A laptop display showing some of the systems data available via a Hybrid Marine interface
up to cruising speeds, the loads are continuously high, especially with displacement-type hulls. Therefore, in most applications it is not practicable to put enough batteries in the boat to maintain this speed for even an hour, let alone several hours, and as such the range under batteries at traditional cruising speeds is typically only a few miles. (Of course, you can always choose to go slower, which dramatically reduces the load with a displacement hull and concomitantly increases the range under electric power, but then you are not comparing apples to apples.)
Once the batteries are discharged the generator must be cranked, but now instead of the diesel engine directly driving the propeller it is driving a generator that is sending power through cables and a motor controller to an electric motor, which is driving the propeller. Of course, all of the additional hybrid system components introduce efficiency losses into the system, not to mention a significant level of complexity and, by extension, a greater number of potential failure points. The end result is that although at low speeds there can be significant improvements in system efficiency over a conventional installation, if the normal duty cycle includes sustained powering at cruising speeds it is next to impossible to make a serial system in propulsion mode as efficient as a conventional drive. Which is not to say a serial hybrid makes no sense: there are other good reasons for this approach, mostly related to lifestyle issues. Timo Jaakkola of serial hybrid manufacturer OceanVolt notes, “Efficiency is not the first criterion for selecting an electric motor system. More important are silent propulsion, less maintenance, no vibration, no exhaust fumes, the ability to regenerate power under sail and sleeping silently overnight without a generator running.”
Also, the higher the house loads relative to the propulsion loads, the better a serial system looks as it can almost always dramatically improve the efficiency with which the house energy is created. With this in mind, an ideal application for a serial system is a fast cruising catamaran with high air conditioning loads: the propulsion loads are relatively light, the house loads are relatively high, and the vessel can regenerate significant amounts of energy when under sail.
ELECTRIC BOATS TO HYBRIDS
In the meantime, while the cost, efficiency and complexity issues pretty much ended attempts to create viable serial systems, there has been a considerable amount of development around pure electric boats. In fact, there have been thousands of these installations, which are very effective for short-distance operation if there is then shorepower avail- able to recharge the batteries. If you want more range on occasion, you can also put a small generator in the boat and use this to charge the batteries (at which point you are operating considerably less efficiently than a conventional installation, so if the generator must be run on a regular basis it makes little sense).
Note that an electric motor with a range-extending generator is not a hybrid system, since a true hybrid requires that you add complex control systems to manage the generator and batteries to optimize system operation and efficiencies; in fact, it is a significant technical leap to go from a range-extending generator to such a system. Looking at the field today, however, there is one company in particular, Fischer Panda, that has been developing these kinds of systems for over a decade now, and two companies that I believe have the staying power to make this step at this time: Torqeedo at the high-powered end of the marketplace, and OceanVolt at the relatively low-powered end. Torqeedo is still in the development phase, while OceanVolt is delivering systems.
The Torqeedo serial hybrid is built around a common control system that provides whole-boat energy management, delivering electric power at any desired standard voltage, AC or DC, to all electrical loads on the boat. This control system can be applied to a series of standardized electric machine, generator and other modules. The idea is to have a system that is scaleable from 27 kilowatts (27 kW; 35 hp) up to 55 kW continuous, including twin propulsion systems for catamarans, using the same controllers, software and building blocks. The high power levels require high voltages to keep cable sizes within bounds—these systems run at hundreds of volts.
Todd Sims, global sales manager, says, “The standardization of components allows for consistent performance, a high level of safety, and easier service and support.” Torqeedo is investing heavily in this system, including in massive lithium-ion battery packs from top-tier manufacturers. If their electric outboards (of which they have sold 60,000 to date) are anything to go by, the finished product will be beautifully engineered.
OceanVolt, a Finnish company, also has beautifully engineered electric machines up to 15 kW and operating at a nominal 48 volts. To date these have been used almost exclusively in electric boats (over 200 systems delivered), running off lithium-ion battery packs. However, OceanVolt has been investigating serial options for some time now, initially in a range-extending fashion, but evolving into a fully-fledged serial hybrid system: the OceanVolt SEA (Silent Electric Autonomy). The system runs at a nominal 48 volts currently using DC generators from Fischer Panda and Polar Power.
OceanVolt’s Timo Jaakkola reports, “The generator operation is controlled with a central unit that communicates on the same CanBus as the propulsion and battery system. The batteries send their state of charge (SoC) information and other important data to the central module which