## 4 Analysis of Experimental Results
### Bearing Force Analysis  
In the ice-propeller contact state, owing to the particularity of the conditions, it is easy to produce a large ice load and axial excitation force, which results in problems such as propeller tail vibration, noise, erosion, and structural damage [35; 36]. Therefore, the analysis of the axial excitation force of the propeller is of great significance to the design of the propulsion system (including the propeller, shaft, and main engine) of an ice-class ship. Accordingly, for operating conditions four and five in Table 3, the time-domain curves of the ice milling load in air, as well as the hydrodynamic load and mixed load in ice-water-propeller interaction under atmospheric pressure, were transformed into frequency-domain curves by fast Fourier transform (FFT). Then, the effects of the ice milling load and hydrodynamic load (separable hydrodynamic load + inseparable hydrodynamic load) on the fluctuating amplitude of the mixed load were compared and analyzed, as shown in Figure 15.  
Figure 15: Frequency domain curves of propeller and blade bearing force: (**a**) \(T\) curve in frequency domain; (**b**) \(Q\) curve in frequency domain; (**c**) \(T_{X}\) curve in frequency domain; (**d**) \(T_{T}\) curve in frequency domain; (**e**) \(Q_{X}\) curve in frequency domain; (**f**) \(Q_{Y}\) curve in frequency domain. Figure 15 shows the frequency-domain variation curves of the ice milling load, hydrodynamic load, and mixed load of the propeller and single blade. Figure 15a,b illustrate that the axial force of the ice milling load and thrust of the mixed load show different amplitudes at the integral multiple of the first-order shaft frequency (rotational speed \(n=22.15\) r/s, 22.15 Hz), and the amplitude at the blade passage frequency (four-blade propeller, 88.6 Hz) is the most evident, which then gradually attenuates. The main fluctuating frequency of thrust in the hydrodynamic load of the propeller is the blade passing frequency (BPF), and the subsequent higher-order fluctuating amplitude can be ignored. By comparing the BPF amplitudes of the three types of thrust, it can be observed that the BPF amplitude of the thrust in the mixed load is the largest, followed by the axial force of the ice milling load, and the BPF amplitude of the thrust in the hydrodynamic load is the smallest. The results show that the axial force of the ice milling load is the main component of the increase in thrust pulsation of the mixed load on the propeller; it also shows that the presence of water intensifies thrust oscillation in the mixed load during the ice-propeller milling process. In the frequency domain curve of the propeller torque, the main fluctuating frequencies of the torque of the ice milling load and torque of the mixed load of the propeller are the axial passing frequency (APF) and BPF, and the fluctuating amplitude of the APF is the largest. The main fluctuating frequencies of the torque of the hydrodynamic load are APF and 2 APF, and the fluctuating amplitude of APF is the largest. By comparing the APF amplitudes of the three types of torques, it can be observed that the APF amplitude of the torque in the ice milling load is the largest, followed by that of the torque of the mixed load, and the APF amplitude of the torque in the hydrodynamic load is the smallest. The results show that the influence of water on the torque pulsation of the mixed load is not evident and only increases the value of the propeller torque (as shown in Figures 7 and 10). Figure 15c-f show the frequency-domain variation curves of a single blade. It can be observed from the figures that the ice milling load, hydrodynamic load, and mixed load show different amplitudes at the integral multiple of the APF, and the fluctuating amplitude of the APF is the largest.