Hi There:
I've been on a mooring for the better part of 20 years...some in Marblehead Mass (open to the NE) and 6 yrs on the Chesapeake Bay (Hartge's on the West River).
Although people say it's overkill for the Bay (not MHD), I firmly believe you should have two
lines going to the mooring. Relative to the rope penant, I recommend 3 strand (1 inch or whatever fits the
cleat). With 3 strand, you can braid the loop that fits over one of your deck
cleats, and the other end can be braided to the metal
cleat that is attached to the mooring (forgot the name of those parts). A braided penant is also not as strong as the equivalend 3 strand. The backup safety
line should be chain that you can attached directly to the chain part of the mooring (often below the mooring ball---in the bay, rope is attached to the chain to go through the buouy). In MHD, there's no rope, it's all chain. The chain is your secondary
line to the boat and goes around the other (opposite)
cleat.
The attachments to the mooring are
anchor shackles that are wire tied shut.
If this sounds like overkill, in MHD, my boat was hanging by the chain after the rope primary was sawed through by the movement of the boat in the no-name (Perfect) storm. Many other boats were lost. Even on the bay last weekend, it blew 49 knots.
Anyway, good luck. As far as
line length, just make sure there is an approx 30 degree angle on the primary and an extra 30% length on the chain. Also, save yourself a lot of aggravation and get a pick-up buoy...then you can sail right to your mooring single-handed and calmly walk up to the foredeck and pick up the penant with your hands.
Good luck.
Moe