Two boats on starboard tack. A F16 is finishing downwind (under spinnaker). Can he legally head up, preventing an A cat (or any other type) from finishing. ... he is not sailing above proper course.
Let's nail down some facts:
1) Both boats are on the same tack (starboard in this case).
2) The boats are overlapped.
3) It is the leeward (L) boat's proper course to finish at the mark.
4a) They are more than three boat-lengths from the finish mark or,
4b) They are at the three boat-length zone from the finish mark.
5) L was able to give W (windward) room at the mark at the time they became overlapped (potentially long before they reached the zone).
Because L is not sailing above her proper course (fact 3), rule 17 does not apply and need not be considered.
While they are outside the zone (fact 4a), rule 18 does not apply. Rule 11 does (facts 1,2) and W must keep clear of L. L may change course provided she gives W room to keep clear under rule 16.1.
When they reach the zone, they are overlapped (fact 4b) and W is inside. Because L was able to give mark-room when the overlap was established, she must give W mark-room within the zone under rule 18.2(b).
In short, you can't pinch somebody off the finish line.
I hope that helps,
Eric
US Sailing Certified Judge,
Member Area D Appeals Committee