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Ship Fin Stabilizer

What is a Fin Stabilizer?

Ship Fin Stabilizer
Ship Fin Stabilizer

Ship Fin Stabilizers or rotors mounted beneath the waterline of the ship which emerge laterally from the hull of the ship to reduce a ship’s roll due to natural currents (wind or waves) are called Ship fin stabilizers. Active fins are generally controlled by a system called gyroscopic control system. If the gyroscope senses rolling of a ship, the angle of attack of the fins’ get changed which exert a force to counteract the roll.

Fixed fins as the name suggests do not move and bilge keels do not move either; they exert hydrodynamic drag which reduces roll when the ship rolls. Mostly ocean going ships use Stabilizers.

Fins principle of work is to produce a lift or downforce when the vessel is in motion at sea. The fins produce a lift which work against the rolling moment of the vessel. To accomplish this lift, two fins are installed underwater on either sides of the ship.

Stabilizers can be of 2 types:

  • Retractable stabilizers – When the fins are not needed, the fins retract into a space in the hull. They retract so that the fuel consumption and the hull clearance is reduced. Most Medium and Large Cruise ferry ships are fitted with Retractable Stabilizers.
  • Non-retractable stabilizers – As the Name suggest contrary to Retractable Stabilizers, Non-retractable stabilizers do not retract into a space in the hull, as usually there is no space on the hull. They are used mostly in small vessels such as a yacht.

The movement of the Stabilizer of a ship is similar to the movement of aircraft ailerons. Mostly the Fins installed on larger ships are provided with flaps, which help to increase the lift by 15%.  Numerous quick changing variables need to be considered by the stabilizer control: Ship Motion, Draft of the ship, Waves of the sea, Wind conditions, etc. High velocities are most effective to bring efficient operation of the Fin stabilizers. Low velocities of the ship are the least effective for fin stabilizers to operate.[1] The Stabilization solutions of a ship at anchor or at low speed include the methods of using rotary cylinders which employ the Magnus effect and using actively controlled fins (such as the stabilization at rest system developed by Rolls Royce[2] which oscillate to counteract wave motion). The latter two systems are retractable stabilizers, this enables for a thinner vessel profile when docking at a port and also reduces drag during cruising at the open sea.

The main purpose of a fin stabilizer on a ship is to give/supply resistance to the excessive rolling of ship at both directions. The fin stabilizers is located at the bottom part of the ship’s hull. A fin stabilizer is usually not visible to the naked eye, hence it is usually not visible to the people working on board during regular operations. They are only visible when a ship enters a dry dock for Hull surveys and inspections. It is very important for on-board engineers to understand and have a knowledge regarding components and functioning of a fin stabilizer system.

What are the components of the Fin Stabilizer?

  1. Two fins: They are a part of the hull structure of the ship. One is located at the port side and the other at the starboard. The fins are connected to the fin stabilizer room from where they can be operated/controlled. The Fin stabilizer room are located on both the port and starboard sides of the ship. Every stabilizer consists of a unique fin box. The fin box supports the fin actuating machinery as well as houses the fin. Every stabilizer machinery unit provides support. It also provides the means by which the fin may be rigged in or out of the fin box, the stabilizer machinery unit also enables the user to lock and tilt the fins up to +/-25 degrees.                                                                                                                                        
  2. Two hydraulic power units: The power for the fin to perform tilting and rigging motions is derived from these two power units. The Hydraulic Power units are located inside of the stabilizer room on both the Port Side and Starboard side of the ship.
  3. Bridge Control Panel (BCP): The BCP as the name suggests is a control panel that is installed on the navigation bridge of a ship in the BMCC. The panel provides controls and warning indicators for the operations of the ship’s fin stabilizers remotely.
  4. Two local control units (LCU): The LCUs are control panels which provide local control and warning indication while operating a ship’s fin stabilizers. The LCUs are located at the fin stabilizer room at both the port and starboard side.
  5. Main Control Unit (MCU): MCU is a control panel which provides controls and warning indications for the operations of a ship’s fin stabilizer which is located in the Engine Control Room (ECR) of the ship. The features of the MCU are the same as the features of the Bridge Control Panel. Both the starboard and port side display of the Local Control panel is present in the MCU.
  6. Roll Motion Sensor Unit (RMSU): RMSU is a device which measures the roll acceleration of the ship and provides the control signal for the algorithm which controls the roll. This data is then displayed on the Main Control Unit. The RMSU consists of electronics which condition the signal and a solid state sensor which is located at the Main Control Unit.
  7. Fin Angle Feedback Transmitter:  This device provides a signal to the servo controller about the position of the fin angle. The fin angle indicator is coupled mechanically to the tilt cylinder piston rod. This is how the fin angle transmitter shows the tilt angle of the fin.
  8. The Stroke Control Unit (SCU): This device is provides feedback of the position of the control spindle of the pump. It also helps in providing drive. The SCU is mounted on top of the variable delivery pump.

Operation of a Fin Stabilizer

  • A Hydraulic Power Unit (HPU) powers the tilting and rigging motions of the fin. A high power electric motor is built into the HPU which drives a variable delivery piston pump along with a tandem vane pump which is used to control the pressure. From the other end of the main motor A second vane pump is driven for replenishment of pump and fin rigging.
  • Fin tilting is controlled by a variable delivery piston pump. A rotary valve controls the flow rate of this variable delivery pump. This pump is positioned by the Stroke Control Unit (SCU)
  • The Stroke Control Unit consists of two components i.e. gears with an integral resolver, mounted within rectangular cast box and a servo motor. The Local Control Unit has circuits. These circuits provide microprocessor based control of the Stroke Control Unit, which eventually control the rate of flow and direction of flow from the pump. The rotational velocity of the fins is directly proportional to the flow rate of the pump.
  • The solenoid actuated valves control the Fin rigging. The means for stowage of the fin under emergency power is also provided by the Hydraulic Power Unit. A constant delivery gear pump is driven by a second electric motor (of lower power). An auxiliary set of solenoid controlled valves is connected this pump. These valves are used to tilt the fins’ angles to zero and then rig it inside the fin box.
  • These operations of the fin stabilizers can be controlled directly from the Navigation Bridge or the Engine Control Room. If the communication to the Bridge Control Panel or the Bridge Control Panel itself becomes defective, there is switch located on the Main Control Unit called a “Selector Switch”. This switch allows the Main Control Unit to become the active control station for the ship’s fin stabilizers.

Following preparation and checks must be done before operating a ship’s fin stabilizer i.e. before the Gyro fin Stabilizer System is started, the following confirmations should be given by the Engine Room:

  1. It should be made sure that the moving parts of the Stabilizer Machinery has no obstructions.
  2.  It should be ensured that the cooling water is flowing smoothly through the Hydraulic Power Unit     heat exchanger without any obstructions.
  3. It should be ensured that all local power isolation switches to the pumps and controls are turned ON.

Do Cruise Ships Have Fin Stabilizers?

Yes Cruise ships do have Active Retractable Fin Stabilizers. Cruise ships are designed for passenger comfort hence they do need fins which reduce the roll of the ship and reduce passenger discomfort.

The cruise ships however mostly work in the calm serene waters of the world which are lucrative for tourists which implies that the fin stabilizers are usually not in use. Hence they are retractable so that they can sit in the fin box when not in use to reduce fuel consumption.

However when the stabilizers have to be used they need to act quickly so as to keep passenger discomfort at a minimum for the least amount of time hence the stabilizers also need to be active i.e. they should have a gyroscopic control system which quickly senses the rolling moment of the ship and in-turn send a signal to the Roll Motion Sensing Unit or RMSU and control the fin angle thus reducing the rolling of the ship.

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