# Changes in Propeller Shaft Behavior by Fluctuating Propeller Forces during Ship Turning
## 4 Discussion
### Correlation between Sensor Position and Propeller Thrust Force Generating Position  
Figure 24 briefly shows the change in the displacement of the propeller shaft relative to the stern tube bearing. This describes the correlation between the shaft displacement at the forward-stern tube seal box in the engine room where the laser displacement sensor was installed and the propeller thrust force generating position.  
If it is assumed that the propeller shaft is a rigid beam without deformation, the movement of the propeller's position (the eccentric thrust force generating position) can be deductively inferred to lie in the opposite direction to the sensor position.  
Therefore, while turning to the port side, the shaft of the measurement position moves downward. So, this force is transmitted as a force applied to the propeller shaft to the upward direction, which temporally relieves the stern tube bearing of the supporting loads.  
On the other hand, during a starboard turn, it can be seen that the force is applied in the opposite way such that it increases the supporting load of the stern tube bearing by acting as a force that pushes down at the propeller position.  
Figure 24: Rigid beam assumptions.  
The validity of this inference regarding the shaft behavior was confirmed by its similarity to the result of the calculation of a bending moment near an actual propeller performed by [PERSON] [24].  
In summary, it can be observed that the reaction force state of the stern tube bearing is relieved during a port turn and deteriorates during a starboard turn when compared with going straight ahead.  
Additionally, it can be seen that the after-stern tube bearing reaction force continuously increases for a longer period of time under the normal ballast condition compared with the fully laden straight condition.  
Therefore, we recommend that ships avoid making a rapid starboard turn under normal ballast draught conditions.  
The shaft's moment behavior was determined to fluctuate in the clockwise and counterclockwise directions; however, the measurement of only the vertical displacement of the shaft can be seen as a limitation of this study from the point of view of complete cross-validation. This limitation needs to be supplemented through additional studies.  
The key finding of this study is that, even with the same starboard turn, a greater load is applied to the stern tube bearing under ballast conditions compared with fully laden conditions. In addition, the correlation between bending moment and displacement was determined (Figure 23) under the normal ballast condition and was found to be much more consistent than in the Figure 18 case of the fully laden condition. The reason for this is probably the eccentric propeller thrust fluctuation. This could be a significant contribution to the literature, as the conventional quasi-static analysis methods cannot yield these results.  
Despite its limitations, this study is meaningful. Through our experiments, we found that, at present, the key reasons for bearing damage are not sufficiently considered during propulsion shaft system engineering in a transient state. We also discovered that, even with the same starboard turn, a greater load is applied to the stern tube bearing under the ballast condition compared with the fully laden condition. We believe that this could be a significant contribution to the literature, as the conventional quasi-static analysis methods cannot yield these results.  
In particular, this study offers an insight into the overload of the bearing support load as it reproduced the same situation at the time of the accident with an identical ship, which may help with the development of guidelines for increasing the stability of the shafting system for similar vessels in the future.