This doctoral dissertation provides results of a research on operational reliability and maintainability of a high speed radial diesel engine. To apply the general theory of reliability on engines type ‘’Zvezda’’ M 504 B2, which are the most important part of the propulsion subsystem of missile boats (RTOP 11 and RTOP 12), statistical analysis was performed and scientific analysis of a large number of data taken from practice and all with the aim of improving overall reliability of the whole Fleet. In the analysis proposed model of reliability of missile boats is one where it can be seen all of its subsystem, but practice has shown that the propulsion subsystem has the greatest impact on the overall reliability of these ships, and therefore on the war preparedness. Therefore, as a research subject selected is a main engine which had to be broken down in subsystem and all based on exploitation data and all in order to ascertain the reliability of each individual subsystem. The aim of this research was to determine a reliability function, failure intensity function as well as maintainability function of the engine in demanding operational conditions in the Adriatic sea and to compare the results with those obtained by different researches in order to perceive trends and their scientific relevance. It has been concluded that the Weibull distribution approximates the best the operational reliability and the expected trouble-free period of operation can be well approximated by the mean time between failures. It has also been found out that the normal distribution best approximates the empirical function of maintainability or engine reparability and the mean maintenance time complies with the mean engine repair time by subsystems. The research results can be applied to other similar boat engines as well as to diesel engines of different platforms. This doctoral dissertation connects the research results with scientific reports in the way that theoretical hypotheses from the literature are related to direct applications in order to improve the operational availability. Immediate usefulness is evident in predicting failures in vital subsystems and of different diesel engine parts and in the allocation of necessary ship repair facilities, financial needs and spare parts procurement as well as in improving the quality of training and qualification. Emphasize the importance of high-quality document management in order to improve the proposed model.