Round Turn 2 half hitches 

This knot is called a round turn and two half hitches. And the round turn of two half hitches is a fantastic knot for tying on fenders.
This is a short lesson from our Catamaran sailing Masterclass knot tying lesson. masterclass.nautilussailing.com
It's also a great knot for tying your dinghy up to a piling. So here are the steps to do a round turn and two half hitches. You go all the way around your stanchion or the dock post, leave quite a bit of tail. So there's our round turn we like to go over and you come back through that triangle, tighten it up, and the second time we come under. And then back over and that tightens it up. And that's a clean looking round turn, two half hitches. And you can see how that is a real strong, solid knot that's gonna hold your fenders or your dinghy to a dock.