Why fool around with making a polytarp sail when you can buy used Sabot sails from lofts for $45? I picked up a complete Sabot rig -- mast, boom and sail -- on Craigslist for $65.
The above suggestions are good - either pick up a thrashed ply El Toro (aka 'Bullship') or Sabot and fix it up, or build something similar in size and shape. Put a used El Toro or Sabot sail on it and awaaaaayy ya go!
The main gotcha with the above cunning plan is that the Sabot and El Toro sails are for 8' RACING boats -- so may quickly overpower an 8' boat built for fun. So you may want to consider cutting the sail down if necessary, taking care to keep the critical curves and sail shape intact. I'm no sailmaker, but I'd guess that you'd want to remove 'excess material' from the foot of the sail, requiring sewing up that edge and a making new clew and tack as well.
Here's my 14' dory with the Sabot rig (sail insignia removed) -
Last edited by Thorne; 07-26-2006 at 10:19 AM.
"The enemies of reason have a certain blind look."
Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.