Steering Gear
Content
Steering gear
A hydraulic steering gear consists of a bridge control which applies helm, an engine control
which is operated jointly by the helm and hunting gear (when fitted) and a power pump and
rudder actuator which constitutes the steering engine.
Telemotor systems
The telemotor system consists of a transmitter on the bridge and a receiver fitted on the steering
gear forming a part of the hunting gear. The system may be electrical or hydraulic or a
combination of the two.
Most modern vessels are fitted with electric or electro-hydraulic systems. Due to the increasing
size of vessels pipe runs have lengthen causing lags in the operation of the receiver in hydraulic
systems. In addition hydraulic only systems generally require more maintenance.
Hydraulic transmitter
Shown above is a typical hydraulic transmitter unit. The pinion driving the pistons is turned by
the bridge wheel.
The casing is usually gun metal, with bronze rams, and copper pipes are led in by frilled leads on
the casting.
To test the system, with the steering gear actuating pumps stopped, the wheel may be lashed at
hard over and the pressure recorded. It should maintain this pressure for some time
To allow for expansion in the system and to allow topping up a 'by-pass valve' is fitted. It will
also act as a safety valve.
Author note:
The main problem appears to be the effect of air entrained within it. Thus regular venting of the
system is required.
By-pass valve
The operating rod is pushed down making both line common whenever the wheel is at midships,
generally by a cam fitted to the pinion. This ensures they system is always balanced
The charging valves are opened only when filling or flushing.
The moving cylinder is attached to the hunting gear. When the bridge wheel is turned hydraulic
pressure acts on the cylinder causing it to move. This in turn moves the hunting gear. The
steering gear is then moved to compensate until the hunting gear is moved back to the neutral
position. The total movement of the receiver is limited by stops.
Electro-hydraulic type telemotor system
Shown is a very simple system capable of operating a steering hunting gear. A pressure relief
valve would normally be fitted after the valve and across the pump to prevent over pressurisation
of the system.
The signal is derived from the action on the steering wheel, created by the autopilot or directly
from the non-follow up control levers.
Telemotor fluid
should be a good quality mineral oil with the following properties;
1.
low pour point
2.
non sludge forming
3.
non corrosive
4.
good lubricating properties
5.
high flash point
6.
low viscosity
Hunting Gear
The steering gear system above consists of the telemotor which receives a signal from the bridge
wheel. This acts on the hunting gear.
The hunting gear moves displacing a control rod, this rod acts on the pump displacement control
gear to alter the delivery from the pump. The delivery from the pump causes the ram to move
rotating the rudder stock and hence the rudder. The other end of the hunting gear is mounted on
the rudder stock.
The rotation of the rudder stock moves the hunting gear returning the operating rod for the pump
to the neutral position once the rudder has reached the correct angle.
Rudder Actuators
There are many different mechanisms by means of which hydraulic power can be converted into
torque at the rudder stock some of which are as follows;
Rapson Slide Actuators - Ram type
Steering gear incorporating the rapson slide principle are the most common in use on heavy duty
applications.
The rapson slide acting on either a fork tiller or the more common round arm. The tiller drives
the rudder stock by means of a key or keys. The crosshead is free to slide along the circular arm
of the tiller so that the straight line effort of the rams is applied to the angular moving tiller. Each
set of two cylinders in line are connected by a strong steel girder usually called a "Joist" which
stiffens the system and forms a "guide bar" for the crosshead guide slippers to slide along. The
joist is often designed to incorporate the steering engine stops.
An important consideration in all steering gears is the "wear down" of the rudder carrying
bearing, this bearing takes all the weight of the rudder.
Therefore there must be adequate clearance between the bottom of the tiller and the crosshead
bearing, so as the rudder bearing wears down in service the tiller and crosshead bearing do not
touch, clearance when new can be 22 mm at bottom and 12 mm at top; the top clearance is a
precaution to stop the tiller bumping up the steering rams in the unlikely event of the rudder
lifting in heavy weather. Should the bottom of the tiller and the crosshead bearing touch, then the
weight of the rudder will be transferred from the rudder bearing to the steering rams with
disastrous results such as leaking of working fluid from the cylinders and shearing of the rams.
In the case of forked tiller design, the thrust from the rams is transmitted to the tiller through
swivel blocks. One advantage of this arrangement is that the overall length of pairs of rams is
reduced compared to the round arm tiller design and this can be an important consideration in
some cases. A disadvantage is that where as any slight misalignment in the case of the round arm
tiller is not vitally important, it could lead to uneven loading of the swivel blocks in the forked
tiller design and it is essential that the line of the rams be exactly at right angles to the rudder
stock centre line if this is to be avoided.
With the Rapson Slide the torque reaction from the rudder is taken on the tiller by a force which
is balanced by an equal and opposite force having two components one of which is produced by
the ram and acts in the line of the ram, whilst the other is at right angles to the line of the ram and
is produced by the guide reaction.
Where guides are not fitted as is sometimes the case with smaller steering gears then the guide
reaction force must be carried by bearings or the glands of the cylinders.
a = actuator area
p = Working fluid pressure
n = Number of effective rams ( 1 for 2 ram, 2 for 4 ram)
q = rudder angle
r = tiller radius at amidships
r' = tiller radius at qo of tiller helm
s = guide reaction force
f = force on ram with tiller amidships ( = p x a)
f' = effective force acting at 90o to tiller
r' = r / cos.q also f' = f / cos.q = p x a / cos.q
t = torque available = f' x r' x n
= ((p x a) / cosq). (r / cos.q) . n
t = (p x a x n x r) . (1 / cos.2q)
Showing that the rapson slide effect which gives increase of available torque with increases of
rudder angle
The torque demanded from the steering gear increases and is at a maximum at maximum rudder
angle when the mechanical advantage of the Rapson Slide gear is at a maximum. Ram type gears
are also well adapted to take advantage of the high pressures which are currently available, since
ram diameters and casing are relatively small and leakage paths are small or non-existent.
Oscillating Cylinder Actuators
The use of oscillating cylinders or pinned actuators is a recent development. They can be used as
single cylinder units for hand only steering or two cylinder units for hand and power steering.
While four double acting cylinders can cope with larger torque demands. These units are double
acting because pistons work in the cylinders and pressure can be applied to either side as
compared with ram gears which are single acting.
In these cases, the torque T applied to the rudder stock varies with the rudder deflection angle
and on the location of the actuator. In general the torque developed will be less at the maximum
rudder angle than the maximum possible from the actuator.
Maximum torque from actuator = p.a.n.r.
Torque at 35o = p.a.n.r. cos (35 = o)
where o = angle traced out by the actuator
between o = 0o and o = 35o
Mechanical advantage at 35o = Cos 35o = 0.82
since the actuators are pivoting about their pin centre, they usually have their working fluid tank
and pump mounted on the actuator cylinder, or they are connected to tank and pump by a flexible
pipeline.
Rams Connected To Crossheads By Links
This type of gear is used if the athwartships space is limited, or the head room at the rudder head
is restricted, as for example, in the case of a vehicle ferry having a slip way aft. The design
enables the steering gear to be moved forward where there is reasonable head room for access.
As in the case of the oscillating cylinder design the Mechanical Advantage of the Rapson Slide
gear is lost in the links and the torque output of the gear is at a minimum at hard over when the
torque demand created by the rudder hydrodynamic forces is at a maximum.
Rotary Vane Gear
These consist of two elements:
a cylindrical static casing (stator) with usually three internal vanes which project
radially inwards
a rotor keyed to and concentric with the rudder stock, the rotor has rotor vanes which
project radially outwards into the spaces formed by the stator vanes.
The spaces formed between the stator and rotor vanes are used as high and low pressure
chambers. The main advantage of the system is that it is compact, occupying about 1 / 10 the
space of a ram system. The disadvantages are ;
it has a long oil sealing path
it is a constant torque machine at all angles of helm compared to the ram system
where due to the Rapson slide effect, the torque available increases with
increasing helm.
Where 100% redundancy is required two rotary vanes in piggy back are used.
All vanes are spheroidal graphite cast iron secured to the cast iron rotor and stator by high tensile
steel dowel pins and cap screws. Rotor strength is maintained by keys fitted full length of the
rotary vane. Steel sealing strips are fitted along the working faces, backed by synthetic rubber in
grooves along the working faces which are elastically loaded, so as to ensure that contact with
the mating surfaces is maintained in order to hold the hydraulic pressures.
The chambers are alternately connected to the suction and delivery from the hydraulic pump so
that they can be used to produce the rudder actuating torque. Because the distribution of the
pressure chambers is balanced around the rudder stock, only pure torque is transmitted to the
stock and no side loading are imposed by the gear.
There are two main types of rotary vane steering gear in use today. One has its stator firmly fixed
to the steering flat deck and the stator housing and cover are provided with suitable bearings to
enable the unit to act as a combined rudder carrier and rudder stock bearing support. The other
type of vane gear is supported where the stator is only anchored to the ships structure to resist
torque but is free to move vertically within the constraints of the separate rudder head bearing
and carrier which is similar to the bearing provided for ram type steering gears.
The rudder carrier ring bearing (Pallister Bearing) is taking the weight of the rotary vane steering
gear and the rudder and stock.
Rotation of the stator is prevented by means of two anchor brackets and two anchor bolts . The
anchor brackets are securely bolted to the stool and vertical clearance is arranged between the
inside of the Stator flanges and the top and bottom of the anchor brackets to allow for vertical
movement of the rudder stock. This clearance varies with each size of rotary unit but could be
about 40 mm total . It is essential that the rudder carrier should be capable of restricting the
vertical movements of the rudder stock to less than this amount.
The anchor bolts are fitted with special bushes in halves, shaped externally in order to pre-load
the synthetic rubber shock absorbers , which are fitted between them and the anchor brackets.
The maximum deflection of the shock absorbers under full load is approximately 1 mm.
The working angle of the gear is governed by the number of vanes and their thickness. Vanes act
as rudder stops when a moving vane contacts a fixed vane. Valves at inlet to the chambers may
be shut causing a hydraulic lock. In the rotary vane units the Mechanical Advantage is unity at all
angles and hence torque is constant
Torque = p.a.n.r.
where n = number of rotating vanes
Tendfjord Rotary Piston Gear Actuator
This gear consists of a casing around the rudder stock which contains pistons of rectangular
section sliding in angular compartments concentric with the rudder stock. The tiller projects into
a gap between the cylinder, the piston ends abutting onto the tiller but not being attached to it so
that axial movements of the rudder cannot be transmitted to the pistons. Steering gears of this
type operate at hydraulic pressures up to 41 bar (600 lbf/in2) and are in general restricted to low
power application.
As with the rotary vane steering gears the Mechanical Advantage is unity at all angles and hence
the torque is constant.
Torque = p.a.n.r.
where n in this case is unity.
Components
Relief Isolating And Bypass Valves
Hydraulic actuators are provided with relief and bypass valves between complementary pairs of
cylinders or chambers of vane gears. The relief valves are set to lift at pressures above the
normal maximum.
The bypass valves are normally closed but can be opened on a two cylinder gear to enable
emergency steering to be used. On a four cylinder gear one pair of cylinders can be bypassed
while the other pair provide emergency steering at a reduced torque, an instruction plate is fitted
over the controls valve block giving a combination of failures and which valves have to be open
or shut to cope with the emergency etc. It should be noted that if one ram or cylinder in a four
ram system breaks down, then never isolate the cylinder diagonally opposite the damaged unit,
since the steering gear will not operate due to the fact that the remaining two cylinders will be
either on all pressure or on all suction at the same time.
Isolating valves are provided at each cylinder or rotary vane chamber which when closed will
hold the rudder by trapping the oil in the chambers. Isolating valves are also fitted to pumps so
that a pump can be completely shut off from the circuit and removed for servicing while steering
is continued with the other pump.
In the case of gears with duplicated variable stroke pumps, in order to be able to bring a standby
unit quickly into operation, the pump stroke mechanisms are permanently coupled together and
both pumps are left open to the hydraulic circuit. Thus it is only necessary to start up a motor for
the stand by pump to be operative. It is usual to run both pumps in restricted navigation waters.
As a variable stroke pump can operate as a motor if pressure oil is applied to one side while it is
on stroke, it is necessary to prevent wind milling or rotation of the pump which is on stand by
duty.
Otherwise, the output of the operation pump, instead of moving the steering gear would be used
up in rotating the stand by pump.
One method to prevent this,is using a fixed ratchet is provided concentric with the pump driving
shaft. Pawls that can engage this ratchet are carried in the drive coupling. When the pump is on
stand-by the pawls engage with the ratchet and prevent rotation when oil on the delivery side of
the operating pump is on pressure. In this condition the tendency to motor the stand by pump will
always be against its normal direction of rotation. As soon as the pump is started, rotation being
in the opposite direction, the pawls disengage and by centrifugal action fling out against the inner
flange of the coupling completely clear of the ratchet. When a pump is on stand-by and the
rudder is being driven by water pressure in the direction in which it is being moved so as to
generate pressure on what is normally the suction side of the operating pump, this will cause the
stand by pump to rotate in its normal running direction. This means that the pawls will disengage
and the pump will be motored round, allowing the rudder to move more quickly to a new
steering position than the single operating pump will allow.
Another method of protection against rotation of the stand by pump is to fit Servo pressure
operated automatic change over valves in the pipelines; these ensure that the pump can only be
started in the unloaded condition (neutral) and in addition prevents the stand by pump from being
motored by the pump in service.
On some ships it has been discovered that the ball bearing races on the stand-by pump have been
failing due to brinelling of the ball bearings, caused by ship vibrations, and in these cases it is
usual to fit devices which allows the stand by pump to be motored slowly.
When fixed delivery pumps are duplicated in supplying oil to a common hydraulically operated
control valve, an automatic change over valve can be fitted which will isolate the stand by pump
when it is at rest, but will connect it to the actuator when the pump is started up.
Stops And Limit Switches
External or stern posts stops are set at the absolute limit to hard over movement of the rudder ,
protects propeller and ship stern in the event of metal or other failure which allows rudder to
swing in an uncontrolled manner. Mechanical stops on the rudder actuator operate before the
external stop are reached .these take the form of travel limits. Stops on the bridge control operate
before mechanical stops. local controls are set midway. auto pilot controls are set first. It should
be noted that the vanes act as stops on rotary vane gears.
Drive Back Due To Heavy Sea's
Heavy seas acting on the rudder can force the actuator against the hydraulics sufficient to lift the
relief v/v, in which case the rudder will move. Hunting gear will tend to return the gear to its
correct position.
Hand And Power Hydraulic Steering Gears
For small ships during navigational course keeping hand steering can be used, whist during
manoeuvring power steering can be used. These may take the form of chains or simple
hydraulics operated by a fixed delivery pump attached to the steering gears.
"Follow Up" Steering
This is the normal method of steering and involves the feedback of steering angle to the helm.
This is suited to both manual and automatic operation.
The ships heading may be set into the autopilot which can then compare the actual to desired
heading and adjust the rudder angle to suit
"Non-follow Up" Steering
Normally used for back up purposes only. Consists of a single lever per steering gear unit, by
moving the lever in on direction the rudder will begin to turn, the rudder will continue to turn
until the lever is released or it reaches the limit of its operation
Charging A System With Fluid
. In all cases high quality hydraulic oil should be used , containing inhibitors against oxidation ,
foaming, rust and wear and emulsification.
In order to keep the transmission load as low as possible when hand steering , hand power
systems must have oil of low viscosity.
The condition of the oil should be monitored and ensured at least clean and free of moisture.
Steering gear failure
A study of steering gear defects demonstrates that the most common are related to vibration and
the working loose of components.
The most common source of failure are the pump and the hydraulic system associated with it.
Rudder torque calculations
Formulae for assessing rudder torque's are based upon the expression Ta ACpV2Sin q where:T = rudder torque
C = rudder area
Cp = centre of pressure distance from centre line of rudder stock
V = velocity of ship
q = rudder angle measured from mid-ship position
In practice different constants obtained empirically are used with this expression and take into
account such factors as propeller slip and wake speed as appropriate depending upon the relation
of the rudder and propeller positions. The position of the centre of pressure has a significant
effect upon rudder torque and hence the size of the steering gear required; the greater the
distance of the C of P from the centre line of the rudder stock, the larger the torque required;
therefore designers attempt to bring the C of P as near to the centre line as possible. With the
simple "barn door" type rudder on some single screw ships, no adjustment can be made, but the
semi-balanced and balanced-type rudders can be designed to reduce the torque required; for
instance, with the spade type rudder such as fitted to twin screw ferries, the position can be
adjusted by the designer to give optimum position. This lies between 30 and 32 per cent abaft the
leading edge of the mean chord of the rudder. Such a rudder would have its C of P forward of the
stock position at low angles of helm, would balance around 10o to 15o and drift aft of the stock
at higher rudder angles.
In graph above is shown a typical torque characteristics for a spade type balanced rudder and a
"barn door" or unbalanced plate rudder. The astern torque's should also be calculated since this is
sometimes higher than the ahead torque, this is true for spade rudders.
POWER
The peak power that a steering gear must develop is the product of the maximum torque (T)
usually at hard over with the ship travelling at full speed, and the maximum speed (S) of rudder
movement i.e. Power (max) a T x S.
The combination of maximum power and speed only exists for 2 or 3 seconds during each
manoeuvre; so clearly the average power required to operate the steering gear is considerably
below the peak. Because the steering gear must have sufficient power to overcome friction and
still have ample reserve of power, the value for used in the foregoing expression is significantly
higher than that used in the expression for rudder torque. When considering the diameter of the
rudder stock, bending and shear stresses must be taken into account.
Rudder Wear Down
This refers to the measurements taken generally during a docking period to indicate excessive
wear in the steering gear system particularly the rudder carrier. The significance of this is that for
ram systems excessive wear can lead to bending moments on the rams. For rotary vane systems
it can lead to vane edge loading.The readings taken are offered for recording by the classification
society.
Trammel
This takes the form of an 'L' shape bar of suitable construction. When the vessel is built a distinct
centrepunch mark is placed onto the ruder stock and onto a suitable location on the vessels
structure, here given as a girder which is typical. The trammel is manufactured to suit these
marks As the carrier wears the upper pointer will fall below the centrepunch mark by an amount
equal to the wear down.
Rudder Clearance
Pads are welded to the hull and rudder. A clearance is given ( sometimes referred to as the
jumping clearance). As the carrier wears this clearance will increase
Steering gear Clearance
Direct measurement can be taken from the steering gear assembly. Shown below is one example,
here the clearance will be seen to reduce as the carrier wears and impact his has on the system
can be directly judged
Rules
Design of steering gears have been influenced over the years by the rules and regulations of
national authorities and classification subjects. Any changes of real substance tend nowadays to
originate from the international
Maritime Organisations(I.M.O.) conventions and regulations. Classification society
requirements are as follows;
1.
All ships to have power operated main gear capable of displacing the rudder from
35o port to 35o starboard at the deepest draught and at maximum service speed. Must also be
capable of displacing the rudder from 35o port to 30o starboard in 28 seconds and vice versa.
2.
The auxiliary gear must be power operated and capable of being brought rapidly into
action. The auxiliary gear is only required to steer the ship at either 7 knots or half service speed
3.
If the main gear comprises two or more identical power units, then a single failure of
either power unit or piping must not impair the integrity of the remaining part of the steering
gear
4.
Each power unit must be served by at least two electrical circuits from the main
switchboard. One circuit may pass through the emergency switchboard. All circuits to be
separated as widely as possible throughout their length.
5.
All power operated gears to be fitted with shock relieving arrangements to protect against
the action of heavy seas.
6.
An efficient brake or locking arrangement to be fitted to enable the rudder to be
maintained stationary
7.
the maximum power developed by the gear is proportional to T x S
where T = rudder torque
S = Speed of rudder movement
also T = A x P x sinq x V2
where A = rudder area
P = centre of pressure
q = rudder angle
V = velocity of the ship
Special requirements
Owners may specify additional requirements such as faster hard-over to hard-over time, strength
of components above that required by the Rules, additional control points and additional
duplication,
New tankers of 100 000dwt and above-shall comply with the following
The main steering gear shall comprise of either
two independent and separate power actuating systems each capable of meeting the hard
over port to 30o starboard in 28 sec requirements,
or
at least two identical power actuating systems which acting simultaneously in normal
operation, shall be capable of meeting the hard over requirements. Where necessary to comply
with this requirement inter connection of hydraulic power systems shall be provided. Loss of
hydraulic fluid from one system shall be capable of being detected and the defective system
isolated so that the other system shall remain fully operational
In the event of loss of steering capability due to a single failure other than the tiller, quadrant or
components serving the same purpose (these are excluded from single failure concepts), or
seizure of the rudder actuators. The steering capability shall be regained in not less than 45
seconds after the loss of one power actuating system.
Steering gear other than hydraulic should meet the same standards.
Example of suitable system permissible for all ships
The system shown consists of two sets of rams but could equally be two rotary vane units. With
no power on the solenoids are in by-pass mode with oil being allowed to pass freely from one
side to the other. When an electric motor is started the control pump supplies oil to the solenoid
shutting it. High pressure oil from the main unit is now fed to the rams as required. The other
unit remains in by-pass until the electric motor is started.
Low level alarms are fitted to the tanks. Low low changeovers may also be fitted so that in the
event of oil loss from one system, the other system is started.
New tankers between 10 000gt upwards to 100 000tdwt
For these tankers the single failure criterion need not apply to the rudder actuator or actuators
subject to certain requirements being fulfilled. These include a requirement that steering be
regained within 45 seconds following failure of any part of the piping system or power units and
a special stress analysis of non-duplicated rudder actuators.
The left hand unit is shown in operation.
For this basic arrangement the power units must be identical
New ships 70 000gt and upwards
system suitable for all ships except tankers of 10 000 gt and above
The main steering shall comprise two or more power units and that the main steering gear is so
arranged that, after a single failure in its piping system or in one of the power units the defect can
be isolated so that steering can be speedily regained.
'Speedily' is intended to mean the provision of duplicate hydraulic circuits or , for example, a
conventional four ram steering gear with a common hydraulic circuit with appropriate isolating
valves
New ships of less than 70 000 gt and tankers less than 10 000 gt
suitable system
Single failure is not applicable as a rule, however, attention is drawn to the requirement that
auxiliary steering gear be independent of any part of the main gear except the tiller. There is no
requirement that main and auxiliary power units be identical.
The auxiliary steering gear must be capable of putting the rudder over from 15ofrom one side to
the other in not more than 60 seconds with the ship at its deepest draught and running ahead at
half maximum speed or 7 knots.
Existing tankers of 40 000gt and upwards
The steering gear shall be arranges so that in the event of single failure of the piping or one
of the power units, steering capability can be maintained or the rudder movement can be
limited so that steering capability can be speedily regained by
An independent means of restraining the rudder
or
fast acting valves to isolate the actuator or actuators from the external hydraulic piping
together with a means of directly refilling the actuators by a fixed independent power pump and
piping system
or
An arrangement so that, where hydraulic power systems are interconnected any loss of
hydraulic fluid from one system shall be detected and the defective system shut off either
automatically or remotely from the bridge so that the other system remains intact
Requirements for all new ships
Administrations must be satisfied in respect to the main and auxiliary steering gear
provided for every ship that all components and the rudder stock are of sound construction
Every component, where appropriate, utilise anti-friction bearings which will be
permanently lubricated or provided with lubricant fittings
Parts subjected to hydraulic pressures should be designed to cope with 1.25 maximum
working pressure when the rudder is hard over at maximum draught and service speed
special requirements for fatigue resistance( due to pulsating hydraulic pressure), relief
valves and oil cleanliness
Low level alarm to be fitted to each hydraulic reservoir.
Fixed storage capacity sufficient to recharge on system
Auxiliary steering gear
The other set of steering (auxiliary ) may be an arrangements of blocks and tackles or some other
approved alternative method.
The auxiliary steering gear need only be capable of steering the ship at navigable speed, but it
must be capable of being brought speedily in to action in an emergency. Navigable speed is one
half of maximum service speed ahead or 7 knots whichever is the greater.
The auxiliary steering gear must be a power operated type if the rudder stock exceeds 230mm for
passenger ships and 250mm for cargo vessels. No additional means of steering is required when
electric or electro-hydraulic steering gear is fitted having two independent motors or two sets of
pumps and motors.
Electrical Supply
Short circuit protection and overload alarm are to be provided in steering gear circuits. Indicators
for running indication of steering gear motors are to be installed on the navigation bridge and at a
suitable machinery control position. Each electric or electro-hydraulic steering gear shall be
served by at least two independent circuits fed from the main switchboard. Cables for each
circuit led through a separate route as far apart as possible so that damage to one cable does not
involve damage to the other. A change over switch is fitted in an approved position to enable
power supplies to be interchanged. One circuit may pass through an emergency switchboard.
Rudders
In passenger ships where the rudder stock exceeds 230mm, an alternative steering position
remote from the main position is to be provided. Failure of one system must not render the other
system inoperable. Provision made to transmit orders from bridge to alternative position. The
exact position of the rudder must be indicated at principal steering positions. An efficient braking
or locking device must be fitted to the steering gear to enable the rudder to be held stationary if
necessary. Spring or hydraulic buffer relief valves fitted in steering gear system to protect the
rudder and steering gear against shock loading due to heavy seas striking the rudder. Suitable
stopping arrangements are to be provided so as to restrict the total travel of the rudder. Stops or
cut outs on the steering gear are arranged so that it operates on a smaller angle of helm than the
rudder stops.
Rudder restraint
Since failure of a single hydraulic circuit can lead to unrestricted movement of the rudder, tiller
and rams, repair and recharging may not be possible. Difficulty arises with which the speed a
restraint whether in the form of a mechanical or hydraulic brake can be brought in to use.
Due to the possibility of considerable damage occurring before it could, regulations have
concentrated on continuity of steering rather than a shut down and repair solution
Testing and drills
Within 12 h before departure, the ship's steering gear shall be checked and tested by the ship's
crew. The test procedure shall include, where applicable, the operation of the following:
the main steering gear;
the auxiliary steering gear;
the remote steering gear control systems;
the steering positions located on the navigation bridge;
the emergency power supply;
the rudder angle indicators in relation to the actual position of the rudder;
the remote steering gear control system power failure alarms;
the steering gear power unit failure alarms; and
automatic isolating arrangements and other automatic equipment.
The checks and tests shall include:
the full movement of the rudder according to the required capabilities of the steering
gear;
a visual inspection of the steering gear and its connecting linkage; and
the operation of the means of communication between the navigation bridge and
steering gear compartment.
Simple operating instructions with a block diagram showing the change-over
procedures for remote steering gear control systems and steering gear power units
shall be permanently displayed on the navigation bridge and in the steering gear
compartment.
All ships' officers concerned with the operation or maintenance of steering gear shall
be familiar with the operation of the steering systems fitted on the ship and with
the procedures for changing from one system to another.
In addition to the routine checks and tests prescribed in paragraphs (a) and (b), emergency
steering drills shall take place at least once every three months in order to practise
emergency steering procedures. These drills shall include direct control from within the
steering gear compartment, the communications procedure with the navigation bridge
and, where applicable, the operation of alternative power supplies.
The Administration may waive the requirement to carry out the checks and tests prescribed in
paragraphs (a) and (b) for ships which regularly engage on voyages of short duration.
Such ships shall carry out these checks and tests at least once every week.
The date upon which the checks and tests prescribed in paragraphs (a) and (b) are carried out
and the date and details of emergency steering drills carried out under paragraph (d), shall
be recorded in the log-book as may be prescribed by the Administration.
A hydraulic steering gear consists of a bridge control which applies helm, an engine control
which is operated jointly by the helm and hunting gear (when fitted) and a power pump and
rudder actuator which constitutes the steering engine.
Telemotor systems
The telemotor system consists of a transmitter on the bridge and a receiver fitted on the steering
gear forming a part of the hunting gear. The system may be electrical or hydraulic or a
combination of the two.
Most modern vessels are fitted with electric or electro-hydraulic systems. Due to the increasing
size of vessels pipe runs have lengthen causing lags in the operation of the receiver in hydraulic
systems. In addition hydraulic only systems generally require more maintenance.
Hydraulic transmitter
Shown above is a typical hydraulic transmitter unit. The pinion driving the pistons is turned by
the bridge wheel.
The casing is usually gun metal, with bronze rams, and copper pipes are led in by frilled leads on
the casting.
To test the system, with the steering gear actuating pumps stopped, the wheel may be lashed at
hard over and the pressure recorded. It should maintain this pressure for some time
To allow for expansion in the system and to allow topping up a 'by-pass valve' is fitted. It will
also act as a safety valve.
Author note:
The main problem appears to be the effect of air entrained within it. Thus regular venting of the
system is required.
By-pass valve
The operating rod is pushed down making both line common whenever the wheel is at midships,
generally by a cam fitted to the pinion. This ensures they system is always balanced
The charging valves are opened only when filling or flushing.
The moving cylinder is attached to the hunting gear. When the bridge wheel is turned hydraulic
pressure acts on the cylinder causing it to move. This in turn moves the hunting gear. The
steering gear is then moved to compensate until the hunting gear is moved back to the neutral
position. The total movement of the receiver is limited by stops.
Electro-hydraulic type telemotor system
Shown is a very simple system capable of operating a steering hunting gear. A pressure relief
valve would normally be fitted after the valve and across the pump to prevent over pressurisation
of the system.
The signal is derived from the action on the steering wheel, created by the autopilot or directly
from the non-follow up control levers.
Telemotor fluid
should be a good quality mineral oil with the following properties;
1.
low pour point
2.
non sludge forming
3.
non corrosive
4.
good lubricating properties
5.
high flash point
6.
low viscosity
Hunting Gear
The steering gear system above consists of the telemotor which receives a signal from the bridge
wheel. This acts on the hunting gear.
The hunting gear moves displacing a control rod, this rod acts on the pump displacement control
gear to alter the delivery from the pump. The delivery from the pump causes the ram to move
rotating the rudder stock and hence the rudder. The other end of the hunting gear is mounted on
the rudder stock.
The rotation of the rudder stock moves the hunting gear returning the operating rod for the pump
to the neutral position once the rudder has reached the correct angle.
Rudder Actuators
There are many different mechanisms by means of which hydraulic power can be converted into
torque at the rudder stock some of which are as follows;
Rapson Slide Actuators - Ram type
Steering gear incorporating the rapson slide principle are the most common in use on heavy duty
applications.
The rapson slide acting on either a fork tiller or the more common round arm. The tiller drives
the rudder stock by means of a key or keys. The crosshead is free to slide along the circular arm
of the tiller so that the straight line effort of the rams is applied to the angular moving tiller. Each
set of two cylinders in line are connected by a strong steel girder usually called a "Joist" which
stiffens the system and forms a "guide bar" for the crosshead guide slippers to slide along. The
joist is often designed to incorporate the steering engine stops.
An important consideration in all steering gears is the "wear down" of the rudder carrying
bearing, this bearing takes all the weight of the rudder.
Therefore there must be adequate clearance between the bottom of the tiller and the crosshead
bearing, so as the rudder bearing wears down in service the tiller and crosshead bearing do not
touch, clearance when new can be 22 mm at bottom and 12 mm at top; the top clearance is a
precaution to stop the tiller bumping up the steering rams in the unlikely event of the rudder
lifting in heavy weather. Should the bottom of the tiller and the crosshead bearing touch, then the
weight of the rudder will be transferred from the rudder bearing to the steering rams with
disastrous results such as leaking of working fluid from the cylinders and shearing of the rams.
In the case of forked tiller design, the thrust from the rams is transmitted to the tiller through
swivel blocks. One advantage of this arrangement is that the overall length of pairs of rams is
reduced compared to the round arm tiller design and this can be an important consideration in
some cases. A disadvantage is that where as any slight misalignment in the case of the round arm
tiller is not vitally important, it could lead to uneven loading of the swivel blocks in the forked
tiller design and it is essential that the line of the rams be exactly at right angles to the rudder
stock centre line if this is to be avoided.
With the Rapson Slide the torque reaction from the rudder is taken on the tiller by a force which
is balanced by an equal and opposite force having two components one of which is produced by
the ram and acts in the line of the ram, whilst the other is at right angles to the line of the ram and
is produced by the guide reaction.
Where guides are not fitted as is sometimes the case with smaller steering gears then the guide
reaction force must be carried by bearings or the glands of the cylinders.
a = actuator area
p = Working fluid pressure
n = Number of effective rams ( 1 for 2 ram, 2 for 4 ram)
q = rudder angle
r = tiller radius at amidships
r' = tiller radius at qo of tiller helm
s = guide reaction force
f = force on ram with tiller amidships ( = p x a)
f' = effective force acting at 90o to tiller
r' = r / cos.q also f' = f / cos.q = p x a / cos.q
t = torque available = f' x r' x n
= ((p x a) / cosq). (r / cos.q) . n
t = (p x a x n x r) . (1 / cos.2q)
Showing that the rapson slide effect which gives increase of available torque with increases of
rudder angle
The torque demanded from the steering gear increases and is at a maximum at maximum rudder
angle when the mechanical advantage of the Rapson Slide gear is at a maximum. Ram type gears
are also well adapted to take advantage of the high pressures which are currently available, since
ram diameters and casing are relatively small and leakage paths are small or non-existent.
Oscillating Cylinder Actuators
The use of oscillating cylinders or pinned actuators is a recent development. They can be used as
single cylinder units for hand only steering or two cylinder units for hand and power steering.
While four double acting cylinders can cope with larger torque demands. These units are double
acting because pistons work in the cylinders and pressure can be applied to either side as
compared with ram gears which are single acting.
In these cases, the torque T applied to the rudder stock varies with the rudder deflection angle
and on the location of the actuator. In general the torque developed will be less at the maximum
rudder angle than the maximum possible from the actuator.
Maximum torque from actuator = p.a.n.r.
Torque at 35o = p.a.n.r. cos (35 = o)
where o = angle traced out by the actuator
between o = 0o and o = 35o
Mechanical advantage at 35o = Cos 35o = 0.82
since the actuators are pivoting about their pin centre, they usually have their working fluid tank
and pump mounted on the actuator cylinder, or they are connected to tank and pump by a flexible
pipeline.
Rams Connected To Crossheads By Links
This type of gear is used if the athwartships space is limited, or the head room at the rudder head
is restricted, as for example, in the case of a vehicle ferry having a slip way aft. The design
enables the steering gear to be moved forward where there is reasonable head room for access.
As in the case of the oscillating cylinder design the Mechanical Advantage of the Rapson Slide
gear is lost in the links and the torque output of the gear is at a minimum at hard over when the
torque demand created by the rudder hydrodynamic forces is at a maximum.
Rotary Vane Gear
These consist of two elements:
a cylindrical static casing (stator) with usually three internal vanes which project
radially inwards
a rotor keyed to and concentric with the rudder stock, the rotor has rotor vanes which
project radially outwards into the spaces formed by the stator vanes.
The spaces formed between the stator and rotor vanes are used as high and low pressure
chambers. The main advantage of the system is that it is compact, occupying about 1 / 10 the
space of a ram system. The disadvantages are ;
it has a long oil sealing path
it is a constant torque machine at all angles of helm compared to the ram system
where due to the Rapson slide effect, the torque available increases with
increasing helm.
Where 100% redundancy is required two rotary vanes in piggy back are used.
All vanes are spheroidal graphite cast iron secured to the cast iron rotor and stator by high tensile
steel dowel pins and cap screws. Rotor strength is maintained by keys fitted full length of the
rotary vane. Steel sealing strips are fitted along the working faces, backed by synthetic rubber in
grooves along the working faces which are elastically loaded, so as to ensure that contact with
the mating surfaces is maintained in order to hold the hydraulic pressures.
The chambers are alternately connected to the suction and delivery from the hydraulic pump so
that they can be used to produce the rudder actuating torque. Because the distribution of the
pressure chambers is balanced around the rudder stock, only pure torque is transmitted to the
stock and no side loading are imposed by the gear.
There are two main types of rotary vane steering gear in use today. One has its stator firmly fixed
to the steering flat deck and the stator housing and cover are provided with suitable bearings to
enable the unit to act as a combined rudder carrier and rudder stock bearing support. The other
type of vane gear is supported where the stator is only anchored to the ships structure to resist
torque but is free to move vertically within the constraints of the separate rudder head bearing
and carrier which is similar to the bearing provided for ram type steering gears.
The rudder carrier ring bearing (Pallister Bearing) is taking the weight of the rotary vane steering
gear and the rudder and stock.
Rotation of the stator is prevented by means of two anchor brackets and two anchor bolts . The
anchor brackets are securely bolted to the stool and vertical clearance is arranged between the
inside of the Stator flanges and the top and bottom of the anchor brackets to allow for vertical
movement of the rudder stock. This clearance varies with each size of rotary unit but could be
about 40 mm total . It is essential that the rudder carrier should be capable of restricting the
vertical movements of the rudder stock to less than this amount.
The anchor bolts are fitted with special bushes in halves, shaped externally in order to pre-load
the synthetic rubber shock absorbers , which are fitted between them and the anchor brackets.
The maximum deflection of the shock absorbers under full load is approximately 1 mm.
The working angle of the gear is governed by the number of vanes and their thickness. Vanes act
as rudder stops when a moving vane contacts a fixed vane. Valves at inlet to the chambers may
be shut causing a hydraulic lock. In the rotary vane units the Mechanical Advantage is unity at all
angles and hence torque is constant
Torque = p.a.n.r.
where n = number of rotating vanes
Tendfjord Rotary Piston Gear Actuator
This gear consists of a casing around the rudder stock which contains pistons of rectangular
section sliding in angular compartments concentric with the rudder stock. The tiller projects into
a gap between the cylinder, the piston ends abutting onto the tiller but not being attached to it so
that axial movements of the rudder cannot be transmitted to the pistons. Steering gears of this
type operate at hydraulic pressures up to 41 bar (600 lbf/in2) and are in general restricted to low
power application.
As with the rotary vane steering gears the Mechanical Advantage is unity at all angles and hence
the torque is constant.
Torque = p.a.n.r.
where n in this case is unity.
Components
Relief Isolating And Bypass Valves
Hydraulic actuators are provided with relief and bypass valves between complementary pairs of
cylinders or chambers of vane gears. The relief valves are set to lift at pressures above the
normal maximum.
The bypass valves are normally closed but can be opened on a two cylinder gear to enable
emergency steering to be used. On a four cylinder gear one pair of cylinders can be bypassed
while the other pair provide emergency steering at a reduced torque, an instruction plate is fitted
over the controls valve block giving a combination of failures and which valves have to be open
or shut to cope with the emergency etc. It should be noted that if one ram or cylinder in a four
ram system breaks down, then never isolate the cylinder diagonally opposite the damaged unit,
since the steering gear will not operate due to the fact that the remaining two cylinders will be
either on all pressure or on all suction at the same time.
Isolating valves are provided at each cylinder or rotary vane chamber which when closed will
hold the rudder by trapping the oil in the chambers. Isolating valves are also fitted to pumps so
that a pump can be completely shut off from the circuit and removed for servicing while steering
is continued with the other pump.
In the case of gears with duplicated variable stroke pumps, in order to be able to bring a standby
unit quickly into operation, the pump stroke mechanisms are permanently coupled together and
both pumps are left open to the hydraulic circuit. Thus it is only necessary to start up a motor for
the stand by pump to be operative. It is usual to run both pumps in restricted navigation waters.
As a variable stroke pump can operate as a motor if pressure oil is applied to one side while it is
on stroke, it is necessary to prevent wind milling or rotation of the pump which is on stand by
duty.
Otherwise, the output of the operation pump, instead of moving the steering gear would be used
up in rotating the stand by pump.
One method to prevent this,is using a fixed ratchet is provided concentric with the pump driving
shaft. Pawls that can engage this ratchet are carried in the drive coupling. When the pump is on
stand-by the pawls engage with the ratchet and prevent rotation when oil on the delivery side of
the operating pump is on pressure. In this condition the tendency to motor the stand by pump will
always be against its normal direction of rotation. As soon as the pump is started, rotation being
in the opposite direction, the pawls disengage and by centrifugal action fling out against the inner
flange of the coupling completely clear of the ratchet. When a pump is on stand-by and the
rudder is being driven by water pressure in the direction in which it is being moved so as to
generate pressure on what is normally the suction side of the operating pump, this will cause the
stand by pump to rotate in its normal running direction. This means that the pawls will disengage
and the pump will be motored round, allowing the rudder to move more quickly to a new
steering position than the single operating pump will allow.
Another method of protection against rotation of the stand by pump is to fit Servo pressure
operated automatic change over valves in the pipelines; these ensure that the pump can only be
started in the unloaded condition (neutral) and in addition prevents the stand by pump from being
motored by the pump in service.
On some ships it has been discovered that the ball bearing races on the stand-by pump have been
failing due to brinelling of the ball bearings, caused by ship vibrations, and in these cases it is
usual to fit devices which allows the stand by pump to be motored slowly.
When fixed delivery pumps are duplicated in supplying oil to a common hydraulically operated
control valve, an automatic change over valve can be fitted which will isolate the stand by pump
when it is at rest, but will connect it to the actuator when the pump is started up.
Stops And Limit Switches
External or stern posts stops are set at the absolute limit to hard over movement of the rudder ,
protects propeller and ship stern in the event of metal or other failure which allows rudder to
swing in an uncontrolled manner. Mechanical stops on the rudder actuator operate before the
external stop are reached .these take the form of travel limits. Stops on the bridge control operate
before mechanical stops. local controls are set midway. auto pilot controls are set first. It should
be noted that the vanes act as stops on rotary vane gears.
Drive Back Due To Heavy Sea's
Heavy seas acting on the rudder can force the actuator against the hydraulics sufficient to lift the
relief v/v, in which case the rudder will move. Hunting gear will tend to return the gear to its
correct position.
Hand And Power Hydraulic Steering Gears
For small ships during navigational course keeping hand steering can be used, whist during
manoeuvring power steering can be used. These may take the form of chains or simple
hydraulics operated by a fixed delivery pump attached to the steering gears.
"Follow Up" Steering
This is the normal method of steering and involves the feedback of steering angle to the helm.
This is suited to both manual and automatic operation.
The ships heading may be set into the autopilot which can then compare the actual to desired
heading and adjust the rudder angle to suit
"Non-follow Up" Steering
Normally used for back up purposes only. Consists of a single lever per steering gear unit, by
moving the lever in on direction the rudder will begin to turn, the rudder will continue to turn
until the lever is released or it reaches the limit of its operation
Charging A System With Fluid
. In all cases high quality hydraulic oil should be used , containing inhibitors against oxidation ,
foaming, rust and wear and emulsification.
In order to keep the transmission load as low as possible when hand steering , hand power
systems must have oil of low viscosity.
The condition of the oil should be monitored and ensured at least clean and free of moisture.
Steering gear failure
A study of steering gear defects demonstrates that the most common are related to vibration and
the working loose of components.
The most common source of failure are the pump and the hydraulic system associated with it.
Rudder torque calculations
Formulae for assessing rudder torque's are based upon the expression Ta ACpV2Sin q where:T = rudder torque
C = rudder area
Cp = centre of pressure distance from centre line of rudder stock
V = velocity of ship
q = rudder angle measured from mid-ship position
In practice different constants obtained empirically are used with this expression and take into
account such factors as propeller slip and wake speed as appropriate depending upon the relation
of the rudder and propeller positions. The position of the centre of pressure has a significant
effect upon rudder torque and hence the size of the steering gear required; the greater the
distance of the C of P from the centre line of the rudder stock, the larger the torque required;
therefore designers attempt to bring the C of P as near to the centre line as possible. With the
simple "barn door" type rudder on some single screw ships, no adjustment can be made, but the
semi-balanced and balanced-type rudders can be designed to reduce the torque required; for
instance, with the spade type rudder such as fitted to twin screw ferries, the position can be
adjusted by the designer to give optimum position. This lies between 30 and 32 per cent abaft the
leading edge of the mean chord of the rudder. Such a rudder would have its C of P forward of the
stock position at low angles of helm, would balance around 10o to 15o and drift aft of the stock
at higher rudder angles.
In graph above is shown a typical torque characteristics for a spade type balanced rudder and a
"barn door" or unbalanced plate rudder. The astern torque's should also be calculated since this is
sometimes higher than the ahead torque, this is true for spade rudders.
POWER
The peak power that a steering gear must develop is the product of the maximum torque (T)
usually at hard over with the ship travelling at full speed, and the maximum speed (S) of rudder
movement i.e. Power (max) a T x S.
The combination of maximum power and speed only exists for 2 or 3 seconds during each
manoeuvre; so clearly the average power required to operate the steering gear is considerably
below the peak. Because the steering gear must have sufficient power to overcome friction and
still have ample reserve of power, the value for used in the foregoing expression is significantly
higher than that used in the expression for rudder torque. When considering the diameter of the
rudder stock, bending and shear stresses must be taken into account.
Rudder Wear Down
This refers to the measurements taken generally during a docking period to indicate excessive
wear in the steering gear system particularly the rudder carrier. The significance of this is that for
ram systems excessive wear can lead to bending moments on the rams. For rotary vane systems
it can lead to vane edge loading.The readings taken are offered for recording by the classification
society.
Trammel
This takes the form of an 'L' shape bar of suitable construction. When the vessel is built a distinct
centrepunch mark is placed onto the ruder stock and onto a suitable location on the vessels
structure, here given as a girder which is typical. The trammel is manufactured to suit these
marks As the carrier wears the upper pointer will fall below the centrepunch mark by an amount
equal to the wear down.
Rudder Clearance
Pads are welded to the hull and rudder. A clearance is given ( sometimes referred to as the
jumping clearance). As the carrier wears this clearance will increase
Steering gear Clearance
Direct measurement can be taken from the steering gear assembly. Shown below is one example,
here the clearance will be seen to reduce as the carrier wears and impact his has on the system
can be directly judged
Rules
Design of steering gears have been influenced over the years by the rules and regulations of
national authorities and classification subjects. Any changes of real substance tend nowadays to
originate from the international
Maritime Organisations(I.M.O.) conventions and regulations. Classification society
requirements are as follows;
1.
All ships to have power operated main gear capable of displacing the rudder from
35o port to 35o starboard at the deepest draught and at maximum service speed. Must also be
capable of displacing the rudder from 35o port to 30o starboard in 28 seconds and vice versa.
2.
The auxiliary gear must be power operated and capable of being brought rapidly into
action. The auxiliary gear is only required to steer the ship at either 7 knots or half service speed
3.
If the main gear comprises two or more identical power units, then a single failure of
either power unit or piping must not impair the integrity of the remaining part of the steering
gear
4.
Each power unit must be served by at least two electrical circuits from the main
switchboard. One circuit may pass through the emergency switchboard. All circuits to be
separated as widely as possible throughout their length.
5.
All power operated gears to be fitted with shock relieving arrangements to protect against
the action of heavy seas.
6.
An efficient brake or locking arrangement to be fitted to enable the rudder to be
maintained stationary
7.
the maximum power developed by the gear is proportional to T x S
where T = rudder torque
S = Speed of rudder movement
also T = A x P x sinq x V2
where A = rudder area
P = centre of pressure
q = rudder angle
V = velocity of the ship
Special requirements
Owners may specify additional requirements such as faster hard-over to hard-over time, strength
of components above that required by the Rules, additional control points and additional
duplication,
New tankers of 100 000dwt and above-shall comply with the following
The main steering gear shall comprise of either
two independent and separate power actuating systems each capable of meeting the hard
over port to 30o starboard in 28 sec requirements,
or
at least two identical power actuating systems which acting simultaneously in normal
operation, shall be capable of meeting the hard over requirements. Where necessary to comply
with this requirement inter connection of hydraulic power systems shall be provided. Loss of
hydraulic fluid from one system shall be capable of being detected and the defective system
isolated so that the other system shall remain fully operational
In the event of loss of steering capability due to a single failure other than the tiller, quadrant or
components serving the same purpose (these are excluded from single failure concepts), or
seizure of the rudder actuators. The steering capability shall be regained in not less than 45
seconds after the loss of one power actuating system.
Steering gear other than hydraulic should meet the same standards.
Example of suitable system permissible for all ships
The system shown consists of two sets of rams but could equally be two rotary vane units. With
no power on the solenoids are in by-pass mode with oil being allowed to pass freely from one
side to the other. When an electric motor is started the control pump supplies oil to the solenoid
shutting it. High pressure oil from the main unit is now fed to the rams as required. The other
unit remains in by-pass until the electric motor is started.
Low level alarms are fitted to the tanks. Low low changeovers may also be fitted so that in the
event of oil loss from one system, the other system is started.
New tankers between 10 000gt upwards to 100 000tdwt
For these tankers the single failure criterion need not apply to the rudder actuator or actuators
subject to certain requirements being fulfilled. These include a requirement that steering be
regained within 45 seconds following failure of any part of the piping system or power units and
a special stress analysis of non-duplicated rudder actuators.
The left hand unit is shown in operation.
For this basic arrangement the power units must be identical
New ships 70 000gt and upwards
system suitable for all ships except tankers of 10 000 gt and above
The main steering shall comprise two or more power units and that the main steering gear is so
arranged that, after a single failure in its piping system or in one of the power units the defect can
be isolated so that steering can be speedily regained.
'Speedily' is intended to mean the provision of duplicate hydraulic circuits or , for example, a
conventional four ram steering gear with a common hydraulic circuit with appropriate isolating
valves
New ships of less than 70 000 gt and tankers less than 10 000 gt
suitable system
Single failure is not applicable as a rule, however, attention is drawn to the requirement that
auxiliary steering gear be independent of any part of the main gear except the tiller. There is no
requirement that main and auxiliary power units be identical.
The auxiliary steering gear must be capable of putting the rudder over from 15ofrom one side to
the other in not more than 60 seconds with the ship at its deepest draught and running ahead at
half maximum speed or 7 knots.
Existing tankers of 40 000gt and upwards
The steering gear shall be arranges so that in the event of single failure of the piping or one
of the power units, steering capability can be maintained or the rudder movement can be
limited so that steering capability can be speedily regained by
An independent means of restraining the rudder
or
fast acting valves to isolate the actuator or actuators from the external hydraulic piping
together with a means of directly refilling the actuators by a fixed independent power pump and
piping system
or
An arrangement so that, where hydraulic power systems are interconnected any loss of
hydraulic fluid from one system shall be detected and the defective system shut off either
automatically or remotely from the bridge so that the other system remains intact
Requirements for all new ships
Administrations must be satisfied in respect to the main and auxiliary steering gear
provided for every ship that all components and the rudder stock are of sound construction
Every component, where appropriate, utilise anti-friction bearings which will be
permanently lubricated or provided with lubricant fittings
Parts subjected to hydraulic pressures should be designed to cope with 1.25 maximum
working pressure when the rudder is hard over at maximum draught and service speed
special requirements for fatigue resistance( due to pulsating hydraulic pressure), relief
valves and oil cleanliness
Low level alarm to be fitted to each hydraulic reservoir.
Fixed storage capacity sufficient to recharge on system
Auxiliary steering gear
The other set of steering (auxiliary ) may be an arrangements of blocks and tackles or some other
approved alternative method.
The auxiliary steering gear need only be capable of steering the ship at navigable speed, but it
must be capable of being brought speedily in to action in an emergency. Navigable speed is one
half of maximum service speed ahead or 7 knots whichever is the greater.
The auxiliary steering gear must be a power operated type if the rudder stock exceeds 230mm for
passenger ships and 250mm for cargo vessels. No additional means of steering is required when
electric or electro-hydraulic steering gear is fitted having two independent motors or two sets of
pumps and motors.
Electrical Supply
Short circuit protection and overload alarm are to be provided in steering gear circuits. Indicators
for running indication of steering gear motors are to be installed on the navigation bridge and at a
suitable machinery control position. Each electric or electro-hydraulic steering gear shall be
served by at least two independent circuits fed from the main switchboard. Cables for each
circuit led through a separate route as far apart as possible so that damage to one cable does not
involve damage to the other. A change over switch is fitted in an approved position to enable
power supplies to be interchanged. One circuit may pass through an emergency switchboard.
Rudders
In passenger ships where the rudder stock exceeds 230mm, an alternative steering position
remote from the main position is to be provided. Failure of one system must not render the other
system inoperable. Provision made to transmit orders from bridge to alternative position. The
exact position of the rudder must be indicated at principal steering positions. An efficient braking
or locking device must be fitted to the steering gear to enable the rudder to be held stationary if
necessary. Spring or hydraulic buffer relief valves fitted in steering gear system to protect the
rudder and steering gear against shock loading due to heavy seas striking the rudder. Suitable
stopping arrangements are to be provided so as to restrict the total travel of the rudder. Stops or
cut outs on the steering gear are arranged so that it operates on a smaller angle of helm than the
rudder stops.
Rudder restraint
Since failure of a single hydraulic circuit can lead to unrestricted movement of the rudder, tiller
and rams, repair and recharging may not be possible. Difficulty arises with which the speed a
restraint whether in the form of a mechanical or hydraulic brake can be brought in to use.
Due to the possibility of considerable damage occurring before it could, regulations have
concentrated on continuity of steering rather than a shut down and repair solution
Testing and drills
Within 12 h before departure, the ship's steering gear shall be checked and tested by the ship's
crew. The test procedure shall include, where applicable, the operation of the following:
the main steering gear;
the auxiliary steering gear;
the remote steering gear control systems;
the steering positions located on the navigation bridge;
the emergency power supply;
the rudder angle indicators in relation to the actual position of the rudder;
the remote steering gear control system power failure alarms;
the steering gear power unit failure alarms; and
automatic isolating arrangements and other automatic equipment.
The checks and tests shall include:
the full movement of the rudder according to the required capabilities of the steering
gear;
a visual inspection of the steering gear and its connecting linkage; and
the operation of the means of communication between the navigation bridge and
steering gear compartment.
Simple operating instructions with a block diagram showing the change-over
procedures for remote steering gear control systems and steering gear power units
shall be permanently displayed on the navigation bridge and in the steering gear
compartment.
All ships' officers concerned with the operation or maintenance of steering gear shall
be familiar with the operation of the steering systems fitted on the ship and with
the procedures for changing from one system to another.
In addition to the routine checks and tests prescribed in paragraphs (a) and (b), emergency
steering drills shall take place at least once every three months in order to practise
emergency steering procedures. These drills shall include direct control from within the
steering gear compartment, the communications procedure with the navigation bridge
and, where applicable, the operation of alternative power supplies.
The Administration may waive the requirement to carry out the checks and tests prescribed in
paragraphs (a) and (b) for ships which regularly engage on voyages of short duration.
Such ships shall carry out these checks and tests at least once every week.
The date upon which the checks and tests prescribed in paragraphs (a) and (b) are carried out
and the date and details of emergency steering drills carried out under paragraph (d), shall
be recorded in the log-book as may be prescribed by the Administration.
