Squeezing past an anchored boat
The situation
Two boats reach along, overlapped on the same tack, toward an anchored boat sitting in their path. Both will pass it on the same side. Green is between the anchored boat and Blue.
The question
Who is entitled to room to pass between the obstruction and the other boat?
The ruling
Green — the inside boat — is entitled to room to pass between the obstruction and Blue, and Blue (outside) must give it (rule 19). An obstruction is anything you'd have to make a substantial course change to avoid: a shore, an anchored boat, shallow water, or even a right-of-way boat you're keeping clear of. Unlike a mark, you pass an obstruction — you don't round it — so rule 19 governs, not rule 18.
Passing an obstruction while overlapped — the outside boat owes the inside boat room to fit through.
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 19 — Room to Pass an Obstruction.