underwater diving Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on context ...

, 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 A guideline is a statement by which to determine a course of action. A guideline aims to streamline particular processes according to a set routine or sound practice. Guidelines may be issued by and used by any organization (governmental or pri ...

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 A diving shot line, shot line, or diving shot, a type of downline or descending line (US Navy), is an item of diving equipment consisting of a ballast weight (the shot), a line and a buoy. The weight is dropped on the dive site. The ...

is a special case of downline which uses a heavy weight at the bottom and a float at the top. A

jackstay A jackstay is a cable or bar between two points to support and guide a load between those points, or as an anchor to attach something to be constrained along that line. The term is mostly used in a marine context and originated on sailing ships. N ...

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 In physics and geometry, a catenary (, ) is the curve that an idealized hanging chain or cable assumes under its own weight when supported only at its ends in a uniform gravitational field. The catenary curve has a U-like shape, superficia ...

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

, 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 A sea anchor (also known as a parachute anchor, drift anchor, drift sock, para-anchor or boat brake) is a device that is streamed from a boat in heavy weather. Its purpose is to stabilize the vessel and to limit progress through the water. ...

may be used to limit wind drift, particularly if attached to a boat with significant windage.

Variations

* The anchor cable is commonly used by

divers Diver or divers may refer to: *Diving (sport), the sport of performing acrobatics while jumping or falling into water *Practitioner of underwater diving, including: **scuba diving, **freediving, **surface-supplied diving, **saturation diving, a ...

as a downline for ascent and descent. They may use ''granny lines'' from the entry point of the boat to the anchor cable as an aid for divers in currents, or low visibility. A basic granny line has a large, heavy shackle sliding over the anchor cable to tension it, and is tied off at the boarding area, usually at the stern of the boat. Divers entering the water at the stern of the vessel will be down-current of the anchor cable, but can use the granny line to pull themselves to the downline, which takes less effort than swimming up-current. On the return the anchor cable and granny line will guide the diver to the exit ladder or platform. *

Shotline A diving shot line, shot line, or diving shot, a type of downline or descending line (US Navy), is an item of diving equipment consisting of a ballast weight (the shot), a line and a buoy. The weight is dropped on the dive site. The ...

s are near-vertical downlines that use a weight at the bottom and a float at the top. Several methods can be used for tension control when needed. * A lazy shot, blue-water downline, or midwater downline, is a weighted line supported by a large float or boat, which does not extend all the way to the bottom, and is used for blue-water diving work, usually by scientific divers on scuba.

References

{{Underwater diving, divsup Diving equipment

Arrangement and use

Variations

See also

References