directional stability

Directional stability in nautical science refers to a ship's ability to maintain a set course. A directionally stable ship will only deviate from its course if an external force is applied, while an unstable ship may deviate or continue deviating without such a force. While instability can make a ship easier to maneuver, a stable ship requires less energy for steering. The underwater profile of a ship can indicate its directional stability; a hull with more area concentrated towards the aft is generally more stable.1

To control direction, ships use rudders, typically located at the stern. Rudders function like airfoils, generating lift when turned to create a turning moment. The effectiveness of a rudder is enhanced when placed in the high-velocity water flow from a propeller. For low-speed maneuvering, bow thrusters are often employed, which are propellers in transverse tunnels near the bow that can push the bow sideways without causing forward motion.1