Originally Posted by With regard to blasts on ship's whistles when leaving a pier, you
actually it is a Manoeuvring and Warning Signal under the International Collision regulations act
More precisely, it's a sequence of two whistle signals required by Rule 34
the International Regulations for Prevention of Collisions at Sea
, more commonly known among mariners as the "Rules of the Road," established as a treaty among maritime nations.
>> Rule 34(a): Three "short" blasts = "I am operating astern propulsion."
>> Rule 34(e): One prolonged blast = "I am about to emerge from a location where you might not be able to see me."
Rule 34 also prescribes other whistle signals that you might hear as a ship enters or leaves a harbor and when within sight of other ships on the high seas. Note that a ship that is leaving a pier in the forward direction sounds only the prolonged blast.
For those who care, Rule 32
sets the lengths of the blast and the requirements for the whistle itself.