Chain is made from wire. The cross-sectional area and strength of the wire determines the strength of the chain. The weld of each link should be stronger than the wire, so if a chain breaks, it should break in the body (wire)-usually at the crown, or the curve in the link-not the weld.
The grade numbers commonly used to denote the strength of chain are numerically one-hundredth or one-tenth of the strength of the wire expressed in megapascals (MPa). So, a G3 or G30 chain is made from a wire with a minimum breaking strength of 300 megapascals (or 300 Newtons per square millimeter). Also known as Grade L, G30 chain has similar characteristics to BBB, the short link chain commonly used in anchor rodes.
The most common chain grades used for anchor rodes on recreational boats are G30, G40, G43, and G70. Chain marked as G30, G40, or G43 is made from carbon, or mild, steel. Chains denoted as G70 are heat-treated steel, commonly quenched and tempered (the same process used for some anchor shanks).
Quench and tempered steels are made to specific chemical tolerances, as determined by manufacturer. The tempering temperature fixes the strength of the steel. If some post-manufacturing process, such as galvanizing, raises the chains temperature to the one used for tempering, then the chain is weakened.
Although you can galvanize the high-strength, tempered steel used in G70 chain, tempered steel should never be re-galvanized-a major drawback for anyone who plans to extend the life of their chain by re-galvanizing it. Although there are higher-strength chains- G80, G100, and more recently G120-galvanized versions of this chain are either non-existent or too new for us to recommend.
A confusing array of names applies to the same or similar chains. Some examples: G43 sometimes goes by High Test, HT, or HT4; G70 is often called transport or trawl chain. One useful term is proof test; this means the very chain you are buying has been tested to about twice its working load limit, or about half of the breaking load. In Europe, the two common grades are DIN 766 and ISO4565, the equivalent of G30.
Often, the limiting factor for choosing a chain is the gypsy or wildcat on your boats windlass. Gypsies are sized to fit the pitch (inside opening) of the chain. There are at least four different gypsy types on the market. In the U.S., many gypsies fit short-link BBB size, but gypsies for longer-pitch G30 and G43 chain are also common. Peerless G70 (under its Acco brand) is a not the same size as a G43 chain.
Even Europe, with its quest for manufacturing uniformity, has 10-millimeter chains with different pitches. The bottom line: Test the chain size under load in the gypsy before buying a full length. You might have to buy a 6-foot length to do this test, but some chain retailers will let you have a test sample.
Working Load and Tensile Stress
The working load limit (WLL) is the maximum load-uniformly in direct tension-that should be applied to a chain. This is a percentage of the tensile stress (TS) or minimum breaking stress (MBS). Although manufacturers use mathematical formulas to determine the WLL and TS/MBS of their chains, the formulas are supported by actual testing done by the American Society for Testing and Materials (ASTM) and/or by the National Association of Chain Makers (NACM). The latter group has six members, two of which, Peerless and Campbell, sell short-link, galvanized marine windlass chain. Finally, there is ultimate tensile stress, or the actual breaking load, and this should always be higher than the minimum TS or MBS.
The working load limit is determined by the safety factor that each manufacturer applies to the minimum tensile strength. In the U.S., the ratio of minimum tensile strength to working load limit is 4:1 for G70 and G30 (G80 and G100) chain, and its 3:1 for G40 and G43 chain. In Europe and Australia, a safety factor of 4:1 is used for all grades, although the Italian company Maggi uses a more conservative 5:1 ratio for its G40 and G70 chain.
Surprisingly, West Marine uses a less-conservative 3:1 safety factor for its G70, giving it a very impressive working load-even though its minimum tensile strength is no greater than that of other G70 chains.
The lesson here? Looking only at published working load limits can lead to mistaken assumptions regarding a chains strength.
In our opinion, a minimum 4:1 ratio should be used across the board for windlass chain, and retailers such as West Marine shouldnt be using the 3:1 safety factor for some stock, which only confuses the buyer.
The best way to compare chain is to compare the minimum tensile strength: A 5/16-inch G30 will be 7,600 pounds; and a G40/G43 chain will be 11,600 pounds.