Distance line

A distance line, penetration line, cave line, wreck line or guide line is an item of diving equipment used by scuba divers as a means of returning to a safe starting point in conditions of low visibility, water currents or where pilotage is difficult. They are often used in cave diving and wreck diving where the diver must return to open water after a penetration when it may be difficult to discern the return route. Guide lines are also useful in the event of silt out.
Distance lines are wound on to a spool or a reel for storage, and are laid in situ by unrolling. The length of the distance line used is dependent on the plan for the dive. An open water diver using the distance line only for a surface marker buoy may only need, whereas a cave diver may use multiple reels of lengths from to 1000+ ft.
Reels for distance lines may have a locking mechanism, ratchet or adjustable drag to control deployment of the line and a winding handle to help keep slack line under control and rewind line. Lines are used in open water to deploy surface marker buoys and decompression buoys and link the buoy on the surface to the submerged diver, or may be used to allow easy return navigation to a point such as a shotline or boat anchor.
The material used for any given distance line will vary based on intended use, nylon being the material of choice for cave diving. A common line used is polypropylene line when it does not matter if the line is buoyant.
The use of guide line for navigation requires careful attention to laying and securing the line, line following, marking, referencing, positioning, teamwork, and communication.

Applications

Wreck diving: The use of guide lines in wreck diving is very similar to their use in cave diving.
Diving in low visibility: A guide line may be used for dives where poor visibility may make it difficult to return to the starting point, and there is a safety or operational requirement to return to that point.
Other dives where it is necessary or highly desirable to return to a starting point: This may include dives where a shot line or anchor line is to be used for ascent, and other dives where a specific exit point is chosen for safety or convenience. Working divers may use a guide line to allow confident and efficient movement to, from and around the workplace. This may also be referred to as a jackstay. In these applications the guide line is generally more for efficiency than for safety, but it may also help the divers stay away from potentially hazardous areas.
Guidelines may be used to direct divers on underwater search patterns.
Guide lines may also be used as a means of directing tourists around a dive site, between points of interest which may be difficult to find without the line. This form of guide line may be permanently placed.

Equipment

Cave line and wreck line

, strictly speaking, is line used for navigation in caves, but the term is used generically for the type of line carried by divers on reels and spools for use as guide lines and surface marker buoy lines. It is made using the same machines and materials to the same specifications as the equivalent line used for other purposes. When used in wreck diving, the same material is called, though a thicker line may be used for better abrasion and cut resistance in an environment of rusty metal edges.
;Material:
;Structure:
;Thickness:

Code	Diameter in mm	Diameter in inches	approximate breaking strength	approximate breaking strength
#18	1.58	1/16"	65.4	144
#24	1.86	0.073	104	230-250
#36	2.11	0.083	150	330-360
#48	2.48	0.098	182	405-550
1/8"	3.18	0.125	331	728

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;Marking :

Reels

A dive reel comprises a spool with a winding knob, which rotates on an axle, attached to a frame, with a handle to hold the assembly in position while in use. A line guide is almost always present, and there is usually a method of clipping the reel to the diver's harness when not in use.

The handle may be positioned on top, behind, or at the side of the spool.
The spool is the storage area for the line. It has a hub and two flanges, which prevent the line from falling off the sides of the hub. The size of the spool and the thickness of the line together determine the length of line the reel can hold, though this is also affected by how neatly the line is wound onto the spool.
The winding knob is on the side of the spool. Most reels are laid out for holding in the left hand and winding with the right. The knob is usually attached by a bolt which allows it to rotate, giving a more secure grip to the winding hand.
The line guide is a slotted attachment which guides the line onto the spool. This helps to control the stacking of the line on the spool when it is reeled in.

Other accessories which may be present include:

Ratchet mechanism, which prevents the spool from rotating when there is tension on the line. this prevents inadvertent unreeling of line, as the ratchet lever must be held in the open position to allow unwinding. The ratchet mechanism is usually a ratchet wheel on the spool which engages with a spring-loaded pawl which can either be held open by a trigger lever, or clipped open by a knob or lever on the handle. The ratchet allows the spool to be manually rotated to wind in line when engaged.
A lock down screw or latch is an alternative way of preventing spool rotation if there is no ratchet. This will prevent rotation in both directions.
An adjustable brake may be fitted to control the friction of the spool on the handle, so that some tension is required to unwind the line, and the spool can not spin freely, causing overruns and tangled line. The brake may be released when reeling in.
The line may have a snap hook or an end loop to tie off the free end.
A clip may be attached to the handle to clip the reel off to the diver's harness.
A lanyard may be connected to the handle to connect the reel to the diver while in use.
A wrist loop may be fitted to the handle to attach the reel to the diver while in use.

Reels may be made from a wide variety of materials, but near neutral buoyancy and resistance to impact damage are desirable features, which are easiest to achieve in engineering polymers such as nylon, acetal and polyethylene.
Reels may also be open or closed. This refers to the presence of a cover around the spool, which is intended to reduce the risk of line tangles on the spool, or line flipping over the side and causing a jam. To some extent this works, but if there is a jam the cover effectively prevents the diver from correcting it. Open reels allow easy access to free jams caused by overwinds or line getting caught between spool and handle.

Spools

Finger spools, dive spools, line spools, or cave spools, are a simple, compact low tech alternative to reels best suited to relatively short lengths of line. They are simply a pair of circular flanges with a hole in the middle, connected by a tubular hub, which is suitably sized to use a finger as an axle when unrolling the line. Line is secured by clipping a bolt snap through a hole on one of the flanges and over the line as it leaves the reel. Line is reeled in by holding the spool with one hand and simply winding the line onto the spool by hand. Spools are most suitable for reasonably short lines, up to about 50m, as it becomes tedious to roll up longer lengths. The small, compact size, and low cost make them useful for various purposes where long line is not required.
Spools may be made from any material that is strong enough and suitable for underwater service, but engineering thermoplastics are most common.

Line holders

A line holder is a simple device for storing and deploying line underwater. A simple type is an H-shaped piece of wood, plastic or stainless steel used to manually wrap a moderate length line.
It may be used in conjunction with a surface marker buoy or a delayed surface marker buoy by divers, or for a short guide line. When used to deploy a DSMB, negative buoyancy is helpful in to safely unwind the line underwater as the buoy floats to the surface, as it may not be possible to manually unwind the line fast enough to avoid being pulled upwards. For this method of deployment the line holder is dropped and left to unwind by gravity. This may not work as intended, and the line holder may unwind more line than is useful if deployed in midwater. This problem falls away when used with a regular SMB, or for a short guide line.

Types of reel or spool

Cave reels

Cave reels are reels used in cave diving. They are often specifically designed for laying and recovering line at swimming speeds, and may differ from reels used for other purposes such as deploying a DSMB. They usually have an adjustable drag facility to prevent overrun, and a lock, but may not have a ratchet.
Exploration reels are large cave reels that can hold a lot of line, in some cases as much as, and are used during major exploration dives where this length is needed. They tend to be bulky and impractical for most other purposes. They are often used to lay permanent line.
Penetration reels are used to run line from a point with direct access to the surface to the start of a permanent line. This may be as much as a few hundred feet into the overhead. These reels will usually carry of line. These reels are also used for short penetrations where there is no permanent line.

Spool types

Jump and gap spools are used to bridge gaps between two permanent guide lines. Line length is generally about
Safety spools are used in emergencies such as line breaks, searches for a lost guide line or lost diver. They generally carry about of line, and one should be carried by each diver. They are not intended for use under an overhead except in an emergency.
DSMB spools are used to deploy inflatable surface marker buoys from depth. They have negative buoyancy and enough line to reach the surface from the planned deployment depth, and are often made of injection moulded plastic and sold as a set with the DSMB. Larger versions which can carry more line and are suitable for use from deeper depths are more likely to be machined from aluminium and are both more ergonomically shaped and more expensive.