A rudder allows the ship to turn, simple plates have been superseded by plates welded to cast or fabricated frame. Rudders are hollow and so provide for some buoyancy . In order to minimise the risk of corrosion internal surfaces are provided with a protective coating and some are even filled with foam. A drain plug is provided to allow for the drainage of water , enable internal inspection to be made using fiber optic device and even allow for the limited application of a protective coating. Plates are welded to the frames internally in order to provide flush fitting , the final closing plate must be welded externally. A means of lifting is provided taking the form of a tube as close to the center of gravity as possible. Rudders are tested to a pressure head 2.4m above the top of the rudder.
If the rudder has its entire area aft of the rudder stock then it is unbalanced .A rudder with between 20 and 40% of its area forward of the stock is balanced since there will be some angle at which the resultant moment on the stock due to the water force will be zero. Most modern rudders are of the semi-balanced design. This means that that a certain proportion of the water force acting on the after part of the rudder is counter acted by the force acting on the for'd half of the rudder; hence, the steering gear can be lighter and smaller. A rudder may lift due to the buoyancy effect, the amount of lift is limited by the jumper bar fitted to the stern frame. The jumper/rudder clearance must be less than the steering gear cross head clearance to prevent damage. A rudder is supported by means of a bearing pintle or a lower bearing depending upon the design. Where a lower bearing is employed the rudder is actually supported on split bearing rings fitted on the lower face of the rudder and the upper face of the sole piece ( the extended lower section of the stern frame upon which the rudder sits)
SEMI BALANCED RUDDER
Semi balanced rudder with rudder horn
Fully balanced rudder
To reduce the amount of torque required to turn a rudder the pivot point is moved back from the leading edge. The amount of torque then varies depending on the angle of attack. Zero torque leads to instability with rudder moving within its clearances.
The reduced diameter at the upper part is purely to transmit torque. The lower section must also support bending moments and hence increased diameter. With twin rudder ships the inner rudder must turn through a greater angle than the outer. This is achieved by having the tiller arm at an angle to the centre line of the rudder.
It is possible to have the blades angled in or out when the wheel is amid ships to increase propulsive efficiency.
SPECIAL STEERING DEVICES
THE KORT NOZZLE
Adequate clearance is essential between propeller blade tips and sternframe in order to minimise the risk of vibration. As blades rotate water immediately ahead of the blades is compressed and at the blade tips this compression can be transmitted to the hull in the form of a series of pulses which set up vibration. Adequate clearance is necessary or alternatively constant clearance, this being provided with ducted propellers such as the Kort nozzle Originally designed to reduce erosion on river banks the nozzle has proved itself also able to increase thrust without increase of applied power.
The nozzle consists of a ring of aerofoil section which forms a nozzle surrounding the propeller. The suction of the propeller causes an acceleration of flow in the mouth of the nozzle and hence a drop of pressure in this region. Since the pressure on the outer part of the nozzle remains relatively unchanged, there is a resulting differential in pressure, which acting on the projected annulus of the nozzle, gives the additional forward thrust. This additional thrust is transmitted direct from the kort nozzle to the hull via the nozzle supports ,so that no additional force acts on the propeller and shaft thrust block.
There are two types of Kort nozzles. The fixed type has a conventional rudder behind it, whereas with the swivelling rudder type , the whole assembly is supported by a carrier attached to the rudder stock and actuated by the steering gear.
In the case of nozzle rudders ,when helm is applied , the increased thrust has an athwartship component which has powerful steering effect, so that hard over angles of 25' ( or 30' in special cases ) are sufficient to provide effective steering ahead during a crash stop and ,provided the hull is a reasonable design , astern.
This device is especially valuable for tugs, trawlers, special vessels and more recently ,VLCC, which are required to manoeuvre well , particularly at slow speed , and have the best propulsive efficiency.
Bollard pull gains between 30 and 50% , equivalent to re-engining up to 1 3/4 times the original power , have been obtained in tugs and trawlers and in VLCC gains in propulsive efficiency between 6 to 13% can be expected. The normal method of calculating rudder torque's can be applied to nozzle rudders . The maximum steering effort is required to return the rudder towards midships and not to move the rudder over from amidships. Thus , the steering gear must be designed to keep control of the rudder under these conditions. For diagram and additional notes see 'method of reducing vibration' and 'increasing propulsive efficiency'
A normal rudder can only be effective when the ship is moving, and the torque it exerts varies with the square of the speed ,so that at very low speeds it can be very ineffective. A pleuger rudder incorporates a submersible electrically driven propeller which can be run when the main propulsion is at rest . In order to attain maximum effect and manoeuvre the ship at rest the rudder is able to turn to 90',owing to this normal floating linkage hunting gear cannot be used, and a special cam hunting gear used. For normal course keeping the angle is limited to 35',and a warning signal initiated if exceeded.
THE VOITH SCHNEIDER PROPELLER
This propeller consists of a series of blades of aerofoil section which project vertically downwards from the ship's hull and rotate about a vertical axis. The blades are mounted on axes on a circle around the central axis and are linked together with a mechanism which can cause them to oscillate so as to provide thrust in any direction. The amount of thrust can be varied by varying the degree of oscillation, thus with the blade assembly rotating in the same direction, manipulation of the blades can give ahead or astern thrust, or port and starboard thrust without an ahead or astern thrust component, or any angle of port or starboard thrust with ahead or astern thrust.
The cycloid motion of the blades can be made to produce thrust in any direction round the circle of rotation. This means that the propeller in addition to providing the main drive for a ship provides full manoeuvrability without the need to provide a rudder and steering gear. This unusual degree of control is of particular value for special craft or floating equipment such as floating cranes or drilling ships which must be kept in position.
The location of the propeller depends upon the particular application and it can be placed where the maximum desired effect can be achieved.
JET FLAP RUDDER
Another device which is being investigated at the N.P.L. is the jet flap rudder. In the trailing edge of an otherwise conventional rudder, a fluidic switch is fitted, which can direct a jet of water to port or starboard. The water is pumped into the hollow rudder through a hollow rudder stock.
Considerable increase in manoeuvrability is claimed, especially at low speeds.
ROTATING CYLINDER RUDDERS
This is a device to make a ship equally manoeuvrable at all speeds and was developed in the U.K by the Ship Division of the National Physical Laboratory (N.P.L.).
A normal rudder is effective up to angles of about 35', after which the flow over the rudder stalls in a manner similar to that over an aeroplane wing at high angles of incidence. There are various methods of preventing this from occurring and they all involve feeding energy into the stream of fluid adjacent to the rudder or aerofoil surface. This is called boundary layer control. One such method is to rotate a cylinder at the leading edge of the section at such a speed that the rudder can be put over to 90' without stall, and this is the basic principle of operation of the rotating cylinder rudder. It is, of course, necessary to reverse the direction of rotation of the cylinder depending on whether the rudder is put to port or starboard, and such a system can be fitted to almost any type of rudder, balanced or unbalanced.
The major advantage of putting a rudder over to such a high angle is that the flow from the main engines may be deflected through a much larger angle than with a conventional rudder, and static side thrusts of over 50 per cent of the bollard pull have been measured. Another main advantage is that its effect is independent of forward speed and it works as effectively at zero as at full speed.
The flap is attached to the hull. As the rudder rotates the flap is turned in an same direction increasing the aerofoil shape and thereby increasing lift.
This system can be used at very large rudder angles giving side thrust capabilities
This design allows for larger turning forces for the same sized rudder or reduced size requirements compared to other sizes
Another format allows the flap to be steered independently. This again allows increases in thrust as well as improving steering with the vessel underway as only the flap has to be turned to cause small corrections to be carried out
MANOEUVRABILITY AND STOPPING
The problems of improving the manoeuvrability and braking of ships is of increasing importance as they increase in size. One device which is being tried out to improve stopping power is to arrange the control of twin rudders so that they move outboard simultaneously. This involves two separate steering gears, one for each rudder, the movement of which must be synchronised for normal steering.
With twin rudder ships the inner rudder on a turn must turn through a greater angle than the outer . This is achieved by having the tiller arm at an angle to the center line of the rudder . It is also possible to slightly angle the rudders either in or out to increase propulsive efficiency.
Pumps suitable for steering gear systems
Pumps used for supplying the working fluid to the main steering gear can be of either the variable capacity reversible delivery type or the fixed delivery non-reversible type. For large capacity outputs with high rates of change in demand, the variable capacity pumps are normally fitted. These are of two main types, the Hele-Shaw variable stroke pump having radial cylinders and the swash plate variable stroke pump having axial cylinders.
Left shows the construction and operation of this type of pump which is normally driven by a constant speed electric motor. The pistons are fitted in a row of radial cylinders and through the outer end of each piston is a gudgeon pin , which attaches the slippers to the piston. The slippers are free to oscillate on their gudgeon pins and fit into circular grooves in the circular floating ring . This ring is free to rotate being mounted on ball bearings , which are housed in guide blocks , this reduces oil churning and friction losses. The latter bear on tracks are controlled by the actuating spindles, which passes through the pump casing. The movement of the floating ring by the actuating control spindle (operated by, say, the telemotor receiver) from the central position causes pistons to reciprocate in the radial cylinders so that a pumping action takes place. The direction of the pumping depends upon whether the movement is to the left or right of the central or neutral position.
The action of the pump is shown above.
It should be noted that an advantage with this system is that in reversing the direction of the flow of fluid, the pump moves from maximum delivery in one direction across to zero delivery then through zero delivery to maximum delivery in the opposite direction. The build up in fluid pressure taking place without shock loading of pipe lines supplying fluid to the main steering unit.
The pump is usually provided with an odd number of cylinders, usually seven or nine, which produces more even hydraulic flow and a better balanced pump.
Variable Stroke Reversible Swashplate Pump
This pump is sometimes called the variable stroke gear pump (V.S.G), it runs in the flooded condition, the make up tank being above the level of the pump so that all the working parts are immersed in oil. It is driven by a constant speed electric motor the volume and direction of the oil flow being controlled by means of a stroke control lever.
The VSG pump is stated to have some advantages over the Hele-Shaw , this is due to the fact that the c of g of the Hele Shaw plungers is a relatively large distance from the centre of rotation operating relatively large centrifugal forces . The VSG plunges have a c of g close to the centre of rotation creating relatively small centrifugal forces, this means that the VSG system can be run at much higher speeds and therefore can be much smaller whilst doing the same work as the Hele-Shaw. Due to centrifugal forces acting, the wear on the V.S.G. pump pistons can be greater than that for the radial type pistons There is a small clearance between the valve and cylinder blocks when running off load. When the unit comes on stroke the hydraulic pressure forces the two faces together.
External pipes connect ports to steering gear.
VSG pumps and Hele-Shaw pumps have an odd number of cylinders since calculation shows that this gives better hydrodynamic balancing (and a better starting torque when used in a pump driving hydraulic motor).
Some manufacturers supply an auxiliary pump driven from the main pump shaft, which draws oil from the replenishment tank, delivering through non return valves to each side of the main hydraulic system. A low pressure relief valve opens to return the auxiliary pump delivery back to the replenishment tank if the main system is full, at the same time keeping equivalent initial pressure on the whole system. This ensures the lubrication of the main pumps when at no stroke and resists the ingress of air into the system. Pressure from this pump can also be used to power the automatic helmsman control, to operate change over valves, or to power servo control units which in large installations may be used to operate pump stroke mechanisms and so reduce the force required from the telemotor.
Constant Pressure Pumps
The constant pressure delivery pump is a standard production line , cheap pump; it runs constantly delivering a set volume of liquid whose pressure must be regulated .Recirculating v/v's allow oil to by-pass rams when stationary, an oil cooler may have to be incorporated.
Valves must be incorporated to divert the flow of oil to one side or the other. These normally take the form of electrically operated solenoid valves which are subject to wear, as well as damage to seats and solenoid coils.
Shock loading to rams and pipework causing noise and vibration as well as damage.
Rudder Carrier bearings
The rudder carrier bearing takes the weight of the rudder on a grease lubricated thrust face. The rudder stock is located by the journal, also grease lubricated. Support for the bearing is provided by a doubler plate and steel chock. The base of the carrier bearing is located by wsdge type side chocks, welded to the deck stiffening. The carrier is of meehanite with a gunmetal thrust ring and bush. Carrier bearing components are split as necessary for removal or replacement. Screw down lubricators are fitted, and the grease used for lubrication is of a water resistant type (calcium soap based with graphite).
A small allowance is made for weardown, which must be periodically checked. This may be measured either between pads welded on top of the rudder and onto the rudder horn, or between the top of the rudder stock and a fixed mark on the inner structure of the steering gear flat. The latter generally involves the use of a 'Trammel gauge' which takes the form of a 'L' shaped rod ade to fit the new condition of the gear. As wear down occurs it can easily be checked with this gauge.
The rudder is prevented from jumping by rudder stops welded onto the stern frame.
Rudder movement stops
Rudder stops are arranged as follows;
Angle from centreline Position of stop Note
35o On telemotor system Normal limit
37o On steering gear Prevents rudder striking external stops
39o External, on stern frame emergency stop to protect propeller
These limits refer to rudders of traditional design and is governed by both the physical layout of the rudder and actuator but also due to the stall angles of the rudder. i.e. the angle at which lift ( turning moment ) is reduced or lost with increasing angle of attack. There are designs of rudder such as becker flap which have increased stall angles up to 45o
Critical profiling of carrier bearing
Reasons for critical contouring of thrust face;
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.
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.
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.
The main problem appears to be the effect of air entrained within it. Thus regular venting of the system is required.
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.
should be a good quality mineral oil with the following properties;
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.
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.
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 ACpV2 Sin
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.
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.
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.
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
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;
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
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
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
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 15o from 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
Requirements for all new ships
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.
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.
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.
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.