Development of a custom wireless dynamometer for an outboard motor propeller performance assessment

The measurement of the loads developed by the outboard motors, in terms of torque and thrust at design speeds, are fundamental quantities for determining the propulsive efficiency of the designed system and are the starting point for subsequent improvements. The possibility to assess these quantities in operative, behind hull, conditions can be a great improvement to the accuracy of collected data and a valuable contribution to the understanding of hull-motor-wake-propeller interactions. This paper describes the development of a completely redesigned outboard motor lower case, housing a dynamometric system providing the real time measurement of thrust and torque developed during scaled model, self-propulsion tests. The experimental setup has resulted in the new design of an existing lower case able to house a custom thrust/torque measurement shaft and a tailored highly integrated acquisition system, all rotating together with the propeller. The transducing device is a machined 17-4PH steel rod, sensitive to axial load and torque around its main axis, instrumented with foil strain gauges. A miniaturized acquisition board, positioned in a small waterproof chamber at the end of the measuring shaft, has been developed, which is able to provide signal conditioning, amplification and wireless data transmission. This activity, to the authors knowledge, is one of the few attempts reported in literature for the monitoring of the propeller thrust and torque of a self-propelled, outboard motor driven, model. The system provided valuable data for the quantification of those physical quantities in behind hull conditions providing a non-invasive, real-time measurement in almost real operative conditions.