ankle weight question

[TimAZ]

I can see how accelerating an increased mass repeatedly increases exertion as you fin, but that only applies in any measurable way when finning very hard, as in fighting a current (F=MA). This is the same condition in which trim becomes inordinately important to maintaining the narrowest profile possible. Seeing that every time relative speed doubles, the force of drag squares, I think you do almost whatever it takes to establish good trim and discard whatever "training wheels" used as skills develop.

 

[beezwax]

Luis, I am no physics guru, but I did study it for two years in school so I certainly understand what you are saying since it is quite rudimentary. But I'm not able to follow your point. It seems like you are suggesting that adding ankle weights would not impact the overall work necessary to move. I know the difference between kinetic/potential energy, and yes I know that thrust is opposed by drag, but it seems to me that more weight on our thrusters means more energy expended to produce that thrust. I know this is incredibly simplified and that there are a lot more things in play here-- we use fins for example because adding the resistance of that larger surface area to the power stroke equals more thrust. But when you say this:

....If your legs are heavy or buoyant the magnitude of the potential energy required to move them up and down can easily exceed the difference in kinetic energy caused by the use of ankle weights....

It still seems to me that that equals more total work. If your feet are negatively bouyant, then the force of their potential energy would act downwards and you'd have to work to overcome that (i.e. lift them). And vice versa if they were positively bouyant-- the potential energy would act upwards and you'd have to "work" to bring them down. The key question would be whether our feet (without ankle weights) are positive or negative or neutral. Because if they really were "floaty", then ankle weights would actually mean we'd have less work to do to produce thrust because they'd bring the feet closer to neutral by counteracting such floatiness. But I don't believe our feet are positively bouyant (unless you have really fat ankles). And when you add a couple pounds of lead to each ankle they most certainly become negative. The feet drop and your angle of attack may even increase. AND you'd have to work against that load every time you lift your leg.

If you think I've got this totally wrong please explain your argument again because I didn't get it.

 

[Luis H]

beezwax

You are not far of.

As I said: “The potential energy related to moving your legs up and down in the water will only be zero if your legs are independently neutrally buoyant.”

I can’t imagine anyone really needing ankle weight unless they have very buoyant legs, as with a heavy dry suit (but I am not ruling out other possibilities). With a drysuit the diver could be neutrally buoyant and in trim, but with its extremities being very buoyant. In this situation bringing the legs closer to neutral buoyancy will reduce the potential energy used during every kick stroke. This potential energy decrease can easily be much higher in magnitude than the kinetic energy increase.

The kinetic energy of your legs is never very high since you can’t move your legs that fast.

As I also mentioned:
“In any case when we are in a water media, the biggest energy related to propulsion has to do with the hydrodynamic forces (as in hydrodynamic drag).”
This hydrodynamic drag is also involved in the legs moving through the water, especially with the added bulk of a dry suit.


My real point is that nothing is just black or white. What seem like a simple answer doesn’t always apply under all conditions.

 

[Jason B]

I'll be happy to. When a diver is overweighted or has an underinflated BC, they will sink. To keep from sinking, they constantly swim up. They move forward, but with their bodies at an angle (head up/feet down). Divers who are underweighted or have too much air in their BC float up. To keep from floating up, they constantly swim down. They move forward, but with their bodies at an angle (head down/feet up).

Okay, just a misunderstanding. I thought we were talking about a body at rest (hovering), not propelling through the water. The poster said their buoyancy was fine and was now working on trim hence my thoughts.

 

[beezwax]

I can’t imagine anyone really needing ankle weight unless they have very buoyant legs, as with a heavy dry suit (but I am not ruling out other possibilities). With a drysuit the diver could be neutrally buoyant and in trim, but with its extremities being very buoyant. In this situation bringing the legs closer to neutral buoyancy will reduce the potential energy used during every kick stroke. This potential energy decrease can easily be much higher in magnitude than the kinetic energy increase.

Luis-- I think we agree on the physics here under the various possible circumstances, pos/neg/neutral feet. But now you've got me really confused with your comments about dry suit diving.... Your arms/legs only get bouyant in a dry suit if you let the air run down into them. Why would you do that? This goes back to what I said in my first post in this thread-- that the air bubble moves, and you can either use ankle weights to keep your legs down and the air out, or you can learn to control the movement of the bubble and then you won't need those weights. If a dry suit diver has floaty arms or legs they might be neutrally bouyant, but they are certainly not trimmed.

 

[CIBDiving]

....The key question would be whether our feet (without ankle weights) are positive or negative or neutral. Because if they really were "floaty", then ankle weights would actually mean we'd have less work to do to produce thrust because they'd bring the feet closer to neutral by counteracting such floatiness. But I don't believe our feet are positively bouyant (unless you have really fat ankles). And when you add a couple pounds of lead to each ankle they most certainly become negative. The feet drop and your angle of attack may even increase. AND you'd have to work against that load every time you lift your leg......

Your feet/lowerlegs are usualy a little negative ( I'd quess a couple pounds). It would of course very with the individual. But than again, a persons body is almost nuetral to within a few pounds ( at least if it's of a "normal" build ) - So why wear 25+ lbs of lead at all?
If you use a bag suit it has no bouyancy to speak of, so you don't need lead to make it sink.
What keeps you warm in a drysuit? It's not the clothing you wear - in very few cases does the 'clothing' provide Any warmth at all.

 

[beezwax]

CIB, I agree, I think. I'm not sure if there was a question I was supposed to reply to or not. Of course it's the air (or argon) in a dry suit that is your thermal insulation and that also necessitates the weights to counteract. But the whole point of this discussion has been where you allow the air to go in the suit, as in, keep it out yo feet. I'd agree that normal folks' feet would probably be a little negative, which is why I'd also think most folks wouldn't need ankle weights. I've got nothing against them-- one of my dive buddies who has way more dives than me uses them every time. I don't think they're necessary for the reasons already discussed, but if you like them, great.

 

[Luis H]

beezwax

When you are wearing a drysuit you have air everywhere. It doesn’t matter if you are wearing a neoprene dry suit or a bag suit you have air in your legs even if you are in a vertical position. That is why it is called a dry suit. The compressibility of the undergarment and loft of you undergarment will determine the minimum amount of air surrounding every square inch of your body.

Air and only air (or argon) is your thermal insulator. All the other materials are thermal conductors (from a heat transfer analysis stand point). The only purpose of the undergarment is to keep a lofty air space everywhere around your body.

Controlling the air bubble…never mind …that is totally secondary to my point.

Here in Maine in the winter I wear a 7 mm neoprene drysuit with enough undergarment to keep me warm. I have dived with and with out ankle weights and haven’t made up my mind which I like better. I dive many different configurations. I will continue to experiment, with an open mind. I have been experimenting for enough years.

 

[CIBDiving]

CIB, I agree, I think. I'm not sure if there was a question I was supposed to reply to or not. Of course it's the air (or argon) in a dry suit that is your thermal insulation and that also necessitates the weights to counteract. But the whole point of this discussion has been where you allow the air to go in the suit, as in, keep it out yo feet. I'd agree that normal folks' feet would probably be a little negative, which is why I'd also think most folks wouldn't need ankle weights. I've got nothing against them-- one of my dive buddies who has way more dives than me uses them every time. I don't think they're necessary for the reasons already discussed, but if you like them, great.

As was said - if you have insulation on your feet you have AIR there. It doesn't have to be a 'Bubble', in fact, it's not but it is there and in several pounds worth. All the underwere does is to give a space for that air to stay. Positive bouyancy ( and so trim ) in a dry suit is dependent on the underwear, it's thickness, and it's placement more than ANY other factor. Put on thick socks because your feet tend to get cold and you can change your trim by a couple of pounds! Add a thicker undergarment on your torso because you keep getting cold there and not only will you change your trim to foot heavy - you'll need a lot more lead also. Why? To float the extra air in the suit.
You can easily get foot heavy- add insulation up high and/or remove it from the legs.
But if you want minimal lead on you AND warm feet - ankle weights are the best freind you got.

 

[beezwax]

Hmmm, Luis I don't know why but for some reason it feels like we're on opposite sides of an argument...yet really we're not. Yes of course there is still air everywhere in a dry suit, I didn't imagine that when you go vertical your legs enter a vacuum. And yes I know it's the air that is the true insulator. The air BUBBLE may be secondary to whatever point you are making, but it is exactly the point I'm trying to discuss. The bulk of the air in your suit will rise to the highest point as you well know (unless some gets trapped around a bend). As you are also well aware, new dry suit divers aren't used to this moving bubble of air, and until they figure out 1) how its location and movement affects their trim and 2) how to manipulate it, it's pretty easy to get caught unawares with the bubble in the feet/legs and viola they are upside down. Floaty legs (and the comparative advantages or disadvantages of ankle weights as one way to prevent them) is what we're talking about, not whether air or your undergarment is what keeps you warm....