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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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