Pony Bottle Drag?

[Blackwood]

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.

Really depends. If it's all within the wake created by his body its shape doesn't matter a whole lot.

 

[MauiScubaSteve]

Don't know if this is the right forum but my question is about the increase in drag caused by using a pony bottle. I am a recreational diver and use a 30 cu. ft. pony bottle clipped to the front of my B.C.D. to add 37.5% to my air supply.

...

I was wondering if there was any truth to the divemaster's claim on my last trip. She said that I was probably adding so much drag by using the pony bottle that it negated the additional air from the pony bottle by increasing my air consumption. Anyone know if using a 30 cu. ft pony bottle on a leisurely paced recreational dive would increase air consumption by 38% or more?

As usual, the OP is not exactly answered by any replies (including this one), because there is a number of conundrums in the Q. 77.4 cft is the typical capacity of tourist AL 80's, so your pony is not adding 37.5%, it's actually adding 38.8%, but that number may be irrelevant. With regards to useable gas, first let's see what % the Pony is compared to total gas; 30/107.4x100=27.9%.

Now, how much of the air would you use? I'll arbitrarily use 500 psi remaining in all tanks; 500 psi is 12.9 cft in an AL80, 500 psi is 5 cft in a 30. Total useable gas is now 89.5, with 25 cft of useable gas in the pony, so 25/89.5x100=27.9%; same % as above, which is right since we subtracted the same % from each. The interesting number is total useable gas at 500 psi surfacing; 89.5 cft.

Most vacation locations have some AL 100's. Service pressure on those is 3300, but at typical 3000 psi vacation fill pressure the actual volume is 90.9 cft. 500 psi in that tank is 15.15 cft, so useable gas is 75.75 cft; a difference of 13.75 cft compared to the 80/30 rig.

After some simple math, I am of the opinion the DM is right; a 30 cft pony clipped to your BC probably causes more than 13.75 cft additional consumption, compared to diving just an AL 100 cft.

 

[Blackwood]

Halemano -

Given that the OP is using his Al30 to extend dives, can you explain your concept of usable with respect to the bottle?

Seems to me he'd likely be using it the way I use a stage: empty it.


edit: looking back I see he added a post in which he states that he uses about 2/3 of it (i.e. 20CF).

So the question is, on a typical dive, does adding that Al30 cause him to breathe an additional 20CF. Usable gas is irrelevant unless he changes his backgas reserve depending on whether or not he brings the bottle.

 

[MauiScubaSteve]

Let's say Hoover's rule of 10 cf reserve at surfacing (just made that up), no matter the configuration.

16.5 cf would be the 3000 psi starting fill pressure difference. So would the drag of using all but 10 cf for a single 80 cft w/ BC clipped 30 cft cause more than 16.5 cf additional consumption compared to using all but 10 cf just diving a single 100 cft?

Edit; if you could get a full 3300 psi 100 cft, the allowable service pressure difference is only 7.4 cf!

 

[MauiScubaSteve]

Short answer; the question is speed dependent. At rest the diver lasts longer with more air. As speed increases the drag effects rise exponentially. There is a speed where both configurations give the same dive time. How do we account for the wheeze factor of futzing with the pony vs futzing with the 100.

 

[Scott L]

Halemano -

Given that the OP is using his Al30 to extend dives, can you explain your concept of usable with respect to the bottle?

Seems to me he'd likely be using it the way I use a stage: empty it.


edit: looking back I see he added a post in which he states that he uses about 2/3 of it (i.e. 20CF).

So the question is, on a typical dive, does adding that Al30 cause him to breathe an additional 20CF. Usable gas is irrelevant unless he changes his backgas reserve depending on whether or not he brings the bottle.

Good analysis IMO. I go back to my earlier post of locating a dive operator on the island who rents steel cylinders & acquire a 100HP and be done with the pony...

 

[Scott L]

Let's say Hoover's rule of 10 cf reserve at surfacing (just made that up), no matter the configuration.

16.5 cf would be the 3000 psi starting fill pressure difference. So would the drag of using all but 10 cf for a single 80 cft w/ BC clipped 30 cft cause more than 16.5 cf additional consumption compared to using all but 10 cf just diving a single 100 cft?

Edit; if you could get a full 3300 psi 100 cft, the allowable service pressure difference is only 7.4 cf!

That's either spot on or way off. I got a headache trying to reconcile.

 

[1_T_Submariner]

I can't imagine a 30 cuft round bottle causing that much drag diff. what does your air consumption tell you. If it where the same drag as the extra gas you would not get any extra bottom time. Theroy to Practice. Dive and give it a try.

 

[searcher3]

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?

I use flutter kicking. I will try to measure my consumption as recommended with and without the pony. Problem is I won't get to dive again until around Thanksgiving. You have peaked my Geek Engineer problem solving funny bone. Maybe I could get the finite element analysis guy at work to do a flow analysis. I think he got an el cheapo verson of a Fluid Flow software package. I would have to draw a 3-D model up for him. Easy enough.

I often have trouble locating a 100 cu. ft. tank at dive destinations. I have e-mailed every dive operator on several islands and Nada. That's why I went with the pony bottle. Guess I could have got a 40 cu. ft. bottle but I have to lug that thing in my duffle bag. One of the dive instructors on my last trip tried to convince me to sell it to him to use for a pure O2 deco stage bottle and I would have but he got a cheap plane ticket to Panama to party and didn't make it back in time.

 

[Jim Lapenta]

There's a real issue right there. You are out of the water way too long between dives. Increased diving usually leads to better technique, more relaxed breathing pattern, and better trim and buoyancy. You need to get some local in water time. I go a bit batty if I'm not in the water at least once a month on scuba, Even if it's in the pool.