## 1 Introduction  
Recently, the interest in developing low-resistance hull form, high-efficiency propeller and energy saving devices is rapidly increasing in shipping and shipbuilding companies in accordance with rules and regulations that require minimizing fuel oil consumption and greenhouse gas emissions. In general, the speed-power performance of ships is estimated by model tests. It is believed that the evaluation of a ship's hydrodynamic performance with high accuracy and confidence level can be achieved at a model scale through towing tank experiments or numerical simulations. Then, full-scale ship performance prediction can be obtained by extrapolating the model scale results according to the friction correlation line, the law of similarity and the extrapolating method recommended by the International Towing Tank Conference (ITTC). The ITTC-recommended procedures [1] assume that the form factor is the same for the model and full-scale ship and is independent of ship speed. These assumptions have been investigated with model tests [2] and numerical analyses [3]. The scale effect of form factor depending on change in the Reynolds number has been studied and made a comparison with three kinds of friction resistance curves [4]. In order to avoid the uncertainty of scale effect, the simulation of speed-power performance at full scale is necessary [5, 6]. Undoubtedly, the most advanced scenario is to simulate the self-propulsion at full scale without any assumptions, i.e., with the free surface, with the real propeller and free sink and trim [7]. The simulation results at full scale, however, must compare with scaled quantities obtained empirically according to the ITTC procedures [8]. For the substantial improvement of the hull form, propeller and energy saving devices [9], it is essential to estimate ship performance in full scale and understand the scaling effect more deeply [10] because through the full-scale speed trials, the performance prediction from the model test can be verified [11]. In general, up to now, the speed trial has been carried out based on the propeller torque and revolution measurement [12; 13]. Although a reliable measurement of the thrust at the same time allows verifying the prediction propulsive efficiency, the uncertainty of thrust measurement has been too large to obtain a reliable result to be compared with the model test prediction results [14; 15]. The measurement of propeller thrust has two main benefits. Firstly, the thrust measurement gives a way to obtain speed-power performance in line with the model test. The model tests are based on the measurement of propeller revolution, torque and thrust, and the model test results are analyzed based on thrust identity. Without thrust measurement, the full-scale speed trial should be analyzed based on the torque identity. These fundamental differences can cause large errors [16]. Thus, the speed trial analysis procedure can be harmonized with the model test procedure by the thrust measurement. Moreover, the scaling effect can be understood with the measured thrust additionally [17]. Secondly, in the operation view point, the thrust measurement can reduce the operating expenditure (OPEX) through guidance on hull and propeller maintenance. The propeller efficiency can be calculated from ship speed, propeller revolution, thrust and torque. The hull resistance can be evaluated by thrust deduction fraction and propeller thrust [18]. Thus, from accurate measurements of the propeller thrust, the reduction of propeller efficiency and the increment of hull resistance can be found separately. From the monitoring of the resultant propeller efficiency and hull resistance, the time of propeller polishing and hull cleaning can be decided easily, which may save OPEX compared with the regular hull cleaning and propeller polishing by the condition-based maintenance. In this study, to verify the results of thrust measurement using electrical and optical sensors, the relation of revolution, torque and thrust are compared with model test results.