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1 INTRODUCTION
Transportation is the largest contributor to climate
gas emissions in Norway and within this segment,
shipping accumulated 6% of the total emissions in
2018 [1]. Ferries play a major part in the domestic
shipping emissions and in 2019 there were 112
number of ferry connections, transporting 34.249.453
PBE [2]. Even though several modern ferries are built
as electric ships, the annual emission from this sector
accumulates to 2,7 TWH annually, including coastal
passenger ships [3, p. 22]. Naturally, companies and
government agencies work towards lowering
emissions as well as reducing the cost of
transportation. One area of focus has been to reduce
pollution through energy efficiency measures on
existing ships, as stated at MEPC 62 in 2011 [4}.
Energy efficiency of a ship can probably be improved
through careful planning, monitoring and evaluation
of operations. One important focus area has been fuel
efficiency and energy consumption during daily
operations. The scope of this paper is to describe and
assess how small RPM adjustments and power
allocation, affect fuel consumption.
2 FERRY OPERATIONS
This paper delves into ferry operations, which are
usually split into 6 phases [5, p. 3]:
1 Ferry arrives at dock and keeps itself in place
using its thrusters.
2 Hatches and doors open which let the vehicles and
passengers off the ferry.
3 Ferry personnel guides waiting vehicles and
passengers on-board the ship.
4 Hatches and doors close, and ticketing is
performed.
5 Ferry undocks from the current harbour and starts
transiting to the next one.
Optimized Use of Thrusters During Transit with a
Passenger Ferry
E.M. Kløvning
Norwegian University of Science and Technology, Trondheim, Norway
ABSTRACT: This paper aims to address the impact that optimized use of propulsion equipment would have on
a ferry during transit. Assuming that the ship arrives at it’s destination on schedule, a potential reduction in fuel
consumption would cut the overall cost of the voyage. The idea of optimizing propulsion equipment fostered a
series of sea trials on board a passenger ferry in Norway. Modern aft-bow symmetrical ferries allow several
different methods of manoeuvring, each with it’s own advantages and disadvantages. The goal was to uncover
the method that used the least amount of KWh per nautical mile during transit. The results are presented in this
paper and there are several interesting findings. For instance, applying equal RPM on both thrusters gave the
lowest KWh per nautical mile. However, increasing the RPM on the aft thruster and reducing the RPM on the
bow thruster gave the highest speed over ground.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 14
Number 4
December 2020
DOI: 10.12716/1001.14.04.19
938
6 During transit and docking/undocking ferry
personnel takes care to follow the International
Regulations for Preventing Collisions at Sea
(COLREGS) to avoid any collision.
The different phases of operation require unequal
amounts of power. The transit phase often
accumulates most of the energy consumption during
ferry operations. Moreover, when the optimized
transit speed is known, the navigator must choose
how to achieve this speed through the propulsion
equipment.
3 OPTIMIZED USE OF THRUSTERS
While most ships have the main propulsion at the
stern and auxiliary propulsion at the bow, modern
ferries are not always designed this way. They are
often aft-bow symmetrical with equal amounts of
propulsion equipment in both ends [5, p. 9]. This
enables different configuration possibilities regarding
power allocation on thrusters during transit, as
shown on figure 1. In transit mode, most ferries can
choose between the following methods:
− Method 1: Equal RPM applied on aft and bow
thrusters.
− Method 2: More RPM applied on the aft thruster
and less RPM applied on the bow thruster.
− Method 3: More RPM applied on the bow thruster
and less RPM applied on the aft thruster.
It is also possible to use only one thruster during
transit or combining the different methods in a single
voyage. Unfortunately there is a limited supply of
literature regarding propulsion efficiency on car
passenger ferries and which method to adhere to.
Nonetheless, some basic principles of hydrodynamics
are applicable.
To move a ship through water, it is necessary to
overcome resistance [6]. A ship in motion will
experience several forces working against the
propulsion equipment, such as viscous friction,
residual resistance, and air resistance [6, pp. 9-11].
The friction of the hull will create a boundary layer
around the hull consisting of water moving in the
same direction as the hull and at the same velocity.
This velocity is reduced depending on the distance
from the hull, until it reaches zero. The thickness of
the layer increases along the length of the hull. All
this creates a wake field aft, which the stern propeller
often operates in.
Techet states that a propeller working in the wake
field is more efficient and demonstrates this
mathematically [7, pp. 17-18]. To clarify, the propeller
pushes through water going in the same direction as
the hull, thus increasing efficiency. In theory, the
stern propeller would therefore be more efficient
during transit than a propeller mounted closer to the
bow.
Furthermore, the extent of viscous resistance
depends on the type of flow it is experiencing [8, ch 7,
p. 11]. Generally, we have laminar flow and turbulent
flow as shown on figure 2. Laminar flow is
characterized by smooth lines and a minimum of
frictional resistance. Turbulent flow, located in the
boundary layer, provides resistance. If turbulent flow
is increased, the frictional resistance increases as well.
This is important when using a bow thruster. The
bow propeller provides additional turbulent flow that
travels along the ships hull, increasing viscous
resistance and reducing propulsive efficiency.
Figure 1. Illustration of propulsive possibilities, including
methods from this study.
Figure 2. Flow pattern around a vessel in motion. [8, ch 7, p. 11]
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Figure 3. Starbord profile of M/F Korsfjord (courtesy of Fjord1)
These facts suggest that method 2 would probably
reach the highest speed, while method 1 would be
slightly slower and method 3 the slowest. However,
there is a difference between highest speed achieved
and fuel consumption. Model testing on this ferry
manifested that method 1 is more fuel efficient
compared to other configurations [9].
It should be mentioned that these tests investigates
fuel consumption primarily, and velocity secondarily.
Course keeping abilities, vibration or other factors
present within the different methods will not be
assessed. An important moment here is that the
testing is done without changing course. Method 2 is
perhaps the most preferred propulsion configuration
on ferries in general, partially because the aft thruster
has the most authority over yaw moment during
transit [10, ch 1, p. 11].
4 QUANTATIVE RESEARCH STUDY
This study would be characterized as a quantitative
research study with an experimental design [11, ch 7].
The purpose was to examine how different
propulsion methods affected power consumption.
However, considering that transit time is of great
importance to passenger ferries, this variable was also
included. The concept was to have a ferry travel a
certain distance during transit, while logging
consumption and time, for each of the given methods.
The tests would be repeated several times to
strengthen the validity and hopefully a trend would
be uncovered. The method applied can be referred to
as a simple time-series design. [11, p. 208].
4.1 MF Korsfjord
Every observation was made on MF Korsfjord, which
is a ro-ro passenger ferry that runs on liquid natural
gas (LNG). The ferry operates on the Molde-Vestnes
connection as part of E39 in Norway. There are three
other identical ferries on this connection and to keep
the scheduled timetable, each ship must have a transit
speed of around 11,5 knots. The ferry is a monohull,
aft-bow symmetrical vessel as seen on figure 3.
Korsfjord has 2 Schottel STP 1010, N=1000kW
azimuth thrusters, one in each end [12]. Both have
fixed pitch propellers. This design allows fully
actuated manouvering where surge, sway and yaw is
controllable. Power is generated from two
MITSUBISHI GS16R-MPTK (900 kw) gas engines and
a MITSUBISHI S12R-MPTA (1000 kw) in standby.
These engines provide the thrusters with power and
consumption is logged using an energy monitoring
system [13]. Consumption is directly affected by RPM
on the thrusters. An increase in RPM gives an
increase in consumption.
The ship has a set of azimuth thrusters with a twin
screw configuration in each end. Each propeller has a
diameter of 2m, and they are placed 1,8m below
baseline. Each thruster is placed 45,40m from midship
and 15,97m from length overall.
4.2 Data collection
Data was collected over a period of three months.
First, a designated test area was thoroughly marked
on the vessels TECDIS, as shown on figure 4. Because
the ferry still had to operate the route as usual, the
area is part of the normal sailing route. The area is
also in the middle of the transit phase, and the ship
does not need to change course or alter speed during
normal operation. It also allows the ship to reach
transit speed when leaving the quay in Vestnes. The
testing procedure is described as follows:
− Before approaching the test area, the navigator
carefully regulate RPM on each thruster
accordingly to the chosen method.
− The navigator plots the designated course in the
autopilot.
− The navigator then waits for the SOG and the
autopilot to stabilize.
− When the ferry enters the designated test area the
navigator places a mark in the TECDIS.
− When the mark is placed the navigator
immediately logs KW and time in IAS.
− When the TECDIS shows sailed distance of 1
nautical mile the navigator logs KW and transit
time.
Figure 4. Test area for observations. Recreated from ships
TECDIS using online charts. [14]
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When conceding such full-scale testing there are a
number of factors that must be assessed, to avoid
corrupting the data collection. A ship in motion will
experience interactions that affect energy usage, in
both positive and negative ways. Current, wind and
waves are some of the dominant external forces that
affect ship motion, resulting in drift or change in
speed. Internal factors such as trim, heeling angle,
growth or loading condition will also affect overall
performance [15, pp. 34-43]. To reduce the impact of
these interactions, every test was carried out during
calm weather. An upper limit of wind was set at 6
m/s from any direction and wave height at maximum
0,5m. Every test was carried out by the same
navigator and to compensate for current, the results
are based on average consumption and transit time
both northbound and southbound direction, as seen
in fig. 4. Since the ferry transports a number of
different vehicles on every voyage, the loading
condition will vary on almost every single test.
Loading condition is therefore not possible to
compensate for during these tests.
These measures mentioned above will reduce the
impact of interactions but not eliminate them. Any
data acquired in this trial must therefore not be
viewed as flawless.
5 RESULTS
There were conducted 45 valid tests during this trial.
Several other tests were carried out but eventually
removed due to corruption of data from sudden
changes in weather or increased marine traffic in the
area. The results are outlined in the following tables.
Figure 4. Line graph describing consumption.
Figure 5. Line graph describing time spent transiting.
Table 1. Average KWh per nautical mile.
_______________________________________________
Average KWh
_______________________________________________
Day/Method Method 1 Method 2 Method 3
(220/220) (200/240) (240/200)
_______________________________________________
29.04.20 81,5 83 84
30.04.20 80 82 84,5
01.05.20 80,5 80 83
18.05.20 83 84 84,5
19.05.20 83 84 85
20.05.20 83 85 86,5
21.05.20 82 83,5 86
22.05.20 82,5 84,5 85
25.05.20 82 85 87,5
26.05.20 83,5 85 86
27.05.20 83,5 84,5 89
28.05.20 84 85,5 86,5
22.06.20 81,5 82,5 86,5
23.06.20 81,5 82 84
24.06.20 82,5 82,5 83,5
_______________________________________________
Table 2. Average time per nautical mile.
_______________________________________________
Average Time
_______________________________________________
Day / Method Method 1 Method 2 Method 3
(220/220) (200/240) (240/200)
_______________________________________________
29.04.20 310 309 315
30.04.20 308,5 307,5 312
01.05.20 308,5 306,5 316,5
18.05.20 309 307 318,5
19.05.20 312,5 310 314
20.05.20 311,5 308,5 316,5
21.05.20 313,5 309 317,5
22.05.20 311 312,5 315
25.05.20 313 312 319,5
26.05.20 314,5 312 317,5
27.05.20 311,5 310 319
28.05.20 310,5 309 318,5
22.06.20 314 310,5 317,5
23.06.20 314 306,5 317
24.06.20 312,5 308 313
_______________________________________________
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Table 3. Analysis with descriptive statistics
_______________________________________________
Method 1 Method 2 Method 3
_______________________________________________
Statistic: KWh Time KWh Time KWh Time
Median 82,5 311,5 84 309 85 317
Mean 82,26 311,63 83,53 309,20 85,43 316,46
Mode 81,5/ 308,5/ 85 309 86,5 317,5
83 311,5/
312,5/
314
Range 4 6 5,5 6 6 7,5
Standard 1,09 1,95 1,46 1,89 1,56 2,15
deviation
_______________________________________________
6 DISCUSSION
In this section the tables and line graphs from section
five will be interpreted and discussed. As mentioned
earlier the data in this study is gathered through an
uncertain method. Several variables thought to affect
the results are present and can neither be measured
nor ruled out due to lack of equipment on the vessel.
Such variables include, but are not limited to, wind,
waves, current and loading condition. Interactions
would explain why the data is not completely
identical for each column. On the other hand, the
differences are surprisingly small and not severe
enough to invalidate the results.
Table 1 and figure 4 presents the data regarding
consumption per nautical mile. With a few
exemptions, method 1 has the lowest values followed
closely by method 2 and finally method 3. As shown
on the line graph the different methods remain
relatively stable during the tests, without rapid or
substantial changes in value.
Table 2 and figure 5 describes transit time per
nautical mile. Method 2 proved to be the fastest,
except on one occasion, followed closely by method 1
and lastly method 3. Again, the values are quite
consistent, similarly to the results for consumption.
This suggests that the readings underline a trend.
Table 3 presents some measures of central
tendency. The median, the mean and the mode for all
methods do not differ significantly. They seem to
revolve around the same numbers, in their respective
columns. Still, the results are the same, leaving
method 1 as the most fuel efficient and method 2 as
the fastest. Furthermore, table 3 provides measures of
variability. The range indicates the spread of data for
each variable and it seems to be quite low for all
columns. In turn this suggest that the spread is
minimal. This argument is further proven when
looking at the standard deviation. Again, the values
are relatively low, indicating a highly concentrated
set of data. In conclusion the data is mostly
homogenous and without values that deviates far
from the norm. All columns seem to cluster around
their points of central tendency and with a minimal
spread.
Given the above, the results indicate that there are
measurable differences between the aforementioned
methods when it comes to power consumption and
time spent transiting.
Method 1 has turned out to be the most fuel
efficient method during transit and the result is
consistent with the model testing completed by LMG
Marine [9]. A possible cause for this phenomena, is
that the overall engine load is slightly lower when
allocating equal amounts of power to each thruster,
compared to an unequal distribution. Increasing RPM
on one thruster and decreasing correspondingly on
the other one results in a minimal, but still noticeable
change in engine load between 1% and 2%.
On the contrary, reduced consumption is
meaningless if the vessel fails to achieve a sufficient
velocity and uphold the timetable. As stated above
method 1 is the most fuel efficient. On the other hand
it is only the second fastest method, closely beaten by
method 2. An exceedingly plausible explanation for
this is that the stern propeller works in the previously
mentioned wake field, which in turn provides
increased propeller efficiency. Coincidentally the bow
thruster has reduced its RPM, thus reducing potential
hull resistance accordingly.
Perhaps unsurprisingly, method 3 turned out to be
the slowest and least fuel efficient method. As stated
above the unequal distribution led to higher
consumption compared to equal power allocation.
Concurrently, using a bow thruster as the main
propulsion and the stern thruster as auxiliary
propulsion resulted in lower speed during transit. A
noticeable increase in RPM on the bow thruster
probably led to increased hull resistance, whilst the
auxiliary thruster worked in the wake field. This
method is therefore not recommended under any
circumstances.
Ultimately the testing showed that the methods
were not that disparate. A possible explanation comes
from the hull design and placement of the thrusters.
As shown on figure 3, the thruster placement might
be unconventional. Compared to other vessels the
thruster are located closer to midship. Any potential
differences would therefore be minimized compared
to other ferries, where thrusters are closer to the
perpendiculars.
7 CONCLUSION
This paper aimed to investigate how daily operations
could be optimized on a ferry, specifically through
the propulsion equipment. It can be concluded that
method 1 is recommended for fuel efficiency, while
method 2 is recommended for transit time. Method 3
is unfavourable in both aspects and not
recommended. On the contrary, the differences are
surprisingly small per nautical mile and the potential
reward is quite low. Finally, these results only apply
to this particular ship, but it may be relevant to other
ferries with similar design and propulsion
equipment.
8 FURTHER RESEARCH
Considering that the uncovered differences were
minimal, it would be interesting to measure energy
usage and transit time for each method at different
942
velocities. Perhaps speed reduction could have
greater potential for fuel efficiency.
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