hmmmm...
Great forum! Thanks indeed for your further thoughts Brion, and for putting the issue in perspective for the general reader. I still think there is something unexplained going on, and would appreciate further ideas from yourself and others.
So as I understand it for the general reader we have to note that:
1.
Whilst variations in splicing technique will lead to fluctuations in breaking load, so do fluctuations in wire swage tolerances. And its probably a lot easier to tell if a splice has been done badly than it is to tell if a swage has been done badly. Advantage plastic rigging.
2.
Both plastic rigging and stainless wire degrade in the environment - plastic due to u-v and wire due to salt etc. (however, the degradation rate in wire increases as it gets older whereas the degradation rate in plastic is greatest in the first few years then levels off). U-V degradation of Dyneema implies a covering is advisable in extreme sunlight. No advantage to either plastic or steel, just different.
Then to the specific case of the broken spectra backstay:
The load in this case was not applied by the wind, it was applied by me pulling the backstay tackle on in the pen before we even put the sails up. This is just a standard cruiser-racer with a standard block and tackle. I have sailed on lots of boats that apply a very much higher pre-tension in their (wire) backstay. Without revisiting the sums, the bottom line is that the spectra broke when the backstay tackle was applied whereas the wire never broke under very much higher loads exerted by the exact same backstay tackle plus the much bigger loads of wind and waves. This despite the plastic rope having a higher quoted breaking load than the wire. Even when you consider all the factors (u-v –salt corrosion, splice quality, swage quality, creep-induced weakness etc.) it just doesn’t add up that a stronger plastic rope breaks at a hugely lower load than a weaker stainless wire.
There’s something missing in the analysis which needs a bit more investigating. Suggestions, anyone?
Kim
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