In underwater diving, a downline is a piece of substantial cordage running from a point at the surface to the underwater workplace, and kept under some tension. It can be used as a guideline for divers descending or ascending, for depth control in blue-water diving, and as a guide for transfer of tools and equipment between surface and diver by sliding them along the downline at the end of a messenger line. A shotline is a special case of downline which uses a heavy weight at the bottom and a float at the top. A jackstay is a more lateral equivalent, that commonly follows a surface, and will not usually allow materials transfer without a messenger line from the destination end.
Arrangement and use
There is no definitive arrangement. A downline is a generic piece of support equipment that can be set up using available components and is defined by its function. The top end can be secured to any suitably secure point in an appropriate place. This can be a large float or buoy, the dive boat or diving platform, or other substantial item on the shore. The bottom end can be secured to any suitably secure point at or near the worksite, such as heavy structure, the actual bottom rock, a large weight or an anchor. Enough tension to keep the catenary suitable for use is needed, and that may not be very much for a near vertical line.
A downline used for open ocean diving is much the same as a shotline , but does not reach all the way to the bottom. An open-ocean downline is weighted at the bottom, and attached to a substantial float at the surface, which may be tethered to the boat. It may be marked at intervals by knots or loops, and may be attached to the decompression trapeze system. In some cases a sea anchor may be used to limit wind drift, particularly if attached to a boat with significant windage.