We get a lot of questions from divers who want to start diving on twin cylinders and expect there to be a BCD that will hold both a single cylinder and a set of twins. It's a bit more complicated than that unfortunately and your best bet is to go old school with a backplate and wing setup. Backplates are much stronger than conventional BCDs today and much more customisable, another huge benefit is they're mostly universal so you can mix and match brands without worrying too much about compatability.
I won't go into the benefits of diving twins because if you're reading this you've probably heard them all so I'm going to get straight into this...
Apologies for the scruffy hose routing below, it's the only picture of my twins I could find.
BCDS, WINGS, BLADDERS...
The first thing that confuses people is the terms as a term like 'wing' can refer to a BCD that can only take a single or the buoyancy section of a BP/W. In this blog I'm going to refer to recreational BCDs, everything from jacket style to wings, as BCDs. These are designed only to take single cylinders because they have limited lift and limited structure to accommodate twin cylinders.
A bladder is just the sealed bag in both a BCD and a wing that you fill with air to fine adjust your buoyancy.
A Wing is where the bladder is contained behind you and doesn't wrap around your waist, that makes it a jacket style. For twins we only use wing style bladders for the trim characteristics and shape so you struggle to find a jacket style bladder for a twin set.
WHAT YOU NEED FOR TWINS
You need a Backplate and Wing setup, commonly known as a BP/W. This consists of a metal backplate, a webbing harness threaded through it and a wing to act as buoyancy. Your backplate is the strength to hold everything together, the wing size relates to the weight of your cylinders and equipment and the harness holds you into the rig and has attachment points for equipment. You don't usually have integrated weights on the waist like a BCD but use a P-Weight or V-Weight to keep the weight close to your center or just a traditional weight belt.
Backplates are either steel or aluminium, steel is stronger but more importantly heavier than aluminium. By using a steel backplate you don't need to use as much lead on your weightbelt which is why manufacturers like Mares XR make the 4kg 6mm Backplate.
Aluminium backplates are for travelling where you're diving with a thinner wetsuit so don't need as much weight or diving on lighter cylinders. 0.75kg compared to a standard steel backplate at 2.4kg, aluminium plates are much lighter for travelling abroad.
Harnesses range from a simple single piece webbing harness made from 2" Nylon webbing to fully adjustable, fully formed padded systems. Simple single piece harnesses like the PureTek, HD, or Solo are preferred by more technical divers because of their standardisation and they don't have any breaks. Breaks in the webbing are seen as failure points which makes DIR harnesses more reliable but the only adjustment they have is in the waistband.
Other harnesses exist to be more adjustable and comfortable like the Elite II & ProTek that have one or more breaks with a quick adjustment buckles but still use the 2" standard webbing so you can thread and adjust D-Rings where you want them. The far end of the spectrum is the Form & Transpac which are fully padded and shaped harnesses that bolt onto your backplate. They're padded and have quick adjustment but poor customisability as you can't move the positions of your D-Rings.
Around your harness you need some attachment points because you rarely have pockets, Over each shoulder you only need one bent D-Ring, One or two straight D-Rings on your waist, depending on your needs, and a couple on your crotch strap.
There are two unofficial sizes of wing small and larger. Smaller wings are made for single tank setups and can usually be identified by an offset inflator and much smaller lift. Larger wings have a much wider mid-section to wrap around twin cylinders neatly. The inflator is located in the center to allow more space for your valves and 1st stages.
Wings come in two shapes; horseshoe and donut. Horseshoe wings are slowly going out of fashion but can still be found at dive sites, they have more buoyancy down to your lower back that gives you a nice position in the water and an unobstructed access to your buttplate or rear D-Ring. The only 'downside' to Horseshoes is that gas can get trapped down one side, making it harder to dump without changing your position in the water, this can also be seen as a benefit in case of a rupture you can trap the buoyancy in one side.
Donuts are large O shaped wings so gas can travel all the way around the wing unobstructed but other than that are very similar to Horseshoes.
Wings range from small 10kg all the way up to a 27kg with all sizes in between. The size is the amount of buoyancy you need and relates to the buoyancy of your twins and equipment. Smaller bladders have lift up to about 18kg of lift and down to about 10kg for a really nice compact little bladder for diving on singles. Larger bladders start around 20kg and up to 30kg for multiple stages that require plenty of lift at depth. Here you need to understand the difference between weight and buoyancy of your kit and how much positive lift you need to offset that negative buoyancy.
Most wings have a two-part design with an inner bladder and an outer shell to protect the bladder. The outer shell is made from a tough Nylon Cordura material that if torn won't affect your buoyancy but if the thinner internal bladder is torn then your buoyancy can drop as gas escapes. You can access the internal bladder and repair or replace it easily if it gets damaged.
Some training organisations require a double bladder for tech courses as a redundant buoyancy source should your primary bladder get torn. You get two inflators, one for each bladder, one falls over your left shoulder as normal that you use a a primary and the second sits under your right flank to use if your primary bladder is torn.
The debate rages online over the pros and cons of a bungeed wing. Bungees wrap around your bladder to wrap it up neatly when deflated so you're more streamlined and your wing doesn't float around in the water. The downside of bungees is if your bladder is torn then the bungees will squeeze any buoyancy out.
It's easiest if you start from scratch but you can retro-fit new valves and manifolds but it's far easier to get everything together and build your twins from the ground up. You will need two bare cylinders of the same size and spec, both a left and a right handed valve, a manifold that joins the two and a pair of bands to hold the cylinders together.
Your cylinders have to be the same or the spacing will be wrong and you'll be offset in the water, always leaning over to one side. You of course need two valves with opposing handles and they need to face the same direction so you can screw the manifold between them. Manifolds come in a few sizes that relate to your cylinder size so make sure you get the right size before trying to fit them.
Once your valves and manifold is in place you need to be very careful not to twist or turn either cylinder as this will damage the valves and manifold resulting in a catastrophic failure. This is where your twinning bands com in. Again sized specifically to your cylinder size, your twinning bands act as a mounting point and hold the cylinders parallel to one another. Through the centre of both bands you thread a long bolt that will pass through your wing and backplate to bolt everything together, these need to be 11 inches apart to line up with the holes in your wing and backplate. The height of your bands depends on how flexible you are to reach the valves and where you need the trim weight in the water.
Many divers in the UK use twin 12s, you have plenty of gas and don't require any lead, depending on your setup, to get down. They're heavy out of the water but they're a nice length that you can sit down to take the weight off your shoulder. Twin 10s have a little less gas than 12s but are shorter so sitting down on the RIB the bottoms wont touch the side so the weight is always on your shoulders. Twin 7s are smaller and lighter than 10s but the same length as 12s so they sit down nicely but have less volume than a single 15 so only really benefit because of the redundancy. Larger twins like 15s and even bigger exist but are only really used for extended range due to their size.
THE COMPLICATED BIT
Buoyancy is different from weight. To work out the amount of lift you need from your wing depends on the -ve buoyancy of all of your kit in the water. The only true way to do this unfortunately is to get wet and try but I'll get to that later. There is no universal rule on how much lift you need for a certain size cylinder setup because you have to take the buoyancy of all of your kit into account and there are so many variables that there can't just be a table that spits out an exact answer for you.
A few divers online have gone to the trouble of working out the buoyancy of different sizes and makes of cylinder when they're full and when they're empty which makes your life easier. Aluminium cylinders are much more buoyant than steels and can have a +ve buoyancy when empty so you need more lead or -ve weight to stay down at the end of a dive. Steels start the dive very -ve and finish less -ve so you need more +ve buoyancy to compensate at depth.
The most reliable way to test your setup's buoyancy is to either measure each bit of kit in the water and add up the overall buoyancy or you could just jump in the water with everything you intend to dive with in a controlled environment like a pool, inland site or a shallow coast. Best to do this before your big dive expedition too but remember that different waters create different results. Go down to 3m at 50bar, empty your wing and ditch as much lead as you need until you're neutral.
As a simple rule of thumb if you're diving on anything up to steel twin 12s all you need is a 18-20kg wing. Smaller twins may need some lead and larger twins will need a larger bladder
EXAMPLEI dive on:
- twin 12l 232bar steel Faber cylinders,
- a 20kg wing,
- a steel backplate the weighs about 2.5kg,
- a single piece harness,
- 0kg lead
and my membrane drysuit, mask, fins etc. I don't need any lead in fresh water and I still have buoyancy for a steel stage cylinder. If I need to dive with more stage cylinders I'd swap my 20kg wing for a larger wing like a 24 or 27kg.
When you're starting out this all seems very confusing and complicated but once you've built a rig it's really simple and comfortable because you have created a custom rig for your needs that will feel bulletproof. Your rig can be retro-fitted for different size twins and singles without having to get a separate BCD for each, you just need a different wing.
If you're unsure you need to talk to your Instructor or dive team to see what they dive on and take their recommendations. It is important to try to stay as uniform with the team you're diving with for understanding and performance of one-another's kit.