Higgs...
Rule 23.1 states that "If reasonably possible, a boat not racing shall not interfere with a boat that is racing."
A boat is racing from her preparatory signal until she finishes and clears the finish line and marks or retires, or until the race committee signals a general recall, postponement, or abandonment.
This from the US Sailing publication "Understanding the Racing Rules of Sailing through 2012" by Dave Perry.