In the event of fire occurring on board any vessel or watercraft in the Portland harbor, except those under way, it may sound five prolonged blasts of whistle or siren as an alarm indicating fire on board or at the dock to which it is moored. The signal may be repeated at intervals to attract attention and is not a substitute for, but may be used in addition to, other means of reporting a fire. The words “prolonged blast” used in this Section mean a blast of 4 to 6 seconds duration.