A statistical approach to zero adjustment in torque measurement of ship propulsion shafts

•The ship propulsion shaft torque measured on stopped engine varies significantly.•The residual torque after the propulsion unit stoppage is not constant.•The ship propulsion shaft strain relaxation period usually lasts a few hours.•The statistical method of zero adjustment allows for random error minimisation. Propulsion shaft torque measurement is essential in ship energy efficiency evaluation and emission control. Its accuracy and uncertainty influence decision making in terms of voyage optimisation and fuel consumption. For best results, the measuring device should be adjusted and calibrated adequately. This paper analyses the problem of ship propulsion shaft torque meter zero adjustment. Presented results of initial investigation suggest significant unrepeatability of the torque observed when the propulsion unit is at a standstill. Using a recently designed telemetry torque meter and an older analogue device a series of experimental research was conducted on board of a ship and on marine propulsion units installed in a laboratory. A new statistical method for zero adjustment is proposed and evaluated against methods commonly used on ships. The survey results suggest that the new method is burdened with a lower random error compared to presently used methods and uniquely allows for uncertainty evaluation.