roaringgirl
Well-Known Member
I suspect that leeward shroud slackening was a major contributor to the eventual failures. I'm going to tension all to the point there's no slack on leeward when heeled to avoid load cycling.
Rigging failure aftermath
- Thread starter roaringgirl
- Start date
vyv_cox
Well-Known Member
Thinking about it overnight I believe the fundamental problem to be that the rigid construction of the fitting has resulted in unfair loading. The two forces seem to be acting against each other, with the one on the left in the photo being bent to the left and slightly down, and the one on the right being bent upwards and to the right. This would account for the location and direction of the crack propagation. Ideally the two plates would be separate, pivoting at the bolt.
ean_p
Well-Known Member
is it possible to get a side on photo of the edge to see how the tang assembly was formed to accommodate the cable ends and also a side on photo of the remaining insitu tang to see how the remaining cables 'fair' on to the tang please?
Are you motoring or attempting to sail to the riggers?
Are you motoring or attempting to sail to the riggers?
roaringgirl
Well-Known Member
It is literally impossible to give you a picture of the part that's still up as it's totally obscured from all angles by the spreader. The two plates are welded together to form one of double thickness, which has a hole through it to go over the through-mast bolt, like in this link: Google Image Result for http://static1.squarespace.com/static/58b066e83e00be6efbd33633/58b6101385c5bd680c74d564/58b628e685c5bd680c77cce8/1488333030884/RigChk2-Tang.jpg?format=original
vyv_cox
Well-Known Member
Interesting that the link you provided above includes this image. It appears to be similar to the arrangement described.
roaringgirl
Well-Known Member
Yes, I agree. The best thing would be to replace with a t-bar or ball joint. This isn't going to happen in Tahiti, so I think I should tighten the lowers good and hard to minimise the movement that has clearly caused fatigue.
roaringgirl
Well-Known Member
I went up to jury rig the starboard side today, to pre-empt the failure. When I took the pins out of the shrouds, the inner plate of the tang fell out, as it had also cracked right through. The crack-creep pattern across the crack face is *exactly* the same as that seen on its port-side counterpart. I had to leave the outer part of the tang up there as I can't get it off without removing the spreader. Now the shrouds are attached to the soft shackle, it's obvious that the angle the tang makes with the mast is too large. I think this, and the shroud lowers being too loose are what led to the failure.
Last edited:
vyv_cox
Well-Known Member
The one on the right in this photo is slightly different, there are two cracks growing from top and bottom. Can you see a groove stress-raiser that has initiated the cracks as with the port side?
roaringgirl
Well-Known Member
Yes, there's a groove at exactly the same point.
tico
Well-Known Member
Classic fatigue failure striations
vyv_cox
Well-Known Member
They are beach marks. You need an electron microscope to see striations (if you are lucky!)
ean_p
Well-Known Member
Would it be right to think that it is poor manufacturing technique that has directly lead to this Vic, in that they have formed the bend over a 'sharp' edge as opposed to having given it a nice radiused form such as appears to be the case with the other (lower )bend on the outer tang. And is possibly something to watch for when buying new or surveying existing equipment? Though I tend to think / recall that its quite common practice........
vyv_cox
Well-Known Member
Yes, the groove that is present is a stress raiser. I have a wonderful old book that gives values for stress raisers and ones of the type that are visible in the pic in post#5, sharp, narrow V shapes, can increase the stress locally by very big numbers, maybe up to x 15! Line stress raisers like the ones here usually cause multiple initiations, as seen in the one on the right in post #27. If the loading had been fair this would be less of a problem but applying tension in bending at the groove has led to the fatigue.
Several examples on my website here Fatigue
These pics show one of my favourites on stress raisers. An engine component, a large plain bearing, with a maker's part number stamped in it. A fatigue crack has initiated at the '8'. Second pic shows the sectional view.
roaringgirl
Well-Known Member
Well, the soft shackles didn't last long! We managed to sail under genoa and mizzen for about 30 miles inside the flat waters of the lagoon of the atoll, but only about 20 minutes of being pounded by the swell of the Pacific outside the pass and both sides failed. We didn't even have any sails up! It looks like the eyes swaged on to the lower shroud terminals are too sharp and caused rapid chafe under rigging tension as we rolled in the swell. I'm now waiting to hear back from the Selden dealer in Tahiti who ordered the new tangs for me a week ago...
There's a rigger in the anchorage who assures me that we can remove the spreaders to fit the tangs while the mast is still up. (After supporting with 4 halyards).
There's a rigger in the anchorage who assures me that we can remove the spreaders to fit the tangs while the mast is still up. (After supporting with 4 halyards).
roaringgirl
Well-Known Member
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!
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!
Mudisox
Well-Known Member
What an excellent post and replies. Exactly what the forum was set up for.
I will be getting my mast lowered and checked. - it is 20+ years old [but visually checked and photo'd very year}
I will be getting my mast lowered and checked. - it is 20+ years old [but visually checked and photo'd very year}
roaringgirl
Well-Known Member
My rigger friend advocates using some longer levers to get to 15% on the cap shrouds. He sent me the Selden rig tuning guide, which says the same, so I will do it tomorrow!
Supertramp
Well-Known Member
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.
geem
Well-Known Member
Riggers have always recommended sailing in moderate winds to windward and watching the leeward cap shroud and lowers for sag. If you are getting sag, they ain't tight enough.
I was up our mast doing a rig check yesterday and refitting a grub screw that had come out of one of the furler sections. Because of this post, I did a more thorough check of my tangs with a magnifying glass. All good on a 42 year old boat. Next to me is an old Bowman 45 from the 1970s. Those mast tangs looked very undersized and not very good. By comparison, ours are huge. The central bolt through the mast is 19mm. The tangs are also pretty beefy. The same sized are used on the lowers as well.
Kukri
Well-Known Member
This outstanding thread is giving me litters of kittens. Thanks to all concerned.