Electric Propulsion: Solomon Technologies’ High-Wire Act

It's powerful and quiet. It's renewable, and sometimes even free. It doesn't pollute. It's simple enough to understand. How severe are the drawbacks? That depends on several of your thresholds.


As much as we like to believe that we’re purists when it comes to sailing, the auxiliary engine is obviously very important to most of us, whether inboard or outboard, gasoline- or diesel-powered. We rely on the engine to get us in and out of the marina, get us off a lee shore, charge the batteries, dig in the anchor, and provide us with power for all sorts of electrical needs. 

Electric Propulsion: Solomon Technologies’ High-Wire Act

We have standard and virtually universal complaints about auxiliary engines: They’re noisy, often unreliable, difficult and expensive to service, and they require a lot of space, spares, and tools to stay running happily. They need oil and fuel and coolants and filters. They require holes to be drilled through the bottom of the boat. All the moving parts, alignments, plumbing, cooling, exhausts, filters, etc. work together to make the auxiliary both high-cost and high-maintenance. Additionally, access to all the parts that need servicing is usually cramped and miserable.

Nothing says that an auxiliary has to be a fossil-fuel driven system. Indeed, boats have been using electric motors with storage batteries for propulsion for over 100 years (both on top of and under the water). However, the reasons for the popularity of the fossil-fuel driven auxiliary are similar to the reasons that gas engines are still so popular in cars—given their high power output (or “power density”), they’re extremely economical. Put another way, you get a lot of bang for the buck. While this has been true enough for decades, there are times when innovation—often in the form of old ideas updated with modern techniques— becomes viable enough to be lucrative. Witness the increasing popularity of electric-hybrid cars from Honda and Toyota.

In the world of sailing, we’ve seen a gathering ground-swell of interest in the electric auxiliary system designed and installed by Solomon Technologies of Benedict, MD. The company has exhibited its “Electric Wheel™” at many boat shows in recent years, and a number of PS readers have sent letters asking what we know about the system. Until recently, all we knew was that the device reminded us of an article in NASA’s Tech Briefs back in the ’80s, describing an electric motor design that was small, highly efficient, and suitable for installing at each wheel of an electric vehicle. Motors of this type have not yet made it to the consumer automotive market, but they’ve made it to Mars, and now they’re making it into a few sailboats.

After studying what we could about Solomon Technologies via their website, we called the number and asked if we could come for a visit.

The Company
Solomon Technologies is located on the shores of the Patuxent River, about 13 miles from the Patuxent Naval Air Station and the Chesapeake Bay. The company’s offices are in an older house; the testing, developing, and manufacturing are done in an adjacent barn. Across the street is a small marina that serves as the working yard for installing Solomon Technologies Motors and on-the-water testing.

We met with David Tether, inventer, CEO, and motive force behind the company’s products, and with John Finnerty, the company’s marketing director.

Tether was a contractor for the US Navy for 14 years. He invented simulators for thermo-nuclear explosions, and was a specialist in electro-magnetic signatures. He’s a high-energy personality, and in fact can get so carried away with preaching the gospel of electric auxiliary power as represented by his products that it can be difficult for his questioner to get the facts straight. This is where John Finnerty comes in handy.

The first facts that needed straightening, as far as we were concerned, had to do with what, exactly, an “Electric Wheel” is. Solomon Technologies’ website and literature use the term to describe all their products, yet it had seemed to us on boatshow and website visits that there were at least two different bits of machinery that shared the name under the same umbrella. This, as it turns out, is true.

All of Solomon Technologies’ products use a proprietary DC brushless motor. We’ll refer to this motor as an ST Motor. There is also the Electric Wheel, which refers to the ST Motor as the power source, combined with a proprietary internal planetary gear system—the ingenious turning gizmo vaunted at the ST boatshow booth.

The ST Motor
Most of us are familiar with the simple electric trolling motor, in which the motor itself is submerged along with the propeller it drives.

The Solomon Technologies motor is not—by a long shot—a trolling motor. It’s a brushless DC motor with the windings on the stator, and neodymium iron boron permanent magnets on the rotor. The power take-off shaft connects to the rotor.

The ST Motor works by switching the current through the windings, which “pulls” the permanently magnetized rotor along with the switching magnetic fields. The motor shaft has a Delrin  coupling.

The ST Gear Assembly
The planetary gear assembly on display at boatshows is composed of sun and ring gears made of Delrin, and planet gears made of bronze. A demonstration version of the ST58 Electric Wheel, without the case and windings, makes both its operation and the cleverness behind the assembly readily apparent. The operation is a little complicated to explain, but it becomes obvious once you see it in action.

Assume that you’re driving the boat forward and looking at the front of the motor, which points to the bow. In the ST58, one ST Motor drives the ring gear and another ST Motor drives the sun gear. The planetary gear carrier connects to the drive shaft (PTO).

At motor speeds below 600 RPM, the sun gear turns counterclockwise (CCW), the ring gear freewheels, and the planetary gears drive the shaft clockwise (CW). The gear ratio is 2.6:1. Above 600 RPM, a controller switches the drive gears—the ring gear drives CW and the sun gear freewheels. Again, the planetary gear assembly continues driving the shaft CW, though at a ratio of 1.6:1. When the motor speed exceeds 650 RPM, the controller switches both sun and ring gears to drive CW, and the gear ratio becomes 1:1.

Presently, Solomon Technologies sells three different propulsion systems: the ST37, the ST58, and the ST74. (The number in the product name indicates the motor’s torque in foot-pounds.) All systems use the ST Motor as the power source. The ST58 and the ST74 use two ST37 ST Motors combined into one housing. The motors have a water-resistant cast aluminum case. All the motors are 13″ in diameter.

The ST37 and ST74 systems are direct-drive motor systems. The middle system, the ST58, is the Electric Wheel version—two motors plus gear assembly. Again, it has three speed ranges that automatically change according to the motor’s RPM. It has eight moving parts altogether. With the doubled-up motors, if one motor or its associated gear should break, the other gear may be able to propel the boat, albeit at reduced power. According to David Tether, the Delrin gears are self-lubricating and need no maintenance. He estimates that the Electric Wheel has a lifetime of 80,000 hours.

The Electric Wheel is not, in fact, what ST installs in most sailboats that show up for a power-change operation. Most boats need and receive only the ST Motor. It took us quite a while to understand that, but finally John Finnerty clarified it all for us:

The ST58 was the first electric propulsion device designed by Solomon Technologies for the marine market. According to Finnerty, once they started on- the-water testing with the Electric Wheel, they found that the gearing was of little advantage. The development of the ST37 andST74, capitalizing on the electric motor technology in the ST58, deliveredthe required torque in a simpler package. However, the Electric Wheel gearing may have an advantage with regenerating electricity. Solomon Technologies plans tests in the near future to measure the ST58’s regeneration capabilities.

So, as clever and useful as it may be, the gear assembly is more of a red herring when it comes to auxiliary sailboat applications.

Other Elements
In addition to the motor, the propulsion system includes the batteries, the SPMD Panel (Safety, Power Monitoring and Distribution), the controller, the wiring, throttle and shift controls, and house voltage regulators.

The ST Motor requires a higher voltage than what we’re used to on most boats—at least 10 times higher, in fact: The recommended voltage is either 120VDC or 144VDC, with battery banks comprised of 10 or 12 12V batteries wired in series to boost the voltage. After every four batteries in the chain, there’s a circuit breaker to protect the wiring and the boat from a catastrophic battery failure.

The ST Motors use this high DC voltage to minimize power losses in the wiring and the windings, and to reduce the size of the battery cables needed in the installation. In the installations we saw, the battery cables were 4 AWG, much smaller than the 2/0 AWG wire many of us are used to.

To supply the rest of the boat with the 12V power that we’ve come to expect in our vessels, Solomon Technologies uses a DC-DC converter to step the voltage down to 12V. These converters generally have efficiencies better than 90%, so waste heat should not be a problem. Battery size is determined by how much energy you want to carry. One installation we saw used 10 Group-30 batteries.

The SPMD is used for passing the power between the batteries, the controller, and the motor. This is an enclosed wiring panel with bus distribution bars and shunts for measuring current flow both into and out of the batteries.

The brains of the ST Motor are in the controller box. In the box is a commercially available industrial motor controller setup for driving the ST Motor. Additionally, there are two muffin fans for cooling the box. The controller sets the motor’s speed by varying the pulse frequency to the windings. The current flowing through the windings sets the power. You control the motor speed through a potentiometer, which can be coupled to a standard Morse-type throttle control or something similar.

Power Equivalents
As you can see from the table on page 14, the horsepower for these motors is not close to the horsepower we’re used to with a standard auxiliary engine. However, for driving a propeller, the torque number is more important and that’s the strength of the ST Motor. For example, a Universal M-40B diesel engine produces 37.5 hp at 3,000 rpm and 73 ft-lb of torque at 2,180 rpm. Compare this to the ST74 with 12 hp at 1,100 rpm and a torque of 74 ft-lb. Also, unlike a diesel or gas engine, the ST Motor has nearly constant torque, even down at 1 rpm.

The ST Motor spins the prop at a slower rate than a fossil-fuel auxiliary. Tether claims that the best efficiency happens at above 600 and below the 1,100 rpm limit. At this rate, he says that the efficiency of the ST37 and ST74 are above 80%. We were not able to verify this figure.

Reversing the motor is simple: The controller pulses the windings in the opposite sequence, causing the rotor to turn in the opposite direction. The efficiency is the same.

Electric Propulsion: Solomon Technologies’ High-Wire Act

Generating Power
Because the ST Motor’s efficient rpm range is slower than that of a fossil-fuel auxiliary, it requires the largest diameter prop that clearance will allow. Solomon Technologies recommends a three-bladed prop over a two-bladed prop, both because of the rpm issue and because a three-blader will do better at fulfilling the other big promise of the ST Motor: the generation of electricity while the boat is sailing. (Solomon Technologies calls it “regeneration”—not a bad term, considering that energy is retrieved by exactly the same mechanism that uses it.)

As we know, unless the transmission of an inboard auxiliary engine is locked, the propeller tries to freewheel in the waterflow under the boat. The ST Motor makes use of this tendency: The prop is allowed to spin in the flow, the shaft in turn spins the rotor, and the rotor creates voltage and current flow in the windings. Through the controller, this power can be put back into the batteries as a charging voltage. In simple terms, while sailing, you can replenish the charge in your batteries.

Because of inefficiencies in the charging of storage batteries and other losses throughout the system, including the prop and the ST Motor, the ratio of powering time to recharging time is far from equal. According to Tether, depending on the wind speed, it may take two to four hours to replace the power used during an hour of motoring. A fixed two-blade prop won’t work nearly as well as a three-blader, and a folding or feathering prop won’t work at all, because they won’t spin as the boat moves through the water (unless you have a manual feathering control).

For the best regeneration potential, then, you would use the largest diameter three-bladed prop you could fit—and you’ll have maximum drag going through the water. If you care only about propulsion, and not regeneration , a two-bladed prop will do. Tether says his company has successfully fitted a folding Martec prop and a Maxiprop feathering prop.

From the two installations we saw, repowering a boat with an ST Motor appears very straightforward. The owner must remove the original engine and all of its assorted paraphernalia, such as fuel tanks, filters, drip pan, etc. Also, with everything out, it’s time to clean the bilge. Once all of this is completed, Tether says that it takes two Solomon Technologies installers two days to install an ST Motor, SPMD, controller, and E-Meter. The E-Meter, a stock product from Cruising Equipment, shows the power used and the power remaining. You use it just like a fuel gauge.

The owner is responsible for installing the batteries, AC charger, and the throttle controls. According to Tether, the new installation weight is “usually net-even” compared to a diesel engine, fuel tanks, plumbing, and house and starting batteries.

About half the installations that Solomon Technologies has done include a generator as a backup charging system. The gensets have been in the 4- to 6-KW size. Tether recommends that a boat with a genset use ten batteries (120VDC) in its battery bank, whereas a boat without a genset should use 12 batteries (144VDC). In addition, he points out that one additional 12V battery is needed for the bilge pump and to start the generator.

Motor and shaft alignment are accomplished in a unique way: Since the motor can be spun at any speed with unwavering accuracy, an ST installer can simply bring the motor and prop shaft coupler together into crude alignment, then spin the ST Motor at 4 RPM and watch for wobble, making fine adjustments to alignment until the wobble is completely eliminated. The result is shaft rotation with no noise and minimal friction, while perfect control of the motor’s speed is maintained.

A 120VDC or 144VDC system obviously packs a big wallop, and there are naturally safety concerns about such a high-power system on a pleasure boat. The ST Motor comes with a user’s manual, warning labels, and recommended circuit breakers after every fourth battery to meet UL safety requirements. Although any type of storage battery can be used, all installations so far have used absorbed glass mat (AGM) batteries, which are designed for hardy use at a variety of operating angles, and which don’t require special boxes or pans for containing spilled acid. (Still, you would want to make sure that the battery compartments were well-ventilated.) According to David Tether, ST installations have not caused any new insurance issues for the boat owners.

An interesting by-product of using the ST Motor is that the system needs no winterization. In fact, Tether uses the ST Motor and prop on his boat as a bubbler system to keep his slip ice-free in the winter.

Another good feature is that at least two through-hull fittings upon which an auxiliary engine depends—the raw water intake and the exhaust port— can be shut permanently.

On the Water
On our visit to Solomon Technologies, we saw a water tank used for testing each motor before it’s installed on a customer’s boat. We also watched two motor installations and went for a ride on Tether’s cutter.

The 2,000-gallon test tank lives in the workshop. It has a prop in it, with the shaft coming through the tank wall, through a stuffing box, and to a flange, so that new ST Motors can be coupled up and checked before delivery, and new designs checked out in “lab” conditions.

We put an ST37 through some simple testing with the tank. The time it took the motor to spin-up from 0 rpm to full speed was quicker than we could measure—much faster than a diesel or gas engine. Also, we could run the motor at a very slow speed with no unusual jerkiness or hesitation with the motor’s rotation. ST claims that the current usage is around 1.1 amps per foot-pound. Since this number is actually voltage dependent, it’s difficult to verify. However, when we saw the ST37 running at full speed spinning the prop, the current usage via an E-Meter was in the mid 30-amp range, which is close to the claim for a motor with 37 ft-lbs of torque.

Electric Propulsion: Solomon Technologies’ High-Wire Act

The two muffin fans used for cooling the internal electronics in the controller consume around 10 watts with the controller powered. Tether says that most people just leave the controller on all the time, since the 10w draw is very small compared to the 4.5kw or more power that the motor uses. Even so, we’d want the fans switched off until the temperature in the box reached a certain point.

The motor is quiet, but not silent—it does emit a low buzzing sound. Most likely this noise comes from the current pulsing through the windings. The noise is very acceptable, especially when you think about the noise that comes from a diesel or gas engine.

Tether retrofitted his 1939 33-ft. Casey cutter, Seaward, from a 22-hp diesel to an ST37 with 12 batteries (144VDC). He uses this heavy boat as a demonstration platform for his technology. As we boarded Seaward, he goosed it slightly forward, which startled and unbalanced us. There was no engine noise and yet this 19,000 lb. boat moved smartly. We tossed off the docklines and took Seaward out into the Patuxent River.

We powered along at 5 knots with barely any noise. The sound was like the whine of a car’s tires on a highway. According to the E-Meter, at full speed we were drawing 35 amps at 144VDC, which is approximately 5KW. We threw the boat into reverse and stopped in about a boatlength.

We tried to sail to test the regeneration capability, but the light winds, typical of a Chesapeake Bay morning, could only move Seaward at less than two knots, which wasn’t nearly fast enough to get the prop spinning.

The full-keeled Casey had unexpectedly good control in reverse. We attributed this to the very slowly turning prop minimizing the prop torque force that would normally cause our stern to walk to port.

The repeatability of the motor’s operation is “very predictable,” according to Tether. For the same power demands from the controls, you’ll always get the same power out. Also, since the controller sets the speed based on the frequency of the windings switching, the operating speed is also repeatable.

For dockside charging of the batteries, you need to use a specialty charger to handle the 120 or 144VDC. These chargers are supplied by Solomon Technologies as part of the installation package.

Thus far, Solomon Technologies has installed its electric motors in 32 boats, ranging in size from an Ericson 27 to a Cherubini 44. Reports from the few owners we’ve been able to contact directly have been favorable. John Conser of Conser Catamarans in California says the system is “very, very quiet,” and “absolutely the only way to go.” ST Motor systems are now the standard power plants on the Conser 47—there is no diesel option. Conser believes that the loss of boatspeed to prop drag while regenerating power in 18-knot windspeeds is about a knot. This “one knot” refrain was one we heard several times when looking for specific, quantitative information on the actual speed loss for a medium-sized displacement boat regenerating power in a good breeze with a three-bladed prop: “About a knot.” “One knot.” The owners say it, and Tether says it.

It would have been nice to provide, for example, a chart with curves showing boatspeed gain and loss in relation to windspeed and charging rates in a “typical” displacement boat, but the rough data for that doesn’t appear to have been generated yet by owners of the system, much less compiled by Solomon Technologies. Aside from the variables of boatspeed and windspeed there’s the matter of prop type, size, and pitch—so it may be that the hope of perfect data here is fanciful, as it often is in sailing. Still, “about a knot” seems a bit loose-ended…

David Hadfield, owner of the first-ever ST installation in a consumer boat, wrote PS a letter offering to share his thoughts with us when we mentioned in our July 15 issue that we would be looking at Solomon Technologies. Hadfield repowered his 1973 Islander 30 Mk II about two years ago, replacing his Palmer 18-hp engine with an ST Motor (a prototype called the Solo 6).

Here’s an e-mail Q&A session between PS and David Hadfield:

PS: What’s your experience with regenerating power?

DH: At hull speed of roughly 7 knots my 30′ Islander will regenerate at a ratio of approximately 2.5 to 1 hour of motoring. At less than 7 knots, say 4 or 5 knots, the ratio goes to 3.5 or 4 to 1.

PS: Does regenerating offer enough power input to keep the batteries charged?

DH: Yes.

PS: Does the ST Motor develop enough power for your powering needs? Too much power?

DH: Yes — and at 60 % I am normally at hull speed. Any additional power is wasted in a normal transit while powering.

PS: What’s the range, i.e., time or distance, of the electric propulsion system?

DH: Average, with my ten Group 27 batteries, is 6 to 7.5 hours of range.

PS: Any issues with so many batteries on the boat?

DH: None. Batteries were placed centrally and weight has “centered” the boat.

PS: Any other safety or insurance issues with the electric propulsion system?

DH: No vibration; silent, no smell, no visits to the gas dock. My four-year-old plays down below when powering out of the channel—something he never did before. The security of instant power forward and reverse can not be overstated.

PS: Would you install the motor again?

DH: Yes, without reservation. In the last two years I have had zero maintenance and a building confidence in its viability and versatility as anauxiliary.

The ST Motor system, with or without the Electric Wheel gear assembly, looks like a reasonable alternative to the diesel auxiliary. There are the usual trade-offs: In some areas where the tradewinds blow and the sun shines enough to make solar panels reliable sources of energy, it’s quite possible to imagine the system being entirely self-sufficient. In other areas it would certainly be more prudent to carry a small generator, although not necessarily permanently mounted. It’s also easy to see how coastal-sailing marina-dwellers with access to AC shore power would be able to “motor” and sail happily in most circumstances without a fossil-fuel burning device on board.

Obviously, a careful assessment of one’s motoring habits and other DC power needs is called-for, although it seems reasonable to assume that a generator could be added later if energy needs turned out to be overwhelming for the system. (It seems to us that a permanently mounted genset in addition to the ST Motor system simply adds back most of the complication that one is presumably trying to escape in the first place. A smaller generator, stowed elsewhere in case of emergency, is different.)

Other advantages were mentioned earlier: fewer holes in the boat, less pollution, less noise, less smell, greater operating and maintenance simplicity, and so on. Also, since the motor controller and other system parts are built from commercially available sources, the teething pains for a new technology have been minimized: Most parts can be had from other sources if needed.

Disadvantages include at least a moderate boatspeed loss while regenerating power. If “about a knot” covers a typical loss, it’s significant. A knot means a lot to boats with typical cruising speed ranges of 5-7 knots.

We also have some questions about the viability of using a DC-DC converter to step the voltage from 120 or 144VDC down to the 12V that most of our equipment uses. Will the DC-DC converter be capable of supplying power to some of the larger power users in today’s boats, like SSB radios and refrigeration? What about the regulation of the 12V output of the DC-DC converter at low power versus high power? We weren’t able to test for these concerns, so we don’t know whether they’re valid.

Finally, we wonder about the longevity of the battery banks, given the kind of use (and disuse) they’re likely to encounter with this system. It’s one thing to replace a $90 battery every few years; it’s another thing to replace 10 or 12 of them.

For someone considering any of these systems, the best thing that Solomon Technologies could do would be to address questions and concerns of this type with hard data. It’s possible that this small company is on the verge of something big, and it would be a shame for it to stumble by spending too much energy on mere boosterism and not enough on clarity about the quantifiable pros and cons of the products. It would do Solomon Technologies good if the following were widely available:

1. A clear separation of the ST Motor technology from the Electric Wheel technology, and an explanation of the advantages of using the gear system.

2. Graphs or charts comparing the ST Motor to conventional diesel or gas engines. There is a graph on Solomon Technologies’ web site, but the Dynamic Output graph is difficult to understand.

3. Graphs or data comparing torque output to power input. For example, does a slower prop speed mean less power usage?

4. A chart or series of charts (as mentioned earlier) showing regeneration capability versus loss of boatspeed, in relation to windspeed, for a variety of boat types and propeller options.

5. Hard data from present users regarding power usage, presented in an easy to understand format, allowing comparisons between boat size, type, and conditions.

We would welcome more feedback from current owners of ST systems. Please e-mail us with your appraisals.

Contacts- Cruising Equipment Company, 5916 195th Northeast, Arlington, WA 98223; 360/925-5000; www.cruisingequip.com. Electric Boat Assn of the Americas, P.O. Box 4025, Lantana, FL 33465-4025; www.electricboat.com. Solomon Technologies, Inc., 7375 Benedict Ave., P.O. Box 314, Benedict, MD 20612; 301/274-4479; www.solomontechnologies.com.


Also With This Article
Click here to view “Electric Wheel Drive Characteristics.”
Click here to view “ST Motor Comparisons.”

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


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