Choosing the best surface marker buoy (SMB) for your dive
Surface marker buoys are handy for a number of occasions, whether it’s to signal your position to your boat at the end of your dive or during emergency ascents.
When you need immediate help or when someone in your dive group needs to get to the surface quickly, you’d want to be able to deploy the bag without it causing other problems or making matters worse. That’s why choosing a suitable SMB for your dive is so important.
There are three main factors a diver should consider when choosing a surface marker buoy: its size, method of inflation and the length of its line.
- What size do you need?
To be useful, the marker must be visible on the water’s surface, so it should be as large as you can manage. My emphasis is on what you can manage. You could have the largest marker on the planet, but if you can’t inflate it properly, it would simply be a limp piece of plastic floating on the surface. That certainly won’t be noticeable from a distance — unless the boat crew is consulting Google Earth.
Therefore, the size of the SMB you choose should be the largest that you can reliably fill from depth. Personally I prefer a smaller, thinner bag that will stand upright when filled with a relatively small volume of gas. I use the Halcyon 1m SMB on all my dives. This bag will fill up completely with only a single regular breath at a depth of about 10m. If you are at a shallower depth, then you’d need to blow in a larger breath of air, but it’s definitely still possible to fill up the bag quickly and send it on its way without much stress.
Now you might be wondering, what if you’re diving at sites with large waves and surge? Wouldn’t a larger (more visible) surface marker be better? In response I would ask: Is the dive worth putting yourself in such a situation? If the answer is yes, then make sure you are prepared for it with a larger surface marker.
Remember that a larger bag must be filled with a greater volume of gas in order to stand upright on the water’s surface. This means you’d need to either blow more gas into it (by exhaling or purging from the regulator) or release the bag when you are at a greater depth to allow for more gas expansion. The latter would be a good option, but the downside would be the much longer line that you would have to reel in as you ascend.
I would normally only inflate these larger surface markers once arriving on the surface and the boat is further away. The bigger marker will be easier to spot from a distance, but will also be harder to manage while underwater.
- Which method of inflation should you use?
Open circuit SMBs are some of the commonest in the market. To inflate an open circuit SMB, a diver typically purges air from his alternate air source into the bag. However, I do not recommend this method of inflation for a number of reasons.
The large amount of air purged from the alternate air source would likely cause a fairly significant and sudden buoyancy change. In an emergency or stressful situation, a sudden change in buoyancy could easily spiral out of control.
The force generated by the large volume of gas also means that the SMB will take off towards the surface really quickly. If the diver does not manage the line well, the line could get tangled with the BCD or tank, and drag you up to the surface. Nobody wants to be a human SMB!
I would always opt for the oral inflation method, because it is quicker and safer. The amount of gas that you exhale would not cause a significant buoyancy change (unless you inhale an extra-large breath of air right before). This makes the buoyancy change much more manageable. You wouldn’t need to find your alternate air source, too.
If like me, you prefer the oral inflation method, what’s left is to choose an SMB that works best with it. It’s possible to orally inflate an open circuit SMB, but due to the line being attached near where the open end of the SMB, there is a higher risk that your regulator might get caught in the SMB line or the SMB itself. Definitely not my first choice.
There are many SMBs on the market that are designed for easy and effective oral inflation. They normally have an oral inflator with a one-way valve. This, in my view, is the best one to use because it guarantees that the gas goes directly into the SMB. It’s also really easy to use. The Halcyon SMB I use does not have a locking mechanism, so you won’t need to fiddle with extra bits to inflate the bag.
- How long should the line be?
Some SMBs come with a 5m line, which means you can deploy the SMB only at the 5m safety stop. Obviously this is limiting, as you might run into other situations where you need to deploy the SMB at a greater depth. My advice: Always get a longer line that’s wound around a spool.
SMBs do not always take a straight path up towards the water’s surface, and might ascend at an angle when there is a current. As such, always make sure your SMB’s line is longer than your deepest planned depth. This means that if you are doing a 30m dive, then a 30m spool is not going to be sufficient. A 45m spool would be a better choice. I would use a 30m spool for dives to depths shallower than 25m.
In summary
My recommendation is to get a small thin SMB that you can reliably fill via oral inflation, and use a spool with an appropriate amount of line. Also, remember to practise deploying your SMB after buying one! A seasoned diver can easily get a bag deployed in less than 15 seconds, so keep practising until it becomes second nature!
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