Backmount Double Stability Issues

[divingx]

Hey guys, I just started diving twinset. Before that I did most of dives using sidemount since I did a lot of my dives in caves. I read many threads on this forum about twinsets. I found a lot of people think stability as a pro for backmount double. But I found my double 'rolling' on my back, especially when I tried to swim 'sideways'. The tanks were like pulling me down. The only time could keep the tanks stable was when I was in perfect trim. But when I'm in caves, sometimes I have to swim 'sideways' and I cannot do that with doubles. I even had this problem when I was on surface.

I have tried double FX100s, which are like HP100s I think, and double LP50s. I dived these sets in cold water in Ontario. I don't need a lot of weight with double 100s but I used about 10 pounds in my v-weight with double 50s. I had problems with both sets but I felt much worse with FX100s. Maybe it's because they are much more negative than 50s. I'm 5'5 (165 cm) in height and about 127 pounds (58 kg) in weight. Perhaps I had this problem because I'm a small guy and the tanks are too heavy? I'd like too dive double in open water because of its simplicity and it's easier when I do boat dives. Could anyone give some suggestions or share some experience about how to master twinsets? Thank you.

 

[MichaelMc]

Those doubles make you roll over because they are negative, not because they are heavy on land. So they are likely putting your center of mass above your center of buoyancy (the center of the water you displaced). So gravity constantly works to roll you unless you are balanced just right and keep it there. The LP tanks tend to be modestly negative. The HP tanks tend to be rather negative, so worse.

If your gas needs are modest you can shift your center of mass forward. One way is moving any lead you have more forward on your body to the sides or front of your harness or belt. Such as with the LP50s (or say LP85s). The next solution is avoiding negative doubles. Double AL80, AL72 and AL40s are closer to positive so help on your back but also come with extra lead you must carry. Their needed ballast might be fine on the plate, or some on your sides or front to make your mass distributed evenly front to back. That is the simpler solution, that it is just a matter of keeping center of mass at or below center of buoyancy.

If you have large gas needs, the harder view is that this is just doubles, you will need to deal with balancing them, you will get so used to constantly adjusting you will no longer notice it, , just do not get off of level. If you need to dive massive HP tanks to have the gas you need, this may be your only choice.

I subscribe to the simpler solution of having gravity work slightly for me (center of mass at or just below center of buoyancy) instead of strongly against me (center of mass above center of buoyancy). I have that luxury because I do not need massive gas loads.

But my only doubles experience is double AL40s which are very easy. I started there instead of double LP50s for this very reason. Though mainly I sidemount. My tank(s) trying to roll me annoyed the hell out of me when I dove single LP85s or steel 72s, so I don't do that unless I can set my weights to prevent the roll.

Welcome to the Board!! Lots of good advice here. Sometimes there is some consensus, sometimes not.

 

[Doppler]

A reasonably common issue especially when switching from SM to BM regardless of size (within reason). I have similar issues myself since SM has become my default configuration over the past several years. Fitting tanks to the task at hand (smaller, lighter is easier to control given the dive's gas requirements) will gain you some relief of course.

But the best advice I can offer, mate, is practice.

 

[kensuf]

also the shape/design of the wing may play into this.

 

[AJ]

Having experience with both BM (double and single) and SM, I don't think you will ever reach the neutral stability with BM that SM offers. Centre of gravity is completely different. Double BM is more stable than single BM (with same weight distribution of course).

I like SM for the freedom it gives in every possible position. BM is however more relaxed when boat diving imo. Different tools for different goals, none is better or worse than the other.

 

[divezonescuba]

Definitely what everyone else has said.

I had a student who tried to do TEC45 in a set of backmounted hp100s. He had a problem turtling as well. When I had him switch to AL80s, he did fine. He was about 5’5”, but weighted 190 lbs.

I am 5’7”, 165 pounds. I normally use AL80s. But I have also used LP85s without much difference except being slightly more head heavy. No turtling problems. I am sure I would have an issue if I used hp100s.

 

[divingx]

Thank you guys for suggestions! This center of mass thing makes sense to me. So is it possible to solve this problem when I have stages cuz they are sort of hanging below me?

 

[divingx]

also the shape/design of the wing may play into this.

I used donut shape wing.

 

[Jack Hammer]

Practice breeds familiarity which leads to being more comfortable. Also be sure your waist belt is tight enough. Elsewise the tanks can move around and create instability.

 

[divingx]

Definitely what everyone else has said.

I had a student who tried to do TEC45 in a set of backmounted hp100s. He had a problem turtling as well. When I had him switch to AL80s, he did fine. He was about 5’5”, but weighted 190 lbs.

I am 5’7”, 165 pounds. I normally use AL80s. But I have also used LP85s without much difference except being slightly more head heavy. No turtling problems. I am sure I would have an issue if I used hp100s.

Thanks for the experience! I'll try the LP tanks. But here in cold water I need about 10 pounds, even more with stages. If I put the weight in v-weight porch, it will only make things worse right? Since they are on my back? Where should I put the weight so that they are evenly distributed? I don't really like weight belt but if it's the only solution then I'll try it.