Anti-Rolling Devices

Q. What are the different anti-rolling devices used onboard ships?
Ans. Different anti rolling devices include the bilge keel, anti-rolling tanks (active or passive), stabilizing fins (retractable & active).

Q. What are the objectives of various anti-rolling devices?
Ans. They either reduce the amplitude of roll or increase the roll period or do both.

Q. In what way the bilge keel accomplishes the two objectives?
Ans.Bilge keel virtually acts to form a tank outside the width of the ship & thus, in rolling, in a way, it lifts or lowers the water in that tank. This is, loading away from the roll axis. This increases the radius of gyration or in effect, mass moment of inertia and thereby increasing the time period of roll. The bilge keel also increases the hull area & resistance to roll, thereby reducing the amplitude of roll.

Q. How do the partially filled anti roll tanks accomplish these objectives?
Ans.Firstly, the free surface in both the tanks (port & starboard), causes virtual loss of GM. Roll period is inversely proportional to the square root of GM. In general, the reduction in GM due FSC increases, the roll period. The size of these tanks is a function of the size of the ship. If tanks are full, no objective will be met.

Because the tanks are partially filled, there is always flow of liquid, one or the other way as the ship rolls. The size of the connecting pipe between the two tanks causes the phase lag between the rolling of ship & the cycle of ‘liquid transfer’ between the two tanks. Thus, as the vessel rolls from starboard extreme to upright posture, during this phase, the liquid still flows to starboard in spite of vessel rolling in opposite direction (towards port). Thus, the listing moment acting in a direction opposite to roll curtailing the amplitude of roll.

Moreover, the positioning of tanks close to shipside also increases radius of gyration or mass moment of inertia, increasing the roll period.

Q. In what way is the active anti rolling tank system different from the passive one?
Ans. In an active system of anti roll tanks, there is a sensor employed to sense rolls, normally, based on the precession principle of gyro. The gyro senses the roll to eventually direct actuator. Actuator, in the case of anti roll tanks system is an impeller, placed in the middle of connecting pipe, which runs clockwise or anticlockwise. This means the impeller can be used to throw the liquid or to pull the liquid based on the results from roll sensing.

Q. In what way is the active fin superior to the retractable fin system or bilge keels?
Ans.In active fin system the pitch of the fin changes & the oncoming water is deflected upwards or downwards causing a thrust (lift) in downward or upward direction, causing the ship’s body to move in downward or upward directions, respectively. (A system perpendicular to the rudder system). Since, this system is active (not passive), the efficiency increases due to the inputs by the sensors, in respect of sensing the roll.

Q. How is the gyro used as sensor in anti roll system?
Ans. Precession is one of the inherent properties of a gyroscope. The precession occurs in a direction perpendicular to the plane of applied torque. The rolling is a rotatory motion. This tendency of ship to roll provides torque to gyro & in return the gyro axle precesses. The direction of precession by axle decides the appropriate signals to direct the actuator. As he direction of roll changes (CW to ACW), the direction of precession reverses.

Q. Why a swimming pool cannot be considered a safe & approved anti rolling device?
Ans.As the water sloshes in the swimming pool the sloshing period would match the ship’s roll period, which is not of any use. Phase difference between the slosh cycle and the rolling is necessary (as discussed in case of anti roll tanks). Moreover, the centerline position not involving loads near ship side will not serve the purpose to increase radius of gyration.

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