A full century before the hybrid Toyota Prius went into production, Ferdinand Porsche had developed a gasoline-electric automobile. Similarly, diesel-electric propulsion for ships and submarines has been around since the turn of the 20th century. So the underlying principles of hybrid drive are not new. What is new are the advances in systems technology and power management to make them more efficient and sophisticated than ever before.
The number of diesel-electric superyachts launched over the past decade has been small. Interestingly, while hybrid cars are designed to save fuel and reduce emissions, hybrid or diesel-electric propulsion at sea also offer other advantages that are just as significant. Eco-friendliness is still a vital criterion, but given the nature of a vessel that floats on water—a yacht cannot, for instance, recharge its batteries when braking or coasting downhill—other key attractions include more flexible layout solutions, less noise and vibration and better power management.
“Hybrid” and “diesel-electric” are invariably used interchangeably (and inaccurately) when applied to yachts. Some overlap exists, but there are also some differences. “Hybrid” commonly defines a system drawing on two or more power sources for propulsion and on-board “hotel” services—usually diesel engines and electric motors that can be used together or individually. Importantly, the drive shafts form a direct mechanical connection between the diesel engines, electric motors and the propellers. A “diesel-electric” system, on the other hand, lacks a mechanical connection. Instead, diesel generators produce electricity for hotel services, and for propulsion via flexible cabling to electric motors that drive the propellers, removing the need for drive shafts. Both systems may use battery banks for auxiliary power needs.
A prime example of a superyacht with diesel-electric drive is the 73m (240ft) explorer motor yacht Grace E. Her owner was attracted by the higher redundancy and reliability of multiple generators. Grace E’s six generators can be loaded at optimal efficiency and comfort on board is enhanced by near silent operation. Her Azipods (electric thrusters that swivel through 360 degrees) also provide Dynamic Positioning (a computer-controlled system that automatically maintains the yacht’s position and heading) without the need for rudders.
Because there are no mechanical drive shafts, a diesel-electric set-up allows for more freedom in the positioning of machinery outside the traditional engine room. On another recent diesel-electric yacht of 45m (148ft) the principle power generation units are housed in the bow, allowing more space for guest use. The system basically consists of four generators producing electricity to drive two Azipods with contra-rotating propellers that recoup the energy lost in the swirling water created by standard propellers. The electrical energy generated can also be stored in battery banks, allowing the yacht to cruise using battery power alone in completely silent zero-emission mode.
More distinctive still is the 83.5m (274ft) Savannah. Her innovative propulsion system comprises three generators and one medium-speed main engine (instead of two higher-rev diesel motors) driving a single shaft and a propeller that has a 40 per cent larger shaft than the norm. Combined with an azimuthing, contra-rotating thruster mounted behind the main propeller, the result is higher efficiency and lower vibrations. The gearbox is connected to an electric motor that can supply electrical power for the complete hotel load, or to drive the main propeller. In addition, two banks of batteries can be used in combination with the other power sources and act as buffers before the generators start up during spikes in demand. With five distinct propulsion modes, the hybrid set-up provides high levels of redundancy and fuel savings of up to 30 per cent. The system also offers guests the prospect of much quieter and more comfortable cruising.
None of these systems is particularly new. Yet the way that they have been combined is avant-garde, assisted by developments in battery technology. Bulky, heavy and quick to discharge, batteries have traditionally been the weakest link in the energy chain. The latest generation of lithium-ion and lithium-polymer batteries, however, are smaller and lighter with a higher energy density that is typically guaranteed for ten years.
It is not just motor yachts that can benefit from hybrid propulsion; sailing yachts also require generators for hotel services and engines to drive propellers when not sailing. A groundbreaking hybrid sailing yacht is the 58m (190ft) Ethereal, launched in 2008, she has since logged up over 100,000 miles cruising the world’s oceans. In addition to sailing systems and anchoring functions, her battery bank provided power for the entire hotel load, enabling the yacht to operate in true stealth mode. The freewheeling propeller also acted as a generator to recharge the battery banks while under sail.
Another example of a cutting edge sailing yacht is the beautiful 52m (171ft) Elfje designed by Andre Hoek. The design brief called for a luxury sailing yacht that would deliver across all criteria and remain cutting edge well into the future. To highlight her owner’s focus on maximizing innovation, sustainability and energy efficiency, the shipyard dubbed Elfje a “NextGEN” ketch. Her advanced systems architecture features lightweight and low-noise variable speed generators backed up by a lithium-ion battery bank and efficient power management. The energy storage system is designed to absorb peak loads, significantly reducing fuel consumption.
Sybaris, an Italian built 70m (230ft) ketch launched in 2016, features variable speed generators with the potential to store excess power in lithium-polymer batteries. These battery banks, which can be recharged in less than one hour, offer “silent running” mode at anchor and while sailing.
Although the vast majority of existing superyachts are equipped with conventional diesel propulsion, other hybrid and diesel-electric projects are under construction or in design. As international regulations governing emissions become more stringent and owners expect noiseless, more flexible and more efficient yachts, the pace of change is likely to accelerate in the future. The focus of research now is on systems that rely on electricity for all the power requirements, from propulsion to hotel services and even sail handling. That day is close at hand, if not already achievable.
This article originally appeared in The Superyacht Book – an insider look into some of the most luxurious floating residences. Get your copy here.