In recent years, growing interest in engine efficiency, because of international regulations in the maritime sector, necessitates studies about new operational techniques. Ships have been operating at reduced speeds in recent years and, the liner temperatures of marine diesel engines vary significantly at reduced loads, which increase the heat loss from main engine. The research focuses on revealing the energy efficiency enhancement potential by decreasing heat loss through jacket water during reduced loads without any permanent modification. The heat loss reduction is achieved by means of higher coolant (glycol-water mixture) temperatures, to keep the liner temperatures at their maximum permissible temperatures. The methodology is calculating the energy balance and efficiency increment at reduced loads of main engine by modelling. The model have been validated with the digital twin of a two-stroke Sulzer-12RTA84C marine diesel engine. The simulated results comprise in-cylinder pressures, exhaust gas temperature and pressure, scavenge pressure, engine speed, turbo-charger, indicated power and heat transfer rate. Liner temperatures, affecting the heat transfer rate, are investigated in variable operating loads of the main engine. The study shows that keeping the liner temperature of the engine at the maximum continuous rating's temperature, has respectable efficiency advantages under different operating loads. 0.5% reduction in fuel consumption could be achieved, coming with 127.8 tons fuel and 398 tons CO2 reduction in a year. Additionally, waste heat recovery system calculations were carried out and additional 48.8 tons of fuel could be saved in the generation of electricity. In total 176.6 tons of fuel and 550 tons of CO2 emission reduction could be achieved. As a result, the controlled cylinder wall temperatures can be considered as one of the methods to solve the efficiency and emission problems in ships in the near future.