because...
Dave C.,It's my guess that you used a cast not forged shackle and that's why it broke. Most, if not all, of the imported shackles are made of cast iron and as such can and do break. A shackle should meet the minimum federal load rated spec of RR-C-271D and of course should ALWAYS be of the forged type NOT cast. Shackles from Columbus McKinnon (CM)or Crosby are very high quality, forged and do meet the federal spec. The general rule of thumb here is DON'T buy imported shackles and make sure they are US Spec by reputable shackle manufactureres like CM or Crosby. Many suppliers & chandlery's are now stocking cheap imported cast shackles and it's tough to tell between a cast and forged shackle by an untrained eye! A 3/4 shackle NEVER should have parted in 40 knots if it was a proper forged shackle... If it snapped it was most likely cast and NOT a "great shackle" as you say. As a simple safety measure you should always know:1) The name of the shackle manufacturer and this name should be proudly embossed into the shackle during forging.2) If it meets the MINIMUM specs set forth in RR-C-271D. (don't always trust this though because some seedy companies will sneak a product through a spec then change the build). You want a shackle with a NAME and a company standing behind it.3) FORGED no CASTAs to the OP's question it sounds more like you lost your shackle. I've never heard of or seen Acco chain breaking especially in benign conditions. Acco is top quality chain and even if they missed a weld, which automated welding robots don't generally do, and then the automated inspection machines missed it too, there is still no way a Hunter 27 would have stretched out a link enough to make it part in the conditions you describe. Chain is not made of cast iron, it's made of steel so it does not "snap" or "break" off in benign conditions. Steel generally stretches and or distorts well before it breaks unless it was hardened and was high test chain but that would make your scenario even less likely.In general chain links will bend and distort under severe load but links don't usually just break or snap off. Chain will break under load testing or severe loads but if you were to look at the links after testing or breaking under severe load they have become somewhat longer and narrower and more pinched at the ends before failure. To do that to a chain, in a boat, with a anchor, in mud or sand is very, very difficult if not impossible especially in benign & calm conditions.You need to secure all shackles with seizing wire after tightening with a large wrench..