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

[eab]

Hi. Hoping this is the right spot for this question. I couldn't really find a home for it.

I'm curious about the science ... or maybe physics? ... of drift diving. If three very differently shaped people dive together on a drift dive, how do they stay together? Who drifts the quickest? Who drifts the slowest? Is it weight, mass (as in width & height) that's perpendicular to the current direction, how close one is to the reef, something else?

I seem to be able to convince myself in various ways, and wanted to see what others think.

Let's go with:
Person 1 ... short, fat, and round
Person 2 ... tall, skinny, hardly any body fat
Person 3 ... average height & weight

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.

Many thanks!

Eileen

 

[Insta-Gator]

Doesn't matter.

Think of the current as a car. If these three people are in the same car, they all go the same speed. The only way the divers might experience a difference would be if you could calculate the net of pushing force to resistance force. But since all the water is moving in the same direction and speed there is no (practical) resistance, it's a mute point.

 

[ianr33]

 

[eab]

Doesn't matter.

Really? That just baffles me. I get your concept, but can't seem to wrap my head around how that makes sense and don't recall that's how it worked last year when we were there. Now I'm extra excited to get to Cozumel!

Thanks. I will stop wondering how I'm going to stay with the group, and will just enjoy the ride. Sweet.

Eileen

---------- Post Merged at 09:32 AM ---------- Previous Post was at 09:30 AM ----------

ianr33 ..... *love* it!

 

[TSandM]

Well, I'm going to differ with the answer you've been given. Think about sailboats -- if one has up a small sail, it will move slower than the one with a big one. How fast the current moves you depends on your mass and the resistance you provide to the water. People WILL move at different speeds, and in addition, current is laminar, so water close to the reef will be moving slower than water further away.

If you want to stay together in moderately strong current, stay CLOSE to one another, and try to offer a similar profile to the water.

 

[eab]

How fast the current moves you depends on your mass and the resistance you provide to the water. People WILL move at different speeds, and in addition, current is laminar, so water close to the reef will be moving slower than water further away.

If you want to stay together in moderately strong current, stay CLOSE to one another, and try to offer a similar profile to the water.

That's what I was thinking! As well as experienced last year, but thought that I was doing something different from the others.

I can't figure out where or how to look this up, so I thought I would find the answer here. Sigh.... The debate in my head still rages on.

Eileen

 

[The Chairman]

There are more factors here than body size. 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.

As for Lynne's observations about bigger sails etc... put the same sized sail from a large boat onto a small boat and those won't wash. A diver's mass usually affects their profile in the water so it probably evens out.

HOWEVER... the problem is not with body size, but with a diver's relative positioning in regards to the reef. The bottom tends to slow and divert currents. Ergo the closer you are to the bottom, the less of a current you are in. Duck behind a reef and that current may completely cease. The best way to keep together is to reef hop... but don't forget about your flag man. While your bodies may be shielded from the current, they are still holding on to that sail of a dive float/flag. Again, the guy with the flag is NEVER lost.

 

[redacted]

mass and shape have no effect in homogenous current as long as the body is not inputting any force. if the diver is providing any propulsion, then size and shape will have an effect just as in still water. more importantly, the current is not homogenous. it is like a big river that has many different vectors caused by fixed structures. as individual divers will likely encounter different vectors, you will need to stay aware of each others location and progress and make adjustments to stay together. in stronger currents, you will need to recognize stopping points where you can get out of the current to wait for slower divers. avoid trying to xwim into the current whenever possible.

 

[skearse]

Oh boy my inner geek :geek: twitched...flashbacks to fluid dynamics classes...but the short answer is, it depends. In general, though, the factors that affect it are diver 'density' (and anything that affects buoyancy), 'fluid' (water) density and viscosity (which will both be constant), fluid velocity (and whether it's a 'weak' current [laminar] or strong [closer to turbulent], 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).

In short, the drift and 'where you end up' is probably most affected by profile...the overall forces at work are essentially buoyancy versus gravity (up and down) and the current working against the diver profile (drag).

Sorry for the probably WAY too technical answer; couldn't help myself...

 

[The Chairman]

mass and shape have no effect in homogenous current as long as the body is not inputting any force. if the diver is providing any propulsion, then size and shape will have an effect just as in still water. more importantly, the current is not homogenous. it is like a big river that has many different vectors caused by fixed structures. as individual divers will likely encounter different vectors, you will need to stay aware of each others location and progress and make adjustments to stay together. in stronger currents, you will need to recognize stopping points where you can get out of the current to wait for slower divers. avoid trying to xwim into the current whenever possible.

Kind of what I said, but I used English.