USA measure is based on one yard length, at 27 inches (0.75 yard) wide, simply because that's the width that the first sailcloth weavers in the USA decided to produce, so as to get the loom to pack the threads tightly and get the most stable result. 

[edit] ... or so I had always understood. But looking at Wikipedia, I see that it should be 28.5 inches x 36 inches as the unit of USA sailcloth measure. And to make it more confusing, the standard width for cotton sailcloth was 23 inches in the USA and 24 inches in the UK. The heavier grades of Clipper Canvas are 60 cm (23.6 in) wide, so as to reproduce the effect of cotton cloth for traditional boat's sails. Keeping up so far?

UK measure nowadays is actually based on a square yard. Easy.

So, 1 oz/square yard is the same as 33.91 gm/square metre.

And 0.7916666 oz USA measure is also the same as 33.91 gm/square metre.

As soon as the USA goes metric, we can all turn over to metric and everything will be easy. But don't hold your breath. I haven't seen whether this is another way in which the President-Elect will try to prevent globalisation, but I wouldn't be surprised.

I've found a source for Weathermax here in NZ and they will sell it to me for NZ$20 a metre ($18 if I buy the whole bolt).  It's 8oz per sq yd, 152cm wide and comes with a 5 year warranty.  Sounds like the go.

It's all right, Graham.  I've changed my mind about purple - I'm thinking more along the lines of buttercup yellow, now :-). Or tan if I go for the cheaper option of Clipper Canvas.  The Weathermax does sound appealing, though, and the fact that your sailmaker and Sebastian both reckon it's OK for a junk sail ...  It sounds a lot more suitable than acrylic. Unfortunately, Contender in Oz don't give any prices.  I'll try contacting them here.

Aaah - Graham, I should have said that Clipper Canvas is Terlyene (Dacron).  Interesting about the Weathermax, however, but it would be cheaper for me to import the Clipper Canvas than to buy Weathermax.  However, if I could find some way to get it from USA, then that could be the way to go.  I've been quoted NZ$30.50 here - it's about $12 from Rochford textiles, but with only three (boring) colours to choose from.

For all those weeping into their beer about the unsuitability of Odyssey and worrying about other exotics, there is some good news: rumours about the demise of Clipper Canvas have been exaggerated.  It is only the very light weight cloths that are no longer made.  Shame, they were lovely, but still, I've got it from the horse's mouth that the following are still manufactured.  (From personal experience the tan is much more UV resistant than the natural.

Clipper Canvas

100% Spun Polyester

 

Quality                                  Width    US oz     GSM      UK oz                           

Natural / White

13440 Superlight                 150cm   7.2          310         9.1                                         

15450 Lightweight              060cm   9.8          420         12.4                                       

16545 Extra                         060cm   12.2        523         15.4                                       

22600 Heavy                       060cm   13.5        580         17.1                                       

24800 Superheavy              060cm   17.4        750         22.1                                       

 

Quality                                  Width    US oz     GSM      UK oz                          

Tan

13440 Superlight                 150cm   7.2          310         9.1                                         

15450 Lightweight              060cm   9.8          420         12.4                                       

16545 Extra                         060cm   12.2        523         15.4                                       

22600 Heavy                       060cm   13.5        580         17.1                                       

24800 Superheavy              060cm   17.4        750         22.1