the dividends
of delay

They say we shouldn't put things off, but sending up a delayed surface marker buoy is often preferable to towing a buoy around throughout a dive. John Liddiard looks at the development of this simple piece of equipment, and offers his own 16-step guide to using it correctly

I was diving off the Torran Rocks at the south-west corner of Mull. It was 1980 and we were on a summer club trip, with two inflatables, an old GPO van dangerously overloaded with diving gear and 12 soggy divers trying to survive in tents.
The weather had turned unusually good, and we drove our boats as far south and west along the rocks as possible. We picked a flat-topped rock that would barely show at high tide, dropped anchor on the steeper-looking eastern side of it and snoozed in the sunshine.
Well, I did, because everyone else was diving except my buddy, and she was keeping watch.
Half an hour later, it was our turn. We descended a steep, rocky slope to 25m and found a seabed of coarse sand. Visibility was excellent and marine life plentiful.
We followed the join of the rocks, away from the boat. Nearing the end of our no-stop time, the sand ended and a wall dropped below us. We checked air, gestured at our deco tables and decided to stretch our dive over the wall for a few minutes.
It seemed a good idea at the time; the view was fantastic. But five minutes later, rising above the wall, we found ourselves heading into a rapidly building current.
On our tables we had already accumulated 10 minutes of stops, and could afford no more. We made a bluewater ascent and deco stops, while swimming against the current on a compass bearing.

The reel is fastened to wreckage , not the diver, while the DSMB is inflated and released.

We eventually surfaced a couple of hundred metres off the opposite side of the island. The sea was flat calm, but there was no way we could be seen from the boats. We shouted and blew whistles while swimming towards the island. No response.
Beginning to feel nervous, we started swimming harder, moving into top gear to make headway. It didn't cross our minds to ditch any kit. I used to play a lot of octopush and was quite fit at the time. My buddy had recently been told to slow down by the rest of the women's octopush team because they couldn't keep up with her (you wouldn't get that from the current crop of tough female players). It wasn't easy, but we made it to the island and scrambled ashore. Fins in hands, and with the rest of our kit still in place, we walked back across the island to our boats.
"Wondered where you'd got to. We were giving it another five minutes before we came looking," was the comment when we joined the rest of the group.
Now, all sorts of dive-planning and execution points could be made from this near-incident, but the real point is that with one item of equipment now taken for granted, we wouldn't even have noticed anything abnormal about this dive. I'm talking about the delayed surface marker buoy.
OK, we could have taken a conventional SMB at the start and towed it around for the dive, but we had planned to follow the rocks down and back. The only time an SMB would have had a role to play was during the unplanned method of our ascent.
A delayed SMB would have allowed us the freedom to make the dive, while giving the cover boats something to follow while we were decompressing in the current.
The principle behind the DSMB has been with us for a long time. In the old days wreck divers with a spare lifting bag would attach 5m of light rope and hang on the bag for their shallowest decompression stops. It saved crowding on the shotline and removed the need to return to the shot at the end of a dive.
The hassle of laying a bottom line was no longer an absolute requirement for divers making long deco stops. The worst case was that you missed the shot and had to ascend in blue water until shallow enough to hang on the lifting bag.
Pretty soon, however, the weaknesses of this system were exposed. Lifting bags required a lot of air to stick up well above the surface. Inflating a bag at a shallow stop depth placed divers at risk of losing control of their buoyancy.
Specialised decompression bags, what we now know as DSMBs, were developed - long and thin, with less lift and more sticking out of the water. At the same time, divers started using longer lines, and reels with which to hang on the bags.
Safe use was now much easier. Less positive buoyancy was involved in sending a DSMB up, and it could be released from greater depth, giving more time to bring things under control before breaking a decompression ceiling.
Another advantage of the narrow tube pattern became evident in heavy seas. Whereas a lifting bag would ride at a constant height in the water, taking the divers hanging below up and down with the waves, a dedicated DSMB could ride the waves more easily, smoothing out the worst of the movement.
Divers using reels to release a DSMB made the next step. Rather than playing with buoyancy, they wanted to hang on to a rock or piece of wreckage when inflating it, but this required more than one pair of hands. The answer was to tie the reel to a wreck temporarily using a short piece of line while releasing the DSMB. An added advantage was that this allowed time to sort things out should the reel jam.
It was at this stage of using DSMBs that I had another "near-incident". My buddy and I were diving the Aeolian Skye, a large freighter in 35m off the Dorset coast. We had built up 15 minutes or so of planned decompression and it was time to ascend.
I tied my reel in, inflated my DSMB fully and let it go. It shot up and a few seconds later the line went slack, indicating that it was at the surface. I untied and started to ascend, reeling in line as I went. Halfway up, we met the bag coming down.
I re-inflated the bag fully and let it go again. As it ascended, I could see it tumble in the water, tipping air but still buoyant enough to reach the surface. During our 3m stop I could just see it above us. It was almost empty, with only the tip showing above the water. We were all right decompressing, but in a strong current and a choppy sea, could the boat see us?
Fortunately, I could hear the engine and knew it was close by. Our diligent skipper had seen the tip of the DSMB and was staying nearby. When we got back to shore I dropped in to the local dive shop and bought a DSMB with a closed end.
These closed-bag variants are not strictly "closed" - a set of flaps allows air to enter the bag while preventing it escaping, and overpressure is handled by a separate relief valve. The important thing is that such bags stay fully inflated, no matter how they tip on the way up.
The current state of the art is a self-inflating DSMB, with a closed end and small air cylinder attached. Rather than having to fill it from a DV or air gun, you simply crack the cylinder.
DSMBs are now an almost essential piece of equipment for deco dives, and many skippers require divers to "ascend on a bag" rather than on the shotline. The last thing a skipper wants is half a boatload hanging on to a line while the other half drift off on DSMBs. What was once an item of precautionary equipment has now become the preferred method of ascent.
DSMBs have given us the option of conducting in freedom and safety dives like the one I began by describing. Which brings me to a question I would like regular users of such equipment to ponder: are you a safer diver by virtue of any degree of caution, or is it just that modern diving equipment lets you get away with it?

SAFE PRACTICE WITH YOUR DSMB Always use a reel rather than a length of loose cord, which tangles easily. Some makes of reel are prone to jamming - get one that works. Often the simplest and cheapest are best. If the reel is attached to you, have a quick-release coupling in case it jams. Otherwise disconnect the reel from your harness before inflating the bag. Where convenient, tie it in to the wreck or rocks before inflating. If making a mid-water release, start well below your deepest deco stop to allow a margin for error. Be prepared to dump your own buoyancy as you fill the bag and correct your buoyancy when it reaches the surface. If ascending into strong current, make sure your buddy is holding on to either you or the reel before you set the DSMB off. You don't want to fly off and leave your buddy behind. In a strong current, be prepared to untie from the wreck and start drifting while the DSMB is still ascending. The current could drag the line out sideways and prevent it reaching the surface if you are stationary on the seabed. Both members of a buddy pair should carry DSMBs, in case separation occurs. You don't have to send both up, but this provides redundancy in case of jammed reels, loss, torn bags or just for convenience during long stops. Some skippers like every diver to release a DSMB, so they know when everyone is accounted for and on the way up. Write your name on your bag. At busy dive sites your boat needs to know which one to follow. If making extended deco stops, use a closed-ended bag and inflate it fully. There is nothing worse than looking up to see only 10cm of bag breaking the surface. On a stop, make yourself slightly negative to keep the line in tension and the bag standing upright. Staying neutral will allow it to lie flat, and you run the risk of ascending above your deco ceiling should your concentration lapse. Clip the reel to your harness rather than hanging on with your hand while on a deco stop. Movements of the bag could aggravate a bend in that arm. Your buddy can clip on to the tie-in line used to launch the bag, to ensure that both of you are at the same level on a stop. Some groups have a convention of using orange bags on which to decompress, with a spare yellow bag to signal for help from a standby diver. Make sure you know the procedures of the group with which you are diving. Some divers carry two reels, with the line from the second attached to the handle of the first. If the first reel jams while deploying a DSMB, you can let it go and deploy line from the second; possibly useful for midwater deployment when deco stops are necessary. Another trick is to calibrate your line, by tying knots or using coloured tags. I have one knot at 3m, two at 6m, three at 9m and so on down to 18m, so if necessary I could manage my decompression with just these and my watch. If you decide to try this, make sure that your reel is not the type to jam on knots! Practise deploying your DSMB in a swimming pool or shallow water with no decompression penalty, then practise with your eyes shut - you might just have to do it all in zero visibility!

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Appeared in DIVER - April 2000

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