Backmount Double Stability Issues

[MichaelMc]

if you do a lot of work to get the balance between ballast on a belt in front of you equal to that on your back, there will be a small window during the dive where you can roll on your side and the tanks will be comparably negatively buoyant to your body. Practically though it's not going to happen.

I agree, rolled on the side perfectly trimmed on weight has a very small window during the dive. You've got some gas stuck off to the side, so only one pressure makes it the right amount of gas.

If you chose to dive double AL80's in a drysuit with a lightweight plate and had a ton of lead on a vest or belt, then you would have all of the ballast on your front and when you tried to roll on your side, the lead would just pull you face down again.

(added bolding is mine)

This though is my main point. For a range of modest size doubles, you can make yourself belly heavy instead of back heavy. Thus self righting instead of self turtling. Thus STABLE by most of our definitions. Very belly heavy is very annoying as you can not easily roll to the side. But modestly belly heavy makes modest doubles easy. But only up to the limit of those modest gas loads. Double 80s seems a decently useful gas load.

I'm sure guys like @kensuf can back me up, but after you spend enough hours in doubles, even double 104's while cave diving with light undergarments, your body learns what it needs to look like to be able to stay on your side for extended periods of time. It's not something I like to make a habit of, but it's not something I'd consider a problem either.

I believe this. In a single I've made myself way too belly heavy yet could still manage to swim on my side.

 

[tbone1004]

@MichaelMc that logic is sound, but it's the practicality of it. One of the things to emphasize is that there is a LOT more mass on your back and when you roll. In scuba there should be no motion that is erratic and all motion should be deliberate. When diving doubles it is especially critical because inertia is not on your side and if you roll too fast, you won't be able to stop it. If you roll at an appropriate speed then you will find that you can arrest the roll before you turtle over. Lots of practice, same with the ability to maintain exact attitude despite the natural tendency of doubles to tip you head down, even in a drysuit with neutral fins *though admittedly heavy fins are much easier*

 

[MichaelMc]

@tbone1004, I get what you are saying about slow rolls so it does not get away from you, with back heavy doubles. And learning to do that and all the other deliberate moves to keep negative doubles well behaved above you is likely a challenging bit of what you have to learn to dive heavy doubles to get the gas they give. I do not disagree with any of that. But you are talking about momentum + gravity getting away from you.

But if all you want is doubles off a boat, as that is easier than sidemount, with say 160 cu ft of gas, you do not need back heavy doubles. The situation is then reversed.

If you are belly heavy and wanted to dive inverted facing the surface you would need to not roll quickly less the mass of your roll and your center of mass being to the side of your center of buoyancy got away from you. So you need to make deliberate motions. Otherwise you will reverse-turtle. This is the mirror situation of trying to stay face down with heavy doubles. This is your momentum + gravity getting away from you.

But if you are belly heavy and want to be face down, then the momentum of any quick turn to the side is counteracted by your center of mass trying to stay down, so momentum away from face down is fought by gravity not helped by it. Depending on how belly negative you are, you now need deliberate and strong action to not be face down. And strong momentum or twisting to get all the way into the rotation/turn that you wanted.

A less excessive belly heavy weighting obviously makes more fluid diving. Being slightly belly heavy you need no constant unconscious corrections to stay level. In fact, you need to not make unconscious motions as they just upset your natural slight stability. If you are very belly heavy, you can make all the small flailing you want and you will stay belly down, but that is not a very fluid way to dive.

Now, none of this belly heavy diving, ie., stable because of gravity, helps prepare you to balance very negative doubles. But you do not need them to dive modest doubles off a boat.

 

[tbone1004]

@MichaelMc all true, but then you do need to wear a belt or harness with enough lead to keep you stable. The question circles back to whether or not we can apply @cerich 's famous of quote of "this is an equipment solution to a skills problem". A set of double 80's with a weight belt is going to be heavier on the surface than a set of double HP100's and will provide about 75% of the usable gas. That is inefficient and if the only reason for doing so is to shift the center of buoyancy to fix a problem that is not difficult to overcome with time in the water, then I think the quote applies.

 

[MichaelMc]

@tbone1004, I agree it is an equipment setup to avoid a skill deficiency. Some places that might be appropriate:
- new to doubles, and want them as fluid as something else, maybe later you will move on to heavy ones.
- diving 80s, or LP85s, instead of pick your favorite massive gas cylinder as they are easier, as mentioned up thread by apparently experienced double divers.
- diving doubles for some redundancy, no intent of heavy ones, figure rebreather makes more sense at that point (of which there are chest and sidemount versions).
- diving air, nitrox, helitrox, instead of full trimix as they are easier, though the DCS risk is higher.
- diving wet instead of dry.
...

ETA:
It is also an equipment setup to avoid constant small corrections just to stay in your dominant orientation. To rest in the water as if you were on the couch.

 

[tbone1004]

@MichaelMc I'd argue that the first point is basically cheating, but more importantly is just bandaiding the problem.
If you need to dive 80's for whatever reason then I'd argue that v-weights are easier to deal with anyway, but that would be a personal preference thing.

I use HP120's because they're long and I don't see a point in LP tanks if you're going to add weight. I use LP tanks for sidemount because of their buoyancy characteristics, but that's special use case. I have the LP121's because 149's are expensive and rare as hens teeth and the 121's were free :-D

Those reasons are all outside of what I understood to be your argument of actively seeking out AL80's only as a bandaid in order to avoid the skill deficiency.

On the constant correction, as I said above, once you have enough hours in them those corrections come subconsciously. No different than using your breath to control your buoyancy or all of the minute corrections while walking or riding a bike.

 

[MichaelMc]

@tbone1004 Big doubles are not a worry of mine. I'm happy enough with sidemount and diving silent with less or similar weight would likely be too attractive as a next step.

I was not comparing your tanks, but more a description that AL80 and LP85 are similar in ease but HP100 would be harder, from one who teaches. So there is a range of what is easier in tanks. Which mirrors their negativity. Sharing that with new doubles divers seems important. That they might phase in from easier to harder.

I read descriptions from divers new to simple sizes of doubles, seemingly on their own, having a big reaction to the instability. And them being told there is no way around that, you have to get used to it, use steels for the negativity plus lead on the plate. Which gives them no notion of the easier (but still negative) to harder tank selection range. My thought is that may cause some to just ditch that idea. When a more phased intro may let them expand their diving.

Yes, if your intent is heavy doubles and stages soon for the gas, then mastering the balancing act is central from the start. If it is not that or you are approaching doubles on your own and do not grasp front/back weight issues then being told put it on the back and balance it is the only way is maybe not best.

In my post above, I struck out the first point as my intent was showing other examples. But I think it is a valid reason as above. We phase people into things in many contexts, giving them what they need at each point and deferring other complexity.

On other reasons for using a less unstable (or even stable) setup, there is still a difference between standing in a meadow and sitting against a tree in one. Or having a few more easy degrees of freedom in your orientation, if you do not need max gas capacity.

Anyway. I'm made my point that decent size doubles need not be unstable. Thanks for the responses. My point was never that that is the way to approach an extensive doubles skill ladder, after hours of balancing practice, if that is a diver's plan.

 

[tbone1004]

@MichaelMc I stand that the selection of tanks is based on a combination of your gas needs balanced with your height. The LP85's and FX100's from Faber are basically identical in terms of dimensions with the difference being 3lbs of buoyancy. If you are filling to working pressure there is no reason to ever buy LP tanks as they are horrifically inefficient from a size:weight perspective. The 6lbs of total buoyancy on those doubles does not outweigh the negative aspects of lack of gas. That is definitely a compromise based solely on lack of desire to hone the skill.

 

[Marie13]

Odd. One issue I’ve never had with my HP80 doubles is rolling.

 

[MichaelMc]

@MichaelMc I stand that the selection of tanks is based on a combination of your gas needs balanced with your height. The LP85's and FX100's from Faber are basically identical in terms of dimensions with the difference being 3lbs of buoyancy. If you are filling to working pressure there is no reason to ever buy LP tanks as they are horrifically inefficient from a size:weight perspective. The 6lbs of total buoyancy on those doubles does not outweigh the negative aspects of lack of gas. That is definitely a compromise based solely on lack of desire to hone the skill.

Joe or Jane diver:
I really like the pretty fish. I've got a great dive op that will give me anything I need on the boat. I'm even really strong. I like how smooth I am in the water. I've got my buoyancy and trim great with my AL80 we used in training from the start. I had great instructors that taught me neutral buoyancy from the start, trim and all in a BP/W. But I really just hose down the gas, I'm a big guy or girl. I usually run short a bit before every one else. None of the big HP single tanks are enough and they do not dive like my 80s, I'm not sure why. My buddy says I could use doubles. That sounds cool. Then I could stay down as long as everyone else, I strapped on some great tech doubles from an elite tech diver. But they just turtled me instantly. I only really dive these day on vacation in Hawaii with a modest suit with my great dive op. Is there no hope Obi wan?

Are there doubles that would not turtle me (so much) without extensive training.
Please Obi wan, you are our only hope. Guide us.

Have I sufficiently made my point? We may not need max gas. Though we might need cooler water.