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I have been in the same situation. The answer is B did not have inside rights, but A needed to close the door and not allow B inside. A did not have the right to tack on B, even though B should not have been there. A should have protested B.
As the boats tacked 18 would be off and if neirher down to clossehaulled the they are tacking at the same time so the boat on the right is in the right.
If so or not, why or why not?