Electric and Hybrid Propulsion for Sailboats
Practical Sailor looks at the players in the developing field of electric auxiliary engines
How soon will electric auxiliary propulsion be available to everyman? That depends on whom you ask. Opinions differ widely not just on what type of drive system might surge to the forefront, but even on whether the concept itself is viable. While a handful of companies forge ahead, notably Glacier Bay and Electric Marine Propulsion on this side of the Atlantic, some expected participants are waiting on the sidelines.
One of the big issues that divides promoters and detractors alike is whether the appropriate way to go in a sailboat is with a pure diesel-electric drive train, with a hybrid electric drive with a diesel generator as back-up, or as a pure electric drive with regeneration capability. We’ll take a look at these and other options later in this article. For now, the short answer is that no single approach suits every sailor all the time.
Simply put, in the diesel-electric system, the electric motor runs only when the diesel-driven generator is running. Such arrangements have long been employed in railway locomotives, submarines, and commercial vessels of many types. In the hybrid system, a large bank of batteries provides the energy for the electric motor and the diesel generator recharges the batteries. On the face of it, the hybrid system offers a certain degree of redundancy in that, assuming the batteries are kept well charged, the boat has a measure of emergency power should the generator fail at an inopportune moment. The hybrid also is capable of recharging its batteries when sailing: Driven by the turning propeller, the motor becomes a generator.
Each of these approaches has its strengths and weaknesses, and while we’ll leave it to their developers to work out the technical issues, we would like to urge anyone contemplating installing an electric drive, or purchasing a boat that has one, to first look very closely at how they expect to use the boat. There’s more entrained in the choice than in picking a flavor at Baskin-Robbins. More on this later.
Among the electric drives currently available in one form or another, or as components, the big variable is operating voltage. Motors are available that run on 24, 36, 48, 72, and 144 volts, and, in the case of Glacier Bay’s diesel-electric system with Ossa Powerlite technology, 240-volt DC. Each supplier will discourse at length on the merits of their voltage choice, but an inconvenient fact haunts the entire field: High-voltage DC is deadly, potentially more so in some circumstances than AC.
While neither form of high-voltage is "safe," we have a lot more experience with AC aboard recreational vessels than with high-voltage DC. An extensive body of knowledge exists on which to base AC installations so as to make them safe as well as reliable. High-voltage DC is used in a variety of marine and non-marine commercial applications, but these installations are well protected from access by untrained operators.
What voltage constitutes high voltage? That, again, depends on whom you talk to. The American Boat & Yacht Council (ABYC), which sets voluntary standards for the marine industry, defines it as 50 volts and above. Prompted by rapid adoption of high-voltage services in small commercial craft and bigger yachts, though not specifically in propulsion systems, the ABYC is in the process of drawing up guidelines for voltages higher than the 48 volts covered by existing standards.
An absence of standards might not deter individuals from installing an electric drive, but it might impede widespread adoption of the technology. If a surveyor can’t state in an insurance survey that a boat is built according to ABYC standards, that could affect its insurability.
Jim Nolan, who manages the underwriting department for BoatUS, said the company has no clear cut guidance regarding insuring boats with electric propulsion. Each boat is dealt with on a case-by-case basis. A new boat with a factory-installed system would be a good deal easier to underwrite than a one-off or do-it-yourself project, especially in the absence of a standard practice. Lagoon Catamarans’ 72-volt-DC hybrid system, for instance, has qualified for the European standard (CE) certification on the strength of following industrial standards that apply to such applications as fork-lift trucks. Anyone contemplating an electric drive would be well advised to discuss it ahead of time with an insurer and even get a surveyor involved from the outset.
Because of the safety issues surrounding the voltages involved in electric propulsion, Fischer Panda has decided to limit its DC product line to boats weighing 10 tons or less. A company representative we spoke to said that while Fischer Panda currently sells DC generators up to 48 volts in the USA for marine use, it "won’t touch" high-voltage DC because it’s lethal.
A proposed collaboration with Catalina Yachts to fit a diesel-electric system in a Catalina-Morgan 440 never came to fruition due to budget constraints, according to Fischer Panda. But in Europe, Fischer Panda teamed up with Whisperprop to equip a Bavaria 49. (Beyond the fact that one of its boats was used, Bavaria Yachts was not involved in the project.) According to Fischer Panda, after evaluating the Bavaria project, the company decided that the diesel-electric AC system is a niche product that wouldn’t interest their prime market: original equipment builders.
"Although the AC system has some advantages in the improved response of the electric motors ... and the quietness of the system, the desired fuel efficiency and weight savings were not evident," Fischer Panda reported.
Fischer Panda considers the DC system to be more suitable for its North American customers. Although it’s limited in output due to its limited battery voltage of 48 volts, it is still able to power multihulls up to 10 tons.
Currently, much of the movement toward electric drives is taking place in the catamaran world. This makes sense when you consider that a single diesel generator can, in theory, provide all the boat’s electrical needs and also take the place of two diesel-propulsion engines. Taking the lead in the field, Lagoon Catamarans introduced in 2006 the Lagoon 420. Originally offered only as a hybrid, it now is also available in two diesel versions. Corsair Marine is building the Corsair 50 catamaran around the Glacier Bay diesel-electric drive, but the boat’s launch date—formerly set for this summer—has been postponed.
Dick Vermeulen, president of Maine Cat, tried the Glacier Bay system in a prototype power cat, but it failed to meet performance expectations, so production models will have conventional diesels. A number of other cat builders have announced hybrid or diesel-electric projects, but feedback on how they perform is scant.
So much for the mainstream—but backwater sailors will go their own way, as they always have. As more vendors and components enter the market, the options for do-it-yourselfers or custom-boat customers become broader and more attractive. However, before going ahead with an installation, make sure it’s appropriate to how you plan to use your boat, and even then be prepared to adapt the way you sail to take best advantage of the system’s characteristics. Here’s a rundown of the various types.
Electric Drive Only
Duffy Electric Boats has for years been building electric launches and lake boats that have the simple capability of puttering around in sheltered waters for a period of time determined by battery capacity and speed maintained. A battery charger powered by shore power charges the batteries overnight. Transferring that approach to a sailboat up to about 25 feet used for daysailing and kept near an electrical outlet shouldn’t be too difficult. It won’t offer the assurance of diesel when trying to get home against current or wind, but a proven 36- or 48-volt system will keep you out of uncharted standards territory.
For a bigger boat, more power, a greater range, or a combination of these requirements, it will be necessary to install a large battery bank and almost certainly will entail going to a higher voltage to keep the amps and the cabling needed to carry them manageable. The boat’s range under power will be limited by the weight of batteries, and while lighter lithium-based technology is on the horizon, for now the standard is lead/acid. The fast charging, but expensive pure lead thin plate (PLTP) Odyssey batteries have attracted particular interest among propulsion enthusiasts.
Electric Drive with Regeneration
The next level up in complexity is a "reversible" system. When the boat is sailing, the propeller turns the motor, which then becomes a generator. The electricity it makes is used to recharge the batteries. The capability to regenerate extends the boat’s potential range, but the drag on the propeller slows the boat measurably. One hour of regen will not restore the power consumed by one hour of motoring, but if sailing time sufficiently exceeds motoring time, this arrangement offers considerable range.
A regenerating system does have the potential to overcharge the batteries once they become fully charged and the boat continues to sail fast. The solution is, ironically, to give the motor some "throttle," which reduces the drag on the propeller and consequently the power output. This phenomenon gives rise to a new technique, that of "electro-sailing" in which sails and an electric motor complement each other. At present, the "throttle" must be adjusted by hand, but developers are working on automatic controls. Field trials of existing regen motors such as the Solomon systems suggest that a small regen motor’s ability to match the output of a much higher-rated diesel have been overstated.
Hybrid Electric Drive
A hybrid system adds to the mix an onboard generator, which is used primarily to maintain charge in the batteries, both those for the propulsion motor and for the house services. This arrangement extends the boat’s capability to lie for long periods at anchor, independent of shore power for electricity and without the need to go sailing for the sole purpose of charging the batteries. A hybrid can motor constantly, as long as there is fuel, but it cannot sustain full speed for long periods. This is because the generator is usually rated at a far lower horsepower than that required to drive the boat at full speed.
In a pure diesel-electric, the electric propulsion motor runs only when the generator is running. Storage batteries are not needed for propulsion purposes, and the generator is the source for all onboard electrical power needs. The rationale behind diesel electric lies in the relationship between a diesel engine’s rate of fuel consumption and the load it’s working under. It burns fuel more efficiently when heavily loaded than when lightly loaded. When the diesel engine is disconnected from the propeller, it can be controlled so that it is working in the upper range of its efficiency regardless of how fast the propeller is turning. Nigel Calder’s series of articles in Professional Boatbuilder magazine (www.boatbuilder.com) beginning with the June/July issue delves deeply into the efficiency discussion surrounding these engines. Systems on large vessels are built around multiple generators that switch on or off according to the power demands of the moment. Translating those efficiencies into a smaller boat scenario has proven to be challenging.
Hype vs. Experience
Maine Cat’s Vermeulen, on the company’s website, describes the sea trials he performed in the Maine Cat 45, a power catamaran. He began with a Glacier Bay diesel-electric system with two 25-kW generators, each weighing about 550 pounds.
"With both generators putting out their full power of 25 kW each ... our top speed was a disappointing 8.4 knots, and the assumption that electric horsepower was somehow more powerful than conventionally produced horsepower was in serious doubt."
He replaced the propellers with a pair with less pitch, which allowed the electric motors to reach their full rating of 1,100 rpm, but that only increased the speed to 9.1 knots.
"These are about the same speeds and fuel burns we get on our Maine Cat 41 sailing cat ... powered by twin 29-horsepower 3YM30 Yanmar diesels with saildrives and two-bladed, folding propellers." At the time he installed them, the 25-kW generators were the highest power available from Glacier Bay.
Vermeulen replaced the diesel-electric system with twin 160-horsepower Volvo diesels. At 9.1 knots, they together burned 2.2 gallons per hour, considerably less than the 3 gallons per hour that the Glacier Bay system burned at the same speed. With the twin Volvos maxed out at 3,900 rpm, the boat made 24.5 knots.
Also among the unconvinced is Chris White, well-known designer of ocean-going catamarans. "To date, I’ve not seen any system that makes sense for a cruising boat," he says, but he might change his mind, "if someone can show me by building one that delivers an advantage in performance, weight, or cost."
White sees the current bubble of interest in diesel-electric drives as a fad. In the end, he says, you’re getting the horsepower the diesel creates at the crankshaft, which is basically the same whether it’s delivered to the prop via a conventional reduction gearbox or via a generator and an electric motor. Besides, he says, diesel engines and diesel fuel are understood and available anywhere in the world you might take a sailboat. Complex, electronically controlled electric motors are not.
White’s reservations notwithstanding, it’s in the world of catamarans that we’re seeing most of the applications. At first sight, it does seem logical that replacing three diesel engines—two propulsion and one generator—on a fully equipped cruising cat would result in fuel savings. Still, if the generator is big enough to drive the boat at cruising speed (which in a cat is expected to be in the vicinity of 10 knots) and run the air conditioning at the same time, it will be overkill for the times it’s only needed to operate the boat’s services. For this reason, commercial and military diesel-electric systems employ multiple generators that can be switched on and off according to the power demand of the moment.
Corsair Marine hopes that by installing a diesel-electric system in its 50-foot catamaran, it will be able to descend the weight spiral. Where a conventional installation would involve two 75-horsepower saildrives plus a 6-kW genset, it’s fitting a pair of 28-horsepower electric motors, one 25-kW generator, and a 40-amp, 230-volt battery bank. It expects to save about 700 pounds in equipment weight, some of it through the use of high-voltage, low-current systems, which will in turn reduce the rig requirement, thus the structural weight, and so on toward an estimated overall weight savings in the thousands of pounds.
Corsair’s David Renouf estimates that the boat will cruise at 8 knots and be capable of short bursts at 10. He admits that, until the first boat is launched, his information is "based on extrapolation, not proven numbers." He says that some clients will add a second 25-kW genset to assure longer periods at 10 knots. Currently, the project is running behind schedule, with a launch scheduled before the end of the year.
Cost and Other Benefits
At the present time, there appears to be no reason to install any proprietary electric drive of any description in the expectation of bettering the economics of a standard diesel drive. The motors and their electronic controllers are sophisticated and expensive. A battery bank sufficient to provide a useful motoring range is a big investment in weight, space, and money. When you add a generator and its peripherals, the cost and weight take another upward leap.
Only the simplest system will begin to pay itself off in terms of fuel not burnt, and then only if the boat sees a great deal of use. A diesel-electric system designed to closely dovetail with the way you use the boat may prove to be more efficient over time than a conventional diesel installation, but until enough systems have been installed and used and data from that use compiled and compared, we can’t know that.
So why even consider going electric? Cleanliness and silence of operation are two qualities that make electric propulsion an attractive proposition for a sailboat, but in order to enjoy them, we have to accept the limitations they impose.
A hybrid or a diesel-electric system enables us to have a single fossil-fuel power source for both propulsion and onboard appliances, but whatever fuel we might save as a consequence of motoring more efficiently for a couple of hours will be inconsequential if we run the generator all night to power the air conditioning.
As we go to press, pickings are slim for sailors looking for an electric solution to the diesel problem. Suppliers of components are few, prices are high, and the feedback on long-term reliability is nonexistent. On top of all this is the elephant in the room: the unexplored safety ramifications that accompany high-voltage DC.
However, none of this should deter the dedicated tinkerer who has funds to match his curiosity and who can live within the parameters imposed by electric propulsion.
Practical Sailor encourages our readers to explore the technology, because ultimately, it is the experimenters who bring us the equipment we eventually come to take for granted.