Rigging failure aftermath

Supertramp said:
Is the rigging tension suggested independent of hull and deck design and the differences in stiffness? I would have thought tie rods and stiffening materials would deform or stretch differently from old fashioned plain grp. I do get the need to avoid slackness although this is near impossible to achieve on some older designs due to flexing.
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On 19 strand wire, 1mm stretch over a 2m section equates to 5% of breaking strain. There is a note in the selden rjgging guide that says if you're using rods, 5% of breaking strain is 0.7mm stretch in 2m.

I don't think the failure we've experienced has anything to do with sizing. The Maxi 120 was built for circumnavigation in the 70s. Everything on this boat is massively overengineered, which is why the mast stayed up in a 2m beam-on swell even when all the lowers came down! The point where the tangs failed is 4mm thick stainless plate, and the pattern on the break surface shows it was due to fatigue. The tangs are bolted through the mast on piece of steel dowel, the nuts on it are M24, the configuration of it has a second through bolt a few cm above that also goes right through the mast and has M19 nuts on it.

The fact that this was a fatigue failure means it was due to movement when the shrouds were too loose so I am particularly interested in setting the tension correctly. The Selden rigging guide is very thorough and does indeed advocate checking the rigging at sea under specific conditions, but only *after* setting the correct static stretch of 15% of breaking strain.
 
roaringgirl said:
The lowers tangs finally arrived and I fitted them today. The spreaders had to come off, so for much of the day I was aloft with 6 of the 8 stays loose around me!

All the reading I've done says that I should tighten the cap shrouds to 15-20% of their breaking strain, which is measurable as 3-4mm stretch over a 2m section. There's no f-ing way I can tighten it past 1mm extension without sliding some big poles over the spanners!
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Don't do that on a Lagoon catamaran. The hulls might fold inwards judging by all the Youtube videos on Lagoon bulkheads.
 
roaringgirl said:
They don't even have lowers, just cap shrouds and a forestay, backstay and a tangle of jumpers all over the place. The Selden guide doesn't even *try* to cover what the hell you do with that!
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Ah, I know about those! I’ve got an original Uffa Fox International 14 dinghy with her original “birdcage” hollow spruce mast. Two jumpers, two diamonds several fathoms of piano wire! Definitely a tangle of jumpers,but on a manageable scale!
 
Interesting that the rigger including Selden describe how to best avoid fatigue in the rigging. However he is only concerned with the rigging and metal fatigue. The sialor is concerned with the end result. ie the whole boat. The hull of which may not be stiff enough to acheive the desired rigging tension. I think you just have to accept that rigging may not be able to be tensioned to optimum for the rigging because of hull distortion. In which case just live with and accept metal fatigue.
As an aside I was inspired by this post to remove tangs on my little boat. A separate post describes getting the bolts long enough to have load on shank. Other wise all looks good after 42 years but then a much lighter boat (1t) ol'will
 
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