HR shackles can explode

[fxykty]

Well, a recent passage has got me going over all of my blocks and shackles with a much closer eye than before. What doesn’t kill you can make you stronger.

Both of our running backstays failed within a 12 hour period during very bumpy conditions with huge shock loading on the rig every second or third swell.

First to go was a Harken HR D shackle, used to connect a block to the lower end of the port running backstay. A segment of the shackle blew out with a sound like a gunshot and suddenly the backstay was blowing in the breeze.

A few hours later we tacked and a few hours after that a Harken fiddle block (that uses HR for all steel parts) at the chainplate to which the running backstay attaches blew apart and showered the cockpit with shards of aluminium and several chunks of steel. At least this time the backstay, albeit very loose, was still attached by its adjustment line and was easier to retrieve.

Taking the broken parts to the local Harken tech, both breaks were blamed on wear and fatigue. I estimate the shackles and failed block are about 10 years old.

The remainder of this post refers specifically to HR, aka High Resistance, marine hardware components from Harken, as described to me by a senior tech. HR components use a special stainless steel grade 17-4PH that is stronger and tends to deform prior to breakage, but is more brittle so once it starts to deteriorate it can lose strength quickly. Wichard HR shackles use the same steel. The steel has more iron in it and develops surface (cosmetic) rust more readily than 316 stainless.

Signs that your HR component is losing strength:
- excessive rust
- pitting
- wear
- deformation

The shackle on the left from the other side - note the deformation of the hoop - it’s getting ready to break. The shackle on the left shows the missing section that blew out.
- elongation/enlargement of pin holes

The pin on the right is brand new and the pin on the left was replaced. Note the rust and enlarged pin hole on the left.
- for blocks, also check the pin seating surfaces, especially if plastic, and the condition of bearings. For the block that failed it was the plastic seating that failed first, concentrating the pin load off axis on the HR body plates.

Most of these checks mean taking apart the shackle or block and looking very closely. I’m doing a once over this weekend of all of the shackles and blocks we’ve got and taking any suspects for a closer inspection by Harken.

[billknny]

It sounds like your blocks and shackles are undersized for the loads they experience.

A cat of the size of yours has VERY highly loaded rigging. You might want to go up a size or two rather that just replace in kind.

[fxykty]

Originally Posted by billknny

It sounds like your blocks and shackles are undersized for the loads they experience.



A cat of the size of yours has VERY highly loaded rigging. You might want to go up a size or two rather that just replace in kind.

No, not correct, all the running gear is correctly sized. That could be a good suggestion but does not apply in our case.

All parts have similar working loads, including the parts that failed and the running backstay wire itself. Those parts that failed have obvious (now that I know) signs of weakening.

I posted to make others aware of the potential for weakened components.

[Dockhead]

Originally Posted by fxykty

No, not correct, all the running gear is correctly sized. That could be a good suggestion but does not apply in our case.

All parts have similar working loads, including the parts that failed and the running backstay wire itself. Those parts that failed have obvious (now that I know) signs of weakening.

I posted to make others aware of the potential for weakened components.

Nevertheless, those Harken 75's are half the size of the blocks in my running backs, for an application with far less (10x less?) dynamic loading than what you will experience there. Mine are 120mm high load Lewmars weighing more than a kilo and with 12mm Wichard HR shackles. Your shackles are, what, 8mm?



My eyeball, at least, for whatever its worth, supports Billkny's suggestion that they are undersized, and significantly so. Those shackles should not be loaded to the point where they start deforming like that -- would you load your anchor chain like that? All those bits should be sized to comfortably take all the loads with a comfortable reserve of strength which covers even rust and a certain amount of deterioration. Something breaking in the rig can cause you to lose the whole rig -- not the place to mess around.


Your boat, of course, your call, but I would in your place think about upsizing all that. The failure just by itself shows the need.

[billknny]

If it breaks it's not "correctly sized."

Just because OTHER parts haven't broken doesn't mean the spec is right....

[MarkSF]

Originally Posted by fxykty

No, not correct, all the running gear is correctly sized. That could be a good suggestion but does not apply in our case.

All parts have similar working loads, including the parts that failed and the running backstay wire itself. Those parts that failed have obvious (now that I know) signs of weakening.

I posted to make others aware of the potential for weakened components.

One key feature of correctly sized rigging is that it doesn't break.

[fxykty]

We’re a performance cruising cat, which means we don’t weigh much and our rig is modest. Not like either yours nor Bill’s boats. Our primary winches are smaller than those on a mid 40 foot monohull.

Are you saying the designer under specified the running gear? Hmmm, you must know a lot more than he did. We have 2K kg working loads on all our running rigging, save the mainsheet that has double that, and our travellers that have less. Increasing any of these would transfer excess loads to other parts of the rig, for which it’s not designed.

Also, our running backstays prevent excess forestay sag and are not primary or even secondary support for the mast. Hence they let go and the mast didn’t come down.

Once again, these two components failed due to wear over at least 10 years, not due to being undersized. Sure, next size or two up would mean more reserves of strength. But remember that this is a performance cruising cat and adding a kilo here and two kilos there is not how to keep the performance and to keep the rig loads modest. The trade off is more frequent gear replacement.

I’m sorry I posted what I thought could be interesting information about HR fittings if the only responses I’m getting is that the gear is undersized.

[billknny]

You are missing a point. "Wear" over ten years does NOT result in deformation of metal. Metal deforms when it is loaded past its yield strength. Period. Full stop. Loading metal to 80% of its yield strength--no matter how often--NEVER results in deformation. That's physics.

Now, you might want to suffer breaking parts occasionally to save the weight. That is a acceptable decision if made with fore knowledge. But the parts are overloaded by every picture and description you have given. That's not bad, it's just true. Saving weight on your boat is important. I get it.

I am not being critical of you or your boat. Just giving you an engineer's viewpoint. You can reject it if you wish.

[fxykty]

Originally Posted by billknny

You are missing a point. "Wear" over ten years does NOT result in deformation of metal. Metal deforms when it is loaded past its yield strength. Period. Full stop. Loading metal to 80% of its yield strength--no matter how often--NEVER results in deformation. That's physics.



Now, you might want to suffer breaking parts occasionally to save the weight. That is a acceptable decision if made with fore knowledge. But the parts are overloaded by every picture and description you have given. That's not bad, it's just true. Saving weight on your boat is important. I get it.



I am not being critical of you or your boat. Just giving you an engineer's viewpoint. You can reject it if you wish.

Thank you for your viewpoint. I certainly do not want to save weight at the expense of breaking parts in the course of normal operation. It’s just that what you suggest is an upgrade to a boat that was designed and built for world cruising by a company that knew all its original owners by name.

Once again, it was a senior Harken tech that inspected and explained reasons for the HR shackle and block failing. I’m paraphrasing what he told me, not trying to justify anything. HR steel is different, according to Harken, hence my post.

He did not suggest I upgrade the size of the replacement components. He does rigging consults for race programmes all over Asia Pacific and understands the cruising that we do and plan to do. I assume that he is not selling me under spec’d gear so that it will break again. Do I trust him less than you?

[Tillsbury]

Originally Posted by billknny

You are missing a point. "Wear" over ten years does NOT result in deformation of metal. Metal deforms when it is loaded past its yield strength. Period. Full stop. Loading metal to 80% of its yield strength--no matter how often--NEVER results in deformation. That's physics.

Now, you might want to suffer breaking parts occasionally to save the weight. That is a acceptable decision if made with fore knowledge. But the parts are overloaded by every picture and description you have given. That's not bad, it's just true. Saving weight on your boat is important. I get it.

I am not being critical of you or your boat. Just giving you an engineer's viewpoint. You can reject it if you wish.

Exactly this. It does appear that some of the French builders go all out for the smallest possible margins. The Beneteaus I know locally have some terrifyingly small fittings (to my eyes). The Moodys are certainly the other way. I know which way I like to go, and expecting steel shackles to break under way after 10 years is not in my maintenance plan. I’ve replaced almost all of my shackles with soft shackles (and that would solve your weight issues), but the smallest was 12mm and they’re all 15 years old but appear as new.

My father was a professor of materials science and it turns out I did learn a bit about metal fatigue from him...

[Tillsbury]

Originally Posted by fxykty

I assume that he is not selling me under spec’d gear so that it will break again. Do I trust him less than you?

You have already proven that the shackles you had are prone to break in normal use within 10 years, so that question answers itself if he’s suggesting you replace like with like.

[LongRange]

A very interesting discussion. Thanks fxykty for psting this, so that others can learn from your experience.

I imagine that relatively senior representatives from the companies involved would be reluctant to say the gear is undersized, being wary of legal and reputational risk. And yet it broke...

Also, I think there is a contradiction in this sentence:

HR components use a special stainless steel grade 17-4PH that is stronger and tends to deform prior to breakage, but is more brittle so once it starts to deteriorate it can lose strength quickly.

"Brittle" is the exact opposite of "tends to deform prior to breakage". If that sentence is more or less verbatim from a company rep, they may not be technical.

[Dockhead]

Originally Posted by fxykty

We’re a performance cruising cat, which means we don’t weigh much and our rig is modest. Not like either yours nor Bill’s boats. Our primary winches are smaller than those on a mid 40 foot monohull.

Are you saying the designer under specified the running gear? Hmmm, you must know a lot more than he did. We have 2K kg working loads on all our running rigging, save the mainsheet that has double that, and our travellers that have less. Increasing any of these would transfer excess loads to other parts of the rig, for which it’s not designed.

Also, our running backstays prevent excess forestay sag and are not primary or even secondary support for the mast. Hence they let go and the mast didn’t come down.

Once again, these two components failed due to wear over at least 10 years, not due to being undersized. Sure, next size or two up would mean more reserves of strength. But remember that this is a performance cruising cat and adding a kilo here and two kilos there is not how to keep the performance and to keep the rig loads modest. The trade off is more frequent gear replacement.

I’m sorry I posted what I thought could be interesting information about HR fittings if the only responses I’m getting is that the gear is undersized.

You are getting correct and helpful responses which go to the heart of the matter. What happened to your gear has nothing to do with the properties of HR fittings.



Your running backs perform the same service mine do, but you are a PERFORMANCE CAT, which means that your rig is much higher loaded than a mono of similar size, which heels to relieve pressure in gusts.


You should listen to the engineers who have weighed in here -- they speak the truth. This gear didn't "wear out"; it was overloaded. There's a big difference.



The whole system should be upsized at least one size. The blocks and shackles will be heavier, but you can get the weight back by replacing the wire rope with Dyneema (something on my list to do to my own boat).



You should thoroughly inspect the whole rest of the rig and see if anything else is showing signs of being overloaded. God forbid you've got a toggle getting ready to give way.



You can, on the other hand, just replace the shackle (and block, and wire rope leader) like for like. But if the shackle was not up to the job, what part of the running back system will fail next? There is no guarantee that the next set will last 10 years like the first set did. The shackle in your photos was exposed to loads exceeding the yield strength, which means the whole system, if as you say it is all designed to the same WLL, has been overloaded, and will be overloaded again if you replace it like for like.


Running backs on a rig with a static backstay and/or aft swept spreaders may not be critical to keeping the rig up, which is probably why the designers felt they could undersize it, but the failure of any part of the rig in a dynamic situation can have unexpected consequences to the rest of it. You just don't want that.

[Dockhead]

Originally Posted by LongRange

. . . "Brittle" is the exact opposite of "tends to deform prior to breakage". If that sentence is more or less verbatim from a company rep, they may not be technical.

Exactly right.



Company reps do not always have even a basic grasp of basic engineering concepts. "They may not be technical" is a tactful thing to say about it.

[fxykty]

Originally Posted by Dockhead

Exactly right.



Company reps do not always have even a basic grasp of basic engineering concepts. "They may not be technical" is a tactful thing to say about it.

The Harken NZ person I spoke with is Greg Blewett. I didn’t quote him verbatim, just my paraphrase. My fault if what I wrote doesn’t make sense. He was trying to explain to me the properties of HR steel and how it’s different from 316. I guess similar to how G7 chain is different from G4. Again, my analogy.

Greg is very technical and I expect knows a whole lot more about Harken gear than any of the posters here. Or it’s all BS and he doesn’t know much at all.