The tank is U-shaped and water moves from one side to the other and then back as the ship inclines first one way and then the other. Because of the throttling effect of the relatively narrow lower limb of the U joining the two sides of the tank, the movement of water can be made to lag behind the ship movements.
By adjusting the throttling, that is by ‘tuning’ the tank, a lag approaching 90° can be achieved. Unfortunately the tank can only be tuned for one frequency of motion. This is chosen to be the ship’s natural period of roll as this is the period at which really large motions can occur. The tank will stabilize the ship at zero speed but the effect of the tank’s free surface on stability must be allowed for.
You may also know about active tank :
The movement of water is controlled by pumps or by the air pressure above the water surface. The tanks either side of the ship may be connected by a lower limb or two separate tanks can be used. A system in which the air pressure above the water on the two sides is controlled to ‘tune’ the system.
The air duct contains valves operated by a roll sensing device. The system can be tuned for more than one frequency. As with the passive system it can stabilize at zero ship speed. It does not require any projections outside the hull.
The capacity of the stabilization system is usually quoted in terms of the steady heel angle it can produce with the ship underway in still water. This is then checked during trials. It is possible to use modern theories to specify performance in waves but this would be difficult to check contractually.