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 triangles? Try as you might, you won't be getting a twist out of a three-sided sail. However, a variation in height will still help minimize the effects of sagging, as the lowest point will be an attachment point as opposed to the center of the sail.
An added bonus to sloping is that is makes for better rain shed. 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 a lower place to go. In a flat sail, that lower 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.
Unlike our first method of tensioning mentioned last week—using perimeter wire rope—the variation in height you will use is a decision you need to make. If you need help making that decision, talk it through with one of our helpful project managers, who can also give you a free no-obligation quote. For more tips in tensioning your sail, see last weeks article and stay tuned for the third and final tip next Wednesday!