The paper presents the specificity of operation of propulsion systems of seagoing ships which causes the need to control the load on them, especially on their engines called main engines. The characteristics of the load on the propulsion systems, especially on the main engines as well as on the shaft lines and propellers driven by the engines, along with the process of wear in tribological joints (sliding tribological systems) of the machines have been described herein. Using examples of typical types of wear (both linear and volumetric) for the tribological systems of this sort, interpretation of states of their wear has been provided with regards to the wear levels defined as acceptable, unacceptable and catastrophic. The following hypotheses have been formulated: 1) hypothesis explaining necessity to consider the loads on the systems under operation as stochastic processes; 2) hypothesis explaining a possibility of considering the processes as stationary; and 3) hypothesis explaining why it can be assumed that a given technical state of any tribological system can be considered as dependent only on the directly preceding state and stochastically independent of the states that existed earlier. Accepting the hypotheses as true, a four-state continuous-time semi-Markov process has been proposed in the form of a model of changes in condition of a propulsion system (PS) of any ship. The model includes the most significant states affecting safety of a ship at sea, such as: s(0) - PS ability state, s(1) - PS disability state due to damage to the main engine (ME), s(2) - PS disability state due to damage to the shaft line (SL) and s(3) - PS disability state due to damage to the propeller (P). Probability of occurrence (changes) of the states has also been demonstrated.
