Dynamic Positioning

Operation and maintenance

C11
Thrusters and Maneuvering Systems,
Power Generation and Supply
 Propulsion and steering systems
To keep steady position when in DP mode, vessel has to contract various environmental forces.
Efficient propulsion and steering system is a must. Traditional set rudder + propeller is not
enough to control all three axes (surge, sway, yaw) in efficient way, therefore modern DP vessels
are fitted with sophisticated thrusters allowing to control ship’s movement.

Types of rudders: Thruster Types:

• Conventional rudder • Main propellers

• Schilling rudder • Tunnel thrusters
• Becker rudder • Azimuth thrusters (in various versions)
• Nozzle rudder • Gill Jet
• Schottel propeller • Water Jet
• Voith-Schneider thruster

In the context of dynamic positioning requirements propellers working with rudders can be
considered as thrusters as well.
 Pivot point

• Pivot Point is the center of lateral resistance

• It is the point around which the vessel will appear to turn
• When a vessel is dead in the water, the pivot point will be
near amidships if the vessel is on an even trim
• As a vessel gains sternway, the pivot point will move
toward the stern
• When a vessel gains headway, the pivot point will move
toward the bow
The type of rudder that would suit a particular ship is a decision that needs to be based
on various factors like hull form, speed, propeller design, the structural arrangement of
the stern, clearance between the propeller and the stern, and also a few 
hydrodynamic factors that dictate the flow of water aft of the propeller.
How ship designers go about deciding the type of rudder, is actually an iterative process.
So what designers and naval architects
 do is, estimate a very approximate
dimension of the rudder along with the
propeller. However, what becomes
significantly important from a designer’s
point of view, is deciding on the type and
location of the rudder, depending on the
hull and propeller design.
The location of the rudder should be
such that it is properly oriented within
the propeller’s outflow, so as to produce
the required turning moment on the ship.
1. Spade or Balanced Rudder
A spade rudder is basically a rudder plate that is fixed to the rudder stock only at the top
of the rudder. The position of the rudder stock along the chord of the rudder (width
meaning, from the forward to aft end of the rudder) actually decides whether the rudder
is balanced of semi-balanced one. In balanced rudders, the rudder stock is at such a
position such that 40% of the rudder area is forward of the stock and the remaining is aft
of it.
The centre of gravity of the rudder will lie
somewhere close to 40% of its chord length from
its forward end. If the axis of the rudder is placed
near to this location, the torque required to rotate
the rudder will be much lesser than what is
required to move it, had the axis been placed at
the forward end of the rudder. So, the energy
requirement of the steering gear equipment is
reduced, therefore lowering the fuel consumption
of the ship.
 Rudders

Conventional rudder
• Simple construction - single piece construction with
optimized shape and no moving parts
• Good course stability
• Effectiveness depends on the water flow speed and
rudder area
2. Unbalanced Rudders
These rudders have their stocks
attached at the forward most point of
their span. Unlike balanced rudders, the
rudder stock runs along the chord length
of the rudder.
In this case, the torque required to turn
the rudder is way higher than what is
required for a corresponding balanced
rudder. So, the topmost part of the
rudder has to be fixed to the spindle so
as to prevent it from vertical
displacement from its natural position.
Schilling rudder

• High lift rudder

• Single piece construction with optimized shape
and no moving parts
• Improves both course keeping and vessel
control characteristics
• Operating angles up to 70° port and starboard
• Efficient ‘side thrust’ effect at a ship’s stern
• Enhanced levels of ship handling and control
 a. Semi- Balanced Rudder:
Researchers and ship operators had found
significant problems with the balanced and
unbalanced rudders. That is, in case there
was a failure of the steering gear
mechanism while turning a ship. The rudder
would remain still with its angle of attack in
that condition. The solution to this was found
in designing an optimized Semi-Balanced
Rudder.
The name semi-balanced itself implies, that
the rudder is partly balanced and partly
unbalanced.

The top part being unbalanced will help in acting as structural support to the rudder from
vertical displacement. And the balanced part will render less torque in swinging the
rudder. As a result, a semi-balanced rudder returns to the centreline orientation on its
own if the steering gear equipment fails during a turn.
 b. Flaps Rudder:
That actually helps in attaining the effective
angle of attack so as to get the maximum lift
force.

Becker rudder

• High efficiency (70% - 90% more then conventional

rudder of the same area)
• Less rudder resistance with the same side force (less
water drag)
• High maneuverability at all speeds
• Rudder angles of 45° and additional 45° of the flap
c. Pleuger Rudder:
Is used in the case of a ship, too large to be
maneuver in a basin with size constraints,
such that the ship cannot use its propeller
during the maneuver (or in any case of low-
speed maneuvers).
A Pleuger rudder, has a smaller auxiliary
propeller housed within it (which runs by a
motor). As this housing is mounted on the
rudder itself, it generates a thrust (which is
smaller than what is generated by the
ship’s main engine propeller) in a direction
that is oriented along the rudder, therefore
allowing effective maneuver in slow speed
condition.
Nozzle rudder

• Increased bollard pull and thrust

• Propulsion/steering efficiency
• Reduced vibrations
• Protection of propeller against damage
• Fuel saving
• Used on vessels with restricted aft end space in
lieu of a conventional rudder
• Ideal solution for vessels which require high
lateral thrust
Schottel rudder propeller

• Combined propulsion and steering systems

• Convert the engine power into optimum thrust
• Can be steered through 360 degrees
• Full propulsive power can also be used for
maneuvering and dynamic positioning of the ship
• Space-saving installation
Voith Propulsion:
This propulsion system is one of a kind, which
acts as a rudder itself. It doesn’t need a rudder
control surface to change the direction of the
ship.
This propeller has a completely different
orientation. To visualize it in simple terms, it
consists of a number of hydrofoil blades
mounted on a disc, which is in turn, mounted
onto the hull. The disc rotates in a horizontal
plane, about a horizontal axis, and therefore
imparts a rotation into the blades.
But how does a rotation in horizontal plane
impart thrust to the ship? The blades themselves
can be adjusted to have a varying angle of
attacks during the operation of the propeller.
Depending on that, the direction and magnitude
of thrust are varied.
The Voith Schneider Propeller allows precise and stepless thrust generation; propulsion
and steering forces can be varied simultaneously. As a result of its vertical axis of
rotation, the same amount of thrust can be created over 360°. Blades with hydro
dynamically shaped profiles and end plates create thrust with a high degree of
efficiency.
 Thrusters

Thrusters principles
The thrusters’ principle is based on Isaac Newton’s 3rd rule:

Each action has an equal and opposite reaction. Other words when vessel is forced into
movement in one direction, the equal force directed into opposite direction exist.

Brick wall effect

There is a delay in obtain thrust after command was given.
This is because before any thrust is achieved the water flow has to be created. To do so,
water leaving the tunnel must create passage through the water outside the tunnel first.
To do such ‘corridor’ energy has to be accumulated and some amount of the water has
to be pumped. The obtained thrust will be proportional to the volume of pumped water.
That’s why rate of turn is slow at the beginning and short thrusters ‘kicks’ are never
efficient. To obtain vessel response to the thrusters’ command, it has to work for some
time.
Thrusters’ Power

Thrusters as all propulsion devices have some characteristic. One of the most important
parameters of those characteristic is power.
Thrusters nominal power is expressed in Watts or rather in kilo Watts [kW], with power
range reach from 1000kW to 15000kW (for big azimuth thrusters).
Pure power will not give any information about vessel’s maneuvering capabilities. To
have idea about the ship’s maneuvering characteristic the thrusters’ power should be
always related to the ship size and weight and propulsion configuration.
Less powerful tunnel thruster placed on ship’s extreme would be more efficient for
heading control then very powerful fitted close to amidships.
Bow thruster – A lateral thruster fitted in an athwartships tunnel near the
bow to improve manoeuvrability. When the bow thruster is used while the
vessel is moving forward the thrust is partially counteracted by a vacuum
created in the wake of the water jet emanating from the thrusters. The effect
is worst when the vessel is moving forward at four to six knots. In such
cases the vacuum on the hull can be relieved by the addition of an anti-
suction tunnel.
Bow thruster should be
located as far forward as
possible. Parallel side walls
have favorable influence.
The suitable tunnel length:
2-3D. In short tunnels the
propeller is located
eccentrically on the port
side, in order to improve
the thruster performance
to starboard.
Tunnel thruster

• Thrust only in one direction, mainly transverse

• Effectiveness depends on the tunnel length
• Effectiveness degrades with the speed
increase
• Simple construction
• Good turning moments as usually installed on
the ship’s extremes
Alternating current
electric motor drives with
pitch control
An alternating current (a.c.)
induction motor of the
(squirrel) cage type is used
for many bow thrust units,
with the motor being
mounted above the
athwartships tunnel. Thrust is
varied in direction and
strength through a
controllable pitch propeller.
This arrangement permits the use of a simple and robust induction motor,
which operates at one speed. Starting current for a large induction motor
tends to rise to about eight times the normal full load figure and to reduce
this a star-delta or other low current starter is used. Low current starting
implies low starting torque as well. It is important that the hydraulic system
is operative and holding the propeller blades at neutral pitch when starting.

Pitch control for a thruster, is very similar to that for a controllable pitch
propeller. The shaft of the lips arrangement shown, is hollow and has a flange
to which the one-piece hub casting is held by bolts. The hub is filled with
lubricating oil and there is free flow from the hub to the pod through the
hollow shaft. The four blades are bolted to the blade carrier and have seals to
prevent oil leakage.

The pitch of the blades is altered by means of a sliding block, fitted between
a slot in the blade carrier and a pin on the moving cylinder yoke. A piping
insert in the hollow shaft connects the cylinder yoke to the oil transfer unit
which contains a servo valve for follow-up pitch control.
A mechanical connection between the oil transfer unit and the inboard servo
cylinder facilitates accurate pitch settings and provides feedback for remote
control. The hydraulic power unit is supplied with two safety valves, suction
and pressure filters, a pressure gauge and pressure switch, as well as an
electrically driven pump with a starter. To complete the equipment an electric
switch is supplied which, in combination with the pressure switch, prevents
the prime mover from starting when the pitch is in an off-zero position and/or
no hydraulic pressure is available.
The regular and frequent use of electrically driven bow thrust units on vessels
operating on short sea routes means that motor windings are kept dry by the
heating effect of the current. This helps to maintain insulation resistance.

There are potential problems with the electric motors and starters of infrequently
used units, particularly where installed in cold, forward bow thrust compartments.
They are subject to dampness through low temperature and condensation. Insulation
resistance is likely to suffer unless heaters are fitted in the motor and starter
casings. Space heaters may be fitted also.

A fan is beneficial for ventilation before entry by personnel, but continuous delivery
of salt laden air could aggravate the difficulties with insulation resistance. Bow thrust
compartments below the waterline should be checked frequently for water
accumulation and pumped out as necessary to keep them dry.

Vertical ducts for drive shafts should also be examined for water and/or oil
accumulation. Flexible couplings with rubber elements quickly deteriorate if
operating in oily water. Thruster shaft seals must be inspected carefully during
preliminary filling of a dry dock. Failure to detect and rectify leakage at this stage
can be expensive later.
Bow thrusters with diesel drive

By installing diesel drives various problems are avoided, for example the very large
power demand of electrically driven bow thrusters, the insulation problems associated
with the windings and the complications involved with starting, speed control and
reversing.
For a conventional thruster in an athwartship tunnel, the diesel engine may be mounted
at the same level as the propeller to provide a direct drive through a reverse/reduction
gear. An alternative diesel arrangement where space is limited, has the diesel mounted
above the thruster. The second arrangement requires an extra gearbox with bevel gears
to accommodate change of shaft line. Flexible couplings are also fitted.

The reversing gearbox has ahead and astern clutches, with one casing coupled to the
diesel engine shaft and a drive to the other clutch casing, through external gear teeth.
The clutch casings rotate in opposite directions and whichever is selected, will apply
drive, ahead or astern, to the output shaft. The engine idles when both clutches are
disengaged.
Diesel bow thruster drive
Hydraulic thruster

An external hydraulic drive motor can be used as the alterative to an electric motor. The
variable displacement hydraulic pump is powered by a constant speed, uni-directional
electric motor or diesel prime mover connected through a flexible coupling.

Pump output is controlled by means of a servo-control operated direct from the bridge (or
locally) to give the required speed and direction to the hydraulic motor inside the thruster.
The pod and propeller are suspended in a conventional athwartship tunnel below the
waterline. 
Rim drive thruster

• Rotating element is the propeller which sits

inside the stator of the motor. This reduce the
central body of the propeller aiding the flow of
water
Azimuth thruster

• Able to direct the thrust in any direction – more

efficient
• Very good dynamics of flow as compared to the
tunnel thruster
• May be used as additional propulsion
• Deeper draught
• May interfere with underwater operation
• More complex construction
• If maintenance cans available – may be serviced
without dry docking
• If retracted no extra drag while in transit
• Complex construction
• Can be combined with tunnel thruster
Azimuth swing-up thruster
Azipods

In azipods the propulsion motor is installed inside

external pod.
• Simpler machinery installation
• Lower noise and vibration
• Flexibility in hull lines
• Fewer components
• Reduced installation time and cost
• Improved harbor maneuverability
• Improved propulsive efficiency
Azimuth
contra-rotating propellers

• Lower need of installed power

• Lower fuel consumption
• Redundancy in propulsion and steering
• Full steering control also at low speed
• Lower noise and vibration levels
The customary transverse thruster has a limited application because it is based in an
athwartships tunnel. It cannot contribute to forward or reverse motion of the ship and
ship speed must be less than four knots for it to be effective. Some schemes to improve
performance have variously used double entry tunnels or curved tunnels and different
flap arrangements.
The White Gill type thruster which is fitted on a number of existing ships, can provide
thrust in any direction and is also used as the propulsion unit for some small craft.
This type of thruster is positioned at the bottom of the hull so that the suction and
discharge are at bottom shell plate level. Water is drawn in and discharged by a propeller
through static guide vanes, much as with an axial pump. The guide vanes remove swirl
and the water passes out as a jet through a rotatable deflector. The latter can be turned
through 360 deg.
The deflector has curved vanes, resembling in section a turbine nozzle, which produces
a near horizontal jet of water. The deflector is rotated by a steering shaft which passes
through a gland in the casing. This in turn is controlled from the bridge. No reverse
arrangements are needed because thrust is available in any horizontal direction. The
drive for the propeller may be applied vertically or horizontally depending on the design
of unit installed.
This particular design requires
installation within a flooded seachest,
taking in water through intake grates in
the ship’s hull before being drawn into
the thruster and discharged through the
flush mounted deflector.
In some instances, operating
circumstances, or ship design, may
require adaptions to design.
The name ‘cross shaft’ reflects its design feature - the
vertical steering shaft is sited on the same end as the
horizontally mounted rotor shaft, with both shafts
crossing one another.
The steel fabricated unit is welded directly to the
vessel’s hull and fabricated seachest, considerably
reducing installation costs.  With options for intakes
on either the baseline or side shell to suit vessel’s hull
design or ship’s needs, this design is a flexible and
versatile option.
The rotor shaft is mounted horizontally, with the
vertical steering shaft driving the discharge deflector
directly to provide 360° maneuverability in all
conditions.
Water is drawn into the heart of the unit through the
integrated intake grille before then passing through
the static guide vanes to smooth out the imparted
swirl. It is finally expelled through the unit’s
deflector.
CONTROL PANEL
Manual control panels are installated into the
wheelhouse, bridge wings or aft control stations. With
a 360˚ rotatable control head for thruster azimuth and
speed control, as well as multi-screen HMI providing
all the required indicators, the control panels provide
easy and precise control of our thrusters.
Thrusters information in DP

DP system offers extended

information related to the thrusters
settings and performance. There are
many views informing about the
thrusters available. DPO has full
picture regarding status of each
thruster (running, ready, enable).
The pitch/rpm and thrust
force/direction are also given.
Generally the pitch is presented in
%, thrust force in tones and direction
in degrees related to the ship’s bow
(‘+’ stbd, ‘-’ port side).
 Set point and feed back

Thruster individual controllers execute the feedback loop steering control.

The DP Operator Station allows to monitor the actual setpoint and feedback of every
individual thruster.
The set point informs about desired thrust for each thruster. Feedback is the information
about current pitch/rpm and azimuth.
By watching the set point and thruster response operator can monitor the system
performance and very quickly notice the system failures.
Thrusters Allocation
• For the azimuth thrusters, you can
choose between various thruster allocation
modes.
• Some azimuth thruster allocation modes
can be configured to comprise thruster
biasing.
Variable Allocation

• The system automatically changes the angle of

the azimuth thrusters so that the thrust is always
angled in the optimum direction. In order to
reduce wear and tear on the azimuth thrusters
due to continuous changes in the azimuth
thruster angles, a dead-band function is
incorporated. Use this mode when the
environmental forces acting on the vessel are
large and are not constantly changing direction
• A set of prohibited zones for each thruster can
be predefined to prevent a particular thruster
from interfering with other thrusters, the hull or
other equipment. What happens to the thrust
when thruster passes a prohibited zone can be
predefined for each zone (for example, the thrust
can be reduced)
Steering
• Azimuth thrusters not used for steering will have predefined fixed angles for use in
Autopilot mode
• This allocation mode is automatically selected when the system is in Autopilot mode or
in Auto Track (high Speed) mode
Heave Red (Heave Reduction)
• When using heave reduction, excessive thrust is applied to increase the hydrodynamic
damping of the vessel. This reduces the motion of the vessel induced by wave forces
• The effect can be used to reduce the motions when particularly critical operations are to
be carried out, for example crane operations, transfer of personnel, etc
• The azimuth thrusters configured to participate in the motion reduction will be at
predefined azimuth angles, and they will as a minimum be run at a predefined force limit,
for example 50% force
• The thruster angles are selected so that the resulting thrust is zero when there is no
thrust demand
Manual Fix
• In this mode the operator can freely set fixed azimuth angles of azimuth thrusters and
rudders/nozzles using the Allocation Settings dialog box
Thruster Bias

Thruster Biasing allows azimuth thrusters to counteract each other in groups so that the resulting effect of
the biasing is zero.
Thruster biasing does not limit the use of the thrusters since the counteraction will be reduced when the
total demand increases.
Function is useful in the following situations:
• When an azimuth thruster cannot give zero thrust.
• When a higher power consumption is required (than what is actually needed for positioning).
• When the weather is calm.
• When heave reduction is required combined with variable azimuth mode.
Thruster Biasing function can also:
• Reduce the turning of azimuth thrusters when the force setpoint is changing, thereby improving the
effective thruster response and the positioning accuracy.
• Improve the damping of vessel motion.
STARTER AND CONTROL CUBICLE