Distance line

A distance line, penetration line, cave 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. 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 (300 m).
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.
Cave diving: Guide lines are used in cave diving as standard procedure whenever it is possible that the divers may be confused about the way out. In effect, this is in any dive where there is no free surface overhead and where daylight is not visible at all points of the dive. By the most common definition of cave diving, this would mean guide lines should be used for all cave dives.
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.
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.
Cave line, strictly speaking, is line used 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. 
Reels.
A 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.
Other accessories which may be present (usually not all on the same reel) include:
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 (delrin) 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 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 by application.
Exploration reels are large 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 purposes.
Penetration reels (also known as primary or lead 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.
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.
Line markers.
Line markers are used for orientation as a visual and tactile reference on a permanent guide line. Directional markers (commonly arrows), are also known as line arrows or Dorff arrows, and point the way to an exit. Line arrows may mark the location of a "jump" location in a cave when two are placed adjacent to each other. Two adjacent arrows facing away from each other, mark a point on the line where the diver is equidistant from two exits.
Non-directional markers ("cookies") are purely personal markers that mark specific spots, or the direction of one's chosen exit at line intersections where there are options. One important reason to be adequately trained before cave diving is that incorrect marking can confuse and fatally endanger not only oneself, but also other divers.
Silt screws.
Silt screws are pegs which are inserted into soft bottom sediments to tie off the guide line when there are no suitable natural formations. A common style of silt screw is a length of rigid PVC tube cut to a point at one end, with a notch at the other to secure a wrap. These are lightweight and durable, and are easily transported by attaching them to a cylinder with bungees.
Procedures.
Procedures associated with the use of guide lines include primary and secondary tie-off, laying line, positioning line, following line, marking and identifying line, identifying jumps and exit directions, searching for a lost line, repairing a break, and recovering line.
Laying line.
Most reels are designed to be held in the left hand, particularly for recovering line. Line should be kept under tension at all times to avoid slack which is more likely to snag on divers' equipment. An adjustable brake may be used to provide resistance to rotation, or this can be done by pressing a thumb or finger against the spool as it rotates, which is quick and controllable, but requires some attention, and fails if the reel is dropped. The line should be kept clear of the divers body during the laying to reduce risk of snagging on the diver's equipment. This can be achieved by holding the reel away from the body in clear water.
The line should be laid so that it can be followed in the worst possible visibility, and allow for other emergency procedures such as gas sharing at the time. This generally means that the divers must be able to follow it by feel.
Line traps are places that a line can pass through but are too tight for a diver, making it impossible to follow the line by feel. Avoiding line traps is a large part of the skill of laying line, and generally require the line to be tied off in such a way that it does not enter line trap areas when under the necessary tension to prevent slack. As a general principle, this requires the line to be secured at any change of direction, by a "placement" or a "tie-off."
A placement is made by running the line past a contact surface in such a way that friction or geometry of the line position prevents the line from moving. This may be adequate, for example, when going past a boulder which has a notch which holds the line in place.
A tie-off generally involves "wrapping" the line round a fixed object once or twice, and may be made more secure by adding a "lock", which is made by looping the reel around the incoming line and taking up the slack, before continuing the lay. A lock also puts the incoming and continuing parts of the line in contact, which makes it much easier to follow by feel.
The security of both placements and tie-offs depends on the detail of the place where they are made, and some may be disrupted by a pull on the line in the wrong direction. This makes the skill of following a line without dislodging it an important safety issue.
Placements and tie-offs take time to set up, and time to negotiate when following the line by feel, so they should be limited to those which are actually useful. To be useful, a tie-off would limit the distance to the next tie-off to a reasonable distance which could be negotiated with a safety spool in case of a break, or which prevents a line trap.
Line routing.
The position of the line can make a large difference to the ease with which divers may follow it. The line would ideally allow divers to see of feel it while swimming comfortably without coming into contact with or being obstructed by, the bottom, sides or ceiling. This is not always possible.
The bottom is often easy to follow by eye and often has suitable tie-off points, but sometimes swimming close to it may cause silting, and it may result in a higher inert gas loading and higher gas consumption than an alternative route.
The sides have a problem in that they constrain divers to swim on one side of the line only.
The ceiling may be difficult to tie-off to, and is difficult to follow, as the divers will generally have to look upwards. Following a ceiling guide line by feel can be even more awkward and tiring.
Entering the overhead.
If the lead diver is the one to lay line on entering the overhead environment, it is possible for other divers in the team to follow the guide line for their safety, and they can check the security of tie-offs and that the line is not moving into traps unnoticed. This order also reduces the risk that a diver will become separated from the team if the line-layer has to stop for any reason.
Primary tie-off.
The primary tie off is made in a place with direct vertical access to the surface, if possible, in a place where it is unlikely to be interfered with by outsiders. This tie-off should be secure.
Secondary tie-off.
The secondary tie-off is made just inside the overhead, where the route to the exit is obvious even if visibility is lost. This is a backup in case the primary tie-off is compromised.
Connection to a permanent guide line.
The temporary entry guide line should be tied off to the permanent line and the reel locked. A recommended method is to make two wraps around the permanent line and then clip the reel back to the line.
Recovering a temporary guide line.
If the last diver out is the one to reel in the line, the lead divers can prepare the way by loosening tie-offs just ahead of the reel, and there is less risk of them becoming separated from the group without the reel operator noticing. This can significantly reduce the risk of anyone getting lost. This order is even more safety critical if the exit is in poor visibility. In an emergency, the reel would be left at a tie-off and the divers would exit without recovering the line, as this would save time.
Navigation.
There is more to navigating by guide line than merely following the line.
Survey of line condition: A permanent guide line may have deteriorated since the latest reliable report on its condition. A complete check of line condition during the penetration will ensure that there are no unpleasant surprises like line breaks or line traps during the exit
Distance from the guide line: Good visibility may be compromised very quickly in some environments. If a diver is more than arm's reach from the line when visibility is lost, it will be necessary to find the line in possibly adverse circumstances. This can be avoided by remaining within reach of the line.
Guide lines are frequently used in areas with heavy silt, and the line may lie on the silt or be close above it. If silt is kicked up everyone following will have poor visibility, and the return along the line may be in low or zero visibility, which is at the very least inconvenient. The ability to fin without kicking up the silt relies on good level trim and appropriate finning techniques.
Orientation.
It can be useful to build up a mental model of the route, even to make notes and sketches indicating major landmarks and changes of direction. This helps to reduce disorientation on return along the line, when the surroundings may look very unfamiliar because they are observed from a different direction. For the same reason, occasional checks in the exit direction will make the route look more familiar on return.
Compass checks.
Use of a compass for occasional checks of direction has the advantage of providing a secondary input for sense of direction. This can be of great value when disorientated for any reason.
Pace and order of activity.
It is important for safety to ensure that the gas supply for the return is sufficient for reasonable contingencies. It is more conservative gas management to do any work on the outward leg, and return directly, unless specifically planned otherwise. As the exit is approached, the gas stock and decompression obligations can be re-assessed, and plans adjusted accordingly.
Safety.
Although distance lines are considered to be safety equipment, there are hazards associated with their use.
History.
In 1977, Sheck Exley published "Basic Cave Diving: A Blueprint for Survival" which pointed out that the lack of a continuous guide line was one of five main contributing factors in cave diving accidents.