Beach cats like the Hobie have large mainsails with many full length battens to support the sail's over size roach. The roach is the curved area outside a line from the top corner to the back corner of the sail (which is called the "leech"). This adds a lot of power to the sail. Beach cat mainsails are not designed to be reefed nor are they designed to be used in a rig with a backstay. The most glaring problem is that the backstay will interfere with the full battens that support the large roach making it difficult, even dangerous, to tack or gybe. The sail will get jammed up and can cause a capsize ............. of course, the sail may have been altered by the previous owner, but you'll need to investigate that before you even think about using it. Check with the person who sold you the boat if possible, otherwise, try making some more sailing friends who can give you more than blind advice. I honestly can't imagine what scenario you would use that beach cat mainsail on your totally different rig. Good luck.