How would you decide this?

Dave Perry’s 100 Best Racing Rules Quizzes highlights specific aspects of the racing rules in a fun format designed to help you become more familiar with The Racing Rules of Sailing. Here is one of the quizzes:

Quiz 49
Boats A and B are approaching a mark to be left to port on starboard tack. The next leg is a downwind leg on which the boats will need to sail on both tacks to reach the next mark. A reaches the zone clear ahead of B, sails to the mark and gybes. After her gybe, B has to luff to avoid A and hails, “Protest!” You are on the protest committee; how would you decide this?

You are on the protest committee…how would you decide this? Answer below.

Quiz 49 Answer:
A is penalized for breaking rule 10, On Opposite Tacks. A reached the zone clear ahead of B, and was therefore entitled to mark-room under rule 18.2(a)(2), Giving Mark-Room. “Room” is the space a boat needs to round the mark in a seamanlike way (see the definition Room). It does not include additional space a boat may prefer to take in order to gain some tactical advantage in the race.

At position 2, B was giving A mark-room; i.e., the space she needed to round the mark on the required side and leave the mark astern in a seamanlike way (see the definitions Mark-Room and Room). Because A did not need to gybe to round the mark to begin sailing the next leg, at position 3 she was not sailing within the mark-room to which she was entitled. Therefore she is not exonerated (freed from penalty) by rule 43.1(b), Exoneration, for her breach of rule 10.

Source: US Sailing