A diving shot is an item of diving equipment consisting of a weight, a line and a buoy. The system used to mark the location of a dive site and allow divers to descend and ascend safely. They are mainly used in recreational diving in conditions of low visibility or strong tides where the underwater navigation to and from the dive site and the surface is difficult or on deep dives where a navigation error would dramatically reduce useful underwater time. The shot has several uses. It marks the dive site for the diving safety cover on the surface. The line is useful for navigation, for buoyancy control and for doing long decompression stops during an ascent in a current. Shots are more difficult to use in strong currents: the buoy may be dragged under and divers may find it difficult to hold onto the line. In strong currents divers often decompress on decompression buoys instead of shot. Typically the weight is lead and weighs 20 kg (44 lb) or more. For security, the buoy should provide more buoyancy than is required to make the complete system float. For example, for a 20 kg weight use at least a 20 litre buoy. The line should be longer than the depth of the dive site and at least 10 mm (0.4 inch) in diameter. The thickness is important because the shot weight is often lifted by hand using the line and the pressure of a thin line causes pain. Floating line, such as polypropylene, although cheap, can foul the propellors of boats. There are several solutions to this: coil the excess line after deployment use a "top tensioning" arrangement: the line runs freely through a ring on the buoy and is weighted at the top end by a small weight use a "bottom tensioning" arrangement: the line runs freely through a ring on the weight and is lifted at the bottom end by a small, rigid buoy Sometimes, a lifting bag is attached to the weight and used to help raise the weight at the end of the dive. At the surface, the direction of the current is indicated by three signs: the direction of the line, a "horseshoe" bow-wave on the front of the buoy and a wake at the back of the buoy. Sometimes, two buoys are used at the surface. The second buoy can further indicates the strength and the direction of the current or be misleading because it may indicate the direction of the wind. It can be used to provide additional buoyancy if one buoy is dragged underwater