There are five types of synthetic ropes, namely
- Nylon (Polymide). Nylon is the strongest of all ropes in common use and is slightly stronger than polyester. However, if it is much used, it becomes weaker than polyester. It is also the most elastic of all ropes and can stretch up to 30% of its length. Because of this, it can take shock loads easily. It lasts up to 4 to 5 times longer than natural fibres and it is not harmed by chemicals, ultra violet radiation and oil. It is also very hard wearing and does not absorb water. It has a melting point around 250° C. It has two disadvantages, it will not float and should it part under stress, it will lash back violently.
- Polyester (Terylene). This is the most durable of all the ropes, but it is not as strong as nylon. Because it does not stretch, it cannot absorb shocks very well. It also does not float. However, it is just as good if not better than nylon in all other respects. Unfortunately, it is very expensive. The fibres are very similar in appearance to nylon and it is difficult to distinguish between the two. Its melting point is around 260° C.
- Polyproylene (Polypro). Polyprop’s main advantage is that it is quite light and therefore floats. It does not stretch very much and is much lighter and weaker than the preceding two ropes. It is however, at least twice as strong as manila. Prolonged hard use will quickly weaken polypro and if left exposed to the sun, it will quickly deteriorate. Its fibres can be coloured black or orange to help prevent its deterioration from UV rays. Its melting point is around 165° C.
- Polyethylene. Polyethylene is very similar in appearance to polypro and its characteristics are very similar, but it is much weaker. It can also float and its melting point is around 120° C.
- Armid group. This is a group of fairly new types of synthetic ropes which are much more expensive than the others. On the other hand they are lighter, stronger and more flexible. They are generally up to five times stronger than wires, weigh less and can stretch slightly more.
The safe working load (SWL) of synthetic rope is equal to the diameter of the rope in mm, squared, ie D2 = Kg.
The following table gives a comparison of strengths.
- The strength of a rope which is properly spliced is approximately 90% of that of an unspliced rope.
- A knot may reduce the strength of a rope by as much as 40%. This must be constantly borne in mind when tying ropes together.