Heesen Yachts is developing its own hybrid solutions in a drive to make yachting cleaner, greener and more efficient.
The demand for hybrid yachts has increased by an estimated 50 percent in the last seven years. The numbers compared with conventionally powered yachts are still small, but the technology is improving all the time and becoming more efficient, reliable and affordable. This is in part thanks to developments in the automotive industry (the hybrid Toyota Prius first appeared on our roads 25 years ago) that have trickled down to the yachting sector, but companies like Heesen are also busy researching hybrid solutions that make the most sense in a yachting context.
Photo: Jeff Brown“Heesen first deliberated building a hybrid yacht seven years ago and rather than leave it on paper, we decided to take the risk and build it on spec,” says Mark Cavendish, Heesen’s Executive Commercial Officer. “Project Nova, now motor yacht Home, was the world’s first Fast Displacement Hull Form with hybrid propulsion and was a huge success in 2017. In fact, we plan on offering hybrid propulsion as an option on all our future models.”
There is some confusion surrounding ‘diesel-electric’ and ‘hybrid’, terms that are often used interchangeably and sometimes inaccurately. In a diesel-electric system, generators provide power to e-motors coupled to the propeller shafts, or to drive electric thrusters such as Azipods. This brings advantages in terms of fuel consumption, flexible layout solutions, and reduced noise and vibration, but because all the power – whether for propulsion or hotel services – are converted from diesel to electric, it is what engineers call a ‘lossy’ system.
Diesel-electric becomes a hybrid system when battery banks are added. Recharged by the electric motors, these batteries store energy that can be used for silent, emission-free cruising at low speed, or to cater for the hotel loads without the emissions, noise and vibration from generators.
The problem is that batteries are heavy, take up a lot of valuable space that can be devoted to guest use, and rely on precious metals that are notoriously difficult to mine sustainably. In fact, most experts agree that their limited use on a superyacht does little or nothing to enhance the vessel’s eco credentials. 
For this reason, Heesen has removed the batteries from the equation. Instead, the propulsion package aboard ground-breaking Home, her sistership Amare II (formerly Project Electra), and the 50-metre steel Project Oslo24, comprises two diesel engines, two diesel generators and two electric motors that can be operated separately or together in various combinations thanks to sophisticated power management software.
“Our focus is on energy saving,” says Erik van Mourik of Heesen’s Design & Development department. “This is what makes a yacht more sustainable – not an ability run for an hour or two on battery power.”
In Hybrid Mode, the generators power the yacht’s hotel services while the e-motors propel the yacht silently at low speed. For faster cruising, Economic Mode brings in the main engines both to propel the vessel and run the e-motors, which power the hotel services with the generators switched off. In Cruising Mode the yacht runs conventionally, with the main engines propelling her up to maximum speed, while the generators provide power for hotel services. If still more speed is needed, Boost Mode sees the e-motors come on line to augment the propulsion power of the main engines. Significantly, conversion is only necessary in quiet cruising mode, at low speed and low power, where diesel generators run electric propulsion motors.
“Diesel engines are very efficient at cruising speeds, but not so efficient at low speeds and low power,” continues van Mourik. “Each power conversion means electrical losses. This is why our hybrid system is the most efficient: at low speed, the win in diesel engine efficiency is higher than the electrical losses, because the diesel generator is running at optimal efficiency.”
Heesen’s hybrid system has another advantage that has less to do with sustainability but a lot to do with production efficiency and cost effectiveness. Heesen is primarily a builder of customisable series superyachts based on common technical platforms. So it was important to devise a hybrid system that would fit into the engine rooms of its existing platforms in order to improve comfort and performance without the need for expensive re-engineering. 
Of course, hybrid propulsion is not the final word in energy efficiency. The 50-metre Project Altea and 80-metre Project Cosmos, both currently in build, will feature ‘peak-shaving’ batteries not for propulsion but for storing excess generated energy that can be deployed during peak periods, saving on generator use. 
Hull design is another critical factor. Heesen’s 65-metre Galactica Star, now Illusion, was the first yacht with a Fast Displacement Hull Form in 2013. Developed and patented by Van Oossanen Naval Architects, the FDHF’s resistance values are typically 20% lower than a well-designed hard chine hull at semi-displacement speeds. 
Van Oossanen further believes that cutting fuel consumption and emissions does not always depend on reinventing the wheel. Systems architect Peter Dijkstra, who has worked closely with Heesen since the shipyard first began contemplating a hybrid future, agrees that making yachting greener is more a matter of mindset than emerging technology:
“Did you know that most of the heat pumped out of a yacht by the air-con comes not from the sun, but from the sea?” he asks. “Simply insulating the hulls would save a lot of that energy.”