Believe it or not, the purpose of rule 18 is to make mark roundings simpler. If you think rule 18 is complicated, try to imagine a gybe or leeward mark rounding without it. As boats are bearing away, gybing, and rounding up, overlaps are constantly being broken, new overlaps are established, and boats are changing tack. Try to figure out the applications of rules 10, 11, 12, 15, and 16 in real time while all that changes -- especially if there are multiple boats involved -- while still performing all the necessary acts of seamanship needed to round the mark. It can't be done.

Now, enter rule 18.2(b). All you have to do is look at the relationship between boats at the moment the first one enters the 3-boat-length-zone. If the boats are overlapped, then the outside boat must thereafter give the inside boat mark-room. If they are not overlapped, then the astern boat must thereafter give the ahead boat mark-room. That obligation holds even if the boat's right-of-way or overlap relationship changes as they maneuver.

Back to the scenario at hand, where a port-tack boat is overlapped inside of a starboard-tack boat at a downwind finish mark, rule 18.2(b) applies. The outside boat (S) must give the inside boat (P) mark-room. If that means that both boats must turn dead-downwind to avoid each other and the mark, then that's what both boats must do. Forget about proper course, as that term has nothing to do with rule 18.2. Don't get hung up on "rights", as both boats have obligations. P must keep clear of S, and S must give P mark-room.

I hope that helps,
Eric Rasmussen
US Sailing Judge