Sail Cloth and Alternate Materials
There are a number of materials and cloths that you can use for making your sail. If you just want to go across the wind or down wind, about anything will work. If you want to sail into the wind, like at 60 degrees or less, then you will need a sail that has an air foil shape to it (which generates lift), and the material it is made out of should resist stretching. If the material stretches, it will loose the nice shape, and stop producing lift.
Is the most common "real" sail material, the average factory sailboat will have their sail made from it. It is a great material, does not stretch that easily and can last for a LONG time. I have tried sewing it with my regular sewing machine, and I find that I can go through 2 layers adequately, but after I stack up 3 layers, it gets kinda tough. You can purchase heavy duty sewing machines which have no problem at all with dacron, but they are rather expensive.
There are many used dacron sails available on eBay. Be careful reading the description and talk to the seller before you purchase. Some of the used sails really are not servicable and should be thrown away, while others are almost new - but being sold for the same cheap price as the junk ones.
This is the big blue sheets of plastic stuff that people cover cars and boats with, and use for all sorts of other purposes. This is the most popular material to make duck sails from, because it is so CHEAP, you can usually buy a single large tarp for about ten bucks. The most common color is blue, but also common is brown, grey and silver.
Not all tarps are the same, the blue stuff is usually the thinnest grade. There are thicker grades, and some of them are extra UV treated and will last a lot longer.
The good parts of using polytarp is that with 1 hour and a roll of duct tape, you can have a finished sail. Most tarps come with edges that already have grommets in them, so if you plan ahead, you can use those for lacing your sail to your spars.
The bad part, is that if you abuse it by storing it out in the sun and weather, polytarp breaks down quickly. Also you should try not to get the duct tape wet. But heck, since the tarp sails are so quick to make, when one falls apart, make a replacement.
Tyvek (and Typar)
This is a construction material used in houses to make the walls waterproof, usually it is wrapped around a house just before the siding goes on. Also it is used for un-rippable envelopes. Tyvek comes in much less variety of colors, commonly available at home improvement stores in white with the Tyvek logo printed all over it. There are different grades and thicknesses available, and generally speaking it is a little more durable than polytarp.
Local to me, it is available in 9 foot x 100 foot rolls for $100, or 36" x 50 foot rolls. Because I am such a boataholic, I got the big roll. I use it for making sails, covers for my boats, and a bunch of other projects that would take too long to list here.
The good part is that tyvek is very easy to sew. The average sewing machine can go through 5 layers of tyvek easily. It has very low stretch, so will hold an airfoil shape longer than polytarp.
Bad part is that duct tape and glues don't seem to stick very well to it, yet when pulling the tape off it will pull free some of the fibers. Also when you flog your sails, the noise they make flapping in the wind can almost make you deaf. When sailing with a full sail, it is very quiet though, so it is fun to sneak up behind someone and then pull through the wind. You can really scare the snot out of people. :)
This is pretty good for spinnakers because it is so light, but not very good for upwind sails because it stretches out quickly.
Materials That Don't Work So Well
Here are some of the materials I have heard suggested in various forums, but they do not work very well because they stretch so easily: Painter's drop cloth, canvas, bedsheets, natural fibers, nylon, visqueen, clear or black construction plastic.
Duckers display their number on their HIN Plate and on both sides of the bow. In addition to that, duckers put their hull number on their sails too. For tarp sails, it is very easy to use duct tape or vinyl tape to make large block numbers. Depending on what color tarp you use, there are various colors of tape that will make nice number. For with dacron sails, they commonly use vinyl letters that stick on.