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Old 20-02-2020, 19:16   #16
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Re: Small genset or additional fuel tank?

I think the only reasonable way to test this is to put half a gallon of gas in the Honda and test how much kWA you can produce at 1/4, 1/2 and full load. Then do the same with the nextgen. I am thinking of doing this with my Yanmar and the alternator but need to run the return line to a day tank. Still, it would be a good exercise one day.

Also, using a combustion engine that is 25% efficient, with the rest going to heat, to produce electricity that you then use to heat water... I know it is convenient but it bugs me.
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Old 20-02-2020, 23:48   #17
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Re: Small genset or additional fuel tank?

If you are going cruising there will be extended periods when you will stay at anchor and in overcast weather will probably need some sort of engine driven battery charging.

The problem with small portable gensets is that the most conveniently stored and quiet running are the inverter type with lots of electronics in them. The one I carried for that purpose is about four years old and quit a few days ago. Since it's out of warranty I know there is probably a big repair bill coming up if I choose to keep it.

Another problem is battery charger size. When my present boat was used mostly for weekend sailing a 20 amp battery charger was more than adequate. However now that it is used for full time cruising the 20 amps is insufficient and I have to charge for extended periods of time.

Consequent to these circumstances I have decided that rather than repair the inverter genset I am building a 12V charger from a 5 hp Honda electric start gas engine driving two 85 amp alternators. I could do it with one large alternator but I have a mixed AGM/lithium house battery system and like the idea of some redundancy. I have a 2,500 Watt inverter to take care of high load, AC requirements.

Unfortunately the new battery charger is going to have to live on deck in an alloy box, but them's the breaks in this cruising business.
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Old 21-02-2020, 01:03   #18
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Re: Small genset or additional fuel tank?

I bought a 3.5 KVA, 240 volt Invertor petrol generator, It has a 12 volt battery charger outlet, It runs every thing on Board, $400-00 delivered, 40 Kgs,
12 hours on a tank full,

I bought it for when the batterys get flat and wont start the main engine,
500 watts of solar, But the sun dont shine all the time,
My diesel has a big alternator, But I only use that to get in and out of the Marina,
If I need extra diesel, I use 5 gallon plastic jerry cans, They can be stowed away in small places,
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Old 21-02-2020, 01:38   #19
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Re: Small genset or additional fuel tank?

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Originally Posted by hpeer View Post
If it were me I would know the answer.

1. 25 gallons of fuel
2. A GOOD wind generator, not one of those 3 bladed things, way too noisy.

The 25 gals of fuel can be used to motor or produce electricity or both. The gen can only use fuel, which is rare for you, to produce electricity. So the extra fuel is a a more flexible use of the space.

The wind generator is IMHO a no brained, do that no matter what you do with the tankage space. Even here in Antigua we have just had a run of days with very little solar power but good wind. The wind generator has tombs capacity to produce power 24 hours a day, the solar about 8. So it can be less efficient and still provide as many amps. And if in dire straights, turn off the fridge, and when truly low crank up the engine. But that will only occur if you have no wind AND no solar.

Thatís my fault analysis anyway.
So much depends on where you cruise. The trades in the east Carib are pretty relentless. Most anchorage are still way windy. So a windgen is useful. In lots of the rest of the world anchorages are calm and a windgen does little.
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Old 21-02-2020, 04:18   #20
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Re: Small genset or additional fuel tank?

If the exam question is which is more fuel efficient - engine ticking over at 1250 rpm driving a 120A alternator vs generator ticking over at 2700 rpm (NextGen) or 3600 (Fisher Panda) driving an inverter/charger delivering a 100A charger (at best), difference is negligible.

Your solar panels (300w) will produce, on average, 1500 Wh/day. Even allowing for invariable shading on a sailboat, should deliver close to half your usage.

Unless you want A/C, would focus on optimizing your system. First, 200Ah/day is a pretty generous energy budget. Biggest draws are refrigeration at anchor, and add electronics when underway.

Second, if not already equipped, you alternator should be regulated to produce 3-stage charging just as a high quality charger does. Auto/truck alternators are designed to fill a big hole really fast or to power concurrent drain. Most are designed for higher RPM than the 1250 rpm on your setup, but I assume your data is correct. That's not the same use case as cruiser, especially a sailor who will have long periods of battery drain.

Third, if at all possible, I would increase solar and make sure install is solid with MPPT controllers to be more efficient.

In the end, unless you want A/C, you can most likely get better results by optimizing your current setup. Will be cheaper and frankly work better. Whether you add fuel capacity is up to you.

Good luck and fair winds

Peter.
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Old 21-02-2020, 10:37   #21
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Re: Small genset or additional fuel tank?

I believe the original question has been answered already but I am still curious about the efficiency of generators at low loads (the exam question). One way to look at it is the size of the flywheel for 1, 2, 3 and higher cylinder engines. It does not seem to increase proportionately, so it seems to me that a large Diesel engine will be not that harder to drive at idle than a smaller engine. Any thoughts?

Basically, I am curious about what is the most efficient way to convert one quart of fuel to electricity for large diesel with an alternator, a generator and a small gas generator.
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Old 21-02-2020, 11:54   #22
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Re: Small genset or additional fuel tank?

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Originally Posted by Pizzazz View Post
I am curious about what is the most efficient way to convert one quart of fuel to electricity for large diesel with an alternator, a generator and a small gas generator.
According to their respective URLs, Honda 2000i burns about 0.3 gph to produce 1800W (15A) of power. The NextGen 3.5 produces 30A continuous, but they only state it burns 0.2 gph on average, which probably means a 15A continuous draw. Part of the difficulty in comparing is generators are more efficient when running at full-rated load. Although the diesel in this example is 33% more efficient than the gas generator, the diesel is running at half-load. If you could compare a gas 3.5kw generator to a diesel 3.5kw generator (or 2.0, whatever), diesel would likely be 50% more efficient or more.

Bottom line: you would save 0.1 gallon per hour with a diesel over a gas generator. Two hours per day for a month - you would save six gallons. Or said another way, you would have six more gallons of gasoline for your dinghy.
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Old 21-02-2020, 12:08   #23
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Re: Small genset or additional fuel tank?

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Originally Posted by Pizzazz View Post
I believe the original question has been answered already but I am still curious about the efficiency of generators at low loads (the exam question). One way to look at it is the size of the flywheel for 1, 2, 3 and higher cylinder engines. It does not seem to increase proportionately, so it seems to me that a large Diesel engine will be not that harder to drive at idle than a smaller engine. Any thoughts?

Basically, I am curious about what is the most efficient way to convert one quart of fuel to electricity for large diesel with an alternator, a generator and a small gas generator.
I did the exercise on the capital cost years ago. I can't recall the exact figures but I think the diesel costs were about four times the gas engine and the weight comparison about 2.5:1. One may purchase a lot of gasoline for the difference in costs between gas and diesel generators.

The decision eventually came down to did I want to accept the cost and weight penalties for a contingency item. The fuel hazard question was resolved by the fact that I already had to carry gasoline for my tender outboard anyway.

Unless you are going to run an aircon system an engine driven high ampage
low voltage battery charging system beats an engine driven high voltage AC genset for practicality purposes.
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Old 21-02-2020, 12:23   #24
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Re: Small genset or additional fuel tank?

As far as the diesel vs gas thing, a gas generator gets much less efficient as you reduce the load. This effect is much smaller on a diesel.

Looking at a pair of current production 1800 RPM generators from Westerbeke, for example (a 7.5kw EFI gas unit and a 7.6kw diesel), we get the following power vs fuel numbers:
  • At full load, the diesel burns 0.95 gph, the diesel 0.93 (barely any difference)
  • At 75% load, we start to see a difference, the gas unit burns 0.76 gph, the diesel 0.67
  • At 50% load, it's 0.61 gph for the gas, 0.5 gph for the diesel
  • At 25% load, it's 0.44 gph for the gas, 0.36 for the diesel

So the diesel fuel consumption scales much more linearly. I've calculated gph per kw of load at the various points here:
  • At full load, the diesel burns 0.122 gph per kw, the gas unit burns 0.127
  • At 75%, the diesel burns 0.118 gph per kw, the gas unit burns 0.135
  • At 50%, the diesel burns 0.132 gph per kw, the gas unit burns 0.163
  • At 25%, the diesel burns 0.189 gph per kw, the gas unit burns 0.235

So we can see that while both units get less efficient as the load gets lighter, the gas unit suffers far more (it's almost half as efficient at 25% load as full load, while the diesel is about 75% as efficient at 25% load compared to full load). And small, air-cooled, high RPM engines like the Honda generators tend to be somewhat less efficient than a typical water cooled engine. Carb vs EFI makes a small difference as well, and the Honda being an inverter generator causes another small loss of efficiency.

I'd say it's unlikely that the Honda would burn less fuel relative to power produced in the real world compared to the 3.5 Nextgen. Most likely, the Honda would burn more (and needs a separate fuel supply from the main engine). From the numbers I can find, the Honda 2200 burns 0.165 gph per kw at 1800w (continuous rating) and at 450w (1/4 of continuous rating) it's up to 0.26 gph per kw. So compared to the examples above, it's pretty thirsty, although percentage-wise, it scales better than the Westerbeke by virtue of the inverter-based Honda being able to reduce RPM under light load.

The Nextgen specs 0.4 gph at full load (they don't give good partial load numbers), so that puts it at 0.114 gph per kw at full load, or slightly better than the Westerbeke diesel example above.

If we extrapolate from the data we have, a continuous 1/4 load on the Nextgen (875 watts) should burn about 0.155 gph. A 900 watt load on the Honda should burn somewhere around 0.18 - 0.2 gph (estimated as I don't have a true efficiency value for the Honda at this load). So the Nextgen would definitely be more efficient (about 12 - 20% better) for that amount of load.

As a note on just how bad small air-cooled engine efficiency really is, from the numbers I've found, modern 6 - 7 kw air cooled 3600 RPM portable generators (non-inverter) are no more efficient than the very old design 6.5 kw gas Onan in my boat (which burns about 1 gph at full load down to about 0.4 gph at no load). And my generator is a heavy, cast iron flathead design with a very primitive carb, so nothing modern or efficient about it. A lot of the design goes right back to the 1940s.
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Old 21-02-2020, 12:49   #25
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Re: Small genset or additional fuel tank?

Hi rslifkin.

I'll bet your old Onan is big, heavy, and noisy.

I have an old what is called in Australia a "tradesmans" genset which I have owned for about thirty years. It is presently on my deck doing battery charging service because my nice compact, light and very portable and stowable, quiet and modern, four year old, inverter genset has failed. The old single phase induction gnerator has withstood decades of exposure and abuse as I would occasionally use it and an inverter welder to crank my main engine. The outcome was usually a blown excitation capacitor which I could replace myself for about $20.

I'd resist any temptation to upgrade to a modern inverter unit and stay loyal to the old Onan.
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Old 21-02-2020, 12:53   #26
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Re: Small genset or additional fuel tank?

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Hi rslifkin.

I'll bet your old Onan is big, heavy, and noisy.

I have an old what is called in Australia a "tradesmans" genset which I have owned for about thirty years. It is presently on my deck doing battery charging service because my nice compact, light and very portable and stowable, quiet and modern, four year old, inverter genset has failed. The old single phase induction gnerator has withstood decades of exposure and abuse as I would occasionally use it and an inverter welder to crank my main engine. The outcome was usually a blown excitation capacitor which I could replace myself for about $20.

I'd resist any temptation to upgrade to a modern inverter unit and stay loyal to the old Onan.
No temptation to upgrade in my case. Only way I'm hauling that 400+ lbs of genset out of this boat is if something goes horribly wrong or if I repower the boat to diesel at some point. Otherwise, the cost of a new genset would never make sense, as I just don't use it enough hours for even a 10% fuel savings to be significant. It's a bit noisy in the boat, but nothing terrible (no worse than the A/C units). And it's quiet from outside, which is what really matters when I fire it up to make coffee and breakfast in the morning.
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Old 21-02-2020, 16:46   #27
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Small genset or additional fuel tank?

Reviewing the numbers above and converting to boat friendly units such as AHrs @14V I come up with the following conclusions:

1. A Honda 2200i, working at max load will produce about 400AHrs per gallon of gas and provide up to 120A of charging current which is in line with most charging systems.

2. Diesel generators are more efficient capable of giving 450 to 600 AHrs per gallon of diesel, provided they are operated at full load. This is rarely the case though and if you operate them at 25% load just to charge the batteries (few banks will take more than 120A of charging current) the efficiency drops to 350-400 AHrs per gallon.

My takeaway is that if you your primary function is charging the batteries, say at 75A @14V (max on my charger) both solutions will be equally efficient. Clearly, if you have a larger boat with air conditioning, electric heat, etc.) then you need to size the generator accordingly. But just for charging and occasional water making, it is hard to beat a cheap gas inverter generator.

My optimal usage scenario would be using the smallest inverter generator (700W @ $225 on Amazon), put half a gallon of gas, set the charger to draw max 5A AC, start the generator and go to shore to drink. You come back three hours later, you have 150AHrs in the bank and no noise.

The alternative way, charging with solar, assuming 25-30AHrs per panel would require at least 5-6 panels and I donít have that much space on the boat.
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Old 21-02-2020, 18:28   #28
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Re: Small genset or additional fuel tank?

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If you want to make hot water while running a diesel engine, you should be doing it with waste heat, not electrical power.
Even if you were willing to deal with the complexity of the hoses and a water heater with two heat exchangers, you canít do it with the little Nexgen, reason is there is no fresh water pump, itís cooled by thermal syphon, there is no thermostat either.
To jury rig something with a pump etc is way more trouble than itís worth, and the motor being so small may not have as much excess heat as you would want.
Far less complex to just use the electrical power it generates, and do without the complexity of hoses and pumps etc.
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