Y on port tack is clear ahead of B on starboard tack when Y enters the zone of a port hand leeward mark. Y sails to leeward of the mark and turns onto her course to the next mark before position 3. B holds her course until, just before position 4, she luffs to avoid Y. B protests. What should the call be?
Mark-room includes "room to leave a mark on the required side". If a boat is still approaching a mark, or is passing a mark, she is still in the process of leaving it on the required side.
Y is clear ahead of B when she enters the zone and rule 18.2
(b) requires B to give Y mark-room thereafter. When Y is at position 4, mark-room has not yet been fully given, so rule 18.2
(b) still applies and rule 18.2
(d) does not. B gives Y mark-room and room for Y to sail her proper course as required by rule 18.2
(c)(2). B breaks no rule.
At positions 3 and 4 Y is sailing her proper course so she is sailing within the room to which she is entitled from B. Exonerate Y under rule 21
(a) for breaking rule 10
When a boat rounds a mark wide, especially an off-wind mark, her proper course may cease to be close to the mark before she has left the mark on the required side. In such a case rule 18.2(b) still applies but the mark-room to which she is entitled no longer includes room to sail to the mark.