Surface marker buoy
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. A DSMB is considered by recreational scuba divers and service providers to be a highly important item of safety equipment, yet its use is not part of the entry level recreational diver training for all training agencies, and there are significant hazards associated with incompetent use.
A "safety sausage" or "signal tube" is a low volume tubular buoy inflated at or near the surface to increase visibility of the diver in the water. A DSMB can be put to this service when necessary. When used by a diver to indicate their position, any of these may be described as a personal marker buoy.
Another function for a buoy deployed by a diver is to mark the position of an underwater point of interest. In this use the buoy is attached to the target until the position has been recorded, or until the marking function is no longer required. A wider range of equipment is used for this function, including the same equipment that would normally be used for marking the position of the diver. A problem associated with this use is how to hold the buoy in position at the point of interest but still be able to retrieve it from the surface.
Standard buoy
Primary function
Surface marker buoys 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 which makes it easier to stay clear. In some waters divers may be required to use a float to mark their presence. The US Coast Guard has conducted a public education campaign to get divers to add identification information to their dive-floats, to help them identify and find lost divers, and so divers who lost their floats can advise the Coast Guard that, if found, their float should be regarded as a false alarm.Other uses
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 SMB 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 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 line 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.
A surface marker buoy is considered an essential tool in the initial project phases for establishing a coral nursery, when exploratory dives are being conducted to find suitable sites for nurseries, donor sites and transplantation sites. The SMB is used to mark these areas until a GPS position can be recorded.
Construction
Buoys for this use are usually either inflated and sealed by a valve or cap, or made from buoyant material, so they cannot 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 small 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. The torpedo buoys used by lifesavers are sometimes used as surface marker buoys as they are visible, tough, available, and reasonably low drag.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. If the lanyard clips to the buoyancy compensator, the user should take care to release it 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 in waters where boats may operate, due to high risk associated with snagging the object on a boat and dragging the diver upwards in spite of their decompression obligation or maximum ascent speed limit.
Hazards
- Although a primary function of a SMB is to notify boat users of the presence and location of divers in the water, it can happen that the buoy or line becomes snagged on a moving vessel. This will pull the line upwards, and if it is fastened to the diver, will pull the diver upwards, probably at an unsafe speed, putting the diver at moderate to severe risk of decompression illness.
- In areas where the variation of current with depth is strong, a diver being towed by an SMB may not be able to control their speed and direction of drift sufficiently to avoid being dragged against obstacles on the bottom, or to stay with other members of the group.
- In areas of high relief, the line may snag far above the diver. If the current is strong the diver may have to abandon the buoy to avoid being swept upwards in an arc.
- Loose line in the water is an entanglement hazard. A diver's equipment can snag on even a taut SMB line. In most cases this does not prevent the diver from surfacing, but it is possible to snag on both the diver and part of the fixed environment. Usually this can be cut loose if the diver has an appropriate cutting tool.
Decompression buoy
A closed DSMB, 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. A decompression buoy is not intended to be used to lift heavy weights: for this purpose divers use a lifting bag.
Shape and size
To serve effectively as a marker of the diver's position, a decompression buoy must be visible to the intended observers. This requires it to project above the surface as much as reasonably possible, and to have a sufficient projected lateral area above the water and a good contrast against the background.The shape which easily provides this functionality for an inflatable object is a tall cylinder or gently tapering cone. The preferred height depends on the expected sea conditions – in larger waves a taller buoy will be more visible, and for best visibility it should be close to upright, which requires a downward tension on the line in proportion to the height of the buoy. The lengths usually marketed range between 1 and 2 metres, with the larger models better suited to rougher open water and longer range visibility.