In collision-avoidance maneuvering in congested waters, an own ship 501 is decelerated by astern power. The own ship 501 is continuously navigated on a current target course 502 with a propulsion propeller 101 always rotated forward at the stern of the own ship. The astern power is generated as the propulsion of a propeller slipstream with rudder angles formed at a pair of right and left high-lift rudders 102 and 103 disposed behind the propulsion propeller 101. In the decelerating maneuvering, the rudder angles formed at the high-lift rudders 102 and 103 are controlled within a range from a rudder angle for applying a maximum propeller slipstream as the astern power to a rudder angle for eliminating the ahead power of the propeller slipstream, and the deceleration of the own ship is controlled by changing the astern power according to the rudder angles.
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