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

[eab]

First, the guy with the flag is who you HAVE to keep up with. He's actually holding on to a sail on the surface and doesn't always get to pick which way he's going! Remember the guy with the flag is NEVER lost.

Our guide in Cozumel shoots the flag up once the first person starts his/her safety stop. Until then, it's nicely rolled in his pocket. However, he is the one who we're always sailing past. I thought maybe there was some secret he's learned over the years, but when I asked he just winked and said no. I was hoping to find the secret here, and be able to stick with him throughout.

By the way, loved the "but I used English" further down.

---------- Post Merged at 11:20 AM ---------- Previous Post was at 11:18 AM ----------

avoid trying to swim into the current whenever possible.

Thanks awap. Key point for me is that the current is not homogeneous. I wasn't factoring that in.

But I do enjoy facing into the current and doing my best to "stand still". It's kind of fun, but depending on the current, lasts but a few moments.

---------- Post Merged at 11:24 AM ---------- Previous Post was at 11:18 AM ----------

Oh boy my inner geek :geek: twitched...flashbacks to fluid dynamics classes... and the divers' profile(s) in the water [horizontal in trim and you may be more like a wing, and will go further with the current rather than someone 'standing up' in the current).

Not to worry skearse, I loved the geeky answers too.


Surprised though that someone more like a wing in the water will fly faster than someone standing up. Why wouldn't the current just roll over the wing person but propel the one standing up even more? If I put a piece of plywood in the water, wouldn't it go further standing up rather than lying down? What else am I missing? Come on ... here's your chance to geek out!

 

[The Chairman]

Our guide in Cozumel shoots the flag up once the first person starts his/her safety stop. Until then, it's nicely rolled in his pocket. However, he is the one who we're always sailing past. I thought maybe there was some secret he's learned over the years, but when I asked he just winked and said no. I was hoping to find the secret here, and be able to stick with him throughout.

While that's appropriate in Cozumel (different laws) it would not be here in Florida. That being said, he's probably learned to duck under the reef line. After all, if he's any good as a DM, he's actively looking for critters for you to see. You'll usually find them tucked UNDER the rocks and crevices and you just naturally stay out of the current when you get that low. Yes, it takes some skill, great trim and breathing technique to get down that low, but these are essential skills to master if you want to maximize your fun.

By the way, loved the "but I used English" further down.

I can't remember the name of the movie, but I have always love the quip: "It's all Geek to me!" As an instructor, I try to help everyone understand the environment they will be playing in.

 

[skearse]

Surprised though that someone more like a wing in the water will fly faster than someone standing up. Why wouldn't the current just roll over the wing person but propel the one standing up even more? If I put a piece of plywood in the water, wouldn't it go further standing up rather than lying down? What else am I missing? Come on ... here's your chance to geek out!

Drag. The more drag force on the object, the less 'forward' momentum (velocity). In the case of the plywood, there's also going to be a significant shift in the relative magnitude of up/down forces (buoyancy/gravity) relative to the drag, causing it to float up faster than it would move 'forward.' In the case of a diver, there's enough friction to carry the horizontal diver forward (theoretically, I guess, an ideally, perfectly, infinitely thin flat piece of something without friction would be 'skimmed over' by the fluid rather than carried by it) at closer to the current velocity than the vertical diver.

 

[socaljohn]

Boy, I this one going on and on so here is my 2 cents. I agree with one of the theories offered by other posters.
Seems to me if you are completely within the current, there should be no resistance to the water as the water in front of you (down current) is moving the same speed as the water behind you (up current) as long as you are not trying to swim with or against it. That being said, the water will move at different speeds within the current as the water at the edges is affected by friction of moving past the stiller water outside the current.
Just stay close and I think there will be no problem.

 

[eab]

I guess it's clear now why I got a wink instead of an answer when I asked the DM the trick! Holy cow. Thanks everyone.

 

[ianr33]

mass and shape have no effect in homogenous current as long as the body is not inputting any force.

Have to disagree with that.

A current will move a feather much faster than it will move a lead weight.

 

[The Chairman]

Have to disagree with that.

A current will move a feather much faster than it will move a lead weight.

If the density remains the same, then he's right: mass and shape will have no effect. In fact, the feather, since it is floating on the surface will have to answer to the vagaries of any wind it might encounter. Divers are more or less neutrally buoyant so density is not the issue here.

 

[redacted]

Have to disagree with that.

A current will move a feather much faster than it will move a lead weight.

yes, the body does have to be neutral buoyancy.

 

[skearse]

If the density remains the same, then he's right: mass and shape will have no effect. In fact, the feather, since it is floating on the surface will have to answer to the vagaries of any wind it might encounter. Divers are more or less neutrally buoyant so density is not the issue here.

Which weighs more, a pound of feathers or a pound of lead? ;-P

Agree that, in general, the density effect is probably negligible, especially for faster/more turbulent currents if we're talking about divers only here (roughly equivalent densities). Shape, though, by affecting drag, will affect it...again, though, it's probably a more and more minimal effect as the current increases.

 

[The Chairman]

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.