cause of spectra backstay failure?
Hi,
I am a relative newcomer to this group; the answer to my question should be out there somewhere, but I haven't been able to find it. I have had some brief discussion with Brion Toss, but I still don't have a very convincing answer. The problem: why did my 2 year old spectra backstay break when I was still dockside? Some background...
My unsheathed Dyneema 6mm backstay (I believe it is SK75) from a reputable supplier broke after less than 2 years of light use. The failure occurred at the start of the upper eye splice when I applied the backstay purchase by hand when tied up in the pen. So far I have identified the following possible causes of failure, none of which seem entirely convincing to me.
1. Overloaded
The stay was definitely not overloaded – probably less than 30% of breaking load at the time (see end-notes 2 and 3 for details).
2. Chafe
There is nothing near the backstay up at the top splice so chafe is almost impossible to occur. A rigger specialising in synthetic rigging said it might be the cockatoos (parrots) pecking it. This happens to the dinghies in the dinghy park but I have never seen the cockatoos on the jetties here and there was no evidence of beak damage on the broken stay.
3. Badly made splice
The splice was longer than standard recommendation, though the amount of taper was not very much from what I can tell (Brion notes this is an important issue). It was made by a rigger who does a lot of plastic rigging work on both racing and cruising yachts, and has a good reputation.
4. U-V degradation
This is my main concern. There is a lot of variation in the data about how Dyneema etc. degrades with U-V exposure. Some reliable sources suggest a gradual decrease in strength by 20- 30% over 2 years, then stable thereafter (Brion mentioned similar figures). The stabilisation is probably due to the oxidised material forming a protective barrier over time, a bit like the oxide on aluminium alloy. I guess the actual figures depend on the amount and type of U-V stabiliser added to the rope. The u-v out here in Western Australia is truly fierce (the annual average noon u-v index is 8, and over the summer it averages 12 with many days over 13. I understand this is worse than Miami and much worse than the Med).
5. Fatigue
I have not investigated this possibility, but I noticed that one of the disadvantages of switching from a stainless wire to a braided rope backstay is the humming noise off the backstay in the wind. Given that the tension and the windspeed are the same for both materials, the two differences are the diameter and the surface roughness of the rope/wire. This may affect the Karman vortex street generated, which is a main cause of vibration, which in turn is a source of fatigue cycling. Fatigue properties of Dyneema?.....
My solution has been to revert to a stainless steel backstay seeing as I never has a strength issue with it. That is a pity given all the claimed advantages of dyneema, so I’d like to get to the bottom of this puzzle. The most likely reason I can come up with so far is a combination of a less than ideal splice, severe u-v degradation and possibly a helping hand from fatigue cycling. If that’s the case, we have a time bomb of rigging failures for uncovered spectra ticking quietly away here in sunny Western Australia.
Note 1: Materials selection criteria
An adjustable backstay is a different application from other standing rigging because stretch and creep are largely irrelevant, it is just a weight v windage trade-off (plus longevity and price).
Note 2 – Load calculation:
I applied the 24:1 backstay purchase by hand on my Van de Stadt 34 (when tied up in the pen). I estimate the load at failure (50kg*24=1200kg, less block friction) was at most 40%, of breaking load more likely 25% (see note 3 below).
Note 3- breaking load:
It is difficult to determine the true breaking load of the rope used - the supplier quoted 2,900 kg; Amsteel-blue shows 3,500kg; Melbourne Rope and Splicing Dutch SK75 says 4,200kg. I suspect the huge range may in part be due to some of the figures being safe working load whilst others are breaking load, or one of the many variants in between. Suppliers can sometimes be ambiguous in their definitions, and retailers are much worse. The bottom line is that the 7/32'' 1x19 316 stainless backstay at 2,450kg " breaking load" has worked fine for 30 years of use on this boat (rigging replaced every 8-10 years), so I should be able to account for safety factors etc. by sizing the plastic rope using the stainless as my benchmark
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