Rudder angle indicator | Definition, Working and Circuit Diagram

A device to display the current position of the rudder installed in the control house of the bridge. In the angle indicator system Rudder is independent of the steering system and is only for the display. There is an IMO requirement which states regardless of the steering control system there must be installed steering indicator system installed on board. The specification of the rudder angle is required for each cockpit and steering position in the rudder emergency room.

The various regulatory authorities vary the requirements of the Rudder Level Indicator (RAI). DNV needs a second angle indicator independent bar on deck. Regulation of the Panama canal requires large instruments (min 192x192mm) Bridge, visible wing operator and IMO pull to train / MED (ISO 20673) requires that the accuracy of the system is over a degree.

Rudder angle indicator consists of a control unit, a transmitter and receiver. This includes indicators with different dimensions, different scales and rudder angle display.

A rudder angle indicator consists of a transmitter and a receiver direction of the rudder rudder. The emitter is connected to the steering head by means of a lever, etc. in the wheel housing. Communicates rudder direction and rotation angle for self-synchronization of the machine in the sender and causes the self-synchronization of the machine, in the receiver that I have in the wheelhouse or another that is for with their directions of synchronicity. Self-synchronous transmitters and display units used in the control angle display systems are manufactured by various companies refer to names such as Selsyns, Synchro, and Automatic Syns Telmotors.

Non Follow-up Steering System

Control equipment – conveys a signal of the desired rudder angle from the bridge to the steering flat where it is received to activate the power unit and transmission system until the desired rudder angle is reached. This equipment can be of 2 types (1)hydraulic telemotor systems (tele- means far away in Greek & motor means motion or movement) & (2) electrical electronic control equipment.

Hydraulic telemotor systems:  The telemotor employs master and slave principle. The transmitter is situated on bridge and the receiver at the steering gear unit. Mechanical movement is transduced hydraulically or electrically for distance telemetering and is then transduced back again.

Auto & Follow up Steering System

                            

Hydraulic Transmitter: As the bridge steering wheel is moved to starboard the rotating pinion causes the RH ram to move down, pushing oil out to the receiver unit along the RH pipe. The LH ram moves up, so allowing a space for oil to come from the receiver unit. The fluid being virtually incompressible, any down movement of the RH produces an identical movement at the receiver unit. This in turn displaces the same quantity of fluid which is taken up in the extra space created by the LH ram moving up. The fluid in the replenishing tank acts as reservoir. The casing is usually gunmetal with bronze rams, and copper pipes are led in by drilled leads in the casting. A device (called bypass valve) is required in the system to allow for variation in oil volume due to temperature changes and also to allow for equilibrium between both sides of the system. This bypass valve also has function of topping up the system in the case of leakages and acts as relief valve in case of pressure rise.

Bypass valve: Operation can only be carried out when the wheel is in the mid position. This is achieved by having the operating rod butting against a circular disc, in mid position of the wheel the slot in the driven revolving disc allows the operating rod to be depressed through it. With some types the operating rod is depressed by hand, whilst with some types the rod is automatically depressed by a cam each time the wheel passes mid position. In case of hand operated types the rod is operated at regular intervals and must be operated when either pressure gauge registers above 4.5 bar with wheel in mid position. When the rod is depressed both sides of system are connected thus giving pressure balance. The connection to the replenishing tank is also joined to both sides of the system, so that any expansion or contraction of the oil can be compensated.

Hydraulic Receiver: Consider the starboard (clockwise) movement of the bridge wheel. The depressed RH ram pressurizes the right hand side of the system. The pressure force acts on the central web of the moving cylinder until the movement caused corresponds to the movement of the ram in the steering telemotor. Oil is pushed back on the left hand side of the moving cylinder central web to the steering unit. After a small initial movement the LH sleeve butts against the nut and further movement by the moving cylinder to the left compresses the springs. When the steering wheel is returned to mid-ship the springs, which are under initial compression, return the moving cylinder to mid position. For port wheel rotation the LH ram of the steering unit moves down and the receiver moving cylinder goes in the opposite direction i.e. in this case left to right.

The moving cylinder is connected by a linkage to the control unit of the steering engine. Thus any movement of the bridge telemotor unit by wheel rotation is almost directly operating the control device which causes rotation of the steering engine and rudder movement.

electrical electronic control equipment: This system is based on the electrical and electronic circuits, the monitoring and control of the valves, which control the movement of the rudder. The system also includes a logic circuit that prevents the side rudder from reaching its physical limits. In the steering system can only work, under preset electronic limits. When the bar has reached a limit, the power of the solenoid valve closes automatically.

Electronic Steering Control

The system consists of the following parts: The control unit transmits to the angle of the desired direction from the bridge in the direction of the plane,Hand Steering without follow-up, Hand Steering with follow-up and Auto Steering using Gyro Compass.

The power supply provides the energy to move the rudder at the desired angle and the transfer unit to move the motions of the rudder.

The central steering of the movement of the rudder affect the ship's engines control. And rudder actuators single unit servo steering is the torque means is applied to the i-th rudder shaft. Lance or Quadrant

Control system of the device by means of which the commands from the computer to the power units of the steering mechanism and other necessary parts are transmitted to operate the steering. Including transmitters, receivers, hydraulic control pumps and motors associated hoses and cables.

The rudder actuator, the element that directly moves to a hydraulic pressure on the mechanical effect moves the rudder. Wheel drive means that the parts that transfer the force from the actuator to the aileron of the flow including the rudder. Power unit means:

• in the case of electrical steering gear ; an electric motor and its associated electrical equipment
• in case of electro-hydraulic steering gear ; an electric motor and its associated equipment and connected pump
• in case of other hydraulic steering gear ; a driving engine and connected pump

Steering Gear Regulations

1.Every ship is to be provided with two power unit (main and auxiliary) steering gear, each independent of the other. This is because if one fail another can take over the power. If however two identical power unit is available, the presence of stand-by(Auxiliary) is not required.  

2.The capacity of the unit must be such that, it may swing the rudder from 35º on one side to 35º to the other side of the ship with the maximum speed and at deepest draft.The time required to do so must not exceed 28 Seconds.

3.Steering gear must be power operated if rudder stock diameter is greater than 120mm. Mostly we use hydraulic power for operating the rudder post.

Power of the auxiliary steering shall be such that rudder can be swung from 15º on one side to 15º to the other in 60 seconds at deepest draft and of 7 knots.

5.Steering gear should be prevented from any abnormalities, such as overloading, short circuit, overload, and visual and audible indicator should be available on bridge, ECR and steering gear room, alarms and trips is also present to minimize the damage.

6.For Hydraulic Oil tank, Low level alarm is present.

7.A Tanker of 10000 GRT and more must be provided with two steering gear systems, So in case of failure the steering gear change automatically to the stand-by Steering gear system within 45 seconds, along-with alarm for indication. It is provided for a reason, that if in case failure occur the ship keep moving in the same direction.

#Cover image credit: http://marine-data.co.uk

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Author Arpit Singh & Amit                                                                Article Requested by: kesavan s

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What is stern tube? its working and function

The Stern tube is a narrow hole in the hull structure at the rear end of the ship/boat that connects the propulsion shaft and the engine and the propeller. Purpose of Stern tube is to support the shaft. It forms the after bearing for the propeller shaft and to make water tight joint/gland where the shaft passes through the hull.

Two types of stern tubes are in common use, one with water lubricated bearings with the after end open to the sea & the other type is closed at both ends and has metal bearing surfaces lubricated by oil.

Water lubricated stern tube

Image credit: http://www.marineengineering.org.uk

• The tube is usually constructed of cast steel with a flange at its forward end & a thread at the aft end.
• Also at the aft end, tube has small flange cast on it. ( we can say this as collar)
• the tube is inserted from the forward end and the flange is bolted to the aft peak bulkhead with a gasket to ensure water-tightness.
• A steel nut placed on the thread at aft end retain the tube in position with its collar hard against the stern frame.
• Inside the tube is a brass bush which has grooves in it, running fore and aft.
• Strips of lignum vitae ( a resinous dense hardwood) are fitted into these grooves to act as bearing for the shaft.
• Small spaces are left between the lignum vitae strips. Through these spaces water can enter to lubricate and cool the shaft.
• A check ring bolted on the after end of the tube, keeps the strips in place.
• A stuffing box/water tight gland  is fitted at the fore end of the tube ----- to prevent water from getting into machinery or tunnel space but slight leakage is allowed to ensure cooling of this packing.
• A brass/gunmetal liner is shrunk fitted to the tail end shaft, usually for the full length of the stern tube.
• Bearing length is 4 times the shaft diameter.

Advantage:

1. Natural lubrication assisted by SW.
2. Predictable wear rate allows scheduling of dry-docking in advance. 
3. No sophisticated forward/aft seals required. 
4. No chances of pollution due to leakage of oil into water as in case of oil lubricated type.

Nearly all outboard bearings were water lubricated until about 1960, when transition to oil lubricated bearing begin due to following drawbacks:

Excessive wear-down of bearing materials was caused by the trend to larger ship sizes, which had higher bearing loads. Also Larger ships generally operate at deeper drafts; and with less clearance between the hull and the channel bottom more contaminants, such as silt, mud and sand are drawn into the bearing clearance causing their wear down.

Poor quality of work when rewooding, inferior materials, presence of sand, sediments in the water required early rewooding. Vessels with engine aft, and particularly tankers & ore carriers which spend long periods in ballast sometimes rewooding is required in 18 months. Shaft needs extra liner for SW corrosion protection.

By using oil lubricated stern tube bearings above problems can be eliminated. Oil lubricated stern tube bearings also reduce the power loss in the shafting system. Also oil is better vibration damper & superior lubricant than water

Oil lubricated Stern tube

• White metal lined with oil grooves , cast iron or bronze  bush replaces wood lined brass bush.
• Shaft is protected from metal-to-metal contact  with bearing by oil lubrication which  is supplied from a header tank in the Engine Room.
• a minimum bearing length of 2 times of shaft diameter will ensure that bearing load does not exceed 0.8N/mm2
• propeller shaft has 2 short rotating liners of chrome steel.
• Liner at after end is bolted to the Propeller boss. Inboard liner is fixed by clamping ring.

Aft Seal

It is composed of casing fixed to stern frame and chrome steel liner which is  fixed to propeller boss & rotates with propeller shaft. Ingress of sea-water is prevented by fitting a rubber sealing ring between Propeller boss and the chrome liner.

The casing which is stationary part consists of 3 types of metal rings:-Flange ring, Intermediate rings and Cover rings which are tightened to each other with bolts. These metal rings contains the sealing rings which may be 3 or 4. In our case we discuss 3 sealing rings. The leading edge(lips) of the rings are pressed hard against the rotating liner by the water and oil pressures, elasticity of the rubber material and the tightening force of the springs to maintain sealing effect.
The sealing rings are numbered 1,2,3 from the SW side.

Two outer sealing rings (made of VITON or NITRILE BUTADIENE RUBBER also called NBR which is oil & water resistant) are fitted to prevent SW penetrating the lub oil space. Viton sealing rings are particularly excellent in heat resistant property.Sealing ring also has the function of protecting the inside of the stern tube from the foreign matter in the SW.

Inner sealing ring fitted in the opposite fashion, prevents the oil in the stern tube leaking into the sea. Garter spring --- held the sealing rings on rotating chrome liner.

Normally 2 gravity tanks (High & Low) are provided for oiling for deep draft and shallow draft condition so that oil pressure can be set approx 0.2 to 0.3 bar more then SW pressure applied to the centre of tail shaft so as to prevent SW from penetrating into the S/T. Please follow the specific instructions while changing over from high to low gravity tank & vice versa.

Forward Seal

It is of similar construction to the aft seal. Only 2 sealing rings are used running against on chrome liner preventing leakage of oil into Engine Room. Oil is supplied by gravity tank installed approx 0.5 m above the top of the casing.

Disadvantage of Oil lubricates seals:

1. White metal debris may choke and restrict oil supply, speeding up failure.
2. Contaminated oil supply, causes abrasive wear.
3. Lack of oil supply can happen due to low level in header tank/obstructed flow/damaged pipework.
4. If using high gravity tank in ballast condition, chance of oil leakage into sea (in case of seal failure), causing oil pollution.

Stern Tube wear down measurement 

Stern tube bearing wear down is measured by Poker gauge. Values measured are recorded on the recording board in the wear down gauge. Initial measurement is taken when ship has been completed and the shafting/propeller and the seals have been installed completely.  This is the reference measurement. Measurements thereafter shall be taken normally each time the ship goes to the dock, and values measured are compared with those measured previously.

Split type stern tube (Ross-turnbull)

The roller lower half is machined in the two horizontal front and rear surfaces machined in the rear frame. The bearing is held in a vertical position by two bushes 50 tons of tap pilger, determines the thickness of the spacer around the height of the bearing. These cats also hold the two halves of the camp. The lateral positioning of the bearing is formed by two types of 30 pilgrim tonnes arranged on both sides of the carrier cat.


A bearing ring is arranged above the bearing to facilitate easy transport of the upper half of the rollers, are provided at the bottom with rolling bearing to enable transport from the lower half. After removal of the lower half of the bearing, the first cylinder is arranged below the elevator free of its mounting gaps. Then the washers are removed and the base is resting on the rollers. The weight of the propeller and the shaft is integrated into the rear by a fork support structure.

Stresses in tail shafts and stern tube

Due to the considerable weight of the propeller, the axis of the tail is subjected to bending. However, there are also other reasons which are likely possible. There is a torsional stress due to the drag torque of the propeller and the motor/engine, and a compressive stress due to the pressure support.

All these tensions, and that the tree can be in contact with highly corrosive seawater, makes the probability of an attack corrosion is very likely.

Examining a tail shaft and stern tube

1. It is necessary to measure bearing wear before regular inspection.
2. To remove the stem after thorough examination.
3. In setting bronze coating integrity of the water lubricated shaft must be checked by knocking with a hammer on its entire length to hear its hollow indicating separation.
4. Measuring the wear of the shaft.
5. Examine the button shaped cracks.
6. Replace the rubber rings.

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Author Amit                                                                            

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