The Offshore Voyaging Reference Site

Efficient Generator-Based Electrical Systems For Yachts

In my induction cooking article I shared the results of the experiments I conducted that showed that electric cooking with an induction cooktop and small oven powered from renewables was not practical, at least without festooning our boats with an unseamanlike array of solar panels and wind generators, and marginal even then.

And therefore induction cooking is not green, at least unless we are willing to settle for very limited stovetop cooking, and no baking or roasting, as well as cutting our other electricity use to the bone.

And given that, I realized, particularly after the excellent discussion in the comments, that much (maybe most) of making a rational decision about switching to electric cooking is dependent on understanding what’s required for an efficient generator-based electrical system.

And then I realized that induction cooking was a red herring since most of this analysis will be relevant for any live-aboard cruiser who is considering a lifestyle that will require more than about 250 amp/hours at 12 volts (3 kWh) of electricity daily.

I emphasize “about” because numbers like this vary based on usage, and user, profile. For example:

  • the number where a generator is required is less than 250 amp/hour for people like Phyllis and I who abhor the clutter, windage and vulnerability to storm damage of more than a couple of decent-sized solar panels securely mounted to an arch;
  • but probably more for those cruising trade-wind areas, where wind generators can do more, assuming you are willing to take the windage hit and put up with the noise—modern wind generators are quieter, but I don’t classify them as quiet.

So let’s dig into generators and what changes we need to make to our electrical systems to use them efficiently—based on my nearly 30 years of reasonably efficient generator use—and then I will circle back in the next article to electric cooking with solid recommendations for several different use profiles on whether or not to convert from propane.

Why The Big Deal?

At this point, many of you are probably wondering why I’m making such a production out of this. Surely we just install a generator and have near unlimited power, end of story?

True, but in so doing you would be following in the footsteps of most marine generator users of the past: producing a bunch of electricity in the cheapest possible way (capital cost) and the planet be damned.

You see this often on large motorboats where many electricity consuming loads are installed—electric cooking, household fridges, freezers, icemakers, air conditioning, etc.—and the generator(s) are sized to supply the peak loads. This is horrible since there are few things more inefficient than an underloaded generator.

Wait, it gets worse, because someone might want a cup of tea or to turn on the air at any time, these generators are often run for hours or even days under tiny loads—bad for the generator as well as the environment.

The Best Solution

Obviously the greenest possible solution is to simply cut our consumption, including scrapping the idea of electric cooking, to the point where a seamanlike renewables array can do the job, perhaps supplemented with an occasional charge from the main engine; however, if you do the latter every day, you will be greener with a generator (done right).

Bottom line, a fundamental that often gets missed in all of our excitement about cool technology is that cutting consumption is always the greenest option.

A Better Solution

But given that, most cruisers still seem to want one or more of the following:

  • Electric cooking
  • Washing machines and dryers
  • Big freezers—Phyllis’ and my environmentally bad habit
  • Watermakers
  • Air conditioning
  • etc.

The good news is that in recent years smart yachties, like our friends Steve and Linda Dashew and others, have been working with good companies like Victron to at least make generator usage more efficient and less environmentally damaging than it is on most boats.

Big Alternators

By the way, while it’s tempting to think that the Dashews supply the huge electrical loads on their new boat with an equally huge solar array, the fact is that much of what makes their generator usage as low as it is, is the two massive ElecTroDyne alternators, one on each main engine, that recharge their huge battery bank every time they go anywhere, which they do a lot, being comparatively restless cruisers.

The combo of solar and massive continuous-duty alternators are a good system for them, and anyone with a larger motorboat, but not one that scales well to smaller boats—not enough space for renewables or the size of battery bank this approach requires—and particularly not sailboats.

Backup Alternator

That said, even the smallest sailboat that makes the transition to a lot of electrical consumption and a generator should have a large and properly regulated continuous-duty high-output alternator on the main engine for backup and to save a generator run when the engine must run anyway. See Further Reading for recommendations.

What We Need

OK, enough lead-in and myth busting. What do we need to use a generator efficiently?

Login to continue reading (scroll down)

More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. 11 Steps To Better Lead Acid Battery Life
  36. How Hard Can We Charge Our Lead-Acid Batteries?
  37. How Lead Acid Batteries Get Wrecked and What To Do About It
  38. Equalizing Batteries, The Reality
  39. Renewable Power
  40. Wind Generators
  41. Solar Power
  42. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  43. Battery Monitors, Part 1—Which Type Is Right For You?
  44. Battery Monitors, Part 2—Recommended Unit
  45. Battery Monitors, Part 3—Calibration and Use
  46. Battery Containment—Part 1
Inline Feedbacks
View all comments
Dick Stevenson

Hi John and all,
I have lived for most of the last 20 years on an all DC boat (the occasional AC by invertor) and am a strong advocate of DC generators when living aboard: they just make sense when all you want is to charge batteries. And I have reasonable large DC needs with a big freezer and we are not camping and we move slowly, so we spend lots of time at anchor. I also have a 40-foot boat, so large AC generator with a good track record (such as Northern Lights) is just too big while the smaller AC generators seem often to have problematic track records.
So, I have followed the possibilities of DC generation closely. It should not be hard to take a small diesel (Kubota for ex) and connect it to an appropriately large alternator, but the product seems elusive. (Probably a small market phenomenon.) That is essentially what my 12v genset is presently after stripping off a multitude of problematic add-ons that were designed to make the unit trouble free.
I have made my 12v genset work for 15+ years now (and continue to do so) but not without huge effort and much headache (it was an Ample Power unit now out of business, rightly so). ZRD makes a similar design, but getting details out of them was nearly impossible and they would not discuss how many units were built and sold, etc. etc. Hamilton Ferris makes a DC genset which has escaped my notice in the past. When I just called HF for information, the phone was answered by the owner, who was very pleasant to talk with and who answered questions in a lovely easy-going manner. I was impressed.
Which brings me to Fisher Panda, who have a very good-looking design and an impressive marketing program.
That said, I can point to approximate l/l positions where 2 FP DC gensets have been summarily tossed overboard and a handful of other skippers who have sunk impressive amounts of time and money into their units. I do know of two skippers who report few problems with their FP units and their fortune is unexplained, but their numbers pale in comparison to the numbers of FP owners I have known who have had significant issues.
My best, Dick Stevenson, s/v Alchemy

Jeffrey Stander

Hi Dick, I might be one of the FIscher-Panda owners who tossed it; not over the side, but I did get rid of it on eBay because it kept belching black soot into the pristine waters and had never run right since the initial install. The FP people were very supportive, but it simply was a poor (but expensive) design. The last techie to work on it said the problem was in the single-cylinder diesel which needed rebuilding (yes REBUILDING) annually. I think they may have dropped that unit as the one that John suggested is based on the excellent Kubota 2-cyl 15hp Z-482.

I wanted a DC generator with a 2-cyl engine but none was available 5 years ago, so what I ended up doing was building my own DC generator out of a Z-482 (which I marinized with Beta parts) coupled with a large alternator and regulator. I used the wet exhaust system from the dead Panda. My new DC genset works great, but it took a LOT of work to build and I would not recommend doing that unless you enjoy such things.


Dick Stevenson

Hi Jeff,
Sounds good. But agree, fooling in this area is not for the faint of heart.
My best, Dick

Dick Stevenson

Hi Mal,
Yes, I forgot about Polar Power, but in all my years of knocking about and asking questions of boat owners, I have never come across a PP. When looking, way back, they were built tall and with a smaller footprint where my space lent itself to longer with a bigger footprint.
My best, Dick

Philip Wilkie

I think you outline the problem space very well. If you stick to ‘tried and true’ technology, then a generator becomes inevitable. But in my relatively low volume 40ft boat a generator just sucks; it takes up valuable space and the only location it could go, under the cockpit floor, is very constrained for maintenance.

Without lithium the weight and efficiency of the battery system becomes impractical. Without a substantial solar install, there isn’t enough charging. And absent a strong tech focus on reducing the loads wherever possible the total load is too high. So something has to give.

The path I’ve selected attacks all three constraints; one is that I made the decision to go 24vDC Lithium ( 240AH) storage. The money I’m not spending on a generator is going into the best lightweight solar array I can install, plus the largest sodding alternator (an ex-Isuzu 24v 180A truck alternator) I can possibly fit on the engine. All the electrical system is 100% Victron/Wakespeed for complete charge/load management information. The Ozefridge refrigeration is exceptionally efficient, and the autopilot will be a windvane servo/autopilot hybrid.

This results in a boat that is breaks the ‘tried and true ‘ rules, but I’ve also endeavored to keep it as simple as possible. Many of the underlying ideas I’ve picked up from reading here at MC (for instance no integrated charger/inverter). I can also accept this arrangement will sometimes fall short, but if it delivers better than 98% of the time in most realistic scenarios I’ll be happy enough.

Worst case with an alternator this large, I just have to motor sail somewhere for an hour or so and all is good again. 🙂

Lee Corwin

Hi John
Have come to believe any high rpm genset is going to be a constant thorn in your foot. Replaced a high rpm lombardini with a low rpm northern lights with a massive improvement in quality of life. Noise, service and hassle factor reduced with reliability. service life increased.
With two D400s and two solar panels genset runs are decreased to once a week. Experience consistent with Nigel’s experience. Solar does best during the long summer days in New England and wind does best during winters in the Leewards. We oversized our alternator. We make water, vacuum, turn on the AC, charge portable electronics, enjoy music and do what we can to make sure the NL is fully loaded when run. Think if you’re lithium or firefly based you can be fairly green and not destroy your genset by being underloaded.

Steve D

I’ve gone down the DC genset vs. AC genset rabbit hole several times in my career. Each time the issue is DC generators are low volume production products, some could even be described as prototypes if you look at the construction, with all the problems that go along with those. NL veritably perfected their design by the time they built the 10,000th genset, how many DC gensets has FP (I tried one of these) or Polar Power built in the last 10 years?

I help design cruising vessel electrical systems and while I lean toward a mainly DC boat up to a point (these have gens, they are just used less often), and up to about 65 feet, it depends to some extent on the user. Non-technical vessel owners have trouble managing the complexity of large DC, battery, inverter systems, and it’s easy and reliable to simply turn on the genset when you need power.

You mentioned large yacht gensets and underloading. This happens on almost any yacht with air conditioning regardless of size. The genset needs to be sized to carry most common loads and the HVAC, so when the HVAC isn’t running, it’s almost certainly grossly under-loaded. Some larger yachts get around this with load banks, which simply load the gen and transfer heat to the seawater, it’s horribly inefficient, but deemed necessary for the health of the gen engine.

Inverters that can share/parallel with genset loads are one solution, it enables the installation of a smaller genset, as the inverter can carry the start up loads alone, after which the genset can carry the running load. This works, but it means if the inverter fails heavy start up loads, HVAC, washing machine etc. will not work.

Matt Marsh

As soon as HVAC gets involved, loading a diesel generator properly becomes a nightmare. There really is no good answer there, at present. You end up with a 100% generator-dependent AC main bus and either a usually-underloaded diesel generator, or you install a bank of very expensive inverters (possibly paralleling the generator) with a very expensive DC battery bank behind them, or you have a little generator in parallel with a big generator (with some fun control & sync logic between them) and double your maintenance bills.

A gasoline generator is somewhat better behaved under low & intermittent load, but let’s be honest, how many gasoline generators are actually legal and approved for marine use, and how many people actually carry them? Mmhmm, right.

I suspect that in 5 to 10 years, as the automotive industry begins switching to a 48 V main bus, we’ll start seeing marine HVAC and refrigeration based on 48 V DC refrigerant compressors, and ~10 kW DC diesel gensets based on 48 V DC MGUs, originally engineered for mild-hybrid cars. We’ll get a lot of boats with messy, half-baked, multi-voltage DC systems before the industry figures out how to standardize on a 48 V DC main bus plus auxiliary 12 V systems powered by DC/DC converters.

Eric Klem

Hi Matt,

In theory, part of the solution lies in cranking the rpm way back on the diesel to get the load % up because you will run much higher cylinder temps if you crank up the work per powerstroke.  Of course, the problem in this is that it is hard to do in reality.  For starters, as John rightly points out, non-constant speed generators have a poor reliability track record.  Then there is the question of how you maintain the frequency if you are AC or the voltage if you go DC.  To do it mechanically, this means some form of a cvt which has been done in 1-off stuff but I don’t know of any production one doing this (I built a 1-off one of these a while ago).  You can also do it in electronics with the solution being somewhat dependent on the motor type chosen but it can be hard to do well.  

Of course, for really large yachts, the answer becomes 2 generators, potentially of different sizes and you size them so that you only start the second when needed for HVAC loads.  Personally, I have no interest in ever owning a boat that requires this level of systems and it is out of reach financially to almost all of us anyways.  When you are at this money level anyways, underloading can be solved by replacing the genset prematurely too, not exactly an environmentally friendly solution but then again most things about a boat of that size are pretty bad anyways.


John Tully

Im in the process of switching to all electric cooking. This is not a small subject that I can put in the comments .
I had talked to Paul and Cheryl Shard about my generator lithium setup long before the “intregal” setup came to market , its surprisingly similar to my solution ( I have no side loading of the crankshaft which was a main design goal) but I did utilize a separate Yanmar genset engine to drive the alternator.
I also have a 600 watt solar array on top of a hardtop bimini . The choose of induction hobs and the convection oven is crucial , that’s another long conversion to much for the comments page . All this makes throwing my propane bottle overboard a logical and doable option for us . Our intended cruising grounds are to the warmer latitudes so the solar will be of more use there then here in Canada

Andre Langevin

Very interesting. I just received a quote for new solar panels (39″x67″) that will output 380 watts each and i have space for two = 760 watts. So far i have been using 2×250 watts panel and 10 years later i still got 14.5V @ 33 Amp in our northern latitude. And the salesman told me… if you wait next spring, i’ll have the same panel size for 400 watts each.

So with the new 400 watts panels + 2 flexibles it would yield about 1Kw x 5 hours per day = 5Kwh of power that would be used to keep level 8 GC2 batteries yielding about 800 Ah at 50% charge (12v) The basic Volvo D2-75 diesel output almost 1.6 kw (115 A at 15V) if required.

Then enter the induction cooking plate… the small Ikea one run at 1.5 kw on the 110v meaning that i can cook without restrain while under engine or sunny day, the remaining discharge is minimal. I love induction and will use one on board but i haven’t seen a gimbaled electric oven for boat with an induction top yet.

Andre Langevin

Hello John i was puzzled as well and i’m not versed in the multiple way to measure solar panels but the number on the sheets are impressive. Enough for me to change my 500 watts solar controller to a 1000 watts because i will need one soon:

Regarding hours of output i may miss the point… you know the limiting factor is the temperature of the silicon. I have 230 watts panels currently and after 10 years i still measure 30-32 Amp sometimes more when batteries are discharged and the voltage is around 13 v. This is a first time a product perform better than theorical 🙂

I’m anxious to see what you will found on the LG. thanks for pointing out.

Matt Marsh

I’m in general agreement with John’s numbers here. One must be very careful, when looking at solar panels, to pull out the numbers that represent reality. “STC” (standard test condition) figures are only achievable under perfect cloudless tropical skies, with the panel on a water-cooled tip/tilt mount and a good buck/boost MPPT controller for each individual panel.

Andre Langevin

Thanks for the link and the explanation John – another value of AAC which is to learn and share everyday. So indeed these are much more 280-300 watts panels than 380 w in real situations. Good to know.

William Koppe

Hi John,
All batteries take a hit if discharged below 50% but at that level Firefly has 3900 cycles to Lifeline 1000. At 80% Firefly is 1150 cycles to Lifeline 500. So in real world use you would discharge Lifeline to 50% which does compare to Firefly at 20%. Then Firefly is 25% cheaper weighs 1/2 and takes up 1/2 as much space. Usable power is 1/2 of 255 a/h compared to 80% of 116 a/h or 128 to 93 a/h or 27% less. Then we can also charge at a much higher rate and we don`t need to fully charge all the time.
As a long time user of Lifeline I became an early adopter of Firefly and bought through
Ocean Planet who arranged shipping to Australia and Bruce was very helpful in addressing the bank sizing given the massive electric winches we have.
TwIn Electrodyne 150A @ 24v mounted to the 215 HP Perkins and a 12 kw NL generator will recharge in fairly short times. At anchor solar on the coach house roofs will reduce charging times. The altenators take 10 hp.
The generator has a watermaker mounted to it and a 2nd watermaker is run on 240V . That and heating 2 hws should give the generator enough load.

Matt Marsh

Part of the appeal of Firefly’s carbon foam tech, to me, is that it tolerates the *occasional* deep discharge very well.

Normally, you’d figure your on-passage and at-anchor consumption, then use the worse one as the basis for a 50%-80% cycle routine. So 5 kWh/day on passage and 3 kWh/day at anchor points you to 1,400 Ah @ 12 V, or 700 Ah @ 24 V, nameplate capacity.

But with the Firefly, while you do get a hit in total lifespan for repeated daily deep discharges, the occasional one is relatively easy to tolerate. You can get away with sizing that pack for the 3 kWh/day at anchor on a 50%-80% cycle, so 830 Ah @ 12 V or 415 Ah @ 24 V. Then you just draw it down to 40% or even 30% state of charge on those days when the autopilot’s working hard and the solar panels aren’t doing much. Yeah, it’ll age at 2 to 3 times the usual rate on the days when you use it that way, but that should be OK for occasional use on a battery that takes 10+ years of simulated daily cycling to kill in the lab.

William Koppe

Hi John,

Source was Lifeline Technical Manual, grapth at the end.

Robert Withers

Some years ago, I was looking at Nanni engines who had developed an in-line generator. It sat between the engine and the gearbox, and could generate as much power as the engine could put its way. See

Looks like a great idea – why bother with an extra internal combustion engine if you don’t need it? Except that link was from 2008 and I can’t find anything recent about this or similar solutions. Anybody know why it didn’t catch on?

Anthony Mercurio

You encourage us to read on before we consider Nigel Calder’s system. Yet there is no direct comparison. Is the fact that his system would lead to one IC engine the main downfall? Do you or any of your readers have experience or opinions on this system?

Niels Rasmussen

Can’t resist to add this comment I just came a cross on Instagram:

Point of no return… There goes our 6-cylinder Perkins diesel engine. Next in queue is the Westerbeke AC generator. All this will be replaced by electric propulsion from Ocenvolt supported by a large 22kW DC generator and 28kWh lithium battery bank. Welcome to the future.

(It is for a Swan 57 German Frers design)

Dick Stevenson

Hi Niels,
Very interesting. I very much admire and appreciate early adapters/innovators as they allow me to indulge in a favorite activity: spectating.
Thanks for sharing, Dick Stevenson, l/v Alchemy

Alex Borodin

So true…

Steve D

Agreed, reminds me of the series-hybrid craze 12 or 14 years ago when it seemed every power boat builder had a hybrid in the works. When I would ask them why, they would often say, “cruise ships do it”, “tug boats use it”, and that’s it, no math. When I would say, “but there’s no real advantage, no regenerative braking, in fact there a conversion loss, and those vessels have full time engineers, and they need them”, I’d get a blank stare. To the best of my knowledge, every one of those that were built were later changed to straight diesel.

It’s hard to beat the efficiency of a properly sized diesel engine.

Philip Wilkie

Still the example mentioned above is not necessarily all bad. While I agree the conversion losses are the obvious downside, there are two upsides. The now much smaller diesel engine will be running at constant speed at optimum load and efficiency, and the much slower rotating electric motors will run a well matched prop at a higher efficiency. A straight shaft probably still wins at full load, but the gap may not be all that large at lower powers.

Plus of course now virtually unlimited house electrics when at anchor.

I’m not trying to advocate for these solutions now; Steve D is right too many of them were just way ahead of the technology. But in another five to ten years maybe these hybrids will become more feasible as the EV industry gears up with mass produced systems that can be adapted to marine use. Just like the vast majority of diesels we use are adaptations from land based applications.

Steve D



Matt Marsh

I’ve looked into multi-speed marine transmissions a few times, and the answer always comes back the same: Yes, they’re a good idea. Yes, they help keep the diesel properly loaded, thus saving both fuel and maintenance. The only downside I can find is that it’s awfully hard to actually find them on the market in sizes that will fit a vessel originally designed for a compact single-speed marine gear.

I’ve come across a few commercial / fishing installations based on old 3- and 4-speed GM light and medium truck automatic transmissions. As long as you have a separate thrust bearing to take the load from the shaft, and an intermediate shaft with double Cardan or Rzeppa joints between that and the transmission, they work fine and last basically forever.

Every conversion of energy incurs a cost in equipment, in maintenance, and in efficiency. A good near-universal rule of thumb is to only convert energy between forms if the 2nd form is genuinely more useful to you than the 1st. On a cruise ship, where 40% to 80% of your demand is the distributed house load from hundreds of air conditioners, lamps, pumps, etc. it makes total sense to go all-electric. On a yacht, where 90% of your demand is shaft rotation to turn the prop, just keep most of the energy in that form to begin with, and only extract what you actually need for other purposes as electricity.

Eric Klem

Hi John,

Good points.  It took me too long to realize that putting these discussions in perspective of what can be gained is very important.  From that end, if you use a diesel engine as your source of energy, comparing the couple of the fuel island plots and load curves that I have examined of cruising sailboats, there is a maximum of 15% fuel efficiency gain to be had from making the engine operate at its most efficient operating point compared to the normal cruise point.  This means that any solutions to improve the engine operating point which are not highly efficient themselves will be worse rather than better, it turns out that a straight diesel installation is reasonably efficient to start with.  As you point out, one of the few areas which could move the engine operating point but also have very high efficiency itself is with a transmission that can vary the gear ratio.

Of course, this all assumes that the setup is reasonably well done to start with, if you are underpropped, have too large of an engine, or pick an engine where they tried to squeeze out too much power and the power/rpm slope got to low at the top end, then you need to fix those issues before looking at any of this.  These issues can leave you with 30% or more gains to be had.


Eric Klem

Hi Matt,

I remember delivering a boat that had an old manual gearbox, clutch and a separate thrust bearing in it.  I was initially worried about heat as automotive gearboxes don’t tend to have high loads for very long but then remembered that they are pretty efficient so overall temp was not the concern, just localized breakdown of the fluid at the gear interfaces.  Using it was kind of awkward but the principle worked.  My memory was that it was a 4 or 5 speed and that second had the right reduction ratio in drive and that reverse was a bit deeper so you had to use a lot of revs for that.  I hadn’t really looked at this stuff then but it would have been interesting if we could have cruised in 3rd and then used 2nd for when higher power was needed like docking or pushing into snotty weather.

As you know, there is no technical reason why one of these solutions can’t be created, it is just a matter of availability and demand for them. I have not looked recently but am disappointed that there are still not options but not surprised.


William Koppe

When I was designing the systems for Taniele I Iooked at using a Hundested prop and a windmill generator driven from the shaft. The hundested can be rotated when sailing to keep the shaft rotation the same as when engine driven. It can also be adjusted to give variable shaft speed. In light winds or when fully charged, put in feathering mode. The windmill generator can be driven at shaft speeds, so less noise and no requirement for large pulleys to get the high speed that altenators need. In the end I was put off by the hight cost of the Hundested, at A$80K, but particularly, the bronze unit which would have reacted with my Duplex stainless hull.

Eric Klem

Hi John,

Just to be clear, the 15% is for the single cruise operating point compared to the max efficiency of the engine in energy/unit of fuel and not requiring the power to be equal (best case scenario to allow for hybrids with energy storage).  My memory is that I was looking at speeds around 1.1*WLL^0.5.  If you go faster than this cruise, there is typically some modest drop in engine efficiency and if you go significantly slower, it can drop off a lot but this is only regarding the engine.  Of course, the slower you go, the overall efficiency as measured by mpg gets better but that is because the hull efficiency increases by far more than the engine efficiency drops.

If anyone wants to look at this, my memory is that an example can be found in the Hymar report although I don’t remember them explaining how to read it, my apologies if my memory is faulty. Looking at an island plot with a load curve overlaid, you take the highest efficiency on the plot and then divide by the efficiency at your chosen operating point on the load curve and then compare.  It is also interesting to compare your operating point to the highest efficiency point along the line of constant power as that tells you how much there is to gain by decoupling speed and power with a cvt, hybrid or similar (assuming that you do not introduce a conversion efficiency loss).  In order to operate the boat and the engine at different power points, that requires energy storage so accumulator efficiency comes in if you actually want to get the full 15%, otherwise you are looking at something more like 10% if you stay on the constant power curve.

Bottom line is that there are only modest gains to be made if you have a well set up system and run at a normal cruise speed most of the time. If you violate any of this, there can be big gains but then that suggests you should fix the root of the problem with the only hard one to solve being having a use case where you operate over a wide speed range.


Dick Stevenson

Hi Mal,
You make a good point, however most DC gensets I have come across are Kubota based (small tractor/lawn mower diesels) and these units have passed the test of time in land uses. From what I can see it is all the marinization, especially the design for cooling, where water is used rather than air.
My best, Dick Stevenson, l/v Alchemy

Steve D

Interestingly, several years ago, long before the hybrid mania, ProBoat carried an article on the efficacy of multispeed marine transmissions, the author contended they did indeed have significant merit; the author was Nigel Calder.

Steve D

Agreed, and in that case, if I recall, it was sport fishing applications that were being evaluated.
With any of this out of the ordinary gear, and especially hybrids and electric propulsion, I ask the manufacturers, ‘what is your plan for support, dealers, technical training for mechanics and electricians?’. In almost every case it’s nothing or next to nothing save, ‘we will provide remote support, we can connect to the system via the internet’. In mid ocean that does little good. I’ve been aboard some submarines that were less complicated than hybrid propulsion systems that include, among other things, raw water cooled voltage converters. Most of the companies offering this gear simply underestimate the need for tech support, and as a result the user experience is diminished.

Michael Albert

No doubt NL is the gold standard if you want continuous duty. But, for those where weight is an issue NextGen makes a nice medium RPM (2800 rather than 3600) by using a belt reduction gear. So my 5.5 KW uses a Kubota Z482 which is pretty bulletproof and significantly lighter and more compact than a NL, important since mine is in stern lazarette. And while it will never be mistaken for a heavy duty round the clock genset, it’s a nice compromise for charging batteries and running AC- usually 2 hours at end of day pulls summer heat out of cabin before sunset and tops off batteries.

Brian Russell

Thank you for the mention of the idea to install more than one battery charger when running a largish AC generator. I had never considered that! Our 9kw Kubota is on the large side because I wanted a 240V welder onboard (very compact little Miller Multimatic). Last year we switched to 580Ah of Firefly batteries which can accept 0.4C charge, so our 120A Victron Multiplus is undersized. I really like the idea of a second, stand alone redundant charger, probably a Skylla 70A. Victron informs me that as long as the charge parameters are identical there should be no issues with the use of the second charger. It will be terrific to squeeze those extra amps into the bank during our hour long genset time every other day or when making water. Only issue is getting one here in da islands!

Carl Linley

Hi John,
we are coastal power boaters but tend to use the boat more like a live aboard.
it’s funny but I have always looked at the care and feeding of the equipment as a primary goal inadvertently operating in a greener fashion. I have always select the smallest generator possible and plan for best loading. We would run loads sequentially to extend run time for battery charging. Yes we had a 4KW single cylinder FP and it was a horror show. Now running a Northern lights and agree that it’s great equipment. I agree that restraint is key in power usage but am a bit of a hypocrite with a gyro on board now. Even so we load up the genny to 80 % on a regular basis. I think that most owners go with what they have as changing over can be very expensive.
thanks for an interesting article that’s fun to read.
carl Linley

Dan Stinebring

Hi John and others,

I’m sure John knows this, but I wanted to correct something that could be misleading to electrical system newbies. In the “click here” description of this article it is stated that “will require more than about 250 amp/hours at 12 volts (3 kWh) of electricity daily.” The numbers part of this should be changed to “250 amp-hours at 12 volts (3 kWh).” Why? Because, as most of you know, amp-hours is the number of amps flowing in the system times the number of hours it is flowing. Multiply amps times volts and you get watts. So, 250 amp-hours x 12 volts = 3000 amp-volt-hours = 3 kWh, as stated. It would be good to stay away from amp/hours — which doesn’t make sense — to avoid any confusion. This is a small point, but it allows me to be a bit pedantic. And, as a retired physics professor, that apparently is important to me! Keep up the good work, all!

— Dan (skipper of Polaris, an Aloha 34)