On September 7th, there were two great examples of Rule 18 and how to apply it (and how not to apply it!). One occurred at the windward mark and one occurred at the leeward mark. I do not claim to be a rules expert, but I did stay at a Holiday Inn Express, so bear with me - I think it is worth reviewing this since they are both pretty common occurrences not only on our courses but in other races as well.
First, the definition of overlap from the RRS:
"Clear Astern and Clear Ahead; Overlap - One boat is clear astern of another when her hull and equipment in normal position are behind a line abeam from the aftermost point of the other boat's hull and equipment in normal position. The other boat is clear ahead. They overlap when neither is clear astern. However, they also overlap when a boat between them overlaps both. These terms always apply to boats on the same tack. They do not apply to boats on opposite tacks unless rule 18 applies or both boats are sailing more than ninety degrees from the true wind. (Emphasis mine).
The implication here is that in the absence of marks, two boats on opposite tacks on a BEAT cannot be overlapped but on a RUN most certainly can be.
Rule 18 - Mark Room. I think it's important to know when this applies and when it doesn't, and fortunately it is in 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
Note again that 18.1a is consistent with the definition of overlap - that is, neither overlap nor Rule 18 apply at a windward mark. However it is equally important to note that it DOES apply to leeward and gybe marks.
In the case of the windward mark, we need to look at rule 18.3 Tacking in the Zone:
If a boat in the zone passes head to wind and is then on the same tack as a boat that is fetching the mark, rule 18.2 does not thereafter apply between them. The boat that changed tack:
(a) shall not cause the other boat to sail above close-hauled to
avoid contact or prevent the other boat from passing the mark
on the required side, and
(b) shall give mark-room if the other boat becomes overlapped
It's worth remembering that the zone is 3 boat lengths of the boat nearest to the mark. For most of our fleet, the zone will be somewhere around 100' plus or minus. As a homework exercise, measure out 100' on your dock some time to give you a good idea of just how far 3 boat lengths is.
Back to our Windward Mark rounding. A boat coming in on port tack has no rights to room over a starboard tack boat that is fetching the mark. If a port tacker tacks below the starboard tack boat inside the zone, they do so at their own peril: not only is there a potential to break 18.3(a) but also 13. The situation is changed slightly if the starboard tack boat has overstood the mark and is not sailing close hauled - port has the ability once it has met its obligations under 13 and 15 to get room as long as starboard doesn't have to luff above close hauled to avoid contact. The port tack layline can sometimes play well but be aware of the burdens on you: whether rule 18 applies or not, rule 10 NEVER turns off.
This is important when we look at the gybe-mark rounding. We've already established that 18(a) does not apply because of the clause on a beat to windward and the definition of overlap allows that on a downwind leg boats can be overlapped on opposite tacks. Since the rounding passes 18.1, we need to look at 18.2(a) which states When boats are overlapped the outside boat shall give the inside boat mark-room, unless 18.2(b) applies.
US Sailing Case 12 noted that it doesn't matter if boats are approaching the mark at wildly different angles, only that the overlap is established before the zone. In the case of two boats sailing perpendicular to each other downwind, they are most definitely overlapped based on the definition of clear astern and clear ahead.
So 18.1 tells us the boats are overlapped and 18.2 tells us that the outside boat, regardless of tack, is required to give the inside boat room at the mark. The rule states that while rule 10 is still in effect, as long as the port tack boat takes only the mark room to which she is entitled (including room to gybe) then she shall be exonerated for breaking rule 10. See Quiz 36 in Dave Perry's 100 best racing rules quizzes for a full explanation.
I'm sure there will be plenty of debate about whether I've interpreted these rules correctly, but please save it for the party - you can excoriate me all you want there, as long as you're buying the dark n stormies ;)