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We love diving, whether it's technical diving to explore deep wrecks or just sport diving to search for seahorses or nudibrancs, we can't get enough. We also like to try different equipment and enjoy sharing what we have experienced and learned. We hope you find our blogs interested and that they help you make the most of your equipment and diving.  If you like our blog posts, please feel free to 'like' them and share them with your friends.

 

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Backplate and wing versus jacket BCD

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With such a topic there will be opposing views and I don’t aim to convert the unconvertable.  This story is about my experience to find ‘the right BCD for me’.

I learned to dive in a simple jacket-style BCD. Dive shops like these for training, simple with not much to explain.  My first BCD was a jacket with integrated weights, slightly more complex but otherwise much like I had learned with.

Technical diving introduced me to the backplate, harness and wing (BP&W) BCD. Initially I didn’t think much about it, probably because I was so busy with procedures and safety drills that I didn’t give it a thought. The only thing I recall being inconvenient was a lack of pockets, but my jacket pockets had mostly been taken up by the integrated weights anyway, and I soon got some leg pockets.

My first tec BCD was a harness and wing without a metal backplate, but I soon added a stainless steel one. It was very well made but with back-mounted twins, a pony, reel, and leg pockets I accepted that there was just too much stuff to be streamlined.

Hoki_1_sm

Anyway, diving has always been about exploring and finding something new; there were wrecks in Truk Lagoon and all this equipment was accepted as the price of admission.

Later, when using the BP&W with a single tank, it became obvious that this configuration really was more streamlined.  Experiments ensued: a complex harness versus simple webbing, moving the tank position, relocating weights for trim, and so on.  Unexpectedly, the simpler the system the better it performed.  Then came a home-made sidemount experiment – not perfect (far from it) and a challenge for the (then) untrained, but the result was astounding, real streamlining did exist and gliding was fun!

Sidemount diving is another story but I still do a lot of single tank backmount dives, teaching courses, or just drifting around the shallows taking photos.  However, even for simple dives I want the best trim, buoyancy and streamlining that I can get, so I achieve lower air consumption, better distance and longer bottom times (enabled by Nitrox if need be).

My original jacket BCD is of good quality and it looks like new even after years of use, but it now feels like a parachute underwater and it only gets wet for open water courses.  All other backmount dives are now with a BP&W system, and these are the main reasons why:

  • When trimmed there’s a lot less frontal area than a jacket, and a lot less drag.  A BP&W, properly configured, really is streamlined - there’s less effort when you glide after every kick.
  • With a wing on the back it’s easy to obtain horizontal trim in the first place.  A few jacket BCDs have rear inflation for this reason too but are still bulky.
  • A BP&W system is easily customised.  Change wings for single or twin tanks and keep everything else in place. Add D-rings to connect accessories wherever it works best.
  • Without a jacket that encircles the diver, a webbing harness gives a great sense of freedom.
  • In cold water with a drysuit, a heavy stainless steel backplate reduces the lead needed and it distributes that weight evenly.

I also use a modern wing design that’s narrower across the top and bottom and shaped down the sides, in contrast to my first ‘old style’ wing (an oval shape with even thickness all round). For about 3 to 4 litres of buoyancy, the old style wing leaves much of air between my head and the tank valve, so the weight at my waist drags my feet down.  The narrow top of the new wing forces the same amount of air into the sculpted side sections, moving the buoyancy closer to the centre of gravity and making trim easy.  I like easy, and I have no desire to fight my BCD for decent trim.

But what about those missing pockets?  I now use a dive pouch that I can fit more into and clip onto a shoulder strap D-ring and have a good look inside – try doing that with jacket BCD pockets!

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Which Wing? How much buoyancy?

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One of the advantages of a backplate, harness and wing system is the ability to choose your own wing and change wings when you need to. BCD jackets are fitted by diver size with no way to change the buoyancy for the diving that they will actually do. But if a harness and wing system is being considered for its advantages, a common questions is: which wing do I need?

While they look simple, wing design is somewhat more complex and minor changes in shape affect the diver’s trim and streamlining. Materials and construction are important – a single fabric layer is light weight and great for travel BCDs and many jacket BCDs, but if you plan to penetrate wrecks or caves you’ll want double-layered construction with a tough outer shell to protect the inner bladder.

However, let’s just consider the most basic factor and purpose – buoyancy.

The lift capacity of buoyancy compensators may be specified in litres, kgs, lbs and/or even Newton-metres, but how much is enough lift?  Unless you dive with no weights, the answer depends on how your buoyancy changes during a dive. The function of a BCD is to compensate for those changes with a safety margin, not to carry weight unnecessarily.

Correct Weighting

Sadly, many divers dive over-weighted and lift from their BCD is used to compensate for lead ballast that they don’t need.  When did you last do a buoyancy check? At the end of a dive with minimum gas in the cylinder (approx. 50 bar), can you hover at 3 metres with almost no air in your BCD? If you sink, take out some lead and try again next dive.

Gas Use

Now, assuming that a diver is correctly weighted, how much does buoyancy change during a dive?

As you use air / gas during a dive your cylinder gets lighter (the aluminium or steel cylinder itself does not change weight, only the air inside it).  For an average size (10.5L) cylinder I allow for 2.5kg of change (allow 3kg for a 12.2L). Some manufacturers list empty and full weights on their website – note that it’s the difference that is important here (and you shouldn't plan to breathe a tank to empty!).

Wetsuits

Another factor is wetsuit crush – in a dry suit you can add and remove air and keep buoyancy constant.  The amount of crush depends on the thickness and type of neoprene and the depth of the diver (i.e. water pressure).  This is hard to measure but allowing up to 2kg change is probably more than enough for a 7mm suit.  So for 2.5kg of gas used and 2kg for wetsuit crush – that’s only 4.5kg or about 10lb. 

Zeos_300

Other factors

Even small BCDs have 12kg (26lbs) of lift, so what’s all of that additional buoyancy for?

Firstly there’s safety, as we’ve already discussed many divers are over-weighted, but we also change configurations and suits, and often add some lead (just a couple) to be sure that we don’t float.

Secondly, what else might you carry that changes your buoyancy?  For example, if you have a deco cylinder that you will leave at a stop then your BCD needs to carry that extra weight, but if it’s weight you keep for the whole dive, you should reduce the lead that you carry. As your dives become more technical, with more cylinders, the greater the possible change in the weight of gas.

However, the reason for extra buoyancy could be as simple as recovering a weight belt lost over the side of a boat – do you have enough lift to carry a second weight belt without fining up? Let’s say that you work out that you would need 6kg (13lbs).  Do you have enough buoyancy to lift a weight belt from the bottom (potentially up to 25lbs where I dive) – or will you use an SMB instead?

What if your buddy’s BCD fails, can you lift the both of you on a slow (non-emergency) ascent to the surface?  You may need to double your lift, or more – and that’s for single tank diving!

Note that this is a simple explanation and does not consider factors like salt versus fresh water.

Finally, if we consider a larger wing may be more bulky and possibly less streamlined (so small can be better), what size wing do you need?  It depends on how much your buoyancy will change and an added margin for safety.

Summary

In summary, you need to find your end of dive weight (almost no air in the BCD and neutrally buoyant), and then for the BCD capacity work out:

  • change in the weight of air / gas during a dive;
  • change in buoyancy of wetsuit / exposure suit;
  • what else you might carry that changes your buoyancy; and
  • a margin for safety.

Then find the wing size to suit.

Hopefully this will help you to determine what wing size you need. And as your diving skills grow and you progress from sport to technical diving, you only need change the wing to meet your new requirements.

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