starboard rounding rights

Originally posted by Sam.Spoons But is Blue allowed to change course to prevent Yellow from keeping clear (which is what a luff would be)?

True, while they are on different tacks, the definition of keep clear does not allow for the RoW boat to be able to alter course.

However, the boat on port's entitlement to be able to keep clear does not mean the starboard boat has to allow it room to tack. If the port boat can only keep clear by aborting its tack and carrying on on port, it would probably have to do so? So blue can head up at least until yellow has no option but to complete the tack?

In practice, if this becomes a protest, it will be a case that blue alters course believing yellow has tacked too close. It will be different views of exactly how the boats were positioned.

At no time is Y a right of way boat:  she never has any obligations under rules 15 or 16, nor is she ever overlapped outside with mark-room obligations under rule 18.  At no time is Y required to give room to B to do anything.

@2 Y, still on port is required to keep clear of B still on starboard (rule 10), both boats are on a beat to windward, on opposite tacks and rule 18 does not apply (rule 18.1a).

@3 Y, on the same tack as B and overlapped to windward is still required to keep clear of B to leeward.  Boats are no longer on opposite tacks, and are in the zone so rule 18 applies.  Boats  were neither overlapped nor in a clear ahead/astern orientation when the first of them reached the zone, so rule 18.2b and c do not apply.  Rule 18.2a applies, B, the outside overlapped boat is required to give Y, inside, mark-room, and if, while sailing within the mark-room to which she is entitled, Y fails to keep clear of B, she shall be exonerated (rule 21a).