Sidemount VS Backmount

[Bob Cree]

Now we are getting a bit of balance to the discussion...

I want to start with Forrest and Sludge's point - paraphrased - tank side failures rarely, if ever occur to properly maintained equipment. Due to several of my dive buddies diving a different configuration (BM w/o crossover isolation and SM), I did a fairly involved configuration probability analysis. I used feedback from all my dive buddies to determine the statistics. What I found was that the probability of tank side failures has the highest degree of sensitivity to the result of which configuration was better. I ended up using a probability of tank side failures of 1:100. Just to give you a feel, the analysis took into consideration the number of failure points of each type, valve accessibility and shutdown time (SM is more accessible and faster), mixing issues (I will get to this), rate of loss as a fn of pressure (SM gas loss drops off quicker due to 1/2 the tank volume - until central crossover isolator is shut), even took into consideration the order that valves are closed.

Before I give the results - since it was brought up by several here - the crossover isolator...BTW, I dive with one... What we are talking about are things that have the distinct potential to kill you - so Forrest, I do take the even the small probability as still being possible. I also dive with a buddy or buddy bottle to take care of that by agreed upon very small possibility. The BIG advantage of the crossover (with or without valve) that really shows up in the analysis is total availability of all gas to a still working regulator...cannot do that SM. The big disadvanage of the crossover valve happens before you dive - mix issues - and people have died because of this! If it stays open until really needed - it is an in water advantage. The point of not having one is that you cannot mess up you mix - there are good divers that would still be here today if they did not have the valve! Admittedly it seems counter-intuitive to not have additional in water protection - but it is like using an inner tube for a condom - you loose alot in the process. Also mentioned was head movement...a straight bar gives you the same thing BM...or I like Sludge's inverted valve approach in this regard, although it migh be more confusing and thus deadly on the mix side (unless you don't touch it).

The result...and the judge was "how much usable gas remains for exit" - if it is in the tank but cannot or is very difficult to access - it does not count or counts, but with some of it being lost along the way (another sensitivity)...BM with straight bar was best, SM and BM with isolating bar were tied for second and Independent BM was worst (access to remaining isolated gas is difficult compared to SM due to location). Also, interestingly enough, what showed up was go for the isolation at the post FIRST - shutting the crossover isolation first just wastes time since the problem almost all the time is reg side...what Sludge said!

...dare I...ok, back to the streamlining and drag thing. There is a BIG if there - have a look at most SM'rs and the bottles are not tucked behind their armpits. The bottles show a full 8" diameter face under the armpit. It is hard to see that on yourself - a bit like watching a sports video of yourself - you see plenty wrong with what you did, even when you thought you were near perfect. But - let's take Forrests earlier point that he has been able to tweak things over time and get it to where it is all in the trim - he has enough time and experience to have perfected his trim - most of us mere mortals have not (same point as on the training video). Try an exercise - park yourself off to the side and watch all the divers go by - observe how truly horizontal they are. It is a VERY rare few who will be within 10 degrees of horizontal, most are 15 to 20. Now we all know that you can really notice the drag of a stage bottle if placed in the flow. Let's do some simple math, SM versus BM. For sake of discussion, let us assume the two are equivalent as long as you have near perfect trim - what Forrest said. Now - less than perfect trim, how much does 10 degrees of angle add to the surface area (drag). In both cases the diver themself adds the same amount. In BM the tanks are in the slipstream of the diver and add nothing extra. In SM, the tanks are in the flow and add drag. How much? Each tank is about 8" X 36" and at 10 degrees you add 17.4% of this area to act as drag (17.4% is the sin(10 degrees) for those who care)...that is 50 square inches times 2 botttles = 100 square inches. The added area of one stage bottle also adds 50 square inches of drag surface - so being out of trim by only 10 degrees means the SM diver is effectively thrown two stage bottles on compared to the same diver in BM. For 20 degrees you add 4 stage bottles over your BM'd cousin. Either way you cut it, unless you develop trim as good as Forrest, BM wins over SM for streamlining and drag.

BTW - BM does not have to have gauges you can monitor easily. IT stays tucked nicely out of the way as opposed to SM that don't. Plus BM has fewer failure points.

One last thing - and I know this is going long - but it is good discussion - Solo SM. Oh no!, here comes the legions...lol. I know that the last bastion of why SM is greatest relates to solo diving. You have two totally independent sources of gas. In BM, not totally independent and therefore most carry a buddy bottle. Again, I see this as solving (almost - I give Forrest the point that it can happen) a non-problem and creating a problem at the same time. For the much more likely event of a Reg side gas loss issue - the BM is better. But I said specific to solo - I have to agree it is reassuring to have independence in the gas supply - so - why do several SMr's feel that a buddy bottle is not required? Maybe I am wrong and a vast majority always carry a buddy bottle as wel - then I agree SM wins - but if not, you really are more unsafe than a solo BM with a buddy bottle. Would you dive with only 2 lights? Then why dive with only 2 regs? I think we are all fixated on the gas loss side, when the real issue should be triple total gas system redundancy, and SM solo with no buddy bottle is only double redundant. I would rather lose all my lights than all my regs.

[stairman]

Oh brother! This always happens on these SM VS. BM debates. Seen it before, itll come around again.

[phillip1]

In my humble opinion if you look at a BM configuration and a SM configuration from the front, the surface area is about the same, what the SM will have is increased horizontal drag and BM vertical.
Having dove both extensively I really don't feel any difference, having said all there is to say, I still think that SM is better for cave diving because there really are some places you physically cannot get into BM.
Plus logistically doubles is a nightmare, if you need to walk down dry caves, sinks etc..
But really it boils down to whatever one feels comfortable with, but don't ask me to help you carry your doubles,lol.

[SuPrBuGmAn]

I also dive with a buddy or buddy bottle to take care of that by agreed upon very small possibility.

Sidemount divers have their buddy bottles built in...

The BIG advantage of the crossover (with or without valve) that really shows up in the analysis is total availability of all gas to a still working regulator...cannot do that SM.

Granted, more gas is always better. However, if you gas plan the way you should. Losing an entire tank with the SM configuration will still get you home safely.

Gas loss at the tank, while unlikely, happens and the tanks empty quite fast. In BM, you better hope you close the correct post first and do it fast. I understand you are carrying a buddy bottle, in a sidemount or independant BM configuration it wouldn't be necessary.

The result...and the judge was "how much usable gas remains for exit" - if it is in the tank but cannot or is very difficult to access - it does not count or counts, but with some of it being lost along the way (another sensitivity)...BM with straight bar was best, SM and BM with isolating bar were tied for second and Independent BM was worst (access to remaining isolated gas is difficult compared to SM due to location).

BM with any type of crossbar has the potential of losing more than half your total gas with a catastrophic loss from a tank, hence your need for a buddy bottle.

Diving SM with proper gas management allows you potential of losing a tank completely and you can still make it out.

Failures at the regulator side are easier to overcome(in SM) and manipulate and there is no chance on losing more gas than necessary to exit without multiple failures. This is without an additional buddy bottle.

...dare I...ok, back to the streamlining and drag thing. There is a BIG if there - have a look at most SM'rs and the bottles are not tucked behind their armpits. The bottles show a full 8" diameter face under the armpit.

I don't find SM to be any more or less streamlined. I do have to disagree that most SM'ers have their tanks showing a full 8" under their arms. Most SM tanks are showing some tank from below their shoulder's slipstream, but not the entire tank. You can spit all the math out you want, SM tanks aren't hanging like stages, backmount tanks don't fall within your slipstream unless you are severely out of trim, and it gets even worse if you're diving with a buddy bottle for that independant scenario. In SM, we can swap a regulator out if we HAVE to.

BTW - BM does not have to have gauges you can monitor easily. IT stays tucked nicely out of the way as opposed to SM that don't. Plus BM has fewer failure points.

Some SM divers tuck their gauges out of the way, some have them right out in front of them to monitor.

BM with any type of crossover has the same failure points as SM, with the additional Orings and joints associated with the crossover(and isolator) if used. The only additional failure point on SM is one additional guage, compared to all the additional crossbar hardware. Not only that, we have easy access to being able to identify, manipulate, and hopefully any problem from the regulator standpoint.





The reason I went sidemount? Its much easier to dive out of a small boat along the rivers here. It allows me access to far more caves. I find it safer. I don't find it anymore or less streamlined in the water - this is in practice, not postulation. Longer walks will kill your back, especially over rougher terrain with BM. There are NO caves I can't dive in SM that I COULD dive in BM, the same is not true if I were diving BM, comparing to SM.

Reasons I miss Backmount? I'd rather a short walk with two tanks on my back than two tanks in my arms(or two trips with one tank each)...

Still, nobody is waiting on me diving with a SM setup. If they are, they shouldn't be in such a hurry :P

[Spd 135]

Now we are getting a bit of balance to the discussion...

I want to start with Forrest and Sludge's point - paraphrased - tank side failures rarely, if ever occur to properly maintained equipment. Due to several of my dive buddies diving a different configuration (BM w/o crossover isolation and SM), I did a fairly involved configuration probability analysis. I used feedback from all my dive buddies to determine the statistics. What I found was that the probability of tank side failures has the highest degree of sensitivity to the result of which configuration was better. I ended up using a probability of tank side failures of 1:100. Just to give you a feel, the analysis took into consideration the number of failure points of each type, valve accessibility and shutdown time (SM is more accessible and faster), mixing issues (I will get to this), rate of loss as a fn of pressure (SM gas loss drops off quicker due to 1/2 the tank volume - until central crossover isolator is shut), even took into consideration the order that valves are closed.

Before I give the results - since it was brought up by several here - the crossover isolator...BTW, I dive with one... What we are talking about are things that have the distinct potential to kill you - so Forrest, I do take the even the small probability as still being possible. I also dive with a buddy or buddy bottle to take care of that by agreed upon very small possibility. The BIG advantage of the crossover (with or without valve) that really shows up in the analysis is total availability of all gas to a still working regulator...cannot do that SM. The big disadvanage of the crossover valve happens before you dive - mix issues - and people have died because of this! If it stays open until really needed - it is an in water advantage. The point of not having one is that you cannot mess up you mix - there are good divers that would still be here today if they did not have the valve! Admittedly it seems counter-intuitive to not have additional in water protection - but it is like using an inner tube for a condom - you loose alot in the process. Also mentioned was head movement...a straight bar gives you the same thing BM...or I like Sludge's inverted valve approach in this regard, although it migh be more confusing and thus deadly on the mix side (unless you don't touch it).

The result...and the judge was "how much usable gas remains for exit" - if it is in the tank but cannot or is very difficult to access - it does not count or counts, but with some of it being lost along the way (another sensitivity)...BM with straight bar was best, SM and BM with isolating bar were tied for second and Independent BM was worst (access to remaining isolated gas is difficult compared to SM due to location). Also, interestingly enough, what showed up was go for the isolation at the post FIRST - shutting the crossover isolation first just wastes time since the problem almost all the time is reg side...what Sludge said!

...dare I...ok, back to the streamlining and drag thing. There is a BIG if there - have a look at most SM'rs and the bottles are not tucked behind their armpits. The bottles show a full 8" diameter face under the armpit. It is hard to see that on yourself - a bit like watching a sports video of yourself - you see plenty wrong with what you did, even when you thought you were near perfect. But - let's take Forrests earlier point that he has been able to tweak things over time and get it to where it is all in the trim - he has enough time and experience to have perfected his trim - most of us mere mortals have not (same point as on the training video). Try an exercise - park yourself off to the side and watch all the divers go by - observe how truly horizontal they are. It is a VERY rare few who will be within 10 degrees of horizontal, most are 15 to 20. Now we all know that you can really notice the drag of a stage bottle if placed in the flow. Let's do some simple math, SM versus BM. For sake of discussion, let us assume the two are equivalent as long as you have near perfect trim - what Forrest said. Now - less than perfect trim, how much does 10 degrees of angle add to the surface area (drag). In both cases the diver themself adds the same amount. In BM the tanks are in the slipstream of the diver and add nothing extra. In SM, the tanks are in the flow and add drag. How much? Each tank is about 8" X 36" and at 10 degrees you add 17.4% of this area to act as drag (17.4% is the sin(10 degrees) for those who care)...that is 50 square inches times 2 botttles = 100 square inches. The added area of one stage bottle also adds 50 square inches of drag surface - so being out of trim by only 10 degrees means the SM diver is effectively thrown two stage bottles on compared to the same diver in BM. For 20 degrees you add 4 stage bottles over your BM'd cousin. Either way you cut it, unless you develop trim as good as Forrest, BM wins over SM for streamlining and drag.

BTW - BM does not have to have gauges you can monitor easily. IT stays tucked nicely out of the way as opposed to SM that don't. Plus BM has fewer failure points.

One last thing - and I know this is going long - but it is good discussion - Solo SM. Oh no!, here comes the legions...lol. I know that the last bastion of why SM is greatest relates to solo diving. You have two totally independent sources of gas. In BM, not totally independent and therefore most carry a buddy bottle. Again, I see this as solving (almost - I give Forrest the point that it can happen) a non-problem and creating a problem at the same time. For the much more likely event of a Reg side gas loss issue - the BM is better. But I said specific to solo - I have to agree it is reassuring to have independence in the gas supply - so - why do several SMr's feel that a buddy bottle is not required? Maybe I am wrong and a vast majority always carry a buddy bottle as wel - then I agree SM wins - but if not, you really are more unsafe than a solo BM with a buddy bottle. Would you dive with only 2 lights? Then why dive with only 2 regs? I think we are all fixated on the gas loss side, when the real issue should be triple total gas system redundancy, and SM solo with no buddy bottle is only double redundant. I would rather lose all my lights than all my regs.

Do you sidemount as well as backmount? There are a lot of assumptions based on square inches of drag, possible entanglements and the "perfect everything" that does not exist in reality. In a high flow "pull and glide", or scooter cave lessening my drag doesn't really make much difference with high flow. Having said that If I am totally sloppy, yes it does.

If my spg's are hanging down a bit, or poking out, it is of no issue if there is nothing to become entangled on or if it does not drag the bottom. I only have to know that when it gets small, I need to manage my gear by holding it or moving it so as not to become entangled. This seems to be talking about a setup that is not "pretty".

"Pretty" does not matter nearly as much as having the right "kit" for the dive. As far as getting your gas when a reg fails, regs can be changed on sidemount with little effort in which case the dive has been turned and my only goal is to exit. There are caves that you cannot carry a "buddy bottle" due to its size. Two bottles are enough and the plan is based on managing that gas. There can always be one more, but that is not always practical or realistic.

Personally it seems you have talked to people and read about these two setups, but don't have the personal first hand knowledge / experience to make some of the statements you have. If you have I appologize, you seem to stay on the b/m side. I don't mean this as a flame, just food for thought.

This is a good discusson, but in reality both setups have their place for diving on the technical side. To say one is better than the other is just misinformed or talking out of turn.
If you are only playing the devils advocate.........well done.

There are people on this board that have been diving, both styles for (probably) 40 years or more, adapting each option for the job, or dive, at hand. You may be one of these, I don't know, and don't know you to say either way. Some don't say much, but when they do we should all listen closely. These folk have forgotten more than most of us have ever learned or even heard about.

No matter what style I dive I have to be able to manage my gear and accomplish my dives. I also, no matter which setup I use, have to become skilled at using them. This only comes through, diving, mentoring, tweaking, etc.

[FW]

Do you sidemount as well as backmount?

I think this is the most important question!

I suspect most of the sidemounters that have put in their opinions have done a lot of backmounting, so are qualified to compare systems. On the other hand, the rabid backmounters have had little or no experience with sidemount.

I rest my case...

[phillip1]

Now I am only speaking from personal experience but the day I tried SM I was wondering why I had not done it earlier. I find it is much better in every cave and dive (ocean) so that is why I choose to dive this way.
I have never been to Florida or dove caves with very high flow, I am assuming BM might be better for this as the setup is more rigid on your body, but for exploration and regular low to no flow caves and ocean boat diving I find SM to be much better.
Even if SM were more hazardous than BM etc.., I would still choose to dive SM for the comfort, convenience and back saving advantages.

[FW]

....I have never been to Florida or dove caves with very high flow, I am assuming BM might be better for this as the setup is more rigid on your body, .....

I actually find I can make a longer penetration in a high flow cave with sidemount. FWIW, I dove backmount for 30 years before switching completely to sidemount.

[phillip1]

I think this is the most important question!

I suspect most of the sidemounters that have put in their opinions have done a lot of backmounting, so are qualified to compare systems. On the other hand, the rabid backmounters have had little or no experience with sidemount.

I rest my case...

All the guys here in DR who make fun of me & my team for turning sidemount, they all argue that BM is better and we are not serious etc.. but in reality they are jealous and don't want to admit to themselves they're days as backmounters are numbered, lol...

[Sludge]

If I were to make fun of someone sidemounting in the ocean it would be because wearing a backmounted single and a single stage would make about a thousand times more sense, and the only reason they used sidemount configuration would be to compensate for some perceived personal inadequacy...

But in fact I would NEVER make fun of someone's gear configuration choice. That's their business.