TABLE OF CONTENTS Bilge keels are the simplest of the roll mitigation devices. They are quite effective at reducing roll motions near resonance, where the most severe motions generally occur. However they add to the resistance of the ship, and are vulnerable to damage. There are numerous of other devices currently in use that are more effective, at the penalty of increased complexity. We will briefly introduce each of these here.
Passive devices are advantageous because they have no moving parts and require no power or control systems for operation. Popular passive devices in addition to bilge keels include free surface tanks and U-tube tanks (Figure 5.16). These anti-roll tanks are based on the fact that at their resonant frequency, the roll moment applied by the tanks to the ship is 180 out of phase with the roll velocity and thus increases the total roll damping. A disadvantage of these anti-roll tanks is that they reduce the roll restoring moment, like any tank that has a free surface. Imagine that the entire hull was filled with water: The hydrostatic pressure on the inner surface would cancel that on the outer surface, resulting in no net hydrostatic force or moment. If only a portion of the ship is "flooded", the effect is of course reduced proportionally. This effect leads to increased rolling due to the stabilization device at low frequencies, which may be a limiting factor on the size of the tank.
