PADI buoyancy exercise troubles after switching rigs

[selytch]

I used to dive dry, in cold water, having ~10# of lead in addition to 5# BP to keep neutral.

Now with I'm in a tropical 3mm wetsuit, doing rescue refresher in freshwater.

Performing PADI exercises is unusually hard:

To start, trying to hover I am tilting backwards - the backplate is too heavy and its weight not opposed by weight on the front or inflated wing.

Gear exchange was very hard to do as well - taking the rig off makes me positive - and I know there are ways of keeping the rig close to you, but it is still not optimal.

Now, one solution I see is adding more weight to the belt than necessary. This way I will be able to inflate the wing and compensate for backward tilt. Also, having more weight on the waist will keep me grounded during gear exchange. But then such a setup (being overweight) is not good for real-life bouyancy...

Another option - getting a lighter backplate and keeping all the weight on the belt - but again, with the SS backplate I wouldn't need weight belt at all with tropical diving. And real-life horizontal (but not vertical) hovering is easy with the BP.

So I'm kinda torn between options:
1. getting a AL/plastic BP instead of SS one, and putting weights on the belt
2. Using extra weight just for PADI exercises

I appreciate any advice on the problem!

 

[selytch]

Well...
Looked at my own lift calculator from http://www.scubaboard.com/forums/bu...ems/158370-ultimate-wing-lift-calculator.html

I guess I should have used it before...
First, with the current setup I still need ~5# of lead in addition to the backplate - which will hold me great on the bottom and also counterweight the BP for vertical hovering.
Second,using a lighter plate will only aggravate the issue - I will need more weight on the belt to balance the suit AND the AL80 tank - which is quite positive at the end of the dive.
Third, I cannot see a way of NOT being significantly negative in the beginning of the dive with AL80 tank, which kinda makes the predive routing of neutral buoyancy check inadequate....

Please correct me if I'm mistaken.

 

[don Francisco]

As you know, weight distribution is critical in oprimizing trim. As a cold water diver you had lots of weight to work with and could distribute it pretty much as you needed to manage both pitch and roll.

The only way you'll get that movable weight back on warm water dives is with a lighter backplate offsetting the reduced weight with lead. Putting the lead in front will manage the roll, and if moving it down a few inches on your body creates a pitch problem, which I doubt it will, you can adjust your tank height or spread the weights differently, or both.

Whether you use a steel tank or an aluminum one doesn't effect the weight change during the dive. The weight of either tank is a constant. The weight loss is due only to the removal of the air inside, and breathing a 3000psi 80cft tank down to 500psi is functionally equivelant to dropping a theoretical 5# lead weight off the center of the tank, regardless of the material or shape of the tank.

 

[S. starfish]

When you are trying to hover are you trying to do so in the buhdda position? If you have two options to sort out the hovering issue. 1) Hunch your back a bit to try and get the weight more centered or 2) quite that and hover like a normal person.

For the remove and replace gear I would just add some weight, no point in spending a whole bunch of money just to complete a PADI exercise that's not that important anyway.

 

[-hh]

As you know, weight distribution is critical in oprimizing trim. As a cold water diver you had lots of weight to work with and could distribute it pretty much as you needed to manage both pitch and roll.

The only way you'll get that movable weight back on warm water dives is with a lighter backplate offsetting the reduced weight lead. Putting the lead in front will manage the roll, and if moving it down a few inches on your body creates a pitch problem, which I doubt it will, you can adjust your tank height or spread the weights differently, or both.

Agreed. This is a generic problem that tends to show up more in warmwater, because as you point out, there's less ballast to work with.

In actuality its not the ballast alone, but also the air added to the BC during the dive, and coldwater generally means more thermal protection to get squeezed, which means that there's generally more air in the BC's wings.

This 'extra air' is located in the Wing bladder, which is right next to the otherwise "top heavy" configuration of a steel tank + steel BP, which makes it less noticable in coldwater conditions.

But when you now go to warmwater and much thinner suits, you end up dropping both lead and that compensating air in your BC, so this localized neutralization of the tank/BP "top heavy" condition doesn't happen ("as much").
Shazam! Trim problems crop up.

Whether you use a steel tank or an aluminum one doesn't effect the weight change during the dive. The weight of either tank is a constant. The weight loss is due only to the removal of the air inside, and breathing a 3000psi 80cft tank down to 500psi is functionally equivelant to dropping a theoretical 5# lead weight off the center of the tank, regardless of the material or shape of the tank.

True, the weight change is constant, but the respective differences in tank buoyancy does contribute to changing the overall trim configurations.

For the "top heavy" trim problem produced by a steel backplate & tank, changing to an AL80 will make this trim issue less bad while swimming horizontally during the dive, but it isn't a free lunch: this change also makes the post-dive vertical float trim situation worse, because with all that bouyancy now on the diver's back (wing + floating AL80 tank), the system has an overturning moment ("torque") and it becomes a "face dunker".

So the question remains - what to do about all of this?

In general, you don't want to simply add more ballast. What's more needed is to shift the existing ballast from the rig to you (hello weightbelt), and minimize buoyancy differences, which the OP has already noticed as present when he ditched his gear.

The lowest impact change here would be to replace the steel BP with a lightweight one, and add that back as lead on a weightbelt. Possibly also an ankle weight around the tank's yoke too (for fore/aft trim).

The other factor is the post-dive floating trim problem, since AL80's are pretty much a given at tropical destinations. In general, the only practical alternative is to change your BC's bladder to a design that is more able to allow its air bubble to move to a more "forward" bladder chamber on the diver when he's in the vertical orientation. Unfortunately, wing designs can't do this, so you're looking at going to a Jacket style BCD.

The relative importance to the question of how 'drownproof' do you want your equipment configuration to be for the post-dive float portion will have to how you assess your risks...the reliability of the local dive boats to not leave divers floating, if you're boat diving vs shore diving, etc. YMMV to what degree this is a required capability for the particular dive environment, but it can't be argued to never being a requirement for anyone. FWIW, I personally didn't really appreciate this all that much until I helped prevent a surfaced diver from literally drowning on a 30 minute float because their post-dive floating trim was a mess, and they fatigued out.


-hh

 

[LowVizWiz]

When you are trying to hover are you trying to do so in the buhdda position? If you have two options to sort out the hovering issue. 1) Hunch your back a bit to try and get the weight more centered or 2) quite that and hover like a normal person.

I agree with you completely, who ever dives like a genie? I know it's supposed to show how balanced you are, but has no real life application unless you're an actual underwater genie I suppose! I've even seen people do it upside down, just float like you normally would float.

 

[TSandM]

Well, as has already been said, I wouldn't worry about having trouble hovering vertically. The only requirement is to show that you can HOVER. If you can do it horizontally, that is completely acceptable.

Second, your weighting should be such that you are about five pounds NEGATIVE at the beginning of the dive. That is the swing weight, or weight of the gas you are willing to consume during the dive. If you are neutral at the surface with full tanks, you are underweighted.

In warm water, with a 3 mil suit, I use my SS backplate with 2 lbs on the cambands, and an Al80. That setup trims out perfectly for me, with my Jet fins and spring straps (which act the same as small ankle weights). If I ever had to remove my rig, I WOULD have the positive diver/negative rig issue. I haven't felt the risk of needing to do that was high enough to change my configuration because of it.