1-1/8" stud link chain. The links are 6-3/4" long and it weighs about 12 pounds a foot. I can't think of anything more perfect for the job of ground chain to provide the cushion of catanary in a big blow. This will form the ground chain element of the mooring system. Ideally we'll be using 60'... if we can find a way to move that much weight. When new this chain would have a breaking load of over 100,000 pounds! Even though this is pretty old chain it's still insanely strong.
Next is the top chain which at 5/8" thick with 3" long links. I has a working load in the range of 6,800 pounds.
For more on the mooring build check out these prior posts.