The ship’s stabilizers are active underwater fins that are designed to reduce the amount of rolling that the ship experiences during an ocean voyage, where rolling is brought on by the effect that sea action has on the hull of the vessel. This becomes especially apparent when the frequency of the waves draws closer to the ship’s natural frequency. During a roll, actively reducing the amount of roll that occurs is accomplished by tilting the extended fin. The effect that the ship’s forward motion has on the surface of the fin produces a lifting moment that acts against the roll of the ship.
Some vessels are outfitted with two retractable tilting fin units, one on each side of the vessel. The tilting of these is accomplished by hydraulic units that are powered by electro-hydraulic pumps. When the fins are not in use, they are stored in fin boxes that are located in the hull. These fin boxes have been specifically designed to become a part of the hull and the structure of the ship, which helps to reduce the amount of drag that the hull experiences.
By folding the fin through an angle of 90 degrees, a hydraulic ram mechanism enables the fin to be rigged in and out (housed and extended).
Fins produce a hydrodynamic lift effect as a result of the flow of water over the hydrofoil shaped fins. One fin is in the nose up position, which exerts an upward thrust, and the other fin is in the nose down position, which exerts a downward thrust.
Because the up and down thrust is produced by the flow of water over the fin, the ship must be moving through the water at a speed that is at least sufficient to maintain the flow. If the ship is traveling at a speed of less than 12 knots, the fin stabilizers will not be able to do their job effectively.
The fins have a design similar to that of a symmetrical hydrofoil, and inside the housing for the fin shaft there is a rotating shaft that is supported by two bearings. Additionally, the housing for the fin shaft is mounted in two bearings, which together make it possible for the fin to rotate into and out of the fin box. The required housing for connecting the fin to the hull is provided by the steel casing, which also provides support for the bearing structure of the fin.
To enable the fin to be tilted in the desired direction, a hydraulic tilting cylinder has been coupled to the fin shaft. The trailing edge of the fin is movable, so it moves along with the rest of the fin and helps to increase the hydrodynamic lift effect. The tilting shaft has lip seals installed in it to prevent water from the sea from getting into the machinery and also to prevent oil from leaking out into the water.
A sliding block is rotated by the rigging lever through the use of a mechanical linkage, which is moved by a rigging cylinder. When everything is completely rigged out, this linkage will put the fin in a position where it cannot move. After the fin has been completely rigged in, a latch is used to secure the sliding block in its position.
The stabilizers’ operation is monitored and controlled to ensure that cavitation does not occur, which helps to ensure that erosion issues are avoided.
The vessels are equipped with a gravity header tank that is located above the water line in order to supply the machinery with lubricating oil. The header tank can be broken down into two distinct parts. The first section supplies LO for the mechanism that controls the fins, and the second section supplies LO for the rigging system. In the event that the seal fails to keep water out of the housing, the presence of the pressure caused by the positive head of oil will prevent seawater from entering.
The electro-hydraulic power unit can be found atop the fin housing box in its designated location. Each unit has a main variable delivery piston pump, a tandem vane pump, and an oil cooler. These three components make up the unit.
The tandem vane pump is used for controlling the pressure, rigging the fins, and replenishing the pump; the electrical motor that drives this pump is the same as the one that drives the main pump.
The oil that is fed to the tilting circuit by the variable delivery pump is of the axial piston type and is controlled by servo. The pump is responsible for delivering oil to the various chambers of the tilting cylinder. The chamber that is assigned to receive oil pressure is determined by the required attitude of the fin at that particular time and, as a result, the position of the pump stroke control unit.
An emergency pump that is powered by electricity and has manual controls is made available. In the event that the primary hydraulic unit fails to operate, this pump will be able to rig the fin into or out of position.
The emergency circuit has a safety valve and manual distributors, which allow the fin to be reset to the zero angle position, the fin to be rigged in or out, and the fin lock to be enabled. This is all made possible thanks to the emergency circuit.
LCD touch screens are utilized by the system in order to facilitate automatic control of the extension and storage (rigging) of the fins. The following three positions have the ability to exercise control over this:
- Bridge control panel, including operations for starting and shutting down the entire system.
- Main control panel (which can be found in the ECR): Used for starting up and shutting down the entire system, as well as controlling individual fins for purposes of maintenance.
- Local control panels: Individual fin control for maintenance purposes.
In the event that power fails, the control system also allows for the storage of the fins. This can either take place automatically upon the connection of power to the emergency motor starter or be controlled from the various local control panels located around the facility.
Must be remembered that if either the port or starboard LOCAL CTRL indicators are on, that fin cannot be controlled from the bridge. In the bridge and main control panels LOCAL CTRL is displayed if local control is on.
In the event that the stabilizer power is switched off with the fin not housed, the stabilizer control system will trigger the ship’s general alarm via the FIN NOT IN alarm relay.
The fin control can be transferred from the bridge to the main control panel in the ECR, even when the fins are in operation, by switching the main control panel selector switch to the MCU position.
It is important to note that when the fins are out, they extend beyond the beam of the ship and are angled downwards, with the tip near the keel line. As a result the fins can create a navigational hazard if extended when the vessel is operating in shallow or confined water.
Normally, the system provides the following features to reduce the risk of damage to the fins:
- Key locked power control.
- Power ON interlocks that can prevent operation of the stabilizers at the same time as the thrusters.
- Alarm indicators that appear on the bridge control panel if the ability to stow the stabilizers is impaired.
- Power OFF and fin NOT IN alarm contacts can activate an alarm if the key switch is turned off while the fin is out.
- An alarm that sounds when the ship’s speed drops below a preset level.
- The control system can be configured to automatically rig in the fin when the ship’s speed drops below a preset level.
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Source and Bibliography:
- YouTube video – Marine Insight
- YouTube video – SKF Marine