Author's note:

I'm not a big fan of polytarp sails. I don't believe the tarp material has adequate strength or stability to make a quality sail and firmly believe that anybody who tells you it does is either lying to you or doesn't know much about sails or sailmaking. The photos and building instructions that I've seen on the subject posted on the Internet just reinforce those beliefs. Be that as it may, a very large percentage of what these people are teaching is just bad sailmaking—in any kind of fabric or material. Many of the sails they are making are absolutely atrocious and it quite often has nothing whatsoever to do with the materials they're using. They simply don't have the knowledge that's needed to make a decent sail.

This article deals with one particular style of sail and won't apply to all situations or all types, but much of the information may be useful and can be applied in the production of other styles. It's mostly basic, simplified traditional sailmaking and sail design—the skills and information these people should be using to design and construct their sails in the first place. These are generalized instructions and there are no specific sail dimensions given. I would suggest obtaining copies of Jim Grant's booklet Make Your Own Mainsails and Emiliano Marino's The Sailmaker's Apprentice for a more in-depth look at the subject. Both are excellent and will fill in a lot of blanks if you're seriously interested in making a real sail—from any material.