Diesel-Electric Finding Place in Cruising World

A growing number of sailors and boatbuilders are being drawn to the promise of diesel-electric technology as clean, quiet, efficient, and requires very little maintenance, as well as can be regenerated while under sail.

Published: 09-Mar-2005

Marine diesel engines have millions of hours of reliable running time to their credit. As a class, they represent a mature technology--so much lighter, smaller, and safer than their forebears of a generation ago that the gasoline inboards they replaced are virtually gone but for the history. When it comes to building new auxiliary sailboats, it would seem that the propulsion question is a problem solved. End of story.

But a growing number of sailors and boatbuilders, drawn to the promise of diesel-electric technology, are beginning to think otherwise. At the center of this trend is a 10-year-old company called Solomon Technologies Inc. (STI; www.solomontechnolo gies.com), which went public in January 2004 and has attracted the attention of such boatbuilders as Alliaura (Privil├Ęge), Hinckley, Island Spirit, Lagoon, Manta, and others who are beginning to install STI's electric motors at the factory.

"I'd been documenting electric propulsion for 15 years when I first saw the Solomon solution in 2000," says Bruno Belmont, who directs research and development for Lagoon, Wauquiez, and CNB within Groupe Beneteau. "It was the first setup that made sense to me. I strongly believe that electric propulsion is the future and that all the Lagoon boats will be electrical within five years."

The Promise
Advocates for diesel-electric propulsion list among its virtues that it's clean, quiet, efficient, and requires very little maintenance. Another advantage frequently noted in connection with the STI system is the ability to make electricity--to "regenerate"--when the boat is under sail.

There's nothing new about electric propulsion per se. In 1879, a Philadelphia man named William Woodnut Griscom invented the first electric motor for marine use. Since then, electric motors have pushed tugboats, icebreakers, submarines, cruise ships, and other commercial and military vessels. In 1893, 55 electric boats built by the Electric Launch Company, or Elco, ferried passengers around the Colombian Exposition in Chicago; a reorganized Elco still builds electric boats on the Hudson River today. For decades, companies like Siemens and General Electric have built electric motors for commercial use. Since 1970, the Duffy company in Southern California has been building electric boats from 16 to 30 feet. In Europe, electric power has been used in small recreational boats for decades. Today, electric motors are installed in classic runabouts that can attain planing speeds.

But until recently, no electric-motor system convinced boatbuilders that it was viable enough to be installed in production cruising sailboats.

David Tether set out to change all that. In 1994, he founded Solomon Technologies. Tether is a big, engaging fellow with a mad scientist's air. "I have pattern-recognition capability," he says, by way of introduction. He speaks passionately, often a step or two ahead of his listener, and makes quips like "Time flies when you're in a paradigm shift." He gives the impression of a man who sees the world not as it is but as it could be--and who knows how to get it there.

"Meeting with Dave Tether led me to believe that his system could well be the best there is," says Belmont. "That man is a genius."

Beginning in 1969, Tether wrote software and conducted research for the U.S. Navy. One of his projects was to study the infrared and electromagnetic spectrosignatures from various craft. "I noticed that the emissions from diesel-electric tugboats were far reduced from other boats," Tether says. "Studying it further, I noticed that it was all directly due to efficiency." That was in the early 1980s, and for seven or eight years, he says, "I just pondered that."

Toward the end of the decade, he teamed up with a man he describes as a junkyard inventor who had an idea for an infinitely variable transmission employing a system of planetary gears driven by two electric motors. In the early 1990s, they patented the Electric Wheel; since then, NASA has put a version of it on the Mars Rover. Says former NASA administrator David Golden, "The Electric Wheel has application to everything that moves."

As Tether was developing the Electric Wheel, he had grand visions about revolutionizing the car industry. He even met with automobile executives. Though he's reluctant to speak about exactly what was said in those conversations, Tether reckons he was lucky to get out of Motown alive. He went back to his whiteboard and determined that the marine industry, being unregulated, was perhaps the better place to start putting his ideas to the test.

"What we did then," Tether says, "is we designed an electric motor specifically to push propellers." He believes that was something new in the marine industry. "If you see the diesel-electrics that Siemens or GE have done," he says, "they've taken a motor off the shelf that used to push a bus or open a bridge or run a monster conveyor, a motor that was kind of almost what we needed, and they've emulated what had been done with fossil-fuel motors."

Tether's approach was to start with a blank sheet and look at the whole boat. His line of thinking went like this: "We've got a propeller. What exactly does it need? And in order that I can have renewable input, I have batteries. What exactly do they want?"

Following this process, he says, he worked toward the middle. "When I got to the middle, I knew how much torque, what rpm, what voltage, and what amperage I needed to design an electric motor for."

The solution that he found wasn't a geared transmission like the Electric Wheel but a mechanically simpler system based on a brushless 144-volt DC motor that uses powerful permanent magnets instead of field windings and an electronic commutator instead of brushes. The result is a motor that demands very little maintenance. Roller-thrust bearings at either end of the stainless-steel shaft--among the motor's very few moving parts--are rated for 150,000 hours of use. Compare that to a diesel engine's valves, pistons, cylinders, rings, crankshaft, camshaft, fuel injectors, gaskets, filters, pumps, and transmission.

The difference, Tether says, accounts for not just an improvement in the maintenance schedule but also a dramatic increase in efficiency over diesel engines.

"It was almost our nemesis early on," he says, because nobody would believe the results he was getting. He eventually found that he could install his electric motors in boats that called for diesels with horsepower ratings that were four times higher.

Tether prefers to describe the output of the STI motors in terms of torque (force times distance) rather than horsepower (force times distance per time). While internal-combustion engines are typically described by their horsepower rating, STI's motors are named for the torque they develop. An ST 37 puts out 37 foot-pounds of torque or 6 horsepower; Tether recommends using it on monohulls up to 32 feet and 10 tons or to replace diesel engines of up to 24 horsepower. An ST 74 puts out 74 foot-pounds of torque or 12 horsepower; Tether recommends using it on monohulls up to 50 feet and 16 tons or to replace diesel engines of up to 48 horsepower.

What accounts for the difference in efficiency between electric and diesel power plants? Consider the typical internal-combustion engine. From the time a charge of fuel ignites in a cylinder, it has to push pistons, turn a crankshaft, turn a camshaft, open valves, pump water, pump oil, turn an alternator, and submit to reduction from a transmission to step the engine's thousands of revolutions down to something a propeller can use. By the time that's done, the engine's efficiency is somewhere below 25 percent. Also, diesel engines are rated at their maximum rpm--and on sailboats are rarely operated at that speed.

By contrast, Tether says, the efficiency of the STI motor is a percentage in the low 90s. Here's how it works: When the system is switched on, DC current from the batteries enters an electronic controller, which produces expanding and contracting magnetic fields in the motor's stator windings. These magnetic fields attract and repel the fields from three permanent magnets, made from neo-dymium iron boron, that are attached to the rotor. The controller electronically modulates the pulse width to increase or decrease speed. At 13 inches wide, the motor provides ample contact with the shaft to produce high torque at low rpm, enough for the motor to turn particularly large propellers. Fixed three-bladed 18/18 (diameter/pitch, in inches) propellers are typical in many of STI's installations. From the flowing electrons to the turning prop, the shaft passes through only two bearings and a stern gland--and no transmission, all of which accounts for its high efficiency. Furthermore, with the electric motor, the relationship between rpm and torque is linear: You can use it to turn the boat's prop at 1 rpm or 10 rpm or 50 rpm or 100 rpm. An internal-combustion engine needs to cross an rpm threshold before its propeller is put in gear; otherwise, it would stall.

One topic that proponents of the STI system rave about is "regeneration"--its ability to produce electricity when a boat is under sail. We described how current from the batteries turns the prop. When the boat's under sail, the same process works backward, converting the prop's rotation into stored energy.

"The way this thing regenerates," says Tether, "is it builds up energy in the windings, then dumps that energy in a big pulse, which is the best way in the world to charge batteries. If you put a steady charge on them, you end up wasting a lot of energy and you heat them up too much and you boil them and do all kinds of crazy things that they don't like. But if you zap them with pulses, they love it."

Dennis English was the first customer to have STI motors installed in a production boat by the factory, a Lagoon 410 called Waypoint (www.sailingwaypoint.com). He was originally drawn to electric propulsion because he thought it was an environmentally friendly technology. "Regeneration is really the cool part of this system," he says. 

He describes his experience from a recent passage: "We sailed from Norfolk, Virginia, to Tortola in 8 days, 2 hours. For a couple of days, we had almost no wind, and we ran the genset 24/7. But for two straight days we had 20 knots of breeze and 15-foot swells. On those days, we put the motors on slightly, which gave us an extra knot of boat speed up the waves. Down the waves, though, it regenerated. For two days we ran the refrigerator, freezer, air-conditioner, stereo, and nav electronics without making electricity from any other source."

"It was gorgeous," he says. "That's what it's made for."



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