The effects of wave height and forward speed on the seakeeping performance of a small fishing vessel in irregular waves are evaluated using computational fluid dynamics (CFD). The wave height effect changed linearly for a forward speed in the head sea and beam sea. In the stationary state, the heave and roll motions attributed to the wave height appear nonlinearly. The effect of the speed showed a non-linear shape wherein the heave motion became larger with an increase in the forward speed in beam sea. The seakeeping performance of pitch motion is greatly improved at forward speed rather than in a stationary state. The seakeeping performance of the roll motion is more dangerous than the pitch motion, regardless of wave height and vessel speed. The mean roll period in irregular waves is obtained through this study, and it is longer than the natural roll period in still water. It is necessary to be careful as the probability of exceeding the limit is high and GM is decreased in transverse waves.

Fishing vessels with a length $\left({\mathrm{L}}_{\mathrm{BP}}\right)$ of less than 100 m are generally not required to comply with the mandatory IMO Ship Maneuverability Standards. Therefore, an analytical method using empirical formula is preferred rather than a model test, which consumes a lot of time and monetary resources in estimating the maneuverability at the design stage. However, most empirical formulas have been derived from the model test results of merchant ships, and in the process, estimation errors may occur when hull-form parameters ($\mathrm{L}/\mathrm{B}$ or ${\mathrm{C}}_{\mathrm{b}}\mathrm{B}/\mathrm{L}$) with high correlation are applied to fishing vessel hull form whose characteristics are different from those of merchant ships. Therefore, a modified empirical formula was derived from previous research by including major parameters of fishing vessel hull form in the Kijima 90 empirical formula. In this study, maneuverability of stern trawler hull form is estimated for validating a modified empirical formula. The study confirmed that including characteristic parameters of the fishing vessel hull form in the empirical formula developed for merchant ships could improve the accuracy of estimation.

Because of the relative location between the ship and the bank, the fluid flow becomes complicated such that unstable hydrodynamic forces result in the yaw movement of the ship in an unintended direction. To consider the nonlinear effect, this study calculated the lateral force and yaw moment of barges with different shapes in confined waters, using computational fluid dynamics (CFD). We analyzed the effect of the reflected waves from the bank on the barges. The sway force tended to increase as both barges became closer to the bank, because it worked as a suction force that pulled them toward the closest bank. The yaw moment increased as the barges became closer to the bank, regardless of the shape of the bow. At y′ = 0.2B, when the barges were at the closest to the bank, it rapidly soared. The wave pattern showed that the diverging waves from the shoulder did not disperse, and were blocked by the bank and returned to the ship; such phenomena resulted in changing the hydrodynamic force on the barge. It is determined that the effect of free surface must be considered when conducting a comprehensive analysis of the bank effect when the ship is close to the bank.

To accurately estimate the navigation safety of a barge in still water and waves, computational fluid dynamics (CFD) was used to calculate the hydrodynamic force on the barge and simulate the flow field to comprehensively study the effect of skeg for wave drift force and directional stability of the barge. With respect to the wave drift force, the steady surge force and steady sway force are found to decrease due to the skeg effect in both short and long wavelength regions. The overall steady yaw moment tends to be reduced by the skeg. As the drift angle increased, the sway force and yaw moment also increased. The changing trend in the sway force varied with the angular velocity, depending on the installation of the skeg, and it was noted that the yaw moment value significantly increased because of the skeg. Owing to the effect of the skeg installed on the barge, the yaw damping lever became larger, while the sway damping lever became smaller regardless of waves. It was confirmed that the directional stability was improved both in still water and head waves.