Regarding your recent article on joining chain segments (see Reliable Chain Connections, PS June 2018), I’ve been coastal cruising from Canada to Panama since 1983, and continuously since 2005. I have wondered about using a single 5/16-inch shackle, which will connect two lengths of 3/8-inch, G-43 chain. The Crosby top of the line 5/16-inch will go around the gypsy pretty well. I tried it, but I am reluctant to use it. Any thoughts?
Someday, Gulf Star 41
Puerto Madero, Mexico
Your reluctance is warranted. The maximum size of your anchor shackle is limited by the size of your anchor-chain links. Typically, you need to go up one size greater than the nominal size of our links-i.e. your 3/8-inch chain can take a 1/2-inch shackle-to match the working load limits for G43 chain, and this over-sizing is recommended for lower tensile strength G30 chain as well.
When matching shackles and chains by using the makers published working load limits (WLL), confirm the safety factor used to determine the working loads. Multiplying the WLL by the safety factor provides a minimum tensile stress, which is a closer approximation of ultimate tensile strength (UTS, see Anchor Shackles, Round Two“, PS October 2016; Making Sense of Chain Standards, June 2014; and Inside Practical Sailor Blog, Tips on Choosing and Sizing Anchor Shackles).
Be aware that manufacturers often use very different ratios for calculating safety factors for chain (3:1 to 4:1) and shackles (4.5:1 to 6:1). One reason for the higher safety factor for shackles is that side-loading a shackle can dramatically reduce its strength. According to warnings posted by several manufacturers, a 45-degree side load can reduce the shackles rated strength by 25 percent, and a 90-degree load can reduce it by 50 percent.
For chain rodes, we generally recommend the largest possible Grade B shackle be paired with proof-tested G30 hot-dipped galvanized chain as the most reliable and cost effective option. You, like many boaters, have opted for higher strength G43 chain, which is stronger, but also more expensive.
Unfortunately, the smallest Grade B shackle that we’re aware of is 3/8-inch. You would then have to opt for a lesser-grade 5/16 Grade A shackle (yes, Grade A is inferior to Grade B), and this would be weaker than your G43 chain.
In our testing, we found several 3/8-inch shackles that should fit common 5/16-inch G43 chain (11,400 lbs. nominal breaking strength) and afford some confidence, even when side loaded. For example, a Crosby 209A 3/8-inch Grade B shackle with a 4.5:1 safety factor (4,400 lbs. WLL, 19,800 lbs. UTS) would match the strength for 5/16-inch Peerless Boatmans Pride G43 chain with a 3:1 safety factor (5,400 lbs. WLL, 16,200 lbs. UTS). But, as you are aware, this shackle will not fit your gypsy.
A connecting link or C-link might pass through your gypsy, but again, Crosby 3/8-inch connecting links or missing link joiners do not match G43 WLL. A Dyneema soft-shackle would appear to match strength, and will probably go through the gypsy with ease, but we worry about abrasion.
Another option is a 10 mm G100 Omega link, which has a WLL of 8,800 pounds, but is a semi-permanent connector. For a permanent solution, the link would need to be galvanized, which will reduce WLL to 6,600 pounds. If the Omega option is of interest, you can try Spencer Industries in New Jersey, which uses the Armorgalv process to protect the US military equipment. It may have some 3/8-inch (10 mm) Omega links.
All of this again raises the problem of upgrading to a smaller-size, higher tensile strength chain.