## 1 Introduction  
The today's cost of fossil fuels is constantly evolving. The present economic crisis, combined with political pressures, make the price of oil strongly uncertain. In addition, in recent years, polluting emissions produced by merchant ships have to be contained in increasingly restrictive limits to comply with the new regulations coming in force [1]. In this changing scenario, the need arises to explore the possibility of using alternative fuels, instead of the traditional oil (heavy fuel oil -HFO) and to increase the whole efficiency of the propulsion system. From the point of view of alternative fuels, the research points towards fuels with economically sound potentials and with limited impact on the environment. The technology development occurred in large gas engines during the recent years has achieved high thermal efficiency and low emissions utilizing liquefied natural gas (LNG), suggesting this fuel as a potential substitute of HFO [2; 3; 4]. Modern gas engines abate CO\({}_{2}\) by 30%, NOx by 85% (without after treatments), ensuring the complete absence of SO\({}_{\chi}\) compared to the correspondent engine, fed by liquid fuel oil [5]. In order to benefit from these advantages, the major engine manufacturers have designed several set of gas engine for both ship propulsion and land based power generation, like the Wartsila 50 DF series. From the point of view of increasing the whole efficiency of the propulsion system, energy recovery can be considered of the waste heat usually dissipated, for both improving the propulsion power or for producing useful heat on board the ship [6; 7; 8; 9; 10]. In this context, the use of Organic Rankine Cycles (ORC)as bottom power plant of the main propulsion engine appears a very attractive technology, allowing efficiency gains equal, or higher, with respect to other energy saving solutions, like the adoption of more complex exhaust gas boilers, heat pumps [11; 12], or a bottom Kalina cycle [13]. Many applications of this technology are made in land-based installations, such as biomass, solar and geothermal energy plant [14; 15; 16; 17; 18]. Recently, several studies have been conducted on the possible application of ORC groups coupled to diesel engines, for both stationary and propulsion applications [19; 20; 21]. Specific application to marine propulsion system can be found in [22], where different hydrocarbons and some refrigerant are considered as working fluids in both recuperated and non-recuperated cycles, in [23; 24], where different refrigerant are considered as working fluids in simple and superheated cycles, in [25], where the whole heat exchangers network is designed using the pinch analysis, and in [26], where an important excess of steam is supposed to be produced by the exhaust gas boiler and the energy recovery can be performed by a cascade, made up by a steam Rankine cycle and a ORC. A similar cascade energy recovery system is presented in [27], but the ORC is replaced by an absorption refrigeration machine. This study examines the options of energy recovery from an ICE for ship propulsion by adding a bottom ORC, with the aim of reducing the fuel consumption of the propulsion system during its operation. Different possible bottom ORC configurations are considered and simulated using the Aspen(r) code, and different working fluids are taken into account, jointly with regenerative and two-temperature levels designs, allowing the identification of the most suitable ORC configuration. The proposed analysis is based on the real operating data of a marine engine, considering the needs of its cooling system, and on the use of a realistic design/off-design simulator of the heat exchangers, available in Aspen(r). In this way the expectation is to obtain very realistic values of the actual performance increase of the propulsion system. Therefore, a robust preliminary economic and environmental evaluation of the real operation of the ship engine can be performed.