page_title: Surface marker buoy
section_title: Other uses
hierarchical_section_title: Surface marker buoy / Standard buoy / Other uses
caption_attribution_description: English: Survey SMB with GPS box fitted and GPS - top view
context_page_description: A surface marker buoy, SMB, dive float or simply a blob is a buoy used by scuba divers, at the end of a line from the diver, intended to indicate the diver's position to people at the surface while the diver is underwater. Two kinds are used; one is towed for the whole dive, and indicates the position of the dive group throughout the dive, and the other, a delayed surface marker buoy, DSMB or decompression buoy, is deployed towards the end of the dive as a signal to the surface that the divers have started to ascend, and where they are going to surface. Both types can also function as a depth reference for controlling speed of ascent and accurately maintaining depth at decompression stops. Surface marker buoys are also used by freedivers in open water, to indicate the approximate position of the diver when submerged. They may also be used to support a catch bag or fish stringer by underwater hunters and collectors.
context_section_description: Occasionally an SMB is used in conditions of poor visibility where there is a risk of inadvertently penetrating an overhead environment, to ensure that there is a guideline leading out of the overhead to the surface. 
  In an emergency where buoyancy control is lost due to a BCD leak, the DSMB can be used to compensate for buoyancy loss to the extent of the buoyancy of the surfaced buoy. This may require considerably more effort to wind in the line, but finning upwards will help. The buoyancy of any SMB can be used to help maintain a constant depth at decompression stops, and the process of winding in the line can help maintain a steady ascent rate. 
  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.
  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. Position of a point of interest can be established by the timestamp on a photograph taken of the feature if the camera clock is synchronised with the GPS unit. The position will be most accurate when the buoy is tensioned to float as close as possible to directly above the diver.
  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.
caption: SMB towed with GPS receiver to record the track of the diver during a dive
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