Originally Posted by
addexdiver
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. :-)
