Do shade sails block UV, and do they really work?

Yes. Blocking UV radiation is the core job of a shade sail, and a good quality sail is genuinely effective at it, not just at casting visual shade. The level of UV block depends on the density and quality of the fabric weave, which is why WeatherSafe WA sources from a wide range of shade cloth and PVC suppliers rather than a single low-cost option. The fabric can be matched to how much protection and how much airflow you actually want in that spot.