Concerning on navigational safety of a ship, comprehensive investigation of manoeuvrability of the ship is prominently required. The turning instability due to improper speed and magnitude of the rudder angle is vulnerable to serious accidents such as collision especially in the confined waters. This paper presents a computational fluid dynamic analyses on manoeuvrability performance of a tug in calm water. Here, the characteristics of the turning ability and zig-zag characteristics of the tug has been assessed due to effect of the various angles of twin-rudder and turning speeds. The results revealed that the increase of rudder’s angle resulted in subsequent reduction of her advance diameter from 144 m, 108 m, 96 m to 92 m. While for zig zag manoeuvre, the first overshoot angle is 0.6° and 1.08° for 10°/10° and 20°/20° rudder’s angle respectively. The first overshoot and second overshoot angle are within the IMO criteria which is below 20° and 25°. However, the increase of turning speed from 7 knots to 9 knots has been proportional with the increase of the turning diameter (advance diameter) from 70 m to 105.2 m. Basically, the turning performances of the tug manoeuvring with the turning speed of 7 to 9 knots incorporated with rudder’s angle 20°, 25°, 30° and 35° have been complied with IMO manoeuvring standards. This preliminary analysis contributes very valuable findings at early ship design stage to provide a safety of the navigational guidance for turning ability of the tug.