Nylon fishing line is probably your most vital piece of equipment and yet many anglers know little or nothing about what it is, how it is made and the properties of this critical element of tackle.

Nylon monofilament is an extruded polymer that is durable, resistant to ageing and rot proof. It is very stable and is not affected by micro-organisms or other naturally occurring substances and is totally saltwater resistant.

Monofilament has been used as the main material for fishing line for over sixty years and it is supremely suitable for the purpose for many reasons. Its great strength to its diameter, its suppleness and the fact that it has balanced stretch factor means that it gives good response to bites but still enough cushion to avoid crack off caused by sudden strikes and lunges from powerful fish.

Nylon fishing line has developed significantly in the last 20 years with the introduction of copolymers which have higher numbers of bonds within the material making it significantly stronger.


Nylon Monofilament begins life in granular form comprising different polymers and often a dye material to give colour. The components are mixed and then fed into the extrusion machine where they are heated until molten and extruded through an extrusion head or spinneret. There can be up to 50 lines extruded at one time from one extrusion head. The nylon filaments solidify once they come into contact with air and pass through a fixing bath to further solidify them.

The line must now be stretched or drawn to give it strength and reduce stretch. The drawing process makes the molecules in each filament fall into parallel chains. This is why high strength lines usually have less stretch. Some lines are referred to as ‘pre stretched’, but this is not very accurate. All lines are stretched; it’s just a question of how much. The more you stretch a line the higher the breaking strain becomes but past an optimum point the knot strength begins to fall.

Breaking Strain

The diameter to breaking strain ratio is the true measure of how strong one line is in relation to another. Anglers are constantly searching for a line that has the thinnest diameter for a given strength; a thinner line delivering the required breaking strength gives the advantage of better presentation and lower visibility.

Most stated breaking strains that appear on the label of the spools you buy are the ‘linear strength’. That is the force taken to break the line when pulled at a constantly increasing pressure without any form of knot tied within the line.

You will sometimes see line sold stating ‘wet knot strength’. This should refer to the breaking strength of the line once knotted and submerged in water for a period of at least two hours; however there is no hard and fast rule on this. Once a nylon line is submerged it absorbs water and this water absorption reduces the knot strength between 5 and 10%. Fluorocarbon or fluorocarbon coated lines do not suffer from this as they do not absorb water.

Knot Strength

The goal with the design of any line is to achieve the highest possible knot strength. Linear strength is largely irrelevant to the angler as there will always be knots present in his set up and the knot will almost exclusively be the weakest link.

Knot strength indicates the performance of a line when there is a knot and this is usually referred to as a percentage of the linear strength. For example a line that has a linear strength of 4lbs and knot strength of 3lbs would be said to have a ‘75%’ knot strength.

Different knots will produce different results. The worst performing knot is an ‘overhand knot’ also known as a ‘granny knot’ which should never be used for angling. We use this for our testing purposes to reproduce the ‘worst case scenario’; any angling knot will perform better.

A normal or ‘dry knot’ test is made by tying a knot in the line before wetting and drawing it slowly tight before testing its breaking strain.

A ‘wet knot’ test is made by first tying the knot, then submerging the line in water for a period of at least two hours. During this period the line takes on the maximum amount of water for its diameter and the effect upon breaking strain can be accurately measured.


Stretch is both an advantage and a disadvantage to anglers. Its advantage is that it acts as a safety cushion when bringing a fish in. If there was no stretch at all, the sudden lunge of a fish or a hasty strike could break the line.

The main disadvantage of stretch is that the elongation can be a hindrance to bite detection and hook setting at distance.

Generally, more traditional lines have a stretch factor between 24% and 35% and sometimes even higher. A high tech line will generally have a stretch factor of between 17% and 24%.

Braided lines have a very low stretch factor usually between 3 and 5%. This makes for very good bite detection and hooking at distance. However the downside is that the line has very little give and they can be susceptible to break offs when strong fish make sudden runs and hence a nylon leader is usually advisable.


Fishing line needs to be soft and supple, so it flows easily from the reel, passes easily through the rod guides and lets the bait fall through the water with a natural sinking action. A stiffer line will cause the bait to swing from side to side as it falls through the water and will not rise and fall properly with the flow of the river.

Nylon absorbs moisture from the atmosphere. The speed of absorption depends on the diameter of the line and the level of humidity. The line becomes significantly softer and more flexible when stored in humid conditions. The finer the diameter the more rapidly a state of equilibrium is reached.

Mono will absorb up to around 4% of its weight in moisture from the atmosphere, and this increases to around 10% if stored in water. In order to achieve optimum softness and flexibility when stored for any length of time it in a controlled humidity level of over 60 per at a temperature of 18-22 degrees.

There is no benefit from storing line in cold refrigerated conditions.

Light & Temperature

Nylon is not U. V. stable. In other words the molecular structure within a line will breakdown during prolonged periods of exposure to natural sunlight.

All line should always be stored out of direct sunlight and preferably away from any indirect daylight if being stored for prolonged periods. Do not store your line in green houses or in sheds on a shelf under the window.

Pure Fluorocarbon lines are not affected by light.

Line is ideally stored between 18 and 25°C. Normal ambient extremes of cold and heat do not significantly affect breaking strength until approaching the melting point of the material. Storage at temperatures outside this range does not have any measurable effects on a lines performance. However, a cold line is less flexible and a warmer line more supple and as lines are generally preferred supple this should be taken into account.


All nylon should be stored away from direct sunlight in a shaded or darkened place.

If line has been subjected to direct UV light for any significant period the outer layers of the spool should be removed. Storing for longer periods should be done away from any natural light source even indirect natural light. In doing so you line will maintain optimum performance almost indefinitely.

Temperatures below freezing do not affect the strength of line to any significant degree, although the elongation and softness do decrease noticeably. At low temperature the knot strength remains almost unchanged.

Humid conditions at or above 60 per cent are preferable when storing monofilament to maintains softness in the line.


Nylon monofilament absorbs water and becomes significantly softer and more flexible.

While exposure to high levels of humidity in the air is beneficial as it softens nylon monofilament, when submerged in water the absorption level increases and it does begin to have a detrimental effect on the line.

Both linear and knot strength can be reduced by between 10 and 15 per cent after submersion in water for as little as two to four hours, depending on the diameter.

After use, and back in the atmosphere, monofilament will return to its equilibrium point after a number of days or weeks, again depending on the diameter.

Points to note are that fluorocarbon coating waterproofs the line and prevents this problem. Pure fluorocarbon does not absorb water and submersion for any length of time does not affect the lines performance.