BCD's and lift...

[daco65]

First new thread so pls forgive if a bit simple!
Am just doing my open water part of Ocean Diver and have twice been doing AAS drills ( ie my buddy giving me air and providing the lift ) and they have not been able to get themselves ( or, therefore me ) clear of the surface ( to breathe and inflate my jacket 'manually' ). Whilst I understand that smaller jackets ( for smaller people ) will have less lift, this could surely lead to 2 divers bobbing around just under the surface ( or a choice to jettison one or the other! ). I'm not exactly huge or anything but it does make 'travel' BCD's etc a possible risk ( including for the owner ). I may be over reading this ( as in a real life situation one would dump weights for example ) - or is it an issue?

[Matt-75]

Small question, how much lead are you using and how much lead is he using?

Whenever i've heard of things like this, its usually because the diver(s) are using way to much lead. If you or he are having problems getting your heads above water with your BCD's fully inflated (or in the case of AAS, as near to fully inflated as possible), then i'd say one, or maybe both of you, have a problem somewhere.

The ideal amount of lead (as i remember) is enough that you can submerge, but not so much that you plummet to the bottom uncontrollably if their is very little air in the BCD (or be unable to get to the surface easily).

[David Walker]

Couple of things...

Firstly, you should be able to orally inflate your BC underwater as well as on the surface.

Secondly, as you ascent the air in your BC will expand, so even if you start neurally buoyant on the bottom, unless you dump too much air on your ascent you should be able to stay on the surface. If you are sinking, then chances are you have dumped too much air on your ascent, and therefore probably wasn't a controlled ascent.

Third point, in an AAS ascent, one of the two buddies still has air, and therefore power-inflation of their BC. They should be able to inflate their BC when they hit the surface, and hold their buddy up until they can orally inflate their own BC - don't let go until both divers are secure on the surface.

Finally, remember that if you were in an emergency situation and unable to hold your buoyancy on the surface, DUMP YOUR WEIGHTBELT! Far too many divers forget about that possibility, with sometimes fatal consequences. If you've hit the surface but can't keep your head out of the water, then it is infinitely more critical to maintain your buoyancy than it is to avoid losing a few ££ of lead blocks and webbing!

David

[Edward]

( ie my buddy giving me air and providing the lift )

This is the part I don't understand why they are doing both.

If you are using an AAS then you should be controlling your own buoyancy (Dive 003).

A CBL is for an unconscious casualty, who wouldn't be able to use an AAS (Dive 004).

So I'm not surprised there is a problem, it sounds like tasking overload.

Edward

[Nigel Hewitt]

Am just doing my open water part of Ocean Diver and have twice been doing AAS drills ( ie my buddy giving me air and providing the lift ) and they have not been able to get themselves ( or, therefore me ) clear of the surface ( to breathe and inflate my jacket 'manually' ).Sounds like there is lot too much lead involved somewhere.

Correctly weighted on a single cylinder:
You are neutrally bouyant just below the surface with the BCD/suit empty and the cylinder gaspingly empty.
Hence you start the dive with 3.3Kgs of extra weight which is the gas you carry (12/232) - this makes the initial descent nice and easy.
A jacket, even a small jacket, gives 12Kgs of lift. This can lift two divers worth of 3Kgs and get their faces out of the water for a 'blow up a jacket through the dump' trick.

If this doesn't work for you QED: your are overweighted. The laws of physics do not lie.

[daco65]

thanks all - back to the weight belt i think!

[IainC]

Good luck with your training!

As others have said, too much weight is most likely.

Other possibilities:

You might not be able to fully inflate a jacket if it is too small (it squeezes you uncomfortably before it is actually full) so you can't get all the lift it supplies.

If a jacket is too big you will dangle from it and not get full support at the surface (you can tell this because there will be a space above your shoulders before the shoulders of the jacket).

Iain C.

--awkward physics student mode--
Nigel - to keep your head comfortably out of the water (in vertical position) doesn't the jacket need to provide buoyancy for the weight of your head + your regs (i.e everything above the waterline), in addition to the few kgs of air?
--end awkward student mode--

[Nigel Hewitt]

--awkward physics student mode--
Nigel - to keep your head comfortably out of the water (in vertical position) doesn't the jacket need to provide buoyancy for the weight of your head + your regs (i.e everything above the waterline), in addition to the few kgs of air?
--end awkward student mode----awkward physics graduate mode--
Not feeling that cutting your head off to weigh it is polite I'll settle for the idea that the 3Kgs each ((12-6)/2) my example gave you will get two faces out of the water enough to splutter, swear and blow up the remaining jacket.

If you are over weighted you may not have this so correct weight is not just a good thing for diving but vital once you get into a rescue situation.
--end bloody minded professional physicist mode--

[IainC]

http://danny.oz.au/anthropology/notes/human-head-weight.html

4-4.5kgs apparently.

Even though 2 small jackets should still be enough.
You might have to fin some while blowing up the 2nd jacket though

It's Friday and I am looking up the weight of human heads, I need to go diving this weekend!

[Mike Halligan]

Other possibilities:

You might not be able to fully inflate a jacket if it is too small (it squeezes you uncomfortably before it is actually full) so you can't get all the lift it supplies.



Other possibilities -
- that the BCD once donned had its straps over-tightened and/or
- BCD pockets are over-filled

In either case, this can restrict the expansion of the BCD so that it cannot be inflated fully.

Mike

[Paul Morris]

If this doesn't work for you QED: your are overweighted. The laws of physics do not lie.
That maybe, however we have two small Cressi jackets used for teaching. I hate putting students in them. Even with a small female, no weightbelt, swimsuit only, with 12l tank, the jackets barely hold their chins above the pool when fully inflated.

I use them as a teaching aid to go and buy a jacket with a decent amount of lift! i.e. Buy the biggest size you can still fit!

Regards,
Paul

[Philip Smith]

thanks all - back to the weight belt i think!

I would agree with Edward -- the assisted ascent is not a controlled buoyant lift. Each diver should be looking after their own buoyancy. It sounds as though your would be rescuer is very negatively buoyant when you get to the surface. Check how much weight they are wearing too and whether they are "hanging" on your buoyancy during the ascent.

Phil S

[Almost_ORCA]

This is not exactly a response to the preceeding post but a kind of related subject.
I am getting ready for Open dives and while still in Pool dives I am having difficulty going vertical before descents/assents. I have a large BCD with weights, seem to be inflated well but keeping getting pushed forward by my tank to the point where I have to kick to try and stay in place, getting vertical in one position for descent is near impossible.
As my handle suggests I am clearly overweight, my divemaster has tried different BCD's, weights, weightbelts, and nothing seems to help. We will try a smaller BCD next weekend.
Anybody got any suggestions?

[Michael Purcell]

You could always switch to a wing and enjoy being in a proper horizontal position while ascending :)

Seriously though...are you getting pitched forward (beyond horizontal) while diving or is it only in trying to get vertical on ascent? Also what happens on the surface?

[Nigel Hewitt]

I am getting ready for Open dives and while still in Pool dives I am having difficulty going vertical before descents/assents. I have a large BCD with weights, seem to be inflated well but keeping getting pushed forward by my tank to the point where I have to kick to try and stay in place, getting vertical in one position for descent is near impossible.The only time I've seen this is where the student had far to much lead and all at the front of his rig.
It isn't the tank. If it was as soon as you got vertical it would flip you onto your back and, anyway, tanks don't weigh that much in the water.

The phrase 'have to kick to try and stay in place' screams 'far to much lead' at me. You never need to kick for vertical motion and it makes virtually no difference to your position in the water.

You need a proper buoyancy check. If it's a 12L or such tank at the start of the dive so full tank, inflate jacket - this must set you vertical in the water unless you have hollow legs - cross your ankles so you cannot fin, reg in your mouth, take a full breath in (not 'I'm going to free dive to 160m' just a full breath) deflate jacket. Water level should be roughly at mask level, ideally eye level. If you hit the bottom of the pool like a brick inflate your jacket, back to the surface and remove a couple of kilos and do it again. The important thing is DO NOT FIN. I have taken the fins off students at this point. Those little feet just have to flap.

[Almost_ORCA]

Thank you for your reply. I will try the crossing ankles if needed this Saturday. As I stated we are trying a smaller BCD this coming weekend. The first dive the instructors thought it was the weight belt and the fact that I have no hips so they put an empty weight belt on me and weights in the BCD for the 2nd dive (empty weight belt to prove I could remove/put back on in the water, integrated weights were also tested as being able to be removed and restored). But, when testing the self-inflation of the BCD as well as clearing/using the snorkel I could not float in a fixed position. The instructors major concern is that 'you are going to wear yourself out'. I think what I need most is to drop 20 kilos of body fat.
(which is part of my goal in learning how to dive).

[Nigel Hewitt]

I think what I need most is to drop 20 kilos of body fat. (which is part of my goal in learning how to dive).This shouldn't be too much of a problem.
Fat is about 8% buoyant in fresh water and muscle is about 5% negative so even 20Kgs of spare only needs another 1.6Kgs of lead which isn't going to throw you out too much.

I would say first get the weight right so you are neutral even if it leaves you feet up or upside down then move the weight about until you are comfortable in the water. It may be that you are a candidate for ankle weights. I really doubt that changing the jacket is going to do much good although if it is riding up on you one with a crotch strap to keep it down can compensate for having no hips/waist for it to hold on to.

I sympathise with the weight problem. I'm not heavy but it's a problem keeping it under control. When I was young I stopped eating pudding at the beginning of the racing season and it fell off by the day, now I am older I could probably fast forty days and forty nights in the wilderness and loose three pounds.

[Tony Dwyer]

I wote what follows a fair while ago. I think it's still valid.

Over Weight!

It is common for new divers or trainee divers to loaded up with too much lead.
One of the reasons this occurs is that sometimes instructors deliberately overweight students to help ensure they stay put on the bottom when learning basic skills, such as mask clearing or regulator recovery. The answer to this one is; Don’t do it. It’s bad practice. Teach correct weighting right from the start.

Another reason is that newbies often ‘hoard’ air. That is they don’t breathe properly. Ingrained in their subconcious is the belief that they must hold their breath while under water. The result is that their lungs are nearly always fully inflated and they only breathe in and out a little. For some full grown males that could easily be 3 kilos of extra buoyancy. The tendency is to load up the student to make them sink, no matter what.
In some cases there are multiple problem areas. The student is over weighted in the pool with just a swimsuit & scuba gear, then the poor soul puts on a wet suit for open water and it happens all over again.
A side effect of hoarding air is breathlessness, resulting in rapid breathing and increased discomfort. It can even promote panic.

How to get round this?

My solution is simple. It uses simple demonstrations to convince the student of the maximum amount of weight that may be needed.

1. Student in swimsuit with mask & snorkel. Tell student to float on the surface in the shallow end of the pool, with lungs full of air. Most will float, some rare males will still sink! Tell the student to breathe out most of their air. Most men will sink, as will quite a lot of women. If not repeat the exercise giving them small amounts of weight until they just sink. Strangely, it takes 2 kilos to sink my missus when she’s in her Bikini and she’s a racing whippet! Having done this one should now understand the basic buoyancy of the student.
2. Throw their wetsuit in the pool (without the student in it!). It will of course float. Get all the air out of it and add some lead until it just starts to sink. Now we know the SURFACE buoyancy of the suit.
3. Place the SCUBA unit in the pool with about 50 bar in the tank. Empty the BC. Does it sink? (It probably will). If not add a little weight until it just sinks.
4. Add up all the weight used to sink the student, the suit and the SCUBA set. It the person and or the SCUBA rig sank without adding weight, then one can subtract a little from the total.

This process finds and more importantly demonstrates to the student the MAXIMUM amount of lead they need for their configuration in fesh water. In truth they will probably need a little less.

Weight belts and Trim.

Having the weight at the front will tend to tip the wearer forward. This is a no-brainer and doesn’t need explanation.
Weight should be distributed so that the centre of gravity of the fully kitted diver is just above the waist. If this is done the diver should be able to adopt almost any position underwater and be relatively stable. Provided of course that he or she isn’t doing the hand jive or the fin flutter. The weight should be organised so that the diver is not tipped forward. Staying horizontal while finning becomes a doddle and much more comfortable than if ones feet are pointing at the bottom.

If ones waist is demonstrably larger than ones hips, then the standard webbing weight belt with a flip over buckle is not going to be any good. It will have a tendency to slip south! Especially when the wet suit is compressed at depth. A weight harness or integrated weights are probably the way forward. For one particularly large student of mine (6ft 5” and 28 stone) it was both!

Lastly, it’s a sad fact that women usually need relatively more lead than men. This is not because they are fat, it is simply because men are generally more dense.

[Woz]

I am getting ready for Open dives and while still in Pool dives I am having difficulty going vertical before descents/assents. I have a large BCD with weights, seem to be inflated well but keeping getting pushed forward by my tank to the point where I have to kick to try and stay in place, getting vertical in one position for descent is near impossible.I know what your problem is (at least I can have a blimming good guess) as I see it all over the place.

Your cylinder is set waaaaaaay too low.

Imagine you sat there vertically in the water with a fully inflated jacket. Your lead will be trying to pull you face forwards, and your cylinder backwards. Now. If the top of your cylinder pokes above the surface of the water, all that steel and valveage isn't buoyant any more and is pushing down. If it isn't, it's supported (in part) by the water (especially as the steel is hollow and full of air).

When looking around dive sites, I see cylinders set stupidly low with the camband virtually on the break of the cylinder. No wonder people fin around like seahorses. The jacket should be set so that you can tilt your head all the way back and smack your head on the 1st stage. Alternatively (this works with Buddy jackets) set them at a height where you can just get your fingers through the handle in the top of the plastic backplate.

[Tony Dwyer]

I agree with low cylinders being bad.
I'm not too sure how much effect they have on stability at the surface. I would have thought that low weight would promote stability (as in a boats leaded keel) and higher weight would promote wobblyness.

Having significant amounts of the cylinder and valves out of the water must surely effectively add to weight at the surface, thus pushing the body deaper until an equilibrium is reached. Er??

As Woz says it's all too common. :) Low cylinders that is.

[Woz]

Here you go. Rubbish cylinder position, rubbish trim:
http://ocw.mit.edu/NR/rdonlyres/2E26F177-D7B3-4934-9151-651545DDE920/0/chp_scuba.jpg

Much better. See? Even a cat can do it.

http://jasoncross.com/images/etc/scuba_cat.jpg

[Tony Dwyer]

Yup

I totally agree with the affect of low cylinders on trim underwater. Brings the C of G too low. :)

Like the moggy.

[Nigel Hewitt]

I totally agree with the affect of low cylinders on trim underwater. Brings the C of G too low.Does it?
Even my 12L dumpy steels are only a couple of Kgs negative in the water.
Moving that little about never seems to make much difference.
Your talking what 20cms of shift?

Did like the cat though...

[Tony Dwyer]

Does it?
Even my 12L dumpy steels are only a couple of Kgs negative in the water.
Moving that little about never seems to make much difference.
Your talking what 20cms of shift?

Did like the cat though...

I notice it, if my cylinder(s) is / are not in the usual place.

A full 232 bar 15 ltr has a fair bit of weight. :) and my trim is a fine thing! :)

[Woz]

Don't forget the 2 kg of brass nailed to the top. Makes a difference.

[Almost_ORCA]

Thank you all for your replies.

First; The smaller BCD worked wonders. I was not pushed around and it allowed me to float in place. More important. It allowed me the use of a weight belt instead of the integrated weights. This allowed me to put weights lower.

Second; With the smaller BCD and the weight belt I can now hover in place and can enjoy my experience in the pool (confined dives). I have one more confined dive and then I get to proceed to the Open Water Dives.

I will totally enjoy having this site to gain from your experience.