the science ... or maybe physics? ... of drift on drift dives

[skearse]

I respectfully disagree, but no worries. Cheers!

 

[eab]

I respectfully disagree, but no worries. Cheers!

I agree with your disagree. How could drag not matter? Isn't that the primary difference among the three divers I presented?

If nothing else, I don't feel so dumb for not knowing the answer!

 

[redacted]

Drag has no bearing on this discussion if you assume that each diver is being carried along with the currents they are in. Unless they try to move faster or slower than the current drag is not a factor.

I respectfully disagree, but no worries. Cheers!

drag would not matter if the bodies are neutral and the current homogenous. but when currents are not homogenous, size and shape will result in friction [drag] if the mass is solid.

 

[The Chairman]

How could drag not matter? Isn't that the primary difference among the three divers I presented?

Have you ever heard the saying "Go with the flow!"? Drag only becomes an issue if you aren't going with the current. When it comes to drift diving on the reefs, it's understood that you are letting the current do most, if not all of the work. IOW, you are catching a ride. However, a problem arises when diver A is in a faster current than diver B. This is even more pronounced if diver B ducks below the reef line and is essentially out of the current.

Here's some fun. Stick your hand out of the window while you're driving down the road. Be careful to not hit anything (ouch!), but notice that your hand is being shoved back while the rest of you feels no such pressure. Obviously, your entire body has more drag than your hand, but it doesn't feel the pressure near as much. Why? Laminar flow. There is no apparent air current in your vehicle while the air streaking outside your window can be ferocious. Of course there will be some resultant turbulence caused by the open window and the protrusion of your hand, but don't get distracted. The real difference between how your body and your hand outside the window feels is due to an apparent difference in air velocity (ie current). As long as you "go with the flow" (like your body in the car), drag has no bearing. IOW, a feather caught in a 3 knot current will (ignoring wind and other forces) go faster than a feather caught in a 2 knot current.

As with any river, the current is always fastest in the center. The sides and the bottom tend to slow the current down. Sticking next to the bottom will slow your progress down. Ducking off of the reef and into a depression (like pulling your hand back in the car) may very well put you completely out of the current. It really is that simple.

If nothing else, I don't feel so dumb for not knowing the answer!

There's no reason to feel dumb at all. I remember when one of our users asked (with a straight face) why we didn't use water instead of lead for weights! That was a fun discussion and it was pointed out over and over that he was lawyer.

 

[eab]

There's no reason to feel dumb at all. I remember when one of our users asked (with a straight face) why we didn't use water instead of lead for weights! That was a fun discussion and it was pointed out over and over that he was lawyer.

Oh, that's just fantastic!

By the way, quisiera un otro día en el paraíso.

 

[SeaHorse81]

Regardless of all the theory and variables like drag and density, here's a fact. Without gear, I'm about 110# with very little body fat. Every single time I find myself in current, it threatens to carry me away from the group. I have to turn around and swim back toward the group -- rather hard -- just to break even. I can't possibly swim hard enough to get further back in the group so I have a head start on next time. The only way I can stay with others in any serious current is to hold onto somebody.

 

[redacted]

Regardless of all the theory and variables like drag and density, here's a fact. Without gear, I'm about 110# with very little body fat. Every single time I find myself in current, it threatens to carry me away from the group. I have to turn around and swim back toward the group -- rather hard -- just to break even. I can't possibly swim hard enough to get further back in the group so I have a head start on next time. The only way I can stay with others in any serious current is to hold onto somebody.

current can be managed - it is not the same as riding a train. you need to learn how to find the weak areas and strong areas within the currents and use them.

 

[boulderjohn]

I'm soon headed to Cozumel, and hope to be better prepared to stay with the others. Trying to figure out if I need to eat more, get stretched, or just learn something more about body position in comparison to the current direction.

You asked a practical question, and I hope I can give a practical answer.

You are rarely just at the mercy of the current. During a drift dive you will be taking actions that affect the speed at which you drift. You will usually do some kicking as you dive, so the power with which you kick will be a factor. You will be moving in and out of the cover of the different shapes of the coral during the dive. You will be going up to see something interesting. You will be going down to see something interesting. You may go against the current at times to see something interesting. You may turn around and communicate with a buddy as you drift. All of that is more important than any possible differential in the impact of the current on individuals of different size or shape.

Natural gaps between members of the group will grow and shrink throughout the dive--it's not like everyone staying in a tight formation like a parade.

 

[FoxHound]

try not to think of it as just velocity.....you have to factor in acceleration as well...

if you put a submarine and a diver in the water at the same time in the same current, which one gets to the velocity of the current first....

gotta remember too, if you aren't going the velocity of the current, you are creating a disruption of the water in front of you, ergo the water behind you and the water in front of you are not moving at the same rate or direction.


ETA: current is also seldom in one direction.....all sorts of things in the water disrupting current

 

[SeaHorse81]

current can be managed - it is not the same as riding a train. you need to learn how to find the weak areas and strong areas within the currents and use them.

Can you offer any tips on how to learn that? I've had this issue on drift dives, but one of the worst times was actually some current running through a small, very open, shallow wreck. I tried positioning myself in various places around the wreck, hoping to find a spot where the current was disrupted or eddying. Everybody else was tooling around having no trouble while I was literally getting blown away. I had to ask my nice, big insta-buddy to hold hands (one doesn't worry about impressions at times like this) so I could stay with everyone.

If there are skills to be developed to improve this, I'm all ears. At best, it makes the dive less fun and a lot more work; at worst, it can actually get a bit worrisome.