All sail materials stretch to some degree. Once they have
been over stretched, the material cannot return to its
original shape. At UK-Halsey Sailmakers we test our sail
materials so that we know each material's limits.
Here a sample of this mylar laminate shows nearly TWO INCHES of stretch at 100 lbs load on the sample, far too much for a good sail.
After
releasing all that load the mylar did not recover, in fact
it was permanently stretched. This failure to recover after
loading is just why mylar sails have such a short performance
life. Hot sailors even notice them slowing down sailing the
first beat in a fresh breeze. Have a look now at the dramatically
different results with a UK-Halsey tape on the mylar in the
next image.
Using
UK-Halsey's cheapest tape, the Pentex/Glass Tape, to strengthen
the laminate, the sample yielded less than 6/100ths of an
inch under the same 100 pound load. A sail built with this
light laminate and tape will maintain its shape and performance
far longer than an unreinforced sail.
After
releasing all that load the mylar did not recover, in fact
it was permanently stretched. This failure to recover after
loading is just why mylar sails have such a short performance
life. Hot sailors even notice them slowing down sailing the
first beat in a fresh breeze. Have a look now at the dramatically
different results with a UK-Halsey tape on the mylar in the
next image. 
Using
UK-Halsey's cheapest tape, the Pentex/Glass Tape, to strengthen
the laminate, the sample yielded less than 6/100ths of an
inch under the same 100 pound load. A sail built with this
light laminate and tape will maintain its shape and performance
far longer than an unreinforced sail.