The future of boat propulsion seems to be decided, but where is the break-even point, what are the challenges and how will we safely use these systems?
Antonie and Berend Stockman, co-founders of STOK Electric
To find out where the break-even point between thematic and electric propulsion is, who it is for, as well as what such a system actually consists of, we talked to Antonie and Berend Stockman, co-founders of STOK Electric in the Netherlands, a company specialized in making electric and hybrid propulsion systems for both new boats and retrofits.
Elysium
Installing an electric propulsion system on boats is now a challenge, but also a high initial investment due to the high prices of batteries and the optional generator, but necessary for now for an extended range.
How did you come up with the idea of starting the production of electric propulsion systems?
Antonie and I wanted to start a company together, we are both passionate about boats and technology and we wanted to combine these things.
A few years ago, our father bought a classic sailboat that was 20 years old. It had to be completely refurbished and the diesel engine had to be replaced.
We wanted it to be electric, but after many searches on the internet we did not find any system that was suitable for existing ships. We then came up with the idea of doing this ourselves, because there was a lot of knowledge within the company.
For 2 years we have been totally dedicated to developing an electric and hybrid system that is suitable for both existing and new-built ships, because if you can equip an existing ship, for a new one it is nothing.
The transformation of the sailboat and the equipping with an STOK Electric propulsion system can be viewed HERE
This ship has been in Norway and Sweden twice for 3 months, and new projects are already underway. We have also included range specifications in the appendix. This boat has a 60 kW electric motor, a 37 kWh battery and a 20 kW generator.
Who is the target audience, new boat builders or those who want to switch from the old thermal system to the electric one?
At STOK Electric, we recognize two primary markets: existing ships and newly built ships. Worldwide, there are about 30 million recreational boats, a significant part of which must be converted to electric propulsion so as not to become waste.
We at STOK Electric believe it is essential to conserve these vessels by adapting them for sustainability. Therefore, we have designed our systems to be compatible with existing vessels, ensuring that they can be upgraded effectively.
On the other side of the spectrum are newly built ships. Their electrification is particularly advantageous, as our systems can be integrated into the ship’s project from the very beginning. One of our key goals at STOK Electric is to maintain the simplicity of our systems.
By minimising the number of different components, we make our propulsion solutions adaptable for both types of vessels.
We focus on high-quality systems, believing that quality delivery is better and more sustainable in the long run. Therefore, our target audience is the middle and high-end segment.
What are the trends in the Netherlands, where is the greater interest in electric boats? In the leisure area or for commuters?
The market for electric boats in the Netherlands is developing. Small boats are increasingly equipped with electric outboard motors. However, it is still relatively rare to see large boats equipped with electric or hybrid propulsion systems.
But we are witnessing a gradual change in this area, with more and more people starting to adopt this concept. For example, last year, at the Boot Düsseldorf exhibition, we encountered some skepticism from the participants about electric boats. This year, the sentiment has visibly changed, with visitors showing much more enthusiasm and interest in electric propulsion.
A significant trend in the Netherlands, especially in urban areas, is the increasing regulation in favor of electric boats. For example, in the canals of Amsterdam, the use of electric propulsion will be mandatory from 2025.
This regulatory push is a strong indicator of the growing interest in electric boats, both for leisure purposes and for daily commuting.
The Dutch market is also showing a growing interest in sustainable and environmentally friendly shipping options, driven by increased awareness of environmental issues and the desire for quieter and cleaner waterways.
This trend aligns well with the global movement to reduce carbon emissions and conserve natural resources.
In addition to recreational boating, there is a significant application of electric propulsion systems in the professional maritime industry, largely due to the tangible return on investment that these technologies provide. Many companies are starting to realize the economic benefits and are increasingly considering switching to electric or hybrid ships.
What does a customer who wants to switch from a thermal system to an electrical system need to know?
Complete package: When considering switching to an electric powertrain, it is highly advisable to find a company that can offer a complete package. This ensures a fully integrated system.
There are suppliers that could only provide a motor and throttle control, leaving out other essential components such as batteries and safety devices. In this situation, the customer would have to carry out significant integration work on their own.
Therefore, it is essential to look for a supplier that can assist you from start to finish, providing a complete solution that includes not only the main components, but also installation support, system integration, and after-sales services.
Choice of battery chemistry: The choice of battery chemistry is crucial for the performance and safety of an electric propulsion system. The two most common types are NMC (nickel manganese and cobalt) and LFP (lithium iron phosphate).
NMC batteries, often used in electric cars, offer a higher energy density, which means they can store more energy in a smaller volume.
This can be beneficial for saving space, but it poses a higher risk of thermal instability, which raises safety concerns.
LFP batteries are more stable at high temperatures and have a lower risk of overheating. In addition, they can be extinguished with (sea) water, which is not the case with NMC.
Current Strength and Safety: It is essential to keep the current strength (measured in amperes) low in electrical systems, with an upper limit of 300 A, for safety on board. A higher current intensity can lead to increased heat generation and potentially dangerous situations.
When more power is needed, switching to a higher voltage can reduce the current required for the same power, thus increasing safety, as lower currents generate less heat and put less strain on electrical wiring, which also makes the system less efficient.
A high voltage in a system can be effectively protected with proper insulation and circuit breakers designed to safely handle high voltages. High-voltage systems allow the use of lower currents, which in turn reduces heat development and load on the wiring. This is crucial for safety on board, especially in a marine environment where space is limited and the risk of fire is a serious concern.
In electrical systems, especially those with high current demands such as 400 amps, it is essential to ensure that the fuse can safely interrupt excessive currents. If a fuse is set to trip at too high a current (e.g., 800 amps), but the battery can’t provide enough current to throw the fuse, it may melt instead of jumping.
This scenario is dangerous because it allows for prolonged high current flow, risking overheating or fire. The key is to match the fuse rating to both the maximum current needs of the system and the peak current capacity of the battery.
This ensures that the fuse will explode if the current exceeds the safe levels, providing effective protection.
Careful selection of the fuse rating and understanding of the battery’s ability to deliver high peak currents are essential to prevent such hazardous situations, ensuring the safety and reliability of the system in high-power applications such as electric propulsion systems for boats.
Direct or reduction drive: In the case of electric boats, it is possible to use a direct drive, in which the engine is connected directly to the propeller shaft, without the need for a reduction gearbox. Not only does this significantly reduce maintenance, as there are fewer moving parts, but it also provides a quieter browsing experience.
However, some electrical systems use high-speed motors that require a gearbox to reduce rotational speed and increase torque. This can lead to a less compact engine setup, introduce additional noise, and an additional component that requires maintenance and could fail.
More benefits: Electric navigation offers numerous advantages, beyond durability and quiet operation.
– Provides access to natural areas that may be restricted for traditional combustion engines due to environmental concerns.
– In addition, electric boats often have a significant amount of energy on board, which can be used for various domestic applications. This feature is especially beneficial for longer trips.
– Another key advantage of electric propulsion is improved handling. Electric motors can provide instant torque and precise control, which greatly improves the maneuverability of the boat, especially in narrow marinas or during mooring.
– Finally, electric boats generally require less maintenance compared to their gasoline or diesel counterparts. The absence of a combustion engine means that there are fewer moving parts that can wear out or require repair, leading to lower long-term maintenance costs and fewer downtime.
What advantages do your systems have over the competition? Which battery supplier/technology do you use? And why?
STOK Electric offers a complete and integrated system comprising all the necessary components for electric navigation. This approach ensures that a single entity is responsible for the entire powertrain, simplifying the overall configuration by eliminating the need for different products. This not only increases the reliability of the system, but also improves its performance.
Generators for hybrid solutions: STOK Electric has fully implemented generators specially designed for hybrid boats. These generators integrate seamlessly into the hybrid system, providing an efficient and reliable power source when needed, ensuring a balance between electric propulsion and generator use.
Ease of integration into existing vessels: Thanks to the full integration of the STOK Electric system, it has been kept simple, making it easy to integrate into existing vessels.
Antonie and Berend Stockman, co-founders of STOK Electric in the Netherlands
This ease of integration is a significant advantage for customers who want to upgrade their vessels with an electric propulsion system, reducing the complexity of installation.
Scalable system customized to the customer’s needs: The STOK Electric system is highly scalable, allowing customization according to the customer’s specific navigation profile. This scalability means that the system can be adapted to different sizes and types of vessels, ensuring optimal performance for each unique application, whether it is a leisure, commercial or specialized marine use.
Installation expertise: STOK Electric goes beyond the delivery of the drive system. In cooperation with shipyards, we provide on-site installation and testing services. This approach ensures a seamless customer experience, with experts overseeing the entire process to guarantee smooth integration and optimal performance.
Large range of available power: STOK Electric currently has powertrains from 10 to 90 kW, with the range soon to be extended to 200 kW.
Safety: This is due to the built-in safety features, which make working with high voltages easier and safer. High voltage is actually easier to secure than high-voltage currents. The design of the system takes into account various aspects, such as complete isolation of the system, the use of connectors that close the system when disconnected, and the use of LFP batteries.
STOK Electric uses lithium iron phosphate (LFP) batteries for the reasons mentioned in point 4. The company works with various vendors to cater to different applications. For existing vessels, STOK Electric is partnering with MG and Super B, both based in the Netherlands. For newly built vessels, the company is working with US-based ONE.
In terms of safety, what are the requirements for using these systems – bearing in mind that water and electricity are not best friends!
Waterproofing and insulation: In marine electrical systems, the main concern is corrosion due to moist air, especially when salt is present. It is essential to know that all electrical components are properly insulated and waterproof, as it helps protect against the corrosive effects of moisture and salt, extending the life of the components and maintaining the reliability of the system. This approach is especially important in environments where exposure to salt water is common, as it can significantly accelerate corrosion, leading to potential malfunctions and safety hazards.
Residual current devices (RCDs): These devices detect ground leakage currents and can immediately shut down the system in the event of a failure, reducing the risk of electric shock. They can also provide an audible and visual warning, if the situation is not too dangerous at the time.
Using safe types of batteries: For example, LFP (lithium iron phosphate) batteries are safer than other lithium-ion batteries because they are more stable and less prone to overheating.
Heavy-duty connectors and high-voltage lockout: Use of high-quality, water-resistant connectors that are also designed to prevent accidental disconnection. In addition, a high-voltage interlock is integrated. This is a safety feature that ensures that all high-voltage components are properly connected and engaged before the system can be activated. This prevents the system from operating if a high-voltage connection is not secure, significantly reducing the risk of electrical hazards.
How do you see the development of this segment for sailing boats, but also for motor boats?
Early stages: The market for electric propulsion in boats is still emerging, with a growing interest but with less mature technology and infrastructure compared to traditional combustion engines.
However, for recreational boats, a notable advantage is the ability to use the standard shore power connections available in marinas. These connections allow the batteries to be recharged at night, which is often convenient since pleasure boats usually remain anchored in the harbor for at least one night.
Thus, the current infrastructure is quite adequate for the needs of electric recreational navigation. On the other hand, the requirement for fast charging is a challenge for professional vessels, where the existing charging infrastructure may not always meet the time requirements for fast charging.
Motorboat segment: A key challenge for motorboats is to provide sufficient power and range to rival that of combustion engines. While electric systems are already efficient enough for short-range or lower-speed operations such as fishing, port tours or recreational cruises, high-speed vessels could benefit from a hybrid approach. This could be either a parallel hybrid system, which uses a large diesel engine for propulsion alongside an electric system, or a series hybrid system, in which a large generator recharges the batteries. These hybrid solutions offer a practical balance, combining the efficiency and environmental benefits of electric propulsion with the power and range of traditional motors.
Sailboat segment: Electric propulsion is very attractive for sailboats. Sailors often appreciate the quiet, emission-free operation that perfectly complements the sailing experience. The advancement of regenerative charging systems, in which the propeller generates energy while sailing, further enhances this attractiveness.
Additionally, the use of diesel engines in sailboats is not ideal, as they are often used for short periods of time and at speeds close to idle, leading to reliability issues. Additionally, sailboats are inherently hybrid by nature, relying on their sails for propulsion. Therefore, a long electric range is less critical, making electric propulsion systems an even more attractive option for sailing boats.
Hybrid systems: In both segments, hybrid systems that combine electric propulsion with traditional engines could be a transitional solution, offering the benefits of electric power while alleviating range anxiety. However, this comes at a higher price.
Regulatory changes: Governments and regulators are increasingly focusing on reducing carbon emissions. This could lead to stricter regulations on traditional combustion engines in marine environments, further pushing the market towards electric options.
Environmental Concerns: Increasing awareness of environmental issues is driving the shift to greener technologies in all sectors, including the nautical sector. Electric powertrains offer a cleaner alternative to traditional combustion engines, reducing both emissions and noise pollution.
Electric navigation is an emerging trend and, although it is still in the early stages of development, especially in the higher power segment, starting from 30 kW and above, the market is progressively switching to electric propulsion. For sailboats, fully electric systems are fully feasible and offer an environmentally friendly and efficient alternative.
However, for sailing boats that travel long distances, it is advisable to have a generator on board as a backup. This generator does not have to be particularly large; It should be adapted to the average energy consumption of the boat, as peak energy demands can be met by batteries.
ÎOn the other hand, for motor boats, especially those that require higher speeds, the integration of a generator is highly recommended, forming a hybrid system that combines the best of both worlds. Currently, it is estimated that about 2% of boats worldwide are electric. This figure reflects the potential for growth and development in this sector.
We anticipate that in the coming years the increase in the number of electric boats will gradually increase, with a greater acceleration expected from 2028 onwards. This increase will likely be driven by advances and cost reductions in battery technology, as well as stricter regulatory measures.
Is it cheaper to use electric propulsion for motorboats? From what number of operating hours/day or distances are these systems more cost-effective?
In order to simulate where the threshold is at which the costs related to fuel and maintenance of a thematic system will exceed the initial cost of electric propulsion, a simulation was carried out.
This simulation takes into account a modern 110 hp diesel engine, including a reducer and an 8000kVA diesel generator needed to be able to operate the same household appliances that would operate on an electrically powered boat. The material cost of this system is EUR 36 052.91.
Taking into account a diesel price of €2.00 per litre, a generator run time of 10% for all hours of navigation and a total maintenance cost for both engines of €5,500.00 per 1000h.
In comparison, a 60 kW electric powertrain with a 79 kWh battery and no backup generator, the cost of the system would be €70,314.
An electricity cost of €0.40 per kWh and a maintenance cost of €500 per 1000h.
With this data, one can plot the graph in Figure 1, which shows that in this scenario, the payback time for electric propulsion will be ~5300 hours.
In this situation, the batteries have performed only 386 cycles and are at 97.4% of their original health. Battery replacement should not be considered in the near future, because at 15000 hours of running the engine, the battery has performed 1092 cycles and has a health level of 92.7%. If well maintained, the battery should last at least 3000 cycles and still have 80% of its original health
If a hybrid propulsion system using a 20 kW diesel generator is considered, the intersection point moves towards 10 000 operating hours. Depending on the type of boat and how often it is used, this recovery time may or may not be acceptable.
From your experience, what is the price difference between a new boat with electric propulsion and a thermal one?
The cost of STOK Electric systems varies depending on the power required. For higher power systems, a higher voltage is used, which usually means more expensive components. For a 30 kW system, which is comparable to a 55 hp diesel engine, the total cost for a complete system is around 32,000 euros. This includes a 24 kWh battery. If you opt for a hybrid system, there is an additional cost of around €19,000. However, a hybrid solution is mainly necessary if you plan to travel long distances.
For a more powerful configuration, a 60 kW electric motor with a 62 kWh battery and all the necessary components starts at around 62,000 euros. Adding a 20 kW generator to this configuration brings the total to around €90,000. So, the cost varies depending on your needs. But on average, the initial investment for an electric or hybrid system is about 2-4 times higher than for traditional systems.
Are there any other things you would like to mention or share with our readers?
Installation expertise: Currently, there is a limited number of qualified professionals in the installation of electric propulsion systems in boats. This shortage can lead to higher costs and longer wait times for conversions, which can discourage boat owners from switching to electric systems.
Infrastructure: Infrastructure for electric navigation, such as charging stations and maintenance facilities, is not yet widely available. This lack of infrastructure can be a significant barrier to adoption, especially for motorboats that typically require more power and range. Slow charging is not a problem, but fast charging is often not possible.
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