The S flag, accompanied by two sounds,
is used to shorten the course, often due to a lack of wind.
You’ll generally round the next mark and head right for the
finish. Look for the RC boat.
The C flag, accompanied by a series of
sounds, is used to change the course, often due to a change in the
wind direction. Course
changes (including shortening) must be made before the lead boat has
rounded the mark before the leg that is changed.
The M flag, when flown from a boat
means that boat serves as a mark (the old mark has drifted away or
is missing).
The N flag means races are abandoned,
and the N over H means races are abandoned and racers should return
to shore.