The paper describes the properties of semi-Markov processes and the opportunities and benefits from their use as models of operation processes for marine combustion engines and other machines of ship power plantshe importance of the theory of semi-Markov processes for development of the theory of marine combustion engines and other machines of ship power plants, as well as for development of progress of the machinessefulness of the theory of semi-Markov processes theory and operational practice of ship main engines and other machines of ship power plants to define reliability and safety indexes for the machinesonditions with regard to the properties of semi-processeswhich should be satisfied models of the real processes running during operation of marine main engines and other machinery. The suitability of decision (semi-Markov processes for making operating decisions is also proved herein, as their aim is to obtain a rational process of operation engines . the . The emphasis is put on tthe operational . The uin the is exposed, operational . There are defined the cMarkov , to enable the use of them as ship’s controlled) of the mentioned and machines.