Larger people pushed faster in current?

[rx7diver]

You're all wrong! It's all about Bernoulli. More like a sail. Or an airplane wing. As the water flows past you, it creates higher pressure on your windward side and lower pressure on your leeward side, which creates lift (suction) so you are actually being pulled backward into the current. You are actually being pulled up-current, back to your entry point. The larger the diver, the greater the effect. This is so obvious. How can anyone be confused by this! Oh... wait...!

Safe Diving,

rx7diver

 

[TimAZ]

Based on your details, I have a minimum of almost 3,000 more dives than you, so I do have a bit of experience. This has been in cold dark waters of northern Scotland and Norway to the tropical water of PNG, Chuuk and last week, the far north Great Barrier Reef. Last week we did quite a few drift dives, and the observation was again, that smaller divers were a bit slower than the larger ones, and we were not finning at all.

I am fully aware that a badly trimmed diver will be affected more by current. I am talking about properly trimmed divers of different sizes (and even gear configuration). Even going with a current, a smaller diver will be more streamlined than a bigger diver or one with lots of gear. If streamlined, then logic says that a lot of the waterflow will go around you, starting from your fins.

Of course. Sorry pal. I assure you I meant neither to condescend nor offend; Basic Scuba Discussion thread and all, I was proceeding from a false assumption. FWIW, your logic seems sound. Happy diving.

 

[Lorenzoid]

You will if you maintain your buoyancy and quit kicking

True. But I believe everybody kicks a little, even on a passive drift dive. If you didn't kick, you would likely spin around. Differences in kicking and streamlining/trim together account for perceived differences in the speed of a group of divers "drifting" in a current.

 

[Bison Ravi]

I am 6'6" (about 198 cm) and 250 lbs (about 110kg). I swear I get pushed faster in current than other divers, who are of course usually smaller people. I frequently find myself being pushed ahead of the group and the DM for example, even when I am at the same distance from the reef and the same height in the water column.

Does anyone else agree?

I could bet money I know what is happening.

Many have said it already but many others don't understand it: In a drift dive it doesn't matter how big or heavy you are, you will go at the same speed than the current if you're not moving.

Once you accept that, you have to look elsewhere to find the problem.

Another thing most people don't realize, is how bad their trim/buoyancy is. Even most professional divers (DM, Open water instructors) very often have poor trim. Even a good proportion of technical divers have poor trim.

What happens when your trim/buoyancy is not perfect, is that you fin to maintain stability. Every time you fin, you move in the water. You won't notice it unless you train in a pool with references.

If your trim is worse that the others in the group, you'll fin more. And if you buoyancy is bad too, you'll fin twice as much.

Very often the experienced DM have really good buoyancy even if their trim is bad, so are very slow drifting in the current. The fact they are the ones leading, and the first to react (like slowing down or stopping to see something), creating an accordion effect makes it worse for all the people following.

Now maybe I'm wrong and you have good trim/buoyancy, but I see it happening all the time in drift dives...

 

[dmaziuk]

you aren't moving faster than the current, just at a higher proportion of the current speed than a less streamlined diver.

I'm sure Einstein would have loved this thread in general and this type of comment a-specially.

I'd expect in addition to everyone not just hanging in there perfectly motionless: we do move around to look at critters, make signs to each other, and so on, -- the current itself is not a steady-state even stream either. I guess once they get going, the heavier bodies should be less affected by minute speed changes in the current (inertia).

 

[Slow]

I'm sure Einstein would have loved this thread in general and this type of comment a-specially.

I directed some physicist buddies to this thread for morning amusement. They demanded apologies from me for ruining their day...

 

[tursiops]

I directed some physicist buddies to this thread for morning amusement. They demanded apologies from me for ruining their day...

And what was THEIR answer to the OP's question?

 

[KeithG]

And what was THEIR answer to the OP's question?

their answer will be that the "fast" people are finning with the current and the "slow" people are finning against the current.

i think too many people are over thinking this and getting confused by some red herrings. the system here is quite simple. a neutrally buoyant diver is only affected by the surrounding water flow.

the person with the most friction presented to the current will get up to speed soonest. once you are up to speed, there is no drag as you will be moving at the same speed as the water. so size does not matter at this point. the only way the diver can affect their speed is by pushing against the surrounding water. you can fin/scull either with or against the current.

 

[tursiops]

their answer will be that the "fast" people are finning with the current and the "slow" people are finning against the current.

i think too many people are over thinking this and getting confused by some red herrings. the system here is quite simple. a neutrally buoyant diver is only affected by the surrounding water flow.

the person with the most friction presented to the current will get up to speed soonest. once you are up to speed, there is no drag as you will be moving at the same speed as the water. so size does not matter at this point. the only way the diver can affect their speed is by pushing against the surrounding water. you can fin/scull either with or against the current.

Yes, I agree 99% with what you've said. But I wondered what those physics-friends said.
My 1% disagreement with you is that it is not friction that accelerates them in the current, but form drag, the old VxVxA. the more relative current, the more acceleration; the more frontal area presented to the flow, the more acceleration. Crappy trim will also increase their frontal area, as will big tanks and bulky BCs, so the bigger divers likely accelerate to the current speed more quickly.

 

[BRT]

Yes, I agree 99% with what you've said. But I wondered what those physics-friends said.
My 1% disagreement with you is that it is not friction that accelerates them in the current, but form drag, the old VxVxA. the more relative current, the more acceleration; the more frontal area presented to the flow, the more acceleration. Crappy trim will also increase their frontal area, as will big tanks and bulky BCs, so the bigger divers likely accelerate to the current speed more quickly.

So a dead whale will accelerate to current speed faster than a dead minnow?