Serial or parallel hybrid marine propulsion – how do they work?

Hybrid marine propulsion systems are able to use two different energy sources for their operation; a combustion engine combined with a battery bank and an electric motor, for example. This provides flexible and reliable propulsion for a wide variety of marine vessels. The configuration of the driveline and its components determines which hybrid marine propulsion system it is. While there are several variants and sub-variants, serial hybrids and parallel hybrids are the two most common configurations. So, how do they work? What sets them apart from each other?

How does serial hybrid marine propulsion work?

Serial hybrid marine propulsion systems use electric motors, one or more, for their propulsion. The combustion engine and propeller shaft are not mechanically connected, which means that the electric motor drives the propeller. The main purpose of the combustion engine is to generate electricity for the electric motor, and for charging the battery bank.

Large batteries can allow for long periods of fully electric propulsion. The engine can operate at a steady rpm with high efficiency, which is optimal from a usage standpoint. This also allows operators to lower their emission levels – possibly to a minimum, depending on the battery capacity.

Serial hybrid marine propulsion is ideal for, for example, short-distance public transportation vessels, such as city ferries. Their batteries need to have a high power density and large capacity, in order to keep up with demand. Most hybrid ferries have batteries that can be charged both via the engine and an on-shore charging station. From an environmental and emissions perspective, I would say that onshore charging is essential in this context.

How does parallel hybrid marine propulsion work?

Parallel hybrid marine propulsion systems can use both the electric motor and the combustion engine, simultaneously or separately, for their propulsion. Both the combustion engine and the electric motor are directly connected – via a gear with a PTI (Power Take In) for an electric motor – to the drive shaft for the propeller. This gear allows power transferral between two sources. Accordingly, the electric motor, the combustion engine, or both in parallel, can power the propeller.

During normal operation, the parallel hybrid marine propulsion system will typically run on both the electric motor and the combustion engine throughout the driving cycle. This in order to achieve maximum efficiency and performance. The combustion engine will run during high speed transportation. Similarly, the electric motor is ideal to use at low speeds, but can also provide extra power when required. The combustion engine’s operation varies, as it is more affected by the driving cycle (or driving pattern) compared to that of the fixed rpm of the engine in a serial hybrid marine propulsion system. This often means more emissions, by comparison. That said, the batteries do not need to be as large as in a serial hybrid marine propulsion system, thereby reducing the weight of the vessel. This may also have a positive effect on its fuel consumption.

Parallel hybrid marine propulsion systems are used in a variety of vessels – from leisure boats to tugboats, wind farm boats and high-speed passenger ferries.

Volvo Penta Hybrid Installation

Key benefits of hybrid marine propulsion

Hybrid marine propulsion systems have many advantages, compared to conventional drivelines:

  • The combustion engine can be used when it is most efficient to do so. Electric propulsion contributes to reduced fuel consumption and less emissions.
  • Operators can switch to electric propulsion during low-speed cruising, or in the event of a combustion engine failure, for example. This is highly useful for ferries and other vessels that regularly start and stop along a particular route.
  • Electric propulsion also reduces noise levels, increasing the comfort onboard for operators, crew and passengers. In addition, it allows for possible boat or ferry traffic in areas where operations must be very quiet.
  • Parallel use of electric motors and combustion engines allow for increased torque.
  • Hybrid marine propulsion systems help industries both on land and at sea adjust to new means of operation, such as electromobility.

Choosing the right hybrid marine propulsion system for your operation

Deciding which hybrid configuration that works best for your boat or business boils down to some key factors: type of operation, operation speed, expected travel distance and duration of operations, available charging infrastructure, regional or national emission standards, and more.

Note that hybrid drivelines, more specifically the electric motor and the batteries, inevitably add weight to their vessels. A smaller and/or lighter combustion engine can partly compensate for this added weight, however.

Make sure to keep an eye out for my next article here on the Volvo Penta Professional Power blog. There, I will go into greater detail about what to consider when investing in a hybrid marine propulsion system. If you have any questions about Volvo Penta’s work in this area, you are welcome to contact me at If this topic is of interest to you, I can also recommend our articles on four key things to consider when repowering a boat, how operational profiles help operators choose the right drivetrain configuration, and how exhaust aftertreatment systems prepare the marine industry for tougher emission standards.

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