[Bob Pingel]

I do a lot of dock lines like this.

I use a thimble, shackle the thimble to the chain, and then shackle the chain to the dock.

I typically use 5/8" or 3/4" cordage and 1/2" galv shackles.

I like to use 3/8" acco mooring chain. The links are large enough to allow you to shackle in anywhere along the length -- in normal chain you can only access the ends.

Bob