Impact of sea waving at fluctuation of instantaneous angular speed of ship’s propulsion shaft

The paper presents results of analysis how sea waves affect variation of angular speed of a propulsion shaft of the ship during sea passage. That information is necessary to evaluate the level of credibility of measurements taken as reference value for detection of main engine combustion failures and evaluation of cylinders’ contribution to total power based on shaft’s angular speed irregularity observations. Utilization of IAS (Instantaneous Angular Speed) for diagnostic purposes is mostly based on comparison of actual state measurement with characteristics taken during healthy engine run in certain outer conditions. All variations of atmospheric pressure and air temperature have noticeable impact at instantaneous value of angular speed, they and must be considered during comparison procedure. For marine propulsion set, two weather deriving factors, having impact at angular speed of the shaft, can occur. First one is mentioned above values of air temperature and air pressure, the second one is torque variations caused by changes of propeller’s draught due to pitch of the hull. Any deviations from sampling course of angular speed are treated as malfunction symptoms. The aim of analysis was to evaluate how pitching frequency affect run of IAS and how whether omitting of that impact can lead to wrong conclusions.