## 3 Result and Discussion
### Optimizing the Vessel's Operation Condition
Based on actual measurement data, the speed, fuel consumption, and CO\({}_{2}\) emission factor are classified according to the rpm. The total sailing duration and the total CO\({}_{2}\) emissions for each rpm range can be calculated according to the amount of fuel consumed in the corresponding duration. According to the calculation formula shown in Figure 12, the total operating time and CO\({}_{2}\) emissions according to the rpm range of the five experimental ships in the Far East (Ulsan, Republic of Korea) to the Middle East (Ras tanura, Saudi Arabia) operation  
Figure 11: Sailing route for simulation on Google Earth. Figure 12: Total CO\({}_{2}\) emission calculation method as rpm range variation. section are shown in Figure 13. It is a general fact that the lower the rpm, the longer the operation time. But, as shown in Figure 13b, lower rpm does not mean that the total CO2 emissions are lower. This appears to be because fuel consumption does not have a completely linear relationship with rpm changes. In other words, it means that lowering the speed unconditionally does not lower the total CO2 emission. The main purpose of shipping is to transport goods for profit. Therefore, shipping companies operate their vessels under optimal operating conditions, taking into account operating time and fuel consumption. However, as global warming is currently accelerating due to the indiscriminate use of fossil fuels, it is not considered appropriate to operate vessels by approaching only from an economic perspective. However, setting operating conditions considering only an environmental point of view is not suitable for the purpose of operating vessels. In other words, vessels must be operated taking into account both economic and environmental standpoints. This study presents a method to set optimal operating conditions as follows. First, based on constant speed operation conditions on the high sea, the ratio of the operating time and total CO2 emissions corresponding to each rpm range is set as a factor. The lower the rpm, the longer the operation time, so the factor value becomes lower. As for the factor for total CO2 emissions, the smaller the amount, the higher the factor value. As shown in Equation (5), an operation coefficient is derived by multiplying the two factors calculated for each rpm range, and the rpm range in which the highest operation coefficient is derived is proposed as the optimal operating condition. \[CF_{OP}=F_{SD}\times F_{CO2} \tag{5}\]  
where FSD = sailing duration factor; FCO2 = total CO2 emission factor; and CFOP = operation coefficient. Figure 14 shows the operation coefficient results according to each rpm range change in the simulation operation section of experimental vessels SSD 1-2 and HSD 1-3. Range 3 for SSD 1, range 2 for SSD 2, range 2 for HSD 1, range 6 for HSD 2, and range 3 for HSD 3 were found to be the optimal operating conditions considering both economic and environmental perspectives. The results shown in this study are the optimal operating conditions and are not applicable to all types of vessels. However, as in this study, it is necessary to suggest the optimal operating conditions for each vessel that can satisfy economic aspects and protect the environment under various conditions. As shown in Figure 14, the optimal operating conditions are expected to be different according to the shape, tonnage, engine type, and power of each ship. Therefore, it would be practically appropriate to calculate the optimal operating conditions of the vessels by approaching it from various perspectives rather than simply lowering the load and operating economically. Figure 13: Sailing duration (**a**) and emission as rpm range variation (**b**).