Decompression buoy
A diver preparing to inflate a surface marker buoy using the primary demand valve
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| Acronym | SMB, DSMB |
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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.
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.
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.
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 dragging the diver upwards in spite of their decompression obligation or maximum ascent speed limit.
Competitors in the underwater sport underwater orienteering are required to tow a SMB with a buoyancy of at least 8kg 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 SMB's are designed for low drag, which is a useful feature in any SMB that will be towed by the diver.
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