wrong. It is generalization like the last part which lead many too disregard well meant advised. With a maximun bottom depth below of 30 fsw. I would say its perfectly safe to dive with a wetsuit and a steel tank. Some of us enjoy long dives in the shallows.

Peter

I agree about steels and wetsuits, I dive that way too occasionally. However, I don't dive that way without 2 liftbags, one for my deco shoot and one in case of a bouyancy emergency.

The danger, as I see it, is in inexperienced divers reading posts from people who wear lots of weight (I've recently seen people admit to carrying 40 lbs!) and thinking that the way to easy diving is more weight rather than proper technique.

Tom

GDI
August 6th, 2002, 11:02 AM
:doctor: Well lets take a look at this problem of swimming to the surface?

First of all if you ever got yourself into that position then you most likely did something wrong in the first place. Never let one mistake lead you into another. If you ever need to make that type of ascent then I suggest that you breath some O2 following the dive for a few minutes to make sure you don't have the DCI bug. You should check your equipment before the dive to make sure the chance of failure is minimized. Should you have a problem that your BCD fails to provide the needed buoyancy Then you should consider having a lift bag or a redundant bladder in your BCD. With a PST 104 and 16lbs of weight you are packing some lbs there. All wetsuits will lose some positive lift at depth and as one kicks to return to the surface that lift will come back, that is a risk. You should not be so heavy that you cannot swim yourself back to the surface. KEEP THIS IN MIND YOUR LIFE IS NOT WORTH YOUR WEIGHTS YOU CAN CHOOSE TO LIVE OR DIE. I will let the big guy upstairs decide when my time is up, and until then I would kick as I needed to and hopefully still be in a controlled ascent, AIR IS A GOOD THING. Plan your dive, check your equipment, make repairs if required have it serviced, and then dive your plan.

Scuba
August 6th, 2002, 12:24 PM
Thank you Spectre for your further explanation. I believe I get it now. I was considering neutrality as though it occured at the beginning of the dive instead of the end. Now it all makes sense.

Thank you all for some good advise.

Peter

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