This paper reports the results of manoeuvrability experiments (spiral and zig-zag tests) of three small tankers, which are course-unstable. Experiment were carried out those with or without skeg, and with single or twin rudders. The major conclusions obtained are : 1. The effect of skeg is notable. The skeg of which area is about Ld /45 reduces the spiral loop width from 910° to 12°. 2. The difference between single rudder and twin rudders is not so larger, as far as the results of experiments concerned. However, the ship with twin screws and twin rudders is better than other ships, when steering near the wharf. 3. Even if an unstable loop exists in γ'δ curve, steering ship is not always difficult, and 23°is allowable in the case of these examples with V / L =0.090.1 (1/sec). But steering ship becomes difficult when the spiral loop width is over than 910° 4. In conclusion, ships of these types with twin screws, twin rudders and skeg are best in steering.