DSMBs - the Essential Safety Kit That Many Divers Don't Know How to Use


A Surface Marker Buoy (SMB) or Delayed Surface Marker Buoy (DSMB), is an essential piece of kit, but it’s only in the last few years that training in their use has become a standard part of some entry-level courses.

Many divers across a wide range of experience levels will therefore never have used one. To them, it’s just the ‘balloon’ or ‘safety sausage’ that their instructor or guide inflates towards the end of the dive, indicating that it’s time to head for the safety stop.

Others carry them perfectly packaged but never used, because they were sold a DSMB as an essential piece of safety equipment but have no idea how to use them – but the guide has one, so that’s okay, isn’t it?

Well actually, no it’s not. There are a number of situations where not having one, or relying on somebody else to inflate one, puts your life at risk.

Imagine that you’re drift diving and the current picks up strongly and unexpectedly, perhaps with a serious reduction in visibility, pushing you away from the reef and out to sea. You can’t see the guide any more, you can’t see your buddy, so you do the right thing and wait for a minute then surface – into a rolling 2m swell beyond the end of the reef. It’s late afternoon and the tropical sun is heading towards the horizon; the sea is a glorious fire-bright orange and the captain of your dive boat is blinded – and looking in the other direction, because you were supposed to surface somewhere else.

It’s doesn’t take all that much imagination, actually. I know this, because I’ve seen it happen. Even with the best planning in the world, sometimes it all goes wrong, and it’s beyond your control.

With regards to the acronyms – the technical difference is that a ‘Delayed’ SMB is one that is inflated underwater at some point during dive – rather than towed from start to finish – usually towards the end of the dive to signal that the end is approaching, and to give your own boat crew advance warning of your location, and signal to other vessels that you are approaching shallow water and therefore they should steer clear.

They might also be used if, as mentioned earlier, the current at a dive site picks up unexpectedly, in which case a DSMB might be sent up early so that the boat can follow. It might be perfectly okay to continue the dive, but the crew will know that you will surface much further along the reef than expected. It might be that you’re being pushed out to sea – or have some other emergency – and preparing to abort. The earlier the boat crew can find your location, the better.

There are many DSMBs available on the market – often overpriced, in my opinion – but a good product in my preferred style is obtainable for around £45 - a minimum of 1m in length, preferably longer (mine’s 1.5m) with a non-return valve and weighted magnetic strip at the open end, a dump valve and a spring-loaded inflator mechanism that allows you to inflate the SMB either orally or with a low-pressure inflator hose.

Delayed Surface Marker Buoy with (L-R): clip for the line, weighted magnetic strips, seal for the non-return valve, low pressure/oral inflator, dump valve

The non-return valve is not complex, simply two flexible flaps of the material inside the tube that allow air to go in but which are sealed by the pressure when inflated, so it can’t leak out. The magnetic strips help to keep the ends together and also help to weight the balloon so it sits upright in the water. The dump valve allows you to deflate it easily both underwater and at the surface, and the LPI/oral inflator connector is a wonderful thing because it gives you an excellent option for inflating the SMB, which I’ll come to in a minute.

Some SMBs come packed with their own line, usually between 5-7m long. It is my considered professional opinion that these are quite useless, very annoying, and potentially dangerous. First, it only allows you to inflate your DSMB at a very shallow depth. Secondly, you have to unravel all the rope, which is time-consuming; there is a risk of entanglement as it floats about in the water, and it’s one of the easiest ways I’ve come across for a diver to have their regulator pulled out of their mouth.

Buy a reel (also called a spool). For warm-water recreational diving, a simple, plastic finger reel is practical, easy to use, lightweight and small in size, and can be bought for as little as £10.

For divers wearing gloves – with cold-water drysuit divers being the obvious case – then a hand-held reel with a clutch is probably a better choice, although there are finger spools available which will take the size of a thick pair of gloves. Either choice is more costly than the most basic devices, but you need something adequate for the type of diving you’re doing.

When it comes to the length of the line carried on the reel, obviously that depends on the type of diving you do and how deep you dive. If you’re a shallow-water fish-botherer like me, then 20m of line is more than enough, but deeper divers will want more.

The most common method for inflating a DSMB is to hold the bottom end open and purge a regulator (usually the alternate) into the opening. The technique varies but I would hold the reel and one side of the SMB opening in my left hand, then use the fingers of my right to keep it open while I use my right thumb to purge my alternate into the SMB.


It takes a bit of practice, especially since once the air starts to accumulate in the SMB, it becomes very positively buoyant, very quickly. Even a short, narrow tube will start to pull you towards the surface. Dumping some air from your BCD just before inflation, and exhaling as you inflate the SMB will help.

This is another problem with the short line of some SMBs. It is much easier to inflate an SMB from depth than it is at 5m for a safety stop, because you will be less affected by the change in buoyancy, and have much more time to react. When I was working as an instructor and guide, I would usually inflate my SMB at around 10m, where possible.

Hence the great thing about SMBs with an oral inflator. As you exhale into the SMB from your lungs, the buoyancy characteristics of the whole system – that’s you, your BCD, exposure suit, weights and now SMB – remain almost the same. The disadvantage is that you have to remove your primary regulator to accomplish the feat, but if you hold onto the hose and keep the second stage clear of the SMB and line, this should really not be a problem.

Once the SMB is up, it’s easy to wind yourself up to safety stop depth, clip the line to the holes in the reel so it’s locked in place, and pay attention to what’s passing overhead. Not all boats (or jetskis, inflatable bananas, snorkelers and curious sharks) will keep their distance.

If you’re deploying your SMB on a night dive, you can shine your torch inside your long orange sausage – now you have a glowing metre and a half of signal light which can be seen for miles, and which you can also use to have Jedi-style lightsabre battles while you wait for the boat to pick you up.

The first rule of SMBs is owning one. The second rule of SMBs is practising how to use it, even if you start in a shallow pool you can at least start to work on the technique. Ask a buddy, instructor or guide to assist and watch as you learn.

The third rule of SMBs is – like the little plastic card some people like to carry, only infinitely more valuable – never leave home without one.




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