Speed control during a drift dive

[del_mo]

Accepting the fact that one can control their speed while drifting by changing their depth, I heard a couple of things recently that my little pea brain is having a hard time accepting.

After a swift dive in Cozumel, a VERY experienced instructor/diver commented that he could speed up by sticking his fins out like a sail. Shortly after that a very inexperienced diver (but highly educated) mentioned she could slow down by sticking her fins out like a brake.

It seems to me that in theory it doesn't matter where you are spread eagle or in a cannonball, if you don't fin or paddle and stay at a constant depth, your speed will not change but continue at the speed of the current you are in.

 

[vel525]

Hmm, good question. I was trying to think when I was in Cozumel and it felt like when I spread my legs that I was slowing down. Not sure about the physics of it, I'm thinking it's drag. I don't know how you would accelerate faster than the current without kicking though.

 

[agilis]

Vel is correct. it is impossible to move faster than the water curent without inputting some complimentary energy, like fin work. On the other hand, it's easy to move slower than the current that is carrying you by using drag. For an obvious though extreme example, if you trailed your fins against the sandy bottom. It's something like being carried by the wind. The physics are more complex, but forward velocity is partly a matter of resistance, both positive and negative.

 

[ronrosa]

Vel is correct. it is impossible to move faster than the water curent without inputting some complimentary energy, like fin work. On the other hand, it's easy to move slower than the current that is carrying you by using drag. For an obvious though extreme example, if you trailed your fins against the sandy bottom. It's something like being carried by the wind. The physics are more complex, but forward velocity is partly a matter of resistance, both positive and negative.

Speaking of drag, how about your dive position ? If I am in a streamlined position, will the current move me slower ?

 

[Santa]

If you dump something into a current it seems to me it would initially take a certain amount of energy to fight the weight of the object before it was "brought up to speed". The shape, in terms of drag, would determine how quickly/efficiently this initial acceleration would take place.

But I don't see how you would be able to "accumulate speed" exceeding that of the surrounding current. Once propelled you are not relative to that mass unless you, just part of it. You wouldn't be moving through the liquid but with it.

Could be that the commotion of changing positions in itself might have a passing effect that way but I think it would be very limited.

But then again my subject is language ...

 

[halocline]

Drag against what? If you spread your fins out you're only increasing drag against the water flow, which is not moving in relation to your body if you're drifting. Now, if you've just ascended/descended into a faster current and you're moving slower than the current, sticking out your fins will speed you up, likewise the reverse is true...so both could be correct depending on whether you're moving faster or slower than the current when you do this. But you can't move slower than the current you're in just by increasing drag. Trying to move slower than the current means increasing your relative speed against water flow; increasing drag will make that more difficult.

 

[TeddyDiver]

After a swift dive in Cozumel, a VERY experienced instructor/diver commented that he could speed up by sticking his fins out like a sail. Shortly after that a very inexperienced diver (but highly educated) mentioned she could slow down by sticking her fins out like a brake.

Only a camera can slow you down so much it's hard to stay with the group even if they try to drift brakes on

 

[kidspot]

maybe not what you're looking for, but on a couple of drift dives I've done I turned into the current and just let it carry me along backwards - if I wanted to slow down I kicked a bit. For direction and extra forward movement I just did a backward kick, which is very easy with the current. I'd check my direction by looking back between my legs occassionaly. I was mostly having fun watching the other divers - sometimes they are more interesting than the fish

Aloha, Tim

 

[Hank49]

If you stay within a foot or so of the wall or bottom, you'll slow down because the water is slowed. Watch your bubbles is you stay close to the bottom, they'll go up more straight until they reach about 5 feet above the bottom and appear to shoot off sideways....in a real strong current.
But you'll go as fast as the water. I don't think you can get a sail effect like a boat in the wind, which CAN go faster than the wind going off a bit downwind.

 

[Cacia]

and then there is always the trusty reef hook, invented by a photoprapher...

you can flip on your back and dorsiflex your fins and get in river position and slow a bit...