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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.
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.
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!).
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.
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.
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:
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.
Taking the Xdeep Ghost on holidays over a year ago I loved it so much that I’ve been using it at every opportunity where a single-tank backmount system is called for. It still looks like new after use in some tough conditions, but by being so good it delayed me from trying the Zeos – but the time has now come.
The Xdeep Zeos is promoted as an “out of the box” system, suitable for the new diver and able to ‘grow’ with their needs. I was after the benefits of a backplate, harness and wing system, and tough enough to handle rough entries and exits, being knocked around by breaking surf and able to withstand getting washed up onto rocks …yes, OK, conditions do need to be quite bad to keep me out of the water.
I chose to keep it simple, a basic harness without the optional deluxe set shoulder or back pads. A steel backplate would mean less lead to carry, but rather than be rational I chose a vibrant blue aluminium backplate with matching single tank adapter (STA), for a bit of bling. I moved the Xdeep weight pockets from my Ghost to the Zeos (interchangeable); why wear a weight belt if you can transfer the weight to a backplate? The left weight pocket has a D-ring, so I shifted the existing D-ring to the right side for another attachment point. I also chose the 28lb / 13kg wing, instead of the 38lb / 17kg wing, with the idea of a smaller wing being more streamlined; I don’t see the need to exert any more effort than necessary!
My test dives were a couple of boat dives with the aim of finding grey nurse sharks just off the NSW South Coast. Although I had a couple of new divers to keep an eye on, it seemed like my first pleasure dive in a very long time.
I’d adjusted the shoulder straps the night before so had no problem slipping into the harness. In the water the fit was secure and comfortable – a 7mm semi-dry wetsuit gave me enough padding, and the backplate is a good comfortable shape with well positioned slots for the harness. The Zeos trimmed to a near-horizontal position but I found my head hitting the first stage regulator – my error by fitting the STA by the top bolt hole of the backplate, it needed to be lower, an easy adjustment (after diving).
The double-layered wing construction of the Zeos is similar to the Xdeep Hydros technical BCD, making it a very rugged unit – you don’t normally see this kind of construction in sport BCDs. The double layer adds a little bulk compared to the Ghost, however the Zeos was nicely streamlined when pushing into a gentle current - my old jacket BCD now feels like a sea anchor.
28lb / 13kg lift is similar to a small - medium jacket BCD, but it's a lot of lift when correctly weighted and the weight of gas in my tank will only change about 5lb during a dive. The less weight the less lift needed. At the end of the first dive, bringing the newer divers to the surface, another diver lost his weight belt climbing on board. So back down to 20m to pick up a 24lb weight belt, being deliberately 2lb overweight for this test dive and with half a tank of gas – that’s about 28lb of lift needed, the same as the wing! At maximum inflation I was neutrally buoyant but with all the lift in the wing sitting behind me I was comfortable and trimmed, unlike a jacket where full inflation feels like being crushed to the point of restricting blood flow. (I had an Xdeep closed SMB if extra lift was needed.)
Some other notes. Some divers worry a back inflation BCD will push them face down on the surface; the Zeos had an upright surface position - the wing shape is well designed for streamlining and when on the surface. The Zeos wing has a large-toothed zip making it easily to unzip and wash out salt between the inner bladder and outer shell. Jacket BCDs have pockets (often half filled with integrated weights); however, more D-rings can be added to the Zeos harness and the Xdeep pouch is also a more versatile option rather than pockets. The Zeos is offered “out-of-the-box” in a couple of configurations, but it can be customised to individual needs. The wing and STA can be swapped for a Hydros wing and twin tank configuration, providing an upgrade path.
In summary, the Zeos appears simple, as a harness back plate and wing, but the attention to detail in its design means that it performs brilliantly and, as a sport diving BCD, it is very rugged. I also think it is a great BCD for new divers - forget the jacket and go straight to the benefits of a tec-style BCD.
Finally, we saw crested horn sharks, Port Jackson sharks and Wobbegong's but no grey nurses. They were enjoyable dives nonetheless because of the gear I was using.