You asked for a case cite to defend my interpretation, and ISAF Case 15 matches my statement. It is a direct answer to your question.


Take another look at Rule 18. RRS 18.2(b) says "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. If a boat is clear ahead when she reaches the zone, the boat clear astern at that moment shall thereafter give her mark-room". RRS 18.2(c)(1) says "when a boat is required to give mark-room by rule 18.2(b), she shall continue to do so even if later an overlap is broken or a new overlap begins".

Once the first boat reaches the zone, the obligation to give mark-room is set. It doesn't matter if later the boats become overlapped or ahead/astern. The obligation is unchanged as long as the conditions of RRS 18.1 and 18.2(c) hold. ISAF Case 15 would not change at all if the two boats became overlapped after reaching the zone.


In a scenario where the outside/leeward boat owes the inside/windward boat mark-room, then the mark room she must give includes room to tack. ISAF Case 21 explains in greater detail how much room the outside boat is obligated to give. The definition of mark-room, however, does not place any obligations on the boat entitled to mark-room. Nor does Case 21. The inside/windward boat is not required to tack. She may, if she wishes, do exactly as the windward/astern boat does in Case 15.

In short, Case 21 is not relevant.

If you still assert that a boat entitled to mark-room is obligated to tack at the mark, then please cite the rule that says so.