###### Abstract  
Based on the analysis of the waste heat distribution characteristics of a typical ship two-stroke low-speed main engine (model: MAN 8865 ME-C8.6 HL, the specified maximum continuous rating SMCR: 21,840 kW) under different loads, two different types of organic Rankine cycle (ORC) systems, namely the basic system (BORC) and the preheated system(PORC), were constructed to recover the ship main engine's exhaust gas waste heat and jacket cooling water waste heat. Using the thermodynamic simulation model of the system, the main performance indexes, including net output power of the two ORC systems were studied with the variation of seawater temperature and main engine load, and the annual ship fuel saving and annual carbon emission reduction generated by the two systems were compared and analyzed. It was found that the maximum net output power of the BORC system and PORC system were 445.3 kW and 491.3 kW, respectively, when the ship's main engine load was 100%, and the outboard seawater temperature was 20 \({}^{\circ}\)C; the maximum thermal efficiency was 12.84% and 12.71%, respectively; under the annual operation, the fuel saving of BORC system and PORC system can be 456 tons and 510 tons, respectively, and the carbon emission reduction was 1416 tons and 1581 tons, respectively. The analysis found that the net output power of the PORC system is always greater than that of the BORC system. When the outboard seawater is lower, and the main engine load is more than 80%, the net output power difference between the PORC system and BORC system gradually expands, and the improvement of ORC system performance is more evident by adding a preheater. It can be concluded that when the ship was mainly operated in the sea area with low seawater temperature and the main engine was running under high load most of the time, selecting the PORC system to recover the waste heat of the main engine was more advantageous. s 2023 2023 6 July 2023 2023 6 July 2  
the organic Rankine cycle (ORC) technology offers distinct advantages in recycling low-grade waste heat, ensuring safety, and adaptability. Consequently, it is well-suited for recovering waste heat generated by a ship's main engine [3]. The waste heat produced by the ship's main engine dissipates primarily through various channels, including exhaust smoke, pressurized air, cooling water for cylinder liners, and lubricating oil. Among these, the largest share of waste heat is attributed to exhaust gas [4]. Currently, extensive research is being conducted on the retrieval of waste heat from the main engines of ships. [PERSON] et al. [5] employed a base-type organic Rankine cycle (ORC) system to capture and utilize the waste heat emanating from the turbocharger and exhaust gas boiler, which carries high-temperature exhaust flue gas from a cruise ship. By implementing a reheat ORC system, [PERSON] et al. [6] successfully harnessed the waste heat from a warship's power-generating diesel engine, rated at 1000 kW, resulting in a remarkable net output of 118 kW. [PERSON] et al. [7] investigated the key factors that influence the performance of the ORC system in recovering waste heat from diesel exhaust gas. The exergy analysis revealed the need for enhancing the utilization rate of waste heat, which can be achieved by augmenting the preheater or regenerator components. [PERSON] et al. [8] developed a comprehensive approach by establishing a thermodynamic model, an economic model, and a system optimization model for the ORC system used in recovering waste heat from ship diesel engine exhaust gas. They conducted a comparative analysis of the system's performance using six distinct working fluids, thereby examining the effects and efficiency of each fluid. Aside from the waste heat generated by exhaust gas, other forms of waste heat from the main engine also possess significant potential for recovery. Numerous research studies have put forward the implementation of diverse ORC systems to capture and utilize various types of waste heat produced by ship main engines. [PERSON] et al.