I don't get side mount?

[tbone1004]

There are lots of people that still dive independent doubles. The crossbar is the standard now, but some of the old school divers don't dive with an isolation valve either, that's dumb imho, but it has worked for them for decades.

This style of diving has been gaining traction for the last decade or so, but has been around for quite a long time. The advantage it has over indy-twins is that you can see the valves and have the ability to swap a first stage over if you really have to. You shouldn't ever have to, but the option is there. With indy doubles, you have the same hose routing as you would in manifolded twins, but the second SPG stays on the right side and I prefer to put my drysuit on the right, wing on the left. Reg routing is the same, so you are not always breathing on the second stage you will be donating which turns some off, but it really does work fine.

 

[a878bob]

My road to SM: jacket style, B/P W singles, then to side mount. I'm now working on BM doubles, and I don't get it. The first time I picked up my banded, relatively light, HP00's, I thought they would crush my spine. I figured it would be much easier in the water, nope, still bulky and heavy. And when I roll a bit, they just want to turtle me over. I have no idea if the roll is normal or not, if anyone wants to comment. But it certainly is not a problem in SM. Open water deep wreck in SM, I don't see changing to BM, but I'll keep working with it. I have a lot of time in SM and can manipulate the system easily in the case of problems. I'm hoping for the same in BM, so as to have a another tool in the bag.

 

[tbone1004]

with doubles the center of gravity has shifted forward and up, so now instead of being right around your hip bones, it is sitting on top of your scapulae. This is why diving jet fins, Hollis F1's, Turtle Fins etc work in backmount but not in sidemount, the extra weight at your feet helps to move the CoG back a bit so you don't somersault forward. Because you have now moved ~100lbs of weight from your bodies axis to behind your back, you have moved the CoG up as well and you are now essentially hanging below the unit instead of having it inline.

This is an important question for you, what sidemount unit, and what backmount wing are you using? If you are using a Nomad for sidemount and a circular wing for backmount then you are not used to the rapid gas shift as you roll, if you have a SMS100 or 75 then you are, and in the newer ones that are basically pillows on your back the gas shift isn't that nasty because the wing is narrower.
As you start the roll in backmount, you are shifting air from balanced on either side of you to all of the air on one side. This is a pretty violent shift of balance due to the width of the tanks and is much worse than in sidemount. In sidemount you also have the weight of the tanks out wider, so you are an inherently more stable platform and it is harder to initiate the roll but easier to stabilize. Think semi vs. muscle car. Real easy to tip the comparatively narrow top heavy semi over, but almost impossible to roll a muscle car because the ratio of width to height is favorable.

In twins this is made worse by the fact that the balance is upside-down. The tanks are very negative, you are very not negative, so for the system to be happy, it wants the weight at the bottom and all of the lift up top, so once you are upside-down it is quite hard to reverse it because nature wants you to be on top, especially if you're in a drysuit. You learn to counter all of this instinctively after quite a few hours in the water, but sidemount will always be a more stable platform for yaw because the weight is on the outside and is in the same plane as the body, so it is very difficult to get the platform to roll.

 

[a878bob]

Great Answer! Thanks tbone. My cold water SM is a Manta Wing and BM is new DR Classic, and its heavy in the water with a lot of air in the wing at the moment, I carry little to none in SM, so I can see the air shift making things worse. I have hope then, when I get the weight worked out it will be less of a problem and more stable. If my back will hold up that long !

 

[DaleC]

There are lots of people that still dive independent doubles. The crossbar is the standard now, but some of the old school divers don't dive with an isolation valve either, that's dumb imho, but it has worked for them for decades.

If it worked for decades why specifically is it dumb? Seeing as much of this thread is about what divers "want" to dive instead of "need" to dive I wonder why you would single that one out as dumb. I have dived all three and fail to see it myself.

 

[tbone1004]

neck o-ring or manifold o-ring failure=total loss of gas, at least with an isolation valve you can cut off the other side. Though the same is true of an isolation valve failure, and is the reason I prefer independent twins though valve failures are much less common than an o-ring deciding it has had enough. Seen too many cross bar o-ring failures to not put an isolation valve in the middle. I have a set with an iso manifold on it, but it's an older set of 72's, when my 120's go into backmount they get throw in as independents. They have the OMS valves on them, so it is a captured O-ring, the same type as the DIN O-ring on the 1st stages, so it is quite reliable.

With the manta wing you're used to all of the gas being evenly distributed right down the center of your body, so even when you roll the gas is only moving a few inches. When the gas in the classic wing swings over to the other side it is easily moving half a meter easily so the shift is much more noticeable. This is made worse if you are overweighted which by the sounds of it you are. If you are diving a SS backplate, I would highly recommend going to aluminum or the new DiveRite Lite SS backplate if you're in salt water. I use my Nomad for most doubles diving and have little problems with it. Being a horseshoe wing I can more easily control the flow of gas from side to side which makes it easier to handle the heavy tanks. I'm almost always overweight even in a drysuit sidemounting HP100's, so I have to be able to control the bubble movement to stay stable.

 

[DaleC]

So you are speaking of doubles connected by a non isolating bar, not independent doubles.

 

[tbone1004]

correct, straight bars. Mike O'Leary still dives like that. It's worked for him for decades, but for me I've seen too many manifold O-rings go to dive like that. Indy's or isolation for me

 

[Nemrod]

Thanks.

The reason I asked was this. A few years back I was in a thread about diving doubles and someone asked about rigging up doubles as separate systems instead of having the typical manifold where the two tanks were tied together. The guy was thinking of doing it that way. Along came post after post after post why that was a bad idea and dangerous and why it should never be done. Now comes along sidemount and it seems it's the only practical way of doing it and I have seen various threads why rigging up the two tanks as single entities is a better way to dive.

I would like to know the answer to this as well. Long before side mount was considered useful for anything but cave diving and then still a very limited specialty technique, many of us dived independent doubles and I still do. To even talk of this a few years ago would result in the mightiest of hand wringing, and now, side mount is exactly that.

N

 

[decompression]

Agreed, before SM went mainstream, diving ID was tech diving. Eventually, we started using an isolatable manifold on the doubles which solved many issues and made it more accessible. I have mixed feelings about ID but now that Sidemount has gone rec it's a skillset that has found a revival. Of course, now there is a manifold for sidemount.


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