Propeller Characterization Testing of a Blue Robotics T200 Thruster

Abstract

Over the last few years, the T200 Thruster has become the most popular thruster in the world, propelling thousands of marine vehicles. Their reliability in fresh and seawater, power, compactness, and affordability have made the thruster widely adopted amongst researchers, scientists, and businesses [1]. Given the popularity of the relatively new thrusters, a more complete understanding has large implications for future research and commercial marine robotics applications. Blue Robotics has collected and published data for the thrust force, power consumption, and efficiency for the T200 in bollard conditions. Bollard conditions describe a propeller that is operating at zero advance speed and the only flow over the propeller is that induced by its own rotation [2]. Understanding the propeller performance in bollard conditions is beneficial as it provides insight into vehicles in low-speed conditions where total motor torque and pulling power are maximized. The first portion of this experiment was to validate Blue Robotics’ published bollard test data by testing the T200 Thruster. The thruster was attached to a stationary 6-degree of freedom (DoF) load cell and set to incremental rotational motor speeds to collect data on the force output as a function of rotational speed. An ammeter and voltmeter were connected to the system to collect current and voltage values to calculate electrical input power. The thrust force divided by the electrical input power was used to find an effective bollard efficiency of the thruster as a function of motor rotational speed. After collecting this data, we compared our results to the data published by Blue Robotics. Measuring the thrust force and efficiency in bollard conditions was the basis for the second portion of the experiment, collecting data for a complete propeller performance curve. Blue Robotics does not currently have the testing facilities required to create this curve, but the marine robotics community has expressed interest in the data since the thruster is used in applications other than bollard conditions. For the propeller performance curve, the thruster was attached to a tow carriage in a towing tank and set to a known rotational speed. The tow carriage is set to travel through the water at varying advance speeds to control the velocity of the flow across the propeller. The advance ratio ranged from 0.19 to 0.97 in increments of 0.19. 0.97 is the expected advance ratio where the thrust equals zero. Thrust force and current monitoring data were collected at different advance ratios, which is defined as the advance velocity divided by the product of the diameter of the propeller and angular speed of the propeller. The performance efficiency is measured as the mechanical output power divided by the input power provided to the propeller. The data was used to create a propeller performance curve, which shows the relationship between performance efficiency and the advance ratio of the propeller. Because the maximum speed of the tow tank is 1 m/s and the expected zero efficiency point occurs at the advance ratio of 0.97, 11.6 rev/sec was chosen for the motor speed for testing. The propeller curve for this rotational speed did not exhibit the expected behaviors. One possible reason for this is that 11.6 rev/sec is on the low end of the potential motor speed and running at such low speed could change the behavior of the motor. To test this theory, data was collected where the motor speed was 34.5 rev/sec. The data collected when the motor speed was 34.5 rev/sec followed the expected behavior of a propeller; however, due to limitations of the tow tank only a partial propeller curve could be created for the motor at the higher speed. It may be of interest to use a larger tow tank with a faster tow carriage to complete the propeller curve, building off of the curve established here.