We’ve been reviewing our logic on chain size and thought a few points might be of interest.
To begin with we have a long and successful history of using heat treated Grade 70 chain. But this gets little respect with a regulatory authority with whom we are discussing the classification of one of our yachts – hence the following comments.
When you look at chain in a catalog it is usually displayed with a working load limit (WWL) or safe working load (SWL). To arrive at this figure a factor of safety is applied, a divisor into the ultimate or mean break strength for the chain in question.
These WWL/SWL factors of safety are on part based on service to allow for degradation, in part based on regulations, and part a simple CYA (cover your posterior) for lawsuits.
To compare different sizes and alloys of chain it is necessary to remove the SWL/WWL factors and get at the mean break strength.
Here is some interesting comparative data sent to us by Washington Chain, our source for Acco/Peerless chain products, based on the chain being hot dipped galvanized.
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3/8″/9.6mm Grade 70 – break strength 24,000pounds/10,880 kg
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1/2″/12.6mm Proof Coil – break strength 18,000 pounds/8160 kg
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5/8″/16mm Proof Coil – break strength 27,600 pounds/12.500kg
Note that the 3/8″ Grade 70 is significantly stronger than the 1/2″ Proof Coil and within 13% of the strength of the 5/8″ Proof Coil.
Some will argue that the heat treated Grade 70 is more brittle and less able to absorb shock loads, which is true. But we have the real world experience to indicate that, for our yachts at least, the shock loads are minor compared to the MBS of the heat treated chain.
One other figure to keep in mind. Assuming you carry 300 feet/90meters of chain, the 3/8″ will weight in at 408 pounds/185 kg. The 5/8″ weighs 1107 pounds/502kg. That is a huge increase in weight forward and a negative in terms of motion and steering control (which is degraded through increase bow down trim).
Posted by Steve Dashew (March 4, 2010)
March 4th, 2010 at 12:48 am
Steve,
Always an interesting discussion and tradeoff. What you’re not mentioning is that the primary advantage to chain rodes, other than working nicely with windlasses and stowing themselves pretty well, is their weight. The more they sag into the catenary curve between boat and anchor, the more horizontal the pull at the anchor end, which increases holding power, and the more resistance there is to straightening the rode out, which has to happen before any real shock loading occurs.
I point this out not to disagree with your choices, but simply to mention another variable that gets mixed into one of the more complicated decisions in outfitting the boat.
For a given length of rode, you can certainly save a lot of weight in the bow (always a good thing, as you note) by downsizing the chain a bit, put some of that weight back into a larger anchor, and still come out ahead. OR, would you be better off, in terms of anchor holding, to use a shorter but heavier rode on a slightly lighter anchor? You say you frequently anchor on short scope with your heavy anchors; I suspect the same would be possible with a lighter hook and heavier rode.
Given the options, I don’t disagree with your selections at all, as I always opt for more rode length ‘just in case,’ but I was surprised you left this whole other can of worms out of the discussion. I’m sure it’s at least some of the basis for the class societies’ preferences.
Sorry to muddy the discussion if I have. Please keep up the good work, love the site and your boats. I’m living vicariously here until I can either a) afford one or b) get to design something similar for someone who can.
Best regards,
Chris Critchett
March 4th, 2010 at 9:15 am
Hi Chris:
Our goal is to have the highest holding power, most reliable anchor system, for the lightest weight. Once you have sufficient strength in the chain to connect boat to anchor, the best system for the weight will always be light rode/big anchor. Taking this to extreme, we’d be better with a Spectra rode (much lighter and stronger than steel) connected to an even bigger anchor. The net result would be higher holding for less weight, but there are chafe and self-stowing issues.
The other problem with heavy chain and increased catenary is fouling on bottom debris and/or damaging the sea bottom, which is a byproduct of the heavier chain.