In maritime navigation, hearing three short blasts from a vessel's horn indicates that the ship is operating in reverse (moving astern) or is applying astern propulsion to slow down or stop. While commonly associated with "going backwards," it specifically signals the driver's intent to operate the engines in reverse. This is a critical safety signal in crowded harbors or restricted visibility to warn nearby boaters of a sudden change in movement. It is important to distinguish this from standard fog signals: a power-driven vessel making way through the fog typically sounds one prolonged blast every two minutes to signal its presence. If a vessel is underway but stopped and making no way through the water, it sounds two prolonged blasts every two minutes. Therefore, if you hear three short, sharp blasts in foggy conditions, it serves as an immediate warning that a nearby vessel is actively reversing or maneuvering to a halt, requiring you to maintain a sharp lookout and prepare to take evasive action if necessary.