What does three blasts on the whistle signify?

Three blasts on a whistle signify the maneuver of a vessel operating astern propulsion. This sound signal is specifically defined in the International Regulations for Preventing Collisions at Sea (COLREGs). When a vessel is backing up or moving in reverse, it is important to communicate this action to other vessels in the vicinity to ensure safety and prevent collisions.

While overtaking, turning, or coming alongside are critical maneuvers, they are represented by different sound signals according to maritime communication protocols. Each type of whistle signal serves a distinct purpose in maintaining maritime safety by informing others of a vessel's intentions and movements on the water.