We like nylon for docklines, anchor rode and chain snubbers because it stretches, absorbing jolts that would otherwise be transferred to the boat and ground tackle. But all of that stretching and contracting takes a toll. Fiber wears on fiber. The polymer itself fatigues. Unlike a steel part, which can last basically forever if never stressed beyond its fatigue limit (about 25 to 35 percent of the breaking strength), nylon and polyester wear out. Every load cycle has some effect and the severity depends on the load, rate of pull and range of pull (does the rope go slack or not?). As a result of the process, the rope becomes permanently elongated, and its elasticity and strength decrease. A certain amount of permanent elongation (4 to 6 percent) results from consolidation of the weave, but elongation beyond that indicates fatigue and overloading.
Regarding nylon stretch, my 24/7/365 mooring system includes three legs, each 300’x3/4″ double braided nylon line for my Tayana Vancouver 42. System was deployed 10/28/2007 in the Pacific Northwest. A Danforth type anchor for each leg was deployed under idle power and at low tide. Central bridle replacement and buoy swap is religiously completed every three years. Inspections reveal an astounding amount of sea-growth accumulation and weight gain over the years now necessitating a 30″ Taylor #46375 buoy as well as a need to replace galvanized thimbles as needed. As there is no UV effect and very little movement on the lines, the system has survived very well saving significant annual marina fees.