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Pick a Generator

The diligent reader will know that so far I have picked a motor and a battery. Now let me explain how I picked a generator. I warn you, this is a more lengthy story, and it includes the inevitable engineering calculations.


The earler blogs about batteries and power budget include a rough estimate of the power requred as 6kW. There is a lot of margin here, because higher power will charge batteries more quickly while lower power will result in a smaller generator but take more time to charge. More sunshine will reduce the diesel generation requirement, but, as this is not predictable in England, I like to do the sums as though there is no solar power, then use it as a bonus. The power is therefore about 6kW, where 7kW would be fine and 5kW would also be fine; we'd just lean on solar for extra support.

Therefore it's a 6kW generator...

AC or DC

You can get Alternating Current (AC) or Direct Current (DC) generators, although in fact a DC generator is just an AC generator that has rectification built in. The choice of voltage, given that we live in the UK and we have a 48V motor and batteries, is either 240V AC or 48V DC. To minimize noise while standing at the tiller, Perseverance has the generator in the bow. Therefore the power has to go the length of the boat. 6kW at these voltages is either 26A AC or 125A DC. It's 16 metres from the bow where the generator will sit to the electrical cabinet where the power will be used. There will be a power loss in the cable and if we accept 1% of the power is wasted we need a cable that is either 10mm2 or 150mm2 (apologies; my blog editor doesn't do superscript). To fit 10mm2 cable costs about £65. A 150mm2 cable would be impossible to bend, so we could fit two 70mm2 welding cables in each direction. This would be 72m of cable, costing about £10 per metre is £720.

Historical note: This is exactly why Edison lost the competition to install electrical power to Westinghouse. Westinghouse Electric built high voltage AC power stations that could transmit power across long distances while Edison generated lower voltage DC power and as a consequence suffered huge losses in the distribution cabling.

Therefore, it's a 6kW, 240V AC generator...

Speed - Fixed of Variable

You can get fixed speed or variable speed AC generators, although in fact a variable speed AC generator is in fact a standard AC generator with rectification built in to create DC and an inverter to follow to convert back to AC. This allows the diesel engine to run at lower speeds when the power demand is low, while the inverter provides a stable output frequency. The drawback is (a) we are now thinking of running the diesel engine at lower speeds and powers, which is less efficient than running at high powers, and (b) we've added a chunk of high power electronics which a simple fixed speed generator does not need.

Therefore, it's a 6kW, 240V AC, fixed speed generator...

Speed - 3,000 or 1,500 rpm?

It's a simple fact that if you move a magnet past a coil of wire, the voltage in the coil rises and falls. With two magnetic poles (North and South) and one wire loop, you get one cycle of alternating voltage for each turn of the armature.

To get a 50Hz alternating current you have to turn the armature at 50 x 60 = 3,000 rpm. This is fine, but the diesel engine makes a significant amount of noise at this speed, so an alternative is to fit another pair of poles at right angles and turn the armature at half the speed. (You could do three pole pairs at 1,000 rpm, but this is getting too slow for a small diesel engine). This is why there are just two fixed speeds for a 50Hz generator, and to help with our goal for silent cruising, the lower speed, lower noise level generator is best.

Therefore, it's a 6kW, 240V AC, 1500 rpm, fixed speed generator...

Weeds and Rubbish

Now we need a little detour. Every canal boat I have been on had a weed hatch. Perseverance will have a weed hatch. You need one to clear, well, weeds, but also plastic bags, bits of string and even Sarees.

Sarees, I hear you cry? When we cruised the Coventry Canal, we were told that after a wedding the bride would cast her wedding saree into the waters, the canal being the closest thing to the Ganges that they have available. We did in fact see one, but it was on the far bank and did not get trapped around our prop. My point being that you need a weed hatch to clear debris you get in canals because canals have debris.

Now, I’ve done some sailing in the Solent (racing with the lads) and in warmer seas (holidays with the wife). In the Bahamas I can remember seeing star fish against the sandy bottom. In the Aegean, sailing fast in a Force 4, we saw dolphins swimming alongside the boat. OK, the water in the Solent is not so clear, but I never picked up any rubbish on the prop on any of these holidays. The key thing is that sailing boats don't have weed hatches because the water is free of rubbish.

This leads to the key question when selecting a generator. Which type of exhaust are you going to fit? Generators built for sailing boats have “wet” exhausts, which are usually installed with heat exchanger cooling like this:

Diagram of courtesy Beta Marine

Generators like this suck up sea water, pass it through a heat exchanger to cool the engine and then spit it out through the exhaust where the sea water cools the exhaust gas and, in so doing, reduces the energy and hence noise of the exhaust. Great if you are at sea. Rubbish if you suck up rubbish.

More background information: I plan to leave Perseverance in marinas for months, some of which may not have power available at the mooring, or it may sit by the towpath for some days. In both cases she will be unattended and relying on the generator maintaining energy on board. It is therefore essential, for me, that the generator can run automatically and without attention.

The obvious conclusion is that a generator with a “raw water” intake may not be suitable for use on a canal. To my amazement, of the nine marine generator manufacturers I researched, eight made only raw water cooled generators. I didn't like the idea of a raw water system, but many of these generators are really small, so I tried to find a way to fit them. I spent time at the Southampton Boat Show with the salesman from one of the main suppliers. He recommended fitting two water intakes so that if one got blocked, the other would still work. Each would then have a filter which could be cleaned by closing a pair of valves and taking it out to remove debris. Two filters, four valves and no guarantee of success. There is a point where you know something just doesn’t feel right, and we had passed that point. I then spent time with another salesmen where I took a different tack, asking if the generator could be adapted to run with a dry exhaust. “No” was the blunt reply “that would invalidate the warranty”. Another suggested fitting a “mud box” where the water is brought into the boat and allowed to settle. You just have to wash the mud out occasionally. Once more, past that point where things don't feel right. So I decided not to fit a wet exhaust, because I could not find a way to avoid the risk of blockage when the boat was unattended.

Therefore, it's a 6kW, 240V AC, 1500 rpm, fixed speed, dry exhaust generator...

I should point out that, since doing this research, our builders, Ortomarine, have built a narrowboat with a generator that does not need raw water cooling. It was a unit I had not considered, as this generator is designed for use on large commercial vehicles. In the design use case the heat from the generator and silencer is output through large radiators on the roof of the cab. On the narrowboat, this heat is output through large skin tanks. This achieves the goal of not drawing water in from the canal, but has a drawback in that heat which could have been recovered, for hot water or cabin heating, is lost.

Through this process I eliminated all manufacturers except Beta Marine. With years of supplying engines into the narrowboat world, they understand the problems of canal life and offer their generators with either wet or dry exhausts. I needed 6kW of electrical power, so chose a BetaGen 7 with a dry exhaust. When we get to silencing, I will explain the virtues of 1500rpm generators, but for now please accept that the 4-pole, single phase, 1500rpm model was the one chosen.

Therefore, it's a 6kW, 240V AC, 1500 rpm, fixed speed, dry exhaust, BetaGen 7 generator. Job done.


Jesse Drops a Clanger

And here is my admission. I made a mistake when I selected my generator. I bought a BetaGen 7 thinking that it would produce 7kW of electricity. How naive was I? To save others falling into this trap, here is a discourse on how generators are specified. I am going to use the BetaGen 7 for my explanation but this applies to many manufacturers of marine generators. Oh, and beware, this is going to get complicated.

The generator specification says:

So, OK, it's a 6.7kVA generator which the marketing guys roll up to be the "7" model. Fine, I get that. But notice also that the "Prime Output" is listed as 6.1kVA at 0.8PF. Now we need to have a diversion into Power Factors...


Power Factor

In an AC circuit, it is possible for the voltage and current to reach their peaks at different times. That is, the graph of voltage going up and down might be ahead or behind the current graph like this:

Useful work is only done when the two are acting together, and in an ideal case there is no time difference between these curves. The "Power Factor" is the proportion of useful energy available compared to the energy that would have been available if there was no timing difference.

Low power factor circuits will typically have large inductive loads (transformers, motors etc) but in our case the load is an AC to DC converter which will have a power factor of 1.0, so this should not be applicable in a serial hybrid installation. the 6.1kVA is a measurement of the voltage and current when they are out of phase to the extent that gives a power factor of 0.8. The actual power in this case is given by 6.1kVA x 0.8 = 4.88kW. Now, power factor depends upon the external circuit, which the manufacturer has no control over, but they budget for an external circuit with this low power factor and in so doing they give themselves an apparent extra 25% on the rating.

I therefore bought a BetaGen 7 thinking that I would get 7kW, only to find that it is actually a 4.9kW generator.

Other Manufacturers

Before you rush away with the idea that Beta Marine are the the exception here, I have researched generators from a number of different manufacturers and computed, for similar generators, what I have called the "Exaggeration Factor". This is how far short of the expected power the continuous power is.

You can see that some manufacturers quote power at unity power factor with no exaggeration. Others apply the AC power factor correction to varying degrees. Astonishingly the last example is a DC generator where the manufacturer still applies what looks like a power factor correction even though power factors don't apply to DC circuits.

My message here is to be very careful when reading generator specifications. Ignore any rating with kVA and only believe ratings quoted in kW. If in doubt, insist on a rating with unity power factor.

Now back to the plot...


Beta Marine to the Rescue

Admitting my mistake, Beta Marine took pity on a foolish engineer and upgraded my BetaGen 7. They took the generator back and fitted a larger engine to make it a BetaGen 10. They did this without fuss, for a very reasonable incremental cost, and in a timely manner.

I knew the name Beta Marine from adverts in the press, and from signs dotted around the canal network, but I had never before been a customer. Suffice to say, they could not have been more helpful. It would have been very easy for them to point out my mistake and chuckle to themselves after putting the phone down, but instead they solved my problem. They also helped me to understand the results of my testing and were kind enough to invite me to their factory to see how they test generators up to 1000 kW on their test cells. Much of the data that I will show in future blogs has been shared with Beta Marine through this process.

Therefore, it's a 6kW, 240V AC, 1500 rpm, fixed speed, dry exhaust, BetaGen 10 generator.

BetaGen 10 in More Detail

I will be looking at the silencing and efficiency of the generator in more detail in blogs to come, but it's worth a quick peep to tell you some of the key features of this generator.

Here is the front of the generator with the silencing covers removed. Access is good, and I made sure that when installed in Perseverance it is still possible to get the covers off for maintenance. Practically everything you need to access is on the front side, with filters, pumps etc. all to hand. Beta Marine spray the entire motor and generator green and while this looks attractive, I found it was tricky to spot the dipstick and water drain tap (on the back of the engine), so I painted them yellow for oil and blue for water. I then got carried away and did the filler caps to match.

The brass lift pump on the right allows you to drain the oil without having to get under the sump. Similarly there is a tap on the back of the engine block to drain the cooling water circuit. The Mecc Alte alternator is air cooled, and there is a significant air flow from left to right, through louvres in the left face, through the alternator, across the engine and out of a duct on the right. A fan ensures a high airflow - of which more in a later blog.

As well as the normal closed circuit engine cooling, a hot water supply is provided for the calorifier. These are the two pipes at the top of the motor and, like all of the connections to the generator, they come out of the left side, making installation neat.

A Plywood Generator

To help my friends at Alexander Boatbuilders and Ortomarine to build this unusual generator installation I made a full size space model with all the right connections in all the right places (Andre Previn, anyone?). It has the distinct advantage that you can lift this generator with one hand! The space model only has a rear silencing panel as this is the tricky one to remove with the generator in situ. Here it is, sitting in the bow locker of Perseverance. See how Jim Sparks at Alexander Boats built the bow with an unusually large hatch to suit this generator.


This lengthy blog has built up the specification for the generator and found the only unit on the market which fits the bill. We had Perseverance built specifically to accommodate this generator in the bow.

In future blogs I will explain its thermal performance, how it will be silenced, how it will be controlled and how it will be started, but I really do think this is enough for now!

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John Band
John Band
Mar 27, 2021

Very pleased to see this - I am working up a design for a serial hybrid narrow boat and had figured out that Betagen 10 should be the optimum generator for my purposes - I was nervous that I was specifying larger than needed - I was leaving a safety margin because I didn't understand power factor. Now I do, a bit, and find I got it right by accident! Will work my way through more of your blogs...

John Band


Kevin Wheatley
Kevin Wheatley
Oct 11, 2020

Its certainly a brave an unusual build that Rob at Ortomarine and the people like us behind in build slots will learn from. I'm excited to see how it all works in practice Dave.

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