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 (Privilège), Hinckley, Island Spirit, Lagoon,
Manta, and others who are beginning to install STI's electric motors at
"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."
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
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
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
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
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."
So Where's the Rub?
In general, the STI system trades routine diesel maintenance for a
level of vigilance toward the boat's electrical system that's probably
new to most operators.
The basic STI system includes not just the drive unit but also the
electronic controller, a battery bank, power-distribution buses, and a
shore-power battery charger. This basic system is intended for
daysailors and weekenders who typically return to a dock at the end of
the day. For long-term cruisers and voyagers, STI recommends adding a
diesel genset, which would also provide ample current for
refrigeration, air-conditioning, a microwave, and other big-draw
The Solomon Technologies system is different from most other systems
offering electric propulsion in that it runs on high-voltage direct
current (144-volt DC) supplied by a battery bank. That has several
implications. It reduces the amperage, and therefore the wire gauge,
throughout the system, but it also means that you've got to forget
everything you've been taught about how DC current is safer than
alternating current. At this voltage, it's lethal. What's more, the
organizations that develop standards for sound boatbuilding practices
haven't addressed high-voltage DC, which they define as anything above
50 volts. This is new territory for the whole marine industry; apart
from STI and the boatbuilder, there's no third-party consensus on such
safety issues as grounding.
How do you create a 144-volt DC system? Basic electric theory tells us
that a single lead-acid cell delivers roughly 2 volts of electricity. A
typical marine battery consists of six cells connected in series to
deliver a nominal 12 volts. The size of the battery's plates determines
its capacity, measured in amp-hours. If you connect multiple batteries
in series (the positive post of one battery to the negative post of
another), the voltage of the bank you've created is the sum of the
voltage of the connected batteries. If you connect multiple batteries
in parallel (positive post to positive post), the capacity of the bank
you've created is the sum of the capacity of the connected batteries.
The Solomon system depends on a bank of 12 batteries to run the system,
plus two for the house. So two downsides of the system are the weight
and cost of the batteries: Twelve 4D absorbed-glass-mat batteries weigh
1,620 pounds and cost $3,750. Another potential downside also follows
from basic electrical theory: When a bank of batteries is wired in
series, one bad cell (in this case, just one out of the total of 72
cells) can take down the whole bank. Here's a case where the operator
has to pay close attention to the system.
Then there's the question of depending on a genset for all your
propulsion. When Tether started out, there were no DC gensets available
on the market. That's changing now, but some folks had genset problems
To address these concerns, Tether, knowing full well that he's asking
customers to tread into terra incognita, warrants the entire system.
"It doesn't matter what breaks on the system--whether it's the motor,
the inverter, the generator, the wiring, the switches, the throttle
controls--you call one person. Me."
Dennis English says he's had only two problems with the system. The
first was easily solved once it was identified. The 15-kilowatt genset
performed poorly early on, but it only needed better ventilation. The
second problem English attributes to operator error: He was showing
off. "You can go immediately from forward to reverse without neutral,"
he says. "There's no transmission, so the motors don't care."
True enough. But propeller-shaft shear pins do. English says he was
demonstrating to a friend how the motors can take his 10-ton boat from
8 knots to a dead stop in just two boatlengths. "I slammed into
reverse, and there was so much torque it snapped the shear pin."
Because he put Waypoint into
charter, English says he made one small ergonomic change, swapping
little joystick controls for true throttle arms. "With the joysticks,
people didn't realize how much power they were putting out."
Another drawback, for the moment anyway, is the cost. The initial
installation is more expensive than that of a conventional diesel
auxiliary. For an ST 37, the electric motor itself costs about $10,000.
The entire system--which includes 4D batteries, cables, meters, a
charger, a throttle, a 3-kilowatt inverter, and a 6-kilowatt
genset--costs about $35,000. Of that, the genset accounts for $11,000.
An ST 74 motor, recommended to replace diesels up to 48 horsepower,
costs $12,000. That entire system, including the same extras but with
8D batteries, a 6-kilowatt inverter, and a 9-kilowatt genset, costs
just over $40,000.
Greg Gerken ordered the second electric Island Spirit 401. He reckons
the electric-propulsion option on his boat cost an extra $11,000 when
compared to a standard propulsion package, taking into account that he
would have installed two diesels and a genset anyway. Tether believes
that through production efficiencies, the price will soon be comparable
to that of a conventional diesel.
On the topic of regeneration, of course, nothing is free. When a guest
during a demonstration sail last fall asked Tether whether with the STI
system he'd achieved perpetual motion, he was quick to nip that notion
in the bud. "No, no: perpetuated
motion," he said. Tether reckons that using the motors in regeneration
mode with big fixed-bladed props in light air costs as much as a knot
of boat speed in monohulls and two knots in multihulls. For those who
aren't keen on making that trade, installations with feathering props
are also possible.
One drawback that can't be overlooked is the one that can only be
called the X factor. Today, the number of installations of STI's
electric motors number in the tens, not in the tens of thousands let
alone the hundreds of thousands. At press time, Tether said he's put
roughly 50 STI propulsion systems in 40 boats (10 of these catamarans).
In the arena of recreational boats, these drives must still be
considered an emerging technology.
What the Future Holds
Given the capital that came with last January's public offering and
given the commitments of several boatbuilders to begin offering STI
electric motors from the factory, it appears we can expect to see many
more electric boats in the coming years. What does that mean for the
future of the technology?
There will clearly be competition. Fischer Panda (www.fischerpanda.com)
recently introduced a diesel-electric motor that runs not from a large
battery bank but directly from a running genset.
Certainly, we can expect that improved methods for storing and
producing electricity will come into play. Tether says he's
deliberately created the STI system in a modular way so that, for
instance, nickel-metal-hydride or lithium-ion batteries--which have a
significantly higher power density than lead-acid batteries do--can be
introduced as soon as big enough ones are available. On the
energy-production side, of course, it's all the rage these days to talk
about fuel cells, though Tether is dubious about storing the hydrogen
fuel aboard a boat. So, if not fuel cells, what lies in the future,
"Steam," says Tether, with a twinkle in his eye, obviously relishing the Robert Fulton anachronism.
"I'm looking into steam cells."
Tim Murphy is CW's executive editor.