Canvas, the tightly-woven material used to make the strong sails of sailboats has a very long history. It was used for sails first in Barcelona and Venice, not surprising as Spain and Italy controlled major shipping empires at the time. Today, sail canvas is often cotton, linen or synthetic. It is very durable, but not impossible to damage. Repair rips in your sails until you can afford new ones using the correct supplies.
Slip a leather palm over your dominant hand. Tighten the palm using the adjustable buckle so that it fits you well.
Thread a sail maker's needle with 5-ply waxed twine. Pull the ends of the twine even so that the needle is centered along the strand, just as you would with a regular needle and thread.
Hold the needle between the index and middle fingers of your dominant hand. Use the metal plate on the leather palm to force the needle down through the sail fabric approximately 1/4 inch before the start of the rip. Pull the needle all the way through on the other side with needle-nosed pliers.
Wrap the twine into a slip knot and slide the loose ends of twine into the center. Pull the twine tight to trap the ends and secure the stitch. Whip-stitch the entire rip closed using the leather palm to force the needle down into the fabric and the pliers to pull it out from below.
Continue stitching until you are 1/4 inch past the end of the rip. Cut the needle loose and knot the two ends of the twine securely several times; trim off any excess twine.