Pony Bottle Drag?

[miketsp]

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

I really doubt that. The difference in CD between a flat ended cylinder and a rounded ended cylinder is over 4:1.

See
Aerospaceweb.org | Ask Us - Drag of Cylinders & Cones

So unless the cylinder is mounted inside a water mass already being accelerated by the body movement any streamlining should have a marked effect.

If it's hanging it won't be inside the moving mass of water and should generate it's own resistance.

 

[Blackwood]

I really doubt that. The difference in CD between a flat ended cylinder and a rounded ended cylinder is over 4:1.

See
Aerospaceweb.org | Ask Us - Drag of Cylinders & Cones

So unless the cylinder is mounted inside a water mass already being accelerated by the body movement any streamlining should have a marked effect.

If it's hanging it won't be inside the moving mass of water and should generate it's own resistance.

Read my post again.

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

 

[Mattleycrue76]

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.

My suggestion would be to look into the frogkick. On a non scientific basis I'd be willing to bet that you'd see a huge improvement in gas consumption if you can master horizontal trim and the frog kick/modified frog kick.

YouTube - Propulsion Techniques - Scuba

 

[searcher3]

I really doubt that. The difference in CD between a flat ended cylinder and a rounded ended cylinder is over 4:1.

See
Aerospaceweb.org | Ask Us - Drag of Cylinders & Cones

So unless the cylinder is mounted inside a water mass already being accelerated by the body movement any streamlining should have a marked effect.

If it's hanging it won't be inside the moving mass of water and should generate it's own resistance.

Nice link. I definitely keep the pony bottle pointed rounded end first and hold it up against my chest as I kick in order to minimize drag. I will try out the frog kick again.

 

[MauiScubaSteve]

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...

Not sure on other islands but in Hawaii, a fill station that pumps over 3500 psi is rare. With the typical fast fill, your lucky to get 3000 psi ambient. AL 100's are bigger than HP 100's when the max ambient pressure you can get is only 3200 psi. My steel 120's are only 103's at 3000 psi.

 

[Doc Harry]

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?

Only one way to tell for sure: do a test!

Swim a distance X and measure your gas consumption Y with and then without the pony.

Then you can just compare your gas usage in p.s.i. between the two trials. The longer the X the better the results that you will get.

Since underwater resistence (i.e., gas consumption) increases exponentially with speed, it is very important that the two trials be done at the same speed. If you are swimming laps on an underwater course, you can record your split times from the first trial and try to match the times on your second trial.

If you are using the same circuit for both trials, then it is not important to remain at the same depth over the course of the circuit.

My suggestion would be to look into the frogkick. On a non scientific basis I'd be willing to bet that you'd see a huge improvement in gas consumption if you can master horizontal trim and the frog kick/modified frog kick.

I did similar tests of gas consumption and speed, at constant depth, to try to determine my ideal speed for the most "m.p.g.," (i.e., the most distance traveled for the least amount of gas used, in distance per cubic feet of gas.) I tried frog, modified frog, flutter and modified flutter at 4 different speeds while doing laps back and forth along a 150-foot line while wearing double AL80s.

My personal best (i.e., greatest distance for the least amount of gas) was using the flutter kick at a leisurely speed.