PURPOSE: To obtain a navigation distance per unit fuel amount by finding the pulse ratio of pulse rates of speed pulses from a speedometer provided to a ship body and flow rate pulses from a flow meter in a fuel conduit.

CONSTITUTION: The pulse oscillator 14 of a turbine type flow meter 11 generates one pulse at every interval of specific navigation distance and this speed pulse is rectified by a waveform shaping circuit 15 and then sent to an AND circuit 16. The fuel flow meter 6 generates flow rate pulses at a pulse rate proportional to fuel consumption; while they are integrated by a counter 18, a gate signal proportional to fuel consumption time per unit amount is generated by a gate signal generating circuit 10 and sent to the AND circuit 16. Therefore, the output of the AND circuit 16 is proportional to the navigation time within the time when the unit amount of fuel is consumed, i.e. fuel rate. The navigation speed measured by the turbine type flow meter 11 is a relative speed to the sea surface 2, so the optimum fuel rate is obtained as a parameter which shows the efficiency of a prime mover 7 and a screw 9.