The use of direct current represents a breakthrough in optimized power and propulsion systems, providing a safe and reliable operation with improved efficiency compared to traditional AC systems. The DC marine power system allows engines to be operated at variable speed and the integration with energy storage systems boat results in less fuel consumption and reduction of emissions.
In 1880, the SS Columbia was the first vessel equipped with 120 incandescent lights, after the invention by Thomas Alva Edison 2 years earlier of an incandescent electric light bulb for the consumer market. The lighting system of the ship was powered by 4 belt-driven 6kW dc generators (dynamos), connected to the steam engine driving the propeller. The installation proved to be a success and, shortly after, the direct current (DC) lighting became a standard feature onboard ships.
The lack of practical alternating current (AC) motors led to the adoption of direct current as a standard to simplify the systems and this solution was extended to other applications, such as deck machinery and weapon systems.
AC ONBOARD SHIPS
The discovery by Nikola Tesla and Galileo Ferraris in the late 1880s of the induction motor, together with the development of the diesel engine as a replacement of the steam turbine, made the alternating current the preferred solution for electrical systems onboard ships.
In 1903, the Russian river tanker Vandal was the first vessel featuring a full diesel-electric transmission. Another remarkable installation of diesel-electric propulsion was the celebrated Italian Navy sailing ship Amerigo Vespucci, built in 1933 and equipped with a Marelli (now ABB) DC motor with a double-winding rotor, rated 2x665KW (total power 1.330 KW) at 150 rpm.
The advent of solid-state power electronics in 1958, the thyristors, made it possible to invent devices to convert, transform and adjust the frequency in AC systems. The SS Canberra, launched in 1960, was the first passenger liner using alternating current for auxiliary power and propulsion systems.
SS Columbia, the first ship with an electrical system in 1880
DC SYSTEMS BACK INTO THE SPOTLIGHT
Historically, AC has prevailed in electric propulsion for ship applications, with DC mostly confined to conventional submarines.
However, recent technological advances have brought DC systems back into the spotlight, extending their role to conventional surface vessels. The distributed DC solutions can improve the performance and efficiency of the vessels – a key factor after the implementation of the new requirements for reduced emission levels by IMO as part of the marine decarbonisation strategy.
Efficiency
AC-based electrical power systems require that connected generators match system voltage and frequency. This means that diesel engines must operate at constant speed and the specific fuel oil consumption (SFOC) is lowest at a small operating window, usually 85% of rated load.
Instead, DC marine power systems only require matching network voltage. This means it’s possible to run the engines at variable speed with the lowest possible fuel consumption, at least down to 50% loading.
The immediate benefits of this are:
• Reduced fuel oil consumption by approx. 20% for partial load operation;
• Cleaner combustion process with less soot when operating at partial load;
• Reduced GHG and particle emissions as a consequence of lower fuel consumption and better combustion process;
• Increased temperature of exhaust gases at lower loads, meaning that SCR is operational at all loads, decreasing both NOx and urea consumption;
• Reduction of airborne noise level (estimated by more than 50dB);
• Lower maintenance cost due to less wear and tear on the diesel engines.
Space and weight optimization
Comparing volume, weight and length for a 63 m yacht using DC vs AC solutions, the DC Grid shows:
• 30% reduction in length (≈6m);
• 34% in volume (≈10m³);
This makes DC-based systems extremely appealing for hybrid propulsion yacht projects, where space and weight are key design consideration
DC Grid configuration including fuel cells and shore connection for a megayacht
Operations
The DC grid solution opens up an advanced concept of integration and optimization of vessel operation. The flexibility of the system gives the possibility to combine different energy sources: diesel-driven electrical generators and energy storage systems boat technologies based on lithium-ion batteries, super capacitors, and, when commercially available, fuel cells.
Since these ESS are DC-based, their integration into the DC Grid becomes simpler and more functional. In a simple hybrid configuration, ESS take the role of energy buffering, such as:
• Spinning reserve (backup for running engines);
• Peak shaving (levelling out load peaks);
• Dynamic enhancement (instant power support);
• Zero-emission operation (ESS powering the system while generators are off).
Other benefits of DC Grid installations:
• Power distribution on 1000Vdc instead of 690Vac reduces cable needs by ≈40%;
• Voltage distortion, common in AC systems with frequency converters, is no longer an issue;
• Shore connection on the DC side allows easier compatibility across ports, as network frequency is not a constraint;
• Centralized or distributed converter drive configurations possible;
• System efficiency improves by 0.5–1% compared to AC systems.
SAFETY AND CONTROL
DC is simpler than AC, thus easier to predict and manage in fault scenarios while ensuring safe and reliable operation. This is achieved by:
• Built-in overload protection limiting power output at low loads;
• No need for synchronization, allowing generators to connect in ≈10 seconds;
• Short-circuit currents cleared within 40ms, compared to up to 1s in AC systems.
The integrated automation system manages both Power Management and energy storage systems boat via Energy Management functions.
COST ANALYSIS AND CONCLUSIONS
AC systems have lower CapEx due to standard components and technological maturity, although transformers may increase costs.
DC systems have higher CapEx, especially when integrated with batteries. However, their space and weight savings can lower construction costs.
Regarding OpEx, AC systems are less efficient and have higher lifecycle costs, while DC systems improve efficiency and support integration with other energy sources, including electric vessel technology like battery and hybrid systems.
In conclusion, adopting a DC marine power solution offers flexible, reliable operation and supports the broader push for marine decarbonisation in the industry.
