A guide to hybrid yacht propulsion and alternative technologies
Last updated: 09/01/2020
Like many other industries, yachting has been forced to acknowledge its carbon footprint in recent years, with more and more superyacht owners, designers and builders looking for ways to utilise hybrid propulsion systems to power their vessels, but do the ethical benefits of these alternative technologies truly outweigh the increased cost and investment when it comes to yachting?
Yachting Pages looks into hybrid yacht propulsion, considering the benefits and drawbacks of alternative propulsion systems, and exploring hybrid superyachts.
What is hybrid propulsion?
Although not a new technology, dating back as far as the early 20th century, the definition of hybrid is still very much misconstrued. Hybrid can be understood as the output made by combining two different elements, but, in the yachting industry, the interpretation of hybrid is very much subjective and can be translated in a number of different ways.
When we talk about hybrid technologies in yachting, we are typically referring to hybrid propulsion; for example, the convergence of two or more propulsion systems of differing energy sources, like diesel and liquid natural gas (LNG), waterjets and propellers, or engines and electric turbines, or a mix of the above.
Bjorn Moonen of Ghost Yachts explained to Superyacht Design that, “A hybrid yacht is basically a yacht that can be propelled by two different energy sources. Most commonly, these are a combination of diesel and electric propulsion systems.”
Superyacht Design goes on to explain that “the key difference between a straightforward diesel electric system and a hybrid system is the addition of a battery bank and the option to run on diesel, battery power, or both at the same time.”
Hybrid yacht propulsion: Benefits and drawbacks
Like all technologies, there are both benefits and drawbacks in implementing hybrid propulsion systems aboard a yacht.
Why hybrid? The benefits of hybrid yacht propulsion
Benefits of hybrid superyacht propulsion include the added versatility that hybrid systems lend to yachting, in that multiple modes of propulsion can be enlisted for travel.
Using the electric mode of a diesel-electric propulsion system allows for silent running at low speeds, which is perfect for overnight travel; for emission-free running in harbours and marinas; and to provide a speed boost at full power. This set up also allows for a highly flexible layout on board, as generators do not need to be positioned near to the propeller shafts at all permitting the engine room to be positioned anywhere on board, rather than placing it in the centre of the hull where the comfort levels are typically greatest.
Hybrid systems are therefore believed by many of those owning and operating superyachts to provide significant cost and efficiency savings, as well as addressing two of the largest comfort issues on board – noise and vibration.
Having worked on Heesen's first hybrid superyacht, M/Y Nova (pictured above), Ivo van den Berg of Zenoro explained the necessities and benefits of hybrid systems. He said, "Many superyachts are power boats! They can be characterised by a surplus of installed engine (propulsion) power. However, during everyday operation, this overkill of propulsion power is rarely used: Normal cruising speed only uses part load of the engines, and manoevering in coastal areas and harbors is often done in an almost idle engine mode. As a result of this, a typical load profile will show the larger part of the propulsion engines running time at part load, combined with incidental spikes of higher peak power demand.
"This low engine load often results in low exhaust temperatures and higher specific fuel consumption. On top of this, the usage of exhaust after-treatment systems will be complicated, since they all rely of higher exhaust temperatures for their performance. Future emission legislation rules, like IMO 3, will definitely require larger after-treatment devices for exhaust fumes - an even bigger use of the valuable space on board! Big engines and even bigger after-treatment systems that do not operate optimally. So, there is a challenge to overcome.
"A hybrid powertrain is the solution for many superyacht applications: The electrical propulsion provides silent smooth running and excellent maneuverability. Average low-power demands during cruising can be generated by a (series of parallel) diesel engine generator units, whereas a battery can assist instantly when power demand is high. Specific fuel consumption is optimal, since the generator engines will run at a higher average load profile.
"The battery power capacity can be optimised to both required peak power and duration of higher power demand. Average exhaust temperatures will be higher, which enables the more compact after treatment systems to perform optimal on the generator units. The environmental footprint will also be smaller and more flexible in installation, and noise will be reduced. Maneuverability is excellent, and peak power performance will even be higher. Emissions and fuel consumption will be significantly lower, maintenance costs will be minimised and the cost of ownership will drop - The future starts today as hybrid superyachts will conquer the oceans!"
The drawbacks of hybrid superyacht propulsion
However, according to some experts, those who are hoping to save money by making this large investment in their vessels should perhaps look elsewhere. This is because, while hybrid propulsion systems are believed by many to increase fuel economy by reducing fuel consumption paying back their increased cost over time, many argue that this is not categorically true due to the inefficiencies that arise in electric power generation, conversion and transfer. The output often just doesn’t justify the cost.
In this case, the AMELS shipyard has elected to enlist a different approach to reducing the environmental impact of yachting, by optimising different hybrid systems on board their vessels, including HVAC heat recycling and hotel load power innovations, giving what it states to be a “faster return on investment and lower total cost of ownership.”
Hybrid propulsion systems aboard superyachts
Despite criticism, hybrid propulsion systems have been installed aboard many luxury motor and sails yachts – each with differing results.
In 2008, Ferretti Yachts launched, the 22.86m Long Range 23 motor yacht with state-of-the-art hybrid technology. Timed with the beginning of the worst recession the boating industry had seen, and costing around a third more than the diesel-only option, this negated the green benefits of hybrid propulsion for many and the design was not met with great reception.
Soon after, Royal Huisman’s 58m sailing yacht Ethereal arrived to a much more receptive audience, utilising a hybrid electric-mechanical propulsion system for silent running at cruise speed, while recharging her battery bank through the drive train under the sail. Electric motors work in line with silent thrusters for stability, providing fume-free bathing for guests.
Feadship’s 86m motor yacht Ecstasea uses waterjets and propellers and engines and gas turbines as part of her hybrid propulsion system, allowing her to reach a top speed of 35 knots. While, more recently they launched the 83.5m M/Y Savannah who is powered by a ‘triple’ hybrid propulsion set ups using propellers and thrusters for propulsion, combined with diesel engines and electric motors for drive, and diesel and batteries to store energy.
Lloyd’s Register reported that Savannah’s system allows for fuel savings of up to 30% with fewer vibrations and excellent manoeuvrability in marinas; “The complete design package offers major benefits in terms of fuel saving and consists of five modes: Manoeuvring, diesel-electric, range, high speed and boost.
“It is the way that the individual technologies are combined that is a novelty in the yacht industry. Savannah is also the first yacht in the world to be running with an Azipull and variable pitch propeller. Batteries provide extra speed at the top end, allow proper loading of the generators at any speed, and facilitate super-quiet cruising at slow speeds without any engines turning.”