So we have an anchored boat and a boat underway, you seem to imply the onus is on the anchored vessel rather than the vessel underway?
Thanks for the question,
No, in the maritime world there is usually no 100% fault for a single party in most collisions at sea. While the anchor watch/nav watch on an anchored vessel would probably be cleared of any culpability, he/she must take all action to avoid collision.
In the case of an anchored vessel, about the only thing that an anchor watch person could do is ensure the anchor lights are on, the day shape is positioned, and sound signals are being used in restricted visibility. Next they would sound the danger signal and hail the approaching vessel on VHF channel 16.
Nothing more the crew of the anchored vessel can do. They cannot maneuver.
Rule 16 defines the action of the "Give-way Vessel".
Rule 17 defines the action of the "Stand-on Vessel".
Paraphrase: The Stand-on vessel...so close that a collision with the "Give-way Vessel" cannot be avoided by action of the "Give-way Vessel" alone, shall take such action as will best aid to avoid collision.
The master and/or mate who has the conn must avoid collision even if they are the "Stand-on Vessel."
Again, an anchored vessel can't maneuver, a vessel not under command cannot maneuver, a vessel restricted in her ability to maneuver may not be able to avoid collision, and a vessel aground cannot maneuver (Duh!). Rule 9 is for deep draft vessels or other vessels who can only navigate in a narrow channel or fairway. Not much these vessels can do except make an attempt to stop. They don't stop well.
The point is, a vessel at anchor must follow the rules if they don't want culpability in a collision.
markm