Electric or Diesel-Electric Drives for Voyaging Boats

Solar Panels charging batteries aboard sail boat

I myself, up to a few years ago, thought that diesel electric would become a viable alternative for voyaging sailboats that would solve the age old problem of matching the output of an engine to the power requirements of a propeller across a wide range of RPM settings.

But it turns out that this is a more complex subject than it appears at first look.

Let’s examine the viability of electric and diesel/electric drives for voyaging sailboats.

I’m going to start off with a comment made to an earlier post by Eric Klem, a professional engineer who has actually designed and built electric vehicles—he knows of what he writes—that explains the issues far better than I would be able to:

Hybrids are very hard to justify in sailboats in my opinion as they lack any real advantages and have plenty of disadvantages.

It is important to distinguish between the two types of hybrids, series and parallel. What you are describing with a power pack that has no mechanical connection to the propeller is a series hybrid. A parallel hybrid would have both the electric motor and the engine attached to the driveshaft.

Series Hybrid

Series hybrids tend to have poor efficiency due to the number of energy conversions. Energy starts in chemical form (fuel), gets turned into rotational kinetic energy (engine), gets turned into electrical energy (alternator), and then gets turned back into rotational kinetic energy (electric motor). At every step, you lose efficiency and the steps are compounding.

There are a few applications where series hybrids do make sense. One is big locomotives and other applications where immense torque is required at zero rpm because we have not yet figured out a way to design a good torque converter.

Another is in an application where peaking is required but the nominal load is quite low, in this application the system basically runs the engine at maximum efficiency at all times and then a bank of batteries provides a few seconds of boost when needed.

The final application, that I can think of, where series hybrid makes sense is where an engine won’t physically fit and it needs to be mounted remotely.

Parallel Hybrids

Parallel hybrids don’t suffer from the series efficiency losses but they still don’t make sense in most cruising boats.

At steady state motoring along normally, all the electric motor/generator does is provide extra parasitic loss (things like bearing friction) as it is neither generating nor powering.

If you were to use the [electric] motor to add power to the shaft, then you would be drawing down your batteries, so at some point you would need to reverse the process and draw power out of the system and recharge the same amount of energy. The problem with this is that you would lose energy through energy conversions on every step so you would be making the system less efficient than simply using the diesel engine in the first place.

On a boat, there really is no such thing as regenerative braking during normal use, unless you actively use your propeller as a generator when sailing, so you don’t get a benefit there.

When it Will Work…Or Maybe Not

The major exception to my arguments against both series and parallel hybrids would be if you had an alternative way of charging the batteries and you had an enormous battery bank.

Assuming that you would use an average of 15 kW (20 hp) on a 40-foot boat when motoring, an 8 hour day of motoring would require 120 kWHrs. For reference, a single 8D battery typically has around 2.5 kWHrs of capacity so the bank would need to be enormous.

Gives “Ballast” a Whole new Meaning

Let’s just interrupt Eric for a moment and expand on what he just said: That’s 48 8D batteries at an average weight of 140 pounds (64 kg) or about 4 tons if we include wiring and mounting!

But wait, we need to double that so that we don’t discharge the bank more than 50% during our eight hour run. So let’s just call it 100 8D batteries at an average weight of 140 pounds (64 kg) or over 8 tons if you include wiring and mounting!

All to store the energy equivalent of about 10 gallons (38 liters) of fuel weighing about 60 pounds (27 kg). And that’s assuming you have some way to recharge those batteries that does not involve diesel fuel. If not, you have to carry and pay for the fuel too.  Back to Eric.

Alternatively you could try to generate the power as you went but this is impractical as well as most solar panels that you see are 0.1-0.2 kW, only a maximum of 1/75th the power needed even at maximum solar efficiency.

The Aircraft Carrier Look

To put that in perspective, each panel will be about 6 square feet so the total is 450 square feet. So, assuming we cover our 40 foot boat from bow to stern with panels, the resulting array is going to be about 12 feet wide—going to make sailing the boat a tad tricky.

Actually, it’s a lot worse than that, since if you take into account shading and angle induced efficiency losses, never mind recharging after motoring at night, you would need to at least triple that array size.

How About Water Generators?

Since Eric mentioned using the propeller for generation, let’s take a moment to look at that: You could use a generator connected to the shaft, but for illustration purposes let’s go top of the line with a Watt & Sea hydro-generator, since they publish an output graph.

In fact let’s get two—what’s US$12,000 between friends? And let’s say we sail at an average of 6 knots which means we will get about 500 watts out of the two generators, which, assuming 100% charge efficiency (which we won’t get) will take ten 24-hour days of sailing to recharge from our eight hours of motoring! Oh dear, maybe not. Back to Eric.

The one place where you could store enough energy and recharge fast enough is on a boat that doesn’t motor much, such as a daysailor with a big battery bank, and that has a grid connection at night meaning that it is kept on a dock with shore power.

What About Hybrid Cars?

Hybrid cars are somewhat popular but they have a very different load profile than a boat. The Prius is an example of a parallel hybrid and it benefits from the fact that cars spend a lot of their time at conditions other than steady state. It can have a smaller engine and still get decent acceleration thanks to the electric motor that takes care of peaking and it can also capture some of the energy that would otherwise go into heat when braking.

The Volt is an example of a series hybrid and its real strength is that you can plug it in. The Volt would never make sense for someone who drives long distances regularly but makes sense for commuters who can almost always charge off the grid.

Both cars benefit a lot from improved aerodynamics, rolling resistance and low weight; the actual drivetrains are not as efficient as many think, as the rest of the car accounts for a lot of the efficiency increases.

And, if after reading that, you still have doubts about the problems and challenges associated with electric and hybrid drive systems for voyaging yachts, please read this, by AAC technical correspondent Matt Marsh.

The engineers have spoken and not only that, they have spoken clearly in a way the rest of us can understand. A big thank you to Eric and Matt.

A few more thoughts from me:

Torque Curve

One of the frequently mentioned benefits of diesel/electric is that it allows better matching of engine output to propeller torque requirements.

But as Eric points out, there is a huge efficiency loss in this approach as the energy stored in diesel fuel is turned into rotational force, then to electricity, and then back into rotational force.

By the way, if you are wondering why diesel electric works for cruise ships, but not large bulk carriers or voyaging yachts, Matt’s post explains it far better than I would.

Hybrid Marine (HYMAR) Project

If you are still in doubt, you may wish to read the final report from Nigel Calder’s HYMAR report.

The takeaways from the report are:


Hybrid systems add complexity and lots of it. You still have a diesel engine, but to that you need to add an electric drive, a huge battery bank, and incredibly complex computerized control circuitry to make it all work. Lots of luck getting a system like that fixed in a remote place.

Capital Cost

The report basically dodges the entire capital cost issue, but the bottom line is that any sort of practical hybrid system is going to cost at least two to three times that of a simple and robust diesel engine and probably a lot more than that.

Series Hybrid Is Impractical

Despite spending millions, the participants were unable to realize any significant efficiency gains from a small series hybrid system.

Very Large Battery Banks are Intrinsically Dangerous

Many people assume that batteries are safe because they are low voltage devices. Nothing could be further from the truth. Yes, a single battery can only produce a few volts, but it is also capable of delivering several hundred amps, plenty to start a fire. Put a bunch of them together and just one cell shorting out can have disastrous consequences.

Wait, it gets worse. To be more efficient, many systems put a bunch of batteries in series to produce potentially killing DC voltages. (High voltage DC is more dangerous than AC.)

The two issues above apply to any battery. But the dangers get way worse if you go over to lithium-ion technology, which uses a fundamentally highly flammable (inflammable to you Brits that was educated right) electrolyte that can burn without the presence of oxygen (near impossible to extinguish)—just ask Boeing.

A Pity

To me the HYMAR project graphically demonstrates the dangers of starting a research project with a foregone conclusion. In this case the idea that better marine propulsion efficiency would be delivered by a hybrid electric system.

At the end of that project this was their conclusion:

“After many months of gathering data under different conditions and with a wide range of propellers, it is evident that, when loaded correctly, a diesel engine will always be more fuel efficient than a hybrid.”

If, instead, the millions of dollars had been directed toward a broader mandate like developing a more efficient small vessel propulsion system, I think great work could have been done in the areas of better variable pitch propellers and/or efficient continually variable ratio gear boxes that would have delivered real and practical fuel savings and reduction in carbon emissions.

Both technologies have the ability to solve the torque mismatch problem more efficiently than hybrid systems. And, in fact, Norwegian small boat fishermen, among others, have been successfully using the former for decades.

It Depends on Your Use

So do I now believe that  electric or diesel-electric drives are always a bad idea for voyaging boats?  Absolutely not. But the practicality of electric or hybrid drives is dependent on how you intend to use your boat.

For example, if you will only cruise in say the Caribbean, where there is almost always wind, and you are a purist who is willing to sail for hours when some motoring or motor-sailing would get you to your destination much more quickly, then electric or hybrid drive might make sense.

Even then, when the cost of batteries and related control circuitry is taken into account, the return will be small and there’s an argument that you might as well pull a Pardey and just go engineless, and be done with it.

So what about diesel-electric for the rest of us who want to be able to reliably motor for hours at a time? As a good friend and resident of Brooklyn, NY would say, fuhgeddaboudit, at least for now.

What of The Future?

Everything Eric and I have written above is based on today’s available technology, but what of the future? And no one would like to see a greener alternative to the diesel engine than I would.

Will there be some fundamental technological breakthrough that will finally knock the diesel engine from its position as the most economical and practical option for powering a voyaging sailboat?

Maybe, but the barriers are formidable. For example, let’s say for the sake of argument that a huge breakthrough is made that quadruples the efficiency of generally available solar panels to a point where they capture all of the sun’s energy with no losses—extremely unlikely, I would guess, but let’s be generous. Does that change anything for our purposes? Not really, because you are still going to have to cover over half your sailboat with panels.

The point being that the scientific fundamentals are so heavily loaded in favour of the diesel engine that even very large increases in the efficiency of the alternatives don’t change much.

Real Numbers You Can Use

If you would like to see if a hybrid or all electric drive will meet your needs, Eric Klem, has created a calculator that will answer just that question.


If you want to ask a question about any of this, I am happy to do the best I can, within the limitations of my technical training, to answer it.

And if you have found a way that an electric drive works for you, with significant real world experience to back it up, we are interested.

Like what you just read? Get lots more:

Meet the Author

John Harries

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

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