Frequently Asked Questions…

It is a boat featuring a propulsion system, which combines two or more energy sources: an internal combustion engine, an electric motor and batteries, storing the produced electrical energy.

The aim is to reduce consumption and emissions and maximize comfort, while managing the flexibility of the different power sources efficiently.

Up to this point, the approach is similar to a hybrid car operation.

Marine propulsion world, although using similar technological solutions and often derived from automotive, is based on different physical phenomena: A vessel moves through the water thanks to propeller thrust, which overcomes the hull resistance to motion and allows the boat to move forward through the waves. Endothermic engine power is used on cars to overcome the wheels rolling friction and move forward on land.

Apart from this, the environment (wave motion, currents, no descents and climbs in the case of sailing) also makes the two applications very different. The biggest difference between the world of onshore traction and the naval world becomes clear: the need for standardization vs the need for customization.

The car is a product that, despite the differences between manufacturers and models, is extremely standardized, even in the area of hybrid solutions, with a consequent physiological flattening of performance.

The marine world in general, and yachting is at the forefront, is characterized by an enormous variability of applications: technical solutions, power, geometries and very different operating profiles characterize a 5-metre open yacht with an outboard engine compared to an 80-metre yacht with azimuth thrusters.

This variability implies, for each application and operation profile, the design of a dedicated propulsion solution, while maintaining.

Marine hybrid propulsion is then divided into two macro-categories: parallel and serial hybrid propulsion. 

Parallel hybrid propulsion systems have both the diesel engine and the electric motor connected to the propeller. The vessel could then be propelled purely by the diesel engine, purely by the electric motor, or by a combination of both. This is also the most common configuration in the automotive sector.

A serial hybrid system involves the production of electric power from a diesel engine coupled to a generator: In combination with an energy storage system (batteries), the generator gives power to the electric motor, the only component connected to the propeller. 

Batteries convert chemical energy into electrical energy through one or more cells inside that store and transfer energy externally. There are different types of batteries, each with its peculiarities, that are able to manage specific needs in terms of size, design, operating temperatures and chemistry. 

Lithium batteries are currently having a success moment compared to traditional and more well-known lead acid batteries. Then there are new salt batteries, that can operate at lower temperatures, non-toxic, recyclable and not subject to fire risk. 

Lead-acid batteries are economical and available in many sizes but require significant maintenance. 

Lithium batteries, particularly NMC (Nickel-Manganese-Cobalt), LFP (Lithium-Iron-Phosphate), and LTO (Lithium Titanate), are having exponential growth thanks to the electric mobility boom and their high energy density. These batteries are used to power auxiliary loads and commonly as propulsion batteries. Despite their high performance, they present some safety issues that need to be addressed (especially from a thermal perspective). Significant development margins promise further improvements in the future. 

Solid-state batteries could solve many lithium batteries current problems because they can have even higher energy density without being flammable. They have a more compact and flexible design, with significant advantages for heavy transport and the naval industry. 

Lithium-air batteries represent a potential revolution offering greater energy density. 

The Battery Management Systems (BMS) can monitor and manage the battery pack autonomy and safety, controlling the voltages, temperatures and currents of each cell.

The BMS takes over the state of charge and the battery capacity, indicating its health status and preventing operating outside safety limits.

Fire detection systems, connected to a centralized alarm system, provide and immediate response that ensure onboard safety.

Despite fears and lack of trust, recent research and investments in the battery field demonstrate that more efficient, safe, reliable and sustainable navigation is possible. With safer, sustainable batteries that ensure greater autonomy for the vessel, the maritime navigation future has all the cards to become as promising, efficient, and ecological as the automotive sector. 

 The first advantage of a serial hybrid propulsion system is offering significant flexibility to the propulsion system, optimizing the diesel engines’ performance, improving their efficiency. This flexibility is due to the complete separation between energy sources and users, allowing a more efficient energy management. Providing greater independence to the primary power source, the diesel engine, mechanically decoupling it from the propulsion line, also contributes to a higher level of comfort on board, significantly reducing noise and vibrations.

However, adopting this type of propulsion has a considerable impact on the overall vessel cost: The more complex the system, the higher the level of design required to make it as complete and efficient as possible, and in line with the vessel’s needs and requirements.

The initial investment will thus be higher compared to traditional propulsion.

The challenge becomes even greater for our VULKAN’s Hybrid and Electric Drive Solutions team when discussing the implementation of a serial hybrid propulsion on an existing vessel. 
In this case, the VULKAN experts must carefully consider the existing layout, relative weights and sizes of the machinery, available spaces, and the technologies already in use.  

Imagine sailing on a vessel with a variable operational profile over limited time frames with frequent phases of acceleration and deceleration: Large pleasure vessels (superyachts over 60 meters), passenger transport vessels (cruises), or workboats are the perfect example where serial hybrid propulsion is the best applicable solution.

The situation changes (and significantly) when referring to medium-small units, where the available space is limited, annual operating hours rarely reach high values (5,000 hours), and comfort and relaxation are the user’s priorities. 

In parallel hybrid propulsion systems (where the electric motor shares the same axis as diesel engine), both, the in-line and parallel configurations (with the electric motor shaft being “secondary” to the propulsion shaft), show different characteristics and implications.

The in-line configuration offers evident advantages: 

• Compactness and lightness are perfect for limited spaces.
• The robustness and reliability of the system ensure consistent performance over time.
• Thanks to modularity, obtained through the standardization of hybrid components, and advanced electrical design safety, compliant with the highest industrial standards, an additional safety level is added.
• Customization allows for tailor-made solutions, combining diesel engines of various powers with electric motors suitable for the specific needs of the project and customer.
• Flexibility of the package makes it ideal for both, pleasure and commercial vessels.

On the other hand, the parallel configuraion (with “secondary shaft”) is characterized by greater flexibility in managing the reduction ratio between the electric machine and the propeller shaft, optimizing the propulsion system’s efficiency.

The parallel hybrid is the perfect solution for units requiring electric mode at low speeds and small electric motors and battery packs. The electric machine is somehow connected on the same shaft line of the endothermic engine through a system that is called PTO/PTI (Power Take Off / Power Take In).

Such a configuration will consequently have a greater overall weight, due to the use of an additional mechanical transmission, as well as an electric motor. This affects both: the vessel’s fuel consumption and requires more installation space.