The lee-bow tack
The situation
Crossing on port, Tom decides to tack onto starboard right on the lee bow of an oncoming starboard-tack boat, hoping to slow her and roll over the top. He tacks close ahead and to leeward.
The question
When does Tom actually earn starboard-tack rights, and what's the risk?
The ruling
Not until he is fully on a close-hauled course on starboard. While tacking (from head to wind until close-hauled) rule 13 makes Tom keep clear; if the other boat has to take avoiding action during that window, he's broken rule 13. Even once he completes the tack, rule 15 means he must give her room to keep clear at first. A good lee-bow is completed with room to spare — a late one that forces the other boat to bear away or luff is a foul.
A lee-bow only works if you complete the tack with room — you have no rights until you're close-hauled (rule 13).
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 13 — While Tacking.