What is proper procedure for downcurrent

[ArcticDiver]

. Tell me in this situation without inflating your BC or dumping some weight how one might approach this? These down currents may be interfaces between Humbolt and Panama currents and just keep on going down well past depths I want to try swimming up from! If my bubbles are going down and my SPG needle is too then I'd start inflating rather than finning hard and getting tired. If that doesn't work then start dumping weight. I'd only start finning hard when I see that SPG depth stabilize in one place. Then I'd fin up and out at an angle while starting to dump air as needed.

To say never inflate a BC or dump weight are absolutes that may need to be broken in certain situations. Look at the rules for airplane cabin fires. Prior to the Swiss Air accident it was dump fuel waste time and then look for an airport. Now it is get the bird on the ground asap. Sometimes you just got to just trust your gut and break the 'rules' to live another day to tell the story. Paula if you are on SB thanks for yours

In light of the vectors involved I think the rules actually are: For a Down Current you can't immediately fin out of always inflate your BC. If after a reasonable time there is no effect from the inflated BC then start dropping weight. For a Horizontal Current inflate or not depending on whether you need the bouyancy to keep afloat. Primary escape method for a Horizontal Current is steering out by finning.

Remember, in a Down Current the vertical down force vector may be stronger than the upward lift vector of your BC. So you may continue down after the BC is inflated. That doesn't mean the BC isn't having an effect. It is. It just means that you are going down slower than you would have if you didn't inflate the BC.

Also remember that if you are in a Down Current that is strong enough that you have to drop weight to counter it an uncontrolled ascent may follow. The upward rate of that ascent will depend on the forces involved. But, you have at least given yourself a chance for a good outcome.

And there is no guarantee that even with inflating your BC, dropping all weight and finning upward will get you out of the problem. But at least you have done all you can to meet the situation.

 

[SDAnderson]

What if the bubbles are going down with the current, but at a slower rate of speed than the water is? Which, I assume, has to always be the case.

In other words, a positively bouyant bubble might be getting shoved down by the current but always at a slower rate of descent than the water itself or even a neutrally or negatively bouyant object, eh? Blow all the air out of your BC and aren't you actually assisting the current by reducing your bouyancy? Being positively bouyant is still being positively bouyant and I'm not convinced that I wouldn't rather that than the alternative.

From experience, escaping the current will usually require swimming or crawling out of it, but while you're finning horizontally to the current like a madman, anything that can be done to retard the rate of descent would seem like a good idea. Whether or not inflating your wing will increase your bouyancy enough to offset the effect of the increase in surface area in any specific situation may be in doubt, but it seems likely to me.

It seems we're stumbling around in the dark here, trying to figure this out without much in the way of hard information, a sub-optimal situation. Sometimes the blind have to lead the blind, but it would be nice if there were any sighted folks around that might shed a little light on this issue. :sun_smile

 

[pufferfish]

Yes ArcticDiver I think that is a good protocol for a down current you are unable to easily fin out from.

Interesting point about diving Nitrox in these conditions. Doesn't sound like a good idea does it.

 

[pufferfish]

Reefraff the down currents we are talking about here start at 3 knots. As Gary D said most civilian divers who are out of shape would not be able to hold there own for long in that velocity of current. Finning hard would only increase SAC, heart rate, adrenalin and lower the threshold for possible panic. I'd say follow the depth gauge and reach a neutral velocity situation with the current by whatever means necessary, then think about finning out of there. Remember she dropped about 110 ft in under a minute so things are happening real fast.

Yes it would be nice if the physicists/engineers could model this and give us our options to choose from! Where is Charlie99?

 

[saltwater taffy]

I had this happen once and it was THE SCARIEST diving event ever!!!!! I went over the wall at about 40 feet. I was immediatly sucked to a depth of 141 feet in the blink of an eye. I was able to get myself in closer to the wall. My thinking is that, if necessary, I would grab hold of something. I then started to inflate my BCD. It seemed like forever, but slowly, slowly I started to ascend. I watched my depth very closely and as I came near the edge of the wall, I deflated my BCD. My buddy did not stop until 168 ft.

 

[ArcticDiver]

It seems we're stumbling around in the dark here, trying to figure this out without much in the way of hard information, a sub-optimal situation. Sometimes the blind have to lead the blind, but it would be nice if there were any sighted folks around that might shed a little light on this issue. :sun_smile

Actually we're not stumbling either in the dark or light here. This is a simple vector analysis problem that any pilot is used to calculating. It is exactly the same as calculating wind drift except we are doing it in the vertical plane rather than the horizontal. In fact, when I was in Junior High, ages ago I admit, this was the type of problem we solved in Geometry.

What we are missing here are actual numbers. But we can include a couple for demonstration sake. Lets' say a diver has a BC with 45# lift. We all accept that as long as the downward force on the diver is less than 45# inflating that BC will cause the diver to ascend. In addition, that 45# will be a constant force. How rapidly the 45# will overcome the downward force depends on how much downward force there is. Always remember the mass and speed involved in creating the momentum. So, nothing happens instantaneously.

So, if the downward current is putting a 50# vertical down vector on me and I inflate my 45# lift BC I will continue down. But I will do so slower than if I hadn't inflated the BC.(45-50= -5)

On the other hand if the downward current is only putting a 40# vertical down vector on me when I inflate the 45# lift BC I will go up. (45-40= +5)

Also, remember that the vertical up 45# will be a constant force. So, once downward momentum is overcome and the diver starts up the rate of ascent will increase. The effect is something like vehicle acceleration with a constant throttle setting. Once the vehicle gets moving the vehicle will continue to accelerate until other forces limit acceleration(but lets' not complicate this example by involving them).

How do you tell what current strength is? Well, without an external meter a diver can only inflate the BC and then say that it is either stronger or weaker than the BC's lift.

 

[SDAnderson]

Reefraff the down currents we are talking about here start at 3 knots.

Not exactly. I said:

each knot of current works out to something more than 100 feet per minute, thus a 3 knot current can drag you down 100 feet in less than 20 seconds.

I didn't mean to imply that 3 knots is the starting point for trouble, I picked that rate because it more or less equals an easily grasped 100 fpm.

As has been repeated a number of times here, including most recently by ArcticDiver, there are a number of factors that influence what the actual rate of descent will be in a down current and if we could figure out what all those factors were we could plug them into his vector analysis and we'd have an answer. Until that happens, however, I'm going to inflate my wing anytime I find myself in a downcurrent, making sure that I'm first finning like wild for the nearest exit.

 

[simbrooks]

This has been fun to watch, and i will throw my hat into the ring as well with a few points.

1) As you descend, that 45# lift wing, will have less lift as the volume reduces, depending on how quickly you go down will be proportional to the volume reduction, so the constant force is not that. If you have more lift than the current (+5 in last case) then the the force will be constant if you BC/Wing is full, if you are going down, you may start at -5# (in last case) due to the current being stronger than the lift, but that amount will increase -6#, -7# etc as you go down.

2) The force exerted on you by the current is a drag from you profile in the water, so the more surface area you have the more drag, the more force pushing you down as you inflate your BC/Wing. The difference may only be marginal from full to empty though depending on your wing size, 45# + body profile, larger than 27# + body profile.

From what i have read, i would firstly be cautious of the downcurrent if known and told by operator or on a wall (have yet to dive in such), then if i found myself in that, i would either hug the wall and get some anchorage/relief from lesser current due to surface friction slowing the current near a surface, or i would swim out (probably about 45 degrees) to escape and not continue going down and hope the current is contained within a certain distance of the wall (think waterfall flowing and getting outside the falls) due to hydraulics of such things. I would follow the advice, swim in to wall or out of current as mentioned by those more experienced than I, either members on here who had previously posted or take heed to the locals/operators.

We were out this last weekend in "fairly strong" (according to the operators) SE FL currents drift diving, we did a wreck which was fine, and then a regular drift dive over the reefs. We had planned on going to 60ft, but due to a deeper sandy pocket in the bottom the current took us down with the surface profile to near 80ft, we realised almost immeadiately and moved back up to 60ft, but others in the group carried on along that sandy pocket for a little while before coming up. It can creep up on you, particularly as the bottom was slowly getting deeper before this pocket. However this was a mostly horizontal current (at least vector wise, locally horizontal all the time) except down into this pocket and then horizontal again. We applied hydraulics to it and saw what was happening, even if it was after the fact, but it was a new experience!

 

[Otter]

A good rule of thumb - when your exhaust bubbles are heading down, inflating the BC won't help.

Bubbles and BCDs suddenly loose their buoyancy in a downcurrent? It might not stop ya, but it has to be better than deflating and have the buoyancy characteristics of a rock....well maybe not a rock, but at least let Archimedes help a little bit.

 

[Gary D.]

For this post let's break the currents into two different types: First, the original post, a Down Current. Second, a Horizontal (or nearly so) Current. In both currents what the diver is attempting to do is regain control by either overcoming or exiting the current's force. For this discussion I'm assuming the current is strong enough finning by itself is inadequate to do more than possibly steer.

In a Down Current one should inflate their BC completely and early. This provides the maximum up force available to counter the down force of the current. Finning is useful because in the process you might steer yourself out of the current.

Horizontal Currents are another matter. The BC's force vector (lift) is vertical up whereas the current's force vector is horizontal. So, inflating a BC will do nothing to counter the effect of the current. Nor will it help the diver steer. It might help the diver stay afloat if fatigued. Finning is useful because you might be able to steer yourself out of the current.

So to inflate or not to counter the current really is a function of the relative force vectors. How effective any escape method will be is a function of current strength and available lift for a Down Current and finning strength for a Horizontal Current.

As for the idea that inflating a BC is a bad idea because it just gives the current more surface to push on, well let's examine that for a minute. Once in a current the size of the object makes no difference in its' speed. The current will move all items at the same speed regardless of size. If you have a river nearby you can demonstrate this for yourself.

Gary, I'm with you, if you are in a horizontal current, which is what I think you were talking about, relax and play awhile!

The current we have been refering to was a very unpredictable go everywhere but up current. It was two currents that came together with one rolling over the other. On my original video I got a quick shot of the surface line in the distance.

We were down for a while when it appears to have come across the bottom headed out to sea. When it hit the reef it went up and then sharply down where we got caught. On the way up I got another shot of the line from below.

If I remember right everyone who inflated their BC's had problems over and above everyone elses.

What a person needs to do is learn how to "Ferry" across current and keep your fingers off the inflator. It doesn't matter what direction the current is going in but you have to be able to identify what it's doing first. It takes training and practice but it works.

Your right about various size object moving at the same rate along the current. But the object is to counteract the current and swim in a direction other than the direction the current wants you to go. In that case a very small change in size, streamlining and body position make a big difference in the forces applied.

And do not drop your weights!

Gary D.