It's been a week since I posted the leeward mark question, so I think we've received all the answers we'll probably get.. If you haven't read the preceding post and responses by John, Hans and me, you'll need to do so, to understand this follow-up post.
John says: X breaks no rule, A breaks rules 11 (windward/leeeward) and 18.2 (mark-room) with respect to X, and Y is exonerated for breaking those same rules, plus rule 31 (touching a mark) because she was taking room to which she is entitled.
This was, in fact, the call made on the water and was my original opinion, as well.
Hans agreed with this in his first comment, but in his last comment, some doubt creeps in. 'It is not quite clear from the drawing if Y [gives] mark room to X,' he says.
And that's where the other umpires discussing this call finally ended up. It's a well-established principle of the RRS that a boat's obligation to give mark-room is not restricted to the boat right alongside her, but applies to any boat affected by her behavior. For example, if there are 5 boats coming into a leeward mark side by side, all of them overlapped for a long time beforehand, and the outside boat crowds in so that the innermost boat can't pass the mark on its required side, we DSQ that outside boat even though she isn't in the zone. She was overlapped with the inside boat when the latter entered the zone and thus, owes her mark-room.
In the present case, X entered the zone clear ahead of Y, so Y owes her mark-room. Because X cannot sail her proper course when Y goes in between A and the mark, Y breaks rule 18.2. Y was not compelled to go between X and the mark, so she's not exonerated under rule 64.1(a).
Final answer: DSQ both A and Y.
But this raises an interesting additional question: At some point X is no longer rounding the mark and thus is no longer entitled to mark-room under rule 18.2 (see my post, 'When does rule 18 turn off?'). Or, suppose X's course is above her course to the next mark (which, according to the facts, is off to the right somewhere)? Then A and Y don't break rule 18 with respect to X. A still breaks rule 11 (Windward/Leeward) with respect to X, but Y doesn't. So NOW do we exonerate Y?
The answer is “no”, and the reason is rule 19. As a leeward boat, X is an obstruction to both A and Y. When A and Y become overlapped to windward of X, the outside boat Y owes the inside boat A room to pass the obstruction X. She could have easily given that room by passing the wrong side of the mark, so by “going in there” she voluntarily breaks rule 19. No exoneration under rule 21 for that, and none under rule 64.1(a), either. So the answer is the same: DSQ A for failing to keep clear of X and DSQ Y for not giving A room to pass X.
If we back away from the specific rules for a moment, we see that this is the answer we'd like to get. Neither A nor Y had any rights in there with respect to X, who was both a leeward boat and entitled to mark-room, and neither of them were compelled to go between X and the mark. So we'd expect them to both be DSQ'ed, and that's what we're going to do.