Escaping Down Drafts

[Kevrumbo]

How do you guys manage to dive walls that have a hard bottom below recreational depths? Some of the walls in places like Bunaken, Maldives, T&C, and even Cozumel extend well below ranges that would be safe on any breathing gas ...

... Bob (Grateful Diver)

Especially along the walls of Peleliu (Palau) --when you see your Dive Guide gesturing to ascend up and away from the wall, do so without delay and stay together with the Guide. There can be a strong down current on the Peleliu Express drift dive near the end, where the wall starts to quickly descend beyond 18m/60' to well over 39m/130' & deeper (i.e. Past Nitrox32 MOD, and getting low on tank pressure). And do not forget to bring an SMB and know how to deploy it from depth.

Too many divers (usually novices) start the Peleliu Express dive using the wall as reference, and before they know it, they inadvertently follow it as it slopes away, finding themselves at 30m/100' deep AND caught in a stiff downcurrent, drifting right to left past the south point plateau descending into blue water abyss (see map link).

Deploy your SMB at least at your safety stop if you get separated from the group; if you're not low on gas or fightng a downcurrent, deploy earlier at depth depending on how much spool/reel length you've got. You gotta get that SMB up so the dive skiff driver can see you early, before passing the wall drop-off & into blue water open ocean with potential surface swells (makes seeing a small SMB harder to see). For this dive site, I always carry extra length spools, a Halcyon inflatable Life Raft, and a surface deployed Personal Locator Beacon (PLB -Mcmurdo Fastfind Plus in a dive canister; some of the Liveaboards are utilizing similar units).

Last time I was there was an early AM special advanced dive charter to see the Black Snapper schooling/spawning, during a full moon (i.e. very strong currents); got caught and swept over the south point plateau in a 3 knot current and decided to ride it out as it it pulled me down to 36m --on Eanx32-- and hence the worst case Nitrox Diver's Dilemma & Nightmare: You just inflate your BCD/Wing and do the best you can to slow or maintain depth; try not at all to physically over exert yourself which induces Hypercapnia which increases the chances of Oxygen Toxicity; hope the downcurrent slacks off in time to ascend back above MOD; and deploy your SMB from depth quickly so the boat crew sees you early for eventual pick-up (or else next pick-up point is 1000km to the west in Mindanao Philippines). . .

 

[TMHeimer]

Those of you limiting yourself to hard bottoms and air, may need to get out more and expand your diving horizons.

Money may be a factor.

 

[chillyinCanada]

Might be Tom but if a poster is limited in their experiences as diving locales, perhaps they've no understanding of why divers might choose to dive nitrox despite no hard bottom.

 

[REVAN]

Down currents are not a boogy man to be feared by all, but they can create life threatening situations. It is good that you are learning about them before you encounter them unprepared.

First off, don't dive over weighted. Too much weight + currents is asking for problems, especially if down currents are a possibility. If the water starts doing something you don't like, swim out of it and get to someplace better. If you are on a wall and can swim over and grab it, that's a good plan. Just don't stay in a bad place for longer than necessary.

I have seen several videos of people caught in down current. It seems they always add air to the BC in short spurts in response. It is interesting to watch the timeline. You can see when they realize they are going down and see the stages of response that sometimes lead to a full pannick. There is often about a full minute that elapses between when the situation draws a noticeable level of attention and when they hit that Oh S&!t moment. All the while they are adding meager amounts of air to their BC instead of making an attempt to get out of the situation.

A BC is actually the least useful tool for getting out of a down current situation. They respond very slowly and are not that effective. The average rec BC will have between 20 and 30 pounds lift when fully inflated. This is something you will probably never do, as it just doesn't seem right or safe (because it's not), so you will likely never get more than 10 to 15 pounds lift in reality. Unless you are pointed straight up, this is not going to move you up very fast at all. At depth it may take an additional 20 to 30 seconds or more to get max lift from your BC when you finally decide to fully bear down on the inflator button after procrastinating for about a minute to get to that point. Things can get pretty out of whack in 90ish seconds of strong down current if you are not reacting effectively to the situation. If it's a strong down current it is actually possible for the BC to compress faster than you can add air. This is why it is so important to not be overweight to begin with. BCs are unstable, the more air you need to have in it to achieve neutral buoyancy, the more unstable they are, and if bad enough, it can get uncontrollable in a fast descent regardless of your premo BC hovering skills over a calm reef.

In comparison, you can get about 40 pounds thrust from you fins and it is nearly instantaneous and always aimed in the direction of your most streamlined profile through the water. Your fins are much more effective than a BC for getting up as well as across, whichever direction you decide to go. In strong currents, your fins are your friends. Use them to get to somewhere you are comfortable being.

 

[lermontov]

Down currents are not a boogy man to be feared by all, but they can create life threatening situations. It is good that you are learning about them before you encounter them unprepared.


In comparison, you can get about 40 pounds thrust from you fins and it is nearly instantaneous and always aimed in the direction of your most streamlined profile through the water. Your fins are much more effective than a BC for getting up as well as across, whichever direction you decide to go. In strong currents, your fins are your friends. Use them to get to somewhere you are comfortable being.

good point i guess its a trade off effort/gas

 

[REVAN]

good point i guess its a trade off effort/gas

Remember, the BC uses gas also, and the deeper you are, the more it uses. Filling a 30 pound BC to max capacity is about the equivalent of 6 breaths at a heavy workload respiration level (nearly double that if you are breathing efficiently). It is a trade off. You can breath the gas and use it to get somewhere safe, or fill the BC and then dump it later to, hopefully, arrest an uncontrolled ascent before blasting through your safety stop. I like staying in control of the dive.

 

[Kevrumbo]

Remember, the BC uses gas also, and the deeper you are, the more it uses. Filling a 30 pound BC to max capacity is about the equivalent of 6 breaths at a heavy workload respiration level (nearly double that if you are breathing efficiently). It is a trade off. You can breath the gas and use it to get somewhere safe, or fill the BC and then dump it later to, hopefully, arrest an uncontrolled ascent before blasting through your safety stop. I like staying in control of the dive.

Even if you were an Olympic Champion Sprint Swimmer --There is no way you can physically sustain a kick to counteract the effects of a strong downcurrent (given you are now in bottomless blue water with no reef topography to climb/haul yourself back up).

Your chances of exertion Hypercapnia, CO2 Narcosis and ultimately unconsciousness go up exponentially the more you try to fight it along with increased work of breathing, density and consumption of gas supply (and such can be the dilemma of trying to fruitlessly struggle to get out of a strong downdraft).

Conserve strength & energy, do your best to maintain depth and trim WITHOUT OVER EXERTION, inflate your BC/wing, find the edges of the downcurrent's flow and/or wait 'til it slackens --and then pick the moment to start kicking hard & try to escape it.

 

[REVAN]

You do your best to maintain depth and trim WITHOUT OVER EXERTION, inflate your BC/wing, find the edges of the downcurrent's flow and/or wait 'til it slackens --and then pick the moment to start kicking hard & try to escape it.

I say there is no benefit to waiting. If the situation doesn't look good, you should start doing something about it right away, not 1 or 2 minutes later when the situation may be much worse. The sooner you start swimming for the edge of the downdraft (or the wall), wherever you decide to go, the sooner you'll get out of it. The longer you wait to react, the faster you may have to swim to compensate for your hesitation.

 

[Kevrumbo]

I say there is no benefit to waiting. If the situation doesn't look good, you should start doing something about it right away, not 1 or 2 minutes later when the situation may be much worse. The sooner you start swimming for the edge of the downdraft (or the wall), wherever you decide to go, the sooner you'll get out of it. The longer you wait to react, the faster you may have to swim to compensate for your hesitation.

And I say it's a dynamic circumstance when & where you decide on the moment to act --it may be now, or to wait a few minutes until you have less flow and some slack to make a timely & better tactical effort to escape.

In my case I had other compounding issues as well --again they were approaching Nitrox32 MOD at 39m and risk of Oxygen Toxicity Seizure, with further exacerbation in the onset of Hypercapnic CO2 retention & resulting sudden narcosis --the only relief is to relax & try to recover with nominal breathing and less physical exertion. Even if it resulted in being pulled deeper in the downdraft by waiting a few more extra minutes.

 

[John C. Ratliff]

Okay, I have no experience in ocean down currents. From what I can determine here, these are localized currents near a reef. It seems that these currents are limited in width and height (cross-sectional height). If so, they are very similar to classic "rip currents," except they go down, not at the surface. To escape a classic rip current, we teach to swim at 90 degrees to the current. Would this work for a down current? If so, it lends credence to not using the BC inflation to escape it, as this is directly opposing the current. If swimming to get out of the current, move your arms over your head and lock your hand together to get as streamlined as possible, then use several hard, powerful dolphin kicks to initiate the swimming escape, possibly at a 90 degree angle to the current rather than directly opposing it, to gain the edge of the down current.

SeaRat