Integrated Design of Asymmetric Aftbody and Propeller for an Aframax Tanker to Maximize Energy Efficiency

With the implementation of the EEDI, energy saving and emission reduction of ships, especially merchant ships, become more and more important. To achieve high efficiency and low emissions, recently Energy Saving Devices (ESDs) have been re-studied and installed to many ships, both new buildings and also retrofits. Various ESDs, including new concepts, have been tested in model scale and large improvements on energy efficiency have been confirmed. However due to the fact that most ESDs are fitted in the wake field, the performance of the ESDs is influenced by scale effects. For the operators, the fouling and the structure integration of the ESD’s with the hull are the important issues to make decisions on applying ESDs to their ships.
Distinguished from the ESDs where extra ‘appendages’ have to be added in front of and/or behind a propeller, an asymmetric aftbody can also change the flow towards the propeller without appendages. The wake with pre-swirl generated by an asymmetric aftbody is in general more uniform than that by an ESD (such as a pre-stator with finite blades) and with almost no penalty on the ship’s resistance. By integrating a propeller, a ship with asymmetric aftbody can be designed so that the hull-propeller interaction is optimized for its total propulsive efficiency and the required shaft power is minimized at given speed.
In this paper, discussions have been given on the optimization procedure by using the Computational Fluid Dynamics (CFD) towards a fully-integrated hull-propeller design to maximize the energy efficiency of a single screw ship. Comparative model tests, carried out with optimized symmetric and asymmetric ships, showed more than 6% gain in efficiency with a moderate asymmetric aftbody, without detriments to its course stability.