2 more upper keys dive fatalities, 8/6/2011

[dumpsterDiver]

Ahhh. Well I always start a dive ~10# negative. ~7# is gas in my 100CF and a couple or three helps me keep my sausage upright when I hang with a near empty cylinder.

You are going to DIE!!!

Obviously there is nothing excessively dangerous about a diver being able to attain 10 lbs of negative bouyancy at depth. So for once I am not in the minority of opinion???

 

[dumpsterDiver]

I really don't understand all of this. What is wrong with starting the dive 8 lbs negative from gas? .

What is wrong with starting the dive 8 or 10 lbs negative from gas AND lead...My whole point in this thread was to try to explain how it is conceivable that an experienced lobster diver, wearing little or no compressible wetsuit, may choose to wear a little extra lead to provide stability on the bottom (since he can not rely upon suit compression, as many people would while using a wetsuit at depth).

 

[DoNotDstrb]

Perhaps I could have chosen better wording.....

The weight of the gas in the tank is not really relevant to the point I was trying to make. If your tank is so positive empty, you have to make up for that with lead no matter now much the gas inside weighted when full. A 3,000 PSI AL100 is almost 3 pounds positive empty while being 5 pounds negative full. My 12L steels (about 100 cu ft) are 8 pounds negative full and near neutral empty.... I will have to carry an extra 3 pounds with the AL100. The AL100 weights 41 lbs. My 12L steel weights 32 lbs. The gas inside weights the same.

Diving with an AL100, all other things being equal.... I will be carrying a total of 12 extra pounds of mass (9 tank + 3 lead). That's 12 extra pounds I have to fin around for an hour (or whatever).... that takes energy which causes me to use more air and makes me less "well trimmed" .....

If you want a "real world" example of what that is like.... Pick up 12 pounds of lead and walk around with it for a while.... tiring, isn't it? In the water, this isn't so obvious because you are buoyant and you're not holding up the weight against gravity. Newton's 2nd law of motion, however, tells us that the amount of energy required to move 12 pounds (or any weight) is constant relative to the weight (mass) in question.... meaning that moving 12lbs of "stuff" takes a certain amount of energy....... the fact that in water the "stuff" is buoyant means little. I'm also not taking into account that the AL100 is a lot bigger physically.... the 3 pounds also makes YOU a bit bigger.... this translates to a lot of extra drag in an environment that is 800 times denser than air... further compounding the energy consumption problem.

This comment: "Diving 8-10 lbs negative is a bad idea under ANY circumstances......"

... was not meant to imply that this is DANGEROUS or that you are going to die (ROFL!) --- its simply a bad idea because you're wasting energy (and therefore gas) and making yourself less manouverable in the water.

Putting on EXTRA lead beyond your buoyancy requirements... like to "stay down with the bugs and work the hole better" makes the energy issue even worse AND can be dangerous if you are so heavy that you can't swim to the surface without ditching your weights as this unfortunate individual apparently could not do....

Forgetting all the physics... if you can't swim to the surface WITHOUT DITCHING ANYTHING... you are TOO HEAVY.... this is "OOA Plan A" -- a controlled ascent will full gear breathing from your buddy's octo. Ditching weights is "Plan B" and ditching the whole kit is "Plan C" -- you don't want to be in a situation where B better become A otherwise you die.

 

[Blue Sparkle]

The weight of the gas in the tank is not really relevant to the point I was trying to make. If your tank is so positive empty, you have to make up for that with lead no matter now much the gas inside weighted when full. A 3,000 PSI AL100 is almost 3 pounds positive empty while being 5 pounds negative full. My 12L steels (about 100 cu ft) are 8 pounds negative full and near neutral empty.... I will have to carry an extra 3 pounds with the AL100. The AL100 weights 41 lbs. My 12L steel weights 32 lbs. The gas inside weights the same.

Diving with an AL100, all other things being equal....I will be carrying a total of 12 extra pounds of mass (9 tank + 3 lead). That's 12 extra pounds I have to fin around for an hour (or whatever).... that takes energy which causes me to use more air and makes me less "well trimmed" .....

I'm still confused. So let me explain what I'm thinking and see if someone can perhaps figure out where I am going wrong.

I see where the AL100 + lead necessary to compensate for gas used will weigh close to 48# on land, whereas the HP100 (PST) will weigh about 41# for the same. [see chart; I am taking empty land weight of tank, plus the weight that one will "lose" by breathing the gas down]

So that is an extra 7# to haul around on land, PLUS since the aluminum tank is around 4.5# less negative when empty, a diver may need to carry another 4.5# with the aluminum tank, which makes for a 12# difference. I see that. On land.

But, I don't see where that equals 12# more in the water. I see only the "extra" 4.5# that a diver may need to carry because the aluminum tank is less negative (by 4.5#) so that is 4.5# that can't be "supplied" by simply diving the steel tank.

So it seems to me like a diver may, in the water, need to carry 4.5# separately (i.e. on a belt, etc. instead of being a part of the tank material), but that total "weight" (buoyancy) in the water will be the same.... won't it?

 

[DoNotDstrb]

I'm still confused. So let me explain what I'm thinking and see if someone can perhaps figure out where I am going wrong.

I see where the AL100 + lead necessary to compensate for gas used will weigh close to 48# on land, whereas the HP100 (PST) will weigh about 41# for the same. [see chart; I am taking empty land weight of tank, plus the weight that one will "lose" by breathing the gas down]

So that is an extra 7# to haul around on land, PLUS since the aluminum tank is around 4.5# less negative when empty, a diver may need to carry another 4.5# with the aluminum tank, which makes for a 12# difference. I see that. On land.

But, I don't see where that equals 12# more in the water. I see only the "extra" 4.5# that a diver may need to carry because the aluminum tank is less negative (by 4.5#) so that is 4.5# that can't be "supplied" by simply diving the steel tank.

So it seems to me like a diver may, in the water, need to carry 4.5# separately (i.e. on a belt, etc. instead of being a part of the tank material), but that total "weight" (buoyancy) in the water will be the same.... won't it?

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12 pounds of extra mass (weight -- whether in the tank, or part of the tank, or on a belt) is 12 pounds of extra mass. IOW, just because the whole rig, including you, is buoyant... it STILL takes more energy to move if it weights more. Try pushing a boat in the water... a bigger boat is harder to push than a smaller one.... but yet they are both buoyant... same principle... more mass takes more energy to move, even if buoyant.... and if that heavier mass is also physically bigger as an AL100 is bigger than a steel 100, it takes even more energy to overcome the added drag.

 

[Blue Sparkle]

Aha, okay, I see what you are saying. I'm not really used to thinking of it that way and I have been concentrating on buoyancy characteristics. Also, with, say, displacement-speed boats the weight really doesn't make that much difference to the speed/hp needed to push it (within reason) - it's more about length and shape. But then as human swimmers I suppose maybe we are sensitive to even small differences in "horsepower" needed?

Thanks for explaining.

 

[dumpsterDiver]

A lot of back pedaling from DND. He is making the arguement that the diver's goal is to minimize their total weight including body, gear and lead because the less mass you push the easier it is to move.

I suspect that this is a considerable over simplification and that the affects of bouyancy mitigate some extra mass to a very large degree and I also suspect that the configuration of the mass (i.e., steamlined shape and trim) has much, much more to do with the efficiency of the diver than an extra few % points of mass.

Also the analogy of comparing the burden of carry a chunk of lead around the house and then equating that to a comparable increase in effort underwater is "idiotic". If this is not intuitively obvious, then you haven't been diving much. This is one of the reasons why ships are so efficient for moving massive loads.

It is also the reason why some fat (or clearly over weight) divers can move very well through the water, often as well or better than skinny and weak people who would leave the fatties in the dust on a tred mill. Active and over weight divers often have strong legs from fighting gravity and when they get in the water the burden of an extra few dozen pounds becomes inconsequential.

If we follow this logic, people DEFINELTY shouldn't be diving doubles when a large single would do. Minimizing mass is a very useful goal when you have tough entries/exits and/or trying to navigate on a rocking boat, but worrying about a few pounds of mass in the water makes little sense to me.

 

[DoNotDstrb]

A lot of back pedaling from DND. He is making the arguement that the diver's goal is to minimize their total weight including body, gear and lead because the less mass you push the easier it is to move.

I suspect that this is a considerable over simplification and that the affects of bouyancy mitigate some extra mass to a very large degree and I also suspect that the configuration of the mass (i.e., steamlined shape and trim) has much, much more to do with the efficiency of the diver than an extra few % points of mass.

Also the analogy of comparing the burden of carry a chunk of lead around the house and then equating that to a comparable increase in effort underwater is "idiotic". If this is not intuitively obvious, then you haven't been diving much. This is one of the reasons why ships are so efficient for moving massive loads.

It is also the reason why some fat (or clearly over weight) divers can move very well through the water, often as well or better than skinny and weak people who would leave the fatties in the dust on a tred mill. Active and over weight divers often have strong legs from fighting gravity and when they get in the water the burden of an extra few dozen pounds becomes inconsequential.

If we follow this logic, people DEFINELTY shouldn't be diving doubles when a large single would do. Minimizing mass is a very useful goal when you have tough entries/exits and/or trying to navigate on a rocking boat, but woNotrrying about a few pounds of mass in the water makes little sense to me.

Not backpedaling.... just explaining further.....

Of course the analogy about carrying lead on land is far from perfect (I said so) -- it was just to illustrate that 12 pounds adds up. Fat divers that are used to moving 300+ pounds on land could very well be better underwater than a skinny but out of shape guy.... 70 lbs of gear added to a 300 lb frame is a much smaller percentage of the total than the same 70 lbs of gear on a 100 lb person. That alone would make a huge difference. Diving doubles increases both mass and size. You shouldn't dive doubles unless you need the added gas and/or redundancy for some concrete reason....

As far as "suspecting" what the extra mass does... there's no need to suspect..... Here's the math....

Newton's 2nd Law of Motion: F = M * A where F is force, M is mass and A is acceleration....

By following that formula... to accelerate and maintain a 200 lb object to a speed of 1 mph takes 38.7 Newtons of energy. A 212 lb object takes 41.03 Newtons.... an increase of 6.02% -- This equation does NOT take into account increased drag (the math gets very complicated with that one) which would have an even greater effect than the increased mass alone. I suspect that the same diver with an AL100 versus a steel 100 plus the lead to compensate would take about 15-20% more energy to move through the water. Whether that is "easy", or "hard", noticeable, or even a big deal depends on the fitness of the diver -- as you correctly pointed out...... That increase in effort would also translate to VARIABLE increases in gas consumption depending on the fitness of the diver.... HOWEVER, even the FITTEST diver WILL consume more gas pushing more mass....

The whole boat/ship analogy is not perfect either.... I was not talking about a ship moving under power versus some other type of conveyance.... All I said there is that you can shove a canoe around by hand with MUCH LESS ENERGY than my 35' Sea Ray..... Again to illustrate than an increase in mass takes more energy to move REGARDLESS OF BUOYANCY.

My only points in all these posts is that A) being overweighted at all is a bad idea because it wastes energy, makes you less manouverable, and needlessly decreases your supply of gas. and B) being SO heavy that you are glued to the bottom while working a hole CAN be unsafe IF you cannot EASILY swim to the suraface OOA with an empty BC and without ditching anything..... as the unfortunate gentleman that died evidently could not do.

 

[TSandM]

I think we are all in accord, that for the majority of diving applications, people should be properly weighted -- and that is defined by carrying the amount of weight that will allow them to control their depth in very shallow water, with a nearly empty tank. (The precise depth and psi preferred may differ.) I think a lot of us also agree that aluminum tanks are annoying, because they are physically heavy AND require more ballast. And a lot of people are also aware that, if at any point in a dive you are significantly negative, you need to be sure that, in the event of a single failure, you can get back to the surface . . . which means have weight you can ditch, or redundant buoyancy, or strong legs

There ARE diving applications where being able to get negative is useful. I dive at least five pounds overweighted when I dive with students, because being perfectly weighted means that, if I chase somebody who has lost buoyancy control up into the water column, when I catch them, I can't stop them. (Being more negative really helps me put the auger in, too!) It may very well be that lobster hunters are more effective when they can get quite negative, and there is nothing wrong with that, if they have considered the implications of gear failure, and provided for those contingencies. It would be my guess that the last thing a lobster hunter is thinking about is hydrodynamic efficiency in swimming

I think, in most diving situations, the difference in efficiency between a similarly equipped diver at one weight, and five pounds more, is not going to be noticed. The inefficiencies of the shape of humans in dive gear are far going to outweigh the change due to a couple of three pounds weights. The key to efficiency in diving (and extending your gas supply) is to SLOW DOWN, because resistance increases exponentially with speed.

 

[dumpsterDiver]

Not backpedaling.... just explaining further.....

My only points in all these posts is that A) being overweighted at all is a bad idea because it wastes energy, makes you less manouverable, and needlessly decreases your supply of gas. and B) being SO heavy that you are glued to the bottom while working a hole CAN be unsafe IF you cannot EASILY swim to the suraface OOA with an empty BC and without ditching anything..... as the unfortunate gentleman that died evidently could not do.

F=ma pertains to acceleration NOT swimming or moving at a steady speed. So leave newton out of it.

Again there is absolutley NOTHING wrong with being heavy on the bottom. Also there is absolutely no need to be able to swim up from this state as long as someone can ditch their lead (and then swim up). Where do you come up with this BS idea that somebody should be able to swim their rig up without ditching any lead? ANYONE who has dove in cold water knows that you are now spewing crap... A thick suit compressed at 100 feet is going to make you 12-15 - 20 lbs negative...I am beginning to doubt that you have a clue what you are talking about with all these sweeping generalizations..

Doesn't PADI say, always maintain neutral bouyancy at depth? If you are simply parroting that guidline, then fine, just say so, but your arguement is sounding more and more ridiculous.

Next you will tell me it is idiotic and dangerous to hold my breath when I take a picture or stalk a fish with a gun or camera because PADI says "never hold your breath".