Many thanks Deb and Ian.
We put a lot of thought into the swing moorings -- pre-allocated, Ian, and permanent. They have been there for 30 years at least and work well with minor changes every year. For those facing similar decisions, here's what we do:
The water is 12 feet deep, sitting on mud, with limestone about 20 feet down. No tides; we're spoilt. We have a collection of steel railway wheels that weigh about 700 pounds each that sit on the bottom with a welded rebar yoke, a heavy, high-tensile u-bolt and a heavy chain. At the top of the chain is a car tire or, lately, a round mooring buoy that is topped off with another u-bolt. You need a crane barge to install these railway wheels.
We tried hard plastic buoys last year but the ice destroyed 3/4 of them. They sank. The round mooring buoys and car tires survive longer but they do sometimes leak and then sink. The cause is usually easy to find: propeller slashes from a passing motorboat.
We use a variety of mooring lines, depending on the whim of the owner. One of mine is 5/8 inch with a spliced loop that I can pick up and loop over the cleat quickly. The other line is 1/2 inch and long enough to tie around the mast in a hard blow. I have whipped red thread around the 1/2-inch mooring line at the fairlead so I can adjust it to the right length quickly and easily. I use a 1/2-inch line because it is more flexible that 5/8 and perfectly strong.
The lines are generally twice the height of the freeboard plus the length from fairlead to cleat. Some prefer shorter; I like longer because the boats bobs more comfortably. I install net floats on my line to make sure they float up and away from the mooring chain. Without the floats, the lines will sink and twist around the chain, rip on the mussels and cause endless havoc. I know; I've been there.
We have about 60 of these moorings behind a seawall and they work well. They do, however, need checking about once a year, a job for a volunteer scuba diver. The main wear point is, surprisingly, about a foot off the bottom, probably because sand and grit work on the chain where the wave action moves it most.
The annual checks are important. Chains have been known to break. But the more serious problem is the reluctance of some owners to safety wire their shackles. We check the lines every morning, an important precaution.
Why swing moorings instead of docks? I love them. The boat bobs nicely with the wind, I see ducks and geese and egrets and turtles and fish. I'm not bothered by my neighbours on the next dock and the boat handles waves and wind far happier than at a dock.