## 4 Conclusions  
Thrust measurements are a very challenging task, as the acquired axial strains are usually very small and sensitive to the zero setting of the sensor. To investigate the accuracy and reliability of propeller thrust measurements, the strain gauge-type system was installed for the twin skeg LNG carrier at PORT and starboard side, and strain gauge and optical sensor-type systems were mounted on two vessels of a crude oil tanker series. The measurement systems were installed on propeller shafts in the engine rooms between the propeller and the thrust bearing. The summary and conclusions from this study are as follows:  
Firstly, for evaluation of the thrust measurement, the relations of thrust-revolution and torque-revolution are compared with model test results for two vessels of a crude oil tanker series at designed and ballast draughts. The characteristics of torque-thrust by strain gauge show a good correlation with those of the model test. For the optical-type system, the measured thrust at the same torque show approximately 15% difference at designed draught and 5% difference at ballast draught compared with the model test results due to the uncertainty of the zero setting. Secondly, the thrust, torque and revolution of the propeller at PORT and starboard side were measured during the speed trial of the twin skeg LNG carrier. The torque-thrust from both sides showed the same tendency. The difference between the prediction results from the model test and measurements during the speed trial increased gradually as the engine load increased. This comes from the difference in the thrust measurement. The torque from the speed trial and model test agreed well with respect to the revolution of the propeller. Finally, the relation between propeller thrust and torque is an important index that can decide the accuracy of the thrust measurement and can also be a good index for checking the status of propellers. In general, the relation of the propeller thrust and torque due to draught and trim conditions is rarely changed, which can be confirmed by the prediction results from the model tests. As a result, the strain gauge-type sensor shows good correlations with the model test results. The experience with the strain gauge and optical sensor shows that extensive effort is required to realize precise thrust measurement. For accurate and reliable measurements of thrust and torque, it is essential to determine the zero level carefully. The thrust measurement can reduce the operating expenditure through guidance on hull and propeller maintenance due to the evaluation of the propeller efficiency and hull resistance. In order to monitor the power performance of ships in operation at sea, the durability of the thrust measurement system should be verified. For further study, the long-term stability of the thrust measurement will be investigated. Conceptualization and methodology, S.-M.O. and D.-H.L. ; investigation, S.-M.O. and D.-H.L. ; writing, S.-M.O. ; writing--review and editing, supervision, B.-K.A. ; project administration, H.-J.K. and B.-K.A. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by the research fund of Chungnam National University. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** The study did not involve humans. **Data Availability Statement:** The data presented in this study are available on request from the corresponding author. **Acknowledgments:** All authors thank Chungnam National University for the funding. **Conflicts of Interest:** The authors declare no conflict of interest. The funder had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.