I don't mean to single you out, but there have been several mis-statements so far, and this post seemed like a good starting point.

To begin with, Rule 18 applies between boats when at least one of them is in the zone (RRS 18.1). If boats are overlapped when the first of them reaches the zone, the outside boat at that moment shall thereafter give the inside boat mark-room (RRS 18.2(b)). It is not necessary for both boats to be in the zone, only the boat closer to the mark. The obligation to give mark-room under rule 18.2(b) begins when the first boat reaches the zone and does not end until the boat entitled to mark-room has passed the mark, left the zone, or passed head-to-wind.

The rules are applied between pairs of boats. When three or more boats meet, you need to examine the relationships between a boat and other boats individually, and separately determine the obligations for each pair. In this example, the initial relationships are:

• Gray is on the same tack and overlapped to leeward of both Green and Yellow. Gray is on opposite tacks with Blue with Gray on starboard tack. Gray is not required to keep clear of any of the other boats.
• Green is overlapped on the same tack as Gray and Yellow. She is windward of Gray and leeward of Yellow. Green is on opposite tacks with Blue with Green on starboard tack. Green must keep clear of Gray.
• Yellow is overlapped on the same tack as Gray and Green. She is windward of both. Yellow is on opposite tacks with Blue with Yellow on starboard tack. Yellow must keep clear of both Gray and Green.
• Blue is on opposite tacks with Gray, Green, and Yellow, with Blue on port tack. Blue must keep clear of Gray, Green, and Yellow.

Inspecting the diagram, Gray is just over three boatlengths from the mark. Assuming that the sailing instructions have not changed the size of the zone, rule 18 does not yet apply. Gray is, however, on the verge of entering the zone. As soon as she does, not only will she be the right-of-way boat, but the other three will owe her mark-room. No rule dictates when Gray must gybe, but if I think her best course of action would be to make a wide-entry, smooth, momentum-retaining gybe inside the zone, passing right under the mark. That way, she exits as fast as possible, and prevents any boats from getting to windward for the short reach to the finish.

Green's best tactic in the absence of Blue, would be to drive Yellow past the layline to the mark, so that after gybing, she is ahead. If she can get Blue to gybe away, Green will wind up ahead of both. If she goes too far, however, Blue might head up and roll her. If she gybes too soon, Blue might be able to sail through her lee. If I were Green, I think I'd gybe slightly above Blue's line, such that Yellow would be almost in front of Blue at the time. That would put the most pressure on Blue to gybe, and leave her the least room to sail above or below Yellow and Green.

Yellow doesn't have many options. Green is controlling her gybe. All she can do is gybe when Green does, or (if Green goes too far) slow down and gybe above her. She's outside the zone, so if Green gets out in front after the gybe, Yellow will not get mark-room later.

Blue is in a risky spot. Her only safe course of action is to let all the other boats go first - either by slowing way down, or gybing away. From the diagram, her chance of crossing Yellow and Green is mediocre at best. If she bears away, Yellow and Green will simply delay their gybes. If she heads up, they will gybe sooner. She stands a good chance of rolling them while they gybe though. Blue is more than a boatlength below the layline, so she has some room to maneuver. I think Blue's best bet is to shed some speed, encourage Green and Yellow to gybe right in front of her, and then head up to cross their transoms as fast as possible. Gray will likely cut Blue off at the mark, but Blue can still probably dive down to round behind her. That puts Blue in second position. If Gray makes a mistake, and passes the mark too low or too slow, then Blue might get an opportunity to roll Gray, or drive through her lee before the finish.

Regards,
Eric