Sailing with a diesel auxiliary, as well as engineless, led one couple to make a third choice: fully renewable electric propulsion.

As far as I can tell, my fiancée, Alison, and I aren’t typical sailors. On our 1969 Luders 33, Ben-Varrey, it’s not unusual for us to sail when others would fire up the engine. Instead of racing to the next anchorage, we pull out a book, splice a line, clean the decks, or make a nice meal while underway. True, there are times when we are entirely becalmed while some slow sea creature, like a sunfish, does laps around us. But we spend more time on the water and let the journey create an incredible experience.

This philosophy has driven the changes over time in our relationship with Ben-Varrey’s auxiliary propulsion, from diesel, to no engine, to an electric propulsion system. When we bought her, she had a 28-hp diesel that possessed its own personality and demanded regular attention to keep running smoothly. That game lasted for two years before the desire for simplicity and the need for a new challenge took over. We went engineless. Yes, I hauled a perfectly good engine out of the boat, sealed up the shaft log, and bought a sculling oar.

Sailing on and off anchor was standard practice already, but this change did force me to refocus on light-air sailing. There was no option other than to sail or scull up to docks for water or pump-outs. It was a quick way to get to know intimately how the boat handles and to develop new skills in the warping department. Ben-Varrey was noticeably lighter and faster, thanks to that 550-pound diet. And a spacious new storage area for folding bikes was a bonus!

Granted, it takes a special mix of patience, experience, and craziness to enjoy it, but I would rather work hard to keep the boat sailing than spend time buried in the engine compartment. And the change came with unexpected benefits. Every trip required better planning; wind was always helpful, and current could be an ally or enemy. This level of forethought led to quick local sails and a great head start on anything with distance. It was also safer navigationally. We didn’t tempt a lee shore with an engine not starting, and any oncoming traffic earned high attention.

But, after sailing for three years with only a sculling oar for auxiliary power, Alison challenged me to come up with a solution to expand our cruising options. There are some channels that are just too tight to short tack and days when the breeze is just out of reach. It was a matter of convenience. There was always the option to wait for conditions to change, but perhaps there was a better balance—a level of modest convenience? The solution would also need to be as environmentally ethical and thoughtful as possible.

The Calculus

Regardless of what we would install, we would not fall into the trap of relying on it to get us out of trouble. All navigational decisions would still need to be sound, and we would preserve redundancy through our main propulsion system: sails and rigging.

We needed to maintain simplicity and keep maintenance and operational effort to a minimum. All avenues led to an electric propulsion system that was self-sustaining while on a mooring, anchor, or underway. It needed to be charged by renewable power. This represented complete independence.

An electric motor doesn’t have a crankshaft, pistons, injectors, a fuel pump, fuel valves, a camshaft, rocker arms, or a crosshead bearing, among many other common moving diesel engine components. The only moving parts to wear out on an electric motor are a set of bearings and potentially a belt if a gear reducer is used. As for electrical components, modern diesel engines are just as dependent on them as a full electric propulsion system. Combine these two factors, and an electric motor is fundamentally more reliable and requires less maintenance. It also eliminates issues like contaminated fuel that will stop a diesel engine.

Another bonus is the cleaner boat environment. Nothing makes me seasick faster than being trapped in a hot, small compartment that reeks of diesel while rocking back and forth and getting covered in oil or grease. Likewise, no exhaust in the cockpit. We’d experienced the clean boat smell after we extracted the diesel engine and were not willing to go back.

Finally, there was the money factor. Since we were looking at a partially self-built electric system, the cost was well under half of the cost of a new diesel.

With our commitment to wind propulsion firm, this new system was to be a true auxiliary. Range would be limited by our battery capacity. We wouldn’t get anywhere close to the power density of diesel with a similar volume of batteries, but given our goals, this concern could be left ashore. With our intentions clearly defined, the system began to take shape.

The Motor

The heart of any propulsion system is the power plant. This is where I began the project’s design: the electric motor. When sourcing a diesel, the language is horsepower; when selecting an electric motor, it is kilowatts (kW). Some translation was needed to determine the size of the motor I would need, and it’s not as simple as multiplying by a conversion factor.

There are two approaches to this. One is what most sailors probably will use, a table that electric propulsion manufacturers provide to help determine the conversion’s parameters. The second requires more work but is more accurate. However, it also requires a four-year degree in naval architecture, or at least the ability to follow a set of formulas from a book on yacht design. I took the second route, as I’m a bit of a boat nerd; that degree was expensive, and I need to make it count anywhere I can!

The process also provided me with a propeller and shaft specification, which made it simple to order a new Campbell propeller. After crunching the numbers, I knew that the 28-hp diesel that I once had could be replaced with an 11-kW electric motor, with the added benefit of instant torque.

With the power determined, I narrowed the search by choosing between a brushed or brushless motor and the voltage. Brushed motors were less expensive, but a brushless motor carried significant advantages, among them higher efficiency, higher torque-to-weight ratio, increased reliability, a longer lifespan, and no ozone production. Why worry about ozone? In addition to higher concentrations being harmful to humans, it posed a risk to a fair amount of equipment on board. Ozone can quickly break down certain rubbers like nitrile, which was the material of the diaphragm for my primary bilge pump (close to where this engine would be installed). I’d look for a brushless motor.

In the brushless family of motors, I could opt for a brushless DC motor or an AC motor with a DC controller. The voltage would need to be much higher than the existing 12-volt power in the boat, as the current draws would be too large for any reasonably sized wires to handle without suffering major voltage drops. Existing electric vehicles and boats had a range of voltages, but 48 volts seemed to be emerging as the leader on the marine front.

As I dug deeper into options, I came across Thunderstruck Motors (, a California-based operation specializing in DIY electric drive systems for vehicles and boats. They offered kits for boats that included a 48-volt brushless motor, controller, and throttle, all bench-tested before delivery. They also had gear reduction kits, which I would need to reduce the rpms on the propeller shaft per my earlier calculations. I purchased the 10-kW kit, as the next size up, 12 kW, required water cooling for the motor, an added complication not in line with my propulsion needs of occasional and light use.

The gear reducer acts as the primary frame of the installation. The motor is bolted to the upper smaller gear, and a coupling secures the propeller shaft to the lower gear. The two gears are connected by a carbon-reinforced rubber belt. Installing the gear reducer frame required custom brackets that my brother, Ryan, fabricated (he’s a marine mechanic, machinist, and boatbuilder). He also assisted with some custom tools for the propeller shaft installation. The alignment of the gear reducer to the propeller shaft frame is critical and made slightly easier by the flexible shaft coupling provided. A few dry installations went a long way in making sure that all the components would line up correctly.

We built a custom box to house the throttle, accessible through a latched waterproof door in the cockpit. This also housed the main power switch for the engine, which activated a solenoid to allow power to flow to the controller. I installed a second, manual, switch in line with the solenoid to ensure the power was off when I was doing any maintenance.

I mounted the motor controller safely to the side of the motor but close enough to minimize the distance of the wiring runs. Such high current flows can result in large voltage drops if wires are not large enough and if wiring distances are too long. By keeping the components in proximity, I could maximize efficiency with heavy-gauge wire. And, since the new electric motor configuration was so much smaller than a diesel, I could install the batteries right next to the motor, allowing for another short wiring run to the controller.

The Batteries

Aside from emissions considerations, we also wanted the life cycle of the entire system to be as environmentally responsible as possible; each component should be as low impact as possible to manufacture and be readily recyclable. If the batteries were just going to end up in a landfill, require a tedious recycling effort, or run the risk of poisoning the environment, we missed our goal. That immediately removed any form of lithium batteries from the list, despite their capacity and efficiency. This technology is still young, and I bet there will be appropriate and widely available recycling, especially for the LiFePO4 versions, in the near future. It’s just not available today. And although the mining performed to create any battery is far from ideal, that is universal.

At the top of the list were flooded and AGM lead acid batteries. They are almost entirely recyclable (and previous emissions from the process have been greatly reduced by regulation). We settled on flooded lead acid batteries for the cost per amp-hour and relative cost of the efficiency when compared to AGM. A Duracell group 27 flooded deep-cycle 90-Ah marine battery cost slightly over $100. There is undoubtedly a performance difference when compared to other battery types, but as with any outfitting choice, we compromised based on our priorities.

The motor required 48 volts to operate. Shy of power loss through a DC-to-DC converter, that means that the boat needed a power bank comprised of intervals of four batteries. So, would we have four, eight, twelve batteries? At 50 pounds each, this needed to be carefully considered. This was altogether separate from the existing solar-charged house battery bank that already included three of these batteries. It was effectively replacing the long-
forgotten full diesel tank and the single engine starting battery (330 pounds).

Battery bank capacity needed to be equated to range. Also, since resistance is not linear but rather exponential for this and other displacement boats, I needed to determine the range at different speeds. The motor could deliver a maximum of 10 kW. Using the vessel’s particulars and a velocity prediction program (later verified by sea trials), I estimated this to equate to 6.4 knots in flat water.

In a 48-volt battery bank, 10 kW requires approximately 208 amps. Given that flooded batteries should not drop below 50 percent capacity, a bank of four 90-Ah batteries (using only 45 Ah) wired in series would provide just under 13 minutes of motoring at 6.4 knots, or a range of 1.4 nautical miles. Pulling that many amps out of a deep-cycle flooded battery will cause sulfation on the plates and reduce battery life considerably, even with occasional equalization, but since I had no interest in motoring that fast, I wasn’t too worried about this.

The table below breaks down the calculated range at other speeds following the same logic.

With eight batteries, multiply the ranges by two. With 12 batteries, multiply by three. Most importantly, this is based on calm water (if it were not calm, we would be sailing), and slow and steady winning this distance race.

Based on our minimal engine use and the desire to be able to charge this battery bank entirely on renewables in a reasonable period of time, we opted to install just four batteries. In existing space within the engine compartment, I fabricated secure foundations for the batteries using wood, fiberglass, aluminum flat stock, and nylon webbing. Fitting additional batteries would have been possible but challenging. And remember the weight factor. Lighter means faster, so why add more weight to slow us down? The future of energy storage is promising, and we will be keeping a close eye on what is coming next.

Renewable Charging

Any battery storage bank still needs to be charged, and we wanted our installation to be fully off the grid. That meant solar, wind, or hydro generation specified such that the batteries could be recharged within a short passage.

Expanding our existing solar array would be the most cost-effective first step. We opted for twin Renogy 175-watt solar panels, the largest we could fit on board. These replaced our existing twin Renogy 100-watt panels that charged just the house bank and moved us from a combined 200 watts to 350 watts. The panels are attached to the pushpit, port and starboard, with customized adjustable rail brackets. The panels tilt to help optimize power generation and fold all the way down for docking.

The panels are connected in parallel and linked to a two-way switch. One mode charges the house bank with a 12-volt charge controller (Victron 100/30) and the other charges the engine bank with a 48-volt boost charger (Renogy Rover Boost). The generation varies considerably with the time of year, cloud cover, and point of sail. For example, sailing south in the fall wing-and-wing with a northerly breeze is tough on generation. Alternatively, a completely clear summer day at anchor will provide the maximum amount of energy. The panels can add anything from a few watts into either bank to our current best of 1.75 kW in a day. Just like any exchange, you must budget use based on generation. A series of cloudy days equates to less motoring unless there’s another charging source.

A wind generator was next. This took a fair amount of research and would be a completely new form of power generation for Ben-Varrey. The wind generator would be dedicated to the engine battery bank, and a 48-volt unit would offer the best charging efficiency. The market is flooded with options but shrinks considerably when looking for 48-volt models. (On a side note, I installed a crossover switch that can charge the house bank from the engine bank via a Victron 75/15 charger, allowing the wind generator to effectively charge the house.)

After focusing on efficiency and noise level, we installed a Silentwind 400-watt model that included a charge controller specific to the unit. It consistently scored well in professional reviews and in various forums, with an emphasis on decent generation at lower wind speeds and quiet operation. Shortly after installing it, we were in the Vineyard Haven anchorage among a few other boats with wind generators, one at least a thousand feet away. From our cockpit, we easily heard the others above ours. Along with Silentwind’s great customer service, this performance reinforced the choice.

I mounted the wind generator on a 9-foot-tall, 3.5-inch-diameter aluminum Edson pole secured to the deck and the reinforced pushpit using Edson Marine’s stock fittings. The height places the spinning blades out of the way of any person or sail, yet I can just reach it when standing on the pushpit. This maximizes wind velocity while also ensuring that I can physically secure the blades in heavy weather or during maintenance.

A wind generator’s power generation is exponential relative to wind speed, and it does not start generating any power until a certain wind speed. In most sailing environments, it’s the lower wind speeds that count. A wind generator’s ability to produce an impressive number of watts in 40 knots isn’t all that helpful (although on a recent heavy-air passage, I had the entire boat running off of the wind generator, but that’s a rare exception). In most cases, we were concerned with how much power could be generated in 5 to 22 knots of breeze—at least 90 percent of the sailing conditions we experience. The Silentwind 400W estimated production as follows, and has proven very accurate based on our four years of use:

We didn’t expect the wind generator to hit the same power generation peaks as the solar panels, but because it can generate power anytime, it becomes competitive in total generation. On a cloudy and windy day, it will be doing almost all the work. On average, we generate 1.1 kW per day with the wind generator.

A third possibility would have been a hydro generator, which uses a propeller to generate power while under sail. Unfortunately for us, the math didn’t work out. It was too costly for our budget to go with a separate unit, and using the new propeller and motor was equally problematic. The ideal propeller design for driving a boat forward is very different from the optimal design for power generation under sail. With the fixed-blade propeller that was in our budget, and consideration of the aperture in which it would sit, regeneration by the motor wasn’t worth the effort to develop.

Time with this renewable package has taught us that we still need to be conscious of our energy use, but we have sufficient generating ability to fill our needs. The solar is directed approximately 90 percent at the house bank, as the wind generator charges the engine bank well on its own. We have increased our house electrical usage with time and find that we have topped up batteries earlier in the day. Combined, we generate a typical 2.2 kW per day, split evenly between the generation sources over a long enough period. This number will be lower when cruising in Maine (1.8 kW) and higher when wandering in the Caribbean (2.6 kW)—a perfect fit for the additional refrigeration draw in warmer waters.

With our occasional usage of the electric propulsion system (typically five to ten minutes at low speed), it can easily be charged up within the same day, typically in a matter of hours. If we were to run the engine batteries to our imposed minimum level of 50 percent, it would take a couple of days for them to recharge, on average. Under certain weather conditions, that could be shortened to only a day or lengthened to a week. Therefore, we remain conservative with usage, which fits well with our sailing style.

We are still sailing with this system four years later and we love it. Maintenance has been nearly nonexistent, we sustain adequate power levels on a mooring and at anchor, and there seem to be no limits on our sailing destinations.

This has been our experience, and we hope it inspires others to think about breaking through resistance to change and innovating when it comes to repowering.