STEFFI SCHWABE STRODE MANFULLY through the Bahamian scrub, and I staggered along painfully behind, wiping away the sweat that was dripping through my eyebrows and stinging my eyes, and ineffectually fending off the attacks of endless voracious insects.
The least of my worries was the weight of the two tanks swinging from my hips, as we headed for the hidden entrance of one of those Blue Holes, a subterranean cave system flooded with both fresh and sea water. That was a long time ago but, until my autumn trip to Vobster Quay, it was the last time I used my main and only tanks side-mounted.
Why do cave-divers like to side-mount their tanks Because it puts their weight in a better position for buoyancy and trim than if they were back-mounted.
Most importantly, the system gives instant access to the tanks, with all the safety advantages that implies.
For example, if you have trouble with a regulator free-flowing, you can simply turn off that tank, but still get access to the gas in it by turning it on for each inhalation and then off again, just as a rebreather diver might do if an oxygen solenoid got stuck open.
The tanks are totally independent from each other. There would be no loss of gas across a manifold, and shut-downs are quick and easy, because the tank valves are right there next to you.

THIS INSTANT ACCESS TO EITHER tank is very important to anyone who might be struggling to climb through a restriction under water. You can instantly unhitch them at their bottom end and, without interrupting your breathing, pass them ahead of you.
The tanks are still attached to your BC D-rings at their tops.
Of course, putting their weight low down when you need to walk a little way with them, as I had to in the Bahamas, has its advantages too. I didn’t have to stoop to balance the weight on my back, as I might have to do with a twin-set.
Wind forward 14 years. I’m older, and less inclined to want to go into deep dark places, with no escape other than via the way I came in. I’m not a real cave-diver.
However, David Jones of Triton Scuba in Southsea, near Portsmouth, rang me to ask if I’d like to do a side-mount course.
This coincided with a new Hollis SMS 100 wing-style BC arriving at the office, a unit designed specifically for use with side-mounted tanks.
What would I need to know when diving in the open waters of Vobster Quay that I hadn’t needed to know deep in a Bahamas Blue Hole It had all seemed so obvious and simple all those years ago, but I wanted to try out the Hollis SMS 100, and I have a lot of respect for David as an instructor, so I got up early one autumn morning and drove down to join him.
I was accompanied by my erstwhile Dr Watson, retired fire-fighter Nigel Wade, who I had detailed to take the photographs.
We have got so used to seeing technical divers side-mounting their travel and deco gases, and rebreather divers side-mounting tanks for open-circuit bail-out, that the idea now seems to have become firmly linked with advanced diving techniques. But this need not be the case.
The PADI Open Water Side-mount Diver speciality course is a two-day affair comprising four dives. It simply teaches a diver to substitute side-mounted cylinders for the one or ones you might have previously mounted on your back.
This is not a technical diving course, although the techniques learned can easily be translated into a technical-diving situation. The course was written by US cave-diver Jeff Loflin.
David Jones started by giving me the background, and explaining the aforementioned advantages of side-mounted tanks.
He also pointed out that, when ordinarily RIB diving, it is very convenient to sit in the boat already wearing the BC, and to simply bend forward and hitch on the tanks from where they stand, without having to heave anything up on to the back.
He made various promises concerning unencumbered swimming, good streamlining, better trim and easier buoyancy control. Side-mounting two tanks with identical gas is an alternative method available for every diver.

The Hollis SMS 100 wing BC has a doughnut-style bladder toughly encased in an outer layer of 1000 denier Cordura. It has a familiar corrugated hose for the direct-feed, although the direct-feed hose is a lot shorter than would be needed for a back-mounted tank. There is one dump-valve operated by a toggle at the end of a cord at the lower back.
The buoyancy-cell is restrained by an elastic cord threaded around it, so it stays small and neat, with no tendency to flap when it’s not inflated.
The advantage of the doughnut shape is that it’s easier to get air to migrate to the dump-valve when you need to jettison it on an ascent, although as we were using drysuits we never really needed to use this feature during the dives we did.
Fully inflated, the SMS 100 will give nearly 24kg of lift. By over-inflating it at the surface, with my weights and tanks trimmed for a perfect horizontal underwater swimming position, it tended to push me onto my front.
I did encounter a moment of difficulty locating the pull-cord for the rear bottom dump at this time, but practice makes perfect.

This held no real surprises, apart from the fact that it is extremely robustly made. It has a conventional sternum strap, a 5cm-wide webbing waist-strap closed by a conventional quick-release buckle, and a 5cm-wide webbing
crotch-strap (with a ring for possible attachment of a DPV lanyard) through which it passes.
Harness breaks are supplied by muscular pinch-clips in the shoulder straps, which make it easy to get out when you need to. Six large stainless-steel D-rings are mounted on the front of the shoulder facings, and four smaller ones are arranged around the removable soft back-cushion.
Every clip is adjustable for position by means of an H-clip retainer, so that you can tailor this equipment exactly to your needs.

This is where the Hollis SMS 100 differs from a more conventional wing-style BC. The soft backpack is so arranged that you can use it with a single tank or with a pair of twins mounted on your back, as well as with side-mounted cylinders. It’s just a matter of how you organise the two cambands supplied with it. It will even fit a conventional metal backplate if you want that.
With eight small stainless-steel D-rings arranged around it, the fitting of ancillary equipment such as a light battery canister is no problem.
Even rebreather divers can adapt it for use. It really is intended to be a single answer to your diving BC needs.

HOWEVER, THE OBVIOUS DIFFERENCE when using side-mounted cylinders is the absence of cambands of any sort at the back, and the presence of two stainless-steel rails either side of the beaver-tail that also mounts the crotch-strap, and another movable D-ring.
The raised rails have three sets of holes so that they can be positioned conveniently wherever you need them. There is also a bungee on either side.
This stretches round to the front, where it is attached to a D-ring by way of a piston-clip. More on these items later.

We walked our 12-litre tanks over to the edge of the water, having first fitted each one with a camband at the lower end and a double-ended piston-clip at the top held by the loop of cord provided.
The substantially made camband buckles are of stainless steel. I’m never too old to learn a new trick, and David showed me how he attached the double-ended clip to these cambands by way of an H-clip and a cord with a stopper knot passed through the loop in the H. It’s a very neat solution.
Incidentally, if the dive had demanded only a single tank in terms of gas capacity, think how convenient and comfortable it would have been to employ a pair of 7-litre cylinders
We then put on our drysuits. With our weight systems in place (I used a weight-harness with a drysuit) we each donned our Hollis SMS 100s, put our masks and fins somewhere handy, and fitted the regulators to the tanks. It felt a bit strange to be walking about wearing a BC without any tanks. In fact, it was rather nice!

The first thing to do was to fit the regulators to the tanks and check their contents. The regulator rig, to my mind, is the only part of the process that might provide any difficulty to someone less dived-up with unfamiliar equipment than me.
This is because the regulator to each tank has a long hose. It may not be strictly necessary, but it stems from the cave-diving tradition.
Each long hose has to be brought up round the back of the neck in the Hogarthian style. The length is designed to take the stress out of air-sharing, especially if narrow restrictions have
to be negotiated, because one diver can follow behind the donating diver.
We sat on a bench with our tanks lying one on each side. Each tank is then hooked to a convenient front D-ring by its upper hook, and the rear end is lifted and hooked onto the rail behind by the piston-clip attached to the camband.
This is where the importance of the raised rails becomes apparent. They are very easy to locate by feel, and to clip onto.
The regulators are then positioned. The first one on the 1.2m (shorter) hose to be donned is routed round the back of the neck and retained on a necklace.
The second is on a longer 2m hose, with the extra length stowed neatly under a bungee strap on its tank. This is also routed round the back of the neck.

DAVID LIKES HIS REGULATORS to be fitted with omni-swivels at the second stage to take any stress out of the routeing. Direct-feed connections to both drysuit inflator and wing inflator are made. Modern braided hoses make it possible to use short routes for this.
What of those mysterious bungee straps These are simply reached for and hooked forward with a thumb to engage round the handle of each tank valve, keeping them where you expect to find them. David had thoughtfully provided tanks with right-hand and left-hand valves, although this is not strictly necessary.
With complete access to the tank valves, one could leave it to this moment to open them. The pressure gauges are on short hoses, and are very readable.
Fins and masks in place, we were ready to go, but David was keen to demonstrate that it was even easier to rig the tanks when we were already in the water, mask and fins in place.

The first of four dives involved swapping from one regulator to the other, closing down the tank and opening it again appropriately. I found that it was useful to take out the first regulator with my right hand while replacing the other on its necklace with my left hand. That reduced any risk of momentary tangles.
We did this negatively buoyant while perched on a training platform, but I was soon doing it for Nigel’s camera while neutrally buoyant in midwater.
Other party tricks involved unhitching the tanks from the back rails and positioning them ahead of me, as if ready to pass them through a narrow opening. Naturally, I had to reinstall them afterwards.
Air-sharing using the long hose from the right-hand tank was pretty straightforward, too. I can see no reason why you shouldn’t do the same with the left tank, as it too is on a long hose.
Getting the trim perfect is easily achieved once you are able to unhitch a tank and alter the position of a camband along its length.
I found this useful, because I felt head-heavy at first and had to move the tanks back a little so that I didn’t do an impression of a plough with my nose.
I could still pull them forward easily for complete access to the tank valves.
All in all, it was jolly good fun, and nothing any diver with proper in-water confidence should not be both able to achieve and enjoy doing.

Experienced divers might find the whole thing much too easy to justify two days out, but remember that this PADI Open Water Side-mount Diver course is available to anyone certified beyond PADI OWD or equivalent, and side-mounted cylinders can be used on almost any other PADI course.
It progresses steadily, taking into account that some people might find difficulty in doing things in a new way.
Side-mount diving may look like technical diving at first glance, but there is no additional set of tanks on your back. The whole point of side-mounting cylinders is to make everything easier. The dives on the course are no-deco-stop.
Hollis also provides a dual-bladder version of the BC for multiple (that is, four) tank technical diving.
The two-day course costs £275, and a Side-mount Instructor course is also available for those suitably qualified. Visit Triton Scuba’s website,

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The use of the Hollis SMS 100 BC, and all the possibilities it presents, got me thinking.
In the days before “technical diving” was invented, I used to sling my bottom-gas
side-mounted and wear my travel gas and my deco gas on my back – in the belief that
I would always want to travel and decompress, but could in an emergency abandon my bottom gas. Training agencies have universally adopted the converse configuration.
I am also regularly pilloried for diving independent twins with a lean mix in one for use at depth and a rich nitrox mix for the shallower part of my dive.
This “Bantin-rig” may be scorned by Internet divers, but it has been seen used by many holiday divers who want, for example, to have plenty of time on the Rosalie Moller wreck in the Red Sea, but don’t need the gas of three or more tanks. It is not a procedure ratified, to my knowledge, by any training agency.
However, the SMS 100 allows the diver to side-sling a couple of tanks and also wear another on the back.
Imagine diving with 21-litres of gas (at 200bar) distributed between three little independent 7-litre cylinders.
Sounds like a very comfortable way to dive in certain locations. It’s only an idea, and not part of the course!

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