Surface marker buoy

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search
Surface marker buoy
A diver preparing to inflate a surface marker buoy using the primary demand valve
AcronymSMB, DSMB
Other names
  • Delayed surface marker buoy
  • Decompression buoy
  • Deco buoy
  • Blob
  • Marking the position of a group of divers
  • Marking the position of a diver making an ascent
  • Signal to the surface party that assistance may be needed

A surface marker buoy, SMB or simply a blob is a buoy used by scuba divers, with a line, to indicate the diver's position to their surface safety boat while the diver is underwater. Two kinds are used; one (SMB) is towed for the whole dive, and indicates the position of the dive group, and the other (DSMB) is deployed towards the end of the dive as a signal to the surface that the divers have started to ascend. Both types can also function as a depth reference for controlling speed of ascent and accurately maintaining depth at decompression stops.

A "safety sausage" is a low volume tubular buoy inflated at or near the surface to increase visibility of the diver.

Standard buoy[edit]

Surface marker buoy

SMBs are floated on the surface during a dive to mark the diver's position during drift dives, night dives, mist or disturbed sea conditions such as Beaufort force 2 or greater. The buoy lets the dive boat follow the divers and highlights their position to other boat traffic.[1]

Buoys for this use are usually either inflated and sealed by a valve or cap, or made from buoyant material, so they do not deflate or flood during the dive, rendering them ineffective. High-visibility colours such as red, orange and yellow are popular. Sometimes the float includes a diving flag. If the buoy is to be towed by the diver at any speed, a low drag float and small diameter line can reduce the drag significantly.

To avoid losing the reel, a lanyard may be used to attach the diving reel to the diver. This lanyard can clip to the buoyancy compensator or go around the wrist. Alternatively, the lanyard can be long enough to float above the diver and stay out of the way. If the lanyard clips to the buoyancy compensator, the user should take care to release if there is surface boating activity, as boats may drag divers up by their SMB reels.

The DIR diving philosophy considers unsafe any attachment of the diver to equipment or objects which end above the water surface, due to high risk associated with snagging the buoy on a boat and dragging the diver upwards in spite of their decompression obligation or maximum ascent speed limit.[2]

Other uses[edit]

underwater hunting buoy with flag, catch bag line and grapnel for hooking to the bottom
dive reel with rolled up DSMB stowed

Competitors in the underwater sport underwater orienteering are required to tow a SMB with a buoyancy of at least 8 kilograms (18 lb) during competition swims. This is both for safety, and to allow the judges to monitor the route taken by the diver and to score the points for time and accuracy. Such SMBs are designed for low drag, which is a useful feature in any SMB that will be towed by the diver.[3]

A GPS tracker can be mounted on the SMB to record a dive track. This can be downloaded and used to establish positions of underwater landmarks with reasonable accuracy depending on surface conditions and current.

Spearfishers also use surface marker buoys to mark the position of the speargun in case it is necessary to let go after spearing a fish or for any other reason. These are towed on a line attached to the speargun handle. Similar buoys with catch bags are used by freedivers for other underwater hunting and gathering activities. They serve as a place to gather and transport the catch, and may be equipped with a means of hooking to the bottom to stop them drifting away while the diver is busy.

Decompression buoy[edit]

Packed Surface marker buoy
Delayed surface marker buoy ready to be inflated
Delayed surface marker buoy inflated
Deco buoy with diver at the surface

A delayed surface marker buoy (DSMB), decompression buoy or deco buoy is similar to a surface marker buoy but is deployed whilst the diver is submerged and generally only towards the end of the dive. The buoy marks the diver's position underwater so the boat safety cover can locate the diver even though the diver may have drifted some distance from the dive site while doing decompression stops. A reel and line connect the buoy on the surface to the diver beneath the surface.[4][1][5] Alternative means of marking one's position while doing decompression stops are shot-lines, uplines and decompression trapezes.

A closed SMB, inflated through a valve, is likely to be more reliable, by remaining inflated, than an open ended buoy which seals by holding the opening under water.[2] A decompression buoy is not intended to be used to lift heavy weights: for this purpose divers use a lifting bag.


There are at least four methods of keeping the air in the inflated deco buoy. The buoy can be:[2]

  • open ended (preferably with small independent weight to keep the opening submerged to prevent the air escaping);
  • open ended self-sealing buoys (the air in the buoy expands as the buoy ascends sealing a neck at the bottom of the buoy);
  • sealed, with an inflation valve and a pressure relief valve;
  • sealed, with a built in air supply and a pressure relief valve.

Divers following the recommendations of some training organisations carry two differently coloured deco buoys underwater so that they can signal to their surface support for help and still remain underwater decompressing. For example, in some circles in Europe, a red buoy indicates normal decompression and a yellow buoy indicates a problem, such as shortage of gas, that the surface support should investigate and resolve. Although in other circles, two buoys (any colour) up one line means the same, currently the protocol is not universally accepted even within Europe.[1]

Some types of buoy provide an attachment for a strobe light, cyalume stick or writing slate, which can convey signals to the surface support.

Deployment problems and hazards[edit]

Several problems may be encountered when deploying deco buoys.

  • The diving reel jams after the buoy is inflated, dragging the diver up. This can be compounded by expansion of the gas in a dry suit, and gas in the buoyancy compensator, all of which must be dealt with at the same time, and fast enough to prevent an uncontrolled ascent. To avoid this, a diver can:
    • use a simpler system or a reel which cannot jam (e.g. a weighted spool of line);
    • detach the lanyard connecting the diver to the reel before inflating the buoy and ensure no equipment is snagged on the buoy or reel. If the reel jams it is simply abandoned;
    • attach two reels to each other in series. If one fails the other is unlocked to reel out its line.
  • The diver or part of the diver's equipment gets snagged or entangled on the deco buoy, dragging the diver up,[6] with possibly fatal consequences.[7] To avoid this, a diver may tie the lanyard of the reel to something solid on the sea bed before inflating the buoy, giving time to sort the problem out. This is not always practicable.
  • The diver removes the primary demand valve from his or her mouth to inflate the buoy, and is therefore at a disadvantage in dealing with any other problems that might arise as the deco buoy goes up. The ways to avoid this include:
    • using a deco buoy with its own air supply;
    • using a secondary demand valve, such as an octopus, to inflate the buoy;
    • using a sealed buoy with an inflation valve, which is filled by blowing directly into the valve inlet or by attaching a medium-pressure inflation hose from the buoyancy compensator (BCD) or dry suit (the valve does not retain the hose connector, like the BCD or suit inflator valve, and the hose can be easily pulled off the valve when the buoy is sufficiently filled);
    • holding an open ended buoy above the primary demand valve and direct several exhalations up into the open end of the buoy. This technique is also useful in cold conditions to prevent a freeflow caused by high flow rate due to pressing the purge button.
    • Filling an open-ended DSMB from the buoyancy compensator power inflator head. This also allows the diver to dump gas from the BCD into the DSMB, keeping a constant buoyancy for part of the filling procedure. If there is not enough gas in the BCD, the power inflator can be used to supply more gas to the DSMB by pressing the inflate and deflate buttons simultaneously, diverting the inflation gas through the exhaust valve to the DSMB. Once the DSMB reaches the surface, it will support the diver and buoyancy can be readjusted under controlled conditions.
  • The demand valve develops a free-flow while filling the DSMB. This can deplete the contents of the scuba cylinder rapidly, and since the DSMB is commonly deployed at the end of the dive, when the breathing gas has reached a critically low level, this can put the diver in a situation of inadequate gas supply for a safe ascent. This can be avoided by:
    • not using a demand valve to fill the DSMB;
    • using a desensitised demand valve which is less likely to free-flow;
  • The reel over-rotates, causing a loose bight, which can snag or jam the reel. This can be avoided by having sufficient friction in the reel spindle that the drag slows rotation rapidly when there is not enough tension in the line to maintain unrolling.

Safety sausage[edit]

A safety sausage is an inflatable buoy used when the diver is at the surface to indicate the diver's position to the dive boat, reducing the risk of losing contact when air, light or sea conditions decrease the visibility of the divers from the boat.[8][9] The sausage is a plastic tube that is normally inflated by putting one end under water and purging the second stage underneath to inflate it. Inflated tubes are normally about 6 feet (2 m) tall. Uninflated sausages roll up and fit in a buoyancy compensator pocket. Commercial boat dive operations, especially at offshore reefs or areas known for strong currents or mercurial weather, may require divers to carry safety sausages. A safety sausage is not a substitute for a surface marker buoy or diver down flag.

See also[edit]

  • Line holder – Simple device for storing and deploying line underwater


  1. ^ a b c Staff. "Recommendations Concerning the Use of Surface Marker Buoys" (PDF). British Diving Safety Group. Retrieved 7 March 2016.
  2. ^ a b c Marshall, Dean (2005). "Lift Bags and Surface Marker Buoys". DIRquest. Global Underwater Explorers. 6 (1).
  3. ^ "ORIENTEERING Rules Edition 2009/01". Confédération Mondiale des Activités Subaquatiques. p. 13. Retrieved 2016-06-13.
  4. ^ Staff (2015). "Delayed surface marker buoy". BSAC Safe Diving. British Sub-Aqua Club. p. 18. Archived from the original on 3 April 2012. Retrieved 7 March 2016.
  5. ^ Nawrocky, Pete (2014). "We're Over Here!". Alert Diver online, Spring 2014. Divers Alert Network. Retrieved 7 March 2016.
  6. ^ Staff. The Diver Clinic. Bournemouth, UK.: Atlantic Enterprise UK Ltd. Retrieved 5 January 2018. Missing or empty |title= (help)
  7. ^ Concannon, David G. (18–20 May 2012). Vann, Richard D.; Denoble, Petar J.; Pollock, Neal W. (eds.). Rebreather accident investigation (PDF). Rebreather Forum 3 Proceedings. Durham, North Carolina: AAUS/DAN/PADI. pp. 128–134. ISBN 978-0-9800423-9-9.
  8. ^ Davies, D (1998). "Diver location devices". Journal of the South Pacific Underwater Medicine Society. 28 (3). Retrieved 2009-04-02.
  9. ^ "The Safety Sausage Story" Archived 2007-07-15 at the Wayback Machine,


External links[edit]