Question
Boats A on port tack and X on starboard are sailing parallel courses on a run and are approaching a starboard-hand leeward mark. A is clear ahead when she reaches the two-length zone. A slows and X becomes overlapped outside her. A now sails past the mark and luffs slightly. X has to change course and protests.
What should the call be?

Answer
When A reaches the zone, X becomes keep clear boat under rule 18.2(c) until both boats have passed the mark. When X becomes overlapped, she must also give A room. However, rule 18.2(c) only applies when the boats are passing, or about to pass, the mark or obstruction.
At position 3, the boats courses are consistent with passing the mark and A retains her rights under rule 18.2(c). However at position 4, the boats courses are clearly inconsistent with passing the mark, so rule 18 no longer applies. A is therefore the keep clear boat under rule 10, and by causing X to change course A fails to keep clear. Penalize A.