Acoustically the submerged components of an Azipod propulsion unit comprise two uncorrelated noise sources, namely the propeller and the electrial motor. The propeller hydrodynamic noise is highly dependent on the load & speed whereas the magnetic noise of the electric motor exhibits a smoother dependency on the load & speed. The electromagnetic noise of the motor is highly dependent on the converter supply properties such as the number of voltage levels and current/voltage modulation parameters.
The paper deals with the challenge of estimating the total underwater sound emission of an Azipod unit using both the airborne sound power measurements and underwater sound emission simulations and current hydrodynamical tools available.
The airborne electromagnetic noise emission of the unit can be measured at a factory testbed using the designed converter supply together with correct shaft loading. Sound intensity based methods are favoured in the paper due to their ability to exclude the extraneous noise from the results (factory background noise & noise from the loading machine). Once the airborne noise figures are obtained, a conversion to corresponding underwater noise figures is needed. This is accomplished by computing the airborne and underwater noise radiation with same electromagnetic loading using FE-method for the structural dynamics and BEM for the acoustic radiation. The conversion factor K from airborne to waterborne sound emission is finally obtained for each of the electomagnetic excitation patterns. The accuracy or goodness of K is then validated by comparing future underwater noise measurements during sea-trials and the measured airborne emission at the test floor.
Separate analysis of hydrodynamic acoustic characteristics are performed with three different tools and finally combined to total URN level with electromagnetic noise emission estimation.
In this work, the underwater fluid loading on the Azipod unit surface plates is included by using a very simple approach, which is to be addressed in future studies.