A U-tube water tank is first designed to roll a ship floating on still water. The 6-degree of freedom (DOF) dynamic model of U-tube tank is derived and the effects of its parameters on ship roll motion are studied. Numerical simulations show that the U-tube tank is an effective stimulator to roll the ship on still water. For a rolling ship, the U-tube tank can be used as a damper to reduce ship roll motion quickly.; Active control of ship roll motion with a proportional and derivative (PD) controller, linear quadratic regulator (LQR), generalized predictive control (GPC), and deadbeat predictive control (DPC) is studied using a U-tube water tank as actuator is studied. For the predictive control, system identification is applied to update the parameters of the linear ship roll model with a U-tube tank when the ship dynamics changes. Numerical simulations show that GPC has the best performance and the U-tube tank is effective in ship roll mitigation.; Nonlinear ship roll mitigation with passive U-tube tank, U-tube tank using feedback linearization with completely known system parameters, and U-tube tank using adaptive fuzzy feedback linearization control with unknown system parameters, are also studied. In numerical simulation, a passive U-tube tank and feedback linearization help to reduce ship roll motion and capsizing compared to a ship without the U-tube tank. Feedback linearization is the most effective means of controlling ship roll motion, and adaptive feedback linearization is more effective than a passive U-tube tank.
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