All divers should carry a compact scuba finger spool with 30m | 100ft line. Cave divers should carry a durable primary reel with minimum 100m | 330ft line and several jump spools with various line lengths. Use braided nylon line. Line markers should be personalised for look and touch.
Finger spools hold up to 25m-50m of line, and are used with a stainless steel double ended bolt snap . Most divers will require a 30m | 100ft finger spool for shallow water (20m | 66ft) SMB deployment.
In cave diving, a finger spool is also known as a jump spool, and is used to bridge two unconnected main lines (e.g. to follow a side passage) and as an emergency reel for cut line, entanglement, and lost line drills. Cave divers will carry a selection of different sized finger spools with different line lengths.
The regular centre hole diameter is 2.5cm | 1in, but larger 3.2cm | 1.25in holes are available for cold water (thick gloves).
Spool and reel materials
For spools and reels it is necessary to consider which material achieves the right combination of price point, weight for travel, buoyancy underwater, and durability:
Plastic – Cheap, light weight, positive or slightly negative buoyancy, not durable.
Aluminum – Expensive, light weight, slightly negative buoyancy, very durable.
Stainless steel – Expensive, heavy weight, negative buoyancy, very durable.
Braided nylon line is best
Nylon is negatively buoyant so will sink if it comes loose underwater. It is better to have a line laying along the floor where you can see it instead of floating at the ceiling. Nylon is also very resistant to the elements and can be left in place for years without noticeable deterioration.
Dyneema line is stronger than nylon for any given line thickness (so you can get more length on the same reel or spool), is negatively buoyant, and resistant to the elements. However, Dyneema is, for the moment, unjustifiably expensive for scuba diving applications. Other line types are not suitable for various reasons (e.g. polypropylene floats and cotton line rots very quickly).
The two main types of nylon line are twisted and braided. Twisted nylon line has a high tensile strength and braided nylon line is more abrasion resistant. The purpose of laying line is not to hold any great weight or tension, it is to provide a reliable marking through cave or wreck environments. Therefore braided line is more appropriate.
Line thickness is measured in gauge. For scuba diving we use 24-gauge for most applications, and otherwise:
18- gauge to achieve extra distance on the same spools where abrasion is less of a concern,
26-gauge for wreck penetration or other circumstances where abrasion and sharp edges are a concern,
36-gauge and above for permanent fixed lines in high traffic areas.