Issues with synthetic rigging?

[Brion Toss]

Hello,
Thanks for all the details. For the information of most of the people reading this, what follows is a follow-up to an earlier contact by email, and a delightful Q and A, via Skype, with the Freemantle Sailing Club.
So, to take things in order, "probably less than 30% is distinctly overloaded; 20% is a good design load for most rope (or wire rope), and 10% is good to shoot for with Spectra, in order to minimize creep.More on this later.
Glad to hear that the cockatoos hadn't been attacking your rope. We don't get a lot of that here in Port Townsend, either, though we do have problem with bird poop in blackberry season.
The splice does not appear to have been tapered at all (I have a picture under separate cover). According to Starzinger's tests, this could weaken the rope by at least 15%.Note also that, as creep progresses, the rope is somewhat weakened (https://www.dsm.com/content/dam/dsm/...plications.pdf). So now we have a stress riser, plus the effects of creep.
Next, a biggie for Spectra: UV. I agree that this is a time bomb in your part of the world; after two years with no treatment, uncovered Spectra could easily lose 25% of more of rope strength.
Finally, note that it is not you who applies load to that backstay, it is the wind. I recommend having someone monitor a tuning gauge next time you are under way, so see what the loads get to.
In sum, we have a not-great splice, plus undersized rope, plus time in the sun, plus uncertainty on loading. It does not seem surprising that the rope failed.
Try this with your wire backstay: use cable clamps instead of swages for your terminations. Use an undersized thimble, for a too-tight radius. Apply some tape along the standing part, to trap water and salt inside. In other words, do the things that are known to weaken wire rope. Of course that is a bad idea, but that kind of describes what happened to your Spectra backstay. I recommend HSR or similar, not Amsteel blue, for standing rigging, and I recommend sizing it appropriately. For your part of the world, a covered version seems important; you'll still save considerable weight.
Fair leads,
Brion Toss