Tips on Choosing and Sizing Anchor Shackles

Posted by at 06:39AM - Comments: (8)

Jonathan Neeves
Jonathan Neeves

Stainless-steel shackles, even high-quality ones, don’t belong in the anchor rode in our view because of the limitations of the material itself, and susceptibility to forms of corrosion that are nearly invisible.

No matter what brand of anchor shackle you decide to use, understanding sizing and grades of shackles is essential, since this connector between your rode and anchor is often the weakest link in your ground tackle. The maximum size of your anchor shackle is limited by the size of your anchor-chain links. Typically, you can go up one size greater than the nominal size of our links—i.e. a 5/16-inch chain can take a 3/8-inch shackle, but the surest way to ensure that you are getting the right size is to take a few connected links of chain with you to the chandlery and make sure the shackle pin actually fits the hole.

The most common types of anchor chain BBB, G30, G43, and metric chain all have varying chain-link hole sizes for ostensibly the same diameter (thickness) wire. Shackles of the same nominal size also vary; even the shackle-pin diameter varies. Except when buying matching chain and shackles from the same manufacturer, take nothing at face value—test fit first. When buying chain in bulk, some vendors allow you to order chain with oversized links at each end, making it easier fit the larger shackle pins; this is a good option for those who prefer high-strength G43 chain. Quality control is imperative. As we discovered in our most recent round of anchor shackle testing, there is a wide range of quality among shackles that are nearly identical in appearance.

The other critical factor is strength. Ideally, the shackle should be stronger than the anchor chain. However, achieving this becomes difficult once you start moving into chains rated G43 or higher. The 3/8-inch shackles we look at in our test should fit common 5/16-inch proof-tested chain stamped G30 (7,600 lbs. nominal breaking strength) or G43 chain (11,400 lbs. nominal breaking strength). For more on chain standards and breaking strengths, including those for metric chains, see “Making Sense of Marine Chain Standards.

However, safety margin becomes even smaller when the effects of side-loading are taken into account.

When the shackle pin or body is loaded from an angle, the shackle body can be forced open, causing the pin to break or fall out. This is the most common failure we have seen in our tests—even though our test involved a straight pull. According to warnings posted by several manufacturers, a 45-degree side load can reduce the shackle’s rated strength by 25 percent, and a 90-degree load can reduce it by 50 percent.

Fortunately, high quality shackles have a safety factor of between 4:1 to 6:1, meaning the actual failure point is four to six times greater than the rated working load; so, in theory, a good shackle is capable of handling side loads.

How do we identify a good shackle? In the U.S., shackle standards are spelled out in RR-C-271F, “Federal Specification for Chains and Attachments, Carbon and Alloy Steel.” In our most recent report, we are focused mostly on Class 2 (screw-pin) shackles that meet or exceed RR-C-271F IVA Grade B Class 2 specifications.

Other classes will meet these working load requirements— Class 3 shackles, secured with bolt, nut and cotter-pin instead of a screw pin, for example—but the essential nomenclature here, is Grade B. This grade has twice the strength of the more common Grade A shackle. This may seem counter-intuitive, but one way to remember this is to remember: “B is Better.”

If you are serious about anchors, anchor chain, and anchoring accessories like snubbers, chain hooks, and windlasses, or you are just curious about the best anchoring technique—check out our 4-part ebook featuring a complete selection of tests of anchor chain, swivel shackles, and other anchoring accessories. Profits from this and all our other ebooks, available in our online bookstore, help support further tests like this anchor shackle test. Practical Sailor accepts no advertising.

Comments (8)

It is a pleasure to see so many posting out the obvious errors in the lead photograph. Every time I visit a marina I see too many examples of similar assemblies - and some are much, much worse. Hopefully the owner of this yacht (from the US East coast) and others with a similar practice will take note of your comments and improve their rode security.

Every time I visit a marina I see unbranded, undersized shackles, no name swivels, and swivels attached directly to the anchor shank matched with G43 chain - it makes no sense to me either.

A good Grade B shackle costs peanuts (Campbell, Crosby) - why try and save money - but I see very few Grade B shackles in use (and I never see them in chandlers - though I am sure there must be exceptions).

But don't entirely knock the absence of seizing wire - Loctite is a perfectly acceptable alternative, search our archives (and for our report on swivels)

Jonathan

Posted by: neevesip@bigpond.com | August 22, 2019 6:47 AM    Report this comment

Also....what happened to the seizing wire in your photo? A really bad example of what to do!

Posted by: Rweinc | August 6, 2019 3:16 PM    Report this comment

I continue to see million dollar boats using a cheap stainless steel swivel directly attached to the anchor! No shackle between the swivel and the anchor is looking for a disaster. Lateral forces must be considered, in addition stainless is more susceptible to breaking. Most agree that there is no place for stainless steel on chain or anchor.

Posted by: Rweinc | August 6, 2019 3:12 PM    Report this comment

Regarding the idea of using soft shackles, would suggest that anchoring gear may be subject to significant chafe against the bottom (sand, rock, coral, etc.) which running rigging and standard rigging are not.

Posted by: JonathanBresler | August 1, 2019 1:31 PM    Report this comment

small question:
why would you use a stainless steel shackle as in your photo, where this would introduce galvanic corrosion of the zink coated mild steel?

Posted by: Alex Blackwell - Happy Hooking | July 25, 2019 1:10 PM    Report this comment

Standard procedure for Port (Port San Luis) supplied and maintained moorings is to use ss seizing wire. That has also been my preferred seizing material for shackles although I have witnessed plenty of plastic ties used. But these have been for moorings in sand areas (e.g. Diablo Canyon waverider buoy). I think if you keep an eye on things, you'd have to regalvanize your rode before the seizing wire ate up your shackle,

Posted by: Hedgie | July 25, 2019 12:32 PM    Report this comment

Another reason to avoid stainless steel: mixed metals promote galvanic corrosion. SS, being more noble than steel, will promote corrosion of the steel. This is also a good reason to avoid using SS and Monel seizing wires.

Posted by: Optoeng | July 25, 2019 10:41 AM    Report this comment

How about soft shackle or lashing?
Intuitively, I wouldn't trust it. But if I wouldn't trust it here, why would I trust it elsewhere?
Maybe we could have some long-term tests...

Posted by: ilader | September 21, 2016 9:22 AM    Report this comment

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