What is proper procedure for downcurrent

[pasley]

Many have given excellent advice on how to handle the situation based upon their personal experience. I have not had that experience yet, but will offer one additional piece of advice if caught in a down or up current.....pray, stay calm, follow the advice the others have given of swimming away from the wall and across the currrent and then pray some more!

 

[ArcticDiver]

Taking into consideration my somewhat minimal dive experience, counteracted by my quite extensive flying experience, and considering that the difference between air and water is about 33% in density, I can tell you that in theory alone, this is only true when the mass of the object is undisturbed. So if we're talking only about the size of the diver, then the statement is true.

This is a question of area. Newton's second law of motion states that Force=Mass X Acceleration (F=MA). The product of greater mass, times the acceleration (the current), will result in greater force, which will affect the diver when you add the coeficient of drag to that equation.

Inflating a wing in a current is no different than trying to land in high winds. When I flight instructed, I taught my students to use 1/2 the amount of flaps used in normal landings when landing in gusty conditions, or in high winds, if any at all. Applying this to diving, in a current of about 1 - 2 knots, or better yet, as Walter puts it, "when the bubbles are going down," there is no question that inflating a wing will require stronger finning, as in the aircraft, using full flaps in high winds will require about 30% more power to land at the same speed (Newton's third law of motion, "for every action there is an equal and opposite reaction"). Hence, if you inflate your wing in a down current, and you're unable to keep up the propulsion necessary to counteract for the increased drag, there is no way you're going to avoid a greater speed going down than if you don't inflate the wing and try to swim out of it, or do something else to avoid it.


Semper Safe,

Rick

Ah, you'll understand then that inflating the BC does a similiar function to adding power. Deflated the BC has minimal lift(power). In fact it may have negative lift(power). Inflating the BC is very analagous to adding power in the situation you describe. Further extending the analogy, inflating the BC has a similiar effect to reducing wing loading. The mass involved stays the same.

So, the net effect of inflating the BC is to "add throttle" in an upward direction and to "reduce wing loading". Thus, the diver has increased propulsion to the surface and is more subject to course displacement by burbles in the current.

If the sole thing that happened when a BC was inflated was to increase its' size then I'd agree with you. But, that is not the case. Changing BC inflation is primarily changing lift vectors with changes in size being only a peripheral issue.

So, if in a downcurrent: First, try to fin out of it(change course). If not successful continue to fin and inflate your BC(add power in an upward direction). If still not successful continue finning and inflating your BC and sequentially drop weights. Of course if diving dry by all means use your dry suit inflation too.

 

[DocVikingo]

(nt)

 

[Scubaguy62]

Ah, you'll understand then that inflating the BC does a similiar function to adding power. Deflated the BC has minimal lift(power). In fact it may have negative lift(power). Inflating the BC is very analagous to adding power in the situation you describe. Further extending the analogy, inflating the BC has a similiar effect to reducing wing loading. The mass involved stays the same.

So, the net effect of inflating the BC is to "add throttle" in an upward direction and to "reduce wing loading". Thus, the diver has increased propulsion to the surface and is more subject to course displacement by burbles in the current.

If the sole thing that happened when a BC was inflated was to increase its' size then I'd agree with you. But, that is not the case. Changing BC inflation is primarily changing lift vectors with changes in size being only a peripheral issue.

So, if in a downcurrent: First, try to fin out of it(change course). If not successful continue to fin and inflate your BC(add power in an upward direction). If still not successful continue finning and inflating your BC and sequentially drop weights. Of course if diving dry by all means use your dry suit inflation too.

I don't disagree at all. Finning is always the most effective means of propulsion, especially when deep. Just trying to illustrate the flip side of the scenario you illustrate.

Take into consideration something though, notwithstanding the positive buoyancy propulsion, or "adding throttle" effect of inflating the BC, as you put it, in the scenario of a strong downcurrent, the tendency to inflate the BC to it's maximum capacity will be close to, or greater than 90%, which in esence has the same effect as the difference between 10 degrees of flaps and 40 degrees of billboard. With that in mind, there will be more drag than propulsion.

In airplane terms, it is no different than throwing the throttles into Beta (or reverse if you would) on the landing rollout; power increases (slightly), but drag increases greatly.

In the air, or underwater,

Semper Safe,

Rick

 

[ArcticDiver]

I don't disagree at all. Finning is always the most effective means of propulsion, especially when deep. Just trying to illustrate the flip side of the scenario you illustrate.

Take into consideration something though, notwithstanding the positive buoyancy propulsion, or "adding throttle" effect of inflating the BC, as you put it, in the scenario of a strong downcurrent, the tendency to inflate the BC to it's maximum capacity will be close to, or greater than 90%, which in esence has the same effect as the difference between 10 degrees of flaps and 40 degrees of billboard. With that in mind, there will be more drag than propulsion.

In airplane terms, it is no different than throwing the throttles into Beta (or reverse if you would) on the landing rollout; power increases (slightly), but drag increases greatly.

In the air, or underwater,

Semper Safe,

Rick

This is where the airplane example meets its limitations. First of all flaps, as you know operate on relative wind(air flow). If there isn't any relative wind the flaps don't function. Second, unlike your airplane a BC can't reverse its' thrust.

So, a better example would be an outboard motor boat floating down the river off and with the prop out of the water. It moves at the same rate as ther river. Put the shaft and prop in the water and it still moves at the same rate as the river, even with the increased surface area. But when the driver starts the motor and points the bow upriver the boat goes that direction at a rate equal to the speed of the boat minus current velocity.

In the same way when a diver inflates a BC the diver will go up at a rate equal to the upward lift of the BC minus current velocity. If the lift exceeds the down current's force the diver will go up. The size of the BC has little or nothing to do with the equation.

I admit I've an advantage in conceptualizing this as I've spent time on a river bank watching the debris being carried downriver. It all goes at the same rate, from the tiniest branch to the largest tree.

A homely demonstration would be to put an air hose into a gentle downward water stream. Watch the bubbles. They will all go up at a similar rate even though size varies. In a BC certainly the increased size will result in more friction drag. But, in comparison to the lift(power) involved I suspect that is truely minor.

 

[Otter]

Is the key factor revolving around to inflate or not inflate. the ratio of increased lift vs. increased drag. If the amount of drag is greater than the amount of lift, then you will actually descend faster.

 

[Otter]

This is where the airplane example meets its limitations. First of all flaps, as you know operate on relative wind(air flow). If there isn't any relative wind the flaps don't function. Second, unlike your airplane a BC can't reverse its' thrust.

So, a better example would be an outboard motor boat floating down the river off and with the prop out of the water. It moves at the same rate as ther river. Put the shaft and prop in the water and it still moves at the same rate as the river, even with the increased surface area. But when the driver starts the motor and points the bow upriver the boat goes that direction at a rate equal to the speed of the boat minus current velocity.

In the same way when a diver inflates a BC the diver will go up at a rate equal to the upward lift of the BC minus current velocity. If the lift exceeds the down current's force the diver will go up. The size of the BC has little or nothing to do with the equation.

I admit I've an advantage in conceptualizing this as I've spent time on a river bank watching the debris being carried downriver. It all goes at the same rate, from the tiniest branch to the largest tree.

A homely demonstration would be to put an air hose into a gentle downward water stream. Watch the bubbles. They will all go up at a similar rate even though size varies. In a BC certainly the increased size will result in more friction drag. But, in comparison to the lift(power) involved I suspect that is truely minor.

On your boat analogy, the 'speed of the boat' is a function of drag and hence it affects the upstream progress. The increased power from the motor obviously overpowers the increase drag of the props etc... The boat obviously won't go any faster than the downstream current, but thats not the question...Another example is say, you were dragging a sheet of plywood facing parallel to the current and then moved so that it was perpendicular. Your mass hasn't increased, but you drag has....and hence the speed of your boat drops since there is no offseting increase in power.

On your bubble analogy, I suspect that bubbles rise and the same rate because the ratio of their size (surface area) to their lift is constant. As the size of the bubble increases so does their proportional lift.

 

[Otter]

The question boils down to

Does the increase in lift from the inflated BCD offset any increase in drag? I believe that the drag is soley a function of surface area (and hence frictionable contact area) and is constant with respect to the speed of the current. That is my drag (not speed) is constant regardless if I am in a 2 knot downward current or 20 knot downward current....or in a 0 knot downward current.

Since I know that a fully inflated BCD in a 0 knot downcurrent current will ascend, I know that the upward force (lift) of the BCD is greater than the increased drag.

What I don't know is whether the net increase in lift will be sufficient to offset the downward current, but thats not the point. The point is that adding air to your BCD is not wrong due to increased drag.

If this is wrong, tell me where I goofed.

 

[Michael Schlink]

I was just in a "down current" in Dec. in Indo. Not even close to my first. 500+ dives in Cozumel, Pelelui "express" that was a ripper! So you guys bat this around BUT the first thing you better do is STOP going down, some way. I add air to my BC to get some lift -I don't believe the increase in mass is significant but the added lift has helped. Dropping weights is not an option, I don't dive with any. The MANY currents I have been in I wouldn't have been able too overcome by kicking. SOME currnets will diminish when you get away from the wall but some DON"T. As soon as you kick off the wall you no longer have anything to grab on to which in a few occasions would have been a problem for me.
The reason I said not to bother with a lift bag is that I can't imagine being able to pull that off while dealing with the problem with the current. If you can deploy a lift bag while getting blown downward you're good. The currents I've been in I was more concerned with stopping my descent, than I work the rest out based on what options I have-gas supply, deco obligation, surface support (like where's the boat). Do what you think is best but for me it's 1.Stop the descent then figure the rest out-Dive Safe-Dive Alot-M

 

[Scubaguy62]

Otter hit the nail on the head, although drag is relative to both area and speed; the greater the speed, even at a constant area, the greater the drag (Newton's 3rd law of motion). 8 degrees of flaps will produce more drag at 240 knots than at 140. The flip side is that if the area increases, the drag will also increase. 8 degrees of flap will produce the same drag coeficient at 240 knots as 30 degrees will produce at 140 (speeds being hypothetical), but even if structuraly possible, the airplane will not fly with 30 degrees of flap at 240 knots, unless you have a PW F100 engine on full afterburner, and then you're likely to damage the A/C. Maybe applied to diving this is not a good example; I mean, we'll never hit 140 knots under watter, but again, the drag of the fully inflated wing is relative to the speed of the current (F=MA).

If the ratio of lift vs. drag falls on the side of drag, there is only one way the diver will go on a down current, down. This will happen when the fully inflated BC doesn't deliver enough lift to counteract the effect of the down current. So, in a 5 kt down current, chances are that the drag being excerted on the diver with a fully inflated wing will be greater than the lift capabilities of the BC, making the drag coeficient greater than the lift coeficient, rendering the wing useless.

Boyle's law complicates this even further. The deeper we are, the less displacement the wing has, so while there will be less drag, less lift will also be available, so again, if the current is stronger than the BC's available lift at that depth, the diver will go down current.

In Nassau, the wall starts somewhere between 50 - 80 fsw (at least at the sites I have visited), and in Coz, it can start as deep as 90 (again at the sites I've visited). If at 66' we only have 1/3 the available lift, and 1/4 at 99', that means that my 40 lb wing will only provide about 13.3 lbs of lift at 66 fsw and 10 lbs at 99 fsw. In other words, if there is nothing to which hang on, and the bubbles are going down in a hurry, the BC might not have enough lift to get a diver out of the down current, so it will be better to just hang on and wait, gas supply permiting.

Michael's experience is one of those where he had enough lift available from his wing to get him out of a jam, but that's not always going to be the case, so with that in mind, the rule of thumb Walter mentioned seems to me it will work best. I hope I never have to find out for myself.

Semper Safe,

Rick