When does rule 18 turn off?

A question I hear often is "When does rule 18 turn off?"  At the simplest level, the answer is given by rule 18.1:

18.1 When Rule 18 Applies
        Rule 18 applies between boats when they are required to leave a mark on the same side and at
        least  one of them is in the zone.
        However, it does not apply
             (a) between boats on opposite tacks on a beat to windward,
             (b) between boats on opposite tacks when the proper course at the mark for one but
                   not both of them is to tack,
             (c) between a boat approaching a mark and one leaving it, or
             (d) if the mark is a continuing obstruction, in which case rule 19 applies.

If rule 18 does apply at some point during a mark rounding, part (d) clearly never applied.  Condition (b) is really just an extension of (a) for situations where there might be disagreement about whether boats are on a beat to windward, so we get the following summary: "Rule 18 turns off when either boat tacks at a mark to be rounded on its windward side, and when the leading boat leaves the mark while the other boat is still approaching it."

However, that's not the whole story.  The real question is, "When does the obligation to give mark-room end?"  And while that seems like the first question, it's actually quite different.  Basically, the obligation to give mark-room ends when the boat entitled to mark-room no longer needs it to pass or round the mark. 

Consider ISAF Q&A 2013-017.The animation at the left was used to draw the scenario in that Q&A; it shows two catamarans rounding a port-rounding offset mark, with the next leg directly downwind, in 10-12 knots of wind, no current.  At position 4 Yellow has passed the mark and both boats are on their downwind courses (which, they being catamarans, is not straight downwind).  Yellow jibes directly in front of Blue, breaking rule 10 (Port/Starboard).

Blue is required to give Yellow mark-room at this mark because Yellow is clear ahead at the zone.  According to the definition Mark-room, Blue has to give Yellow three things:

1. Room to sail to the mark if her proper course is close to it;
2. Room to round the mark as required to sail the course; and
3. Room to leave the mark on its required side.

(I've changed the order of the requirements in the definition Mark-room so that they appear in time order, making them easier to discuss here.)

Consider the situation at position 4 of the animation.   Yellow's proper course is no longer close to the mark, so she no longer has the right to room to sail to it.  She is on her downwind starboard-tack course, so she has rounded the mark as required to sail the course.  And Blue has clearly given her room to leave the mark on its required side. Thus, Blue's obligation to give Yellow mark-room has been discharged and she is no longer obliged to do so.

That conclusion is particularly important because of rule 18.2(c)(2), which says:

(c) When a boat is required to give mark-room by rule 18.2(b),
      (1) she shall continue to do so even if later an overlap is broken or a new overlap begins;
      (2) if she becomes overlapped inside the boat entitled to mark-room, she shall also give
            that boat room to sail her proper course while they remain overlapped.

At position 4, Blue has become overlapped inside Yellow.  But because Blue has discharged her obligation to give Yellow mark-room, she is no longer required to do so and the initial phrase in rule 18.2(c) does not apply to her.  Therefore rule 18.2(c)(2) does not apply.  Yellow's breach of rule 10 is not exonerated under rule 21 and she should do her turns.

 Now consider a situation at a port-rounding leeward mark, with the next leg a beam reach (as with Mark 3 in a standard digital-N course in team racing). Blue is overlapped when she enters the zone, so she is required to give Yellow mark-room.  By luffing Yellow at positions 3-4, does she break rule 18?

I think the answer is no.  Refer again to the list of requirements from the definition Mark-room.  Blue has certainly given Yellow room to sail to the mark, and just after position 3 Yellow is on her course to the next mark, so she has been given room to round it.  At position 4 Yellow  has not yet passed the mark on its required side, but Blue is not preventing her from doing so.  Thus unless Blue luffs Yellow into the mark she has given mark-room as defined and has no further obligations.  

Note that this isn't as clear-cut as the previous example because not all the conditions of the definition Mark-room have been met when Blue luffs Yellow above her course to the next mark: Blue still has to allow Yellow to leave the mark on its required side.   So one could argue that until all the parts of the definition have been met, Blue has no right to interfere with Yellow.  But I don't think that conclusion is supported by the wording of rule 18 and the definition Mark-room.

If I'm right, this has an interesting consequence for team-racing: it essentially allows the outside boat to set a "post-mark trap" at Mark 3.  Suppose Blue has a team-mate close astern; then this play allows that team-mate to pass while Blue holds up Yellow.  You might say, Blue could have accomplished this by luffing later, along the leg, and that may be true -- but it may not, because if Blue allows Yellow to trim in and reach along the next leg, she may lose any real opportunity to luff Yellow above close-hauled, which she needs to do if she wants to hold Yellow up.  It's only the presence of the mark that forced Yellow to come down into the vulnerable position she's in at position 4.