Fortress anchor: what material for shackle?

[UNCIVILIZED]

Okay, here's where my question stems from:
If I'm using 5/16" (8mm) G4 chain, which has a WLL of 3,950lbs +/-, & a breaking strength of 3x this.
It'll accept a high load shackle, with a 1/2" (12.7mm) cross pin, which has a WLL of 2 2/3t. And a rated breaking strength of 5x this.

In theory (& via the math), the shackle wins. Albeit, no, it's cross pin is far from being ideally supported, but, the shackle's 2.5x stronger than the chain. Or so it would seem.
And yes, in an ideal world, there would be plenty of room to fit a compression sleeve onto the crossbolt.

Then again, I suppose the above "case story" doesn't lie about how the various bits fared in the real world, either.
Which makes me wonder which shackles, & of what size, type, spec., & connected, how, & to what, failed, in said tale.
Albeit, regardless, there are so many; unknown, & uncontrollable elements in such a circumstance as to make drawing any real conclusions nigh on impossible.

I must say though, that's some BEEFY ground tackle for a 10m catamaran, assuming that such was the same boat in the story. Most boats that size don't carry anchors weighing even half as much. Especially multihulls.

[darylat8750]

Originally Posted by UNCIVILIZED

Okay, here's where my question stems from:
If I'm using 5/16" (8mm) G4 chain, which has a WLL of 3,950lbs +/-, & a breaking strength of 3x this.
It'll accept a high load shackle, with a 1/2" (12.7mm) cross pin, which has a WLL of 2 2/3t. And a rated breaking strength of 5x this.

In theory (& via the math), the shackle wins. Albeit, no, it's cross pin is far from being ideally supported, but, the shackle's 2.5x stronger than the chain. Or so it would seem.
And yes, in an ideal world, there would be plenty of room to fit a compression sleeve onto the crossbolt.

I'm having trouble understanding why you think the shackle is the weak point here. I thought that the rated strength considers the shackle as a system and the pin should stand whatever load the shackle is rated for.

[Snowpetrel]

I think a more rounded eye to the fortress would have another plus, enabling a bow shackle to sit properly in the hole as well as for a soft Shackle or straight rope at a pinch.

I think fortress could do a good line, selling lightweight plastic coated g7 chain and shackle combos that perfectly fit their great anchors.

I used duralac on all the Stainless bolts on my g37. Suprised they don't seem to specify this as standard. There is some minor corrosion on my old fx11 near the bolts, due to the PO not using any anticorrosive paste, and mistreating the poor wee thing terribly.

Kudos to fortress for replacing the mangled stock. Thanks Brian.

[UNCIVILIZED]

Originally Posted by darylat8750

I'm having trouble understanding why you think the shackle is the weak point here. I thought that the rated strength considers the shackle as a system and the pin should stand whatever load the shackle is rated for.

I posted the math on the strength of the shackle vs. the chain, as a previous poster was stating that the shackle IS the weak link. And me delineating the #'s was to question the "why" behind his statement that the shackle is/was, the weak part in the ground tackle package.
And the only conclusion which I could reach as to why the shackle might be the weak point, even given it's stength numbers, is that it's cross pin is very poorly supported where it passes through the chain.

I should think that if the chain had an oversized link in it's end, which allowed a compression sleeve to be fitted over the shackle's pin (assuming a true crossbolt in the shackle), then the real world strength of the shackle would climb greatly. And then, the weak point in the system would be the chain (most likely).


Snowpetrel,
Might I inquire what specific make & type of stainless shackles you're using? Particularly as, at times, I've been tempted to use some of the very high strength ones used for deck gear. That, or possibly even sourcing some WAY overstrength Titanium ones.
-> The catch(es) being: their price, experimenting with gear in a Very critical application, & their possible brittleness.

But much like you, the idea of using a thread sealant; one which possibly also aids in securing them shut, has crossed my mind, more than a few times.

[TheThunderbird]

The shackle's pin must enter the chain with some practical tolerance... thus its section will hardly match the two sections of the chain together.

And, on top, the pin is longer than the chain, and less stiffly secured than the ( welded ) portions of the chain components.

Three reasons, therefore, which all conspire to make the shackle the weakest portion of the anchor line. Unless titanium is used.

[four winds]

Originally Posted by TheThunderbird

The shackle's pin must enter the chain with some practical tolerance... thus its section will hardly match the two sections of the chain together.

The section of the pin doesn't match the 2x section of the link. But the section of the pin and bow do.

And, on top, the pin is longer than the chain, and less stiffly secured than the ( welded ) portions of the chain components.

Three reasons, therefore, which all conspire to make the shackle the weakest portion of the anchor line. Unless titanium is used.

Thinking about the first paragraph here, I can't say I agree with the statement. Not sure I can explain why though.

But the pin is pulling on one end of the last link, and the second link is pulling on the other end of the last link. So the 2x sections of the last link don't add up to resist the pull from the pin.

Three points of contact..... second link to last....last link to pin.... and bow to anchor, all see the same force applied.

Two sections of the last link doesn't resist the force from the pin. It splits it's duty between the second link and the shackle pin.

The pin section doesn't match the 2x section of the last link, but the pin section and the bow section do.

[Snowpetrel]

Uncivilized, my Shackles are galvanized, high tensile. I had to hammer open the end links of my 8mm chain slightly to take the 2 t shackle pin. I used a much bigger straight shackle on the anchor.

The duralac was on the bolts for the stock, and the mud palms.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[UNCIVILIZED]

Originally Posted by Snowpetrel

Uncivilized, my Shackles are galvanized, high tensile. I had to hammer open the end links of my 8mm chain slightly to take the 2 t shackle pin. I used a much bigger straight shackle on the anchor.

The duralac was on the bolts for the stock, and the mud palms.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

Roger on that.
Might you be better off, removing a bit of the galvanizing on the end link, instead of beating some of the strength (& shape) out of it, in order to make enough room for the shackle pin? Or have you tried that already?

That, & this thread has me pondering doing some research into Ti shackles, & their properties under heavy, repeating, cyclical loads. Especially embrittlement, & shock load behavior.
Anyone got some research studies they'd care to share

[Cruisingscotts]

the google knows

https://www.google.com/#q=practical+...r+shackle+test

[noelex 77]

Unfortunately, the most corrosion resistant aluminium alloys (the 5 series such as 5083) cannot be extruded so they are not suitable for anchors like the Fortress. As far as I am aware, it has never been released what grade of aluminium is used in the construction of the Fortress, but it strongly rumoured to be 6061. Probably 6061 T6.

With the lifetime guarantee I don't think it should be a concern, but a galvanised shackle is the best choice. Providing the galvanising surface is in good condition this gives a galvanic cell of Zinc verses aluminium. In this case the zinc will preferentially corrode, protecting the anchor.

A stainless steel shackle is less good. This will create some galvanic corrosion on the anchor, although with galvanised chain the zinc on the chain will provide reasonable protection. The effect will be minimal.

I think it is wise to avoid an aluminium anchor with stainless steel chain and shackle. Here the relatively high mass of stainless steel in relation to the mass of aluminium will potentially cause some practical problems.

Try not to have any stainless steel (even a shackle) resting on an aluminium anchor in a wet anchor locker.

[Snowpetrel]

From memory the 8mm g3 chain has a wll of 800kg. The titan black pin bow shackles have a wll of 2000kg. I am not worried about the shackles, as long as they are wired properly.

I should be able to go g4 or even possibly g7 chain and still have a margin of safety.

I had to open the links about 0.5 of a mm by hammering a 12mm rod through them so the pins fit through properly without binding. I don't think it's weakened anything, especially with the ductile grade 30.

I guess if the chain gets overloaded it might close back up, but then I'll know I need to replace the 2.5 meter length, no big cost.

When I can I'll track down some maggi g7 8mm chain to replace the g3 stuff.

[estarzinger]

Originally Posted by Snowpetrel

I think a more rounded eye to the fortress would have another plus, enabling a bow shackle to sit properly in the hole as well as for a soft Shackle or straight rope at a pinch. .

Agreed.

The way it is designed now - I presume they expect you to use two (steel) shackle, one with the pin thru the shank hole and the other with the pin thru the chain, and the two bows interconnected?

I stowed the anchor in its (very nice bag) with a shackle on the shank, and a shackle on the piece of chain. So it was all there and ready when I pulled it out. Again, that may be why I had some corrosion, because it lived pretty much full time in contact with the shackle and some contact with the chain. And it was stowed/lashed to the sail locker watertight bulkhead (immediately available and easy to pull it out the sail hatch) - so got damp occasionally.

[Fortress]

Originally Posted by noelex 77

As far as I am aware, it has never been released what grade of aluminium is used in the construction of the Fortress, but it strongly rumoured to be 6061. Probably 6061 T6.

Noelex, that is correct.

Originally Posted by estarzinger

The way it is designed now - I presume they expect you to use two (steel) shackle, one with the pin thru the shank hole and the other with the pin thru the chain, and the two bows interconnected?

Evans, Fortress has an oval hole opening in the shank and owners will oftentimes connect the bow of the shackle through this hole, and then the shackle pin through a link in the chain.

We also manufacture the Guardian, which has a circular hole opening in the shank (both ends), in which the pin of the shackle will be attached, and then the two shackles connected as you described above.

Brian

[Steve Bedford]

This has been an interesting thread to follow. It seems to shift from questions about dissimilar metals and their reaction to each other and then to weak links of the various components. I believe Brian had an excellent answer to the original question about dissimilar metals of the shackles and the Fortress anchor.

Most metal components become a "weaker link" when they asked to do tasks for which they are not originally intended to perform. Namely lateral forces. Swivels of course are intended to swivel. They all have a certain amount of resistance to lateral forces but lateral forces put much greater strain on the swivel that straight line pulls. Same can happen with shackles if their arrangement in connecting various components creates a situation where the screw pin gets locked and wedged and lateral forces put added forces on the connection.

Proper arrangement of shackles and swivels (if a swivel is indeed required) can significantly reduce lateral stress and sheering forces on the components. This arrangement will put added length between the anchor and the chain and that is a consideration to make sure it fits properly prior to the windlass (if using a windlass). An arrangement of anchor, shackle, swivel, shackle, chain creates an almost impossible situation for lateral sheering and stress. Remove the swivel and the arrangement of two shackles is also excellent. Remove one of the shackles and lateral stress is greatly increased.

Steve

[estarzinger]

Originally Posted by Fortress

Evans, Fortress has an oval hole opening in the shank and owners will oftentimes connect the bow of the shackle through this hole, and then the shackle pin through a link in the chain.

But that does not sound at all 'proper' - The 'oval' is in the plain of the shank, and the edges of the hole are square (90 degrees) right? So if you put the bow thru that hole, it will only bear on the 2 square edges - right? It will surely wear those two square edge contact points down (including taking off the anodizing).

What I was suggesting was rounded edges - which would match a bow profile for full contact . . . and also allow a dyneema connection (which would not rub the anodizing and be entirely corrosion free, and stronger than the steel shackle).

I guess you stamp out that hole today. You would have to do some sort of milling/grinding to get the rounded profile I am suggesting. Perhaps it is not worth the cost. But I do think it would be an improvement.