Fine tuning of buoyancy distribution.

[REVAN]

@REVAN I think the point is most of us believe we don't want to swim fast while on scuba. It burns air faster, you see less stuff, and other than fighting currents, serves very little purpose. In that regard, hydrodynamics simply don't matter much, despite all the "streamlined" nonsense you read on scubaboard. That's mostly about not dragging stuff on the bottom/reef rather than actual swimming efficiency.

I know, but you are wrong and I'm going to attempt to change what you believe.

 

[fjpatrum]

Sorry, you will never prove that swimming faster burns less air. You may be able to prove efficiency can increase speed without burning more air.

You also will never convince me what I want to SEE can be seen better at a faster pace. To even assume that you could know what I want to see takes a level of arrogance (or ignorance) I don't even want to consider. I will give you the benefit of doubt that you are not doing that and you are just meaning more efficient diving will be better, which I can at least agree to in theory.

 

[sea_ledford]

A couple points:

Streamlining absolutely makes a HUGE difference in both speed and efficiency. Look at the great lengths and cost triathletes go through to get their bikes and riders as streamlined as possible, and they are moving through a much less dense medium than we are in water.

Sometimes getting from point A to point B as fast as possible is what you need to do. There is a lot of very fast (relatively) swimming going on in cave diving for example. You might not want to blow over the reef you want to see, but if that reef is 1/4 mile offshore over sand flats, you'll want to get out there as fast as possible. If adding some silly looking tail cones to your tanks and emphasizing hydrodynamics in your gear would allow you to get 800 ft further before reaching your turn pressure, would it be worth it?

Back to the OT, instead of moving weight around to counteract sinking feet (or worse adding additional weight you don't actually need), learn to use your muscles to shift your center of gravity closer to you head. This is a totally cheesy swimming video, but it works:


Basically, flex your back and use your hamstrings and butt to bring your legs in line with your body.

-Chris

 

[tursiops]

Sorry, I have no desire to go faster or further. I already know the hydrodynamics can be more efficient, but this is relevant only at speed. Some of my best dives have been for an hour in a area smaller than my living room. Some of you guys are obsessed with your own ideas, and are not listening.

 

[fjpatrum]

A couple points:

Streamlining absolutely makes a HUGE difference in both speed and efficiency. Look at the great lengths and cost triathletes go through to get their bikes and riders as streamlined as possible, and they are moving through a much less dense medium than we are in water.

Yes, and they are also traveling at significantly higher speeds (20+ times as fast) where the streamlining actually matters. At speeds people use for SCUBA, generally, it's completely unnecessary and has very little impact. The higher the speed the greater the impact. The only exceptions I can see would be fighting currents or trying to chase sea-life while filming.

Sometimes getting from point A to point B as fast as possible is what you need to do. There is a lot of very fast (relatively) swimming going on in cave diving for example. You might not want to blow over the reef you want to see, but if that reef is 1/4 mile offshore over sand flats, you'll want to get out there as fast as possible. If adding some silly looking tail cones to your tanks and emphasizing hydrodynamics in your gear would allow you to get 800 ft further before reaching your turn pressure, would it be worth it?

Most of us just surface swim in these instances, where tail cones and efficiency have zero bearing at all. When the tank's out of the water, a tail cone isn't doing anything for you. And again, at the speeds most people can swim, it will have very little impact. These streamlining efforts are expensive, cumbersome, and will provide very little return on investment for a scuba diver. When speed really is your concern, that's when these count. If you're scootering, add the tail cone, streamline your gear, and make it worth it. If you're using your own power, you're never going fast enough for it to matter.

 

[tbone1004]

I go 50fpm in a cave, I was unaware we are trying to break speed records. In fact, the fastest I'm aware of people kicking regularly is around 80fpm, which is still not that fast... If we need to go faster we use a DPV.... If there is a 1/4mile kick to get somewhere, I'd do it on my back on the surface where I can go much faster than I could underwater without having to worry about how much air I use....

 

[REVAN]

Sorry, you will never prove that swimming faster burns less air. You may be able to prove efficiency can increase speed without burning more air.

Well, I actually was implying your second sentence above. Through better hydrodynamics, you can be more efficient, thus allowing for faster swimming speeds while at the same time using air at a lower rate.

But, it is also possible to be more efficient even without hydrodynamic improvements, lending a potential falsehood to your first statement as well. To prove this, just look at the limits. Take a diver at rest. What is his air consumption per foot traveled? It is infinite. Now take the same diver swimming at 60 fpm. In that case, the cubic feet of air used per foot traveled will be a real number, i.e. - non infinite. Faster can be more efficient!

You also will never convince me what I want to SEE can be seen better at a faster pace. To even assume that you could know what I want to see takes a level of arrogance (or ignorance) I don't even want to consider. I will give you the benefit of doubt that you are not doing that and you are just meaning more efficient diving will be better, which I can at least agree to in theory.

Hydrodynamic efficiency simply opens more options; more options for things to see, to follow, to get to, places to be or to extract yourself from. It is well within the scope of operations to go as slow as you want, including hovering at a standstill or crouching in the sand. However, as soon as you start moving, regardless of how fast or slow that is, a diver with poor hydrodynamics will always have to produce more physical power to move at any given speed than would a diver with better hydrodynamics. This is true at 30 fpm, and also at 120 fpm. At some point the dirty diver will limit out and have no more power available to go faster.

What do you do if you find yourself in a statistically common current moving at about 200 fpm? Give up, and hope someone else saves you? That's what divers generally do these days, and most of the time, if they are diving from a boat with a good crew it works. But, it doesn't always work out. With good hydrodynamics, I can swim at 240 fpm and still make 40 fpm progress against the current. That may be enough to get me to where I need to be to extract myself from the situation without incident. If it doesn't work, I still have the option to quit and hope someone picks me up, but I would consider that situation a failed dive plan, not a standard M.O.

 

[sea_ledford]

Sorry, I have no desire to go faster or further. I already know the hydrodynamics can be more efficient, but this is relevant only at speed. Some of my best dives have been for an hour in a area smaller than my living room. Some of you guys are obsessed with your own ideas, and are not listening.

To be clear, I'm actually not advocating for hauling ass underwater. I'm more of a slow diver myself. I'm just saying there are times when moving quickly can be a priority. Even if it's just to get from one patch reef to another 100 feet away.

Yes, and they are also traveling at significantly higher speeds (20+ times as fast) where the streamlining actually matters. At speeds people use for SCUBA, generally, it's completely unnecessary and has very little impact. The higher the speed the greater the impact. The only exceptions I can see would be fighting currents or trying to chase sea-life while filming.

Probably closer to 40x as fast on average, but the density of water is almost 800x that of air, so it actually affects divers at normal diving speed more than it does to an elite athlete going 40 mph. But they are also racing, so speed is the goal... not so much for diving. Efficiency should be a priority though.

Most of us just surface swim in these instances, where tail cones and efficiency have zero bearing at all. When the tank's out of the water, a tail cone isn't doing anything for you. And again, at the speeds most people can swim, it will have very little impact. These streamlining efforts are expensive, cumbersome, and will provide very little return on investment for a scuba diver. When speed really is your concern, that's when these count. If you're scootering, add the tail cone, streamline your gear, and make it worth it. If you're using your own power, you're never going fast enough for it to matter.

I think we are just used to the amount of drag that is standard with a set of scuba gear and think it is insignificant at the speeds we are going. It certainly does matter though. Ask any freediver how much it matters. Would you rather do that 1/4 mile surface swim in scuba gear or snorkeling gear? it's not the weight that makes the difference, it's the drag. But scuba divers have a lot of gas to waste, so it isn't of as much concern.

Now would there be "stuff" to add to scuba to make it more efficient, and still be worthwhile? That I don't know, but probably. We haven't even gone after the low hanging fruit on hydrodynamics. I am surprised that doubles don't have covers though. There is probably some reason that they stopped being used... perceived risk of line entanglements or something. Or maybe aqualung just copyrighted it.

-Chris

 

[cool_hardware52]

But, it is also possible to be more efficient even without hydrodynamic improvements, lending a potential falsehood to your first statement as well. To prove this, just look at the limits. Take a diver at rest. What is his air consumption per foot traveled? It is infinite. Now take the same diver swimming at 60 fpm. In that case, the cubic feet of air used per foot traveled will be a real number, i.e. - non infinite. Faster can be more efficient!

You are aware that delta drag is a square of velocity and delta power is velocity cubed?

I've spent a bit of time at greater than 200fps using high performance DPV's and it's very challenging.

Tobin

 

[REVAN]

You are aware that delta drag is a square of velocity and delta power is velocity cubed?

Yes.

I've spent a bit of time at greater than 200fps using high performance DPV's and it's very challenging.

I assume you mean 200fpm. It is very challenging when you are very draggy. For contrast, dolphins do 200 fpm when they are sleeping. So, the level of challenge for a particular speed is very dependent on the total drag of the system.

I'm working on an article regarding all of this. I'll link it to a thread in the "basic scuba discussions" section when it is ready.