Chris,

I can see the point in using dead cheap material for experimental sails for dinghies. The very first sails I made, at the age of 16, was from of a sheet of yellow polyethylene, meant for camping. The sprit-sail and jib was not sewn at the hems, but partly glued and partly ‘melt-welded’ with a home-made solder iron. Anything works when below 10m².

However, if you need serious sails for a serious yacht (your 34-footer?), I suggest you don’t invest a lot of time on making sails which will last for less than 5 years. The cost of the sail material will anyway be small (when spread over the years of use), compared to the other costs of keeping the yacht operational.

Arne

Painted Tyvek is  also used for fabric covered canoes and dinghies I see. It performs as well as or better than canvas perhaps?

I have some spinnaker nylon and a Tyvek envelope. I just managed to tear the nylon. I could not tear the Tyvek. That is a cheap test anyone can perform.  Try rubbing and perforating both samples.  Put them in the washing machine. You will be amazed at Tyvek . Non UV treated Tyvek has a sunlight rating about the same as Dacron. My first experience with the material was a caravan cover which was replaced after five years because of grime and mould. The material did not breakdown but that (grey one side  and white the other !) sample was a little too thick for a sail and I did consider it.  Tyvek has largely replaced other fabrics in that caravan cover application. The cat loved the cover - soft and cosy to lie on in the sun and excellent for sharpening claws! He liked the grey colour so I am glad I did not buy the olive green alternative. 

I have new sails now but one day might experiment with camber when I finish sailing back to NZ. By then my flat sails will be shot (but I have treated the Dacron with space age UV protector which could well be snake oil). Hoping there is a Tyvek knowledge base relevant to Junks when the time comes to replace those sails. Retired, I will never be able to afford again the Scanes manufactured sails l have now. 

So I am very interested in weights of cloth, longevity and experience with the material etc. Someone out there is cruising around with Tyvek sails who could either inspire junk experimentation or  end Tyvek speculation. Certainly there are enthusiasts who think it is a wonder material but more hard  data is needed from genuine cruisers. 

http://www.materialconcepts.com/tyvek/uv-soft/

Comes in black as well. It can be dye printed permanently any colour and there is guidance. Link is to the heavier fabric. That is three times price of the light fabric which has been used for mainsails on 30 ft yachts. They also make a " woven" fabric . Lots of choice. Drill down and there is every fabric for every engineering situation. It is even used as barrier on ground and sharp gravel poured over it then subjected to traffic. It is tough. 

According to Wooden Boat Forum the lighter is impossible to tear or deform even when old and worn. It also has limited stretch that amateur sail makers find useful for designing headsails. Some appear to be very knowledgeable.  Articles on broad seaming Tyvek are interesting and relevant for junk cambered sails no doubt. Lots of PDFs can be downloaded for guidance as been used by amateur sailmakers in US for decades.  There is a cult following.

 Fabric has been  used offshore because it can be engineered to take reinforcing on sail corners without tearing and reef points and eyelets do not pull out.  

Importing anything made in China is very, very much a case of "buyer be aware" because the results are extremely variable, even with supposedly the same product from the same factory. Quality varies from batch to batch.

I work for a company that imports extensively from China and just about every year we land up dumping two or three entire 40ft containers of stuff... and we have a representative in China which supposedly should prevent the problem.