SMBs and Reels Explained

SMBs and Reels Explained

Reels and SMBs…. For such an innocuous piece of kit nothing quite inspires the mixture of opinion, fear and a sprinkling of instructional show boating quite like SMBs and reels. During training, they’re barely mentioned at entry level, leaving many a student diver to regard SMB deployment as a complicated fast ticket to a rapid ascent, whilst at more advanced, technical levels they’re approached with a level of refinement close to reverence, where even a slight change in buoyancy during deployment is tantamount to heresy.

All the average diver needs to know though, is how to cut through the bewildering array of shapes, sizes, colours, inflation and deployment methods.

To skip through to find out which set up is right for you head to our blog about 'How to choose an SMB and Reel'. Otherwise, read on...


SMB stands for Surface Marker Buoy. You may also hear the term DSMB which stands for Delayed Surface Marker Buoy. The difference is simply that a DSMB is designed to be deployed at depth and sent to the surface. In reality though, don’t get too hung up on the terminology, most divers tend to use the terms fairly interchangeably and almost all the sausage shaped SMBs we sell are DSMBs. All a DSMB needs to distinguish it from an SMB is a way to dump excess gas as it expands on the way to the surface. Some SMBs are torpedo shaped and designed to float on the surface as a dive marker.

The Colour

Red or orange is the way to go. In some parts of the world (the UK included) yellow is intended to indicate an emergency. Some tec divers will carry a yellow SMB which ,if deployed, sends a pre-agreed signal to the surface (for example to drop over an extra tank of deco gas). There are some funny colours out there including some that are red one side and yellow the other, which I can only imagine mean you’re fine or you’re not, or you’re fine or you’re not again depending on which side is visible to the boat….

The Size

For sea diving, especially in lower visibility conditions, you want your SMB to be well over a metre in height. There are some smaller ‘single breath’ SMBs on the market, usually around a metre in height. These are fantastic to use as training aids or backups and are designed to be fully inflated on the surface when deployed from around 10m or so with a single breath in them (more on deployment later). Some SMBs also have SOLAS tape on them or space to put a wetnote or strobe at the top for added visibility.

Inflation Methods

There are 4 basic types of SMB:

  • A very basic, open-ended design, essentially a long red sock.
  • A self sealing design where the SMB is still inflated through tan opening at the bottom which then doesn't let the gas escape back out.
  • The types with an LP inflator port that can be inflated orally or using an LP inflator hose (the valves to do not ‘connect’ to the hose so you don’t need to worry about the inflator hose getting stuck). Gas is then drained using a dump valve.
  • The auto inflating kind that use a small cylinder of gas to inflate the SMB.

Many SMBs have a combination of these inflation methods so you can decide which one works best for you. Following on from that, there are 5 basic methods to inflating an SMB:

  • Orally using an LP inflation port. This is my personal favourite (doesn’t mean it’s the best before ‘angry of Milton Keynes’ gets in touch). You inhale from the regulator and exhale into the LP valve on the SMB. The advantages of this method are that it’s very controlled in the amount of gas that goes into the SMB and even on some larger ones, a single breath will often be more than enough gas to fill the SMB adequately when sent up from around 20m or so. The downside is that it involves taking your reg out of your mouth.
  • Orally using an open-ended SMB design. This is where you exhale your exhaust bubbles into the bottom of the SMB before sending it up. It’s fairly simple in that you don’t take your reg out of your mouth but it does involve bringing the SMB close to your regulator which could be an entanglement risk.
  • Using your alternate air source: This method is very popular around the world and involves using a 2nd stage to fill the SMB through the open end. It’s probably the simplest way to do it although there is a little less control over the amount of gas that enters the SMB and whenever you purge a regulator, especially in cold water, you can risk a free flow
  • Using an LP hose on the LP inflation port: It’s easy, but it can be less controlled and requires you to disconnect an LP hose unless you happen to have a spare (it’s a popular method with rebreather divers who carry an inflation hose on their bail out tanks)
  • Using an inflation cylinder: Again easy to do and the cylinders don’t carry enough gas to really be an ascent risk. Downsides are cost and a more cumbersome piece of kit.

Reels and Spools

The arguments over whether spools are better than reels have sent many a new diver, who was eager to discuss what they actually saw on a dive, to an early bed. I refuse to be drawn and will simply say that one isn’t better than the other they’re just different (spools are better though).

Essentially a reel usually has a handle, a ratchet and a grip to help wind the line in. The advantage to a reel is that they’re easy to use, the line is much less likely to spill anywhere, and they’re often more practical for bigger amounts of line so if you like sending up SMBs from the bottom and you like deep diving then reels maybe more your bag. The downsides to a reel are they tend to be bigger and more cumbersome, they have moving parts so can break and jam and winding up 30 odd metres of line from the bottom with a small knob can be really quite fiddly and tiring.

Spools are just a solid piece of metal or plastic with line. By contrast, they’re more versatile, they don’t tend to break or jam, can be easily carried in a pocket or clipped to a D ring and are actually quite easy to wind in. The downsides are that they do need more practice to use and if not used properly are more likely to be dropped, lost or end up with line everywhere.

Good things to look for on a spool are stainless steel boltsnaps (brass ones, tend to crud up quickly which can be a real pain when using a spool) and a decent clearance of the lip of the spool to the amount of line on it (if the line is flush to the edge of the spool, you won’t be able to clip off the boltsnap).

At the end of the day either method will allow you to send an SMB to the surface and then wind in the line, the method that suits you best will probably depend a lot on the type of diving you do which we'll deal with in a separate blog