Steering
Units 10.1 to 10.7
Types of Steering Gear
• A steering gear is a machinery fitted on ships to facilitate the
movement of the ship’s rudder which is used to steer the ship on its
desired course.
• There are three types of steering machineries used on merchant
ships : All electric steering machinery, Ram steering systems, and the
Rotary Vane steering gear.
• The most commonly used types are the Ram type and the Rotary
Vane type.
• In Ram steering systems there are further two types – The Two Ram
steering gear and the Four Ram steering gear.
• The number of rams used in dependant on the amount of torque
that is required to turn the rudder.
• This means when the torque required in large the steering usually
employed is the Four Ram type.
• For moderate torques the Two Ram system is sufficient.
Two Ram Hydraulic Steering gear
Four Ram Electro Hydraulic Steering Gear
Telemotor System
• This consists of the transmitter and the receiver unit.
• The transmitter is equipment, which sends a signal for the change of
direction of motion of the ship and
• the receiver is the equipment, which receives this signal and makes
the actuating system to work and respond to this order.
Hydraulic
transmitter
• The transmitter consists of two rams ending in racks, which mesh with a
primary pinion attached to the steering wheel shaft and housed in a
casing.
The unit consists of two parts:
• The top which houses the racks and the pinion and serves as the
replenishing or make up tank, the racks and pinion being lubricated by this
fluid.
• The bottom half which is made of two cylinders in which the reciprocating
rams pass through sealing glands.
Bypass Valve:
Since the cylinders are always full of the hydraulic fluid a device has
to be fitted to allow for the variations in the oil volume due to
temperature changes, and relief in case of a build up of pressure and
for equilibrium between the two cylinders when the wheel is at
midships. This device is called a “Bypass Valve”.
Telemotor
- Receiver
Receiver – Construction & Working
• It consists of two hollow rams in line, fitted to two fixed crossheads.
A cylinder closed at mid length and open at each end, fits over and
floats between the rams. Sealing glands are fitted at each end of the
cylinder and air cocks provided on both sides.
• The crosshead and the rams are kept fixed and aligned by two parallel
rods. The rods have two sleeves each fitted over them and sliding
through the cylinder casing.
• The stopper nuts as shown restrict the movement. Each set of sleeves
carry a strong spring between them. The crossheads are connected by
supply/return pipe, to the telemotor transmitter.
• Slight movement of the moving cylinder causes the left hand sleeve to
butt against the stopper nut and further movement causes the spring
to get compressed.
• When the steering wheel is returned to midship the springs, which
are under compression, return the moving cylinder to its mid position.
Two Ram Hydraulic Steering gear
In a two- ram system the operating principle is as follows:
Considering the telemotor receiver cylinder moving from left to
right, this movement moves the lever, which has a fulcrum at the
center in a clockwise direction. The floating lever top moves to the left
and along with it pulls the control lever of the pump out (or also to the
left), pivoting at the bottom point.
Considering a Hele Shaw pump, which is running all the time, when
the floating ring is moved it starts the pump. Hydraulic fluid is sent to
the right hand cylinder and sucked back from the left hand cylinder.
The starboard (or right) ram, not shown is pushed from right to left,
which in turn pushes the port ram as both are connected by a crosshead.
The tiller arm, which is at right angles to the center line of the rams and
fitted to the crosshead, thus moves to the left. To the other end of the tiller
arm is keyed the rudderstock which then rotates and turns the rudder to
starboard.
The tiller arm is connected to the bottom portion of the floating lever by
a buffer spring and link. This is called the hunting gear.
The floating lever movement to the left pulls the pump control lever
also to the left with the pivoting taking place at the bottom point.
When the tiller arm turns it pushes the hunting link to the right, which
in turn pushes the pump control lever Inwards and to mid position with
pivoting taking place at the top point.
At mid position the pump is at “no stroke” and thus the rudder is
stopped at that helm angle.
For the rudder to be brought back to the midship position, the
telemotor receiver cylinder will have to move to the left, the floating
lever top to the right, pushing the pump control lever further in.
Pump will start and the tiller will move in the opposite direction,
pulling the hunting link to the left.
This will in turn, will pull out the control lever of the pump back to
mid position, pivoting about the top floating lever point.
NOTE:
The buffer spring in the hunting gear link in provided to take up any
excess movement beyond the maximum stroke of the pump.
This extra movement is stored by the compressed spring and used
to reset itself as the hunting gear approaches the no effect point.
It also takes up the effect of heavy seas on the rudder
Depending upon the torque requirement, a ship may have 2 ram or 4
ram steering gear arrangement
Some ships are provided with a 4-ram hydraulic steering gear. Here
there are four rams instead of two. Such a system gives freedom of
isolating any leaky part from the system and thus ensuring availability
of steering power at all times.
Rotary Vane Steering Gear
It consists of a rotor which is a taper fit on the rudder stock and
keyed to it and
a stator of larger internal diameter than the outside diameter of the
rotor, to form an annular space between them. The stator is firmly
fixed to the ship’s structure by anchor bolts to prevent it from rotating.
A variable delivery pump delivers into one side and receives from
the other side, till the rudder is at the required position.
At this point the feed back lever L3 brings the pump to neutral
(zero pumping position) and the rudder gets hydraulically locked.
Compared with the 4 ram hydraulic steering gear, the cost of the
rotary vane type to produce the same torque on the rudder stock is
generally less.
It is lighter in weight, takes up less space and requires less
maintenance.
SOLAS rules pertaining to steering gears
• All vessels must be provided with efficient main and auxiliary
steering gear of power operated type. An auxiliary gear is not
required if the main gear is provided with duplicate power units and
duplicate connections up to the rudder stock.
• The main steering gear must be able to steer the ship at maximum
ahead service speed and be capable at this speed, and at the ship’s
deepest draught, of putting the rudder from 35 degree on one side
to 30 degree on the other side in no more than 28 seconds.
• The vessel must have means provided to allow steering from a
position aft.
• Power operated gears must be fitted with a device to relieve shock.
• Any lead connections, steam, hydraulic or electric should be
independent to the gear only. Electric leads and fuses are to allow
100% overload.
• Moving parts of steering gears should be guarded to avoid injury to
personnel.
• Hydraulic systems should employ non-freezing fluid. Steering
room floors should have wooden gratings to provide for easy
movement of personnel in case of oil spill.
• A clear view from the steering position is required and the wheel, tell-
tale indicators, and rudder movement must correspond in the correct
amount and in the correct direction for the ship’s head.
• Operating trials should be carried out on steering gears to ascertain
degree of action time of operation, angle of heel at various speeds, etc.
• The emergency steering gear must be capable of being brought
into operation speedily and be able to steer the ship at a navigable
speed and move the rudder from 15 deg on one side to 15 deg to
the other side within 60 seconds while the ship is travelling at half
speed or 7 knots which ever is greater.
• 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 capability can be maintained or speedily regained
• The steering gear must be easily and quickly changed over from
automatic control to hand control. The vessel must be capable of
being steered from position aft and have a system fitted where orders
can be received from the bridge to the position. The exact position of
the rudder must be indicated at the main steering.
Effect of a wave hitting the rudder
At any position of the rudder, both sides of ram cylinders are full of
oil and rudder is hydraulically locked. Rudder therefore cannot be
disturbed from it’s position even if external forces like wave pressure
etc. try to do so. The tendency of rudder to move will only cause oil
pressure in the respective ram cylinder to increase.
However, during such tendency the rudder stock will actually experience
twisting moment.(Wave forcing the rudder to move from the bottom end of
the rudder stock and simultaneously the locked rams preventing the rudder
to move from top end).
Now, if this twisting torque is excessive the rudder stock may twist or
even shear off. To prevent shearing off, and other damages, a set of relief
valves are incorporated in the hydraulic system.
Mechanical Stops & Limits in a Steering Gear
• As per regulations a steering gear system should be capable of
putting the rudder from an angle of 35* on one side to 35* on
the other side (statutory requirement is to put rudder from 35* on
one to 30* of the other side) in 28 seconds. To ensure that these
limits are maintained there are a number of stops and limit
switches.
Mechanics of a ship turning due to the rudder
• Configuration: normally one rudder for each propeller, mounted
directly astern of the propeller.
• Range of motion: 35° Port or Starboard of CL
• Hard rudder: 35°
• Full rudder: 30°
• Standard rudder: 15°
• Rudder effectiveness is dependent on flow velocity over the
rudder surface.
• “Bare steerageway” is the minimum speed at which a rudder
is still effective. This is generally 2 to 3 knots for most ships.
• Rudder angles must be reversed for astern operation
When the helmsman changes the rudder angle from zero to some angle towards
the starboard, at that very moment, a lift force acts on the rudder. The direction
of the lift force is towards the port side, as illustrated in Figure 1. Rudder force
on a ship with rudder to starboard.
This rudder force, is directed along a transverse direction to the ship. In
other words, this force will cause the ship to attain a sway velocity
towards the port side, because the rudder force is nothing but a sway
force towards the port side. It is because of this, a ship will sway slightly
to the port when the rudder is turned over to hard starboard.
Other than this, the rudder force has another effect on the ship. It creates a
moment about the centre of gravity of the ship, in the direction as shown in
Figure 2. The centre of gravity of the ship is forward of the rudder, and given the
direction of the rudder force, the moment it will create about the C.G will be
along the direction shown.
• Acts a wing
Water
Flow
High Pressure Area Low Pressure Area
Rudder
Force
Propellers / Rudders
• Primary means of controlling the stern
Thrust
Side Force
Rudder Force
Options during operational leaks
With regard leakage in the system, we have following options:
i) Isolating valve 1, 2, 3, & 4 open: By pass valve 5 & 6 shut. All four rams in
operations.
ii) Isolating valve I & 2 shut. By pass valve 5 open: Isolating valve 3 & 4 open,
By pass valve 6 shut. Only Ram 3 & 4 in operations: Rams I & 2 are isolated.
(may have to be isolated because of leakages).
iii) Isolating valve 3 & 4 shut, By pass valve 6 open: Isolating valve I & 2 open,
By pass valve 5 shut. Only Ram I & 2 in operation. Rams 3 & 4 are isolated
(may have to be isolated because of leakages).
Safematic Design of Steering System
If, in the 4 Rams hydraulic system described, the leakage had
developed at say point "X" (as happened in case of `MT Ameco
Cardiz' that resulted in total loss of the tanker and its cargo), there
would be no way to isolate this leak to have the steering capability
of the system available to the ship after the leak.
To take care of such leaks (i.e. leaks outside isolating valves of the
rams), safematic designs have been developed.
