Safety ropes and UV degradation

You are putting the boat to bed for the winter. You don’t want to leave the life-sling out for the winter. As you undo the knot securing the line to the pushpit, the rope crumbles to powder in your fingers… What has happened?

This experience is from RATS member Richard Ritchie.

When this happened to me some years ago I was astonished. My lifesling had been new that spring. The sun’s ultra-violet (UV) rays had destroyed the exposed part of the yellow floating rope in a single season. I had no idea that floating ropes degraded so fast. Was this normal? And how do I stop this happening again? Have you checked yours?

Plastics serve us well if chosen correctly. Dacron sails and Braid-on-Braid ropes (both polyester) stand up well to both abuse and sunlight. Nylon has good stretch with reasonable strength and abrasion resistance, great for anchor rodes. New materials like Dyneema or Vectran have minimal stretch and remarkable strength. Dyneema (UHMWPE) as used in rigging is actually the same base material as used in many shopping bags, but drawn into long parallel fibres, processed, and pre-stretched. None of these are fully ultra-violet (UV) stable, so degrade slowly with time, but they are good enough for their purpose.

But what if you want the rope to float? Sadly only three synthetic polymers will float. Of these, LDPE (Polythene) is not strong enough and UHMWPE (Dyneema) does not float when made into a rope. Which leaves Polypropylene (PP), the familiar, often yellow, life-ring rope, as the only one available for such safety uses. BUT it is not UV stable, and the loose weave allows damage to penetrate throughout the rope. Some versions of PP rope have a level of chemical surface treatment but it is generally wise to assume that all PP should be kept from the sun… as I discovered, fortunately before the life-sling was needed.

An in-depth article on ropes, materials and their application is planned for the September edition of Cruising.