Propeller torque is most commonly measured by strain gauge techniques.
The strain gauges are placed on the propeller shaft and measure the shaft elongation due to forces and moments.
The deformations are transferred into voltages which determine the strain on the shaft.
The typical torque measurements on a propeller shaft are in the order of 330 micro strains and the strain gauges are able to detect changes in the order of 1.5 micro strains, Wärtsila (2009).
The Maihak torque meter is measuring torque by strings (strain gauges) and RPM by laser, SICK/Maihak (2010).
Shaft rings are mounted and they are placed as close as possible to the main engine.
The measuring accuracy is in the order of 0.5 % and the update period of the measurements can be varied.
The RPM / Torque meter is calibrated at installation and depending on manufacture and type, various calibration intervals and methods are suggested.
The torque meter and RPM measurements are used to find the shaft power