CALL UMP 18              

Rule C2.1, Changes to the Definitions - definition Finish
Definitions, Clear Astern and Clear Ahead; Overlap

Question
At a close downwind finish, Yellow eases her spinnaker halyard, sheet and guy so that the sail is further ahead of her bow than normally. When does she finish? What action should the umpires take?

Answer
Yellow finishes when her bow crosses the line. The Definition of Finish makes it clear that as the spinnaker is not in normal position it cannot be considered for the purpose of finishing.
Judging the finish is solely a matter for the race committee. However, if the umpires see the halyard eased they may advise the race committee, which will not be in such a good position to see until the boats cross the line.

The same reasoning would apply when two boats approach a leeward mark and there is a question about whether they are overlapped.