Pony Bottle Drag?

[H2O 70]

The full configuration, which "hangs", was found to have 12% less drag than the "float in the armpit" configuration.

I would have not guessed at this. When it is full and horizontal it has less of a profile than 1/2 full. Makes sense, but counter intuitive.

So fdog, sounds like you guys are just creating excuses to get in the water up there. How much beer was involved to get you guys in the water to test this one out?

 

[iztok]

In fact, if you want to really improve your consumption, get rid of the divemaster.

I'd be happy hanging out for the whole hour just watching a single area. For some reason, DMs want to chase you around.

Yup! I was upset when diving in Thailand when DMs wanted to run around and I just wanted to hang in there and observe animals around me.

 

[Jim Lapenta]

What others have said, tell the Dm to take a hike. I'm guessing she has never slung a bottle. And ask her why drag is such a big deal on simple reef dive that she should not be rushing around on anyway. If you are slinging it DIR style and it is not hanging down and hitting the reef a 30 should not cause much drag at all. If you really want to mess with em go to a plate and wing with a long hose and sling the bottle. If you are ok with it and it is a well thought out part of your gas plan there is no issue with it. But at the same time do try to relax, breath deep and regular, and stay horizontal as much as possible to try and reduce your air consumption. And keep it slow. I think some Dm's and guides purposely try to rush people around to burn up their air supply. Sorry not me. If I think that is what is happening you can bet your butt that I will go as slow as possible and do everything I can to maximize my air supply that is safe. That's if I even follow the DM at all. Most times I plan my dive an go my own way. I try to avoid boats that put a DM in the water.

 

[Scott L]

During testing at the Tahoe Benchmark 2009, we tested several Technical configurations. Specifically, we tested a stage as:

• Luxfer Al 80, 3000 psi of 32%
• Luxfer Al 80, 1/2 full of 32%

The results were a surprise - the half full configuration, which tucks nicely into the armpit, was found to generate significantly more drag. This goes against common knowledge.

The full configuration, which "hangs", was found to have 12% less drag than the "float in the armpit" configuration.

Also, the Technical configuration (seen above) generated 26% to 30% more drag than the Tahoe Standard configuration of drysuit, BP/W, & single.

Hope this helps. Given the numbers we generated during testing, I can't see your pony creating more than 22% more drag.


All the best, James

That sounds spot on to me. What would be the extra gas consumed with all other varibles being the same?

 

[Blackwood]

That sounds spot on to me. What would be the extra gas consumed with all other varibles being the same?

James was testing scooters, not kicking. I don't suppose there'd be any additional effort if you're on the trigger the whole time.

Kicking, drag will significantly increase you consumption IF you are trying to match speed with a lower-drag configuration. The power required to overcome drag varies with the cube of velocity. When I strap on bottles and kick, I tend to go a little slower and my gas consumption doesn't change a lot.

 

[Mattleycrue76]

If I'm not mistaken (correct me if I'm wrong) drag increases exponetially with speed. Going twice as fast means overcoming four times the drag. This is why a smoth deliberate pace is so much more efficient than a full speed kick. Also the reason why it's so difficult to make scooters ever faster. I see no issue with your stage use and because of the way you are using it I would definitely NOT consider backmounting it.

On a side note do you use a frog or a flutter kick for your primary means of propulsion?

 

[Blackwood]

If I'm not mistaken (correct me if I'm wrong) drag increases exponetially with speed. Going twice as fast means overcoming four times the drag.

Correct, and requires 8 times the power.

 

[Scott L]

James was testing scooters, not kicking. I don't suppose there'd be any additional effort if you're on the trigger the whole time.

Kicking, drag will significantly increase you consumption IF you are trying to match speed with a lower-drag configuration. The power required to overcome drag varies with the cube of velocity. When I strap on bottles and kick, I tend to go a little slower and my gas consumption doesn't change a lot.

Well then, I have to now disagree with Jim's post. Assuming the OP does not scooter or stay in one place his loosely affixed pony was causing significant drag and thus materially increased gas consumption...

 

[Blackwood]

Well then I have to now disagree with Jim's post. Assuming the OP does not scooter or stay in one place his loosely affixed pony was causing significant drag and thus materially increased gas consumption...

Agreed, it is likely. Whether it increased to (or beyond) the point of diminishing returns, I have no idea... but it seems like something the OP could easily quantify with a couple dives and a calculator.

Anyone know if using a 30 cu. ft pony bottle on a leisurely paced recreational dive would increase air consumption by 38% or more?

(AIR_start - AIR_end) / TIME*(1+DEPTH_avg/33)

Do a dive with it and a dive without it in equivalent conditions (current, exposure protection, exertion, etc.). Compute your gas consumption rate for both dives. If you are using anywhere near 38% more gas due to the stage bottle, ditch it.

 

[miketsp]

Slightly off-topic but if this is your usual configuration then I'd look for something in the way of a plastic cone to fit onto the flat base of your pony. I'd say most of the additional drag would come from there and having a tapered rear end would really improve things.