A fundamental requirement of fabric architecture is that the fabric should be in a 3D form which is referred to as “double curved” or “Anti-Clastic”. Sounds technical but, it’s really quite simple.
All it means is the shape is formed by applying tension to the fabric along opposing radii (curves). In the example of the simple 4 sided “Hypar” Sail think of it as two curves – one between the 2 low points (the low radius), and one between the 2 high points (the high radius). When we tension the 4 corners the 2 radii “balance” each other in a state engineers refer to as “equilibrium” – in the fabric industry we often refer to it as the “pre-stress” state. The fabric is held firmly in place by these balanced forces until a “load” is applied such as wind. A load will cause the sail to change shape until it finds a new “Force Balance” shape. In the case of the Hypar when subjected to wind uplift, tension in the low radius will increase while at the same time, tension in the high radius will decrease thereby allowing the fabric to change shape without “stretching”.
The triangle is not so lucky. It is flat or “planar” (single plane) even if it is sloping. In order to prevent the fabric from sagging in the middle it must 1st be subjected to higher stresses than an anti-clastic shape. Then, when subjected to loads, it can only change shape by stretching. Just like our own skin, the older it is and the more it is stretched, the more saggy it will become. The result is there for everyone to see – just drive down any suburban street in Australia and you will see any number of saggy baggy sails most of which are triangles, the rest are just poorly designed or flat/planar sails. These sails are destined to a short and unattractive life as the more they stretch and sag, the more they flog in the wind which accelerates the deterioration of the fabric.
So, for the your sail to look good and last a long time make sure it is a 3D shape therefore it must have more than 3 sides and must have lots of height difference. The shapes which qualify as anti-clastic are Hypars (or saddles), Conics (umbrellas – make sure they are curved surfaces not flat fabric resting on the arms), Barrel Vaults (arches) and any combination of these.