BP/W: I officially don't get it

[Adobo]

Exactly, and even though you can put trim weights on the cambands, it's not as good as having the ballast evenly spread out against your back. That's ideal weight distribution, especially with an AL tank, and I can definitely tell the difference between using my steel plate as opposed my AL plate with camband weights.

Why is that ideal? I recently dove a single tank rig.. I had weights in trim pockets and on my waist harness. I also had weight on a weight belt. It was the weight distribution that gave me the feeling of being balanced from head to toe. Should I be going for having the weights evenly distributed on my back or should I be going for having the weights distributed such that I can achieve the desired trim? (These two things aren't always the same.)

The soft harness BCs simply don't couple the tank to your back in the same way.

Is that ideal or is that preference?

 

[peterbj7]

To adobo's list of benefits, I would add the stability of the tank on your back. Soft BCs just don't give the same sensation of tank, BC and diver being a seamless whole.

Why is that necessarily ideal? I don't know about you, but my spine bends quite a lot and I don't like being in a straitjacket that prevents that. That's probably my main reason for not being keen on backplates - the rigidity that is supposed to be their main point.

 

[halocline]

To me the steel plate's distribution of weight over your entire back is preferable to having weights in camband pockets, that's what I meant by ideal. I meant it's the ideal way to put the weight on your back. I was not saying that all divers should have all weight on their backs, sorry I realize that my wording could have been better.

My statement about soft BCs not coupling the tank to your back in the same way as a rigid plate is not "ideal or preference" it's an easily demonstrated fact. A rigid plate acts differently than a soft surface. What were you referring to?

 

[halocline]

Why is that necessarily ideal? I don't know about you, but my spine bends quite a lot and I don't like being in a straitjacket that prevents that. That's probably my main reason for not being keen on backplates - the rigidity that is supposed to be their main point.

Well, the only problem with this logic is that you still have a rigid surface against your back, the tank. The difference is that it's now a single point of contact along your spine able to roll around rather than a flat surface that spreads the contact out over a large area.

I would hardly characterize a hog harness as a "straightjacket", in fact, it's lack of clutter and simple waistbelt promotes a greater freedom of movement; there's simply much less in front of you. Contrast this with a vest and cummerbund that wraps around your midsection, which I find much more constricting.

 

[D_B]

Well, the only problem with this logic is that you still have a rigid surface against your back, the tank. The difference is that it's now a single point of contact along your spine able to roll around rather than a flat surface that spreads the contact out over a large area.

this must refering to soft BC, not ones that have a "backplate" inside like a Knighthawk

 

[MSilvia]

Why is that ideal?

It's ideal because it keeps the weight and lift in the same place, so your trim is static even when you change the volume of gas and alter your buoyancy. Think of a diver as a seesaw. In order to be horizontal, there needs to be equilibrium in the forces applied to each side of the center of balance. If your lift and ballast are collocated at the center of balance, you can apply upward or downward force without tipping the seesaw out of horizontal orientation. If your ballast and lift is distributed at various points across the board, applied force at those points will cause the board to tilt.

In other (oversimplified) words, if you have your weight directly above your lungs, you can inhale or exhale without changing your trim. If you have your weight on a belt, you might be trimmed out with half-full lungs, but a deeper breath will act like a lever and fulcrum, and tilt you head-up (or vice-versa for fully exhaling) unless you compensate by dynamically adjusting the center of balance.

Should I be going for having the weights evenly distributed on my back or should I be going for having the weights distributed such that I can achieve the desired trim? (These two things aren't always the same.)

It isn't an either/or answer... it's both. You should be going for having the weights as close as possible to your lungs while still achieving the desired trim. Trim is more important, but the greater the percentage of ballast is located above (or below) your lungs/wing, the easier that trim is to maintain.

 

[MSilvia]

I would hardly characterize a hog harness as a "straightjacket", in fact, it's lack of clutter and simple waistbelt promotes a greater freedom of movement; there's simply much less in front of you. Contrast this with a vest and cummerbund that wraps around your midsection, which I find much more constricting.

Agreed... in my experience, a jacket BC is much more restrictive of movement than a BP/W. In fact, proper horizontal diving technique for a BP/W requires the diver to arch their back while wearing it. Claiming it prevents a diver from bending their spine is total nonsense.

The only effect of the plate's rigidity I notice is that it keeps the tank(s) from flopping around.

 

[peterbj7]

In the case of a single tank and no backplate there is no pressure on the back of my shoulder blades, something that I welcome. And I have never had trouble with a decent jacket BC properly fitted with the tank "rolling around" on my back. With some very cheap and ill-fitting rental ones, yes. Mine has a shaped plastic holder to locate the tank positively.

With doubles the effect is less because the tanks are further apart, but I still feel less constrained with a soft harness than with a backplate. On occasion I have dived with more than two tanks in a line across my back, and in those cases there is indeed no alternative to a backplate.

I do still have a backplate rig and use it, and it has manifolded steel 95's so it is quite heavy, but I use my Transpac with manifolded aluminum 80's perfectly happily. I am not in any way opposed to the use of backplates, but to the idea that there isn't any sensible alternative. I suppose I am fundamentally opposed to the prescriptive DIR/GUE approach, and much more attuned to the more relaxed "laissez-faire" approach of the training I had.

 

[MSilvia]

I am not in any way opposed to the use of backplates, but to the idea that there isn't any sensible alternative.

I don't think you'll find anyone sensible suggesting that backplates are the only way to dive... especially for recreational single tank divers.

I suppose I am fundamentally opposed to the prescriptive DIR/GUE approach

I think it's a very sensible approach when it comes to expedition dives, but I think it's overkill for the vast majority of recreational divers. There's certainly no harm in applying the methods to recreational dives if that's something that interests folks, and there's a lot to be gained by becoming familiar with it, but it isn't necessary for recreational divers by any means. Although I took a GUE fundamentals class and learned a tremendous amount, I don't dive DIR either, and don't assume any connection between BP/W and any particular training agency or methodology.

 

[Mayor]

Here is a diver that is just ok in the water but you can see the difference from RI to BP/W. Take it easy I know the diver!

Rear inflation / steel hp120 / 4lbs added / 80 feet / Cozumel

BP/W / Alum plate 2lbs / STA weighted 6lbs / Alum 80 / 6lbs added / 60 feet / Turks & Cacios