Twynkle,
Not sure exactly what you are asking but ship's whistles are used in the maritime rules for general messages such as 1 prolonged blast (5-7 sec) is change in status (from moored to underway, underway to anchored, etc), 3 short blasts (3-5 sec each) is "I am operating astern propulsion", 5 or more short blasts " you standing into danger' or inother words get out of my way, you are about to be run over with 70,000 tons of QE2.