Offshore Log: Anchor Shackle Warning
The link between rode and chain is vital.
After six months of daily use, we were surprised to discover that the shackle pin on the high-tensile galvanized shackle connecting our anchor chain to the 60-lb. CQR was rusting. Fortunately, we could still get the shackle apart, and a careful examination with a high-powered magnifying glass revealed some disturbing problems.
Although the shackle bow is hot-dipped galvanized steel, the high-tensile shackle pin has a thin electrocoating that is unsuitable to prolonged immersion in saltwater. After half a year, this coating has virtually vanished, and although the pin shows no structural wasting, it will have to be monitored, and replaced on a regular basis.
Furthermore, it appears that the threads in the shackle bow were re-cut after the bow was galvanized, and have no anti-corrosion protection.
By comparison, the conventional hot-dipped galvanized anchor shackle that we used last year is still in perfect shape, with virtually no corrosion on the bow or the pin. This standard 7/16" hot-dipped shackle was galvanized after all threads were cut.
The thread profiles of the standard and high-tensile shackle pins are slightly different. Threads on both the pin and bow of the standard shackle are “softer” in profile, with more clearance to accommodate the roughness created in the galvanizing process. This makes it difficult—fortunately—to screw the high-tensile pin into the standard shackle, or the standard pin into the high-tensile shackle bow.
As we reported in the November 1, 1997 issue, we chose these high-tensile galvanized shackles because their strength more closely matches the working load limit of our 3/8" high-tensile galvanized chain. Now, however, we must add the caveat that if you use high-tensile shackles, you should lubricate the pin threads with waterproof grease prior to assembly, and disassemble them on a regular basis for inspection and re-lubrication.
For a boat like Calypso, which lives on the hook in tropical waters, this examination should take place at least every three months.
Don’t forget to re-seize the shackle with stainless or Monel wire after re-assembly.
If you intend to use high-tensile galvanized shackles, carry plenty of them. You’ll be replacing them much more frequently than standard galvanized anchor shackles.
Another option is to use a forged stainless steel shackle in place of a galvanized shackle. The best forged stainless shackles, hands down, are from Wichard. For 3/8" high-tensile chain, you can use either the 10mm bow shackle (part #1245), which has a working load limit of 4,190 lbs. and a breaking strength of 9,480 lbs., or the 12mm bow shackle (part #1246), with a working load limit of 5,730 lbs. The pin of the 12mm shackle may be too large to fit through some 3/8" chain.
The downside of stainless steel shackles is that they will chafe the galvanizing off your chain and anchor. This will probably not present a structural compromise for the anchor, but could mean that you will need to cut off the last link of anchor chain every year or so—not a bad idea in any case.
Unless you use high-tensile chain, the best choice of all for anchor shackles is a US-made, drop-forged, hot-dipped galvanized bow shackle meeting federal specifications as a Type IV shackle. Use one size larger than the nominal chain size—7/16" shackle with 3/8" chain.
Contacts- Wichard, 507 Hopmeadow St., Simsbury, CT 06070; 860/658-2201.