Why exactly are jacket BCDs "better" for beginners? I never quite worked that out..
IMO, there's three factors:
a. They're a lot more comfortable than Horsecollars, which is what they replaced;
b. They permit a (higher and) more comfortable float position;
c. They continue to have easy/quick adjustment points;
The net sum of the above is that its the fast path to a setup that a novice will feel comfortable with.
I was diving double steel 72s on a plastic backplate with 6 lbs of lead attached to it. I had nothing on my belt, yet the tendency was still to tilt forward. Was I doing something wrong? Maybe. Even probably. However, for various reasons (not just the faceplant thing), I found that the BP/W setup was not right for me. Personal choice. I should have found a way to try one out before buying the wing. I lost almost a hundred dollars when I sold it.
Faceplant a myth? Hmm. Most realistic myth I've ever encountered, then.
You are not alone. I spent nine (9) seasons mucking with my wing trying to get the blessed thing to work as well - - for what I wanted - as my Jacket and Horsecollar BCD predecessors. I’m back in a Jacket now and while it isn’t perfect in all ways, it did reconcile the surface float characteristics that I didn't like the wing for.
There are millions of divers around the world and each one has a slightly (or markedly) different style of diving and the choice of gear will naturally reflect that style.
Agreed, but unfortunately, the small(?) percentage that have tried a wing but found them unsuitable for their needs and have said so, tend to be vigorously criticized by others, frequently with suggestions that they’re guilty of "doing something wrong" and/or are incompetant. The reality is that something probably is different, but the logical fallacy is that different doesn’t automatically equate to being wrong.
I have often heard someone advise a new diver to buy gear that can be used for more ambitious things like tech or cave diving, just in case the newb decides to go that way some time in the future. That's kind of like building a hangar for your Chevy just in case you decide to learn to fly and buy a Cessna one day. IMO, one should get what he or she needs, and will use and enjoy, now and let the future take care of itself.
Very well said, and I find this attitude (that everyone must become a jet fighter pilot) to be disconcerting and quite disappointing, because it carries an assumption behind it that ‘most’ divers eventually go in a particular direction ... with zero substantiating statistics to support their assumption.
Sure, some do go on, but
most don't...let's not permit the tail to wag the dog.
My perspective is that if we’re going to be brutally honest with novices and share with them the things that there are statistics for, the situation is quite bleak: there’s better than 50% odds that five years from now, their gear decisions are irrelevant, because they’ve dropped out of the sport. Whatever gear they bought will be sitting in the bottom of a closet, waiting for a garage sale to get rid of. Sorry, but that's the grim reality of this hobby, even before we consider what the percentages of (rec-vs-wreck/tech/cave) of the survivors of that attrition, and how there's stastical skewing of that population on discussion groups.
Having floatation forward of your lungs will prevent "face planting" if the weight is improperly distributed. Think of the moment arm (tau = r x F) between the weight and the weighted center of the sources of buoyancy (your body, your suit, and your wing). The greater the separation between the weights and the weighted center of the sources of buoyancy (i.e., at large r) the greater the tendency to tip (again, for improperly distributed weights).
Actually, having floation forward will do that regardless of weighting distribution. But I neverthless agree even though unfortunately, going into technical discussions of this sort has not been historically well received by much the SB audience, possibly because it has never been seen before in this form.
That leaves you modifying a static condition (weight distribution) with a dynamic condition (buoyancy in the variable-buoyancy parts of the system) though, right? So you are going to trim out differently depending on the inflation of the BCD....and the pressure in your tanks?
I'm trying to picture how that works out. Seems as though an empty tank will push you forward more than a full tank so you will need a fair amount more volume in your BCD, and will be bobbing on the surface like a cork.
Want pictures? Here you go:
http://www.huntzinger.com/dive/sb/BC_physics2a.jpg
http://www.huntzinger.com/dive/sb/BC_physics-centroid.jpg
http://www.huntzinger.com/dive/sb/BC_physics1a.jpg
FYI, no need to look at these links quite yet - I've applied these in the discussions below in their appropriate place.
OTOH if the buoyancy control was very near the tanks (short arm) it seems like it could vary the... 200 cubic inches or so... needed to offset the air in the tank without really changing the overall balance. If the full tanks didn't pitch you forward, the empty tank wouldn't pitch you forward, and you would be able to use the air in your lungs to establish a head-back posture on the surface (just as you do when floating in a swimming pool).
That seems like an argument for the BP/W.
Not quite – you got the moment arm’s directions transposed:
A full tank would be negative and using
the first illustration, result in a clockwise (‘face up’
torque. But make it now an empty tank (end of dive) and instead of it pushing down, it is floating up, so it will become a counter-clockwise (‘faceplant’
torque.
Ant to take the next step, if the torque is meaningful requires looking at all of the diver and all of his gear – what it all comes down to is that every piece of negative/positive gear will contribute (
detailed weighting example) to where their centroids are for center of mass and center of buoyancy. It is the distance between these two that creates torques that messes things up - - such as the imbalance that we talk about as being ‘trimmed’ during a dive (
here's the third one).
Expanding a bit further, it isn’t the absolute difference in location between the two centroids (although this helps): the piece we’re really interested in is the force breakdown of just the “orthogonal to the gravity vector” portion...and the key insight here is that as a diver rotates from horizontal to vertical, this value changes because gravity "moves" (so to speak). What this means is that just as we work to ‘trim’ out our dive while horizontally, we can go through the same exercise for when we’re oriented vertically...and hope (pray) that a change to improve one doesn't mess up the other.
Finally, this all is looking at the problem from a simple ‘static’ condition and not dynamic. Yes, a diver’s breathing will be an input that will perturb the system, and similarly, a diver can also choose to input some energy to counteract any undesired torque. For the latter, the question then can be asked of ‘for just how long?’, as people are not perpetual motion machines. Pesky Laws of Entropy!
-hh
