Explanation (6 marks)
- Construction:
- Active fins are mounted externally on each side of the ship, usually amidships below the waterline.
- Each fin is shaped like an aerofoil and is connected to a hydraulic actuator that alters its angle of attack.
- Controlled by a gyroscopic roll sensor and control unit.
- Operation:
- When the ship rolls to one side, the control system detects the roll angle, roll rate, and acceleration.
- The fins are rotated by the actuators so that water flow across them generates hydrodynamic lift.
- The lift force acts opposite to the roll motion, producing a restoring moment that damps the roll.
- The system operates continuously, adjusting fin angles dynamically to match sea conditions and ship speed.
- Fins can be retracted when not in use to reduce drag.
Hydrodynamic Principle (2 marks)
- The stabilising effect is due to the hydrodynamic lift generated by the fins:
- Lift force ∝ surface area of the fin × water velocity² × angle of attack.
- Properly phased fin angles ensure the lift is always counteracting the roll.
Sketch (2 marks)
Your exam sketch should include:
- Hull cross-section showing two fins amidships.
- Arrows showing direction of roll.
- Fins angled to generate lift in the opposite direction.
- Labelled parts: fin, hydraulic actuator, control signal from gyro sensor, hydrodynamic lift force.