(R)evolutions in Naval Propulsion – It’s Electrifying!

Hybrid propulsion is installed on a wide number of European frigates, including the FREMM. (Naval Group)
Hybrid propulsion is installed on a wide number of European frigates, including the FREMM. (Naval Group)

Changing the type of propulsion used by warships to ones more environmentally friendly pose difficult challenges, especially if power hungry weapons/systems are to be introduced.

Shortly after his election in July 1984, New Zealand Prime Minister David Lange implemented a policy banning nuclear-powered and/or nuclear weapon carrying ships from docking in New Zealand’s harbours. In February 1985, New Zealand authorities turned USS Buchanan away after it refused to confirm it was not carrying any nuclear weapons or power; the incident sparked a diplomatic spat. In September 2021, when Australia announced that it would acquire nuclear-powered submarines, New Zealand stated that it would not allow them in its waters.

Over the past decades, ships’ mission profiles have largely dictated technological advances in naval propulsion. Flexibility in the operational spectrum, together with increasing energy needs, have driven forward electrical propulsion. As environmental concerns continue to gain prominence in governments’ agendas, naval propulsion is likely to undergo another (r)evolution.

Multimission Drive

Over the past decade there has been a significant trend toward capital naval surface ships being capable of conduction a wide range of missions. “This has had a big impact on the propulsion system,” Luca Mattei, vice president Design and Engineering for Fincantieri Naval Vessels, told Armada (AI). “Different prime movers have to be installed in order to cover efficiently the whole operational load profile.”

For destroyers, frigates and certain types of corvettes, this means being able to function both in open crisis times and during peace time in patrol or coast guard mode. In the latter mode, these ships generally move at cruising speed – typically between 8 knots (kts) and 18kts. As they shift to open crisis times, however, these vessels are generally required to move from cruising to higher speed – typically 22kts to 30kts – imposing significant sudden energy demands on the propulsion system.

Yet speed is not the only consideration when it comes to naval propulsion; greater fuel efficiency is also a critical element to consider. “Operationally, greater fuel efficiency means greater range, endurance, and time on station for the same amount of fuel,” Matthew Caris, senior director at Avascent, told AI.

As a result, finding the right balance in the midst of these sometimes rather contradicting requirements is the key naval propulsion challenge highlighted by Fincantieri, Naval Group and Rolls-Royce Power Systems. Kevin Daffey, director Marine Systems and Automation at Rolls-Royce Power Systems told AI that, “key requirements are: maximum range, which defines the most efficient machinery line up and propeller characteristics; maximum speed, which sets the peak propulsion power; and, [limiting] maximum underwater noise and vibration levels [which] define the size and characteristics of mounts, rafts and couplings.”

As a consequence, “the trend has clearly been a move from mechanical propulsion to hybrid propulsion,” Bertrand Lars, Energy Technical Domain director at Naval Group, told AI. “In fact, there is even a clear tendency to move toward all electric.”

Hybrid propulsion, known as CODELAG (Combined Diesel Electric And Gas), is the propulsion system installed on a wide number of frigates such as the French and Italian FREMMs (European multi-purpose frigates), the German Thyssen-Krupp and Lürssen F125, and the UK BAE Systems Type 26. But Europeans are not the only ones using hybrid propulsion; “this is also an area where the US Navy (USN) has been experimenting,” added Caris, with the future ‘Constellation’ class frigates benefitting from CODELAG technology.

USS Buchanan was turned away from New Zealand when it refused to confirm that it was not nuclear armed or powered. (US Navy)
USS Buchanan was turned away from New Zealand when it refused to confirm that it was not nuclear armed or powered. (US Navy)

One of the key advantages of this propulsion system is the fact that electric propulsion is much more efficient at lower and mid-range speed than a four stroke engine; the latter cannot go below 60 percent speed, so it has to be revved up to get the right thrust necessary to maintain low speed, making it inefficient. Additionally, because the need to shift to higher speed is “unpredictable,” Caris added, CODELAG brings another important benefit to naval propulsion: since both the turbine and the electric motor to the propeller shaft can run at the same time, the shift between cruising and high speed can be done swiftly. Finally, hybrid propulsion is also particularly interesting for Anti-Submarine Warfare (ASW), with electric propulsion ensuring a certain level of acoustic discretion while mechanical propulsion enables sudden shifts to faster speed.

Beyond CODELAG, both the USN (on the DDG 1000) and the UK’s Royal Navy (on the Type 45 destroyers) and the Queen Elizabeth-class aircraft carrier have also started to explore true integrated electric drives. With this system, all the power minus the gas turbine is being transitioned through electric motors that can then be used to create electrical power for the ship’s systems or drive the propeller shaft. “These systems are particularly interesting because of the flexibility of electric operation medium to low speed,” John Buckingham, chief mechanical engineer at BMT, told AI, “but also increasingly for warships where lasers, direct energy weapons and other devices need pulsed energy.”

Power Hungry

For Anti-Air Warfare (AAW) and Anti-SUrface Warfare (ASuw), for instance, a growing number of weapons developed to carry out these missions is electrically powered. “Electric weapons typically have a limitless magazine, which makes them very attractive because it means they never run out of bullets,” Buckingham noted. Similarly, as sensor performance required to carry out all these missions continues to improve, these systems – radar, sonar, etc – also become progressively more power-hungry. Finally, the integration of unmanned systems will also require power to recharge their batteries, Lars pointed out. Consequently, the electrical power load for warships is going to vary often and within very short timeframes, explained Caris; “as these vessels shift from mission control mode to combat mode in a matter of seconds, the electrical load demanded is going to be important.” Both CODELAG and true integrated electric drives can serve to provide that power.

One key issue with growing electrical load demands, however, sheds light on a bigger challenge for warships in the coming years: ensuring that there is room onboard to store all the power necessary to maintain autonomy. “Electric solutions typically take up more space than mechanical drives,” Buckingham said, and while DC systems, better designed converters, and other innovations are helping reduce the size of these converters, power storage remains a major challenge.

The UK Royal Navy Queen Elizabeth class aircraft carriers are fitted with true integrated electric drives. (Royal Navy)
The UK Royal Navy Queen Elizabeth class aircraft carriers are fitted with true integrated electric drives. (Royal Navy)

Greening the Blue Water Navy

Although operational concerns are – and will always remain – the key consideration for navies choosing how to power their capital surface ships, progressively environmental considerations are gaining prominence. “Tomorrow’s challenges will also be closely linked to the environment,” noted Lars, “and while navies have been slow in taking this up, increasingly they are becoming more attentive to International Maritime Organisation (IMO) regulations.”

New Zealand’s attitude to nuclear power is one example of how environmental concerns can represent a key strategic consideration for blue water navies. New Zealand’s stance against nuclear power dates back to its opposition, in the 1960s and 70s, to US, French and UK nuclear weapon testing in the Pacific. As noted in the introduction, this has led to the country declaring its territory – including its waters – a nuclear-free zone; it has also resulted in a few diplomatic spats with allies such as the UK and, much more recently, Australia (the latter being opposed to nuclear power until recently).

Today, through its Tier III standard, the IMO has defined a number of Emission Control Areas (ECA) throughout the world: ships transiting through those zones must comply to restrictions applying to emissions of Sulfur Oxides, Particulate Matter and Nitrogen Oxides – or all of the above. Navies do not have to comply to IMO regulations, yet it is within countries’ and port authorities’ rights to refuse transit and docking access to ships, including naval, that do not comply.

“This possibility is progressively leading to more environmental considerations being included in requirements for new ships and ship propulsion systems,” Lars added. A sentiment echoed also by Buckingham, who said that, “navies are a great part of the diplomatic service, so having a navy that is clean and in line with environmental standards is very important.” From the point of view of engine manufacturers, Daffey noted that, “in the naval world, with the long lifecycle of vessels, this means that not only future vessels but also current new builds should be capable of being climate neutral in the future.”

Future Fuels: Fact or Fiction?

To tackle present and future environmental constraints, navies that have made the choice to fit their capital ships with CODELAG systems or true integrated electric drives are already slightly ahead: if they enter ECA zones, they can simply continue to navigate with electric propulsion and turn off all other means. However, neither of these solutions is fully green, as both rely on diesel or gas generators to produce the electric energy that will then propel the ship. They also continue to imply a dependence on fossil fuels with dwindling resources and volatile prices.

As such, shipbuilders such as Naval Group and Fincantieri, but also engine developers such as Rolls-Royce Power Systems and naval architects such as BMT, are already exploring alternative solutions. “This is an area where Europe is simply ahead of the US,” Caris pointed out; “European navies benefit largely from the experience of shipbuilders and industry that also work in the commercial sector and therefore already face the challenge of having to find greener solutions.”

Naval Group is working on its Blue Ship concept, which according to its website seeks to “guarantee energy autonomy required for all its current and future missions in terms of energy efficiency and power, with the smallest possible environmental footprint for the whole warship.” According to Lars, this concept focuses on: increasing ship’s energy efficiency; thinking about tomorrow’s ships’ electric architecture – in particular energy storage; and, more long-term research on future fuels.

Currently, Fincantieri is working on ZEUS (Zero Emission Ultimate Ship), an experimental oceanographic research vessel powered by batteries and fuel cells. “ZEUS is an opportunity for us at Fincantieri to experiment with fuel cells and hydrogen onboard a ship, and understand how this can be installed safely,” Mattei told AI. The latter is being done in collaboration with RINA, the Italian classification society. The biggest challenges in using fuel cells, much like it is for electrical power, are transfer and storage, Mattei added. “Technologies to safely store hydrogen do exist, though they might today still be too heavy, but transfer of hydrogen remains a delicate operation,” he said, especially as navy ships have weapons onboard. “At the moment, we think that the most interesting use of fuel cells would be as a distributed power source onboard the ship for small applications, rather than for propulsion,” Mattei concluded.

Daffey told AI that Rolls Royce Power Systems is also taking concrete steps towards a climate-neutral future, seeking to cut greenhouse gas emissions by 35 percent by 2030 compared to the 2019 level. “A key element in achieving these goals is the certification of the most important mtu engine productions, which will run on sustainable fuels from as early as 2023,” he added. Concretely, this means that new generations of Series 2000 and 4000 engines will be qualified to run on second-generation bio-fuels and on E-fuels.

Bio-fuels and E-fuels are likely to be key to the transition toward greener navies according to both Lars and Buckingham. “This will come from the maritime domain, rather than the military one,” added Caris, “and navies will eventually have little choice outside the use of those fuels because key engine manufacturers like mtu will design engine lines conforming to emission standards.”

Rolls-Royce has been selected to supply its mtu naval generator sets for phase one of the U.S. Navy’s Constellation class frigate program. (Rolls Royce)
Rolls-Royce has been selected to supply its mtu naval generator sets for phase one of the U.S. Navy’s Constellation class frigate program. (Rolls Royce)

However, Buckingham cautioned: “There is a large number of pathways that can be achieved with synthetic drop-in fuels, but it also means that navies will have to take a greater interest and responsibility for the supply chain of their fossil fuels.” A sentiment echoed by Caris, who added that while navies can now easily replenish their ships with fossil fuels everywhere in the world, this will not be true in the short and medium term for bio-fuels and E-fuels.

As for other types of propulsion, such as hydrogen – which Fincantieri and Naval Group are also exploring together – and fuel cells, while the technology is there, safety and logistical considerations remain.

Getting Creative

“Whatever ship we design, it is going to have a lifetime of 25 years or more, which means it will have to undergo an upgrade to update the combat systems, weapons, and sensors,” Buckingham concluded. “Consequently, when designing ships we have to build-in margins for these updates and for the corresponding available power.” Energy storage, in other words, will be one of the key naval propulsion challenges of the coming years for capital surface ships – whether it is electricity, fuels cells, hydrogen, etc.

One potential solution, in the meantime, is also greater use of distributed unmanned systems with high autonomy to carry out certain missions such as Intelligence, Surveillance and Reconnaissance (ISR) – or to access ECA zones. Saildrone, a US-based company, has developed a wind-powered Unmanned Surface Vehicle (USV) with autonomy at sea of 12 months, high stealth capabilities and, when wind propelled, a zero operational carbon footprint. “The secret is in harvesting energy from the environment,” Richard Jenkins, founder and CEO of Saildrone, told AI, “so there is no need to carry that fuel reserve, allowing for a system with higher endurance and lower drag as well as low propulsion needs.”

Saildrone has developed a wind-powered USV with autonomy at sea of 12 months, high stealth capabilities and, when wind propelled, a zero operational carbon footprint. (Saildrone)
Saildrone has developed a wind-powered USV with autonomy at sea of 12 months, high stealth capabilities and, when wind propelled, a zero operational carbon footprint. (Saildrone)

The requirements for large capital surface vessels and smaller patrol, research or unmanned vessels are clearly different. Yet as navies face increasing logistical and environmental constraints in their naval propulsion needs, the use of the latter – for tactical, such as Saildrone, and research, such as ZEUS, purposes – may well be key to such critical transition toward greener blue water navies.

Dr. Alix Valenti