How We Got Here
In 1996 our Amel Super Maramu was equipped at the factory with an Onan MDKD-P 220V/50Hz/6.5kW generator. This was a standard, low speed, 1500 RPM, generator coupled to a three cylinder Kubota engine. These small Kubota engines have a reputation of running for 10, 20, even 40 thousand hours. Certainly the one we have has never shown a sign of an issue after almost 8,000 hours. It seemed our generator might just run forever.
The problem for us started last year. We needed to replace the exhaust elbow which had sprung a salt water leak. This is part of the Onan marinization of this engine, not a Kubota part. We expected to pay $400 to $600 for this piece. The first price we got was: $1600. Extensive searching, turned out nothing cheaper. Eventually we found out the issue was that this generator was "obsolete" as far as Onan was concerned. This meant that they were no longer making spare parts, and supply was limited to stock on hand. We have since discovered that several models of generator--some a lot newer than ours--have limited parts availability from Onan.
We had several major maintenance issues coming up which would have needed Onan parts, that were no longer available. This left us with two options: Run the generator until it died a natural death, or replace it now. Running it out until it died certainly would be a help with cash flow in what has been an expensive maintenance year for us. On the other hand, we NEED a reliable generator. Having a generator die in a remote location and needing a replacement could be a very inconvenient and expensive situation with limited options.
In addition, our high capacity (100 Amp) battery charger had proven unreliable, with two warranty replacements in as many years. It needed to be replaced with something else.
Considering all this, we decided that replacement--now--was the better option.
Efficiently Using a Generator
Most AC generators used on boats spend a lot of time, frequently most of their time, running at very low or even close to zero load. They are waiting for something to turn on or they are slowly charging batteries. This is inefficient, and bad for the diesel engine that runs the generator. With modern equipment, we can do much better while maintaining the quality of life features that are important to us.
Victron Energy published a whitepaper on small generators and the results are very interesting. It is well worth a read, for anyone shopping for power on a cruising boat. Across a wide range of small generators (From 3kW to 12kW) the amount of fuel needed to generate one kiloWatt-hour of electricity is independent of generator size when the generators are run at a high load. About 350 grams of diesel fuel is required to make one kW-hr of electrical energy. To put this in units that might be more meaningful, that is 0.41 liters per kW-hr, or about 0.11 US Gal per kW-hr, or at current USA prices for diesel fuel, that is about US$0.30/kW-hr.
All the generators Victron tested from 3kW to 12kW use this much fuel IF (and only if!) they are generating at least 2000 Watts of power (that's about 9 Amps at 220 Volts). Below this number, things get worse very quickly, and the bigger the generator, the worse it gets.
When running with a 500 Watt load with a 3kW generator it will take about 1000 grams of fuel to make a kiloWatt-hr of electricity, THREE times more than when running at full load. If we go up to a 10kW generator at that that same 500 Watts load, fuel usage doubles again, needing 2000 grams of fuel per kW-hour.
In a real world perspective, on our boat we need about 1.5 kW-hr of power per day on average from our generator. If we run an AC generator at its most efficient, that would require about 0.17 gallons of diesel a day, or roughly a gallon a week. The issue is that it is really hard with the system as designed by Amel back in the day to run your generator at its most efficient.
Smaller is Better
So... our first lesson is that smaller is better! From a fuel consumption standpoint, the smaller the generator we can use, the better off we will be (from a fuel consumption standpoint) any time we are consuming less than 2000 Watts of electric power.
There is another piece to this: Any diesel engine will run better, happier, longer, pollute less, and just generally cause less trouble if it is loaded significantly. Diesels that spend a lot of time at low or zero load have a lot of problems that increase maintenance and limit their lifespan. Just like with people, a healthy diesel engine is NOT a couch potato! A diesel run between 35% and 75% of its full power rating is running in its “happy place.”
But... there is another significant issue that needs to be considered. Some kinds of electric loads require a LOT more power to start up than they take to keep running. Electric motors are the typical problem here. An air conditioning motor that needs 3.5 Amps (700 Watts) to keep running might need 7, 10 or even 15 Amps (3300 Watts) to start. Now, if you have a battery charger that might need 12 Amps of steady power, and two Air conditioners that each might need 12 Amps to get started, and... anything else, suddenly you need a really big generator to supply all those loads. And we already know that large generators are inefficient...
But... that was THEN...
How Things have Changed
There are now many, many off-grid electrical systems of a size similar to a cruising boat like a Super Maramu. Many companies are making equipment for this market, and in the past decade a lot of engineering time and money has been invested. Inverters (that convert battery DC power to AC) and battery chargers (that convert AC into battery friendly DC power) have gotten infinitely more reliable and sophisticated. We can now easily, efficiently, and economically, turn AC power into DC power and back again.
In the 1990's this was what the world of power on a larger cruising boat looked like:
ALL of the AC loads were supplied directly by either shore power OR the generator. All the DC loads were run from the battery. Except for the battery charger there was no connection between the two systems. This was the way boats were built given the limitations of the equipment available.
Gradually, as the state of the art advanced, it became normal to add an inverter to the system so small AC loads can be run from the battery without having to start the generator.
Now, just adding an inverter to the system is a fairly small change. But... when larger capacity inverters become cheaper, and more reliable, we have a paradigm shift. We can have the inverters run ALL the AC loads on the boat. Shore power and the generator are relegated to the job of battery charger. Like this:
This might seem like another fairly small change, but it has very far reaching consequences.
Our generator no longer has to be able to supply the peak starting loads for all the equipment on the boat. It only has to supply the average load. Peaks loads are pulled directly from the battery by the inverter, and then the battery is refilled by the charger. The generator can be half the size! This is a huge savings in capital cost, maintenance, weight and fuel usage.
There are also a lot of advantages on the shore power side because with proper selection of a battery charger we have totally isolated the boat's AC system from shore power, without the addition of a seperate isolation transformer or galvanic isolator.
But, if you go this far...
Wait! There's More!
We can take this one step further. There are now very compact, and highly efficient, generators that output DC power suitable for charging a battery bank directly. They gain that size and efficiency by using permanent magnet alternators that can run at a range of speeds. While all of the AC power generators run about 330 grams of fuel per kW-hr, the DC generators run about 275 grams per kW-hr, about 15% more efficient.
Now our imaginary system looks like this:
Now things are, potentially, significantly more efficient. We have essentially created a "hybrid" system.
Just like Toyota made a huge impact in the automobile world with the hybrid Prius, we are working the same way here. A hybrid automobile takes advantage of the fact that a car only needs a lot of power when it is accelerating or climbing a hill. To run at a steady speed, even 65 miles an hour, only takes a dozen horsepower. The magic of a hybrid car is that it has a little tiny engine to supply the steady loads, and uses batteries to pump up the power output when needed. Our “hybrid” electric design uses a small engine (generator) to take care of the average load, and we use batteries to supply the short term peaks.
And still more... with a little design thought and selecting the right gear, the generator can be automated. It can start itself when the batteries need charging, run until they are appropriately full, and then turn itself off.
Design Considerations
Pulling all this together in a way that makes sense isn't easy. There are a lot of things to be considered. First is your “lifestyle objective.” In our case we want a system that requires little or no intervention by human intelligence when we are at the dock. We want to be able to turn things on and off without concern for popping a breaker, or overloading anything. While we are out sailing, we are comfortable with a bit more active human management of the system.
One of our primary concerns is for fuel efficiency. Cost is, of course, part of the reason, but even more important is the ability to be self sufficient for longer periods of time in places where good quality diesel fuel might be hard to come by.
We also wanted something lighter than the original installation. We have always had an issue with a small list to port (the side the generator is on.) Our older Super Maramu carries nine batteries on the starboard side. That’s just not quite enough to balance the 600+ pounds of the generator installation. Later model Super Maramus carry 13 batteries. That extra 300 lbs of lead on the starboard side cure the list. Unfortunately, there is no economical way to add extra battery weight to our boat. Because of this, we are looking to reduce our generator’s weight by as much as we can.
Design Process
First, set aside those loads that you would ONLY use while connected to shore power. In our case this is the electric hot water heater. We will be making hot water from waste heat from the generator, so there really is no need to consume the HUGE amount of battery power it takes to heat water.
We rarely (VERY rarely) run the air conditioners while underway or at anchor, but we decided that keeping that as a viable option is a good idea.
Now, look at expected usages. Here are numbers from our boat. Yours will certainly be different depending on how it is set up, and how you use it. For our design purposes we want to be sure we understand what our highest expected sustained usage is, and our peak usages.
We know from experience the following usages:
At the dock, on shore power:
Normal steady DC load is about 7 Amps at 24 volts. (About 200 Watts). This is mostly to power the freezers.
Maximum steady AC load with two 8000 BTU air conditioners running is about 7.5 Amps at 220 Volts (1650 W).
Maximum expected AC load would be one air conditioner running, PLUS the 2700W electric kettle PLUS the 750W water heater (5300 W or 25 Amps) all while trying to start the second air conditioner. In this situation, our peak load is close to 6000 W, about 27 Amps at 220 Volts.
Underway or at anchor
Our 464 Amp-hr Firefly battery bank is rated to accept a maximum charge rate of 220 Amps at 24 volts (5300 W). Minimum recommended battery charge rate for these batteries is about 90 Amps (2250 W)
We run the watermaker for an average of about 1 hour per day, and it uses 25 Amps at 24 Volts, so about 0.6kW-hrs per day
Average DC usage at anchor is about 6 Amps (144 W, or about 3.6kW-hrs per day, and adding the watermaker, that makes 4.2kW-hrs per day)
While underway we use an average of 11 amps at 24 volts or about 6.4kW-hrs per day, plus the watermaker, gives us 7kW-hrs per day. Underway our AC power usage is usually tiny.
Solar power produces about 3000 W-hour on an average sunny day. (25 Amps Peak output)
Shore Power Design
Our first design challenge is for our shore power connection. We use a 30 Amp/120V shore power connection, which after running through our transformer, gives us 15 Amps/220 Volts, or 3300 Watts. While this is certainly sufficient for our average needs, peak loads can easily overwhelm it and trip a dock side circuit breaker. The primary culprit here is my 2700W electric kettle (I hate waiting for my coffee in the morning!)
Fortunately, we can get more than enough power with modern equipment. Enter Victron Energy, a leading manufacturer of hardware needed for off-grid and power sharing applications based in The Netherlands.
Victron makes a line of combined inverter/chargers (the MultiPlus, the Quattro, and the MultiPlus-II) that sit in the shore power circuit and do a large number of very helpful things:
Use incoming AC shore power to intelligently charge the batteries.
Convert battery power to AC power to run loads if shore power is not present.
With two AC outputs, one is used ONLY when shore power is connected.
A high speed internal transfer switch starts the inverter immediately if shore power is lost.
If the AC load is higher than a preset amount, the inverter draws power from the batteries to add to the incoming AC power to make up the difference.
That last bit is the real magic. It lets us use more power than we have available from shore power to level out the peaks. I can make hot water for my morning coffee without worry about tripping the shore power circuit breaker, or needing to turn off the air conditioner or water heater. The MultiPlus pulls the extra power from the batteries, and as soon as it is finished, it automatically puts the power back into the batteries with no user intervention at all. Like magic, we can get over 6000 Watts (27 Amps) of 220 Volt power from a line rated at only 3300 Watts (15 Amps).
With this set up we do lose one advantage of a completely separate charger and inverter system: We no longer isolate our AC ground from shore power. Because of this we need to add an isolation transformer that also gives us the ability to use either 120 Volt or 220 Volt as our shore power source.
Our Hardware Choices
Once we were seriously considering using a permanent magnet alternator DC generator we sorted through the major suppliers. For a variety of reasons we ended up choosing WhisperPower. This is a Dutch company that was a spinoff from MasterVolt 10 years ago when they decided that making and selling generators was a distraction from their core business.
We chose one of their marine models, an M-GV-2 which can output 150 amps of battery charging current. At 132kg (290lbs) it is about half the weight of the Onan MDKD, and is significantly smaller in overall size.
This system consists of three major components: the engine, permanent magnet alternator, and a battery charger that accepts 3-Phase AC current from the alternator, or standard shore power, and outputs 150 Amps at 24 Volts for charging the battery bank. The engine is a standard, reliable 2-cylinder Kubota diesel
Running at about 2500 RPM, this is probably best catagorized as a "mid-speed" generator falling in between the low speed synchronous units running at 1500 or 1800 RPM and the high speed units that run at 3600 RPM.
Details for the Nerds
Despite everything written above, there is no such thing as a "pure" direct current generator. To get into this a little bit, we need to understand how generators work, but without getting too deep into the weeds.
Moving a magnet past a coil of wire makes a pulse of electricity move through the wire. The stronger the magnet, and the faster it moves, the more electricity we push out the wire.
We can use either a "permanent magnet" which is a hunk of metal that is always magnetic, or we can use pass an electric current through a coil of wire and make a magnet that way.
Most AC generators use electrically generated magnets since they let us control the output voltage by adjusting the amount of current in the magnet coils. With a DC generator, we are going to control everything outside the "box" so we can let the voltage go where ever it wants (within limits, of course!) We can then use a permanent magnet alternator, which is much simpler and more efficient.
Because the "DC Generator" doesn't need tight control of its voltage or frequency it can be a much smaller, simpler machine. Basically they consist of a plate with permanent magnets that rotate (the "rotor") inside a cylinder with the coils attached to it that stays still (the "stator"). Wires collect the power from the coils which is typically three phase AC power of 80 to 300 volts and 300 to 500 Hz. This is fed to a transformer that adjusts the voltage, and then a rectifier to convert it to pure Direct Current that can be fed to the batteries.
The traditional AC generator has current flowing through the coils on the rotor. That means they get hot and need to be cooled. Because they are spinning, the only way to easily cool them is to blow air across them. So the whole generator is air-cooled. Which heats up the engine room.
The magnets in a permenent magnet alternator’s rotor don’t need cooling, only the coils in the stator. Since they are not moving, we can cool them with the same cooling water that cools the Diesel engine. Now all the heat we are removing from the system leaves with the cooling water, and this dramatically lowers the engine room temperature, greatly extending the life of all the electronic parts installed in there.
The higher speed engine does inherently make more noise than its low speed cousin, but because it is smaller engine and does not need to be air cooled, we can now put it into sound-proof box and the whole installation is a MUCH quieter both inside out outside the boat.
Is This for You?
Is this the right solution for you? Well, maybe, and maybe not.
If you find that almost all of your generator time is charging batteries this is an option you should seriously consider. If on the other hand you run your generator frequently to keep the boat cool (or warm!) and are regularly running one or more air conditioner units when away from shore power, or have very large consumers (like compressors for scuba tanks) a traditional AC generator might be the better choice.