A dock line ties your boat to a fixed dock or slip. A mooring line (or pennant) connects your boat to a mooring buoy or piling where it sits for extended periods. Mooring lines run heavier and take sustained load, so they are sized up from dock lines and usually spliced to a thimble at the mooring end.

Size by boat length and displacement: 1/2 inch up to 25 feet, 5/8 inch for 25 to 35 feet, 3/4 inch for 35 to 50 feet, and 7/8 to 1 inch for 50 feet and up. Heavier or high-windage boats should move up a size.

3-strand twisted nylon stretches the most, which absorbs surge and shock, and it splices easily to a thimble. Double braid nylon handles more smoothly and is stronger for its diameter. Both are nylon so both give under load; 3-strand is the traditional mooring pennant choice.

A common guideline is a pennant about 2.5 times the boat's freeboard height so the line leads at a comfortable angle. Many boats run two pennants for redundancy. Add enough length for the tidal range where you moor.

Inspect every season, especially at the thimble eye and anywhere the line passes through a chock or bow roller where chafe concentrates. Add chafe protection at those points and replace the line when you see glazing, flattening, or broken fibers.