Stainless-steel failure

[Krusty]

Seeking well-founded technical knowledge; but relevant practical experience would be welcome too.

Consider an inner forestay held by a shackle at one end and a (larger) pelican hook through a U-bolt at the other.
Both the shackle and U-bolt are the usual candlery quality.

Under extreme storm-jib loading, which one would give way? And why? (NOT trick questions posed for fun!)

 

[prv]

Check the max working load of each part. The lower one will (probably) break first.

If they don't have published working loads, don't use them for rigging.

Pete

 

[Krusty]

Check the max working load of each part. The lower one will (probably) break first.

If they don't have published working loads, don't use them for rigging.

Pete

Why would the lower one fail first?

Personally, I would never use either of them for rigging unless "in extremis".
Published working loads are sometimes no real guide to "failure load".

So the question remains.

 

[prv]

Why would the lower one fail first?

I meant the one with the lower maximum working load, not the one at the lower end of the stay. Sorry if that was unclear.

Obviously there are tolerances involved and it's possible the lower-rated one might actually hang on past where the higher-rated one breaks. That's why I said "probably".

Of course, you haven't considered the strength of the wire itself. I heard somewhere that things ought to be sized so that the wire is the weakest link (which makes sense, as it costs the most in weight and windage to upgrade). I've just done this on my own shrouds, which are all held on by shackles at the top. The random no-name shackles previously in use played on my mind a bit last year, so I've just bought all Wichard ones whose published working load is higher than that of the wire.

Pete

 

[Bosun Higgs]

Seeking well-founded technical knowledge; but relevant practical experience would be welcome too.

Consider an inner forestay held by a shackle at one end and a (larger) pelican hook through a U-bolt at the other.
Both the shackle and U-bolt are the usual candlery quality.

Under extreme storm-jib loading, which one would give way? And why? (NOT trick questions posed for fun!)

Impossible to answer without detailed testing of all components. And the stainless steel bit is not really that relevant since you can have austenitic stainless steel to any strength from 30 TT to well north of 100TT.

 

[Krusty]

Of course, you haven't considered the strength of the wire itself.

Now that's quite an assumption! The wire and its terminations can be taken to be above suspicion.

Impossible to answer without detailed testing of all components.

Really? It's only about the shackle pin and the U-bolt.

And the stainless steel bit is not really that relevant since you can have austenitic stainless steel to any strength from 30 TT to well north of 100TT.

I did state: "Both the shackle and U-bolt are the usual candlery quality."
Perhaps I did not go to the best places, but I don't recall any chandlery displaying comparative data for their pre-packed chandlery items.

I should have made clear that the shackle and U-bolt are the same nominal "size": say 8mm.

 

[Tranona]

So on the information I gave, the question remains!

But there still is no definitive answer! "Chandlery item" is not a technical term that tells you anything about the loads the components can carry.

So the only "answer" is the one you already have - that is the component with the lowest breaking point - and you don't know which one that is until you test it.

 

[prv]

Perhaps I did not go to the best places, but I don't recall any chandlery displaying comparative data for their pre-packed chandlery items.

Wichard shackles have the data printed on the display card.

ProBoat ones do not, but it can be looked up on their website (which I did, on my phone, in the chandlery and discovered that the working loads for the same sizes were much lower than Wichard, hence the significantly lower price).

No-name Chinese shackles have no load information, which is why I would not use them for critical rigging purposes like shrouds and stays. Even chunky-looking fittings can break surprisingly easily if badly made from cheap metal.

Since I was buying shackles to hold my mast up, I bought the Wichard ones, despite the eye-watering price.

Pete

 

[wklein]

Tricky one! Here's my 2p.

Pelican hooks generally have lower working loads than shackles, having said that if they are equal i would say the shackle would give first as it likely to be unfairly loaded by the forestay attachment as they are likely to be a lot thinner than the tang(which is why they now produce swage forks)

 

[Krusty]

.....Wichard shackles have the data printed on the display card...
Pete

So they do! Overlooked those.

OK; based on experience of U-bolt failure in a different application, my thinking has been that it would most likely fracture at the start of one of the screw threads, or by stripping of threads due to unequal loading; as the stay is, of course, angled.

I seem to recall that Vic's testing of shackles, (published some time ago in PBO or YBW) showed failures where the U joined an eye for the pin.

For practical implications at sea, failure of the shackle at the top would be the least-worst scenario with every chance that the stay and sail would be recoverable.
Failure of the U-bolt, leaving a sail flogging from the masthead, whipping a wire with it, is (to quote the venerable Tom Cunliffe) "not for contemplation by the squeamish".

So the shackle should be sized to fail before the U-bolt: but how much smaller to be fairly certain? 8mm shackle v 10mm U-bolt?

 

[Daedelus]

The higher one ie out of reach would be the one to fail first.

 

[Deleted member 36384]

...... So the shackle should be sized to fail before the U-bolt: but how much smaller to be fairly certain? 8mm shackle v 10mm U-bolt?

No thats flawed logic. All items should be selected based on the strength required for the applied loads and direction of pull. For an inner forestay, like most rigging, one needs some sort of articulation in the connection to allow for the resultant force due to any forestay sag.

If you were designing something to fail before something else then you can not do it only by size (cross sectional area), you must take into account the material yield strength.