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

[boulderjohn]

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.

Here are a couple of tips.

1. Just as you might duck behind a building for relief on a windy day, you can duck behind natural coral or rock formations for the same sort of relief.

2. Current is usually weaker nearer the bottom. Stay low.

3. The pull and glide technique is more effective than kicking when going into a current. If you are going through or over a wreck with current, look for good places to hold and pull yourself across one hand at a time, your legs streamlined behind you and not kicking. You'll be surprised how easy it is. I have been in fairly big currents on drift dives in places like Cozumel and had to go against it briefly to see something of interest. You can find spots of dead coral below you. All you need to do is hook it with one finger usually and pull yourself along. I remember a dive in which we had to cross an opening between two coral formations as the tide was going out. The current was ripping. We went to the sandy bottom and pulled ourselves along by digging our fingers into the sand and pulling while keeping a low profile. It was surprisingly easy.

4. Hold onto something if you need to hold position or rest. Again, one finger hooked on dead coral is all it usually takes.

 

[SeaHorse81]

Thanks, John.

 

[redacted]

john covered it well. only thing i would add is to not let yourself get too upset over those time when it is difficult. but do discuss current management with your buddy. should too much separation occur, the up-current diver needs to join the down-current diver who needs to find a place to hold. also, if you do need to move up-current, look for a route that provides some rest points. that usually means a zig zag course with cross current legs.

 

[tracydr]

Seahorse81, you might consider some heftier fins if you're having that much trouble. What kind of fins do you use?

 

[SeaHorse81]

Thanks so much for the info. The fins I use are Cressi Frogs, circa, oh, 1999. I actually like them a lot and have found them to be very fitting for all the rest of my diving. On occasion, I find them tiring, but not usually. That makes me think that going with a stiffer or longer fin might not work well for me.

 

[eab]

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.

Thanks boulderjohn. Definitely starting to make sense.

I was at the shop yesterday filling my tanks, and asked the shop owner/course director this question. He agreed that everyone would travel at the same rate as long as we assumed neutral buoyancy for all. His final comment was to observe everyone on a safety stop. Hate to say it, but he's right ... I don't recall the group being too far apart on those.

Great discussion!

 

[supergaijin]

If the bottom is completely flat and the wall completely straight, divers in single file drift at the same speed but the reality is that the bottom and walls are irregular creating little whirlpools etc. much like a river. Your position relative to the wall makes a big difference in your speed.

Furthermore many divers readjust their body position in current by using their fins in some way which propels them forwards.

 

[vladimir]

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

Great answer. Now I'd love to hear somebody (who knows) resolve the theoretical question. I think the "drag isn't a factor" answers are incorrect, although it makes some sense intuitively. But it assumes that the divers move at the same speed as the water, without friction. Perhaps close enough, practically speaking, but theoretically?

 

[ianr33]

I agree with Vladimir. IMHO drag IS a factor.

I've observed this exiting Jackson Blue cave when the flow is up. Align yourself with the flow ,you move at a certain speed. Go sideways and you move faster.

Maybe it's time for an experiment. 2 divers next to each other on a drift dive. One holds up a "sail" (trashcan lid maybe) who moves faster??

 

[eab]

Maybe it's time for an experiment. 2 divers next to each other on a drift dive. One holds up a "sail" (trashcan lid maybe) who moves faster??

Sadly, no drift here or else I'd be out trying it! I will definitely be playing around when we hit Cozumel. Probably drive the DM crazy, but hey, I tip well.