Fine tuning of buoyancy distribution.

[tursiops]

Could very well be. Unless there was some material that is buoyant and extremely thin?

Well, the physics don't work out that way. The most buoyant material you can get is....nothing at all....just a vacuum in the water. And even then, the buoyant force is just the weight of the water in the volume now occupied by a vacuum. That means that if you want 2 pounds of buoyant force, you need to displace two pounds of water, which is 1/32 of a cubic foot, or about 54 cubic inches; this is a cube that is about 3.8 inches on a side, or a sphere that has a diameter of about 4.7 inches. (The sphere is the most compact shape to enclose a particular volume.) So if you wanted to lift your feet by about 2 pounds each, you'd have to tie a sphere of vacuum about the size of a big orange to each foot. Much easier to find lighter fins....they do vary by a pound or two.

 

[boat sju]

Agree, and we do this at times for OW students. Don't see how adding weight to the top area of a tank helps with heavy legs.

The valve wt is a counterweight to the heavy legs. It can actually help quite a bit. I use SP Jets in warm water and it balances them out.

 

[TMHeimer]

Well, the physics don't work out that way. The most buoyant material you can get is....nothing at all....just a vacuum in the water. And even then, the buoyant force is just the weight of the water in the volume now occupied by a vacuum. That means that if you want 2 pounds of buoyant force, you need to displace two pounds of water, which is 1/32 of a cubic foot, or about 54 cubic inches; this is a cube that is about 3.8 inches on a side, or a sphere that has a diameter of about 4.7 inches. (The sphere is the most compact shape to enclose a particular volume.) So if you wanted to lift your feet by about 2 pounds each, you'd have to tie a sphere of vacuum about the size of a big orange to each foot. Much easier to find lighter fins....they do vary by a pound or two.

Thanks, I get it. So maybe there really is nothing that you could invent to wrap around ankles for buoyancy.

 

[TMHeimer]

Well, the physics don't work out that way. The most buoyant material you can get is....nothing at all....just a vacuum in the water. And even then, the buoyant force is just the weight of the water in the volume now occupied by a vacuum. That means that if you want 2 pounds of buoyant force, you need to displace two pounds of water, which is 1/32 of a cubic foot, or about 54 cubic inches; this is a cube that is about 3.8 inches on a side, or a sphere that has a diameter of about 4.7 inches. (The sphere is the most compact shape to enclose a particular volume.) So if you wanted to lift your feet by about 2 pounds each, you'd have to tie a sphere of vacuum about the size of a big orange to each foot. Much easier to find lighter fins....they do vary by a pound or two.

Thanks, I get it. So maybe there really is nothing that you could invent to wrap around ankles for buoyancy.

 

[USFpsychDiver]

While 'sinky feet' can be an issue (I've had to switch from Scubapro Jets to DiveRites for most of my configurations), it can be misleading to think of one part of the body as too sinky or floaty, but rather to think about weight distribution about the entire body. If the feet sink too much then there's probably too much weight lower on the torso, perhaps from where the tank is riding and/or where the lead is. I've got small (XS Scuba) weight pockets on the shoulder straps of my BCs, held in place by triglides, so that they ride just behind my shoulders. If some configuration has me pitched up (feet down, head up), a one or two lb lead in each pocket brings the head back down. Yes, skills are important, but no amount of practice and training can balance an out-of-trim rig.

 

[tbone1004]

Here is why recreational divers have sinking legs. Three reasons
1. Starts with tank. They have their valves way too low, if you can't reach the valve when you reach back, the tank is too low. This makes the problem worse.
2. They dive with far too much weight, and where does this weight go? Their hips. This is made worse with jacket bc's because they have no inherent negative buoyancy so you have an extra 6lbs on your hips that you wouldn't need with a BP/W where it is nicely distributed from your shoulders to your waist.
3. They have terrible instruction courtesy of the dive agencies abysmal training of instructors and they don't know how to hover. If your thighs droop and your legs are straight out, you will ALWAYS be foot heavy. The only way to combat this is to extend the thighs so they are in line with the spine, and bend the lower legs 90* to move the moment arm closer to the center of the body.

If you don't do all three of these things, tank at appropriate height, proper weighting, and proper body positioning, everything else you put on there whether it be trim weights, gaiters, or the proposed ankle floats, are equipment solutions in search of a skills problem. Conversely, if you do all three, you technically should be ever so slightly head heavy with an AL80 as soon as you get below about 2000lbs, and this is where Jet Fins are helpful for backmount diving because it helps to bring your feet back to normal.

 

[TMHeimer]

I don't disagree with the last 2 posts on anything. Other than-- If in fact anyone ever does figure out how to invent ankle floats that don't inhibit fin movement, drag, I wouldn't say they would be an equipment solution to a skills problem. It's possible that that often used phrase is at times overdone on SB. Lost of equipment innovations could be seen as solutions--like the LPI. My brother dived a lot decades ago and says he doesn't like pushing a button to inflate the BC. But it does make it easier to add air descending without doing orally.Doing it orally may have once been a skill I suppose.
It just might be that "ankle floats" would be preferred by someone who likes his tank positioned where it is, wants to keep negatively buoyant fins because he likes them, etc. Who knows?

 

[67macoshark]

Here is why recreational divers have sinking legs. Three reasons
1. Starts with tank. They have their valves way too low, if you can't reach the valve when you reach back, the tank is too low. This makes the problem worse.
2. They dive with far too much weight, and where does this weight go? Their hips. This is made worse with jacket bc's because they have no inherent negative buoyancy so you have an extra 6lbs on your hips that you wouldn't need with a BP/W where it is nicely distributed from your shoulders to your waist.
3. They have terrible instruction courtesy of the dive agencies abysmal training of instructors and they don't know how to hover. If your thighs droop and your legs are straight out, you will ALWAYS be foot heavy. The only way to combat this is to extend the thighs so they are in line with the spine, and bend the lower legs 90* to move the moment arm closer to the center of the body.

If you don't do all three of these things, tank at appropriate height, proper weighting, and proper body positioning, everything else you put on there whether it be trim weights, gaiters, or the proposed ankle floats, are equipment solutions in search of a skills problem. Conversely, if you do all three, you technically should be ever so slightly head heavy with an AL80 as soon as you get below about 2000lbs, and this is where Jet Fins are helpful for backmount diving because it helps to bring your feet back to normal.

I do think there also could be a 4th reason. Some of us are inherently negatively buoyant. The controversial Fred Lanoue study shows a 12lb difference in buoyancy between individuals (-5# to +7#). I dive dry around home, and heavy legs are not a problem, but the annual family vacation in the Caribbean is a different story. I am looking for an equipment solution...

 

[archer1960]

I had big issues with sinking legs, but ended up resolving it by moving weight from my BC dumpable pockets, to trim pockets on the tank bands, and moving the tank band to a different set of holes in the BC's mounting plate. No weight added, just moved around.

 

[Dnaber]

I also think that when trying to fine tune your trim make small changes. I just adjusted my BC to sit lower on my hips and it has completely thrown my trim off. I'm now so azz heavy. My next trip in I will move my tank up to help get me back in trim.