As sail cloth gets older exposure to sunlight, salt and wear and tear will eventually make it rip easily. If this is the stage your sail is at then any repair will not last long. If the leach tape is off then see if the material can be torn by hand. If so then not much lost, if you can rip it by hand it is "rotten" and stitching will not hold to it, if it does not then you can decide if you want to spend money on it at a sail loft or try to fix it at home. If you try at home be careful at head and clew patches. Extra layers of cloth can break neddle in machine. Go slow or hand stitch. If in doubt put on a pair of safety glasses if you use a machine.