Round turn with two half hitches: bollard, mooring pin
The round turn with two half hitches hardly looks like a knot that can be relied on to secure a boat, but it does work and can be tied with one hand, the other hand taking up the strain (keeping the boat close). You can add additional turns in windy weather or if there is heavy traffic.
Step 1
The two round turns go around the mooring spike. Add more turns as necessary.
Step 2
Tighten the turns.
Step 3, 4
Pass the end of the line over the line from the boat and then through the opening to make a half hitch.
Step 5
Add another half hitch. Two half hitches add up to a clove hitch, which by itself could slip, but not with the round turn. If you have a lot of extra line, you can make the two half hitches with a bight (a loop of line). A clove hitch made with a bight makes for a quick release.