Diving Shot - Construction

Construction

Typically the weight is lead or iron, and weighs around 20 kg (44 lb). To prevent the assembly sinking and being lost, the buoy should provide more buoyancy than is required to make the complete system float. For example, if a 20 kg/44 lb weight is used, the buoy must provide more than 20 kg buoyancy: it needs to be at least 20 litres/4.4 gallons in volume.

Some agencies and codes of practice recommend a buoy that can not be dragged underwater by the divers relying on it for depth control.

If used in a current, the shot must be prevented from being dragged away. This can be done by attaching it to a small anchor by a short line. Another method used is to tie the shot off to an object on the bottom by a thin line, which can be either released by the last diver, or broken when the shotline is recovered.

The line should be longer than the depth of the dive site at the deepest state of the tide during the dive, and at least 10 mm (0.4 inch) in diameter. A thick line is used because the shot weight is often lifted by pulling the line by hand and the pressure of a thin line causes pain and damage to the skin and the hands when the shot is recovered.

Floating line, such as polypropylene, although cheap, can foul the propellers of boats and is an entanglement hazard to divers if it is allowed to accumulate near the surface. There are several ways of avoiding 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.

A large float will go up and down with the passing waves. This can be disconcerting and inconvenient for the divers if they need to hold onto it in a current, as they will to some extent go up and down with the line. This effect is minimised if a small diameter spar buoy is used as the primary buoy to tension the line, as it will tend to react less to the waves, with a large volume safety buoy on a short tether, which will prevent the spar buoy from being dragged down if the tension gets too much.