Our client wanted to investigate the potential use of propellers with motors encapsulated around their periphery (Rim Drive Propulsors or RDPs) in acoustically sensitive marine environments.
There was a lack of understanding about the Underwater Radiated Noise (URN) performance of propulsors where the motor is effectively ‘in the water’. We put together a team including leading academics, a company producing advanced motor-driven propellers and pumps, and a manufacturer of large scale RDPs, as well as technical experts from the customer.
As well as combining the capabilities of the various team members, we hired a research vessel propelled by two full-scale RDPs, and undertook URN measurements as well as measurements on board of noise, vibration and motor dynamic current and voltage.
We developed multiple models to describe the various noise and vibration mechanisms present in the RDP device. This included detailed modelling of the electromagnetic response of the machine, and the production of unwanted harmonics of Motor Drive Frequency (MDF), Switching Frequency (SF) and propeller Blade Passing Frequency (BPF), as well as intermodulation between these components.
We showed how features in the design of the RDP result in vibration in the machine that can cause high levels of URN, validating this against the URN measurements.
We showed how the design of the RDP could be optimised to substantially reduce the URN, allowing RDP driven vessels to be potentially utilised in noise sensitive applications such as for fisheries research vessels. The work also identified how the noise environment on board the ship could be made more amenable to the crew as well as identifying likely causes of heat build-up and potentially accelerated deterioration of the propulsion installation.