Combined diesel-electric and diesel-mechanical (CODED) propulsion

Wärtsilä has proposed to adopt the CODED machinery consisting of two azimuthing pods and one mechanically driven feathering CP propeller for the next generation of cruise ships. This arrangement offers very high propulsion efficiency. The possibility to split the load between three propellers instead of two yields better propeller open water efficiency. Furthermore, the single-skeg hull form without any open shaft lines has lower resistance than a twin shaft line arrangement. At low speeds, the vessel is driven by the pods alone. The centerline propeller is only used at high speeds and feathered at low and medium speeds. The feathered mode (pitch changed to align the blades with water flow) results in significantly less resistance than for a windmilling propeller.

Other version of CODED machinery with one pod installed in a contra-rotating mode aft of the main propeller has been proposed for RoPax vessels. This configuration offers better hydrodynamic efficiency, compared with twin screws on long open shafts supported by brackets. The aft propeller takes advanta ge of the rotative energy left in the slipstream of the forward propeller improving the rotative efficiency. In addition, the resistance of the single skeg hull form with a single pod is lower than of a twin-screw hull with two open shaft lines, two rudders and many appendages. Such configuration has been used for ferries AKASHIA and HAMANASU; see CRP-Azipod propulsion of the ferries AKASHIA and HAMANASU.