Group of Divers with SMBs

Every year there are stories of divers getting injured by the propellers of boats.  Another incident happened a few weeks ago in Hong Kong.  The details are not well known, but it made me think of the commonly overlooked issue among divers.  How to be more visible to others?

The answer is of course the SMB.

If you take a dive course now, your instructor should give you an introduction to ‘Surface Marker Bouys’ or ‘SMBs’.  That’s a start.  What about the many divers who trained before SMBs use was included in training?  What about you? Do you know what one is and how to use one?

What are they?

An SMB is a simple device used by divers to indicate their position to those on the surface of the water.  It is a long inflatable tube, about a meter long, which is filled using air from the diver’s regulator system.  Normally, it is bright red or some other visible colour.  When not in use, the SMB rolls up nicely and can be fitted into a readymade pouch, or BCD pocket.

SMB and Spool

Once inflated, it will stand proud and erect (with a little pull from below) on the surface of the water and can be waved to attract attention.  When inflated from depth, the diver will attach some light line to the SMB before deploying it.  At night, a light can be shone on the SMB to enhance visibility.

SMB’s serve many purposes.  They are a system for attracting attention.  They can warn power-boaters on the surface of the presence of divers below.  They can provide emergency floatation.

For this reason, SMBs are an essential part of any diver’s equipment.

Types of SMB

Some SMBs are simple tubes with an open end.  These are the simplest, and will do the job, so long as the air doesn’t spill out of them.  They need constant pull from the diver below to remain upright and inflated.

Other SMBs have self-sealing ends which allows them to be inflated, and remain inflated.  With these there is normally a dump valve to empty the air once the SMB has served its purpose.

As with any kind of gear there is a myriad of weird and wonderfully over-complex solutions out there, each with pros which are normally outweighed by the cons, such as price and potential failure points.

I use a middle-ground SMB (self-sealing style with dump valve) which was purchased over 15 years ago, and although a little faded still does the trick faithfully each time.

Types of Lines and Reels

Complicated Divers ReelOf course, for it to be useful, the SMB should be deployable from depth, so that the power-boaters are warned of the ‘diver below’ before the diver surfaces.  This then requires a line to be attached.  The line must be able to pay out freely as the SMB is inflated and let go.  Any snags on the diver, could result in the diver being rocketed to the surface, and of course then face the risk of DCS.

Again, there are many different reel (some unreal) solutions all which promise to be ‘snag’ free, but most of which eventually end up proving to be more of a hindrance due to the finickity parts which break easily and jam the reel, rendering it useless.



Once upon a time, tech divers favoured a simple rugged reel, with one screw type locking device.  Even then, it took some practice to get the SMB away cleanly.


Nowadays, the KISS principal is taken to the extreme with divers preferring just a simple spool, with double-ended dog clips to lock the line when not in use or at a certain depth, say for DECO and safety stops.

Simple Spool type reel

Length of Line

The length of line is more important than one might think, depending on where and how you want to use your SMB.

Let’s say, you dive where there can be strong currents and where the depth is about 20m.  You like to deploy your SMB early (say from the sea floor) to alert the support boat, so they can follow your drift as you ascend.  How much line do you need?

The answer is, you need significantly more than 20m of line.  In a current, the SMB will not go straight to the surface. Instead it will take the longer route via the hypotenuse of the triangle, depending on the strength of the current.

Given that the diagonal of a square is about 1.4 times longer than the side, in this example you need at least 28m of line!

Since these lines are useful for other purposes such as search and recovery, it is not uncommon then for divers who regularly dive deep to carry spools with 50m of line.

Even if the depths are not so deep, the concept remains the same.  Most people do a final safety stop at 5 or 6m and this is a great time to deploy an SMB, a 10m line should be the minimum length on the reel.


Tech diver on deco stopIf you have not ever used one, I strongly encourage some training and practice on how to stow and subsequently deploy your SMB.  There can be problems with the line or reel which, could put a diver at more risk, particularly if caught up as the SMB is filled.  However, with practice it does get easier.  Of course, perfect buoyancy helps.

The Bottom line

In short, if you are diving in open water, places where you may surface away from your support boat, or areas where there is a lot of power-boating going on, you should have an SMB and be proficient with its deployment.

It could save your life.


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