Steering and Primary Thrust 
The sections on propellers, pods and rudders outline how steering commands interact with the primary thrust. The major difference between the classic ship and the pod ship is the need for thrust to be applied in order to have any steering effect. With a large rudder, just moving through the water will provide some steerage. With a pod ship, however, the steering effect of the shape of the pod will give only a negligible steerage effect - you must have propeller thrust.