SUMMARY OF THE FACTS
A number of boats are reaching on starboard tack along a river towards a mark, to be rounded to starboard, laid about 6m (20 feet) from the river bank, approximately in the middle of the river.
Approaching, but more than 3 lengths from, the mark, six boats are overlapped. However, there is only sufficient space between the mark and the bank for 4 boats to pass safely through and round the mark. The leeward boats call for room at the continuing obstruction while the windward boats call for room to round the mark.

QUESTION 1
What rules apply before any of the boats reach the zone of the mark?
ANSWER 1
All the boats are at the continuing obstruction because there is a rules obligation which binds them to the inside (Yellow) boat and the obstruction. Each Blue boat must give room to the boat inside her (in relation to the obstruction) and also keep clear as a windward boat. A boat racing is never a continuing obstruction (see the definition, obstruction). Therefore, the limitation on establishing an overlap when passing a continuing obstruction applies only to a boat trying to come directly between a boat and the continuing obstruction itself (see case RYA 2014/4).
A boat coming from astern between two overlapped boats is entitled to room from the windward of the two boats to pass the leeward boat, which is an obstruction because they are both required to keep clear of her. However, that entitlement is dependent upon the windward boat being able to give room from the time that the overlap begins (rule 19.2(b)).

QUESTION 2
What rules apply when the first of the overlapped boats reaches the zone of the mark?
ANSWER 2
Because all the boats are overlapped and are at a continuing obstruction, they are all subject to rule 19.2(b) and rule 18 does not apply, as stated in the final sentence of rule 19.1. While this situation continues, none of the overlapped boats are entitled to mark-room. Each windward boat must continue to give room for leeward overlapped boats to pass the obstruction as required by rule 19.2(b) and must keep clear of those boats as required by rule 11. Boats are only entitled to pass the mark on its required side if they can do so while complying with those requirements.
If Yellow were to luff away from the river bank, she would no longer require room under rule 19 so that rule would cease to apply and rule 18 would apply, entitling boats to windward of her to mark-room.

QUESTION 3
Would the answers be the same if, with the wind in the same direction but the mark to be rounded to port, the boats were reaching on starboard tack along the righthand bank?
ANSWER 3
Yes, except that the boats towards the middle of the river would be right-of-way boats required to give room. Nevertheless they would not be entitled to markroom because rule 18 would not apply.

QUESTION 4
What would be the situation if, with the wind in the opposite direction, the boats were reaching on port tack along the right-hand bank and the mark to be rounded to port?
ANSWER 4
The situation would be as in Answer 2