Some of the challenging issues when introducing electric podded propulsion systems to new types of vessels are:
・ First cost of equipment compared to conventional solutions
・ Building costs with the podded propulsion concept
・ Reliability and maintainability
・ Operating costs.
Clarification and evaluation of these issues require both extensive studies and experience. A vessel will benefit from electric propulsion when one or more of the following conditions exist:
・ A variable operating and power requirement profile
・ A need to maximise the cargo or passenger volumes within given measurements of the ship
・ A requirement for low vibration levels
・ A requirement for reduced exhaust emission levels
・ Propulsion redundancy and safety requirements
There is a growing demand for building environmentally friendly ships, which produce less exhaust gases, are equipped with redundant propulsion systems and are capable of maneuvering safely in narrow passages and difficult weather conditions.
This paper will present the Contra Rotating Propeller concept applying a podded drive in place of a rudder of a conventional propulsion system. The concept is an alternative to e.g. conventional mechanical propulsion systems where a twin shaftline design is chosen to achieve propulsion redundancy and/or reduce propeller loading in high power applications. A case study of applying the podded CRP concept to a fast RoPax vessel is also presented to specify costs and performance differences in more detail.
4.1 Introducing The podded Contra Rotating Propeller (CRP) concept
A typical way to achieve propulsion redundancy is to double the existing single propeller propulsion plant and design the systems to be independent of each other. Thus a single fault will not result into a total loss of propulsion power. This solution is widely used and is in many types of vessels a standard (cruise vessels, offshore supply vessels). Duplicating the propulsion plant is in many ways a compromise where the following sacrifices are made:
・ Machinery space is increased at the expense of cargo space
・ The aft hull form is more complicated and expensive to build
・ Machinery operation at low speeds can be uneconomical, because of low load of the main engines
・ Machinery weight increases significantly
Fig.2 presents the podded CRP concept principle:
・ The rudder is replaced by a pulling type podded propulsion unit.
・ The main propeller is typically driven by a diesel engine, 2-stroke without a reduction gear or 4-stroke with a reduction gear. Naturally the main propeller can be driven by an electric motor to implement full electric propulsion.
・ Aft thruster is removed.
The vessel's electric generating capacity is upgraded to meet the electrical power demand
Fig. 2 The podded CRP concept principle.
4.2 Benefits of the podded CRP concept
Several areas are affected when the podded CRP concept is applied compared to standard mechanical propulsion solutions.
・ Fuel economy
・ Propulsion redundancy
・ Maximum propulsion power capacity of a conventional single shaft hull form
・ Manoeuvrability
・ General arrangement of the machinery areas
・ First cost
・ Life cycle cost
The benefits are discussed below and more specific data can be found form the case study included in this paper.
4.2.1 Fuel economy
Fuel economy is improved with the contra rotating propeller principle. Depending on the vessel type and speed of the vessel the propulsion power demand can be reduced by approximately 5 to 15%. The total fuel economy depends also on the amount of manoeuvring time and slow speed sailing of the vessel and therefore the improvement is not necessarily the direct impact from the reduced power requirement. In addition the possibility to use only the podded propulsion unit during slow-speed sailing improves fuel economy and decreases maintenance requirements of the main shaft line engines. The losses in the electrical power system from the generator to the propulsion motor are typically 8... 10% and they will therefore be more than compensated with the total efficiency and flexibility of the system.
4.2.2 Propulsion redundancy
Propulsion redundancy is achieved with two independently operating propellers and a single failure in the propulsion power systems will not result in loss of manoeuvrability or operability. The required power of the podded propulsion unit can be set by a minimum speed achievable with only the pod in use. The power of the pod unit then gives sufficient speed for the vessel also in a "take-me-home" mode.
