I would like to run my prep by the community, just to get some responses. We live in Eastern NC and our boat (Heron a 35' sloop at 11,000 lbs) survived Irene by anchoring to 2 trees and 2 anchors in a protected cove in our creek (plus all the other precautions like stripping the boat, etc). But I did loose 1 anchor rode due to chafe (my fault as I was in a hurry to get things done) and because it was anchored fore and aft with 4 attach points.
This time my plan is to use a swivel with at least 3 groundings; 2 anchors and a tree. I purchased the swivel at a local consignment shop and it is huge. It is a Crosby swivel for a crane with a 1" shank; I also purchased over-sized shackles to fit it. I will hang the swivel, on chain, from an inflated mooring ball. The topside of the swivel will have 2 pendants to the boat; one slightly longer than the other. See the picture below; it only shows 1 pendant on top, but imagine 2.
For pendants I plan to use 5/8" 3 strand nylon (the largest my cleats can handle) around 30' long, open on one end so I can change the tie off and chafe points, with plenty of chafe protection.
The bottom of the swivel will have the 3 anchor rodes. The one to the tree is a 3/4" line and I use chain around the base of the tree and shackle the hard eye end to it. I plan to use my existing 2 anchor rodes (5/8" @ 200'); one is 4 years old and the other brand new (after the one broke in Irene) for the other 2 anchor points, and here is where I have a question. The lines have a hard eye on one end for the chain connection. The other end is open, so my plan it so tie them to a shackle using an anchoring knot (2 round turns, pass the free end thru and add a half hitch).
This way I can tie them off at whatever length I need for enough scope based on the proposed water depth. The alternative is to make up special anchor lines, just for this mooring, with hard eye's on both ends and at fixed lengths. The cost is probably a few extra hundred dollars as I have good lengths of chain on the anchors and probably need < 100' line to get 10:1 scope. I question whether that is really necessary as these knots seem to be considered very adequate, the lines are in excellent shape and there is some redundancy in the anchoring. Am I just being "an$L" and worrying about nothing or would anyone else be concerned about this?