Pacific Maritime Magazine - Marine Business for the Operations Sector

All-Electric: The Future of Ferry Propulsion


A simple charging system, in this case the Rolls-Royce SAVeCHARGE system, can drive a short-run ferry exclusively on battery power. Artwork courtesy of Rolls-Royce.

Operational costs, environmentally-friendly solutions, and EPA-regulations are all important factors in the future of ferry propulsion. Double-ended ferries carrying passengers and vehicles across sounds are a vital link in the transportation network on the West Coast. For many of these routes, pure battery ferries are already a realistic choice, charged with shore connections from the grid. Shorter near-coastal ferry routes are ideal for all-electric propulsion systems as they have a predictable operational pattern and a limited power demand per crossing. In the past few years, a transition toward all-electric ferries has started in Norway, where operational requirements are similar to those in the Pacific Northwest.

In Norway, regulations and funding have been drivers in the all-electric ferry development. Several Norwegian operators are making the investment and all-electric ferries have become a reality, the first one already having more than 2 years of operational experience. Rolls-Royce Marine has developed SAVeCHARGE, a full propulsion-system for these ferries, utilizing its extensive background in hybrid and diesel-electric installations.

New Technology

The battery and frequency converters have seen great development in the last years. In a simplified electric propulsion system the three main components are the batteries, a frequency converter and an electric motor connected to a propeller or thruster. To explain how they work, a comparison to a hydro power plant can be made. The batteries do the same as the dam; they store energy and have it ready for whenever you want more power. The frequency converters are comparable to the main valve in the hydro plant. The valve opens or closes gradually to adjust the amount of water that is taken from the dam and then turned into power. The frequency converter controls the amount of electric power that is pulled out of the batteries and put into the electric motor. The next step in your power plant is the generator that transforms the amount of water coming through the valve into the desired electric power. On the propulsion system, the electric motor transforms the amount of electric power coming from the frequency converter into a rotating movement to the propulsor, creating the thrust needed to move your vessel. The control mechanism is a critical part for both systems; the generator will transform waterpower to electric power controlled by the valve, while the electric motor will transform electric power to waterpower controlled by the frequency converter.


Integration of all components in the system can be one of the challenges. In most cases the supply of electric motors, propeller units, frequency converters, energy storage, and control system come from separate entities. The SAVeCHARGE system is a unique arrangement from Rolls-Royce where the thrusters, frequency converters and control system are manufactured and integrated by one entity. Small engines are installed only for backup if, for instance, there is a power outage in the landside grid. A complete system reduces both installation time and commissioning time compared to a split delivered system. The system is optimized toward energy efficiency by minimizing the amount of energy conversions needed and utilizing permanent magnet technology for high efficiencies at all power levels. This will reduce the operational cost of energy and reduce capital cost since smaller battery installations can be used for the same crossing distances.

Rolls-Royce recently signed a deal to supply its first automatic crossing system to two new double-ended battery ferries for Norwegian ferry company Fjord1. Such an automatic crossing system ensures reliable and energy-efficient transit back and forth by automatically controlling the vessel's acceleration, deceleration, speed and track. Such systems ensure consistent behavior during the journey and hence predictable energy consumption.


Lithium-ion battery technology is in most cases the best technology for a near-coastal ferry. It gives a high energy density and the capability to recharge large amounts of power over a short period during unloading and loading of passengers. It is however important to distinguish between different Lithium-ion batteries. A mobile phone battery, an electric car battery, and a marine propulsion battery are all Lithium-ion, but they are manufactured in different ways. In a conventional Lithium-ion battery, a fire/meltdown can occur when one internal cell fails, starting a chain reaction to all the other cells around it. The marine batteries have a passive safety function that prevents this chain reaction from happening. Class societies have been heavily involved with battery suppliers in both developing and testing today's marine propulsion batteries. The solutions we now have are safer than 3-4 years ago with stricter requirements for type approved batteries, machinery space and monitoring. The guidelines provided by class societies have ensured a safe and reliable battery installation.


Electricity is an easily available power source in almost all areas where ferries operate. Even a weak electrical grid can be utilized as a power source with a shore side battery pack. LNG is one of the other options to create lower-emission propulsion systems with a reduced fuel cost compared to conventional diesel. Some of the biggest challenges for LNG are the infrastructure with supply, and the safety regulations from port authorities and coast guard. LNG must be purchased, liquefied, stored and supplied to the ferry, therefore demanding a large infrastructure investment. This fuel complexity doesn't exist with an all-electric system, as the infrastructure of electricity are already there. Economic analysis shows that the long-term operational benefit in only fuel savings for an LNG-solution are about the same 20 to 30-percent reduction as an all-electric system. This is based on the current and future anticipated development in the oil, gas and electricity price. The maintenance benefits are however significantly larger for an electric solution as there are no engines and complex LNG fuel-arrangement to maintain. The electric batteries and converters are static components which require less maintenance, giving an estimated 50 percent reduction in operational cost compared to a conventional diesel.


Lithium-ion batteries can be charged quickly, but on many routes the destination at each end are served by a rather weak electrical grid. Therefore, recharging the ferry's battery, direct from the mains supply during the short turnaround time would be an unacceptable load on the grid. To overcome this challenge additional battery packs can be installed at the terminals ashore. When the ferry docks, its battery will be rapidly topped up by drawing power from the shore batteries. Those batteries can in turn be recharged from the grid at a slower rate before the ferry returns, thereby reducing the power spikes on the grid. A high peak power demand to the grid means a higher cost of energy as the grid owner must charge more per kWh to be able to operate a reliable power grid. A shore-side battery bank will then reduce the operational cost as the maximum power demand is reduced, and it also supports in strengthening the local grid by giving it a constant load. Connecting the charging equipment when docking is an automated system that does not require any breakers or switches to be operated by the crew. The newest concept in charging is even performed wirelessly through induction technology combined with vacuum mooring.


The first large all-electric ferry, Ampere, has already been operating in Norway for more than 2 years. The noise level onboard the ferry is very low as there are no combustion engines running in the engine room. Batteries fill up most of what would normally be the engine room and only a small backup generator is installed in case of power outage on the mainland grid network. The chief engineer on board says, "It is a delightfully quiet operation". Passengers also enjoy this silence, and on a calm sunny day seagulls are the only sound heard from the observation deck. The Rolls-Royce thruster units installed on board "Ampere" also have a quicker response compared to a conventional system as the battery can deliver instantaneous power while a diesel engine has a certain response time when the power adjustments are large. This gives the captain higher controllability while docking the ferry or in case of an emergency maneuver.


There are many long-term benefits including enhanced reliability in the transport network and a reduction in emissions for all-electric ferries, and several funding options exist for these solutions. The Federal Highway Trust, TIGER grants and Washington State's Clean Energy Fund are all examples of possible funding options for an environmental friendly all-electric ferry. The incentives are already in place and the operators of ferries can seek out these funding possibilities in cooperation with the designers and equipment suppliers. A large part of the capital cost can be directly supported, making the payback time short and increasing the long-term benefits for both operators and the society.

Skagit County

The Guemes Ferry operated by Skagit County in Washington State between Anacortes and Guemes Island is a good example of where an all-electric ferry can be an ideal solution. The county has performed studies together with design and integration firms to investigate the benefits and payback times for this specific ferry. The operational and maintenance cost of this all-electric ferry could be reduced by as much as 50 percent, making it a very realistic project in the long-term view. Further investigation into the benefits is now underway while awaiting a response from the funding agencies. This ferry has the potential to become the first all-electric car ferry in the United States


Hybrid plug-in systems can provide the answer when an all-electric solution is not ideal. Ferries with more intensive operations, longer routes, and no overnight layover will find battery capacity and charging time problematic. Hybrid plug-in systems give flexibility and help to reduce both the fuel consumption and emissions. Batteries and internal combustion engines complement each other. The battery can be charged to the extent that time and shore infrastructure allows. The hybrid solution enables the engines to be run with less load change as the battery takes care of the transients. Total running hours and engine maintenance are reduced while fuel efficiency is maximized.

Washington State Ferries

A battery hybrid can also be attractive when recharging from the shore grid is not realistic. The energy storage system can be sized to bridge intermittent load peaks so that the engines can be smaller and operate more efficiently. Apart from improving the operating conditions for the fossil or bio fueled prime movers, this type of hybrid improves safety through system redundancy. This is a technology that can easily be retrofitted onto existing diesel-electric systems. Several years ago the Hyak, a diesel-electric ferry operated by Washington State Ferries, was evaluated and approved for a grant to complete a mid-life refurbishment including a new hybrid installation. This refurbishment fell through due to the total cost of interior and hybrid propulsion renewal compared to the life of the vessel, but it shows the interest in this technology. A newer vessel with a longer life expectancy would be an even better candidate for these systems.

The Steps Forward

In the future, we are going to see a large increase in sustainable and environmentally friendly propulsion solutions. It is becoming economically viable to invest in these systems and the current funding possibilities support this transition. Operators in cooperation with designers and suppliers are strongly encouraged to investigate these options for either new builds or retrofits. The funding options provide a unique opportunity to get the financial support needed to renew aging fleets. The industry is supporting this change and the technology is proven.

Kristian Eikeland Holmefjord has a Master's degree in Electrical and Environmental Engineering from the Norwegian University of Science and Technology. He is currently the technical specialist on battery and hybrid solutions for the North American sector in Rolls-Royce Marine.

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