Materials for Towlines

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There are many different types of rope both in terms of the material and construction used. Outside of the more specialised ropes available the most common materials used are;
  • Polyester. 
  • Polypropylene.
  • Nylon.

Materials

Polyester and Polypropylene are the most common materials used for towlines. Nylon isn’t generally used by Coastguard as a towline, not because of any issues of strength – in fact Nylon is very strong, and often used in commercial towing. The problem is Nylons capacity for stretch. While a towline needs to be able to stretch to absorb some of the dynamic loading imposed on it, Nylon rope depending on its construction can stretch to an excessive degree (60% + in some cases).
The greater the stretch the greater the recoil when a line parts. The danger is not just from the towline, there is also the risk of a fitting such as a cleat being pulled out but remaining attached to the recoiling line.
Polyester
  • Stretches approx 25 to 35% of its length.
  • Does not float.
  • Good resistance to UV.
  • High melting point (friction).
Polypropylene
  • Stretches approx 25 to 35% of its length
  • Lightweight and floats.
  • Poor resistance to UV.
  • Low melting point (friction).
Of the two types of material Polyester is preferable mainly due to its resistance to UV and the fact that it sinks. A line that sinks is far less likely to get caught around rudders or worse propellers when manoeuvring, and a floating line will sink blow the water to allow a catenary in the towline (See Catenary Devices). Another advantage of Polyester is its high melting point. While it is preferable to make any adjustments to a towline when there is no strain on the line this may not always be possible – as in the case of towing a vessel in heavy weather with a following sea. The higher melting point of polyester means it is less likely to suffer damage if surged around a cleat or post while still under load.

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