Choosing guyline cord: stretch, knot creep and construction

Nylon, polyester and UHMWPE guyline cords trade stretch, slipperiness and friction heat against one another. Nylon generally stretches more, polyester generally stretches less than nylon, and UHMWPE is low-stretch and slippery, so ordinary knots may creep or slip on it. Choose by the stretch you want, the termination you can tie and check, and the construction and diameter that your accessories need.

Stretch and what it does to a pitch

Stretch matters because a pitch changes as the weather changes. A line that stretches more gives under load, so the pitch can sag as the cord takes up tension. A low-stretch line keeps the setting you gave it more closely, so the pitch changes less as the line takes up load.

These are general rankings. The figures for a particular cord come from its manufacturer, and construction changes them. Check tension after the first use and again after any change in weather, whatever the material.

A pitch with several guylines depends on each one. A stretchy cord can hide a loose line, because the tarp still looks taut while one line carries less than its share. Look along the pitch as well as at each tie-out. If you want the setting to change least, a low-stretch cord changes least. If you want the line to give under a sudden pull, a stretchier cord gives more.

Knot holding and creep

Slipperiness is the main knot issue with UHMWPE. Ordinary knots may creep or slip, so manufacturers specify terminations for it. Use the termination the manufacturer gives, not a knot you know from nylon, and recheck the tie-out after the first use, since creep shows up as a loosened or shifted knot.

Creep builds up over time, so a check at each stop on a trip shows whether a knot has moved. Note where each knot sat when you tied it, and compare that position later.

The Knot bench has a material setting that shows qualitative notes for each cord. Set the Knot bench to UHMWPE to read those notes beside the step diagrams. The notes describe handling. They do not give a knot-retention figure.

Friction heat and melting points

Friction heat needs particular care with UHMWPE. Rope maker Marlow lists the melting point of HMPE (Dyneema) at about 150 °C, against 250 °C for nylon 6.6 and 220 °C for nylon 6. A line that slides fast through a tensioner, or saws across another line or a sharp edge, heats where it rubs, and the low-melting fibre has the least margin. Adjust UHMWPE lines slowly, keep them off sharp edges, and look for glazed or stiff patches, which can signal heat damage.

Sunlight and water

Marlow rates ultraviolet resistance on a scale of 1 to 5: polyester and HMPE score 5, UV-treated nylon 4. It also notes that UV damage is very difficult to predict, so the score is a ranking rather than a lifetime. Its specific-gravity figures show which fibres float: HMPE at 0.98 floats in fresh water, while nylon at 1.14 and polyester at 1.38 sink.

None of these figures describes a particular guyline. Coatings, sheath construction and colour change how a cord ages, so check the data sheet for the exact cord, and inspect lines that spend weeks in the sun for fading, fuzzing and stiffness.

Construction: core and sheath, braid and coatings

Two constructions are described here. Kernmantle has a core inside a sheath, and a single braid is one woven layer. Construction changes how the line handles, how it wears and how a knot sits in it. Coatings change grip and the surface a knot bears against. Read the construction on the listing, because the material name alone does not describe it.

On a kernmantle line, check the sheath for cuts and abrasion. A sheath that has worn through to the core needs attention before the line is used for a pitch.

Diameter, line-locks and tensioners

Diameter must suit the accessories. Line-locks and tensioners state a range of line diameters, and a cord outside that range may not work as intended. Check the accessory's stated range before buying or cutting cord, and check the fit again whenever you change cord.

Rehearse the knot on the cord you will actually use, not on a sample of another diameter. A knot practised on the wrong cord tells you little about the one that will be on the pitch.

Paracord and reflective tracers

The name paracord does not tell you the fibre, the construction or the diameter. Read those from the listing, then apply the points above. A reflective tracer helps you see the line at night and avoid walking into it. It is a visibility feature. It does not change stretch, knot holding or strength, so do not choose a cord for its tracer alone.

Terms for cord and accessories

• Termination: the way a line is finished at its end or at a fixed point, such as a knot, splice or eye.
• Creep: a slow movement of a knot along a line as the line is loaded and eased over time.
• Line-lock: a friction device that holds a guyline at a set length.
• Tensioner: a device that adjusts how tight a line runs.

Accessories set the limits for diameter, so read their stated ranges before you choose a cord.

Choosing a cord for the pitch

Start with the job. Decide the material from the stretch you want and the termination you can manage. Then confirm the construction and diameter against the accessory the line runs through.

Write down the cord you choose, its listed fibre, construction and diameter, and the accessory it fits. A short record means the next pitch starts from the same facts rather than from memory. Store spare cord somewhere dry and away from heat, and check it again before a trip.

Rehearse the tie on spare cord before you commit to it. Pitch behaviour in wind is covered in pitching a tarp for wind, and the length of line to allow is in how much guyline a tarp needs. For the tie itself, see round turn and two half hitches.

Knot strength percentages are not a basis for choosing cord. The reasons are set out in why knot strength percentages disagree.