Effects of strut cover and lower vertical fin in podded propulsion units on ship maneuverability

Abstract

To investigate the effects of a strut cover and lower vertical fin in podded propulsion units (POD units) on the maneuverability of a POD-driven ship, free-running model tests (turning test, spiral/revers spiral test, and zig-zag maneuver test) were conducted for three different profiles of the POD units: only a vertical strut without the strut cover or fin (F0), smaller strut cover and smaller fin (FS), and larger strut cover and larger fin (FL). The profile area increases in the order of F0, FS, and FL. The tests were conducted using two propeller positions: one with a propeller equipped in front of the POD units (PULL position) and another with the propeller equipped behind the POD units (PUSH position). The steady turning performance at a steering angle of 35° in the PULL position deteriorated in the order of F0, FS, and FL. In contrast, in the case of the PUSH position, the turning performance improved in the order of F0, FS, and FL. Thus, the PULL and PUSH positions demonstrated different tendencies. The non-dimensional steady yaw rate exceeded 1.0 in all cases, which indicates that the present ship has an excellent turning performance. Furthermore, captive model tests (the POD steering force test in straight towing, oblique towing test, and circular motion test) were conducted to determine the effects of the strut cover and lower vertical fin on the lateral force and yaw moment acting on the ship model. It was found that the large strut cover and fin increase the steering force and improves the course stability of the ship in both the PULL and PUSH positions.