Ship and Both Screws Going Ahead (Single Rudder) If the rudder is amidships, the ship will steer a steady course. The forces of the two propellers are equal and opposite in direction. If one screw is stopped with the rudder amidships, the turning of the screw will take charge, and the ship will turn toward the side of the stopped screw.
Ship Going Ahead, Both Screws Backing (Single Rudder) The steering effect of the rudder is the only force turning the ship from a straight course. The effect of the rudder is reduced as the headway is lost until there is no steering control when the ship is stationary.
If only one screw is backing and the other is stopped with headway on, the turning of the backing screw added to the momentum of the ship going ahead will swing the stern away from the backing screw.
Ship Going Astern, Both Screws Backing (Single Rudder) If the rudder is amidships, everything is equalized, and a straight course can be steered. If the rudder is put over, the pressure of the water that the ship is backing into against the back side of the rudder will let you steer that course. Most of the water that passes through the screws comes from free surface and has little effect on the rudder.
If one screw is stopped, the turning moment of the backing screw is added to the effect of the rudder when it is put over away from the turning screw. The swing may be slowed or stopped if the rudder is put over toward the screw. The effect of the rudder is to counteract the effect of the screw.
Ship Going Astern, Both Screws Going Ahead (Single Rudder) The ship will respond to the rudder, a left rudder will throw the stern to port unless you have a lot of sternway on. The forces of the screws will be same. The steering effect of the rudder when going astern will be reduced as the ship loses headway, until all steering control is lost before the ship has lost sternway. This loss of control happens because the discharge flow from the propellers will interfere with the flow of water against the back of the rudder. This by know means covers everything on ship handling, I think a lot of it is you have to have a feel for it. Also this is great if your in a lake.
