This is a somewhat atypical hypothetical example of RRS 17 (based on an actual encounter I had recently), but it illustrates how a right-of-way boat can be limited to not sailing above her proper course.



Prior to position 1, the boats are overlapped on the same tack more than 2 boatlengths apart. Yellow has understood the mark and is sailing slower than Blue. At position 1, the two boats close to within two boatlengths. Because Blue did not establish overlap to leeward of Yellow from clear astern within two of her hull lengths, Rule 17 does not restrict her course.

Between positions 1 and 2, however, Yellow catches a puff, accelerates, and breaks overlap. She hails "no overlap". Blue, thinking that Yellow is about to claim mark-room, replies "we're not at the zone yet".

Between positions 2 and 3, Blue gets the puff, accelerates, and the two boats become overlapped again. This time however, Blue did establish overlap to leeward of Yellow within two of her hull lengths. Therefore RRS 17 prohibits Blue from sailing above her proper course. Yellow hails "proper course".

Blue's proper course (the course she would sail to finish as quickly as possible in the absence of Yellow) is to sail directly aside the leeward mark. Blue turns down to that course between positions 3 and 4 and maintains that course until position 5. Yellow sails a parallel course.

At position 5, Yellow reaches the three boat-length zone. Yellow is overlapped inside of Blue. Per RRS 18.2(b), Blue must thereafter give Yellow mark-room. Yellow hails "mark room".

Between positions 5 and 6, Blue turns down to give Yellow mark-room. She actually turns down a little farther than necessary (aiming for a tactical rounding). Yellow takes advantage of the excess room.

Between positions 6 and 9, Blue gives Yellow mark-room and Yellow sails within that mark-room. Both boats round the mark and Yellow exits inside and ahead of Blue.

During most of the encounter (except for the time between positions 1 and 2), Blue has sailed faster, but Yellow has taken advantage of her position and the rules to come out ahead.

Had Yellow not caught the puff and broken overlap at position 2, Blue could have headed Yellow up and driven her past the mark (provided they remained outside the zone). Then Blue would gybe and come out ahead.

I hope that's helpful,
Eric