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A thumb spool is one of the most diversely useful pieces of equipment a diver can carry. It can be used for multiple applications and is something everyone should carry. It can be used to:

  • Conduct a search
  • Measure a distance
  • Denote a path to an exit or ascent point
  • Launch a DSMB 


A properly prepare thumb spool should have two loops tied in the end. (Figure 1.)

  • A loop large enough to pass the spool itself through easily
  • A small loop at the end of the large loop used to clip a double-ender to and also as a pull tab to untie the spool

A spool should always be used in tandem with a double ender bolt snap. That double ender should be sufficiently large enough to easily use with whatever gloves you are wearing.

The double-ender also serves multiple purposes.

It is:

  • The attachment point of the spool to your d-ring
  • The handle used to wind up your spool
  • A weight to keep your line taught and harder to get tangled
  • A connection point to link two spools together 


Stowing your Spool when diving:

1.       In a pocket or pouch

2.       Hanging from a d-ring

If hanging a spool from a d-ring, use the line (Figure 2.) to wrap over the thumb slider and back through the eye of the bolt snap to prevent accidental opening and unexpected deployment or loss of the spool.


Spool Size:

Thumb spools come in a variety of sizes, anywhere from 30 feet up to 200 feet or more. The right size of spool is determined by the primary role that spool is to play.

For DSMB deployment, a 50-foot spool is ideal. Generally speaking, you would never deploy a DSMB from deeper than 50 feet. And at that depth, even for moderate decompression schedules, you would be able to launch and hold ceilings at the 40-foot mark and above. A spool longer than 50 feet will certainly work for DSMB deployment; however you have a lot more line to wind up if you should have to let go of the spool and you also want to consider the size of the physical spool itself. A smaller spool will fit in your pocket better than a larger spool.

For exploration, a longer spool is better. I like 100-foot spools as the go to when I want to explore. At depth, 100 feet is a manageable distance to venture away from a size without being too worried about going too far from your ascent point.

For marking a route, a much longer spool may be necessary and often several spools may be needed. Keep in mind, that the more line you pay out … the more line you need to wind back up, and this time should be factored in when you are on deeper dives.



 1.       Tying off at the start – using the large loop we created makes deploying the spool simple for virtually any application. Wrap the end of the line around the tie off point – an anchor chain, another rope or line, a piece of wreckage – and feed the spool back through the large loop. The line is now pulled tight and locked onto the tie in securely, and cannot become untied or unclipped accidentally. (Figure 3)

2.       Tying off at the far end – if you want to run a line, say from a descent line to the bow of a shipwreck, you may want to tie off the spool end at the wreck when you get there. Pull the line reasonably tight, so that it is up off the bottom, but not so tight as to become a clothesline and wrap the line three or four times around anything suitable at your far end. The use your bolt snap and one of the holes in the spool to lock it in place. (Figure 4) 

3.       Linking two spools – if you have come to the end of your first spool and need to extend the range, use your double-ender from the first spool to connect the spool body to the end of the line on the second spool and continue paying out the line. (Figure 5) 

4.       Marking your line – when used to measure distance I like to have a 100-foot spool that has graduated markings at 25 foot increments using a Sharpie and simple dashes – for 25, -- for 50, --- for 75 and ---- for 100 feet.

5.       DSMB (see section three)

NEVER clip off a spool to yourself once you have begun to let out line. It becomes a serious entanglement hazard and could cause injury or death! 

Line Colour:

There are a variety of line colours available, the most common of them being white, along with green-yellow, glow-in-the-dark and my favorite, blaze orange.

There is no right or wrong colour to choose and the colours are nothing more than personal preference. There are however, differences in visibility of the lines. The blaze orange lines are by far, more visible in the water column. Next would be white and last would be green-yellow – at least in our local fresh water environment.

Line Weight:

Line weight is the same as the line thickness. Typical spools use a #24 cave line though there are smaller diameter lines that boast the same strength as #24 cave line. You may also see line marked #36 wreck line. #36 line is thicker and is typically more durable when used in environments that may include sharp edges. The best line wait for our purposes is the thinner #24 line.

Line Markers: Arrows and Cookies

Markers are used to denote specific information on a line when that line has been run for navigation.

Line Arrows:

Arrows are ‘directional’ markers used to point to the nearest exit point in overhead environments.


These round markers are ‘non-directional’ and used individually and are not meant to denote anything to anyone other than the diver(s) that have placed them. If leaving a main line to explore, a diver might tight off his or her spool to the main line – this is called a ‘jump’ point. That diver may leave cookie on the main line, on the exit side of where they have tied in their own spool to indicate the exit direction for them when they return.

Square / REM markers:

These markers are also ‘non-directional’ and are used to mark points of interest or relay information within a team of divers. They have a large flat area, idea for writing information on.

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