**1. Introduction**

Most marine vessels have propellers at the stern operating in non-linear wakes. This propeller placement is done primarily for ease of construction and propulsion efficiency. The main disadvantage of such a setup is having to operate in a region where the wake is non-linear. This generates noise from an oscillating thrust and, in terms of hull induced vibrations at frequencies below 100 Hz [1], can have large effects on passenger comfort, mechanical wear and acoustic detection. Developing an accurate vibrational model of the propulsion system in a non-uniform wake is extremely difficult. It is necessary to have a good understanding of the hydrodynamic forces inside the thrust bearing and measuring this force is logistically difficult. This is due to the complex nature of thrust bearings which self stabilise based on operating parameters such as speed and load [2]. To simplify the inclusion of thrust bearing stiffness in analytical modelling, simple linear stiffness representations are used

**Figure 1.**

*The arrangement of the PVDF film within the thrust bearing in our previous work [8] (a) and in this work (b). In both arrangements the PVDF is embedded on the stationary body and 'v' indicates rotational direction of the rotating body.*

[1, 3, 4], however these are limited as it has been shown that due to large perturbations of the oil film thickness, a nonlinear response is required [5–7]. Therefore measuring the internal hydrodynamic forces inside the thrust bearing is ideal. This paper attempts to address this problem by using intrinsic piezoelectric sensors installed inside the thrust bearing itself. Conventionally this would be done with accelerometers however their physical properties make them unsuitable for use inside a moving bearing. Another option is the use of thin piezoelectric polymer film such as PVDF (Polyvinylidene Fluoride).

In our previous paper [8], an in-depth review of the use of PVDF to measure dynamic forces in static structures [4, 9–16] was discussed. It was noted that PVDF cannot measure absolute pressures, only the fluctuations, which is a common use of PVDF in fluids [17–22], due to its piezoelectric properties [23, 24]. The paper reported on the use of PVDF inside a thrust bearing to measure the pressure fluctuations and infer the change in contact force. Due to limitations in the experimental design, namely the placement of the PVDF films in relation to the pads on the rotating body, the signatures captured from the PVDF films were heavily influenced by the Pad Passing Frequency (PPF). Hence, although that study found that the sensors could measure the pressure fluctuations generated by the sliding pads, the true nature of the excitation of interest, the BPF was masked.

Here, the design of the experiment was reworked so that the true excitation force from the propeller and the resulting change in contact force within the bearing could be observed without the influence of the PPF. This was achieved by placing the PVDF in the tilted portion on the stationary body.

The previous configuration (**Figure 1a**), resulted in the pressure profile generated by the pad sliding over the PVDF film. This led to the dynamic pressure fluctuations being subjected to the PVDF from the pad motion relative to the PVDF sensor. The new configuration proposed in this study (b) removes this component and results in the PVDF film being subjected to the dynamic fluctuations arising from the shaft axial load and hence propeller. The static component of the pad's pressure profile cannot be observed in the signature as a result of the intrinsic nature of PVDF.
