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 Twin rudders with twin propeller shafts
Rudders have a generally symmetrical aerofoil shape in their cross section and steer the ship as water exerts more force on one side of the rudder than on the other (or conversely, more "lift" on one side). When the ship is going straight ahead the rudder is also more or less straight ahead with equal water pressure on both sides. When the rudder turns, say with the trailing edge rotating to the starboard side, there is greater water pressure on the starboard side of the rudder than there is on the port. This pushes the stern to port thus steering the ship to starboard. The physics of rudder design and effects are complex. If, for example, the rudder is rotated too far, it will cavitate on the low pressure side with an accompanying loss of steering control. Turning the rudder to any degree also increases its drag and slows the ship.
A rudder can only be as effective as the water that is passing over it. With a stationary ship, the rudder has no effect. (You simply cannot scull a ship as one would a dinghy.) There are two ways in which the flow of water passing over the rudder can be increased thus giving steerage to the ship:
Single rudder ships place the rudder on the center line or amidships. If there are twin propellers, they are also placed near the midline to maximize the effect of pushing a jet of water against the rudder. Twin rudder ships place the rudders directly behind each propeller to maximize the steerage effect. Single vs. dual rudders have a tradeoff effect: Single rudder ships have less drag and should operate more efficiently, but twin rudder ships should be more maneuverable. This is especially true when the twin rudder ships place the propellers well outboard to facilitate their Relative Thrust Effect.
Twin rudder ships can maximize the Relative Thrust Effect of the propellers by turning the rudders toward each other. That is, set the starboard rudder for a hard turn to port and the port rudder for a hard turn to starboard.
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