Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)24

Efforts toward Development of Energy-Saving
Large-Size ROPAX Ferry
MASANORI ONZUKA*1

YOKI UEMURA*2

KUNIAKI YAMATO*3

AKIRA DOI*4TESTUYA MORI*4

Mitsubishi Heavy Industries, Ltd. (MHI) has built numerous ROPAX ferries. With thesocioeconomic background around ROPAX ferry operators, such as fluctuating fuel oil prices anddomestic cargo logistics, the need for energy-saving ROPAX ferries with a high capability ofattracting customers and freight is increasing more than ever before. This paper describes MHI's
efforts on the development of energy-saving ROPAX ferries equipped with new propulsion plants,
highlighting the perspective of a reduction in fuel consumption.

|1. Introduction
Rising price of fuel oil that started in fiscal 2005 seemed to come to an end in the second halfof fiscal 2008 immediately after the failure of Lehman Brothers. In fiscal 2009, however, fuel oil
prices began to rise again and resulted in nearly 10 years of concern for ROPAX ferry operators
until the middle of fiscal 2014 (Figure 1). Taking the existing fleet of very old ships that waste fuelin conjunction with this increase in fuel oil prices into consideration, a reduction in fuelconsumption is undoubtedly an urgent issue facing ROPAX ferry operators.

Figure 1

Transition of C heavy oil priceIn order to survive, long-distance ROPAX ferries essentially need to improve their capabilityof attracting customers and freight, so that they can win the battle for acquiring passengers against
Shinkansen high-speed trains, LCC (low cost carrier) airlines, express bus lines, and expressways,and promote a modal shift policy as a solution to the chronic shortage of long-distance truck drivers
caused by the changing social structure, the decreasing birthrate and the aging population.
Against such a backdrop, MHI has been working on the development of a next-generation
ROPAX ferry in order to achieve the conflicting purposes of an increase in size and theenhancement of energy efficiency. This paper first presents the concept of the propulsion plants forthe next-generation ROPAX ferry that we have developed, and then the most advanced large-size
ROPAX ferries introduced successively to lines between Shinmoji and Izumiotsu and between
Osaka and Shinmoji in 2015 as examples of actual introduction. Finally, we summarize our futureapproach.
*1 Shimonoseki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development Division, Commercial Aviation & Transportation Systems*2 Deputy Director, Shimonoseki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development Division, Commercial Aviation &Transportation Systems*3 Manager, Shimonoseki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development Division, Commercial Aviation &Transportation Systems*4 Chief Staff Manager, Shimonoseki Ship & Ocean Engineering Department, Shipbuilding & Ocean Development Division, Commercial Aviation &Transportation Systems

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)25|2. Next-generation propulsion plants
Typically, ROPAX ferries, which have higher propulsive power and navigate at higher
speeds in comparison with general merchant ships, are subject to draft limits by the wharves being
used and therefore their propellers are limited in diameter. For this reason, it is difficult for a single
propeller to handle the required propulsive power, and a twin-screw, two-engine diesel propulsion
system with two sets of independent propulsion systems located bisymmetrically is normally used.
A twin-shaft propulsion system can suppress vibration and noise due to the reduction in propeller
vibratory force resulting from the separation of the required propulsion power into two. Such asystem has redundancy, where even if one propulsion system or steering equipment fails,
navigation can be continued using the other, providing excellent reliability and safety. There is,however, a problem with this system where the supporting structure of the shaft system, whichincludes shaft brackets and bossings, must be installed below the waterline and therefore theresistance caused by the additional objects becomes larger in comparison with a vessel using asingle-shaft system.
For this reason, MHI has been working on the development of next-generation propulsion
plants that can attain favorable fuel efficiency while maintaining the redundancy of the propulsion
plant. Figure 2 shows the concept of next-generation propulsion plants.1
Single-screw, two-engine
diesel propulsion system

Hybrid propulsion systemDual hybrid propulsion system(Features)- A system where the twomain engines are directly
connected to the one
propeller through areduction gear
- Resistance caused byadditional objects becomes
smaller than that of thetwin-shaft system.

(Features)- The pod propeller (or azimuth propeller) isarranged behind the main propeller.
- The pod propeller (or azimuth propeller) canbe used as a thruster.
- The propulsion system has a redundancy
similar to that of a twin-screw vessel.

(Features)- A system established by adding to theconventional twin-screw, two-engine diesel
propulsion system a shaft generator motor
equipped with a frequency conversion board- The shaft generator can be driven by a wide
range of main engine speeds to supply
electricity to the ship.
- The shaft generator can be used as a motor
for propulsion assist using electric power fromthe main generator.

Figure 2

Next-generation propulsion plants

A twin-engine, single-screw diesel propulsion system adopts a single-screw vessel type toeliminate the shaft brackets and reduce the number of bossings in order to provide higher
propulsion performance than that of a twin-screw vessel. While the twin-main engine construction
secures redundancy against the failure of a main engine, the single-shaft structure has lessredundancy in terms of the shaft system and the steering equipment in comparison with atwin-shaft structure. Therefore, it is important to secure reliability when the twin-engine,
single-shaft diesel propulsion system is adopted. In addition, when the propeller diameter isincreased in order to improve the propeller efficiency, a proper clearance between the propeller andthe vessel bottom outer panel under the waterline must be secured in order to reduce the effects ofpropeller vibratory force.

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)26

A hybrid propulsion system is a combination of the main propeller driven by the main engine
and an electrically driven propeller such as a pod propeller or an azimuth propeller. A pod propeller
is a pod-shaped casing incorporating a motor and an azimuth propeller is driven by the power
motor installed in the ship. This system has mutually independent propulsion systems driven by themain engine and driven by electricity in order to secure redundancy similar to that of a twin-screw
vessel, while taking advantage of the good propulsion performance of a single-screw vessel. A pod
propeller and an azimuth propeller, which swivel 360 degrees, can be used as a steering device or astern thruster and can be expected to improve ship maneuverability in navigation or controlling in aharbor. A CRP pod propulsion system, which is a hybrid propulsion system, coaxially arranges thepod propeller immediately at the rear of the main propeller driven by the main engine and on its
shaft center line, and uses the main propeller and the pod propeller as a single set of contra-rotating
propellers (CRP). The pod propeller recovers the rotating flow generated by the main propeller inorder to improve the propeller efficiency. A twin pod (azimuth) propulsion assist system, whicharranges electrically driven azimuth propellers on both sides of the main propeller driven by themain engine, attains excellent ship maneuverability and redundancy against accidental problems ofthe drive system or the shaft system.
A dual hybrid propulsion system, which is a system established by adding to a conventional
twin-screw, two-engine system the idea of propulsion assist made by a shaft generator, is intended
to reduce the main engine power while utilizing the advantages of a twin-screw, two-engine
system. Typically, conventional twin-screw, two-engine diesel propulsion systems have a shaft
generator that is driven by the main engine through a reduction gear in order to provide electric
power for the side thruster used for ship control in a harbor. The electricity generated by this typeof shaft generator cannot be used in the ship because the frequency of the generated electricity
varies in response to changes in the main engine speed in combinator-controlled
acceleration/deceleration or in stormy weather. A dual hybrid propulsion system can, however,supply energy from the high fuel efficiency main engine for use in the ship during normal
navigation regardless of the main engine speed, using a frequency converting board that adjusts thefrequency of the electricity generated by the shaft generator to a frequency suitable for supply inthe ship. Furthermore, the shaft generator can be used as a propulsion assist motor when supplied
from the main generator, and therefore a more flexible navigation mode is attained by using thefunctions of this shaft generator motor (SGM) properly depending on the required ship speed.

|3. Recent examples of introduction
Among the next-generation propulsion plants described in chapter 2, the dual hybrid
propulsion system was introduced to ROPAX ferries Izumi and Hibiki of Hankyu Ferry Co., Ltd.,
which were brought into service in 2015 for the line between Shinmoji and Izumiotsu.
Furthermore, a hybrid twin azimuth propulsion assist system was introduced to ROPAX ferries
Ferry Osaka II and Ferry Kitakyushu II co-owned by the Japan Railway Construction, Transport,
and Technology Agency and Meimon Taiyo Ferry Co., Ltd., which were brought into service also
in 2015 for the line between Osaka and Shinmoji. On the Setouchi Route where these ROPAX
ferries travel, high ship maneuverability is required because there are many islands, the marine
traffic is heavy, and overtaking is restricted in narrow water areas. For these ROPAX ferries, theoptimum propulsion plants were selected in consideration of not only energy-saving capability, butalso the circumstances of the route described above, redundancy against the failure of the
propulsion system, usability for the crew and the lifecycle cost including the maintenance cost. Inaddition, energy-efficient vessels suitable for their propulsion plants were developed, and combinedwith the Mitsubishi Air Lubrication System (MALS),2 MHI's proprietary energy-saving
technology, in order to improve the fuel efficiency. The propulsion plants of these ROPAX ferries
are summarized in the next section.

3.1

Dual hybrid propulsion system

The planning of the propulsion plant to be used for Izumi/Hibiki proceeded while also
considering the possible adoption of the CRP pod propulsion system based on the required
propulsion power. The dual hybrid propulsion system based on the medium-speed twin-screw,
two-engine propulsion system was, however, ultimately selected because the system had pastMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)27

results and the crew members were familiar with the system. Figure 3 shows the plant structure ofIzumi/Hibiki. This plant is equipped with a thyristor shaft generation system consisting of afrequency conversion board, synchronous phase modifier and shaft generator. In the shaft
generation power supply mode, AC power from the shaft generator is converted to DC power by
the thyristor convertor in the frequency conversion board, then converted to AC power with acertain frequency by the synchronous phase modifier and the thyristor invertor and supplied to themain switchboard. The shaft generator can be operated in parallel with the main generator byautomatic load sharing control of the main switchboard. In the propulsion assist mode, on the other
hand, the shaft generator is supplied with excess electric power from the main generator and is used
as a motor for propulsion assist, resulting in an increase in propulsion output with the main engine
kept in a fuel-efficient output region. As described above, the dual hybrid propulsion system hasfeatures that enable the energy of the main engine and the generator to be used flexibly in responseto the required propulsion power of the ship and its demand for electric power.

Power generation output: 1,600kWe x 2 units, Electric motor output: 960kW (Max.) x 2 units

Figure 3

Dual hybrid propulsion system

Table 1 compares the specifications between Izumi/Hibiki and the ROPAX ferry that they
replaced. Izumi/Hibiki improve fuel efficiency due to adoption of MHI's proprietary energy-saving
technologies, while enhancing the cargo loading capacity by enlarging the vessel by increasing thenumber of lanes from eight to nine. They also attain a reduction in transportation fuel cost per truckof approximately 35% in comparison with the ROPAX ferry they replaced.
Table 1Main specifications comparison table of Izumi/Hibiki

ROPAX ferry name
Length overall
Breadth/Number of Lanes
Main dimensions
Gross tonnage (Domestic)
Gross tonnage (International)
8.5-m trucks
Cargo loading capacity
Passenger vehicles
Passenger capacity
Service speed

ROPAX ferry replaced
189.0m27.0m/8 lanes
15,188219 vehicles
77 vehicles
810
23kt

Propulsion and steering plantTwin-screw, two-engine CPP

Main engine
Shaft generator motorMax output×units
Max output×units

11,915kW×2―Propeller

Type×units

Controllable pitch propeller×2Other―Izumi/Hibiki
195.0m
29.6m/9 lanes
15,89735,327277 vehicles
188 vehicles
643
23.5ktTwin-screw, two-engine CPP＋Shaft generator motor
8,700kW×2
960kW×2
Controllable pitch propeller(with HVFC) ×2
MALS
HVFC: Hub Vortex Free CapMitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)28

3.2

Twin azimuth propulsion assist system

The planning of the propulsion plant to be used for Ferry Osaka II/Ferry Kitakyushu II
primarily proceeded considering improvement in fuel efficiency and also pursuing shipmaneuverability, which is particularly required for the Setouchi Route. Figure 4 shows thearrangement of the propellers of Ferry Osaka II/Ferry Kitakyushu II. The main propeller is acontrollable pitch type and driven directly by the two medium-speed diesel main engines through areduction gear with a clutch. In addition, the azimuth propellers located on both sides of the main
propeller are controllable pitch type and driven by propulsion motors installed in the vesselimmediately above the azimuth propellers using electric power from the main generator. Taking
advantage of the twin-engine, single-screw diesel propulsion system, Ferry Osaka II and Ferry
Kitakyushu II intentionally adopt small azimuth propellers and use the mechanically driven highly
efficient large-diameter propeller as a main propeller in order to reduce fuel consumption. There
are three control modes to make full use of the functions of the 360-degree swiveling azimuth
propellers used as the stern thruster for berthing and deberthing, the rudder and the propeller for
low-speed navigation and the propulsion assist unit for high-speed navigation. Despite the smallsize, the two azimuth propellers contribute to improvement in ship maneuverability in low-speed
navigation zones such as the inside of a harbor or an area with islands, and can be said to be a
propulsion plant suitable for the Setouchi Route.

Figure 4

Propeller layout on Ferry Osaka II/Ferry Kitakyushu II

Table 2 compares the specifications between Ferry Osaka II/Ferry Kitakyushu II and theROPAX ferry that they replaced. This table shows that although Ferry Osaka II and Ferry
Kitakyushu II are approximately 1.5 times larger than the ROPAX ferry they replaced on a gross
tonnage basis in order to provide an increase in the cargo loading capacity and the accommodationarea, the overall power output of the propulsion plant is reduced significantly. The fuel cost
reduction rate per a truck is approximately 30%, and this remarkably contributes to a reduction inthe operating cost.
Table 2Main specifications comparison table of Ferry Osaka II/Ferry Kitakyushu II

ROPAX ferry name
Length overall
Breadth/Number of Lanes
Main dimensions
Gross tonnage (Domestic)
Gross tonnage (International)
12-m trucks
Cargo loading capacity
Passenger vehicles
Passenger capacity
Service speed
Propulsion and steering plant
Main engine
Shaft generator motor
Main propeller
Azimuth propeller

Max output×units
Max output×units
Type×units
Type×units
Other

ROPAX ferry replaced
160.0m25.0 m/7 lanes
9,479
110 vehicles
100 vehicles
713
22.9ktFerry Osaka II/Ferry Kitakyushu II
183.0m
27.0 m/8 lanes
14,92023,984146 vehicles
105 vehicles
713
23.2ktTwin-screw, single-engine CPPTwin-screw, two-engine CPP＋Azimuth propulsion assist
9,930kW×2
7,000kW×2―
1,000kW×2
Controllable pitch propeller×2
Controllable pitch propeller×1―
Controllable pitch propeller×2―
MALS

Mitsubishi Heavy Industries Technical Review Vol. 53 No. 2 (June 2016)29|4. Conclusion
In a long slump in the domestic economy since the failure of Lehman Brothers, MHI has
been developing energy-saving ROPAX ferries with excellent truck transportation efficiency inorder to promote a modal shift policy to coastal shipping and business innovation of ROPAX ferry
operators. Going forward, we will endeavor to meet the needs of ROPAX ferry operators by
expanding the lineup of energy-saving propulsion plants in view of the development ofnext-generation plants including a propulsion plant powered by LNG, which is attracting attention
as clean energy. In addition, MHI believes that it is our responsibility to offer added values thatallow general ROPAX ferry users to fully enjoy sea voyages.

References
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2.

Mori, T., et al., Approaches of the Energy-Saving ROPAX Ferry for the Next Generation, Mitsubishi
Heavy Industries Technical Review Vol. 44 No. 3 (2007)
Mizokami, S., et al., Implementation of Ship Energy-Saving Operations with Mitsubishi Air Lubrication
System, Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (2013)