Understanding Ship Fin Stabilizers and How They Work

Understanding Ship Fin Stabilizers and How They Work

Fin stabilizers are located at the bottom of the hull and their function is to resist excessive rolling of the ship in any direction. Many sailors have never seen a real fin stabilizer until the ship arrives in dry dock for a hull inspection and survey. But this does not detract from the knowledge that every ship engineer and officer should have about the components of the fin stabilizer system and how it works.

Let's break down the main components of a ship's fin stabilizer.

1. Two fins that form part of the hull, one on the port side and one on the starboard side.

These fins are attached to the fin stabilizer compartments located on the port and starboard sides and are operated/controlled from the compartments.

Each stabilizer contains a fin box that houses the fins and supports the operating mechanism of the fins. Each rigging unit provides the support and tools required to install, set and tilt the skeg (up to +/- 25 degrees).

2. Two hydraulic power units located in the main rigging room and the jib rigging room are used to control the pitch of the skeg and the rigging movement.

3. Bridge Control Panel (BCP) provides control and indication for remote operation from the bridge. This panel is installed in the Ship Control Center (BMCC).

4. Two Local Control Units (LCU) are provided in each rigging room. These panels provide local indication and skeg control.

5. Main Control Unit (MCU): The panel controls the skeg through the Electronic Control Unit (ECR) and has the same function as the bridge control. The MCU contains one LCU display for both left and right sides.

6. Roll Motion Sensing Unit (RMSU) measures the roll acceleration of the vessel and provides control signals for the roll control algorithm and interacts with the Main Control Unit (MCU).

The Fin Angle Control Unit (RMSU) consists of solid-state sensors and signal processing electronics. The sensors are located in the Microcontroller Unit (MCU).

7. The Fin Angle Feedback Transmitter transmits the fin angle position signal to the Servo Control Unit. The fin angle indicator is mechanically connected to the tilt cylinder piston rod and the fin angle transmitter displays the fin tilt angle.

8. The stroke control unit (SCU) is mounted on the variable pump. This unit provides operation and position feedback to the pump control shaft.

Working principle

• The hydraulic power unit (HPU) drives the fin tilt and water filling action. The HPU contains a high-power electric motor that drives a variable piston pump and a tandem vane pump for pressure control. Another vane pump is driven from the other end of the main engine to fill the fins and supply oil to the pump.

• The variable piston pump controls the fin pitch. The flow rate of this pump is controlled by a rotary valve mounted on the stroke control unit.

• The pump control unit (SCU) consists of a servomotor and gears, with an integrated resolver, mounted in a rectangular cast housing. The LCU contains circuitry that controls the pump control unit through a microprocessor, which in turn controls the flow direction and flow rate of the pump. The flap speed is proportional to the pump flow rate.

• The installation of the flaps is controlled by solenoid valves. The HPU also allows the flaps to be retracted in an emergency. Another (less powerful) electric motor is connected to a fixed displacement gear pump. This pump is connected to a set of auxiliary solenoid valves, which are used to set the flap angle to zero and install it.

• Operation can be controlled from the cockpit or from the Electronic Control Unit (ECR). A selector switch on the microcontroller (MCU) control panel switches the unit to the active control station in the event of a failure or disconnection of the Pump Control Point (BCP).

Before operating the fin stabilizer , the following preparations must be made:

1. There are no obstacles in the moving parts of the stabilizer.

2. Cooling water flows through the heat exchanger of the hydraulic power unit.

3. The local power isolation switches of all control units and pumps are switched on.