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 Classic design twin propeller ship
Since the demise of stern-wheelers and side-wheelers, the most common design was a direct linkage of the propellers to the driving engines via a metal shaft. The engines might be steam turbines, piston-driven diesels, electric motors or jet turbines. The propellers can be driven at different "turns" or rotation speeds and direction to provide forward and reverse thrust at different speeds. Alternatively, the blades of the propeller(s) could change their angle or pitch (similar to feathering an airplane propeller) to provide the same range of forward and reverse thrust at a constant speed of rotation.
Rotation of a propeller has several effects other than driving the ship forward or in reverse.
Rotational Direction Effect.- The direction of rotation will turn the ship. With clockwise rotation, the stern will turn to starboard, the direction in which maximum sideways thrust is exerted. The opposite is true for counter-clockwise rotation. For this reason, if the ship has 2 propellers, the starboard prop has clockwise rotation and the port prop has counter-clockwise rotation to cancel the rotational direction effect. When only one propeller is in use, this effect must be taken into account. In the picture above, notice the opposite pitch of the port and starboard propellers.
Relative Thrust Effect.- All modern cruise ships have more than one propeller and their placement has a strong effect on turning the ship. If, for example, the propeller on the starboard side has more forward thrust than the port propeller, the stern will tend to swing to starboard thus steering the ship to port. When the propellers are close to the midline or amidships as in the QE2, this effect is minimized. With more outboard placement, however, the effect can be quite strong. Balancing the thrust from the starboard with an equal and opposite thrust from the port propeller can spin the ship in a pirouette.
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