**3. Experimental Works**

Experiments were conducted in the Marine Renewable Energy Laboratory (MREL) at Tianjin University. The scaled twin-propeller system consisted of two single propellers. Two propellers were fixed at the shaft to rotate at the desired rotational speeds in a tank. The experimental setup, measurement method and 3D printed propeller model are discussed in the following sections. The rotational directions of twin-propellers were controlled to rotate internally and externally as internal rotation and external rotation. The propeller at the port side (left from the aft) rotated counter clockwise, while the propeller at starboard side (right from the aft) rotated clockwise to form the external rotation system. Most of the ships used the external rotation system to prevent flow compression in the clearance between two propellers with strong internal mixing jet. The observation of the development of a scour profile with time was made and followed by the measurement of the scour depth.
