Shade sails, shade cloths, and other fabric shade systems are all tensile structures—they rely upon ropes to keep the fabric elements anchored and held taut. While synthetic ropes have eliminated many of the drawbacks common to organic fiber ropes (like susceptibility to mildew or rot), they also have widely varying properties that suit them to unique purposes.
With poorly selected synthetic ropes, these structures risk drooping, wearing out too quickly, or breaking under the shock load of intense wind. Here are the four mechanical properties you’ll need to keep in mind when selecting an appropriate fiber rope for your shade system:
Load on the Line
Also called breaking strength, the tenacity of a rope is a measure of the total force the fiber can withstand before it snaps. Pay close attention to the difference between the maximum load a rope can hold and the recommended working load.
In general, ropes should not typically operate under loads more than 20% of their maximum strength. Continuous stress near the limits of a rope’s strength can weaken it over time. Also, the closer the rope is to its maximum load, the less margin of error there is in the case of a mishap that puts a sudden shock load on the rope.
Synthetic ropes have grown extremely strong in this regard, in many cases surpassing the strength of steel weight-for-weight. Here are a few examples:
- Kevlar (aramid): 5 times steel strength
- Vectran (liquid crystal polymer): 5 times steel strength
- Zylon (polybenzoxazole): 10 times steel strength
- Dyneema (high modulus polyethylene): 15 times steel strength
This is the stretch of the rope. In some industries, this is desirable. For example, Nylon (polyamide) rope has extremely elastic properties that lend it to use as an anchor/dock line or in fall protection systems. In these situations, elasticity absorbs the shocks inherent in the task without jarring the linked forces or breaking under sudden force.
Creep is a term for the tendency of a rope to permanently elongate while under tension over long periods. It’s possible for low-stretch ropes to have heavy creep properties (unless they’ve been specially treated, like heat-stretched Ocean 5000 Dyneema), and for moderately elastic ropes to exhibit low long-term creep (like polyester). In general, ropes creep less at lower loads.
For a temporary, deployable shade system with a lower overall load on the rope, creep would not be a critical concern (SK-78 Dyneema would be ideal for its durability and low stretch). For a more advanced permanent installation, creep can be much more problematic. DM-20 is a Dyneema formulation that multiplies the HMPE fiber’s creep resistance by a factor of at least 100. This, combined with excellent fatigue and weather resistance, makes DM-20 ideal for use as a heavy duty shade sail rope.
Tolerance to UV radiation is essential in selecting a suitable rope for shade systems that face constant exposure to the sun. Synthetic fibers have come a long way through chemical treatments, sleeving, and advances in fiber technology that resist UV degradation, but it remains a concern.
Polyester and Nylon have the strongest natural resistance, but Dyneema has also made incredible improvements in recent years. Vectran and polypropylene ropes tend to fare poorly under continuous UV exposure. One simple method to combat UV degradation is to cover the structural core with a hard wearing outer cover. This increases the diameter of the rope, but in many cases, will also extend the longevity of the project.