To detect power of a main engine at the present point, a rotational speed per minute, a pitch angle of a variable pitch propeller while comparing them with the predetermined performance by inputting and displaying a signal from a pitch angle plotting means, a signal from a rotational speed-shaft power plotting means, and a signal from a predetermined performance plotting means in a display device.

A pitch angle is detected by means of an angle meter 3, computed under a pitch angle computing program 8, and displayed in a display device 16 under a pitch angle plotting program 9. On the other hand, a rotational speed of a shaft is detected by means of a tachometer 4, a rotational speed per minute is computed under a rotational speed computing program 10, torsion of the shaft is detected by means of a shaft torsion meter 6, torque is computed under a torque computing program 11, shaft power is computed under a shaft power computing program 12, shaft power-rotational speed per minute is plotted under a rotational speed-shaft power plotting program 13 so as to be plotted and displayed in a display device 16. On the basis of the previously inputted predetermined performance, the predetermine performance is plotted under a predetermined performance plotting program 14 so as to be displayed in the display device 16.