Hydrodynamics - backmount vs sidemount

[Gary]

If a diver with several hundred dives starts to tell me he or she prefers BM, no problem I will listen and respect that opinion and maybe learn something out of it?

I prefer backmount - minus the manifold.

SM is one of the most adaptable. If I had to choose only one style of diving SM would probably be that choice. But there are a lot of different kinds of diving. Particularly contrasting the two extremes SM vs the BM-stages-scooter-teams which evolved for an organized group doing large-deep cave it is obvious that one type of setup can not be the "best" for all types of diving. There is even a nitch for rebreathers despite the risks and problems they generally face.

I've only got 63 dives in sidemount (39 of those in cave). There is some diving I'd like to do that requires sidemount. I expect that one of the new sidemount rigs on the market will improve things over any of the homemade SM rigs I have.

The most significant reasons for me prefering BM when I can is the ability to hang at any angle/trim I want, comfort, and less drag. I like to look around a lot including rolling over and checking out the ceiling or drifting in different positions. In SM using AL40's or even AL80's it isn't bad but with most larger volume/steel tanks in SM gravity plays preferences on position. Also it is really nice in BM having the tanks locked in place so they don't move around / change buoyancy thru the dive (except when you actually want to move/remove them as in SM).

Without a manifold tank maintenance is easier and it is actually no trouble to take bands on and off and switch between SM/BM. Other than being able to observe the tank valves in front of you and the questionable possibility of swapping regs underwater BM without manifold is really very like SM with easier buoyancy and non-floppy tanks. Larger tanks, staging etc is easier in BM.


While I am against manifolds in general those BM divers who dive exclusively from stages and use the BM tanks for reserve gas only really don't have many of the issues I have concern with. They have independent gas supply and can make an exit from the cave without air-sharing. The possibility of OOA buoyancy issues from empty BM tanks really should not be an issue for cave divers if they have properly weighted themselves anyway and the possibility of loosing both BM tanks is really not realistic - especially with the concentration on gas isolation. However, one major concern remains: Manifolds still can have problems with improperly filled tanks or even problems with the isolator knob. It is actually debatable if a non-isolable manifold may be safer overall than an isolator manifold. SM and independent tanks do not have that concern and gas mixture problems kill a lot of divers.

[RN]

With more experience it's quite easy to invert in sidemount, even with big steel tanks.

[SuPrBuGmAn]

And the no mount(NM) guys have even MORE caves to dive !

Takes like two unclips for the SM diver to become the NM diver, win.

[NEON]

I prefer backmount - minus the manifold.

SM is one of the most adaptable. If I had to choose only one style of diving SM would probably be that choice. But there are a lot of different kinds of diving. Particularly contrasting the two extremes SM vs the BM-stages-scooter-teams which evolved for an organized group doing large-deep cave it is obvious that one type of setup can not be the "best" for all types of diving. There is even a nitch for rebreathers despite the risks and problems they generally face.

I've only got 63 dives in sidemount (39 of those in cave). There is some diving I'd like to do that requires sidemount. I expect that one of the new sidemount rigs on the market will improve things over any of the homemade SM rigs I have.

The most significant reasons for me prefering BM when I can is the ability to hang at any angle/trim I want, comfort, and less drag. I like to look around a lot including rolling over and checking out the ceiling or drifting in different positions. In SM using AL40's or even AL80's it isn't bad but with most larger volume/steel tanks in SM gravity plays preferences on position. Also it is really nice in BM having the tanks locked in place so they don't move around / change buoyancy thru the dive (except when you actually want to move/remove them as in SM).

Without a manifold tank maintenance is easier and it is actually no trouble to take bands on and off and switch between SM/BM. Other than being able to observe the tank valves in front of you and the questionable possibility of swapping regs underwater BM without manifold is really very like SM with easier buoyancy and non-floppy tanks. Larger tanks, staging etc is easier in BM.


While I am against manifolds in general those BM divers who dive exclusively from stages and use the BM tanks for reserve gas only really don't have many of the issues I have concern with. They have independent gas supply and can make an exit from the cave without air-sharing. The possibility of OOA buoyancy issues from empty BM tanks really should not be an issue for cave divers if they have properly weighted themselves anyway and the possibility of loosing both BM tanks is really not realistic - especially with the concentration on gas isolation. However, one major concern remains: Manifolds still can have problems with improperly filled tanks or even problems with the isolator knob. It is actually debatable if a non-isolable manifold may be safer overall than an isolator manifold. SM and independent tanks do not have that concern and gas mixture problems kill a lot of divers.

A couple of points:

1. I would love to see you swimming graciously on your side or maintaining such a position stationary with heavy BM doubles (HP 130's for example) for any length of time.

2. How do you get to 1/2 of your gas if you tear a hose, (or puncture a second stage diaphragm or experience any other failure that renders the regulator useless) and feathering a valve is not a an option.

[phillip1]

I disagree I find it much easier to roll around in SM than BM, with a correctly set up SM rig the tanks and you are one tight unit and you can swim in any position.
I can get perfect trim and can hang in any position for as long as I need to.
But one important point to remember is you should dive whatever feels comfortable to you, as long as you are being safe, , if you feel more comfortable BM by all means dive BM.

Also I think that there have been a few accidents directly due to manifold misuse in which the victim still had on BM tank full but improperly shut down one side thought they ran out etc.. I have never heard of such a thing with independent tanks.
I know I know the whole free flow reg thing and accessing all the air in both tanks on any reg, still in SM a free flowing reg is not much of an issue at all, feathering the valve is sooooo easy to do that that alone is a major advantage I think.

Oh here is a hypothetical situation were I think the BM diver has a slight safety advantage over the SM diver.
The dive is an exploration push dive in a newly discovered cave system, Diver A is diving manifolded BM, diver B is SM, at max penetration both divers get attacked by dormant prehistoric cave monsters, diver B has a Dr Z-knife and an OMS titanium line cutter, diver A has a pair of emergency shears and a DR econo knife who do you think has an advantage?

[Gary]

1. I would love to see you swimming graciously on your side or maintaining such a position stationary with heavy BM doubles (HP 130's for example) for any length of time.

My tanks are HP120's. Next time you see a BM diver drifting by like they were lounging on thier side in a deck chair - that'd be me.

2. How do you get to 1/2 of your gas if you tear a hose, and feathering a valve is not a an option.

First thing is 1/2 your gas should be plenty (for a independently swimming diver) it is the same amount as entered the cave with.
Second - you have a buddy with yet more gas.
Third - feather the valve. (Feathering is not an option? Even with a torn hose I could take the reg off and feather the valve breathing from the hose.)
Fourth - breathe from the LP inflator
Fifth - swap regs
Sixth - cut the HP hose and breathe off that (provides enough gas for a slow swim but nothing strenuous)
Seventh - breathe from the valve - feathering the valve.

There are increasingly insane methods of getting to the gas but there should normally be no need for any of them - just turn and exit.

If you don't loose the gas you always have the option of using it - but if it leaks out it's gone.

[Gary]

The dive is an exploration push dive in a newly discovered cave system, Diver A is diving manifolded BM, diver B is SM, at max penetration both divers get attacked by dormant prehistoric cave monsters, diver B has a Dr Z-knife and an OMS titanium line cutter, diver A has a pair of emergency shears and a DR econo knife who do you think has an advantage?

I saw the movie! The BM diver with the Lithium-Hydroxide canister and the flare has an advantage over the SM diver with the z-knife...

[phillip1]

lol..


I agree with you that independent BM is safer.
My dad used to have a bunch of wreck diver friends in Montauk and they would all dive independent doubles for redundancy, and they dove the Doria etc... at the time I did not really care about diving that much but I do remeber then arguing about how much more redundant this setup was, and I agree.
I think in a real catastrophic fast gas loss situation (burst disc, tank oring failure) no matter how fast you can isolate you will lose more than half if not more of both tanks, instead of gas from just one tank.
How did this thread get to arguing BM over SM safety?
I thought it was supposed to be about a SM vs BM race.

[NEON]

I think that swapping regulators yourself on a single breath, behind your head would not be very elegant if at all possible. Feathering the valve is certainly an option - one that IMHO would be easier to do in SM.

[ops959]

i suppose that you can carry more air on BM

i do not see many videos of SMs going the distance on scooters though

i might argue the theory that BMs are tradition but they are likely more streamlined

SM harness is rad because can mount camelback on xt wing...and butt mount a canister...(jumping through hoops to mount on my doubles...30 mins more air though)