Methods to predict underwater acoustics are gaining increased significance, as the propulsion industry is required to confirm noise spectrum limits, for instance in compliance with classification society rules. Propeller–ship interaction is a main contributing factor to the underwater noise emissions by a vessel, demanding improved methods for both hydrodynamic and high-quality noise prediction. Implicit large eddy simulation applying volume-of-fluid phase modeling with the Schnerr-Sauer cavitation model is confirmed to be a capable tool for propeller cavitation simulation in part 1. In this part, the near field sound pressure of the hydrodynamic solution of the finite volume method is examined. The sound level spectra for free-running propeller test cases and pressure pulses on the hull for propellers under behind ship conditions are compared with the experimental measurements. For a propeller-free running case with priory mesh refinement in regions of high vorticity to improve the tip vortex cavity representation, good agreement is reached with respect to the spectral signature. For behind ship cases without additional refinements, partial agreement was achieved for the incompressible hull pressure fluctuations. Thus, meshing strategies require improvements for this approach to be widely applicable in an industrial environment, especially for non-uniform propeller inflow.

Marine propulsors are identified as the main contributor to a vessel’s underwater radiated noise as a result of tonal propeller noise and broadband emissions caused by its induced cavitation. To reduce a vessel’s signature, spectral limits are set for the propulsion industry, which can be experimentally obtained for a complete vessel at the full-scale; however, the prediction capability of the sound sources is still rudimentary at best. To adhere to the regulatory demands, more accurate numerical methods for combined turbulence and two-phase modeling for a high-quality prediction of acoustic sources of a propeller are required. Several studies have suggested implicit LES as a capable tool for propeller cavitation simulation. In the presented study, the main objective was the evaluation of the tip and hub vortex cavitating flows with implicit LES focusing on probable sound source representation. Cavitation structures for free-running propeller test cases were compared with experimental measurements. To resolve the structure of the tip vortex accurately, a priory mesh refinement was employed during the simulation in regions of high vorticity. Good visual agreement with the experiments and a fundamental investigation of the tip cavity structure confirmed the capability of the implicit LES for resolving detailed turbulent flow and cavitation structures for free-running propellers.

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Today, optimal propellers are designed by using advanced numerical methods. Major revolutionary improvements cannot be expected. More essential are the design conditions and the optimal adaptation of the propulsion system according to the operational requirements. The selection and optimisation of the propulsion system based on a systematic analysis of the ship’s requirements and the operation profile are the prerequisites for reliable and energy-efficient propulsion. Solutions are presented, which accommodate these issues with a focus on steerable rudderpropellers. Considerations include the efficiency potential of the propulsor itself, optimisation of the engine propeller interaction, and optimisation of a demandresponsive energy supply. The propeller-thruster interaction is complex, but offers some potential for optimisation. Results of examinations show this. The power distribution between multiple propellers at high loads of limited propeller diameters increases the efficiency. This can be done by double-propeller systems like the SCHOTTEL TwinPropeller or by distributing the power on several thrusters. This distributed propulsion offers economic operation and an increased lifetime by means of the demandresponsive use of energy. An efficiency-optimized electric motor instead of the upper gear box reduces the mechanical losses in the case of diesel-electric propulsion. An example: the SCHOTTEL CombiDrive.