part of a vessel's propulsion system is the propulsor - perhaps a conventionally shafted propeller, Z -drive, cycloidal propeller, or waterjet. Conventionally shafted propulsion systems terminate in a screw propeller, of either the fixed pitch (FP) or controllable pitch (CP) variety. The rotational motion of the screw is translated into linear thrust; the pitch controls the magnitude and direction of this thrust. A fixed pitch propeller must use a reversing gearbox or reversing prime mover in order to make sternway, while the controllable pitch propeller simply reverses pitch. Fixed pitch propellers are most efficient at one speed and therefore ideal for long voyages at constant speed. They also have the advantage of simplicity that translates into low initial and maintenance costs. Controllable pitch propellers, as their name suggests, are capable of varying propeller pitch to match a given load resulting in better
power utilization over a range of speeds. With controllable pitch systems, reduced fuel and maintenance costs can be realized by running the main engine at a constant speed. In this configuration, an engine driven electric generator is installed to absorb excess energy from the engine to provide auxiliary power. However, this operational flexibility comes at a much higher initial cost, plus considerable complexity in the pitch control system. A conventionally shafted propulsion system must also include a rudder in order to change the direction of thrust, allowing the vessel to maneuver. Historically, rudders were little more than flat plates hinged at the leading edge. This simple configuration worked well for small boats, but the unbalanced nature of the rudder induced large, undesirable steering loads in bigger vessels. Balancing the rudder -- that is, shifting the axis of rotation aft to about 1/3 of the chord length from
A 360-ft. overnight passenger excursion designed by Guido Perla & Associates, Inc. The diesel-electric propulsion system powers the stern wheel, two azimuthing thrusters, and pumpjet bow thruster.
September 2008
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