Misconceptions and Fallacies

[miketsp]

..snip..
Now we can all agree that with severe overweighting you need to fill your BC up a fair bit, increasing the surface area that is exposed to the water at a greater than ideal angle, creating more drag. More drag = increased force exerted by the water on you.
..snip..

Where does the increased angle come from?
With my jacket BC the extra area/volume is under my armpits.
With my wing the shape becomes more streamlined.

 

[miketsp]

Not quite. Your formula is incorrect. You omitted A (surface area):

Mass doesn't affect drag, but surface area does. Surface area is not static.

Increase surface area and you increase drag. Add more air in your BC, and you increase surface area, drag, and gas consumption (for you warm water divers it may not be much, but it's doing it).

Craig

S is surface area in all the international versions of the formula.
As I just pointed out the increased cross-sectional area is minimal.

 

[roakey]

My gawd, this was actually a good topic until it got sidetracked by the overweighting discussion...

Ignoring the inertia issue of toting around a one-ton anchor, it comes down to this:

For an experienced diver, assuming that the overweighting does not profoundly effect their trim position, overweighting will have a negligible affect on air consumption because an experienced diver knows how to handle their buoyancy and can easily accommodate the less than optimum configuration they’re saddled with. Typically, an experienced diver also moves fairly slowly, minimizing the effect of the negligible increase in drag offered by the more-inflated BC.

Now, for an inexperienced diver, everything changes. A greater amount of gas in the BC is more difficult to manage, causing the diver to yo-yo more (especially at shallower depths). This lack of buoyancy control, initiated by overweighting will lead to greater air consumption via tangential mechanisms:

o Greater stress
o “kicking up” to maintain depth rather than adding gas to the BC
o More add/dump cycles
o Perhaps going too deep

Bottom line is that overweighting does not in and of itself, increase air consumption. But given an inexperienced diver, the collateral damage that results from overweighting can cause a huge jump in air consumption. But it’s from other factors that are brought on by overweighting, not the overweighting itself.

Now, can we get back on topic?

Roak

 

[roakey]

Myth: You can explain all diving phenomenon by trotting out complex formulae.

Anything much beyond simple depth versus pressure formulas is completely insignificant when compared to the physiological and psychological effects of the diver in the diving environment. You need to look at behaviors, not physics, for the significant contributors to diving issues.

The vast majority of recreational dives are statically trimmed to be vertical in the water, yet they’ll argue for hours about the evils of increasing your BC’s volume by 10 cubic inches, as if that’s significant, given their trim.

Roak

 

[Temple of Doom]

Bottom line is that overweighting does not in and of itself, increase air consumption.

So we agree that there's an increase in drag while swimming. If you've got a surface swim, this will be significantly increased because you're inflating your BC more.

Further, if you're shore diving you're toting around more weight to and fro the water.

Both of these effects will pile on more than linearly if you're doing multiple dives.

The extra energy required to do this isn't coming out of the ether. You're indisputibly burning extra energy (some cases more than others), therefore using more gas.

Additionally you will definitely be using more air in your BC. Depending on your climate and profile a little or a lot more. If your BC is currently inflated to produce 20lbs of lift, and you add another 10lb weight to your belt, you're increasing the amount of air you're going to have to put into your BC (throughought your entire dive) by 50%. This isn't a very unrealistic scenario.

If you're a warm water diver off a boat and you're in good shape the effect will be minimal, but there will be an effect. If you're an out of shape diver on your second shore-dive in cold fresh water with a long surface swim, the addition of 10lbs is going to take a significant toll.

These are two extremes, but in both cases the increased consumption exists.

Nobody's suggesting it's always significantly increasing consumption. It is always automatically increasing consumption.

Craig

 

[ianr33]

Myth: Buying a rebreather will save you money.

Myth:Breathing helium makes you colder.

 

[roakey]

So we agree that there's an increase in drag while swimming. If you've got a surface swim, this will be significantly increased because you're inflating your BC more.

Further, if you're shore diving you're toting around more weight to and fro the water.

Sure there's an increase in drag. Does it significaly affect air consumption in an experienced diver? No. You then go on to mention a bunch of physiological effects, which is my point.

Oh, by the way, about trotting weights to and from the dive site -- since it appears you need to be told the obvious: we don't care about air consumption then.

Move along folks, nothing to see here...

Roak

 

[matt_unique]

Part of the reason I listed this as a fallacy (and others have also) is because there is such a labratory approach to some aspects of diving which are just silly. You can be 10 lbs heavy and it will not make one bit of measurable/relevant difference in the air remaining in your tank. You can do the test yourself. If you do it honestly, with no other SAC contributor, you will not be able to measure the difference.

Roak's summary above is right on.

--Matt

 

[redacted]

Myth: Your regulator is life support equipment. You are not allowed to work on it.

 

[Temple of Doom]

Oh, by the way, about trotting weights to and from the dive site -- since it appears you need to be told the obvious: we don't care about air consumption then.

Please try not to get snide, this conversation is going in a good direction, let's not side-track it. If you disagree with what I'm saying, say so and explain why (if you choose).

As for obviousness of your statement. One thing that will have a very large impact on consumption during your dive will be how tired you are before you get in the water. If you're breathing harder when you get in the water, you'll be breathing more when you're in the water.

Again, if your point is that under many conditions the effect is small, I'd agree. However, there are plenty of circumstances where adding 10lbs of weight will make a notable difference.

Craig