Man.ch4

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BOWTHRUSTER
Use of Bowthruster / Steering aid
Thrusters can be used :
• to assist steering at very slow speed
• to keep a ship’s position in the channel without
headway. (this capability is put to good use when the
ship is unable to proceed to the berth immediately
because of an unexpected obstruction in the channel
or a delay in tug availability, etc).
• When navigating stern first, a bow thruster is more
useful than a tug, provided the ship is not allowed to
gather too much sternway.
Position of bowthruster: as far as possible forward
Use of bowthruster / Steering aid
•
Altough slow to move a ship, thrusters provide a
helpfull cushion when landing alongside
•
They are useful when a ship has to berth in a very
tight corner with little manoeuvring space for a tug
•
Thrusters are useful when anchoring
•
Thrusters may be accepted in lieu of a tug,
reducing the costs in port.
2 thrusters to increase the power:
Reliability of bowthrusters
• Older units are proned to problems:
– Outdated electrics and hydraulics
– Situated in fore part of ship and subjected to heavy
weather,vibration and damp.
– Negligence of maintenance
– lack of use on long sea passages
• New units:
– More reliable and robust
• Sometimes underpowered and inadequate
Power of thrusters
• More common:
– reversible electric motor driving a fixed pitch propeller
– Reversible electric motor + hydraulically operated variable
pitch propeller
• Limitation of Design parameters:
–
–
–
–
Restricted location
Size of tunnel
Ballast draft
Hull strenght
• Limitation of power → Twin bowthrusters
• Many thrusters are equivalent to 10-15 tonnes
bollard pull
Counteracting forces near the tunnel
These forces will vary with the vessel moving ahead,
astern or stopped.
The effect of the thruster is the greatest when the ship
is stopped
Counteracting forces near the tunnel
In a ship moving ahead,
pressure fields will develop
near the entrance and the
exit of the tunnel
The suction, acting over an
area of plating, creates a
force which is opposing
the bow thruster. This can
reduce the effectiveness by
50% at a speed of 2 knots .
Speed limitation
• Most thrusters loose effectiveness as the ship
increases speed and are little or no help at speeds of
more than 3 or 4 knots
• They may still exert some force until a speed of
between 5 and 10 knots has been reached , but the
rudder would then be far more effective for turning
the ship
Wind speed / Thruster
Thruster A: 1000kw
Thruster B: 2000kw
1. Container ship 210m:
a) Thruster A enough for
a beam wind of max.
3 to 4 Beaufort (11kn)
b) Thruster B enough for
a beam wind of max.
4 to 5 Beaufort (16kn)
2. Car carrier 200m:
a) Thruster A enough for
beam wind 4 Beaufort
(13kn)
b) B enough for beam
wind up to 5 beaufort
(18kn)
Thruster effectiveness
« Rudder action » versus « thruster action »
DEAD SLOW AHEAD
1. Rudder to starboard
Pivot Point moves forward
Center of gravity moves to port
« Rudder action » versus « thruster action »
DEAD SLOW AHEAD
2. Thruster action
Pivot point moves astern
Center of gravity moves to Sb
The ship « turns on her heels »
« Rudder action » versus « thruster action »
FULL SPEED AHEAD
1. Rudder action
Pivot point moves forward
Center of gravity moves to Ps
« Rudder action » versus « thruster action »
FULL SPEED AHEAD
2. Thruster action
Pivot point forward
Center of gravity moves to Ps
due to difference in pressure
fields at the bow.
No effectiveness of thruster with
speed ahead
Lateral motion to Port using rudder and bowthruster
A. With short kicks ahead
and rudder hard to Sb.
Moderate amount
of thruster to port to
compensate swing to
Sb
B. With astern power
• Transverse thrust of
propeller
• Moderate thrust of
bowthruster to port to
compensate possible
swing
Lateral motion to Sb using rudder and bowthruster
A. Short kicks ahead + Rudder
hard to port + thruster to Sb
No problems in lateral motion
B. With astern power
Bow swings to Sb→ compensate
with Bowthruster to Port
Thruster effect + transverse
thrust cause unintentional
lateral motion to port.
This movement can be of
considerable magnitude
Specific aspects of bow thruster work:
• thrusting when stopped
• thrusting with headway
• creating lateral motion
• working the thruster with sternway
Thrusting when stopped
Pivot point position
Thruster is working on pivot
point located one ship’s
beam from stern (due to
underwater profile of the
ship) → large turning lever.
Turning moment with ship
stopped is better than with:
- headway
- sternway
Thrusting when stopped
Ship develops small amount of headway due to a flow of water around
the bow creating a low pressure ahead of the ship
Thrusting with headway / Straight line
• performance falls off rapidly wit speed> 2knots
• pivot point position reduces the turning lever
→ poor turning moment
Thrusting with headway / Perfomance loss
Thrusting with headway / Turning
1. When the rudder is applied with a kick ahead and no speed, pivot
point moves briefly forward to P, close to the thruster position → no
turning lever / bow is only pushed sideways
2. When vessels gets some speed: pivot point moves to P1 but adverse
effect of speed
Thrusting with headway / Turning
3. Thruster may be very useful to develop lateral motion in conjunction
with propeller and rudder.
for example: rudder to port and thruster to starboard → bow and stern
tends to move to Sb and ships moves sideways
Lateral motion to port with kicks ahead
Frequently used when berthing: driving a ship « by the seat of the
pants »
On large vessels moving sideways, the kinetic energy can carry on
for quite some time
Sometimes difficult to balance the forces of rudder and thruster:
Full thruster of 13t is only equal to kick ahead with dead slow
Lateral motion to port with astern power
• When putting propeller astern ,anticipated swing of bow to Sb can
be stopped by applying port thruster.
• Very effective with large tonnage ships / some caution required to
avoid landing too fast and too heavily
Lateral motion to starboard with kicks ahead
Same way as for a lateral motion to port.
Problem arise when the engine is reversed (see next slide)
Lateral motion to starboard with astern power
More problematic to starboard
(with single screw)
When astern power is applied
(to control headway):
• bow swings to Sb
• tendency to give thruster to
Ps will bring ships back to initial
position.
swing
Thrusting with sternway
• pivot point moves aft : big turning lever for the thruster / possible to
steer the vessel with the bowthruster
• beware for poor thruster power and sluggish response of vessel
• avoid the development of a big rate of turn: difficult to control
Thrusting with sternway / Turning moments of
propeller and bowthruster
If the engine is going astern (200m ship/ transverse forces
respectively 7t - propeller and 13t- thruster) :
• transverse thrust of propeller pushes the bow to Sb
turning lever of propeller = 30m → thrust = 7t x 30m = 210 tm
• bowthruster turning lever = 125m → 13t x 125m = 1625mt (contd)
Thrusting with sternway / Turning moments of
propeller and bowthruster
Therefore it is possible to work the vessel astern by overriding
The effect of transverse thrust with the bow thruster.
→ It may often be preferred to back up a waterway or to a berth
Bowthruster at work
Working astern in a channel
Lack of experience ends up with the ship
in a difficult position (pos6)
Working astern in a channel
The process of backing a ship up a waterway with the
aid of a bowthruster can be dangerous:
The inexperienced can unintentionally allow the ship to
become a victim of unexpected influence of
transverse thrust and insidious drift
→ For this manœuvre, large ships use often the
assistance of a tug secured aft to assist steering and
positioning of the stern / the bowthruster controls
the bow.
Position 1
•Ship has entered the channel
•Good position with bow canted to
port to allow for prolonged use of
stern power and transverse thrust
Position 2
• Insufficient correction of transverse
thrust → stern has dropped away from
centre of channel
• If propeller and thruster are now used
simultaneously: lateral motion to port
• this is not intentional and may be
unnoticed
Position 3
• ship steady on correct heading but off
centered
• at this stage: use bow thruster
vigorously / or: kick ahead with rudder to
port to bring stern back to the centre of
the channel
Position 4
Sequence of events in position 2 is
repeated → ship keeps drifting to port
On very large vessels, the kinetic
energy keep the vessel drifting for a
long time.
A light wind on the Sb side will
increase the drift.
Position 5
The ship is getting too close to the
edge and is running out of
manoeuvring space
There is insufficient water to port to
enable the bow to be thrusted to
port
Position 6
The ship ultimately is hitting an
obstruction on the port side.
To avoid this kind of manœuvre:
1. Get the ship moving smartly astern initially and keep it moving
2. Use kick aheads with caution: it takes the way off the ship which
can drift out of position with leeway
3. No attempt in transverse winds overriding the thruster’s power
4. When sufficient sternway: stop the engine to avoid transverse
thrust
5. Watch the heading carefully at all times and use thruster
vigorously to keep the stern on the intended track / the rest will
follow.
Working astern to a berth
The position of the pivot point: ships goes
slowly astern and turns to Sb.
Working astern to a berth
With engine ahead it is easy to correct a too high approach speed
or an unaccurate positioning of the stern
Working astern to a berth
With engine stopped and bowthruster in use: pivot point moves
after and bowthruster works on a large turning lever
Leaving a berth
1. Vessel turns around a pivot point situated forward: stern leaves the
berth
2. « Let go all lines fwd » and thruster to Sb: with engine stopped and
thruster at work the pivot point moves to pos. « b » and the bow
leaves the berth
Leaving a berth
Small ships often use a combination of forward spring +
rudder + propeller + bowthruster
Backing in a narrow canal with beam wind
« Transverse thrust + bowthruster effect + wind effect » push
vessel to port: 2 tons of the propeller and 5 tons of the bow thruster
(pivot point between midships and stern).
This means a total transverse force on the ship of 7 tons to port
Comparison: Turning a vessel with little headway:
1. With bowthruster only
2. With a combination of bowthruster, rudder & propeller
Ship with headway / Using only the bowthruster
• As soon as the ship starts turning, the pivot point will be
situated somewhere between the stern and midships.
• Although the lever of the turning moment is large, the effect will
be very low, because :
1. the effectiveness of the bow thruster is very small
2. the effectiveness is decreasing rapidly with the ship’s speed
ahead
3. there is high water pressure at the bow and consequently
the lateral resistance of the ship is highest near the fore ship
and is increasing with the speed. The bow thruster has to
push the bow against the bow wave.
The consequence is, that at a forward speed of about 4 knots, the
bow thruster is too weak to overcome the lateral resistance and
to push the bow to starboard.
Ship with headway / Combining thruster , rudder &
propeller
• 1. we first start by using only the rudder and the propellor: the
pivot point moves ahead, between the bow and midships, the
exact location depending of the ratio of the ship’s lenght to
beam (L/B). The high lateral resistance at the bow, the bow
wave, will now have a favourable influence on the turn.
• 2. Then, the bow thruster is started to starboard: it will
increase the rate of turn, in spite of a short lever (pivot point
ahead).
Twin screw ship + bowthruster
Crablike movement
New Designs
Anti-suction tunnel
Additional tunnel improves performance
Anti-suction tunnel
• To solve this problem of the counteracting force, an
extra tunnel is sometimes constructed behind the
bow thruster tunnel to connect the negative and
positive pressure fields : this is the AST tunnel
( anti suction) .
K-Jet
Altering the shape of tunnel
apertures to improve the waterflow
through the tunnel and avoid
turbulences
Navyflux
With the help of a
horizontal propeller, water
is sucked up from
underneath the vessel.
The vessel is manoeuvred
by means of a 360 degrees
rotating deflector, which
turns the water through 90
deg. and forces it through a
tunnel.
Veth-Jet bowthruster
maximum thrust can be achieved even at minimum draught, without
any component parts protruding from under the ship. Thrust can be
maintained for longer than a tunnel thruster, even during speed-up,
by virtue of the fact that the water is sucked up from underneath the
vessel.
Veth-Jet bowthruster
Other application: Vessel blocked by ice
Other application: Purse seiner with bowthruster
Ship equipped with a stern thruster
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