Why does a bp work so well?

ok, so i just switched to a bp/harness setup and tried stuff out in the pool and in open water. damn this thing rocks. orientate yourself in any manner and you just stay there. this thing is just flat neutral. doesn't tip your head forward, doesn't let your legs drop. my trim improved 100% when hovering. no more resting fin tips on coral when i am not finning. it was neutral from full tank to empty (in the pool of course), 0 inflation to 1/2 full.

so my question is, how does it work so well?

i used to have a back inflated bc. was great except that my legs would drop when i'm hovering. in fact, it would tip me over till i'm about 30% facing the sky if i completely relaxed. had tried inflating a little bit to see if the air trapped at the bottom of the wing would help - not really.

then i went back to the pool and tried experimenting to see if i could achieve the same horizontal trim as a bp/harness with a rental jacket bc. i placed weights at various positions along the tank to see if it would work. to cut a long story short, only way i could keep perfect horizontal trim while hovering was if i held the weights with my outstretched hands, in front of me. quite tiring after a while . at that point, it was hard keeping my balance anyway, with the tank wobbling on my back and my flimsy 90 lb weakling arms trying to hold the weights steady.

i had even tried moving the tank further up so that the cg of the tank would be at about the midpoint of my back (my estimated cg). doesn't help either. legs still dropped.

then i thought maybe it was because the bp/harness is an exact fit while most other bc's don't stick to you as much. so i used a smaller size and tightened everything so much that i could hardly breathe. on top of that, i pulled on the jacket with my hands so that it would stick to me. nope, doesn't work either.

this seems to be a very interesting design issue to me. can anyone shed light on this?

I can shed some light on this.

Check this link.

http://www.islc.net/~bayne/homebc.html

SeaJay once bubbled...
I can shed some light on this.

Check this link.

http://www.islc.net/~bayne/homebc.html

Click to expand...

You've been waiting for someone to ask that question haven't you

Actually, there was a huge thread about it.

If I recall, the resulting flames weren't much different than today's. Seems that people really have an issue with pertintent, correct information.

Flames? What flames?

I don't see anything here that anyone could consider flames. In the other thread, I offered suggestions to make your excellent work more accurate. I see it is still misleading, but still has good information.

I'm thinkin' that it's a little hot everywhere in the world right now.

*sizzle*

thanks seajay! looks like a lot of effort went into this. in fact, i was thinking along the same lines as well.

but i had trouble working out what would be the "correct" representation of what is going on. i will use myself as an example as i am familiar with my own buoyancy and have relatively "sparse equipment".

setup:

3 mm shortie wetsuit this is compressed till it is more like a 1 mm skinsuit. buoyancy is about 1/2 lb at the most.

27 lb wing, aluminium bp/harness setup

80s single aluminium tanks

No weight belt and no weights worn.

Okie, here's why I am confused about why the bp/harness works better than my previous back inflated bc

1. i do not wear weights in both setups. so there is nothing to rotate me about my center of gravity, so to speak

2. main difference between bp/harness and back inflated bc, is the addition of a piece of metal. bc material is pretty much neutrally buoyant so we can leave it out of the equation

3. objects only rotate when the cg is away from the point of support, in this case, center of buoyancy. in both setups, without the aircell inflated, only point of support is in the center of my chest (lungs).

4. CG of the piece of the metal is at about the same point as my ccenter of buoyancy (in head to toe orientation). Since it is sandwiched between me and the tank, will assume that cg of the plate is also in line with cg of the systme (front to back orientation)

From my description above, there is no qualitative difference between a bp/harness and back inflated bc setup. But the fact remains that it is near impossible to keep my legs up while hovering in a back inflated bc while it is effortless in a bp/harness.

And check this out. Every now and then, I would carry an extra weight in case the divers following me are underweighted at the end of the dive and need it. This is simply thread into the waist webbing before it passes through the buckle.

By right, since my center of buoyancy is in the middle of my chest (wing not inflated at all) and the weight is low down on my waist, my legs should rotate downwards right? Well, empirically speaking, there is no change in buoyancy characteristics!

Finally, when I do inflate the wing (say 1/4 to 1/2), the air bubble in the wing should change the center of buoyance depending on whether i am pointing head down or head up right? In fact, say i am point head up, the bubble moves to behind my head, cg is below that, resulting moment about the pivoting point (center of buoyancy, which has now moved up), should rotate me until i am up right isn't it? If I were pointing head down with the air bubble around my ass, then the opposite should happen right? Have tried both and there is no change.

In short, I'm still kinda confused as to how the balances are maintained, regardless of air added, changes in tank buoyancy and difference with a back inflated bc.

All of these things come into play.

I only put one example of configuration in the explanation, and the one I chose was the one I am most familiar with.

But the ideas are still the same. I'm sure that if we were to sit down and figure it out, we could explain precisely where your center of buoyancy and where your center of gravity were. Simply put, the closer they are together, the more "neutrally trimmed" you're going to feel.

Your case would be especially forgiving of the differences between where the center of buoyancy are and where your center of gravity are, because of the fact that you tend to dive with less material that is buoyant, and less weight as well. Thus, the forces of each are reduced substantially, and differences in distance between the two create less force than they would if there were more weight or more buoyancy.

There's more, too... Some people wear ankle weights or have fins which are substantially negatively buoyant. (Jets and Turtles being the primary example.) These are especially nice for "trimming yourself on the fly" underwater. A simple bend at the knees, plus or minus, helps to control your attitude in the water, and so it's easy to compensate for whatever little bit of tendency there is for you to tip one way or the other.

I think it's the fact that you are diving with such sparse equipment that makes it easier to control that trim... So you're able to fudge a bit either way and still compensate.

...Which in and of itself explains why it's encouraged to minimize your gear... Especially the amount of weight you carry. That way, you can use less air in your BC to be neutral, and the two forces at work are lessened, and it's easier for you to compensate.

Additionally... And here's where the magic really comes in... Changes in depth, which compress or uncompresses the volume of air in your BC, have less effect on the overall buoyancy. For example:

Diver A wears enough weight to be 18 pounds negative at 66 feet... To be neutral, he puts 18 pounds of air in his BC. If he then swims to 99 feet (but doesn't change the amount of air in his BC) his air compresses by 1/3. In other words, he's now got 12 pounds of air in his BC, and he's got to compensate for 6 pounds. This can't generally be done by filling his lungs, since generally there's not an extra 6 pounds of buoyancy available in his lungs. So he's got to add 6 pounds of air to his BC... Or more commonly, he goes vertical and swims up 6 pounds all the time, silting the bottom and generally having a tough dive. He also uses up his air a lot faster.

Diver B wears enough weight to be 6 pounds negative at 66 feet... To be neutral, he puts 6 pounds of air in his BC. If he then swims to 99 feet (but doesn't change the amount of air in his BC) his air compresses by 1/3. In other words, he's now got 4 pounds of air in his BC, and he's got to compensate for only 2 pounds. This can easily be done by filling his lungs, and no additional air needs to be added to his BC, yet he doesn't have to be vertical in the water column.

The difference is even more dramatic if the diver wears even more weight, and it becomes even MORE dramatic at the surface.

To answer your question on why, exactly, you find a difference between a back inflate and a bp/wing:

AL backplates are 1 pound negative in the water. The plastic backplates found in most back inflates are about 1 pound positive in the water. If you were to float each rig (without a diver attached), I think you'd find that a bp/wing with an AL plate wants to float naturally in the correct, horizontal position. I think that a back inflate would want to float upside-down.

I think that this two pound difference is what you're feeling.

At least, that's my hypothesis.

Having the weight moved off your hips and onto your back seems to be one of the main reasons the BP works so well.

Not having all that random crap hanging off you helps too...

yeah seajay, i think you're right about the reduction in weight that needs to be moved around. i'm sure having more weight distributed in various places will screw things up more. just that little je ne sai quoi that i'm bothered about.

in my case, we're talking about 1 lb of buoyancy either way that needs to be "managed". i tried adding 2 lbs to my belt, breathing out all the way and not wearing my wetsuit, so that i could inflate about half of my bc and still be neutral. we're talking about 12 lbs worth of buoyancy in the wing here (at about 1.5 m). moving that air bubble around doesn't seem to affect trim very much.

i'm still totally amazed by this.

ElectricZombie: i thought that was the case too. but not sure if that's the whole story. my backinflated bc has two non-ditchable weight pockets at the back (around the middle of my chest height). I used to have 2 lbs in each pocket, back when i was a newer diver and used more weights. didn't have any weights around my waist. i remembered not being able to keep horizontal trim while in a stationary hover as well. weird huh?