Fishing boats suffer from much capsizing as compared with other vessels. It may be pointed out that the stability of fishing boats tend to be poor as the bases of the severe operations and the poor management of loading condition as well as the problem with the restrictions of principal dimensions of ship. According to the recent capsizing of the trawler “Ryuho-maru V”, Ministry of Land, In-frastructure and Transport of Japan forced to check the stability of existing fishing boats and to improve the stability if not enough. As for the improvement of the stability, the fitting of a bulge on sidewall of the vessel is one of the best solutions. This method was adopted in the old naval fleets and polar research ship “Sohya” in Japan. However, it is not commonly used. It is important to clarify for the design of the bulge what influence it has on the ship's performances not only on the stability. In this paper, the authors have investigated into the effect of the bulge on manoeuvrability and the manoeuvring prediction, based on the model experiments and the full-scale experiments of original and enlarged fisheries research ship “Ushio-maru”. They have also investigated into the predicting method for a fishtail type high-lift rudder is proposed. The concluding remarks are summarized as below. (1) By fitting the bulge on sidewalls of the vessel, hydrodynamic sway damping lever is slightly reduced. In addition, the yaw damping lever is reduced by the increase of ship's mass, which keeps the almost the same stability lever of course keeping as the original ship. (2) For the prediction of the hydrodynamic derivatives of the fishing boat with an initial trim, Kijima's formula are useful except Y'_β, N'_r, Y'_r-m'_x, using the effective draught and block coefficient of the ship. It is desirable that Y'_β, N'_r, Y'_r-m'_x are not corrected by the initial trim. (3) For the simulation of the manoeuvring ship motion with the fishtail type high-lift rudder, the rudder open characteristic is to be altered in the proposed mathematical model. The coefficient becomes 1.4 times larger than the conventional rudder. Even for the large rudder angle such as 65°, the conventional rudder model is still available, because the actual angle of attack is decreased by the turning motion of ship.