It is important to remember that different materials with similar stated weights may be quite different.  It is hard to compare Tyvek with polytarp just on the basis of weight; the materials are so different that the comparative weight is not even an issue.  It does make sense to look at the stated weight when comparing sources for a polytarp, though.

For the dacron sails, the weight of the cloth per square yard becomes even more confusing.  Dacron is rated by the "sailmaker's yard," which is 27" x 36".  That means that a dacron sailcloth rated at 4 ounces per square yard is really 5.2 ounces per REAL sqaure yard.  The laminate sails mentioned in this article drop the "sailmaker's yard" in favor of the standard, 36" x 36" yard, just like the other "alternative materials" we mention.

Many uses of Tyvek:
Never mind the multiple uses Dupont has marketed Tyvek for, you'll also find numerous alternative uses other than sails, including lightweight backpacks and kites that even Charlie Brown's tree won't destroy. Tyvek

Tyvek has its fans.  Tyvek is registered trademark of Dupont.  It is a spun-bonded polyethylene used for envelopes you can't tear, protective overalls in construction, and "house wrap", a material put under siding to provide a measure of insulating value (it prevents air movement).  Tyvek is white, however I have never seen it without large lettering that says "Tyvek House Wrap" on it (see the doghouse on the left).  There is a school of thought that as long as your going to use an alternative sail material, you may as well be as overt about it as possible, so go ahead and flaunt the lettering.  The main advantage of Tyvek is that it is incredibly cheap if you can find a home builder willing to part with a bit of it.  Tyvek sail makers report paying a wide range of prices: nothing, a six-pack, $20, a date with their sister, etc.  The main disadvantage of Tyvek is that it loses its shape fairly quickly, stretching out.  Sometimes this is viewed as an advantage, as it provides some shape to the sail not obtainable easily otherwise, and as long as you can tighten up the lacing holding the sail to the boom, I guess that's OK.  Oh, that's another disadvantage:  it doesn't hold grommets well.  And its noisy as all get out.  Sailors report learning to NEVER let the sail luff, as the noise drives them batty.  Bill Wallace of Shoreacres Texas has a page on making Tyvek sails which explains more about the actual process.

Canvas Tarps (painter's tarps)

I have to admit, at one time I completely panned the idea of painter's tarps. I felt strongly that canvas tarps sold in home center stores are not suitable to make sails out of.  Even though it feels pretty sturdy, the material stretches too easily to last any amount of time. And yet, I have recieved several emails over the years and canvas painter's tarps have their fans. They do have a salty, natural look, and sometimes can be had fairly cheaply. The problem is they do stretch easily, but then, if you made them, you can probably "take them in" when they get too baggy, eh?

Polytarp

The Weekender plans include complete instructions for building inexpensive sails out of polytarp material.  Polytarp is available in most home improvement stores, such as Home Depot or Lowes, and larger discount stores like WalMart. It is inexpensive and relatively strong, but the color choices are usually limited to blue and silver (and, in the sporting goods section of Walmart, camouflage, which I suppose means you could sail up a jungle river undetected.  "Mr. Livingston, I presume?"   "Whoa!  Where did you come from!") Most Popular:
Polytarp wins the "most popular" title by a landslide, even garnering mentions in such mainstream magazines as Sail Magazine and Wooden Boat Magazine (albeit usually in a story that says something along the lines of "Gee whiz! This kook sails with polytarp sails!")

While cost is the main advantage of polytarp, how does it work as a sail?  Of all the non-traditional materials, it is the most popular so there is plenty of experience with it.  And it seems to work pretty well.  Captures just as much wind as standard dacron sails, so all other things being equal, it is just as powerful.

The disadvantages?  It degrades in sunlight, so if your sail is exposed to the sun all the time in Florida or another sunbelt area, you can expect your polytarp sail to last just a season or two.  Kept covered and protected, especially in northern climes, it will last several years.  It won't impress anyone, except fellow cheapskates like those of us here.  And it is harder to shape it for the most performance like a professional sail maker can do with dacron.  And, blue, silver or camouflage are not, well, traditional, sail colors.

One solution to the color issue is to buy white polytarp.  You probably won't find it at Home Depot, but it is available on-line. Here are two sources:  Master Distributors sells white polytarp in a 12 x 20 size for $23.99 (the Weekender plans specify a 12 x 18' polytarp). Note that it is 3.5 ounce polytarp, which is pretty light material, but is UV treated for some extra protection against the sun.

HR Solutions - Polysail, a small company run by a fan of polytarp sails, also sells a white polytarp kit suitable for the Weekender, their 15 x 20 gaff sail kit. You can make both the main and the jib out of this kit.  It is a heavier guage material, nearly 6 ounce (8 mil) compared to Master Distributor's 3.5 ounce. And he includes some extras making it a pretty complete kit:

• 84' of 1 1/2" double-faced tape.
• 108' of 3" wide white vinyl tape for taping sail edges and reinforcing corners and reef points.
• Brass grommet kit with 24 (3/8" hole diameter) grommets, setting tool, and instructions for use
• 100' of 1/4" twisted poly rope (15' x 20', 20'x20' and 20'x30' kits) with knot tying instructions.
• Full-sized utility knife.
• 75 lb. test plastic ties for use in attaching your new sail quickly to your mast or spars for testing and/or continuing light air use. (Number varies by kit size.)
• Set of easy-to-follow, illustrated sail making instructions.
• Marking pen.