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Specific gravity (buoyancy of a rope / material)

Specific gravity is a measure of the density of a material; a Specific Gravity of 1.0 is equivalent to a density of 1g per cm3 (i.e. a Specific Gravity <1 means the material floats). The following table shows the specific gravity of some of the materials commonly used in fibre ropes.


Material Specific Gravity
Polypropylene 0.91
HMPE (Dyneema®) 0.98
Fresh Water 1.0
Sea Water 1.03
Nylon 1.14
Polyester 1.38
Vectran® 1.41
Aramids (Technora®, Nomex®, Kevlar®, Twaron®) 1.44
Zylon® 1.54
Steel 7.85


Ultra Violet Radiation Resistance

All materials are affected by UV radiation to some extent. The following table attempts simply to rank different materials in line with their resistance to UV radiation. Please refer to the Rope Care Advice section for more information.


Material UV Ranking
Polyester 5
HMPE (Dyneema®) 5
Nylon (UV treated) 4
Aramids (Technora®, Nomex®, Kevlar®, Twaron®) 3
Vectran® 3
Polypropylene 2
Zylon® 1


Melting point

The table below shows the typical melting or decomposition temperature of some common rope making materials.


Material Melting Point (Degrees C)
Zylon® 650 (decomposition)
Aramids (Technora®, Nomex®, Kevlar®, Twaron®) 500 (decomposition)
Vectran® 330
Polyester 260
Nylon 6.6 250
Nylon 6 220
Polypropylene 170
HMPE (Dyneema®) 150


Note – the properties of a rope will change before the temperature reaches the melting or decomposition point.


Chemical Resistance

This table shows the residual strengths of synthetic fibres after chemical exposure under specific conditions.


Test Conditions
Residual Strength
Type of Fibre
Chemical to Water %
Deg C
Hydrochloric 34% 20ºC 100 0% 70% 100% 95% 100%
Nitric 66% 20ºC 100 0% 100% 100% 95% 95%
Sulphuric 96% 20ºC 100 0% 100% 100% 40% 90%
Formic 90% 20ºC 100 0% 95% 100% 90% 100%
Acetic 100% 20ºC 10 85% 95% 100% 100% 100%
Caustic Soda 40% 20ºC 100 50% 0% 90% 90% 100%
Caustic Soda 20% 70ºC 150 100% 0% 100% 85% 90%
Caustic Potash 40% 20ºC 100 90% 0% 90% 90% 100%
Trichloroethylene 100% 30ºC 150 100% 95% 80% 100% 100%
Carbon Tetrachloride 100% 20ºC 150 100% 100% 1 98% 100%
Benzene 100% 70ºC 150 100% 100% 100% 98% 95%
Metacresol 100% 100ºC 4 0% 0% 100% 80% 100%
Oxidising Agents
Hydrogen Peroxide 10% 20ºC 100 0% 100% 90% 95% 100%


Elasticity & Extension

Rope extension consists of several components.

Elastic extension: This is the recoverable component of the rope’s extension and is immediately realised upon release of the load

Visco-elastic extension: The contraction of a rope does not follow the same path as the rope’s extension. This results in an element of extension that is not immediately recoverable but will recover if relaxed for sufficient time. If the load on the rope is cycled, a hysteresis loop is formed which will exacerbate this element of stretch

Permanent extension: This is non recoverable. When the rope is initially loaded all the plaits, strands, and yarns become “bedded in”. This results in a small permanent extension. Most of these constructional effects occur within the first few loadings and have little effect on the rope after this time. In addition to this there are some permanent molecular changes that occur to the material that result in creep.


Load-Extension Characteristics


Rope Strengths and Weights

Rope strengths are tested according to Marlow’s QA25 and 26 quality procedures. Generally these procedures are in line with BS EN ISO 2307, however, a number of other internationally recognised test standards are used including EN 1891, EN 1892 and EN 564

Rope mass is determined be weighing a sample of rope whose length has been measured at a reference load. For most ropes this load is calculated as:

Reference Load (kg) = D2/8

Where: D is the rope nominal diameter (mm)


Working Loads: Marlow Ropes specify a minimum breaking load (or sometimes an Average Breaking Load). It is the responsibility of the user to determine an appropriate factor of safety and safe working load. This factor of safety must be determined after considering all the risks, the strength reducing factors, and the expected life of the rope. The following table shows some of the factors that may affect the determination of the factor of safety.


Static load

Dynamic loads

Strength reduction due to splices / knots

Strength reduction due to sheaves

Strength reduction due to Bending

Fatigue over expected life of rope

Consequences of rope failure

Frequency of inspection

Experience / training of operators

Exposure to chemicals

Exposure to UV radiation

Exposures to high temperatures

Intended life of rope



Most rope strengths in this catalogue are given in kilograms (kg). However, the correct measure of force or breaking strength is Kilonewtons (kN). Conversion factors from one to the other are:

Kg to kN x 0.00981

kN to kg x 101.972