I hadn't heard of Weathermax 80, so I looked it up. These links may be of interest:

http://www.safetycomponents.com/Downloads/

WeatherMAX/Fabrication_Manual_2014.pdf

http://www.rochfordsupply.com/shop/

Textiles/Marine_and_Boat_Cover/Weathermax(TM)_80/index.html

http://www.cruisersforum.com/forums/f55/sunbrella-vs-weathermax-95758.html

The yarn is made by Unifi Inc, and I found this on their website:

"SATURA® is a family of solution dyed polyester yarns available in a wide range of colors. SATURA® has superior color consistency, light fastness characteristics and is bleach cleanable. Because SATURA® is solution dyed, additional fabric dyeing processes are eliminated, reducing the product's environmental footprint. For enhanced product performance, SATURA® Max is available."

Its price at Rochford Supply is similar to that of Mustang. It frays (so does acrylic), and needs felled or french seams, or hot-knife cutting. It appears to be softer and more stretchy than other cloths.

Looking at what's happened so far I don't see any point to using any of the awning fabrics over proper dacron sail cloth. It costs the same or more than dacron. Awning stuff has no UV protection advantage, Graham's was covered after the first year (only one year to wreck it?). The only other advantage left was the fact awning cloth came in wider dimensions than the 90cm of dacron. Quick and easy to make whole panels. But that is out the window since horizontal cut is a no-no, (this is not about vertical cut seams stopping a tear from progressing, it just tears easier).

Design it with a bolt rope, yes I get that Arne. Even seat belt webbing (which for awhile was promoted over rope) failed on Arion. The only success recorded for awning cloth has been on 1/ flat sails and 2/ as cambered sail, occasional daysailing up in the Arctic circle. 

Problem is, we didn't know we were 'experimenting' as we hammered our boat night and day 4,000 miles through the tropics.

Duradon was very popular about twenty years ago, because it has the virtues that David mentions.  However, even the lightweight version is pretty heavy. 

Top Gunn also seems to stand the pace: Kurt's sails are pretty old, but again, they are flat.

Personally, I'm going back to acrylic - there are several manufacturers apart from Sunbrella that aren't as expensive.  Although Badger had flat sails, we got 80,000 miles out of the first set, in all climates.  However, I intend to spend a lot of time making a scale model sail and working on it until it hangs without creases of its own accord with the YHP and halyard and, possibly, some standing luff parrels.

I do think Arne has a point (well, he has a lot of good points to make!) about the sun damage.  We Antipodeans are not only at relatively low latitudes, we also have the hole in the ozone layer (still there, if reducing) passing over us quite regularly.  But if Odyssey has proved to be disappointing, it's all part of the learning process and one has to ask how many other sails would have stood being out in all weathers without a sail cover?

It would be great if we put some more research into finding an alternative fabric, but it is rather horses for courses, as ever: flat or camber/latitude/type of sailing/ease of sewing/expense.

When I wrote about how to construct a cambered panel sail, I slowly and clearly spelled out why such a sail needs a strong non-stretch boltrope, either of pre-stretched rope or from thick seatbelt webbing. Still, it seems that some who make cambered panel sails, simply drop the boltrope, or use a thin one, thinking that some tabling will do in stead. It will not. The tabling, normally being of the same material as the sail, will simply stretch too much. The idea with adding a tabling is rather to make the edges heavier and stiffer to keep them from fluttering.

I repeat; if only sailcloth tabling is used, instead of a non-stretch boltrope, the sailcloth near the edges will have a much harder time than it was meant to have.

Now I just checked a piece of Odyssey: Its stiffness (not that stiff) is about the same in both main directions. However, it is no good when teared diagonally. It is simply not a sailcloth. That worries me very little. Remember, when watching photos of the Hong Kong schooners, the cloth they used was probably not half as strong as our synthetic cloth, even the Odyssey, but they still managed. The integrity of these sails mainly sits in the boltrope, battens and Hong Kong parrels, which take most of the loads.

It may well be that the Odyssey is not that good for use under the tropic sun, but in my waters, at 59° north, it appears to do fine. I think that any sun-proof (and rot-proof) cloth, with some chafe resistance will do. Diagonal stiffness of the cloth, as needed on western sails, makes little sense in junksails.

Conclusion
Make your sails with a serious boltrope (rope or webbing), plus tabling for serious offshore work, and also make sure that the sheetlets and parrels are attached to the boltrope  or battens, and not to the sailcloth (as PJR recommends). The only corner I would bother with giving a (2-3 layer) patch would be the throat corner, which is quite busy, with lots of shear forces until the parrels are set up properly. In addition, don’t use metal eyelets, with their ultra-high point-loading. It is much better with hoops from webbing, which spreads the load.

That’s my opinion, at least.
Cheers, Arne

PS: If real sailcloth with diagonal stiffness were important for the set of a junk sail, then Samson, here, could never spread her cheapo nylon sails, year after year, like this:

Hi Arne,

Thanks for your comments.  They make sense.  My black sail has a 75mm wide tabling along the luff and leech, with a 25mm seatbelt webbing folded over the edge.  Obviously this has not been enough, especially with a light and stretchy material like Odyssey.  Also, the entire sail was straight stitched, despite being made by a professional sailmaker (who usually makes beautiful sails), because he said his machines did not like the Odyssey material (he said it gummed them up).  I suspect both these things have contributed to too much stretch in the sail.  I first realised something wasn't right when the throat tore on the first outing, and so I beefed up the throat and peak reinforcing patches.  Then, bit by bit, all the eyelets started pulling out and I have been forever sewing reinforcing around them.  Now of course, the whole upper part of the sail has given up.

Today I am bending on my flat sail and will just potter around with it this summer. I loved the performance of my cambered sail and will miss it dearly.  I'd like to think I get another cambered sail but because of my circumstances, I am not able to build it myself.  If I was going to pay someone else to build it for me I'd want to be really sure I got something that was strong enough.  As you have said before, most sailmakers don't understand this sort of sail.  Chris Scanes builds nice-looking cambered sails out of Dacron and I think they are likely to be good quality construction, though I don't have any references  from people who have pushed them hard in the open ocean, as I did with my black sail.

Anyway, I am not ready yet to commit to a new sail, so will just make do for now.  Because I am having health issues at the moment, I intend to just potter around this summer, though I still need a strong sail, as yesterday's unpredicted, violent thunderstorms showed!