## 3 Results and Discussion
### Crude Oil Tankers  
As shown in Figure 6, the measurement systems were installed in the engine room. The strain gauge and optical sensors are mounted on propeller shafts between the propeller and the thrust bearing. The series of crude oil tanker with identical main dimensions, hull form and propulsion system were built by Samsung Heavy Industries. The speed trial of the first vessel was performed at designed draught and five sets of engine load (50%, 65%, 75%, 90%, 100% of MCR). The speed trial of the second vessel was performed at ballast draught and three sets of engine load (65%, 75%, 90% of MCR). The environmental conditions of the two vessels were moderate below Beaufort number 4. Table 1 shows the conditions of the speed trials in detail. The measurement results during the speed trial of the first vessel are shown in Table 2 and Figure 7, which provide the thrust, torque and revolution of propeller measured simultaneously. The thrust and torque from measurement during speed trials, the relations of revolution-torque and revolution-thrust, are compared with prediction results based on model tests. The characteristics of torque and thrust by the strain gauge-type system show a good correlation with those of the model test. The difference in averaged values of double runs indicates the thrust and torque are within 2.0% compared with the prediction results from model tests. The results of thrust and torque by the optical sensor show approximately 5% higher than model test and torque is 5% lower compared with the model test results. In the optical sensor, the results of thrust and torque measurement at the 100% of MCR show different characteristics from the other engine load. At the 100% of MCR, the thrust and torque show 0.2% and 2.3% higher than prediction result from model test. In the results from the optical-type system at 100% of engine load, there is a change in the zero value, which came from the automatic zero setting function of the system in case of stopping the engine. During the sea trials, there was an event to stop the engine, and the zero levels were changed automatically by the measurement system by 40 kN of thrust and 100 Kn-m of torque from the zero values set before the test. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **1 st Vessel** & **2 nd Vessel** \\ \hline Trial Draught & Designed & Ballast \\ Beaufort No. & 4 & 3 \\ Wind Speed & 5.0–9.0 m/s & 3.0–5.0 m/s \\ Wave Height & 1.0–2.0 m & 0.5–1.0 m \\ Engine Load & 50\%, 65\%, 75\%, 90\%, 100\% & 65\%, 75\%, 90\% \\  & of MCR & of MCR \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the speed trials of crude oil tankers. Figure 6: Location of the measurement systems. Table 3 and Figure 8 show the results of thrust and torque measurements during speed trials of the second vessel compared with prediction results from the model test. Thrust and torque by the strain gauge measurement system are approximately 5% and 3% lower than the prediction results based on model test. In the optical sensor, the results of thrust and torque measurement at the same revolution are approximately 3% and 7% lower than the model test results.