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Old 16-11-2010, 21:01   #1
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Generator vs Inverter Generator (Genset)

I've been looking at boats in the 34' - 41' range. Several of them have gensets in the 6-8Kw range. This seems way excessive for the size of boat, or equipment on board. I think one could get by nicely with about a 3Kw unit. HOw come they installed so much capacity? Are my calcs wrong?

Also, I have been looking for a diesel inverter genset rather than just a generator. I think the current gensets are air cooled, so the 3Kw should be fine. Has anyone used an inverter generator? I know they are twice the price of a generator, but since they only run hard on load, the unit should be much quieter and more fuel efficient. Any thoughts?
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Old 16-11-2010, 22:23   #2
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G'day, Mate. It may look excessive, but my experience living aboard for 13 years, it comes down to the size of the watermaker, fridge/freezer, and battery chargers. 8kw was the perfect size in our case to run a 20 gal/hr watermaker, 1500 watt freezer compressor and 150 amp/hr of battery charging capacity. These kind of liveaboard comforts may not change whether your boat is a 40 or 50 footer, but obviously it's alot more challenging to fit all this equipment smaller in size. All the best with the search. Cheers.
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Old 17-11-2010, 05:15   #3
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Also, I have been looking for a diesel inverter genset rather than just a generator. I think the current gensets are air cooled, so the 3Kw should be fine. Has anyone used an inverter generator? I know they are twice the price of a generator, but since they only run hard on load, the unit should be much quieter and more fuel efficient. Any thoughts?
I'confused, proper built in DIESEL generators are not invertor generators and are the majority of installed units. There are a number of smaller Gas driven generators that use a invertor, but these are not marinised or suitable for permanent installs. ( They are good at what they do though). Theres no particular advantage to invertor generators , they can produce a slightly cleaner signal etc, but its not a particulary bis plus.

Dave
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Old 17-11-2010, 06:37   #4
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Thanks for the feedback. The bigest issue I have with the installed deisel gensets is the noise. They can really disrupt a nice anchorage. Oh well. I'll just have to be carefull about when to run it.

Jon
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Old 17-11-2010, 06:40   #5
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Say you need about 4 kW continuously for an hour a day to charge your batteries, cool the refer down, etc. I would not size the generator at 4 kW. It would be like running your engine at max rpms everytime you use it. A genset would be quieter, last longer, and be happer running at 1/2 the max rated load. So a good size would be 8 kW. Plus you then have reserve capacity for future growth.
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Old 17-11-2010, 06:48   #6
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The generator not only has to run the average load, it also has to handle the peak starting surge load.
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Old 17-11-2010, 06:54   #7
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I am under the impression that generators can briefly support surge loads above the rated max continuous output.
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Old 17-11-2010, 06:54   #8
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Originally Posted by LakeSuperior View Post
Say you need about 4 kW continuously for an hour a day to charge your batteries, cool the refer down, etc. I would not size the generator at 4 kW. It would be like running your engine at max rpms everytime you use it. A genset would be quieter, last longer, and be happer running at 1/2 the max rated load. So a good size would be 8 kW. Plus you then have reserve capacity for future growth.
Depending upon the engine, operating at 50% of nameplate, may or may not allow the engine to come up to operating temperature.

A major cause of diesel generator engine failure is "wet-stacking" ("wet" unburned fuel accumulating in the engine exhaust, or "stack"); caused by under-loading of the generator. Diesel engines that are lightly loaded, or allowed to idle for long periods, never reach their recommended operating temperature. Over time, unburned fuel coats the combustion chamber, reducing engine rating, efficiency, and life span.

One solution is to always run the generator set with an electrical load that reaches the designed operational temperature of the diesel, or approximately 75% of full load.

Standby generators are “excercised” at a minimum of 30% of nameplate loading.

More ➥ http://www.kraftpower.com/pdfs/KPC_I...t_Stacking.pdf
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Old 17-11-2010, 07:02   #9
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The small Honda EU2000's are a inverter generator....we have two on-board as well as a 8KW Northern Lights diesel generator. My take on the inverter generators are that they are very quite without a load but once a load is put on them they get pretty loud. We will run the Honda's during the day as there is usually quite a bit of noise and they are not too bad but mornings or evening we only run the big Diesel as the only noise is quite splashing of water.
Given the choice the diesel generator would be preferred with the little inverter generators used as back up or to keep the hours off the big generator when noise is not too big of a problem.
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Old 17-11-2010, 07:09   #10
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One solution is to always run the generator set with an electrical load that reaches the designed operational temperature of the diesel, or approximately 75% of full load.

Standby generators are “excercised” at a minimum of 30% of nameplate loading.

More ➥ http://www.kraftpower.com/pdfs/KPC_I...t_Stacking.pdf

From the Northern Lights web site, marine generators, "A generator should never run with less than a 25% load. 35% to 70% is optimal."


http://www.northern-lights.com/PDFs/...yers_guide.pdf

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Old 17-11-2010, 07:58   #11
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There are two ways to get a diesel inverter generator.

One way is to make one yourself. Couple a small genset with a Victron or similar inverter that has the capability to sync with the generator to start and run big loads.

Mastervolt has a packaged unit. The small genset of about 3KW is packaged with a 2 kW inverter that can put out 5 KW for short periods. The generator stops and starts depending on the load and battery charge state. This seems ideal to me. But I bet it is quite expensive.

FWIW large generators operated at low loads can not only "wet stack" as described above and develop a cylinder wall glaze that will lower compression and increase oil consumption, but also they are very inefficient at low loads.

David
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Old 17-11-2010, 08:50   #12
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I agree David, We see more service calls on gensets that have either not been used much or underloaded. The repair and maintenance costs are considerable. That is the problem with folks giving advise based on what they find using their web browsers search function rather than real life or first hand experience. We will look at a boats requirements and base recommendations for the genset on the maximum load and peak loads for AC, and other equipment. We instruct the owners to turn on as much as possible when running the genset whether you really need to or not. It doesn't cost any more. Get the genset loaded to 75 or 80% as much as possible. The original posters concern that the genset is too big is a legitimate concern. But all of the numbers need to be crunched to determine that. Chuck
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Old 17-11-2010, 09:03   #13
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Guessing on the size of required generator you need is going to cause you problems. Figure out your proposed electrical demand, the load in watts you will place on the generator from your a/c, refrigeration, tv, etc. In order to have your new generator last you need the load to be above 50% or you are bogging the engine down and will shoten the life of the thing by half. If you don't like the noise and vibration, the small 3 or 4 kw units are the worst. More cylinders 3 or 4 is better. Get a sound shield, and some soundown insulation for the machinery space. Also you should seriously consider a seperate inverter so you can run most anything but the a/c at night with no gen. A 3,000 (3kw) watt pure sine wave inverter is only $1200 with a built in multi bank batt charger. You said it most people have big 8kw gens, that is not a mistake. If the cost of a new 8kw which is around $9k is too much I know a place in Ft Lauderdale that all they do is completely rebuild generators of all makes and models,no core necessary with warranty. You can call them up tell them what you need and pick it up for less than half of the price of a new one. So instead of buying a new 4kw unit for $5k that won't work you can get a 0 hour rebuilt westerbeke 8kw for $3800. Do the math on the demand or you are going to end up with a little buzz box screaming all the time and tripping the breaker before it goes up in a puff of smoke. Good luck
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Old 17-11-2010, 09:05   #14
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I agree David, We see more service calls on gensets that have either not been used much or underloaded. The repair and maintenance costs are considerable. That is the problem with folks giving advise based on what they find using their web browsers search function rather than real life or first hand experience. We will look at a boats requirements and base recommendations for the genset on the maximum load and peak loads for AC, and other equipment. We instruct the owners to turn on as much as possible when running the genset whether you really need to or not. It doesn't cost any more. Get the genset loaded to 75 or 80% as much as possible. The original posters concern that the genset is too big is a legitimate concern. But all of the numbers need to be crunched to determine that. Chuck
Good advice? That said I'm not sure how one can second guess the people that develop, engineer, manufacture, sell, and service their product. Oh by the way, they also have the most hard data and experience with the product. If they recommend a 35 to 70 percent load I'll bet it is based what they consider best for their product.

Technically it does cost more to run a genset at the higher loads as the fuel consumption is higher.

I'm not sure how one comes to the conclusion that people posting on this subject are getting their information from websites and not experience. Many posters tend not to wear their experience on their sleeves. It would be suspect if one uses this same logic process to make recommendations on generator load factors.
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Old 17-11-2010, 09:51   #15
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Jon,
The proper sizing of a genset is a complicated matter. Not only does one have to calculate the pure output in watts but one must often consider power factor and startup loads as well. I have a 5 KW diesel Onan genset on my boat capable of putting out about 40 amps at 110V single phase. I would like to put a dive compressor on my boat that requires 2.2 KW 110V single phase. On the face of it it sounds like a good match, but in reality because of the power factor (Lag of peak amps behind peak Volts) I actually need close to the entire output of the genset at running load and I need nearly twice the output to start the motor. Also I currently have a 40 amp battery charger on a 430 amp house bank. If it gets down 50% It takes nearly 8 hours of run time to even get close to a full charge as the output tapers down after a couple of hours. In theory I can stuff about 200 amps in at bulk phase so I would like to add a 150 amp charger to cut down my charge time substantially, but chargers are very sensative to power factor and I'll be lucky if my 150 amp charger puts out 75 amps when powered by the genset. It will still be better than the 40 amp charger that came with the boat, but still not what I'm paying for. In the meantime my genset will be loafing along at less than 20% output, so I'll heat my hot water, make some ice, wash some clothes, etc. This is also where I wish I had bought a 110Volt water maker. If you have any large AC motors on board you need to account for power factor and startup loads, then you need enough resistance loads to load up the genset. Like I said, comlicated.
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