Hydrodynamics - backmount vs sidemount

[Sludge]

In a thread on TDS concerning advantages and disadvantages of backmount and sidemount, someone mentioned sidemount is more streamlined. I've heard several "experts" say this, and several other "experts" say backmount is definitely more streamlined. I'd like to know which is.

I thought about having a showdown. Maybe at Jackson Blue this fall. We could get six or eight divers that have both backmount and sidemount setups and scooters, and let each diver make one or two runs on each configuration over a closed course.

We obviously want each contestant to be unbiased, because it shouldn't be an "us versus them" deal. Just a bunch of people that want to know the answer. So we could have some divers on hand to examine each setup before their run to pick out any danglies or other things which would affect speed.

The NSS-CDS and/or Cave Adventurers has been having a Fall Social the past few years - this would be an excellent time to do it.

Who's interested?

[phillip1]

That's a great idea, should film the whole thing too and post it on the net.
I personally think that hydrodynamicaly (is that a word?) it makes no difference at all, you are either stacking two tanks on your back side by side or on each side although spread out differently the surface area is pretty much the same. I assume the drag would be identical, however the fun factor is not the same at all.
I would love to come down for that and to dive FL but I still have about 4 months of PT left with my ACL, damn!

[BillBowden]

Good idea. Actually GREAT IDEA. I'd also like to see a swim/pull section. Fatigue factor will become an issue as will familiarity with the cave. We might consider a second test at Ginnie or Little River, or maybe Peacock for the swim test. Above assumes that sm and bm tanks are the same size.

[Kelly Jessop]

stacking two tanks on your back side by side or on each side although spread out differently the surface area is pretty much the same.

I've read a good description somewhere,but unfortunately can't remember where,but the central thesis was that there is more surface area exposed with sidemounted tanks. They actually measured the amount of tank that was in contact with the body in BM and SM,and found that SM had far more surface area exposed. In the context of drag,it is felt that the SM produces more drag.

[flyczek]

I FEEL faster sidemounting, does that count?

Seriously, I'm interested in hearing the answer, but this is going to be hard to gauge. For example, I dive almost exclusively sidemount, so I feel somewhat slow and awkward while backmounting. Then there is the whole tank size variable, stage vs no stage, wet vs dry, temp and charge on dpv, etc. etc.

It is interesting to me how people make bold claims like that, espousing things as facts that are really only nebulous concepts within their own minds.

I think in addition to using a stopwatch, you should ask how the diver felt about their speed. It will be interesting to see if the bias comes out then.

Whether you can draw conclusions or not, it will certainly be fun. If I'm in town I will play.

[jj1987]

In a thread on TDS concerning advantages and disadvantages of backmount and sidemount, someone mentioned sidemount is more streamlined. I've heard several "experts" say this, and several other "experts" say backmount is definitely more streamlined. I'd like to know which is.

I thought about having a showdown. Maybe at Jackson Blue this fall. We could get six or eight divers that have both backmount and sidemount setups and scooters, and let each diver make one or two runs on each configuration over a closed course.

We obviously want each contestant to be unbiased, because it shouldn't be an "us versus them" deal. Just a bunch of people that want to know the answer. So we could have some divers on hand to examine each setup before their run to pick out any danglies or other things which would affect speed.

The NSS-CDS and/or Cave Adventurers has been having a Fall Social the past few years - this would be an excellent time to do it.

Who's interested?

I'm guessing that they show no measurable difference. I think for some reason when people try something "different" that they really want to like, the placebo effect plays strong. Like AL72 vs AL80, not really any difference at all.

The way I see it...certain caves just don't allow back mount. For those, I'm forced to sidemount. Some caves allow backmount, and for those back mount is IMO the better option. I don't like the idea of losing a reg costing me 1/2 my gas volume, nor how sloppy stages carry in SM compared to BM.

I would be glad to participate in the test however. JB is awesome and I don't get up there enough, this is a good excuse to come!

[Sludge]

Fly Check, I've been thinking about scenarios. One is to get a full charge and do a sidemount run then a backmount run. Then charge the scooter and do it in reverse order. You would need to have divers who are truly unbiased. One thing I thought about would be to offer a prize to the diver whose times are closest to each other.

Otherwise, you need to address variables - maybe say same size tanks in each configuration, same accessories (backup lights, reels, etc., and must hold the primary light as if you were in a cave), same fins and all that.

[Tegg]

All I know is that when I switched to SM my SAC rate improved quite a bit... What used to take me double 108s in backmount could now be done in sidemounted 85s... Then again, I stopped diving backmount at about cavedive #40 or so... if that counts...

[diveongas]

For some the efficiency issue is moot. I dive SM because I'm an old POS that can no longer lift BM twins up. OTOH, I can bear hug a pair of SM cylinders (one at a time) to the water and toss them in. Put 'em on where they weigh nothing. Well, sometines I lower them in on lines and then put them on in the water.

[phillip1]

I think there will be little or no difference, when I switched to SM I found it much better for many reasons etc.. but found there was absolutely no difference in drag or swimming speed.
The only comment I can think of is with two AL 80's with my Razor I wear no buoyancy device so maybe I would have a bit less drag than with BM doubles and a wing, but I doubt I would feel any difference in drag though.
Again great idea, I think a speed test swimming, scooter test and swimming against flow test would be cool and would settle it, all timed and by the same select unbiased test subjects of course.