Economic benefits of hybrid drive propulsion for naval ships

The typical operating profile for destroyers and other marine vessels includes a significant amount of time loitering or other low-speed activity. Low-speed operation is accompanied by propulsion system inefficiency, as the propulsion turbines are operating off-design. Low speeds are further enabled by propeller pitch variation, which exacerbates system inefficiency. A practical hybrid electric drive system will be proposed that complements the ship's existing mechanical propulsion and electrical plants. Electrical power generated by the ship service turbine-generators (SSTGs) is used to drive large permanent magnet motors. The motors then drive the propeller shafts during low speed operation. This scheme allows the propulsion turbines to be shut down and the SSTGs to run closer to design point, resulting in fuel savings. Additional savings are possible at higher speeds by operating the motor(s) in generation mode, enabling SSTG shut down and propulsion turbine performance optimization. This paper describes in detail the concept of hybrid drive operation and demonstrates the economic justification for such a system using the DDG-51 class of destroyers as an example platform.