* I would not place a swivel anywhere but at the ball so it can be expected regularly.. SWIVELS MUST BE LARGE!!!
* I would not install any swivel smaller than 3/4". I use an 1 1/4" swivel on a 36 footer. My top chain is 3/4" long link mooring chain and my bottom chain is 1 1/2" US Navy chain at nearly 22 pounds per foot.
* The chain and swivel should be "winterized" by installing a winter spar which allows the chain to be dropped to the bottom, stop wear and sit in the low oxygen mud bottom....
* Moorings should be inspected yearly or bi-yearly at a minimum. Top chain should really not be any smaller than 5/8 and really that is tiny chain for a permanent mooring.....
* Swivels are the highest wear item therefore they need to be directly below the ball. They should also be VERY LARGE so as they wear they don't wear faster than the top chain.
On a permanent mooring a top swivel is usually very necessary especially in areas of high tidal ranges as the boat swings around, and around, and around sometimes for weeks depending upon usage. Do this enough and the chain twists and shortens to 1:1 scope and sucks the ball under. If a storm comes up you simply un-set the mooring and drag it or shock load the gear until you have a failure.
The problem with the vast majority of moorings is they are grossly under designed. Swivels are too small, placed in the wrong locations or are of an unsuitable design.
Chain is also almost always far to light/small for storm purposes and "shock loads" the gear.
At a minimum for our boat (36') my design criteria is:
*8000 Pounds of granite (actual storm mooring is 9600 pounds), 500 pound mushroom (min) or screw mooring
*USCG/USN bottom chain (1 1/2" X 22 pounds per foot) to 1.5 X Depth
*3/4" Top chain to 1X depth
*1" Swivel MINIMUM directly below ball (actual is 1 1/4")
*Dual unequal length polyester pendants with Vectran storm snugs added to go over the chocks in storms.
I use a 1 1/4" swivel for our CS-36. Our town requires a top swivel on moorings as part of the local ordinance.
This is my eye to eye swivel for a 36 footer.
Swivel Eye To Eye 1 1/4"" Galvanized Made In USA
This is the bottom chain on one of our moorings, the lighter of the two. My storm mooring is the type with the cross bars in the links.