I'm trying to work out exactly how 18 works for a windward rounding when two boats approach on opposite tacks: A on starboard layline and tacks in her zone and B on port layline and in her zone. Here is my thinking so far:
Mark is a windward mark to be left to starboard.
Boat A approached on starboard lay line and must tack in the zone to round the mark.
Boat B approaches on the port lay line and does not need to tack.
Indeterminant who reached the zone first... let's say about the same time.
- 18 doesn't apply initially by 18.1(a) & (b), since they are on opposite tacks to windward.
- Once A has passed head to wind, then the boats are now on the same tack, so the exclusions in 18.1 no longer apply and rule 18 switches on.
- 18.2(a) will apply unless 18.2(b) does...
- 18.2(b) does not apply. When the first of the boats reached the zone (either A or B), they were on opposite tacks and thus can't be overlapped as they are not 90 degrees from the wind. Also neither can be clear ahead when they reached the zone as that can't geometrically happen if both boats are on lay lines (unless one of them is already passed the mark).
- 18.2(c) doesn't apply as 18.2(b) doesn't
- 18.2(d) is meaningless without (b) or (c).
- 18.2(e) does apply, but is less so without (b). However it can still be applied to considerations of (a).
- 18.2(f) may apply.
- 18.3 doesn't apply when A tacks in the zone because it is a starboard rounding (and 18 wasn't applying yet anyway).
- 18.4 doesn't apply as gybing at an upwind mark is not the proper course!
Thus it is just 18.2(a), (e) & (f) that apply (plus 10-17 of course). Specifically:
- if either boat establishes an inside overlap (within doubt given by 18.2(e)) then the other must give her mark-room by 18.2(a) unless 18.2(f) applies and it is not possible to do so.
- If either boat establishes an inside overlap from clear astern, then she is constrained by 17 and has to sail her proper course and round the mark.
- If B's gets the inside overlap due to A's tack, then she is not constrained by 17 and can sail past the mark.
- If one boat reaches the mark clear ahead of the other, then 18.2(a) doesn't apply and neither are owed mark-room.
- But when a boat clear-ahead at the mark bears away to round the mark she may be constrainted by 16 to give the other some room to keep clear. Also it is a possible that the bear away establishes an overlap and 18.2(a) switches back on and mark-room is also owed to the now inside boat.
Is that about right?