Misconceptions and Fallacies

[wettek]

I

However without splitting hairs, if we consider the more normal 5-10 lbs overweight and normal slow swimming speeds around a reef or a wreck then I have never noticed any perceptible increase.
Inertia also comes into play. Once you get the mass moving it keeps moving without any additional effort.

I beg to disagree on this one. Inertia will come into play in air or a similarly thin surrounding medium, but water is a few thousand times denser than air, and drag is huge compared to inertia with a diver. Stop finning and see how quickly you stop, or merely slow your effort. You will soon see how much effort it takes to keep the mass moving to overcome drag

 

[Gil57usa]

Using the butt dump is a good way to work around a slug of water in your wing. Which can accumulate, despite good technique, after a couple of days of diving.

If you drain the water out of your BCD after every dive, or at least at the end of a day of diving, there should not be an accumulation of water at all. Remember the part about "proper equipment care" taught in OW class?

 

[The Kraken]

How can you agree with this comment, you have just said your SAC increases when you are in a drysuit?

Colder water has the propensity to cause one to breathe just a little more rapidly. The dry suit isn't as streamlined as a thin wet suit and generates considerably more drag. Keeping "the bubble" has a tendency to generate a position in the water that isn'ts as streamlined, for me anyway, as the wet suit. Just a couple of little things like that . . . but it all adds up.

I consider myself still a new dry suit diver . . . I've only about 40 dives on my d/s so I'm still learning.

Hopefully, we're all still learning . . .

the K

 

[ShoalDiverSA]

Reading through this thread has taught me one thing:

One man's fallacy is another man's fact.



Cheers,

Andrew

 

[TomP]

Fallacy: Dry suits

 

[Hank49]

Does it really matter? What do you think will happen to the water when you add air to the BC? it should be expelled when the bc reaches its limit.

Only if you position the dump valve on your wing/bc at the lowest point, gravitationally speaking.

 

[matt_unique]

How can you agree with this comment, you have just said your SAC increases when you are in a drysuit?

Cold water and the necessary/significant increase in gear (drysuit, etc.) are examples of things that affect SAC long before 10lbs of extra lead.


--Matt

 

[S. starfish]

Inertia also comes into play. Once you get the mass moving it keeps moving without any additional effort.

Newtons first law of motion: An object at rest will remain at rest unless acted upon by an external and unbalanced force. An onject in motion will remain in motion unless acted upon by an external and unbalance force.

In the case of you being still, you exterting more force than the water resistance exerts on you is the unblanced force. When traveling at a constant speed you exert the same force as the water exerts on you - balanced forces, no acceleration. When you slow down the water is exerting more force on you and you are finning.

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. So with the force fo the water against your person increased you have to work harder in order to first unbalance the forces to start moving and then to balance them out to keep moving. So increased weight will increase your sac, how much however depends on a) how overweighter you are and b) How efficient you are as a diver (trim, how streamlined you are to begin with, how efficient your finning is, do your arms flail about etc.)

 

[miketsp]

I beg to disagree on this one. Inertia will come into play in air or a similarly thin surrounding medium, but water is a few thousand times denser than air, and drag is huge compared to inertia with a diver. Stop finning and see how quickly you stop, or merely slow your effort. You will soon see how much effort it takes to keep the mass moving to overcome drag

What I'm saying is that if I am finning at a steady 1/2 knot along a reef it doesn't really matter whether I'm grossly overweighted or not provided my trim is correct.
There would only be extra work done if I had to keep accelerating the additional mass.
For the reasons I gave in an earlier post the impact on the Drag Coefficient is minimal. The drag depends on the drag coefficient CD and has nothing to do with the mass.
1/2 x rho x CD x v^2 x S .

 

[Temple of Doom]

For the reasons I gave in an earlier post the impact on the Drag Coefficient is minimal. The drag depends on the drag coefficient CD and has nothing to do with the mass.
1/2 x rho x CD x v^2 x S .

Not quite. Your formula is incorrect. You omitted A (surface area. Well, the cross-sectional area perpendicular to the direction of motion, but that's *really* picking nits):

Fd is the force of drag,
ρ is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula),
v is the velocity of the object relative to the fluid,
A is the reference area, and
Cd is the drag coefficient (a dimensionless constant, e.g. 0.25 to 0.45 for a car).

(Source Wikipedia: http://en.wikipedia.org/wiki/Drag_equation)

Mass doesn't affect drag, but surface area does. Surface area is not static. If you think about it this makes sense. Without surface area, a mini-cocktail umbrella would give you as much drag as a parachute.

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