Failing anchor chain

[noelex]

As an amateur in the metals field, it's interesting that the links opened just a crack and no further?
I'd have guessed that the weakest link would have peeled right open and let go completely.

I witnessed a swivel that did exactly what you hypothesised .

The defect started as a crack that gradually opened up until the anchor was free. This was all seen underwater. It only took an hour or so in about 20 knots of wind to progress from a crack to a boat that avoided ending up on the rocks by the skin of its teeth.

Unfortunately, neither myself or my wife could convince the commercial skipper that there was a problem.

 

[thinwater]

I witnessed a swivel that did exactly what you hypothesised .

The defect started as a crack that gradually opened up until the anchor was free. This was all seen underwater. It only took a couple of hours in about 25 knots of wind to progress from a crack to a boat that avoided ending up on the rocks by the skin of its teeth.

Unfortunately, neither myself or my wife could convince the commercial skipper that there was a problem.

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Wow.

This also supports B27's observation that the strain was not excessive. The ratio of closed loop/open loop strength is different with a relatively open swivel vs. more compact link, because of differences in leverage. I'm guessing in this case the rode/swivle was also a little light for the boat.

 

[B27]

Was that a stainless swivel?
Would it have always been that weak, or some corrosion of the weld?

 

[noelex]

Was that a stainless swivel?
Would it have always been that weak, or some corrosion of the weld?

Yes, it was a stainless steel swivel.

The photograph does not give a good indication of scale. The swivel was probably an 18-20 mm model.

Viv will be able to comment better about the type of corrosion that ultimately led to the failure.

Here is this earliest photo, showing when we first noticed the problem.

 

[Roberto]

A couple more examples; IIRC they were all from a batch of chain sold in Martinique, French Caribbean. The top image seems to show a chain that has been in use for some time, no idea of the conditions though.



If the thread is about broken anchoring components, I have a number of other photographs

 

[Minerva]

This is a very interesting point. Once the crack opens, the link looses about 70-80% of it's strength. This means it broke very near the SWL. I will entertain theories that this is not correct, but I've bent links enough to be pretty sure, just as Neeves is sure an ultimate strength failure should look different. If the stress had been greater the link would have ripped open and if had been less the crack would be closed. This tells us that the failure was in a relatively narrow stress range. I'd like to present a different failure theory, for discussion. It is probably wrong, but I'm not sure of that.

This chain broke from fatigue. My other guess is that the OP does not use a long rope chain snubber and he has anchored some places with good exposure to small/moderate waves or wakes from passing boats. The water was not overly deep and the wind strong enough to straighten the catenary. The boat surged against the chain. I have measured forces well over the SWL on anchor chains in light winds when the right combination of wakes was present. The funny thing is, these surges really didn't feel like much because the weather was moderate and it was mostly just the momentum of the boat being arrested by the chain. Just a slight jerk. As B27 pointed out, it would only take 0.1 G, depending on the mass of the boat. One of the surges I experienced broke a load cell and shackles.

The links would only have opened if the load was near the SWL. Otherwise, they would be like the image below. Ideally, the crack should not be through the weld. That does indicate a substandard well and perhaps a chemistry problem as well. But the load was near the SWL or the chain would not be bent.

This should take many thousands or tens of thousands of cycles to cause this, but combined with some minor defect, it would be less, perhaps no more than 1000 cycles. Unless the OP is sure this is loading history is incorrect, perhaps it is also an object lesson on the value of elastic snubbers.

fatigue testing mooring chains

(fatigue crack in chain)

Negative.

Every time the anchor is out we rig our snubber; a 12mm 3 stand line, looped on to the stern cleat, then out the fair lead at the bow to a chain hook at waterline; ie around 12m long on a 10m boat. Really nice and stretchy.

 

[Neeves]

I witnessed a swivel that did exactly what you hypothesised .

The defect started as a crack that gradually opened up until the anchor was free. This was all seen underwater. It only took an hour or so in about 20 knots of wind to progress from a crack to a boat that avoided ending up on the rocks by the skin of its teeth.

Unfortunately, neither myself or my wife could convince the commercial skipper that there was a problem.

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I've tested these swivels - they all break at the weld, even swivels to the exact same design from major players in the 'chain' industry. They don't swivel under tension, too much friction. Commonly the shaft is a bolt with the nut welded on. The 'U' parts simply look like chain links welded on to a ring. At 'that' size they have no place on a boat particularly in the rode.

Jonathan

 

[Neeves]

Negative.

Every time the anchor is out we rig our snubber; a 12mm 3 stand line, looped on to the stern cleat, then out the fair lead at the bow to a chain hook at waterline; ie around 12m long on a 10m boat. Really nice and stretchy.

Your snubber sounds great - I wish more people had the same rigged. You could use thinner but if you find it stretchy - its fine.

I'd continue to advocate climbing rope as 3 ply is susceptible to abrasion - and you are abrading the structure of the rope. Climbing rope has an abrasion resistant sleeve and when it abrades the core remains intact. I might add - we used 12mm 3ply nylon in our second rode, 7t x 38' cat, for over 20 years and it never failed but we were careful about abrasion, it was always cleated to a bow horn cleat, there was no abrasion. We also had a 3ply bridle, originally, and it failed where it rubbed on the bob stays - and there was no furring from abrasion - the fibres were sufficiently weakened - and it failed with a sound like gun shot.

Because a snubber can fail - you should always have a back up - some sort of chain lock, best with cordage, and then if, when, your snubber fails the tension is taken by the chain lock - not the windlass. But everyone knows all this ....!!!

The, or 'a', snubber is not a panacea.

When your anchor is well set but the conditions are going south you need to retrieve the anchor and move - you take off the snubber and retrieve the chain. At some point the chain is near vertical and the anchor as if set in concrete. A bit of chop or a MoBo going past developing some larger waves will produce huge snatch loads, the tension could be 600kgs, until the anchor breaks out. Most people will know how difficult it can be to break out a well set anchor. (The tensions are nearing WLL for a 8mm x G30 chain).

The solution is to re-rig the snubber and break out a really deep and well set anchor with some elasticity to cushion any snatch loads - but its unrealistic to think people will have the time and patience to do this - when you feel you have to move you want to do so 'NOW' (and your wife wanted to do so an hour ago ). In order to offer any elasticity you would need to have your snubber rigged as by Minerva - secured at the transom, run up the deck to a fairlead - then you have length and (thus) stretch. But few will have a deck length snubber secured at the transom.

Any damage to the chain will be 'near' the anchor - depends how deep the water. But would not normally occur, say, 20m from the anchor as you will be motoring forward and usually have slack chain, until you reach the point when you are trying to break out the anchor.

One might assume this might all happen, sometime in the past (and maybe long distant past), and the failure occurred then but was simply a hairline crack (which you would never have reason to look for) - the link would then be weak and open up later even under more benign conditions.

Been there done that (without the rode failure).


I suspect some clever person could work out, mathematically the tension needed to bend/straighten a link. Not me - I'd need to set up a chain, cut a link, add a chain hoist at one end with load cell, attach other end to something big and substantial (trees are good) - but too much work.

Jonathan

 

[Ado]

Yet another broken link. I bought 50m of 8mm calibrated chain from my local Chandler in April, used in only13 anchorages. This morning a crew member lifting the anchor in the Scillies spotted a broken link 3m from the anchor. We have removed the broken section, checked each link and are currently anchored in St Martins bay hoping for the best. If you have bought a new chain recently I suggest a quick check is appropriate.

 

[thinwater]

Negative.

Every time the anchor is out we rig our snubber; a 12mm 3 stand line, looped on to the stern cleat, then out the fair lead at the bow to a chain hook at waterline; ie around 12m long on a 10m boat. Really nice and stretchy.

That settles that. Thanks!

But it is still weird that the crack opened just a little but did not rip open. If the load was low it should not have opened (remained a closed crack). If the load was at the WLL, it should have ripped apart, because the open loop strength is not just half, it is many times less. Observations that are hard to reconcile.