Ive been studying your recent articles on chain and galvanizing (see PS June 2014 and January 2015 online), as I am about to replace the anchor chain on my catamaran. When it to comes to the shackles, I am confused.
If I use 3/8-inch Acco G43 proof-coil chain, why is the same-size galvanized shackle so much weaker? West Marine shows the Acco G43 chain rated at 5,400 pounds maximum working load (MWL), but the similarly sized shackle is rated at only 2,000 pounds MWL. It seems as though whatever size chain I select, the shackle is the weak link. How can I use a larger shackle on smaller chain?
Tonic, Catana 431
Comparing specifications between two different types of products can get confusing. Part of the problem here is the data youre comparing. The products working load limits (WLL) and MWL are derived from the safety factors used in their manufacture. Because chain and shackles are made using different safety factors, their maximum working loads arent directly comparable.
Shackle specifications use safety factors ranging from 6:1 to 4.5:1 (depending on maker and size); however, the factors are degraded by 50 percent if the shackle is loaded at a 90-degree angle. G43 chain is made using a 3:1 safety factor; there is no variation for angle of pull.
Because of these very different safety factors, they seem very far apart in terms of strength, but if you compare their minimum breaking strength (MBS), the shackle is nearly as strong as the chain. The minimum breaking strength of 3/8-inch G43 chain is 16,200 pounds (3:1 safety factor). The minimum breaking strength of 3/8-inch Peerless and Campbell shackles are 24,000 pounds or 12,000 pounds (6:1 safety factor) at a 90-degree load, depending on the shackle grade (see below) and size. Crosby shackles, which work to a 4.5:1safety factor, have an MBS of 18,000 pounds to 9,000 pounds.
Shackles are graded according to strength: More common Grade A (3/8-inch) shackles have a WLL of approximately 1 ton, and Grade B shackles have a WLL of approximately 2 tons. (WLL varies by size.) Makers distinguish their shackle grades by coloring the pins: For example, the Crosby uses silver; Peerless (Peer-Lift line) uses blue pin; and Campbell uses orange.
If you use the 3/8-inch Acco G43 chain, we highly recommend going with a Grade B bow shackle one size up. The Peer-Lift Grade B 7/16-inch shackle should fit the hole in 3/8-inch chain and has an MBS of 32,000 pounds, almost double that of the chain (16,200 pounds). This means the shackle and chain will have similar strengths in worst-case scenarios (i.e. side loading the shackle), but in most situations, the shackle will be twice as strong as the chain. If you want overkill, the half-inch shackle likely will fit the chain also.
Always use a galvanized, rated bow shackle for the anchor rode. (Bow shackles articulate better.) Rated shackles have the WLL stamped on their body, along with the country of origin, a code indicating when the shackles were made and by whom, and their size (i.e. 3/8 inches). These shackles are batch tested for failure, and most-at least those made in Europe and the U.S.-are also proof tested (tested to twice the WLL). No stamped rating means theres no way to know whether it was tested or what its WLL should be. Trusting ground tackle to a shackle with unknown pedigree is asking for problems.
Crosby (www.thecrosbygroup.com), Peerless (www.peerlesschian.com), and Campbell (www.campbellchainandfittings.com) are the most easily accessible shackle brands in the U.S. All offer galvanized, rated bow shackles. Youll find these brands atmost chandlers or local lifting-gear specialists that sell lifting cables, chain, and fittings.
In addition to strength compatibility, youll need to check that all ground-tackle components physically match; this includes the windlass gypsy. Shackle and chain dimensions can vary from maker to maker, so it is essential to check. Cut off a link of your chain, and take it with you when shackle shopping; take the windlass gypsy along when chain shopping; or buy the whole lot at the same time from the same place.