Dealing with current

[REVAN]

Thanks for the personal experiences DD. It's interesting how people are willing to dive into such uncontrollable conditions and are OK with the risk to reward ratio. You say that long fins sometimes don't cut it, and when conditions are bad that can be true, but it is still better than having bad fins. Also, from what I have seen, a scuba diver on a dive-X is still not all that fast in the water, generally less than 3 knots. So, there is no surprise that a strong current can still send you backward even with a DPV. IMO, the DPV concept leaves a lot to be desired. It is just not a well thought out or elegant solution to diver mobility.

From the linked post:

Still, the vast majority of divers need help and yet they seem clueless about that fact. Sure, a part of it boils down to pride and some of comes from out and out denial. I think a lot of it just stems from simple ignorance. The divers who need help simply don't know what's acceptable.

I totally agree! Maybe that "vast majority of divers" is an even even larger group than you think.

 

[dumpsterDiver]

I've never been in currents like you describe. Deliberately getting in (with nitrox i assume) to currents that can be expected to throw you down to 50 meters does not sound like something I would like at all. I assume you are putting air in your pony bottle, so if/when you exceed the MOD of your primary, you are still safe?

Also, I would assume that people would want to have larger capacity BC's ?????... In a situation where I was pushed to 50 meters and had the potential to be pushed to 100.. I would much rather be inflating a big BC, then trying to muscle my way out of a very strong current. Exerting yourself below 50 meters is something I try to limit...

 

[REVAN]

And yet, the BC is such a horrible piece of equipment to use. Sort of like the DPV, the BC is also not an elegant design, inherently unstable in the water column and the more you rely on it the more instability it injects into your dive. IMO - the BC is also something that needs to be rethought.

 

[Diving Dubai]

@REVAN

You're right. DPV's aren't the answer. It just happened in this instance we knew the ridge in question would have a current (it's barren) there are two ways to approach 1. drift into it and sling your reef hook at the right time, hang on and watch the big fish. 2. sneak up in the lee - a longer route needing finning or a DPV. The rewards are great. Seeing the big fish coming out to watch the stupid humans.

Some sites you do take the DPV and be cautious. This weekend we went out took the DPV and headed to a point into current to see what we could see. Didn't get to the point as got distracted by a Mola Mola (my first) We'd have never saw him/her or kept up without a scooter

@dumpsterDiver

No we don't willingly go hunting the stupid currents, however some sites benefit from some flow. What we call some flow might be fast for others. The mussandam ignores tide tables. 2 weeks ago we dropped in on Neaps on a day that had 2' swing between high and low tide on the surface all the indicators showed it to be fine as was the first 5 mins then it picks up. I've done the same dive on springs and it was unrealistically calm (we thumbed the dive as its a boring wall. no flow no fish)

Normally we dive 28-32% which is fine. but that's why we take the Air stages .

No we don't dive bigger BC's I have a 36lb dive rite wing - my wife has a 30lb bigger isn't better as you don't want to fly upwards. Manage the descent is the answer then figure how to climb out. A down current can be only 6' wide or its force dissipates as you go down.

Also (like the Pinnacle) you can start the dive and its okay - indeed I've dive that site 50 times and only once has the current suddenly changed to make it a rock climb out.

 

[REVAN]

This is a side note (but relevant to the discussion of down-currents that are part of this thread): I'm an aerodynamicist and control systems engineer. No doubt being a controls & aero guy changes my perspective on things some. As a control systems engineer, I don't like unstable systems, and a BC is an unstable system. Yes, it provides a new actuator to add up/down authority, but it delivers instability along with that authority. This is why so many people have problems with buoyancy control. The very thing that they use to actuate buoyancy control injects a divergent instability into the control loop.

Also, people overestimate the effectiveness of a BC to control depth in something like a downcurrent. There is a big time lag to adding and dumping air from a BC. Control lag is very bad thing to have in an unstable system, making it even more unstable and divergent. A full BC adds a lot of drag and can also pull you out of trim, so a fully inflated BC pulling up with 30 pounds buoyancy is probably not going to get you more than 2 knots worth of 'up' speed. Couple that with 10 seconds to discriminate that you need to add some buoyancy (while actually losing buoyancy from the down current), then another 10 seconds that you are actually in trouble and need to add a lot of buoyancy and another 15 seconds for the BC to fully inflate after you decide to bear down on the inflator button like a middle school noogie (and assuming that you are not descending so fast that the BC compresses faster than it fills), and you can see just how badly equipped a BC is for doing what so many divers rely on it to do. A ripping current can do a lot in 35 seconds. A 2 knot down current can take you down over 30 meters before you get your 'up' authority fully engaged. That's not acceptable.

This is why I like to dive with no BC for any dive that doesn't require it (any dive where I'm not using more than 3mm neoprene). By focusing on swimming authority instead of buoyancy authority, I can be far more stable in the water column than someone who is using a BC. I can easily swim up or down as needed with speeds that exceed what fully inflated BC can achieve and I don't have to deal with the massive time-lag for implementing the 'up' actuation or the instability of the BC. Swimming just works better! In a serious emergency, I can still drop lead if needed, but I have yet to encounter that need. When you can swim at 2 to 3 knots, there is a lot that you can do. My ultimate goal is to be able to swim at 3 to 5 knots. If successful, there will be little that I cannot do.

 

[dumpsterDiver]

A scooter is not inherently unstable in the water? A

This is a side note, but I'm an aerodynamicist and control systems engineer. No doubt, this changes my perspective on things some. As a control systems engineer, I don't like unstable systems, and a BC is an unstable system. Yes, it provides a new actuator to add up/down authority, but it delivers instability along with that authority. This is why so many people have problems with buoyancy control. The very thing that they use to actuate buoyancy control injects a divergent instability into the control loop.

Also, people overestimate the effectiveness of a BC to control depth in something like a downcurrent. First, there is a big time lag to adding and dumping air from a BC. Control lag is very bad thing to have in an unstable system, making it even more unstable and divergent. A full BC adds a lot of drag and can also pull you out of trim, so a fully inflated BC pulling up with 30 pounds buoyancy is probab not going to get you more than 2 knots worth of 'up' speed. Couple that with 10 seconds to discriminate that you need to add some buoyancy (while actually losing buoyancy from the down current), then another 10 seconds that you are actually in trouble and need to add a lot of buoyancy and another 15 seconds to fully inflate your BC (assuming that you are not descending so fast that the BC compresses faster than it fills), and you can see just how badly equipped a BC is for doing what so many divers rely on it to do.

This is why I like to dive with no BC for any dive that doesn't require it (any dive where I'm not using more than 3mm neoprene). By focusing on swimming authority instead of buoyancy authority, I can be far more stable in the water column than someone who is using a BC. I can easily swim up or down as needed with speeds that exceed what fully inflated BC can achieve and I don't have to deal with the massive time-lag for implementing the up actuation or the instability of the BC. Swimming just works better! In a serious emergency, I can still drop lead if needed, but I have yet to encounter that need. When you can swim at 2 to 3 knots, there is a lot that you can do. My ultimate goal is to be able to swim at 3 to 5 knots. If successful, there will be little that I cannot do.

If someone wears a BC and puts very little (or no air) in it, then they are not going to be subject to the unstable equilibrium issue. The instability is caused by the air in the BC, not the BC itself. This is not a minor distinction.

However, wearing a BC provides a lot of capacity to handle problems and situations that are not as easily addressed by simply kicking. In addition, using a BC to address some of these issues (say supporting a diver who is too heavy, offsetting suit compression etc. are going to be addressed more efficiently (wrt air consumption) than simply kicking.

I am fully in agreement that the option of dropping lead should definitely be available if you are not wearing a BC, but so many people now feel that there is no need to carry ditchahble lead AND dropping 6 or 8 lbs will send them rocketing to the surface (which it won't).

 

[REVAN]

A scooter is not inherently unstable in the water? A

Not by buoyancy. But directionally, yes. Scooters are directionally unstable while towing a diver. If you don't actively steer it, you will not go straight. Thrust from front, unstable. Thrust from back, stable. This is why rockets and boats thrust from the back end.

 

[REVAN]

If someone wears a BC and puts very little (or no air) in it, then they are not going to be subject to the unstable equilibrium issue. The instability is caused by the air in the BC, not the BC itself.

If there is no air in your BC then you are not using it. You are just carrying it along on the dive incase you decide to use it. Maybe for dealing with a downcurrent perhaps? But, I already explained why it is inappropriate for that situation.

My M.O. is to not take along any gear that I'm not planning to use.

If you just weight yourself right for your target depth, then a bit of magic happens. Your buoyancy does not need any compensation. It is simple and brilliant. Suddenly, your dives are easy and controllable. Bottom line: BCs are overrated. A BC is a piece of equipment that tends to create a need for itself. For most recreational diving, if you don't have it, you won't need it, and most of the time, we'd be better off without them. Instability is not a trivial thing.

 

[dumpsterDiver]

That comment was not something I intended to post... but.. most of the people I know, who use scooters are now using saddles.. pushing from behind...

 

[REVAN]

That comment was not something I intended to post... but.. most of the people I know, who use scooters are now using saddles.. pushing from behind...

Actually the saddle is even more unstable. The prop is in the front. That makes the combination of DPV+diver unstable. The diver is being pulled from behind. This is distinctly different from being thrusted from behind, and the diver can also be unstable if the towing strap mounts behind the diver's directional center of pressure. But, not a big deal to this discussion and it was an inadvertent post to begin with. I just like details.

You can probably tell that I'm in general very disappointed with the engineering behind most scuba technologies. That's why I make most of my own equipment.