If there is one argument on mark roundings, this is the one that comes up often. More times than not, there isn't a clear understanding on the rights and rules that govern the example displayed below shows.

In the example, the starboard boat can sail in a straight line beyond the mark legitimately, and point the port tack boat up. To back this up, first read Rule 10 and Rule 18.1.b.

Rule 10: On Opposite Tacks
When boats are on opposite tacks, a port-tack boat shall keep clear of a starboard tack boat.

Rule 18.1.b Passing Marks and Obstructions
(b) … boats on opposite tacks when they are on a beat to windward or when the proper course for one of them to pass the mark or obstruction is to tack.

The starboard tack boat is not changing course, and the rules apply essentially as if there is not a mark there. Because this is the case, the starboard tack boat can keep sailing in a straight line as far as he wants passed the mark, and the port tack boats must keep clear.

If the right-of-way starboard tack boat is altering course, it must give the keep-clear port tack boat room to keep clear. The key point here is altering. As you can see in the example, if the starboard boat starts to round the mark, it will be a head of the port boat, and this would not be a problem.