What I (think I) know about chain

I've been reading about chain. Here is what I have been learning:

  • High test chain is a bit of a misnomer but I'll use it as it is commonly used in the US to mean G40 or G43 chain. You can read a good description of the grading system used in the US here.
  • High test chain used to have longer links and thus pile in your chain locker awkwardly and not be as windlass friendly. I think this is why Mahina's Offshore Cruising Companion suggests against it. Acco's high test chain is currently only .75mm longer than its BBB chain.
  • American high test chain manufacturers use a less stringent formula to determine working load from breaking load. Breaking load is tested. Working load is calculated. Most companies around the world taking the breaking strength and divide by 4 for the working load. High test manufacturers use a division of 3. If you want to compare apples to apples, use your own calculation. Find out the breaking strength of both, and divide them both by 3 or by 4...whatever.
  •  If you compare apples to apples, high test is still stronger than BBB chain at the working load level (which is the safe level to compute from). High test will be about 25% stronger. 
  • Companies (and more often re-sellers) that claim that the high test has a working load that is twice as strong as BBB are comparing apples to oranges. The rated working load is twice that of BBB...but they are also calculating the working load differently.
  • There is no reliable way to figure out how much load your boat will put on your chain in various wind/wave/current conditions. People disagree about which factors to include and how much to weight them.
  • Deciding on a good size for your boat can be difficult if you are, like us, interested in knowing WHY a certain size is recommended. Most of the charts that tell you what size chain to use don't tell you how they estimate those forces and they often use very different conditions for their recommendations. For example, Rocna's chart (you need to first see what Rocna anchor size they recommend and then use that in the chain size chart) uses 50 knots of wind and West Marine's chart uses "up to 30 knots".
  • A good online thread about some of these latter issues is this one.

We have 120' of 5/16" G4 high test chain spliced to several hundred feet of rope.

Our current plan is to add 180' more chain and to use this new 180' as our primary road with the current rode in the anchor locker available to extend the primary (so we have 300' of chain easily available) or to deploy with a second anchor. We also have about 20' of 3/8" BBB chain spliced to a bunch of rope which we will keep for a secondary anchor/kedge. And finally, we also already have a bunch of rope which we were using for the stern anchor kedge and are thinking of getting rid of once we buy the new chain. We can only fit so much crud on a 35' boat after all.

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4 comments:

  1. All good stuff and true.

    I have never heard of a boat being lost due to chain actually breaking.

    For me its a question of how much weight do I want in the pointy end vs. how deep am I going to be anchoring and at what scope.

    Its only slightly less religious than the choice of the anchor itself.

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  2. @Sam - I read the same thought (about never hearing of anchor chain failing). I think our choice was easy because we already had the windlass set up for 5/16HT and a good amount of that chain already in the locker so the question "is it worth changing to something else" was easy to answer for us.

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  3. The calculation of SWL from average breaking strength is not as simple as you imply, I think. It is not just a safety number pulled from the air.

    High strength material (heat treated steel) can bear a higher percentage of breaking strength without fatigue cracking; compare spring steel to mild steel.

    A safety factor is also calculated as a number of standard deviations from the average. I believe that 3 standard deviation is used on BBB chain in non-overhead applications, but don't quote me. With climbing gear, the breaking strength is determined as the lower 3 SD limit (97.3% of the gear is at least that strong). Thus, the correct safety factor depends on the level of quality control inherent in the grade of materials used and the manufacturing process.

    Setting a SWL is combination of these. I have to consider boat of these in my work (designing process equipment).

    Regarding load, I think this is the best information out there:
    http://alain.fraysse.free.fr/sail/rode/rode.htm
    However, in waves, in shallow water, with an all-chain rode, ridiculous snatch loads are probable. I believe all of the breakage stories come from that circumstance. A LONG rope snubber (perhaps 8 times the average wave height) is the only way to limit forces, in a bad anchorage.

    But every boat and situation is sure different, an the PNW is certainly different from the Chesapeake, my home waters. Take care!

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  4. Drew - That was extremely helpful. Thank you!

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