Sail shape is one of the most important things that a sailor can be aware of. It's subtle, and all you are going to be able to do is tug a few bits of rope to try to get the maximum efficiency out of this piece of cloth. Something that looks like a simple triangle on the surface is really a complicated assembly of many, many component parts. If it is made out of Dacron, it will have a little bit of stretch and give; the Mylar sails, and the other sophisticated materials that we use for sails today wont give quite as much. But what ever the material, a sail maker spent hours and hours calculating the correct cut to assemble the sail for your specific boat. The sail itself usually has a little bit of belly built into it. And the center of effort, depending on the boat and the sail, usually would be found in this area here. By tugging the outhaul, or pulling down on the Cunningham, or giving a little bit more halyard, you can stretch this material to a completely different shape, depending on the wind and your angle of sail. A quick tug on the outhaul will stretch the sail further along the boom and flatten the sail just a bit. If the air lightens up, you might ease off a bit on the outhaul and sag your sail off some. In light air, it is a good idea to break off the vang and allow the boom to ease up a little bit. This will give the sail a little fuller shape. This is especially useful if you got a kicker, a boom kicker applied to the mast. The backstay adjuster applies tension on the backstay, which in turn applies tension on the forestay. This is particularly useful in heavy air, when you want a cleaner entry into the wind. The backstay adjuster will squeeze the back end of the mainsail, the leech of the mainsai,l and allow the air to fall off a little bit, and not have quite so much sideways pressure on the boat. On the down wind run, it is a good idea to break off tension on the backstay. This allows the mast to pull forward just a little bit, and allows the sail to fill a little bit more gently.
