Hydrodynamics - backmount vs sidemount

[Dave Lizdas]

Read forum title. Stepped in for OFG's comment. Was not disappointed.

[LiteHedded]

Who cares?

^ This.

[DA Aquamaster]

Whether the diver moves through the water, the water moves over the diver or both is not important, it is all hydrodymanics. The major difference is that the flow in a cave can pass over the diver far faster than he could swim through the water and as drag increases as the square of the velocity the increased flow will magnify any differences in configuration. But there are problems with this approach.

A high flow test may be relevant to divers who scooter at 150 fpm, but the differences in configuration are probably small for a diver swimming 50 feet per minute. In this case, tripling the speed will increase the drag by a factor of nine. If the drag in sidemount while swimming is a unit of 1 and the drag in backmount is a unit of 1.5, then the difference may not be noticeable. But when you are scootering and the 1 becomes 9 while the 1.5 becomes 13.5, then the difference may be significant. Of course, the reverse is also true - large differnces on a scooter may not be significant at all at a normal swimming speed.

Also, what is missing in any strain gauge experiment done while just hanging out in flow is any impact of configuration on the ability of the diver to fin efficiently. For example, I notice many sidemount divers who seem to move their tanks as they frog kick and that slight movement of what are relatively massive objects equates to substantial work being done that woudl not be meausured in a static situation. In addition, unless the diver is finning in water flowing at a normal swimming speed, there is no way to measure the impact of a given configuration on the actual kick, recovery and glide phases of the kick itself.

In short, strain gauges sound great but the deceptively simple nature of the approach tends to mask the rather serious limitations involved. At best it may serve effectively as one phase of a larger study. I think time trials under no flow conditions with the diver actually staying over the line are essential to control for both distance and depth to give valid measures of drag (ie speed) versus gas consumption. You'll also need multiple trials per diver over the same course to get sufficiently reliable data to make any reasonable assumptions about that diver in each configuration.

One major problem is that the most efficient speed for a diver in back mount may not be the same as the most efficient speed for a diver in sidemount. inother words a diver making a "speed" run over the course may not tell you anything useful regarding optimum efficiency and utility on real life cave dives. It is the same problem as identifying the best economy cruise speed in an aircraft, but without the benefit of a wealth of aerodynamic theory and data to help you predict the most likely speed(s). At a minimum you'd have to have several divers make the swim at various target speeds to see where the best compromise between speed and gas efficiency occurs in each configuration.

Another issue is that while back mount configurations are more or less standard in terms of general layout and hose routing, sidemount configurations are all over the place with a high degree of variation in wing size, cleaniness of hose routing and stated purpose. Then you have to add in the effects of stages, deco bottles, etc and variable that extend from that (does the SM diver top or bottom clip the stage, how does he/she address a floaty near empty stage, etc). Ads a result, I suspect the variation within the subset of SM divers will be greater than the variation within the subset of backmount divers and as such the greater variation will create noise that will obscure the differences between SM and backmount divers.

In short, you are pretty screwed if you think you will draw meaningful data from what you hope will be a very limited study. Personally, I have a lot of data on dive time and gas consumption in a variety of caves that I dove in back mount. Eventually I will have s similar body of data for similar dives in many of the same caves in sidemount and a comparison of time and gas consumption in both will probably yield anecdotal, yet far more useful data on what works for me. And in the end it is all pretty pointless as the reasons peopel sidemunt dive have nothing to do with greater streamling in the water and information saying SM diving is or is not more streamlined probably would not impact anyones SM versus backmount choice.

At best it is going to be a major waste of time leading to flawed data and faulty conclusions that will just provide more fodder for internet arguments. I think I'll pass on this one, but thanks anyway.

[Unregistered User]

You also have to consider the wet suit vs. dry suit issue. I propose having the same diver perform this experiment in sidemount, then backmount with first a wetsuit, then a drysuit, and as a control function, totally nude while breath holding.

This whole procedure should be done in real-time to piped-in underground background music such as "Watermelon in Easter Hay" or "3rd Movement of Sinister Footwear".
btw-that is an awesome avatar (equally awesome alboum).

[RN]

You also have to consider the wet suit vs. dry suit issue. I propose having the same diver perform this experiment in sidemount, then backmount with first a wetsuit, then a drysuit, and as a control function, totally nude while breath holding. And then, the nude part, you have to repeat with a man and then a woman, just to account for various pieces parts that create drag. And then, a DD cup vs. A cup, nude naturally, to account for the additional bouyancy. And then, a bald nude guy vs a hairy nude guy . And beards, don't forget long flowing beards that reach out and grab the water molecules like a brillo pad on a greasy frying pan. And warts, find a guy thats hairy with a lot of warts.


BTW, the spring thing is not an indicator of anything other than leg strength. The term hydrodynamic implies a moving situation, a spring tests static loads. I propose time trials. Like the tour de france, we can call it a Tour de farce.

I'll judge the nude women trials. You can just the bald and hairy nude guys trials...

[Jay]

JMHO but I find SM much more streamlined for swimming. That is with my configuration (tanks high and tight under my arm)
A lot of SM divers use the droppy option with their tanks much further back. I notice a big difference in efficiency swimming that way (when I experimented with a 'Dillo for a while)
It is even more of a difference scootering. The looser or further back your tanks are the more they are splayed as speed increases. I like having mine adjusted so they do not splay when scootering. They are much tighter to my body then most people prefer.

[phillip1]

even if BM had way less drag I would still SM, it's way more fun amongst other things.
Why not do a simple penetration test, take the same diver in BM go in 1000ft and measure gas used and time for example.
As DA Auquamaster correctly pointed out BM is pretty much standard and as a result will tend to be a more streamlined setup for the masses.
SM being a less standard setup tends to be a bit all over the place for the masses and may be less streamlined for the less experienced, when I saw the light and switched to SM, it took me a while to get it right.
To do the test justice you would need someone proficient in both BM and SM with a nice streamlined SM setup.
Still a great idea.

[Jay]

To do the test justice you would need someone proficient in both BM and SM with a nice streamlined SM setup.
Still a great idea.

As an example I could go another 500 plus feet of penetration on 1/3's easily in Cow on sm 98's as opposed to bm 98's. Yes I gained a few seconds at the first restrictions SM but I lost time at NMF the first few times in SM. With 85's I was easily reaching my typical 1/3's turn of my BM 98's.
I found that to be true at a lot of other caves I dove frequently in both styles.

[Webmaster]

There are underwater strain gauges available. Maybe someone could find one to borrow for a weekend. It would be an interesting project.

[diveongas]

You don't lose 1/2 your gas if you lose a reg. There are ways to salvage it.

Actually jj1987 is kinda, sorta right, you do lose half your gas, at least potentially.

Illustration: Diver A is backmount with twin AL 80s filled to 3000# and Diver B is SM and has two AL 80s filled to 3000#. Both A and B have the exact consumption rate. They both enter JB and when they both reach "thirds", Diver A closes his right post and Diver B then closes his valve on the right cylinder. Diver A then thoughtfully cuts the LP hose on Diver B's right cylinder. Diver B then returns the favor by cutting Diver A's LP hose from his right post. You now mind telling us who now who (in the real world) has more air available to exit?!

To "salvage" that air in SM, are you perhaps, advocating holding your breath to switch regulators, or do you plan to unclip the SM sylinder and then dump the regulator and breath directly off the valve? In the first scenario you're betting you life you don't lose an O-ring and in the second, well, I would just have to see this. Real efficient on exit, I'll bet, and don't bump anything, lest you knock some teeth loose.

Both of these possibilities are tenuous at best. Why not just admit this limitation in SM and advocate something more conservative than "thirds", like "fourths" when diving SM? This is a case where THERE IS a small, potential advantage to BM with a manifold.