BCD and tanks

[tbone1004]

how can you make such claims that the tank is a LOT more than 10% of the total drag? Have you put someone in a wind tunnel to actually measure while they're in good trim? Are the painfully slow speeds that we kick really significant? The fastest divers kick just over 1mph for very short amounts of time, a more typical speed is 0.5-0.7mph. On a dpv that goes up to what 3mph max? You can't tell me that the tank is creating a noticeably amount of drag. If it was, no one would use 8" diameter tanks.
I use HP120's, they're my primary tanks, they're better than 8" tanks, unless you need the volume and aren't tall enough for the 120's, but the drag argument is frankly BS. Show me the science, but right now I think that Colliams numbers make one helluva lot more sense than yours.

 

[REVAN]

For the sake of discussion in the context of the drag equatiuon, the diver's measured semiminor axis (e.g. front of the abdomen, to the back) is measured to be 22 inches, while the semimajor axis (shoulder tip to shoulder tip) is measured to be 14 inches. The calculated elliptical area is 241.9 square inches.

That's a strangely shaped, sized diver. I'm about 160 square inches in frontal area, worst case (about 2/3 your example). But, it does matter what you strap the tank to and how you attach it. The tanks on Andre the Giant will not show very much difference. On a 90 pound female asian diver, it will be very noticeable. I'm closer to average size.

 

[REVAN]

how can you make such claims that the tank is a LOT more than 10% of the total drag? Have you put someone in a wind tunnel to actually measure while they're in good trim?

I have done tow tests to measure diver drag, but the better data actually comes from swimming tests.

 

[The Chairman]

I have done tow tests to measure diver drag,

At what speed? The mass of your tank might play a role, but tank diameter has so little effect on a diver's drag it's minuscule. The quality of the dive is dependent more on trim than anything else. Just a five degree tilt creates so many more problems than any tank ever could. In fact, the awesome trim factor of a steel tank over any aluminum will save you lots of air. Just a five degree tilt is huge in regards to buoyancy and drag.

 

[REVAN]

Force was my constant and speed the variable. We pulled our tests at 20 pounds. We found the variance to be too large to get good data at only a 10 pound pull. But, like I said, the best quality data actually came from doing swimming tests. It is a more relevant setup and incorporates all the changes in the shape of the diver's body when swimming.

The bottom line here is that the scuba tank is the single largest piece of scuba equipment the diver carries and the intrinsic bluff body drag is going to be 22% higher for the fatter tank. This is before we even try to estimate the interference drag which, I think, is where the bulk of the tank's drag actually comes from. The interference drag will also scale with the diameter. If everyone thinks that this is insignificant, then I can see why everyone is in the 'drag doesn't matter" camp. To me, a 22% change in the drag of the single largest thing on the diver is something worth paying some attention to. All these incremental pieces of drag divers strap on to their body add up. If you don't start thinking about it at some level, your mobility situation will never improve.

Anyway... I think I've said all I'm going to say on this subject in this thread. I'm worried that it is getting too far off topic. If anyone else wants to have the last word, feel free to take it.

 

[The Chairman]

Force was my constant and speed the variable.

There has to be something wrong with the way you're doing it to arrive at such numbers. Could you post a picture of the apparatus?

 

[wetb4igetinthewater]

Interesting discussion, and not something I have thought much about. Depending on my head position, my head (a rather large melon, so I'm told) may "block out" a portion of my tank, offsetting the difference between the drag of the two tanks (the cross section of a 7.25" tank is 82% that of an 8" tank).

As a cold water diver, I feel my dry suit introduces the most significant difference in drag. When I'm lucky enough to dive in warm water, I feel like a torpedo regardless of what diameter is on the tank (and I've had both).

 

[tbone1004]

There has to be something wrong with the way you're doing it to arrive at such numbers. Could you post a picture of the apparatus?

with force as a constant and using speed, you better have used a tank of the same mass and length. I.e. use a HP120, and a LP121, with enough lead to make the mass of the HP120 the same as the LP121, or if you are pretending the surface area of the tank on the walls doesn't matter, at least a HP120 and LP95 filled to working pressure to make sure the mass is at least the same

 

[victorzamora]

Poppycock!

The difference in drag for a 7.25 vs. 8 inch diameter tank is a 22% increase in the tank's intrinsic drag. That's before you take into account the increased interference drag between the larger tank and the diver. The severity of that depends on many factors.

As a fluid dynamacist by trade and training, I'd love to know how you got that 22% number. I'd also love to see how you rigged the tow apparatus for the numbers you came to. There's MUCH, MUCH more to drag than the frontal area....especially at the speeds (well, Re's) involved with diving. A 22% area delta does not equal a 22% drag delta.

 

[Colliam7]

That's a strangely shaped, sized diver. I'm about 160 square inches in frontal area, worst case (about 2/3 your example). But, it does matter what you strap the tank to and how you attach it. The tanks on Andre the Giant will not show very much difference. On a 90 pound female asian diver, it will be very noticeable. I'm closer to average size.

Actually, those are my specific measurements. To provide some context for comparison, I am 5'11", 220 lbs. And, I am not by any means 'strangely shaped, sized'.

And, lest we forget where all of this started - the OP stated that he is 6'4", and 250 lbs! I suspect that his elliptical area would be even bigger than mine.

Since you mention that you are 160 square inches in frontal area, what measurements did YOU make to determine that? What shape did you assume? Circle? Square? Ellipse?

The bottom line here is that the scuba tank is the single largest piece of scuba equipment the diver carries and the intrinsic bluff body drag is going to be 22% higher for the fatter tank.

Again, two technically factual, but largely irrelevant, and probably misleading, statements.1) Yes, the scuba cylinder is the 'largest' piece of equipment the diver carries, and the heaviest. But, by no means does it present the largest surface area in the vertical plane, when the diver is swimming in a horizontal position. 2) As I previously noted, the difference in vertical plane surface area between a 7.25" cylinder and an 8" cylinder is, indeed, 21.7%. But, the change in overall area for an average diver, associated with that cylinder change, is trivial, because the surface area of the diver in the vertical plane, while swimming horizontally, is substantially greater.

Where I will agree with you - the drag associated with the size of the cylinder would be greater substantial, as a proportion of the total, on a 90 pound female Asian diver, than on me, or the OP. But, in this thread, we are talking about the a 250 pound male diver (the OP), not a 90 pound female Asian diver.

And, one final point: since you think the numbers I used reflect a 'strangely shaped, sized diver', let's use YOUR numbers - 160 square inches. Therefore, if we assume a total frontal area of 201.26 square inches for YOU with a 7.25" cylinder (160 + 41.26 for the cylinder), and a total frontal area of 210.24 square inches for YOU, with an 8" cylinder (160 + 50.24 for the cylinder), the increase in frontal area, and therefore the increase in drag (remember, in the drag equation, area is a direct, multiplier variable) for YOU - not me, not the OP, not a 90 pound female Asian diver - would be a whopping 4.4% (not 22%) associated with using the larger diameter cylinder. And, frankly, even that number is probably higher than actual, because we are not even accounting for the drag associated with the frontal area of the remainder of the scuba unit - the BCD, etc.