The present study aims to investigate the hydrodynamic effect of the condition of the tank bottom, e.g., false bottom vs true bottom, on the KCS container ship under static drift and static rudder configurations in shallow water (depth-draught ratio H/T = 1.2) using viscous CFD simulation with rigorous validations. The results of the grid sensitivity study for static drift in shallow water with a false/true bottom show that the grid convergence for forces, moment, sinkage, and trim are rather difficult to achieve in the solutions with the true bottom. Among the static drift cases, the computational results of forces, moment, sinkage, and trim generally show good agreement with the experimental data, provided that identical bottom configuration between the experiment and the simulation is set. Parametric study for the effect of gap size between the side wall of the towing tank and the side edge of the false bottom quantifies that the gap size of 10% ship length is still not narrow enough to approximate lateral force and yaw moment of the KCS as if it were in true bottom condition using false bottom. On the tank bottom, negative pressure magnitude and its region become larger and wider, respectively, for the true bottom than those of the false bottom at non-zero drift angle. This yields a difference in the wave profile along the hull at the suction side which results in the increase of lateral force and yaw moment with true bottom compared to those with a false bottom. The vortical structure developed between the hull and the tank bed is also affected by the difference in pressure magnitude and its distribution on the tank bed. Among the static rudder cases, the difference in pressure distribution is quite localized in the stern vicinity at the port and starboard of the hull. Yet it yields 7–10% difference in hull-rudder interaction coefficients with true bottom compared to those with false bottom although the blockage effect to the tank width is less apparent than the static drift configuration. In consequence, the shallow water effects are more pronounced using true bottom than false bottom at static drift and static rudder conditions. The present study indicates the importance of replicating the tank bottom condition in the CFD simulation similar to the experiment. [ABSTRACT FROM AUTHOR]