Electric marine propulsion system

A marine propulsion system for independently driving a pair of propeller shafts, comprising:

• first and second diesel engines, each of said diesel engines connected to drive first and second alternators, whereby a pair of diesel generator sets are provided, said diesel engines having a controlled shaft operating speed set to produce maximum shaft horsepower at maximum efficiency, said alternators selected to have a standard rated RPM for producing a standard rated output voltage frequency substantially less than said operating speed, whereby a maximum KW power generation efficiency is obtained at a voltage having a frequency higher than said standard rated output voltage frequency;
• a pair of frequency converters connected to each of said alternators for providing a variable frequency output voltage, said frequency being selected by an operator of said system to control the RPM of said propeller shafts;
• an AC motor connected to each of said alternators, said AC motors providing an output shaft RPM which varies along with torque in response to a varying input voltage frequency while maintaining output shaft horsepower constant;
• and, a pair of reduction gear transmissions, each coulping a pair of said AC motor shafts to a respective propeller shaft, whereby each of said shafts is driven by a pair of motors at an RPM and torque controlled by selecting the voltage output frequency of said frequency converters.

In a marine propulsion system, a method for driving a solid cast propeller having a fixed pitch comprising:

• coupling an AC motor to said propeller, said AC motor providing a varying torque at varying RPM to said propeller in response to a varying input voltage frequency;
• supplying an AC voltage to said AC motor having a frequency for selecting said AC motor RPM;
• and
• controlling the frequency of said AC voltage whereby the RPM of said AC motor shaft and coupled propeller is controlled.

A marine propulsion system for providing a variable drive to a constant pitch propeller comprising:

• a diesel engine driven alternator for providing alternating current, said diesel engine operated at a speed of a maximum efficiency, and said alternator producing a current having a frequency higher than its standard rated frequency of operation;
• a frequency converter connected to receive said alternator current having a frequency higher than a standard rated frequency of operation, said converter providing an output voltage at a frequency which is selectable;
• and
• an AC motor connected to receive said frequency converter output, and coupled to drive said propeller, said AC motor providing a variable rotational speed to said propeller proportional to a selected voltage frequency from said frequency converter, said AC propeller motor providing a constant horsepower for each selected frequency.