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Nowadays the theme of pollutant emissions from ships is constantly becoming of greater importance because of the increasing awareness of the human health and environmental concerns of shipping, and the consequent legislative actions taken on international and national levels. Although the diesel engines represent the majority of the prime movers used in the ship propulsion plants, in the last years marine gas turbines have registered a growing diffusion in both civilian and, mostly, military applications. For these applications, where the gas turbine may be combined with other different types of prime movers, it could be important the availability, in a design phase, of numerical methods able to simulate the propulsion plant behaviour, including the pollutant emissions evaluation, also in off-design and transient conditions. In this paper, the authors propose corrections to improve some classic empirical equations, published in literature, for the calculation of the main gas turbine polluting substances. The improvements of the correlations have been based on the availability of a reliable and detailed database concerning the pollutant emissions, determined in a wide range of working conditions of a naval, aero-derivative gas turbine. As a first application of this study, the improved emission correlations have been inserted into a numerical simulator for the CODLAG propulsion system of an Italian frigate, presently under construction. The simulation results, regarding the main pollutants produced during some manoeuvres of the frigate, are presented and discussed in the paper