Can shade sails withstand high winds, or do I need to take them down in winter?

A well-engineered shade sail, properly fabricated and tensioned to correctly installed fittings, should be able to handle winds up to around 80 km/h according to WeatherSafe WA. But a shade sail behaves like a yacht sail in strong wind: the fabric and every fixing point come under real load, and in a severe storm it will try to move exactly the way a sail on a boat does.

For this reason, WeatherSafe WA generally recommends taking sails down over the West Australian winter storm season to avoid the risk of them blowing away. Also, note that not all sails are equal, so it is worth monitoring your particular sail when strong. wind is forecast to learn what it can actually handle. Their blog post on preparing shade sails and blinds for WA winter storms walks through a practical seasonal checklist if you would rather leave yours up and just prepare it properly.