Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Development of Contra-Rotating Rim-Driven
Propulsion System to Meet Needs of Electrification
and Diversification of Ocean Mobility

TAKUYOSHI YAMADA*1

KAZUKI HOSONO*2

SHIGEKI SENOO*3

TAKEHIKO NISHIDA*4

In order to realize a decarbonized society, we are proposing the concept of an electric
propulsion system with flexibility to meet various needs as one of the solutions to reduce CO2
emissions from ocean mobility for society as a whole. We designed and conducted performanceverification a contra-rotating rim-driven propulsion system that reduces the design constraints ofship configuration and has robust propulsion performance under operating conditions.
The rim-driven propulsion system does not have a drive shaft and mounts the propeller blades
inside the motor. The system is characterized by its compactness. By applying contra-rotating
technology and other elements to this propulsion system, we aimed to achieve high propulsion
performance under a wide range of operating conditions. Following the verification, we confirmedthat the contra-rotating rim-driven propulsion system has improved propulsion performancecompared to a propulsion system that adapt to a wide range of conventional operating conditions,as typified by a nozzle propeller.

|1. Introduction
In response to the trend toward decarbonization, the reduction of CO2 emissions during ocean
mobility transportation is an urgent issue for ocean mobility, which is represented by ships. The
reduction in CO2 emissions needs to be achieved not only by the product producer through improved
product performance, but also by cooperation of all the stakeholders involved in ocean mobility
through more efficient operation and loading/unloading.
On the other hand, stakeholders need to secure their individual interests and their needs vary
widely. Products had to be designed on a case-by-case basis to meet individual needs. As a possible
solution to these complex problems, we propose the concept of an electric propulsion system that
contributes to the electrification of ocean mobility for decarbonization and is flexible enough to meetthe design and operational needs of ships.
This report presents the design and performance verification of a contra-rotating rim-driven
propulsion system, which has features to reduce the design constraints of the ship configuration andto maintain high propulsion performance under a wide range of operating conditions.

|2. Design concept of contra-rotating rim-driven propulsion system
The rim-driven propulsion system is one type of motor-driven electric propulsion system,
nevertheless, it has no drive shaft and mounts the propeller blades inside the motor, unlike common
propulsion systems which have propeller blades at the end of the drive shaft. As presented in Figure
1, which shows the configuration of the rim-driven propulsion system, this system integrates the
motor and mechanical parts, thus the ship has no need to have mechanical parts onboard. We
considered that the greatest advantage of this feature is that the design constraints of the ship
configuration is reduced by enabling the propulsion system to be more compact and the arrangement*1

Special Purpose Systems Research Department, Research & Innovation Center*2

Fluid Dynamics Research Department, Research & Innovation Center*3

Deputy Chief Researcher, Fluid Dynamics Research Department, Research & Innovation Center*4

Deputy Chief Researcher, Power Electronics Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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more flexible, and aimed to improve the efficiency by applying contra-rotating technology that takes
advantage of the absence of a drive shaft.
This system has an outer peripheral surface called rim on the outside of the propeller blades,
and the propeller blades are rotationally driven by permanent magnets installed on the rim andelectromagnets placed on the outer side of the permanent magnets. The combination of the permanent
magnets and the electromagnets is referred to as a motor. By arranging two of the motors in forward
and aft, two-stage propellers can be driven. Furthermore, by setting the rotation directions of the front
and back propellers to be opposite from each other, a contra-rotating configuration can be made, inwhich the swirling flow generated by the front propeller is absorbed by the back propeller. We believethat this will dramatically improve the propulsion performance compared to a single-stage
configuration.
The shroud covering the motor is designed to be streamlined so as to generate thrust using thesuction force generated along with the propeller operation, thereby suppressing the reduction of
propulsion performance under high load conditions. We considered that this design could be realized
by utilizing the conventional nozzle-propeller technology with a shroud on the outside of thepropeller.
We considered reducing the design constraints of the ship configuration and providing
flexibility to meet operational needs as issues. Our design concept for a contra-rotating rim-driven
propulsion system is to maintain high propulsion performance under a wide range of operating
conditions by combining the compactness and high degree of freedom of the ship layout of the rimdriven propulsion system with the contra-rotating technology and nozzle propeller technology.

Figure 1
Configuration and design concept of contra-rotating rim-driven
propulsion system|3. Design
We classified the design into the following major categories: fluid design, electromagnetics
design, strength/sliding bearing design, thermal design, and manufacturing design.
Since the geometry and structure obtained in each design interact with each other, the design
was carried out while adjusting the specifications of each one. This report describes the fluidelectromagnetic design, which has a direct influence on the propulsion performance, which is directly
related to CO2 emissions.
Among the principal items, the propeller/motor diameters, shroud thickness, length, androtational speed have optimal solutions for fluid efficiency and motor efficiency. Therefore, thesystem efficiency, which is the product of fluid efficiency and motor efficiency, cannot be optimized
by sequential design.
For this reason, we developed and operated a fluid-electromagnetics design tool incorporating
the numerical analysis shown in Figure 2 to optimize the system efficiency in terms of the principal
items. By adjusting detailed geometry parameters based on the principal items obtained with this tool,
we found design solutions for the propeller/shroud shapes and motor dimensions/structure.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Figure 2 Design system flow of principal items|4. Verification
4.1

Verification with numerical simulation

We conducted numerical analysis using CFD to evaluate the propulsion system efficiency asa fluid performance, which is the efficiency gained through converting motor torque into thrust undera given external flow velocity condition.
In general, the propulsion system efficiency is determined by the propeller load coefficient,and the propulsion system efficiency, as typified by an open propeller, often shows a downward trendwith respect to the propeller load coefficient. The load coefficient is the ratio of the amount ofpressure increase per unit area to the dynamic pressure, and the higher the load coefficient, the more
the power applied to a small area. A high load coefficient means that the acceleration of fluid passing
through the propeller disk of the propulsion system is large in relation to the external flow velocity.
In this case, the momentum loss is large due to the large difference in momentum between the insideand outside of the slipstream, resulting in low efficiency.
The propulsion system efficiency of the contra-rotating rim-driven propulsion system obtainedby CFD is low in the extremely low load coefficient range, while it tends to remain high in themedium to high load coefficient range. This is due to the thrust generating effect that is produced by
the fluid force applied to the shroud accompanied by the suction of the propeller, and is the result of
utilizing the design technology of nozzle propellers. The maximum efficiency of nozzle propellers is
low as around 50-60% (1), whereas the maximum efficiency of the contra-rotating rim-driven
propulsion system exceeds 60% as shown in Figure 3. As shown in Figure 4, the contra-rotating
system greatly suppressed the release of the swirl velocity component downstream of the propulsion
system, which is believed to have improved the efficiency.
This characteristic indicates that the propulsion system maintains high efficiency over a wide
range of operating conditions, which means that it has the potential to meet diverse operational needs.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Figure 3

Simulation result of propulsion system efficiency

Figure 4 Simulation result of propulsion
system wake flow field

4.2

Prototype design

For the purpose of performance verification through tank tests, we developed a prototype
through strength/sliding bearing design, thermal design, and manufacturing design, assuming use inactual operation.
This prototype is a one-package contra-rotating propulsion system with a simple shaft
configuration that does not require a drive shaft. It employs a structure in which the thrust from thepropeller is transmitted to the fixed shaft via a water-lubricated plain sliding bearing on the fixed shaft.
In addition, an underwater direct motor cooling system was adopted, in which water is circulated
between the rotor and stator of the motor using the pressure increase before and after the propeller.
In this way, we could realize a compact-design propulsion system connected by six power
cables, which is expected to be almost free from the design constraints of the ship configuration. Thissuggests that the system has the potential to meet a wide variety of design needs. Figure 5 shows theconfiguration of the prototype, and Table 1 shows principal items of the propulsion system.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 2 (June 2024)
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Figure 5 Configuration of prototype propulsion
systemTable 1 Principal items of prototype propulsion system
Output power
Rotation speed
Design ship speed
Motor rotor - Inner diameter
Motor rotor - Outer diameter
Motor axial length
Motor type
Propeller diameter
Propeller boss diameter
Number of blades of forward stage
propeller
Number of blades of aft stage propeller

4.3

4 kW x 2 stage = 8 kW
246 rpm
4.21 m/s
450 mm475.9 mm
85 mm for each
3 phases, 30 poles IPMSM
440 mm90 mm
65Verification by testingThis report describes the results of a towing tank tests aimed at verifying the fluid performance
among various elemental tests. In the towing tank tests, the propeller was rotationally driven by
supplying electric power to the motor while the propulsion system was towed at a predetermined
speed in the water using towing equipment. Figure 6 shows an overview of the test equipment. Since
a contra-rotating rim-driven propulsion system does not have a drive shaft, it is difficult to directly
measure the thrust and torque of the propeller. Therefore, the thrust of the entire system was evaluatedby measuring the fluid forces acting on all the propulsion system components, such as the propeller,
shroud and struts, and the input power to the propulsion system was measured. Then, by using thepreviously obtained input power and speed-load torque characteristics of the motor, the propeller
torque was evaluated.

Figure 6 Arrangement of towing tank tests equipment

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Based on the measurement results, the fluid performance was calculated and are shown in
Figure 7. One of the differences between the test and the CFD is that the tank test took into accountthe friction loss of the bearings, which the CFD did not. Therefore, the test may have resulted in alower evaluation of the propulsion system efficiency compared to the CFD. However, similar to the
CFD results, the test showed that the propulsion system efficiency of the prototype propulsion system
was improved compared to that of the nozzle propeller.

Figure 7

Propulsion system efficiency obtained by test

|5. Conclusion
We have developed a contra-rotating rim-driven propulsion system that will lead to theelectrification of ocean mobility, aiming to contribute to the realization of a decarbonized society.
The issue to be solved by the contra-rotating rim-driven propulsion system is the necessity of meetingthe diverse needs of the stakeholders involved in ocean mobility, and the development aimed toreduce the design constraints of the ship configuration and to provide flexibility to meet operational
needs.
For the design of the developed system, the contra-rotating and nozzle propeller technologies
were applied while maintaining the features of the rim-driven propulsion system, which is a compact
motor-propeller integrated design that eliminates the need for mechanical parts onboard. Throughsimulation and a prototype test, we believe that the developed system can achieve higher propulsion
performance than a nozzle propeller, which has a lower reduction in propulsion performance over awide range of operating conditions.
Finally, we will present the issues to be solved for the commercialization of a contra-rotating
rim-driven propulsion system. This report reported the results of prototyping and verification of asmall (8 kW) propulsion system, but bringing the system into actual use requires consideration of an
entire electric propulsion system, including the inverter, battery, and control. In addition, one of themost important issues for propulsion system users is the cost of installation. To suppress theinstallation cost, it is necessary to thoroughly reduce the manufacturing costs. We will adjust the
manufacturing issues obtained through prototyping and work toward the realization of lower costs.

References(1)

M. W. C. Oosterveld, WAKE ADAPTED DUCTED PROPELLERS, Publication No. 345 Netherlands shipmodel basin (1970) p.33