Height variation is very important in a shade sail. Varying the heights of the corner attachment points certainly adds visual interest, but it is also important for other reasons.
Height variation is good for water shed. When a sail isn’t flat and level, water can shed off of it. While our fabric is a weave of HDPE fibers, and therefore somewhat porous, much of the water that falls on a shade sail will run off given it has some place to go. In a flat sail, that place will be the center and that pooling water will not only weight down the sail, but will drip through it. Add a little height variation and that water will seek the lower attachment points and edges to run off.
For sails with more than three corners, good height variation is obtained by alternating high and low attachment points. This gives the sail a shape called a hyperbolic parabola. Because the two high points are opposite each other, they work together to pull the sail up and out, while the low points pull the sail down and out. The sum of these opposing forces creates tension in the sail and suspends the center, preventing sagging. This also prevents the sail from catching wind or flapping in it.
What is optimal variation? Our rule of thumb is 1:5. That is, 1' of height variation for every 5' of span. This is the reasoning behind our 7' and 10' standard poles. Take an SS-15 for example: the approximate 15' spans call for 3' of height variation.
What about three-sided sails? Try as you might, you won't be getting a twist out of a three-sided shade sail. However, a variation in height will still help minimize the effects of sagging because the lowest point will be an attachment point as opposed to the center of the sail.