The Offshore Voyaging Reference Site

When Electric Drive Works For a Cruising Sailboat

A few months ago several people wrote to me linking to an article by Jimmy Cornell in which he detailed the reasons for abandoning his Elcano Challenge, a project to sail around the world on an all-electric catamaran.

Many people will expect me to start crowing about Jimmy’s failure because I’m on record as being sceptical about both electric drive and electric cooking for offshore cruising boats…for most usage profiles.

Not a bit of it. In fact, I don’t regard Jimmy’s experience as a failure but rather, a real-world demonstration that going cruising with electric drive and sails as the only means of propulsion is practical, albeit at higher capital cost than most other options.

So let’s analyze Jimmy’s experience to figure out where he got it right (most of it) and what we could change to turn failure into success, without cheating by installing a generator.

Making It Work

You will notice that most anytime engineers like Matt Marsh and Eric Klem, who really understand this stuff, or even yours truly who rides on their coat tails, write about electric drive, the qualifier “usage profile” or some equivalent gets used—now you know why I put the term in bold above.

And that’s really all we need to know to make electric drive work:

We must change our usage profile to fit the limitations and strengths of the technology. End of article…

…OK, that’s not going to work, I can hear the howls from here.

So, first off, let’s dig in to understand what went wrong for Jimmy. To do that, I suggest you read Jimmy’s excellent and honest article, but here’s the key quote:

Our 1000-mile maiden voyage from La Grande Motte to Seville showed up some of the weaknesses of the regeneration system when it failed to keep up with the overall consumption on a proper voyage, not during a test in calm waters. The consumption included both the autopilot and instruments, and also domestic demands such as induction cooker, microwave oven, two fridges, etc. My doubts started after we had sailed 82 miles in a period of 10 hours at an average speed of 8.2 knots. The net gain was a disappointing 9.5% of our total battery capacity, equivalent to 5.32 kWh, or 532 Watts per hour.

…I kept a detailed record over this entire passage, and the results were consistently and disappointingly the same…

Jimmy Cornell (emphasis mine).

At first blush that seems like a deal breaker, but let’s not forget that Jimmy and his crew managed to sail the boat all the way from the south of France to the Canary Islands and back without burning a drop of diesel fuel, although, to be fair, I’m guessing they charged the batteries from shore power whenever they were in a marina. If so, some carbon was emitted, but a tiny amount. Whatever, an inspiring achievement.

Now, after that moment of optimism, I need to get realistic—electric drive fanboys will call it pessimistic—but don’t despair, my ending is all optimism, so stay with me.

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More Articles From Electric and Hybrid Diesel-Electric For Cruising Boats:

  1. When Electric Drive Works For a Cruising Sailboat
  2. Electric or Diesel-Electric Drives for Voyaging Boats
  3. Real Numbers For Electric And Diesel-Electric Drives
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Dick Stevenson

Hi John,
You were speculating about 40-foot boat’s average speed on passage. On Alchemy, it is the 2 of us and we try not to press the boat at more than 75 or 80% of capacity (eaking out that last 20% is when damage to boat and/or crew is more likely to occur) and my impression is that we do much less motoring than most.
I have been surprised by the consistency of 5.7kn average when we are on passage for more than 3-4 days.
My best, Dick Stevenson, s/v Alchemy

Vesa Ikonen

Oceanvolt writes about this too, as does Outremer:

Seems that they all agree that the real world tested the setup in ways that all the prework in the world did not.

The good news is that the companies (esp. Oceanvolt) are already studying what went wrong with the regeneration and how to fix that.

I guess somebody had to be a pioneer here too – much like those brave souls who built & flew the first aeroplanes 🙂

It will be interesting to see how a similar endeavour will go like in, say, five years. Perhaps, in addition to getting the regeneration working on the ocean, solar cells integrated in the sails are needed. That has already been done, and it works fine for day sailing and coastal hopping.

Well, it remains to be seen when we can eventually rid ourselves of the diesel engine on boats, but it’s not going to happen very soon.

Matt Marsh

Oceanvolt, at least, appears to be pretty honest with the numbers in their latest marketing materials update.
The published figures for their AXC10 twin system (“replaces two 20-30 hp diesels” in a cat under 12 tonnes) claim that for €61730, you’ll expect 10 miles of range from 10.5 kWh, and should regen about 400 watts at 6 knots. That price includes a 10 kW DC generator, which can keep the drives running at half power for 325 miles or so on 100 L of fuel.

That’s honestly about the best we’ll be able to expect for a long while. Modern electric cars get their range by being incredibly slippery, coasting along at just a couple hundred watts per kilometre. There’s no nice shortcut to that kind of drag reduction on a boat.

Vesa Ikonen

Ah, how did I miss the further reading link.

Yeah, the regeneration seems like a tough nut to crack. What makes me wonder is the discrepancy between the tests and the real world. Is that due to the test setup being altogether unrealistic, or is it a question of what works in a small chop will not work in big waves.
Oceanvolt seems to think that part of the problem is a lack of optimisation – perhaps that has to do with the ocean swell you often mention.

Drag will always be there, for sure.

Michael Lambert

Maybe have a lot of propane and two propane outboards, for emergencies only?

Marc Dacey

I suspect what constitutes inconvenience for the modern cruiser is a greater hurdle than the technology. The pioneers, like the Smeatons and the Hiscocks, were of the Primus stove and kerosene lamp generation, although electrical alternatives existed. Same with provisioning. Ice was for toffs ashore. Today few would find that level of simplicity appealing, so the question for those willing to confront further proof of the no-free-lunch problem is: where will you draw the line on conveniences? Particularly when the pure options are themselves complex and expensive?

Paul Rutherford

All concerned in this project should be proud of what they have achieved and learnt to date. Far from being a failure, I think this is excellent work in trying to achieve ‘no diesel’ cruising and many of us will be grateful one day for near silent power sources. As others have said, the real world experience was far from the theoretical and calm water testing. Sadly it means I will re-power my, much smaller, catamaran with light weight modern diesels for now but keep looking forward to further developments.

Lee Corwin

Still, it’s interesting you can dramatically lower your footprint. Most of your time is at anchor. With panels and a couple of D400s the genset need not go on. Even adding in an efficient watermaker like a spectra Cape Horn.
One does see power moving to greater efficiency. The Arksen line and Greenline production boats are examples. Even the footprint of the Artnauticas is a marked improvement from a more traditional trawler. Think we shouldn’t let the excellent get in the way of the good.

Colin Palmer

My background is wind energy and I sail an Ovni 345. I have done a few basic sums on the fundamental physics of all this. I can supply the full spreadsheets for anyone interested, but these are the headlines.
Assume that around 15% resistance increase is acceptable (that knocks off 0.3 knots at 5 knots). Working backwards, a 0.25 m diameter “watermill” does this for an 11m yacht like mine. At optimum efficiency and assuming 75% conversion losses (shaft power to power in the batteries), this will produce 130W at 5knots, so 256AH per day (at 12v) or 3.1kWh.
If I were prepared to accept 35% increase in resistance (more than half a knot loss of speed), the diameter could increase to 0.4m and 655AH/day at 5knots (or 8kWh).
As noted by others this is all very sensitive to speed since the power output varies with the third power, so going from 5 to 6 knots almost doubles the output and dropping to 4 knots halves it.
For my levels of electrical power usage (autopilot, fridge, iGadgets etc) such a generator is very attractive, but 100% electric power is very different. Even in calm water my boat will need a propulsive power of around 5kW (for 5.5kts), so I would get less than 2 hours under power after 24 hours of charging with the 0.4m diamter watermill, sailing at 5knots and assuming all the power goes to the batteries.
The more practical 0.25m diameter version would give little more than half an hour under power.
Sadly the basic physics says it is little wonder that Jimmy Cornell had problems.

Dave Warnock

Looking at the YiuTube channels of people cruising with electric power (Sailing Uma, Rigging Doctor and Beau + Brandy are the ones I watch with 3 Atlantic crossings) the key difference is that they do not motor at 5.5kts. They motor little and slowly. The reduced speed means the time and distance under power increases a lot.

Colin Palmer

I just gave that as an example. At 4 knots the power required is 1.3kW and 0.7kW at 3 knots, in calm water and no wind. This really means doing a Pardy and sailing without a motor for anything but harbour manoeuvres etc. It does not get you off a lee shore or make progress against a tidal race. I accept this is a personal view, but I see these as safety reasons for having an engine.

Dave Warnock

I accept that for the last few decades that your view of the safety benefits of a diesel engine is conventional wisdom.
However, my opinion is that for some this leads to them becoming over reliant on their engine. Hence, they do not have sails ready to hoist, they do not have their anchor ready, they have not built their sailing skills, they have not kept their diesel carefully enough and filtered it enough. We see the outcome in the annual stats from the RNLI where engine issues are always, by far, the biggest reason for call-outs.
Having an electric motor absolutely means that you have to plan accordingly, if you don’t then of course it is less safe.

Ryan McNabb

Nigel Calder said it best: “Not yet.” Or, hey, maybe don’t model your life afloat on your life ashore? A radical concept?

Matt Marsh

Getting enough electricity from water turbines / propellers to actually run the boat under power for any length of time is not easy!

It’s not impossible, but let’s look at what you need:

  • An easily driven, low resistance hull form, to keep power consumption low when motoring.
  • An overpowered sail plan, so that the boat can maintain speed under sail despite the extra drag of the water turbines.
  • A powerful, high initial stability hull form, to stand up to that overpowered sail plan.
  • Lots of battery capacity to store that energy.

The 2nd point only works if you satisfy the 3rd point. The 3rd and 4th points are very nearly contradictory to the 1st point. Failure to meet the 1st point means the 2nd point and 4th point must spiral out of control.

It’s a very nasty set of conflicting trade-offs, but there is at least one regime where the equations converge. That ends up looking a lot like one of Chris White’s enormous fast “we swear it’s not a racer, honest, see all the pretty varnished wood?” cruising cats, with a quarter of the total weight budget being spent on LiFePO4 batteries and most of the rest being carbon fibre.

For an example of what an optimized all-electric boat looks like these days, here are the figures for one that’s on my drawing board recently. She’s almost 10 metres long and weighs just 1.5 tonnes unladen, with a quarter of that being her 26 kWh battery pack. Her best performance is 50 to 75 miles on a charge at 6 knots, dropping to 15 miles at 12 knots. She has no mast or sails, and would carry a 3 kW solar array that covers the entire cabin top – nearly 14 square metres. On sunny days, that yields about 55 miles a day from solar alone, which is ideal for river and inland lake cruising. But she’ll never be shorepower-independent for long enough to do coastal or ocean passages.

For comparison, my 11 metre sailboat is 7 tonnes, and her 80 litre diesel tank packs 776 kWh. Even after accounting for the diesel drivetrain’s efficiency, that’s still an order of magnitude more stored energy than the electric boat’s batteries carry.

All-electric is certainly possible, but the entire boat has to be designed around it. Not just from a systems standpoint, but from a hull form, stability, rig, etc. standpoint as well.

And you need to change some attitudes and preferences to live with it. The option to fall back to being a diesel motorboat in calm winds simply isn’t there.

Matt Marsh

Nah, no need for $5m. You can do a fine job of it with just two 😉

But yes, the willingness to say “no, we don’t have stored energy to spend right now” is key to making this work. One 20L jerrycan of diesel packs 200 kWh (about 60 kWh usable after accounting for engine efficiency). The equivalent lithium battery, with the best current technology, is one-third of a ton.

Vesa Ikonen

Hi Matt,
just checking: you are talking metric tons, right?

Dave Warnock

However, when the jerrycan is empty of diesel the story changes

  • you will have contributed to climate change
  • you will have used up more of a finite resource (and knowing that we can’t afford to extract all the known reserves because of the impact on the climate- something known by oil companies for decades)
  • you can’t get it filled mid-Atlantic or even in a remote anchorage
  • you have to pay to get it filled
  • you have to move your boat or carry the jerrycan when you want to get it filled
  • you will have polluted the air you and others have to breathe with stuff that is very bad for our health
  • you will have brought closer the next time you have got to spend money, time and pain servicing your diesel engine and feeding it oil, filters etc
Dave Warnock

I didn’t see it as irrelevant because my starting point is different.
My starting point isn’t diesel technology.
My starting point is more holistic. The advantages of diesel are irrelevant if your viewpoint is different.
For me the conclusion is that I’m willing to trade my convenience for a reduction in harm to the planet that I share with you and everyone else.
I’m not willing to say that my convenience is more important than the millions of lives already affected by Climate injustice.
We have to move away from fossil fuels much faster than we have been doing. So we are going to have to make drastic changes and do so before we think things are well enough sorted.
The industry has known about these issues for years and have been slow to react.
Blaming those calling for and working for change for delaying the industry is not helpful.

Dave Warnock

It seems that you and I have trouble understanding each other. Transatlantic English differences, culture, senses of humour may be part of it.
I didn’t understand your article that way and I wasn’t trying to raise strawman arguments.

Mathieu Fortin

The engineer in me was a bit frustrated not to see production/consumption graph for Jimmy’s adventure. I understand the value of the data though, which is probably why his tech partners want to keep that to themselves, at least for a while.
At least this gives me another argument for my dreamboat, the Alubat Cigale, even if it would keep a diesel for the foreseeable future.

Bob Buck

Interesting article in the current (February/March 2021) edition of Professional Boatbuilder about a 42’ all electric power cat. The owners’ plans are to cruise the Pacific Northwest Inside Passage up to Alaska. I’d be interested to hear what the experts on this forum think of their chances of success.

Matt Marsh

I don’t have access to the full ProBoat article at the moment (subscription system problem). But I did look up the details that Sam Devlin’s made available on that boat, “Electric Philosophy 40”, and did a few back-of-the-envelope calculations.

She is purely solar-powered, with 9.5 kW of lightweight high-efficiency PV covering the entire top surface. There is no generator. In fair weather at these latitudes, she’ll have about 50 kWh/day to play with; probably half that on average in the Pacific Northwest climate. That’s combined with a very lightweight, low-drag catamaran hull form. While Sam hasn’t published resistance figures, I wouldn’t be at all surprised to see her do 5-6 knots with under 6 kW, and would guess roughly 1 kWh/mile at that leisurely cruise speed.

The design will work very well if she’s hopping up the coast in 15 to 30 mile increments, and the occasional 50 to 70 mile day is probably doable. She will simply be a bit limited in total daily range in poor weather, and would have to anchor for a while to recharge if she gets a couple of cloudy days in a row.

Dave Warnock

A few points.

Brunton’s now make the Autoprop Eco Star

This is another way to give control over regen or feathering from the propeller. So little drag until you choose to “turn on” regen. And an option that is available to older boats with traditional shafts not just new boats with OceanVolt saildrives.

As for regen worth looking at recent videos from Rigging Doctor. They can’t be accused of sailing fast 🙂 On the Atlantic crossing currently showing, even at low speeds, they are able to recharge their batteries from regen a little and slowly so they can do some motoring when becalmed.

I would not be so eager to write off solar ovens, if nothing else as a way of reducing the energy required for cooking.

Totally agree on the great example from the Pardeys and those like the Hiscocks from a time when the engine was a not so reliable auxiliary.

I think you are still missing the point that this isn’t just about 95% sailing but also about much slower motoring when you do. Slow motoring gives a much greater range from your battery bank. For example, if you look to tests from Beau and Brandy in the Bahamas with their (600w I think) solar angled to the sun they could balance generation and consumption at about 2 knots.

For me the examples I see on YouTube of successful live aboard cruising for 4 years or more with electric propulsion do not fit the new fast boat model (none of Sailing Uma, Rigging Doctor or Beau and Brandy have new or fast boats). Only Rigging Doctor have had regeneration for any length of time, and their speed is very slow 🙂 All look at running more than just the motor on electric (Beau and Brandy have been running AC for their dog).

For ourselves, we have realised that we are going to need diesel heating (Refleks) and will be looking to maximise our time between shorepower charges in a UK winter rather than be fully zero fossil fuels.

Matt Marsh

Good points, Dave.
Here’s an example of range vs. speed for a 32 foot, 2.5 tonne (fully laden) electric multihull with a 26 kWh pack. This boat is exhaustively optimized for the most efficient possible 6 knot cruise while still allowing a 20 knot sprint. Even then, look at the huge difference that slowing down a knot or two makes. Or the difference that running fully laden vs. in light trim (minimum crew & standard stores) makes. That speed dependence of range will be exaggerated when you switch from this long thin multihull to a typical sailboat.

Dave Warnock


Eric Klem

Hi Matt,

Do you know if anyone has created drag/power curves for boats in different wave conditions (I have to think that this is something that at least racers would look at)? I know from experience that we burn more fuel per unit distance the minute there is any motion but I don’t have a good handle on how much it is as I have never had a proper flow meter, only boats with calibrated tank sounding sticks at best. I suppose boats with common rail engines could see some data provided that you have the right ability to hook up to it.

It would also be interesting to know where the extra power goes. For example, how much is lost to a prop which is often not pointing the right direction and doesn’t have flow over it in the intended direction. On a related note, I was thinking about the issue of the effective difference in prop pitch between the top and bottom of a prop with an angled shaft the other day and realized that I had no idea how to really calculate losses from it.


Dave Warnock

I think there have been too many variables and too few measurements. Very hard to get controlled experiments except through expensive tank testing as there are too many variables at sea.

Dave Warnock

John, obviously my English isn’t as clear as I would like. I’m not suggesting no data from voyages. I was responding to Eric’s point which makes comparisons between different real world conditions difficult.
I was simply thinking about the challenges that sea state, wind, currents, boat shapes are so different that comparisons are going to be difficult.
Two different boats in different places motoring into a force 5 will need different power and it will be hard to know why. Was it the fetch, was it weight distribution, hull shape, windage,…

Matt Marsh

As far as I know, virtually all data on drag/power in waves is gathered empirically. You look at the calculations for flat water, then you compare that to the measurements on the real boat in flat water. Then you repeat the measurements on the real boat in various sea conditions. Said real boat might be a scale model, or it might be a full-size vessel instrumented up with load cells on the thrust bearings, but it’s going to be a real hull floating in real water. Your measurements are the sum of all factors working together, and to tease out any individual contribution in the resulting n-dimensional parameter space, you’d have to run dozens or hundreds of trials and then pore over the results via principal component analysis.

Simulating this, to a level of fidelity that could tease out the contributions of individual components at a level that’d allow for quantitatively accurate optimization, sounds hard. It’s probably easier than calculating protein foldings, but I’d guess it’s at a similar level of difficulty to relativistic magnetohydrodynamics or hypersonic turbulent plasma flows. Solvable, to a certain level of approximation, if you throw a billion dollars and a Top 500 supercomputer at a few hundred engineers and developers. But not easy.

Eric Klem

Hi Matt and Dave,

Empirical testing is what I had guessed but it has its drawbacks. I can certainly see where tank testing as Dave mentions could be useful for measuring certain aspects but where it could also be very problematic to try to accurately represent all conditions. I don’t know if a company like Ocean Volt is gathering customer data but that could be one way to build a database, although one likely that is not available to all and also subject to issues like fouling.

We run into these sorts of usage scenarios in the equipment I design with some regularity. Generally, we create a nominal case that we feel is a good representation of what we should be optimizing around and then do some sensitivity analysis around it to make sure we haven’t created too specific of an optimization. In this we vary the environmental or use parameters through different predicted values for different periods of time/cycles and then use that to predict performance first in simulation then in testing. We also do check edge cases to make sure that we don’t have to worry about something like overheating, stalling, etc.


James Peto

I refer you to Youtube Site for MJ Sailing.
These two have had an electrically powered sailing yacht which they fitted out themselves some seven years ago. going through various iterations of motor and batteries. They have sailed it from the Americas – Caribbean – UK – and are now overwintering on it in Norway. They Sail have a Windvane,refrigeration and a watermaker. Seems to work pretty well.

Dave Warnock

I think you have got MJ Sailing mixed up with Sailing Uma

James Peto

Quite right, I realised the error as soon as I pressed send, then went back to change but to late.

Russell Gurstein

Off the shelf tech may not be there yet but I would not throw in the towel until I visited this company ( They just may be able to make this all work with their custom very high efficient (98.7%), high tech patented no bearing neodymium magnets with reverse polarity electric motors, generators, alternators, wind generators, patented wind turbines, custom batteries and many other patents. Plus high energy output at low wind (3mph) breakthroughs and more. They appear to be leading the way where others have failed using their electric drive and generation technology offering sustainability solutions from very small up to 1 gigawatt single wind generation for 750,000 homes and custom design availability.

Matt Marsh

I would be *extremely* cautious about getting involved with that firm. There are so many red flags of sketchiness there that it’d take me longer to write them all up than it took that outfit to make their GoDaddy website and lease their nondescript little office.

Dave Warnock

I follow the logic that leads you to the conclusion of a new boat, either a fast cat or fast long monohull.
However, I think there is a hole in the logic. These boats are so expensive that they all have to be sold as luxury boats that are expected to come laden with lots of luxury equipment such as washing machines, multiple fridges and freezers etc. So whilst they have good generation they also have much greater consumption. I doubt that many in that luxury target market will be willing to be frugal enough with usage to manage.

The other challenge is to balance the increased regen and solar area of a catamaran with the increased consumption of needing a powerful electric autopilot rather than a windvane. I suspect that in many cases the electric autopilot use will exceed the extra generation.

Dave Warnock

Point accepted re have it all. My point is that is inevitable when you look at such high budget boats. People spending multiple millions are not well known for forgetting luxuries and the boat manufacturers know that.
For me the far more interesting stuff is happening among the people people buying boats for under $5000 with non functioning diesel engines and very low budgets.

Dave Warnock

“That said, looking at the prices at Oceanvolt and other electric boat companies says to me that a DIY small diesel replacement is way, way, cheeper than any electric option that will actually work.

Can you clarify the last part of the sentence. I’m not sure I understand it.

Our electric motor, controller and frame was similar to a replacement diesel.

We didn’t price complete fuel system but it would be less than total battery cost. Anyway we would have needed lots of batteries.

We chose a capital expense to reduce running costs (fuel, parts servicing). Confident that the payback will be only a couple of years.

Dave Warnock

Well hopefully we can get back to the refit next month and will be able to get real figures in 2022 when I have a 3 month sabbatical and we are aiming for the Faroes or further.

Michael Lambert

I applaud the efforts to figure this out, but perhaps the first step isn’t to make sailboats more carbon neutral, but powerboats. The shear number of small houses being plowed through the water, creating the telltale wake is insane. Why can’t they be efficient cats? It would be a smoother ride that’s for sure.

Eric Klem

Hi John,

The Pardey comparison is a good one. Afterall, you can cross an ocean with no electrical system at all provided you exercise good seamanship and have a decent sailing boat. The question then becomes what are the needs for safety and convenience. This becomes a very interesting discussion as many people argue powerful engines are safety critical while others feel you should plan to sail yourself out of all situations.

One thing that it took me far too long to learn how to explain was why prop regeneration has so little power output. The most important thing to know is about prop efficiency. Most of the props people run peak somewhere around 50% efficiency when going ahead and often run far below that. So if you want to compare propulsion power to regen power, you have this 50% hit twice and you end up with a 4X spread. That doesn’t include the fact that you can’t actually extract all of the power regenerating as that would stop the boat so you end up extracting a fraction of it. How much you can extract can be seen from the hull drag curve which also tells you the sail power availability. Down at lower speeds, the curve is relatively linear so you can only extract a small fraction of the power without hurting your speed too much. Once you get close to hull speed and the curve gets a very steep slope, then you can start to pull out meaningful power with a reasonable impact on speed. Unfortunately, thanks to prop efficiency though, your ratio of time spent doing regen to time spent powering will never be very good even if you manage to keep your speed up.

One interesting way to think about this in my opinion is whether you would be better off using prop regeneration or just sailing faster. If you pretend that you will have wind for a certain length of time and then no wind where you have to motor, you will find that unless you are sailing very close to hull speed, you will cover more ground by simply sailing and not doing any regeneration. Of course, this is far from the only scenario that you will run into but it is a helpful way to think about it. You can think about this as the sails provide a certain power output and you can choose to use it for propulsion now or to charge batteries to be used later and the reasons why you might save it for later are that there may be no wind later or you are using power really inefficiently now due to approaching hull speed.

If grid power is available such as for people doing weekend only cruising, then the whole equation changes in a good way. Just like we don’t see solar panels covering every square inch of electric cars, renewable sources tend to be too space intensive for mobile applications.


Vesa Ikonen

Thank you Eric, you managed to explain the fundamental regen dilemma in a very easy to understand way (4X spread & drag curve).

Your logic is quite compelling, and it is hard to see that optimising the system would change that more than perhaps incrementally.

Perhaps Oceanvolt should focus more on solar 🙂

Matt Marsh

Nicely put, Eric. And I think you’ve got it exactly right.

To generate power from a water turbine (or spinning propeller) while under sail, you need to have surplus power from the sails to begin with. If you’re sailing in the speed range where a bit more wind gives you a bit more speed, you do not have surplus sail power, so trying to generate electricity will cause drag, which slows you down. Then you lose half that energy in the water flow -> shaft motion -> electricity conversion, and half of what’s left when you run it in the other direction.

So, for it to work effectively, you need a sailboat that’s slightly overpowered for her hull speed in the most common conditions you expect. That means a larger sail plan and a more racer-like hull form. Or, in an existing boat, it means driving her harder and reefing later so that you can run her at hull speed, with the surplus sail power going into electrical generation instead of being spent on making a bigger wake while going 0.2 kt faster.

Eric Klem

Hi John,

Matt’s response does a good job of answering your question. The power comes from the sails so it is a matter of how much power you can get out of the sails and how little needs to go into driving the boat. From that standpoint, a big rig (or sailing where there is always lots of wind) and a boat with a lot of stability are almost more important than the drag up to a point. A super slippery boat can get away with a small rig and lower stability which is good in the grand scheme of this but not good from a regen standpoint. And while Dick’s way of sailing conservatively may be comfortable and seamanlike, pushing the boat hard is key to all of this.

And if you do that, a smaller or higher drag boat can have decent regeneration output, you just need to push hard and have the stability to take advantage of a lot of sail area. Since you are unlikely to notice a 0.2 knot drop, if you can keep it near hull speed, most boats should do okay, you just won’t get much out of speeds of 1XLWL^0.5 and lower.

There is another thing you need to watch in all of this which is the control algorithm for regeneration. My guess is that the ideal algorithm would know the drag curve of the hull or at the very least the hull speed and also your current preference for how much to regen. Without this info, you either work on a canned program which likely isn’t ideal or need it to be manually controlled which would require constant attention as wind is never as constant as we would wish. I don’t know how sophisticated the controls are from companies like Oceanvolt and Watt&Sea, maybe someone here knows. And then of course, stuff like prop size, pitch, etc are also important.


Dave Warnock

“Not cruising places where long periods of motoring are required to remain safe. For example, Greenland, or pretty much any place in the high latitudes, is out.”

I’ve not recently looked at the numbers from Jimmy Cornell’s surveys. However, it is a pretty small percentage of cruisers who visit Greenland or the High Latitudes.

So realistically this isn’t a very big restriction for the vast majority of long term liveaboard cruisers.

Dave Warnock

Again. Sailing Uma have wintered in Norway and are heading further north in their current videos.
I do not believe that I have the right to kill the Arctic for my pleasure. So if I can’t go without destroying it then I shouldn’t go.

Dave Warnock

Then I’m sorry because I misunderstood you.

Dave Warnock

I’m pretty confident that you count me as an “electric boat fan” 🙂

“electric boat fans pretty much always ignore the carbon hit (and capital cost) of building and fitting out the boat.”

Yet this isn’t true for me or many cruisers out there sailing with electric motors. The examples I look to are all old boats where the diesel engine needed replacing (in our case it was cheaper than replacing the whole fuel system, engine controls and engine mounts – all of which were 43 years old, plus we sold the diesel engine anyway).

When we compare a refit of an old boat, that in many cases was heading for break up, for another 40 years of life then the carbon hit looks very different.

Dave Warnock

We bought our motor from a company specialising in electric cars. Same as Thunderstruck motors sell for boats in the US.

Expecting similar performance but better torque response and obviously massively reduced range.

HPEVS AC-34 Motor Package – Curtis 1236SE-5621 (48v, 600a, 40hp) approx £3600

Add 4 of 300Ah lithium batteries, approx £8000

Add materials for motor frame, belt driven reduction gear, cabling, throttle etc say £1000

Deduct £2300 that we sold the old engine for

So far cost of replacing the diesel engine is £10300

But if we hadn’t switched we would have needed to spend

– 2 replacement custom stainless steel tanks with all new plumbing
– new engine mounts
– new exhaust
– new racor fuel filters
– higher capacity alternator
– new starter battery

So after all that is deducted from the £10300 how much will be left for diesel fuel, filters, oil etc?

I’ve ignored upgrading to a dripless seal, fitting aquadrive and new cutlass bearing as all were needed anyway.

I’ve ignored that the diesel engine needed a lot of cleaning and maintenance.

We didn’t get prices for the tank and fuel system because at that point we didn’t want to spend so much money keeping a polluting diesel engine going. However, we have looked at what other cruisers are spending on diesel and we don’t think the payback is going to take very long.

Dave Warnock


1: Regen. Yes this is supported by all the components. But it might need some programming with specialist (expensive) kit. So we have not been banking on it. Once we have it working we will do real world measurements and graph them. I’d take anything generic with a punch of salt for a DIY kit.

2: Full wiring loom and the motor controller were included in the motor kit. We had to buy the throttle (needed one without a spring), but it is a standard one from the motor controller company.

3: We were having to renew the whole wiring anyway and were already committed to Lithium for everything else (entire gas system was condemned by the surveyor). This was essentially all original and 43 years old. All the lead acid batteries were dead. Fortunately one of our sons is an electrician and so helping us sort all the electrics out.

4: So far first focus is solar. Latest thinking is at
for 1460watts at sea (which can be “reefed” in storm conditions). We will also have 70Amps of 48Volt shore power charging (or reduced to whatever the shorepower can provide). Our thinking re wind generators is not at all settled yet.

Tests of success.
1: Yes. However, we have role models who are close to our goals (particularly Rigging Doctor and Sailing Uma). We are not concerned about needing to give up “luxuries”. Our power demands are a fraction of the Cornell cat.

2: Obviously we think we can. We can add an additional 1000w of solar to be used at anchor if numbers show that will help. We can add wind generators. We can head to sunnier climates. We have decided on a couple of “compromises”. A Refleks diesel heater (with stovetop), as due to Brexit we will need some winters in colder climates. It will heat, provide hot water and electric free cooking (if needed). We will carry a small generator (probably a portable honda style that will be able to at least trickle in charge if we really need it. A 48 volt dc generator is beyond our budget, we don’t have enough space and the efficiency is poor.

We keep having people tell us that in their expert opinion our plans won’t work, then we point them to the thinking on our blog and they typically come back with. Oh you have thought this through.

Dave Warnock

Just to be clear. It don’t think the honda portable generator is more efficient than a 48 volt dc diesel generator. Just that the big generator is a huge investment for something that drives a coach and horses through attempting a low to zero fossil fuel boat.

Marc Dacey

Dave, I will follow your improvements and reports with great interest. Although our steel motorsailer is beyond the spec for easy or logical electric conversion, we have extensive clean generating capacity and look forward to eventually taking our knowledge thereby gained ashore to run an off-grid house. We have two of the Hondas (suitable for providing shore power equivalency if linked), but we use them primarily to avoid inverting our large battery bank’s amp back to AC for the larger power tools we carry. As you point out, they are not more efficient than a standard genset, but they are broadly portable and fairly compact, and can provide several hours of steady power just on their internal gasoline tanks.
Dave, I will follow your improvements and reports with great interest. Although our steel motorsailer is beyond the spec for easy or logical electric conversion, we have extensive clean generating capacity and look forward to eventually taking our knowledge thereby gained ashore to run an off-grid house. We have two of the Hondas (suitable for providing shore power equivalency if linked), but we use them primarily to avoid inverting our large battery bank’s amp back to AC for the larger power tools we carry. As you point out, they are not more efficient than a standard genset, but they are broadly portable and fairly compact, and can provide several hours of steady power just on their internal gasoline tanks.

Oh, and we too have a solar arch, although at just four 135W panels, it is quite modest compared to yours, but it makes a servicable bimini of sorts.

Dave Warnock

Initially we were thinking of the portable generator as increasing the range for returning from the Med via the French canals. But Brexit makes that less likely.
Might also be handy for the Panama Canal.
I didn’t know they could used in parallel.
I’m now wondering if we could run separate shore power connections to two Victron multiplus charger/inverters. That way we could charge at full speed from two more limited shore power sockets. I’d need an expert opinion about phases, earths, galvanic issues etc.

Ann Bainbridge

The points made about usage profile in this Noonsite article might prove interesting to this discussion.

Rob Thompson

As an Oceanvolt owner, but not yet a user until the new cat is launched, a few thoughts come to mind. When I read Cornell’s statements re his EP experience, I immediately thought “something is very wrong here!” The reason for that was that before we went ahead with our investment into the Oceanvolt system, the due diligence I did was pretty extensive. I spoke with cruisers in Europe, North America and Australia, about 8 in all, who all related a very good experience. Why was Cornell’s experience so different?
The first thing I wondered was whether the energy budget had been done carefully enough. At the end of the day, that’s the skippers responsibility I believe, especially with a new boat and new tech and planning extensive offshore passages! I wondered if his expectations were based on reality. He seemed to have very limited solar, no genset & lots of modcons on the boat.Hmm?

Upon checking into this, I think that that while the above unrealistic expectations and inadequate energy budget estimation were true, it turns out there is a “rough seas” factor with the OV Servoprops. I believe the problem is this: when you switch on the regen mode, the motor controller goes through a period of “sensing” the waterflow over the variable pitch prop, before settling the pitch to give maximum output. If the boats SOG is fairly constant, the Servoprop does its adjustment and then goes into regen mode, and you get the output values shown on the OV graphs of their testing. However if the boatspeed is being affected by waves while the Servoprop is “sensing”, the controller is “confused”, and keeps sensing, not regenerating. This is clearly a design oversight on OV’s part, and I believe is being worked on as we speak.

We don’t have any data on Cornell’s actual power usage during his legs, but they must have been using some serious kWs if after 10 hours there was “only” approx. 5 kW NET production.

Also John, I think the 30:1 ratio you came up with for regen output to motoring time is a bit pessimistic. You assumed the motoring would be done at WOT. Why? The power curve for any electric motor shows there is a sweet spot of the curve at around half power, so that is where EP is typically used unless WOT is mandatory. The extra speed you get at WOT compared to half power, is hardly anything compared to the corresponding power consumption, so why do that?

There are plenty of EP cruisers with OV systems who are having a much better experience, but alas they do not seem to write much of their experiences on forums or blogs.

Rob Thompson

Apologies, I plead mea culpa on misunderstanding your analysis. When I read the 20HP 15kW, I jumped to the conclusion you meant it was one 15kW motor at WOT.

Dawi Rissik

Check out “sailing Uma”

Iain Dell

Hi John

I’d love to go electric but the associated costs make it as likely for me as Elon Musk accepting my best offer for Tesla.

However, one area that is affordable and environmentally sensible is regen for tender engines. If this has already been covered then grateful for a steer to the discussion but if not then perhaps it could be?

I use an old yet thoroughly reliable 2 stroke that’s easy to swing around to and from the tender. However, I hate having the fuel for it onboard and I’m developing more of an environmental conscience as I get older. Also, the Scotsman inside me just likes the idea of getting something for nothing so when I saw the new range of engines from Epropulsion with regen I was intrigued. I can sense from the comments here that connecting it to the main batteries would be a waste of time but using regen just to charge itself might make a lot of sense. Is this something that could be covered either here or separately?

The idea is to place the engine on a special bracket low at the stern where of course, it would be bound to provoke ‘interesting comments’ from those unaware of the purpose on the sanity of the owner…

Peter Adamson

As a future Hybrid OceanVolt system owner I too was concerned by what Jimmy Cornell reported. The regen numbers did not jive with my firsthand experience and I was surprised by their inability to keep up with electrical demands.
After talking with someone at OV, it seems that the latter is explained by the lack of solar, my boat will have double that and room for more (an advantage of a Cat). I am thinking that they put walkable panels on, but I also redesigned mine b/c the OV system is centered on the 48v and inefficient.
The regen issue sounds like two different issues (1) how they engaged the regen and (2) the initial programming of the feathering system on the new ServoProps. Problem one is caused by the OV options being non-intuitive and sometimes the regen is not properly engaged. Their regen algorithm also needs to be further tuned different conditions, Inconsistent wave periods will make the prop speed fluctuate and the OV software needs to be better programmed to deal with more variable sea states. But one can just tune down the instantaneous regen and get better average regen (and less drag). It will take more data collection for OV to fix this.
If they had sailed with an OV engineer I am confident those regen numbers would be higher. Unfortunately, as with any software written by engineers for people who are not geeks, there is too much room for user error. OV needs to do more to make things idiot proof b/c not everyone is an engineer like me (and they most certainly shouldn’t need to be).
Just like electric cars there will be a learning curve for all-electric propulsion for cruising sailboats and I laud John for noting that we all need to have more realistic expectations for them. That’s why I chose a hybrid system, it took several years of the Prius before people would even try an EV and an EV still cannot be treated like a gas fueled car. They are only good for certain people who have certain needs for a car. Electric marine propulsion will be the same and 5-10 years behind.

Peter Adamson

Yes. I should have been stronger with my endorsement your statements in the article. Electric motors are not for motor cruising, but they can augment a sailboat as well as a diesel in most situations. That is when I attempted to draw an analogy with electric cars. No matter what an electric car owner tells you, you do not have all the capabilities that can in a gas powered with an EV.

And yes, exaggerations by the marine electric propulsion folks is not doing anyone any good. Realistic data is necessary.

I also didn’t mean to say that regen is not going to slow the boat down. Conservation of energy proves otherwise. I was just trying to say that regen, when done correctly, is not as bad as a fixed prop on a transmission. My experience is around 1/2 knot, but it was short and in fairly ideal conditions. I will know a lot more after a year on our boat.

The nice thing about a Cat is you can use a lower regen setting and still generate more power than a monohull. I can’t wait to experiment with using single regen on the leeward drive to offset the typical weather helm of a cat and see if it offsets rudder drag. Perhaps smaller speed change and decent power regen to boot.

Peter Adamson

As per John’s comment about regen and comparing it to a fixed prop. That’s not quite Apples to Apples. Electric motor regen is a little different.
A fixed prop on a boat, even in neutral is not working with the motor, there is momentum but no feedback when the boat speed changes. This spinning momentum will cause increase in drag when the boat changes speed. A good electric regen algorithm is constantly engaging and disengaging the electric motor to keep the prop spinning consistently. An electric car does something similar when you take the foot off the brake and it is slowing car with the motor for regen. It is nice and smooth, even when you hit the brake.

The result is less drag, not 40%, but measurably less that that of a fixed prop. More importantly the governing SW should, in theory, feather (no resistance) the prop in a lull and wait to turn regen back on until the boat is moving again. Just like battery management, marine electric propulsion is way behind in this area too. However this algorithm is more complicated in water and there’s not all that much data to work with, so it’s going to take a few years for marine electric motor makers like OV to get this right.

Seth Winnick

From a slightly different (non-engineering) perspective, wouldn’t it be great if we could buy biodiesel, thereby drastically reducing our net carbon footprint. I sail and cruise a 1986 Sabre 38 with a Westerbeke 35B diesel. Solar panels totaling 240 watts keep my refrigeration and normal daily electric consumption in reasonable balance on sunny days and I am hoping that a Windpilot will reduce autopilot time substantially offshore.  But my boat, and most of the rest of the existing sailboat fleet, will never be a candidate for full electric. As John pointed out above somewhere, the carbon cost of replacing these boats would likely outweigh future savings. I for one, would be prepared to pay a substantial premium to be able to meet my modest fuel requirements with plant-based biodiesel.  

Vesa Ikonen

Hi John.
While the Guardian article you linked reflects the reality of biofuel production in 2015 and, by and large, what it still is for example in the US (subsidized corn being grown for fuel), it is both dated and frankly, a bit too pessimistic by todays standards.

Yes, biofuels have competed with food production, and still do in many countries.
Yes, they are less efficient than solar panels at capturing the suns energy (and panels again have their own problems if installed on boats in large quantities,as you often say).
However, the second gen biofuels that Guardian mentions in a passing are no longer scifi, but a viable commercial product – waste based, not totally unreasonably priced, and a drop in alternative to diesel in existing engines.

They are used on commercial airliners, cars, and – boats. More on them here by the market leader:

Neste has been critisized in the past for the very things that Guardian cites, and they have heard the message. A lot of the raw material already comes from stuff you can’t eat anyways, and they are very much looking into combining the efficiency of the solar panels and the portability and energy storing properties of liquid fuels.
This is called Power-to-X, which basically means creating fuels from (green) electricity.
Eg. creating biofuel from carbon dioxide in the air, using electricity from the sun.
This stuff is just moving from scifi to commercial reality and drawing a _lot_ of investments in R&D, so give it a few years 😉

You can always argue that burning biofuel releases the same amount of CO2 as fossils, but that is actually an incorrect argument, and I am a bit surprised that Guardian made that error. What counts is how much more carbon you put in to the atmosphere by digging up fossils – not how much is circulating between plants growing and being eaten/burnt for fuel/naturally decaying away (which by the way releases that exact same amount of carbon as burning the plant for fuel), and then being grown again.

While we certainly cannot replace all of the gasoline and diesel in the world with biofuels without ridiculous side effects, the good news is we don’t have to:
most cars can run on electricity just fine, and the regen works fine there. For energy production in powerplants currently burning fossils, there are plenty of alternatives that are actually starting to be cheaper than fossils.

Where electricity is not the panacea is aeroplanes (except for short distances), ships, and of course offshore boats. The last one is so miniscule that it is almost a rounding error compared to the first two in terms of global emissions. But still, at least for now, biofuels seem to be not only a realistic, but actually the only realistic way to sailing ecologically for longer distances – besides your obvious recommendation on sailing more, motoring less 🙂

Vesa Ikonen

Neste is actually moving to produce their biodiesel from 100% non-food sources within a few years:

Seth Winnick

Thanks for fleshing this out. And John, I completely agree that the key is to sail a sailboat and not burn dinosaurs. Aventura Zero is listed for sale on Yachtworld for just under € 1 million, no mention of a diesel generator.

Philip Wilkie

Agreed – I admire those brave souls going all electric, but for the time being the limitations are significant.

But doing nothing may not be an option either; over the next decade the phase out of diesel ICEs’s is going to have a tipping point where the dropping volumes are going to mean unaffordable unit prices to meet the fixed costs of all the processing and delivery infrastructure. There may come a point in the foreseeable future where diesel simply stops being available at reasonable prices.

What I’m paying attention to is the very attractive alternative of what are sometimes called ‘electro-fuels’, liquid diesel alternatives produced directly from renewable electricity and CO2 from the atmosphere. If these become available in the next decade, the problem is solved – we keep using our existing engines and delivery infrastructure.

Eric Klem

Hi All,

Unfortunately I am out of date on the latest on biodiesel in terms of lifecycle emissions, land use, cost, etc but I would love it to look good. At one point I thought it would be a very important bridge fuel but battery tech improved so quickly that for cars and trucks, I don’t believe that we need those bridge fuels anymore and it is really down to planes, ships and those of us who can ride on their coattails. I know Airbus is pursuing hydrogen, it will be interesting to see how planes pan out as they are one of the hardest applications.

One benefit I have found of biodiesel is that it seems to be significantly better for seasickness with smelly exhausts. About 20 years ago, a boat I was working on with a rather smelly 2 stroke Detroit Diesel and side exhausts switched to biodiesel trying a couple of different blends. Previously, motoring in rolly conditions in the fog had proved to be a real killer for many passengers but we observed it to get much better after the switch. I have also run it in cars and pickups and found it to be much more tolerable even at concentrations as low as B20.


Vesa Ikonen

I didn’t even come to think of the exhaust smells and assosiated nausea. Have to do some comparison this season!

Mike Maylor

It seems to me from John’s article, Jimmy’s experiment, and the lively input form the readers that living just on electrical regeneration is currently fraught with difficulty in the real world. For seamanlike self sufficiency, it’s not to be relied upon for cooking, life support and especially propulsion, unless resorting to a back up generator, which kills the zero carbon ambition. What is needed is an alternative way of storing energy that can be converted to electricity on demand, a challenge which has already been solved. Has anybody considered including a hydrogen fuel cell in a system for when amps are urgently needed. This link should take anyone interested to a Yachting World article of December 2020 discussing the use of a fuel cell to boost battery range for electric powered yachts in the 40 foot range…

Vesa Ikonen

Hi Mike,
interesting article. Seems like H2 might be an option to extend the range of electrical propulsion boats in shorter distances.

For longer distances, there are many things that may make it unfeasible:

while H2 has a way better mass density of energy than diesel (about 3:1), it has a way poorer volumetric density (1:6 at 700 bar/10 000 psi, and still 1:3 as liquid H2) storing large volumes of gases in confined spaces requires huge pressures -> costly & heavy tanks. The article you linked said 100 kg tank for the equivivalent energy of 50 liters of diesel. I bet the tank costs a lot more than a diesel tankYou really need a sizable battery bank in addition to the fuel cells, since fuel cells are way slower to ’rev’ than diesel or electrical engines. Thus, a fuel cell system likely needs to be a hybrid system (FCEV or Fuel Cell Electrical Vehicle), rather than directly running the propulsion.There isn’t an infrastructure to distribute H2 in a way that would compete with diesel distribution – and likely there will not be either: as long as major car makers do not produce huge volumes of FCEV cars, nobody is going to invest billions to build a parallel network of very expensive gas stations (think high pressure explosive gas of very small molucules that actually slowly flow through metal tanks). Quite a few auto makers are quitting their FCEV programs – Toyota is a notable exception. This not to say that hydrogen will not be used more in future land based energy systems (eg to replace fossil LPGs) – just that it is not likely available in a gas station near you at a random marina. Biodiesel easily could be, since it is a drop in replacement – both for the infra, and your current engine.

Emile Cantin

There are already a ton of comments so this is probably going to get lost, but if I had to repower, I’d really consider going series-hybrid.

I think having electric propulsion makes a lot of sense in a lot of usage patterns, especially if you sail a lot. A very strong advantage is that it avoids the “only run 15 minutes at a time” pattern that kills so many diesel engines.

Combining that with a properly-sized* generator would let us only run the generator for longer trips. Of course, there’d be some efficiency losses in this regime, but from what I understand they’re relatively small.

There are also a lot of off-the-shelf options for generators, which would probably bring the cost down by a large amount (think Honda 2200EUs), and be a lot easier to service / replace than a diesel engine.

(*) It’s hard to find good info on what a good-sized generator would be to propel a boat. If the marine engineers here could post a back-of-the envelope equation I could use, that’d be awesome.

Dick Stevenson

Hi John and all,
You have made some aside comments lately which, to me, are deserving of emphasis: the benefits of an electric outboard. My 4-year-old Torqeedo is light enough to not need a hoist (as my previous gas-powered outboards needed (this is in part, because the Torqeedo’s battery is easily removed and transfer into and out of the dinghy can be done in pieces). It is a pleasure to not have gasoline on board and to not mix oil in when that was called for.
My best, Dick Stevenson, s/v Alchemy

Emile Cantin

Thanks John,

I did take a look at that spreadsheet, and from what I understand the propulsion power only depends on max speed, not boat size / weight? If that’s the case, we can probably downsize the generator a lot, as it doesn’t need to punch through waves; the batteries will supply that temporary power.

Anyway, I wasn’t thinking of removing a perfectly good engine in order to go hybrid; it’s just that I’m realizing that a basic hybrid system (off-the-shelf “marine” motor & controller + off-the-shelf gasoline generator from the hardware store) can be had for less than half the cost of a repower: ~3k for the motor & controller, ~1.6k for a 10kW generator (at that price, you can buy 2 for redundancy!) and put the rest towards batteries (maybe you can score an EV battery from a junkyard!),

Add to that the fact that I’m much more familiar with electronics (I’m a computer engineer) than with engines, and I’d be comfortable enough to DIY the hybrid system. In my (short) experience, diesel engines break much more often than electronics, so reducing that part to an easily-replaceable block with some short-term redundancy via batteries is very appealing to me.

Good point about outboards, though, standardizing on one type of fuel is always a good thing.

Eric Klem

Hi John,

I am not sure I am totally tracking your wording on peaking and serial hybrids. For peaking in a hybrid, I define it as going above the steady state design point which is typically the fossil fuel engine peak or close to. If designed for it, I think that they can peak but it is different than how a parallel hybrid would peak. In the case of a serial, the drive motor needs to be sized to take the full peaking load and then it can handle the normal input from the generator and also some additional peaking power from the batteries. This means that most of the time it will be running at a relatively small percentage of max power which is not nearly as big a deal for an electric motor as for a diesel engine in terms of efficiency but you do need to lug around that weight.

Otherwise, I agree with your thoughts.


Eric Klem

Hi John,

It is hard to know what the real breakdown of how electric and hybrid systems out there are designed but I strongly suspect that you are right that most are not sized to push into big waves. At least what I have seen online (I can’t bear to read much of what is written as it quickly runs up against physics), it seems that the desires are often cost or principle driven, both of which I would expect to result in systems that lack peaking ability. And you are also right that short of a massive battery bank or a generator far bigger than optimal for most conditions, the peaking power is not going to last for very long. While peaking is an incredibly small percentage of your usage, it is also a very important one and it determines your max shaft power so starting with this and then figuring out how to optimize a system around it is key. And it is also true that many (most?) diesel installations are too conservative here resulting in unusably high peaking power and lower efficiency at other operating points.

A major reason that electrics and hybrids work so well in cars is because of peaking requirements. The penalty on having a higher power output motor is relatively low and when comparing average output to peak output, the difference is huge, like 10X. Also, the amount of time spent peaking is incredibly short with units of seconds usually as opposed to hours sometimes in boats so the total energy is modest meaning that batteries of a reasonable size can handle it.


Eric Klem

Hi Emile,

The spreadsheet gives numbers for a Malo 46. If you have a pretty good idea of the drag curve of your hull, then you can replace it in the appropriate tab and it should work just fine for your boat provided it doesn’t put you outside of the bounds of the model. The original intention was simply to create a comparison tool to let people play with different scenarios and see what was optimal and what could be done to improve or make all scenarios worse. From that standpoint, scaling to a different boat size is not important although a major change like going to a planing boat or a cat would obviously make the comparisons no longer valid unless the drag curve is changed. My feeling is that if someone is going to design their own hybrid system, they need to have the knowledge to do this type of modeling themselves so it was not intended as a design tool but simply as a high level comparison tool. For this type of project to be successful, you need clear requirements written down ahead of time and if they are found to be mutually exclusive during the modeling stage, then that must be worked out. That said, the internet is full of people embarking on hybrid or straight electric projects who have no idea how to run these sorts of numbers and they end up with systems that do not meet their requirements if they ever knew what those were. There are also those who do a good job of modeling beforehand and they get setups that work for them (and not always with the type of drivetrain they initially expected to end up with). If you have specific questions on how to modify the model, if you ask them on that post, I will do my best to help you with how to do it.

One other thought is that this is intended for people doing longer distance type cruising. As a result of that, the assumption is that installations would meet ABYC, be long lasting, etc. While it is possible to buy a hardware store gas generator and make a few kW of power with it for a thousand hours, I would not consider it an acceptable solution for longer distance cruisers. I strongly suspect there is no way to make it compliant in a below decks installation due to stuff like ignition protection, exhaust, etc and an on-deck installation is not acceptable to me beyond the local lake. While I agree that in many ways pure battery electric installations are simpler and should be on par or more reliable than engines, once you go hybrid the engine is the clear winner in both these categories to me as there is going to be the engine there anyways.


Eric Klem

Hi All,

I have been meaning to look at the lifecycle numbers of electric boats again as the numbers have changed a lot in recent years and this post got me to finally do it although it did take a few weeks to get to it. A common reason for going electric would be environmental and I believe that the right way to look at this is as a lifecycle analysis (and for the purpose of this I am using carbon dioxide equivalent although there are other emissions too if you want to really be complete). This has been getting a fair amount of press recently in cars and while the numbers used in different analyses can vary a lot, for almost all use cases, battery electric vehicles are superior to internal combustion and in many use cases, they are far superior, like 3X+. So that raises the questions of sailboats. For the sake of the argument, I am ignoring things like cost, weight, size, etc and just looking at GHG emissions. This looks at it from the standpoint of continuing to cruise the same way that we currently do on our CS36T as opposed to behavior modification which is always a tricky subject. Like anything, this is very sensitive to use case so these numbers likely don’t apply to many readers here.

To get the same range as we currently have and at the same speed, we would need a battery pack that is 10kW*60hrs (to dry tank) = 600 kWhrs which is huge, like 6-10 times a Tesla. Let’s assume that we use 100% green energy to charge the batteries too so we will ignore charging in the analysis.
6800 lbs CO2/yr from battery production (600 kWh * 170 lbs CO2/ kWh /15 yrs) This is one of the most important numbers here. I can find reputable citations for as low as 135 lbs CO2/kWh in ideal conditions and a few years ago, people were using numbers in the 330-440 range. Also, the lifetime is very important, how these batteries age out is a big assumption that we won’t have good data on for a while yet.
0 lbs CO2/yr from battery usage
?? motor and other components
?? end of life

2240 lbs CO2/yr from diesel usage (22.4lbs/gal * 100 gallons which is the most we have ever burned in a season, typically we are around 60 so I am being really conservative)
933 lbs CO2/yr from engine and ancillary production assuming a 30 year life (28000 lbs CO2 / 30 yr). I had to do a fair amount of interpolating to get to this number but it should be pretty good and it is not a huge contributor so some error is not a big deal.
?? end of life

Comparing these scenarios, the yearly CO2 emissions of the electric CS36T operated by our family would be 6800lbs and for the diesel would be 3173 which means the electric is just over 2X worse, not good. But the first thing we need to do is look at the assumptions and looking at the single biggest driver which is the battery embodied energy is a good place to start. Range is a big one. Given the way we use the boat, we have cut it pretty tight on range a few times (like motoring back from Shelburne NS to Massachusetts or St. John NB to Mass when there were days on end of flat calm and jobs were beckoning) and we would have to severely curtail our cruising if we gave up more than 1/3 of it. Speed is also a big factor and I might be willing to slow down to 5 knots and use maybe 6kW but below that would again restrict our cruising and is likely not realistic given that we often motor in not flat calm conditions. If purely electric, I think that we could live with 300kWh and have manageable effects on where we cruise (assuming charging magically became available in out of the way places) but that is still a huge bank with lifecycle CO2 emissions that are equivalent to diesel. If these are your only 2 options, then the only great way to decrease emissions is behavior modification and an electric with a significantly reduced pack size and also much more charging infrastructure.

One option that can work is a hybrid where shore power is the main power source and the diesel is really only a range extender for extreme cases. Using numbers that are only educated guesses and not really analytical, if we pretend that we go with a system that has a 40 kWh battery and a diesel that is half the size, our embodied energy will look much more favorable. Then if we assume that we can displace 2/3 of the fuel used with green power, we could get to yearly numbers of 1666 lbs CO2 which is just over half of the number for using a straight diesel, a favorable result but still not perfect. Of course, there are many smaller issues to be solved with this such as what the charge source is, making sure you always have peaking power when needed, etc.

This is a good illustration of the issue of peaking, this time in terms of range. In a given season, it is not uncommon for more than a third of our fuel burn to occur in one stretch with no ability to fuel up or recharge in between because we really do try hard to sail but sometimes there’s just no wind here and we do have time constraints. The issue is that to have the desired range that is only used very occasionally, you have to make and lug around a giant battery of which a small amount is being used most of the time whereas the equivalent range of diesel has much less CO2 tied up in it. What you really want is a usage scenario that is very constant allowing you to use most of the battery capacity on a regular basis. The weekend cruisers who never stray more than 20 miles from home but do motor a decent amount are a perfect example of the right usage scenario. In the electric car world, the increasing range is helping to sell cars but eventually, we shouldn’t all be driving around in super efficient vehicles with 100 kWh+ battery packs that have ranges of 500 miles. If you have to stop at a fast charger for half an hour less than a handful of times a year and can run say a 50 kWh pack, that is much better. The off-grid houses figured this out a long time ago realizing that they shouldn’t design a solar and battery system for the worst case imaginable as it is grossly oversized for normal use and as a result, many have a small genset to deal with those absolute worst cases which is still better from a carbon standpoint than a ton more panels and batteries. Hopefully the making and disposing of batteries will be improved environmentally and there will be more charging options both of which have a big impact on all of this.

I hope this is helpful to the conversation. The point is that you should always run the numbers to the best of your information and then look at how to optimize, you usually get to a much better outcome than using blanket assumptions. Of course we could all stop owning boats but I don’t want to go there.


Marc Dacey

These points are very worthy of consideration and illustrate (thanks for running the numbers) how real-world usage patterns, combined with those pesky time restraints, deeply determine the cost of things when environmental factors of manufacturing are combined.

It’s why we will stick with diesel, because electric is not an option. On the other hand, our large house bank and four panels of solar and a 400W turbine means we neither have to carry an inboard gnerator nor run the diesel to charge the batteries…yet. Thanks for the sobering analysis and the reminder that technology might make one’s lunch, but it’s never free.

Martin Hald

Great topic to discuss. I would think most sailors would love to get rid of the sound of a combustion engine. The electric engine can easily suit the needs of most daysailors, but the paradox is that the development is right now in the ocean cruising community where the demands are much different concering range og reliability.

Alot of good arguments and nuances have been made to the topic. Personally this technology inspires me and the concept of a regenerative propulsion system is mind blowing cool. So some will argue we are not at the point where changing to electric is realistic. In part this attitude is set up on a way of sailing as well. The YT couple sailing UMA have shown that changing the sailing mind-set makes i possible. And fair enough that not everbody can or wish to adapt to the existing technology.

A danish adventure cruising Garcia Exploration called Anemis have installed a hybrid system to adresse the cruising demands. People should check their boat out and their evaluation of their system. They may still use a little fossil fuel, but it is so little and in a system that is much more efficient and delivers so many more advantages.

Kind regards
Martin Hald
Sailing Skjoldmoen

Martin Hald

Hi John
I agree the numbers should do the talking 🙂
But if we do talk this Hybrid setup they have on Anemis as a case study I think it is fair to say they do not save on anything. They use dive compressor extensively, electrical water heater, watermaker, inductionscooking and more. Sure the numbers for having these luxuries are ofcourse that there needs to be either a huge solar production or a gen-set. The 2000 liters are maybe not a fair number in the discussion on weather hybrid or diesel is a more clean option. They used 500 liters of diesel crossing the Atlantic and back in 6 months. The 1500 liters they used in the Med where they had no wind most of the summer cruise. The way I see the numbers( iam no expert, so correct me) the hybrid system without the re-gen is a more efficient setup that still delivers good range.

Also a point I do not understand, that maybe some in here can clarify is: Oceanvolt and others over and over again claim/explain that a Dieselgenerator is more efficient that a diesel motor, because the RPM curve is set at the maximum efficiency.See the link under the FAQ

Ernest E Vogelsinger

Do you refer to their FAQ which states “Electric engines deliver […] more efficiency (93%)”? This means only they convert 93% of the (electrical) energy they are fed into usable propulsion. A modern diesel engine delivers about 43% of thermal efficiency. Set aside the electrical losses during the conversion, this means you get 93% of the 43% of the diesel genset (which is roughly 40%).
Anyhow, as no system is perfect and losses are unavoidable, a diesel genset usually operates at 30% efficiency (source:
I understand that OceanVolt tries to show nice numbers as they want to sell their (quite expensive) systems. But if you crunch the numbers the picture will change quite fast.

Eric Klem

Hi Martin,

I would disregard many (most?) claims like that from Oceanvolt, they have many claims in their FAQ that simply violate the laws of physics or are misleading. For example, they harp on about how torque is more important than power and that their 10kW system “easily outperforms and is more powerful than a 30hp diesel”. This is just easy math to prove wrong, 10kW = 13.4hp period, they are both units of power so you can just convert, it doesn’t matter whether a diesel or an electric motor makes it. By the way, if you want to know how fast your boat will go, it is all about power to the prop, torque doesn’t tell you anything without rpm. And if you know power to the prop and the prop rpm, you also know the torque, again it is simply a calculation defined by the laws of physics. Now, it is entirely possible that someone will install a 10kW Oceanvolt system in place of a 30hp diesel and feel completely satisfied with the performance but what that really tells you is that the diesel was not properly sized/set up or that it was designed around an emergency design point which they have yet to test with the Oceanvolt. The number of improperly setup diesel engines out there is staggering and there are huge gains to be had but they are had not by going hybrid but by good design.

To your specific question, it is true that a typical straight diesel installation is not operating at its optimal efficiency during normal cruise. The installations that I am aware of leave up to about 15% efficiency on the table so if you can do something to change the engine operating point, this is the total possible gain. Of course, you could really screw up a diesel installation and leave a lot more potential improvement but then the answer is that you should fix this. If you try to get that 15% back with a hybrid, this means that if you can move the engine to the optimal point, then you still need to get very high efficiency on all your components for a relatively small gain. The reality is that you rarely can move your operating point to the exact optimal point, you can only get close. I have yet to see a hybrid system that actually keeps the losses under the total 15% there is to gain (I don’t spend hours a day searching through everything but I do pay some attention to people providing credible data but if they screw up things like hp vs torque or kW vs kWh, I don’t pay attention). If you want to dig more into this, the answer lies in what is called a “fuel island plot”. You can find a few on here under some of Matt’s posts and in the spreadsheet that allows you to compare hybrids, electrics and diesels.


Martin Hald

Thanks Erik, Ernest and John for the comments. I have tried to read up. I can see alot of aspects are important to take into account when talking efficiency on engines and e-motors.

I just found an explanation to serial hybrids and efficiency in Nigels Calders Mechanical and Electrical Manual on page 93 og 94. He states the theoretical cross over speed where the diesel is more efficient is at 6.8 knots. This is when the gen-set powers the motor directly. In a longdistance cruise this could be the case. If the gen-set powers the batteries, that then power the motor the cross-over speed is 5.4 knots.

I can see that the diesel still wins this efficiency battle. But not by alot. Many would argue that the price difference is worth a smarter eletrical power system on the boat and the diesel savings. From a safety og range aspect the serial hybrid answers a cruising boats demands.

Nigel Calder concludes on page 95 – “To acheive overall efficiency gains, a serial hybrid has to be operated more or less continously at speeds below the cross-over speed. Typically, for this to happen, the crossover speed must be higher than the boat’s normal cruising speed, which it often is not, resulting in a net loss of effiency.”

When the Re-Gen on motors starts to work, I think we will see many will choose serial hybrid because of a combination of smarter total power system on the boat, the self sufficient nature of both the re-gen and big solar arrays. Plus the large portions of quiet electrical motor sailing.

And now that facts are important. It is a fact that we will need to find other ways to propulsion than fossil fuels. It may not be perfect alternatives and they clearly come with their own set of challenges. But we have to see that the fossil era is ending, just like horses went obsolete some time ago 🙂

Eric Klem

Hi Martin,

I forgot that we can now post pictures. Here is an example fuel island plot from the Hymar report. As you can see, cruise appears to occur at better than 270 g/kWh with a “matched propeller” and the max point for the engine is 230 so about 15% off the absolute max. If you do the max efficiency at the same power point as 2400rpm on the “matched propeller”, you can only get to 240 g/kWh so in truth the difference is only 11% as you have to match power. If you motor more slowly like 2000rpm, the equivalent power point is close to the optimum point but it raises the question of whether the engine is oversized too much.

One other quick thought is that if we really want efficiency and CO2 equivalent reduction, the answer is a multi-speed transmission if all energy comes aboard as diesel and most of the energy goes to steady state cruising. Doing this, you could pick up almost all of that 11% margin noted above. Not enormous but definitely meaningful. The sad truth is that you can get far more than 11% on a lot of boats just by getting the prop sorted out and keeping it clean so that should always be the first real step but it just gets you back to baseline.


Hymar fuel island plot.png
Henry Rech


I’ve come to the discussion late and have only scooted thru some of the comments so I apologize if you go over old ground in answering my questions.

Can you say why a catamaran was chosen as the test vessel?

Which would perform better at sea on average, a monohull or catamaran, that is, which would be better able to average a higher velocity made good in ocean traversing conditions (I presume a catamaran will well and truly under perform upwind)?

Have much did solar power contribute to the energy supply to Cornell’s test. Given that current solar cell technology recovers only about 20% of the energy impinging on it, would a substantial increase in solar cell efficiency change the equation much? (There is new solar cell technology being developed which could double the efficiency.)

Henry Rech


“..there are basic physics limitations that make it unlikely that we will see panels with 40% efficiency at a price that makes them viable for any use other than cost no object space programs.”

Have a read of this:

But as you say, it probably doesn’t matter anyway.

Henry Rech


“On the other hand a well designed cat kept light will generally beat a mono …..”

Would this apply to ocean sailing conditions?