Backmount Double Stability Issues

[MichaelMc]

@tbone1004 a bit a go in a separate thread you relayed a story about a state educational exam that still tested something that had no applicability in current practice in that field. (Or maybe that was BoulderJohn.) Is it possible that balancing negative doubles has no applicability to both:
1) some entry users of doubles, and
2) some paths to advanced diving to huge depths or penetrations of caves.

ETA: It was BoulderJohn, and it was still applicable but not taught as other things were instead.
General thoughts on the dive physics questions

 

[kensuf]

I used donut shape wing.

But not all donut wings are equal.

I'll use Halcyon as an example, I have both a 40# wing and a 60# wing. The 60# wing is perfect for heavy tanks (104s) by being wide enough that it can wrap around the sides of a set of 104s (see photo). This allows the cylinders to be cradled in the center, keeping the weight in the middle of the system.

The 40# Halcyon is not as wide, if I tried to use it with my 104s, the doubles would basically sit on top of the air cell. This would make heavy cylinders relatively unstable because you're trying to balance the weight of the doubles on top of the air cell itself.

OTOH, if I used that 60# wing on top of a set of AL80s, they would completely envelope the cylinders and trap gas in the outer edge. But the 40# wing fits them perfectly like a glove.

IMHO, match the wing to the size cylinders you're using. For heavy/big cylinders, I've found the 60# Halcyon and the Dive Rite Classic and Classic XT wings work really well. For smaller cylinders, the Halcyon 40, Light Monkey Wing, and DR Rec Wing work pretty well.

 

[tbone1004]

@MichaelMc ahh, but your argument is interesting because boats won't let you take a pair of doubles on a recreational boat because you're taking up the bench space for 2 divers, so the argument for being an air hog and a single 130/149 not being big enough but double 80's are.
I don't think doubles take extensive training, they just take a few hours in the pool or some other benign location and you get the hang of it.
Incidentally, the big doubles are easier to deal with in modest suits than they are in a thick drysuit because the drysuit is what makes you want to roll over far more than the rig trying to roll you over. Diving double 104's in a bathing suit is a cake walk. Diving double 80's in ice diving exposure protection would be much less stable.

 

[PfcAJ]

If you’re sideways, heavy doubles are gunna make you roll. It’s just the way it is.

The diver floats, the tanks sink.

The closer your tanks are to neutral (like al80s, or the closer your tanks are to your centerline (like Sidemount), the less pronounced this effect.

 

[EireDiver606]

What @PfcAJ said is a lot better than essays others have written here...

Also, what European equivalent are 104s? Here bigger than a D12, we have D15s, D18s and the insane D20l.

 

[grantctobin]

What @PfcAJ said is a lot better than essays others have written here...

Also, what European equivalent are 104s? Here bigger than a D12, we have D15s, D18s and the insane D20l.

They're roughly 182bar 16.2L but generally filled to ~250bar for ~4000L of gas.

 

[rjack321]

Why are you swimming on your side in OW? Just because? or is this a hallway in a wreck or something?

 

[divingx]

Incidentally, the big doubles are easier to deal with in modest suits than they are in a thick drysuit because the drysuit is what makes you want to roll over far more than the rig trying to roll you over.

Why is the drysuit want to roll me over? I didn't feel it at all.

 

[divingx]

Why are you swimming on your side in OW? Just because? or is this a hallway in a wreck or something?

I'm practicing in OW but I might take double to wrecks or caves. Before I only did sidemount but sometimes sidemount is not really boat friendly...

 

[divingx]

But not all donut wings are equal.

I'll use Halcyon as an example, I have both a 40# wing and a 60# wing. The 60# wing is perfect for heavy tanks (104s) by being wide enough that it can wrap around the sides of a set of 104s (see photo). This allows the cylinders to be cradled in the center, keeping the weight in the middle of the system.

View attachment 591551

The 40# Halcyon is not as wide, if I tried to use it with my 104s, the doubles would basically sit on top of the air cell. This would make heavy cylinders relatively unstable because you're trying to balance the weight of the doubles on top of the air cell itself.

OTOH, if I used that 60# wing on top of a set of AL80s, they would completely envelope the cylinders and trap gas in the outer edge. But the 40# wing fits them perfectly like a glove.

IMHO, match the wing to the size cylinders you're using. For heavy/big cylinders, I've found the 60# Halcyon and the Dive Rite Classic and Classic XT wings work really well. For smaller cylinders, the Halcyon 40, Light Monkey Wing, and DR Rec Wing work pretty well.

I'll try some different wings to see if it make things better