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.
3/8″/9.6mm Grade 70 – break strength 24,000pounds/10,880 kg
1/2″/12.6mm Proof Coil – break strength 18,000 pounds/8160 kg
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)