Which structure functions as the primary means of steering a ship?

The rudder serves as the primary means of steering a ship because it is designed to create a force that changes the ship's direction. When the helm is turned, the rudder pivots, and water flows over the surface of the rudder, generating a hydrodynamic force that pushes the stern of the ship in the opposite direction. This action allows the ship to turn to port or starboard, facilitating navigation and maneuverability.

The main deck is the flat surface that forms the uppermost part of the ship, providing a foundation for operations and activities but does not play a role in steering. The propeller is responsible for propulsion, generating thrust to move the vessel forward but is not directly involved in directional control. The stem post is a structural component at the front of the ship that supports the ship's hull and is not involved in steering functions. Thus, the rudder is uniquely designed and utilized to direct the vessel’s movement effectively.