Line - Markers

Dive Line and Markers: includes; Bulk Line, Line Cookies (Non-Directional Marker) Line Arrows (Uni-Directional Marker) and Line Squares (Non-Directional Marker).

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Characteristics of Line on Dive Reels and Finger Spools

Traditionally, the line on dive reels is a braided thermoplastic fiber made from polyamide (aka Nylon and PA) or more recently polyester terephthalate (aka Dacron and PET) ; each have their pros and cons.

Both are strong, but nylon has significantly more elasticity while polyester has very little stretch. In water, nylon tends to become a little 'soggy' and swell slightly thus losing about 15% of its strength, while polyester retains 100% of its strength remaining 'crisp' and resilient when wet. Polyester has better resistance to abrasion and oil products, plus it is significantly more resistant to degrading from UV exposure. Polyester also noticeably better absorbs and retains the high-visibility color dyes (Orange or Chartreuse, Which is Better?). Many divers prefer polyester line claiming it deploys 'smoother' from the reel and is noticeably stronger; other divers still prefer nylon claiming its elasticity makes it easier to handle and 'tie off' in guideline applications.

A few dive equipment manufacturers have been experimenting with UHMWPE (aka Dyneema®) fiber - Apeks uses it as a leader on their LifeLine spools and Divesoft offers it as an option on their High-Capacity Reel. The prominent characteristic of Dyneema is the exceptional tenacity; in water it is five times stronger than polyester with very good abrasion resistance similar to nylon. Dyneema is also slightly buoyant, particularly in salt water. Some divers describe Dyneema as "slippery" which requires extra consideration when choosing the type of knot to tie. We normally recommend always tying a Bowline knot regardless of material since that knot preserves the line strength, but depending on your application another knot might work better.

Trade name
Dry Tenacity (cN/dtext)
28 - 38
7 - 8
6.5 - 8.3
Elongation at break (%)
10 - 16
16 - 27
Specific gravity (g/cm 3)
Melting point (°C)
Abrasion resistance
very good
very good
UV resistance
very good
Salt resistance
Resistance to oil products
Knot strength (%)
35 - 50
55 - 60
60 - 65

The size code numbers that describe the diameters of nylon guideline used in cave diving are from an archaic nomenclature for what is termed "twine" (braided string) and each # code describes an approximate thickness range in fractions of an inch. There does not appear to be an established standard (if you know of one, please tell us); size charts vary depending on the cordage manufacturers who each publish their own version for their products. There are four sizes normally seen on guideline reels: #36 and #24 are most common for nylon with #21 and #18 most common for polyester because it is stronger. The thinner the line the more length that can be fit on a given size reel or conversely the smaller the size of the reel for a given length. About 1/16" or 1.6 mm is approaching the minimum diameter practical for manageable line handling in diving applications.

Thinner < #18 = .060" (~1.6 mm), #21 = .067" (~1.7 mm), #24 = .073" (~1.9 mm), and #36 = .085" (~2.1 mm) > Thicker