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Old 24-11-2012, 09:10   #1
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Marine Generator. Low revs or high revs?

I have shortlisted my replacement genset to either a Kohler 1500 rpm or a Cummins Onan 2880 rpm. What is the main difference between the two types? Apart from of course - 1380 rpm. My old genset is (near being a was) Kohler 1500 rpm. Great wee machine. Never tried the faster revving things. Any feedback would be greatly appreciated. Thanks for your time. Russell
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Old 24-11-2012, 09:36   #2
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Re: Marine Generator. Low revs or high revs?

slower=quieter?
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Old 24-11-2012, 15:20   #3
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Re: Marine Generator. Low revs or high revs?

Slower is quieter, less vibration and probably longer lifespan. Faster is less weight and smaller.

If you have the room and can accommodate the weight, go with slower - I would get a Northern Lights in a low rev model and a Nextgen in a high rev model.

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Old 24-11-2012, 16:22   #4
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Re: Marine Generator. Low revs or high revs?

There are two types of marine generators: direct drive and belt driven.

The direct drive types have to run the engine at an rpm that matches the output frequency: typically 3,600 or 1,800 rpm for 60 hz and 3,000 or 1,500 for 50 hz. You can either have a slow engine- Kohler, Northern Lights, etc or a fast engine- Fischer Panda with a direct drive. The slow engine gensets at the same KW output are a whole lot heavier.

The belt drive types can run the engine at any practical rpm but most such as NextGen and the small Onans run the engine at about 2,800/2,900 rpm for 60 hz and about 2,400 rpm for 50 hz.

It sounds like you need 50 hz power as you noted that the Kohler runs at 1,500 rpm.

1,500rpm for a marine generator is pretty slow and it requires a big chunk of iron to make enough power at that rpm to run the genset end.

Almost all 1,500/1,800 rpm marine gensets in the 5 kw range have three cylinders and run pretty quiet.

The NextGen/Onan gensets at 2,400 rpm and 5 KW are 2 cylinders and less cylinders plus more rpm means that they are noisier. But they are a whole lot lighter, almost half the weight of the slow rpm gensets.

The slower the genset runs all things being equal, the longer it will last.

So in summary: slow means quiet, heavy and long lived. Faster (either the direct drive types or the belt drive types) means noisier, less long lived and lighter.

It all depends on what you want, what room you have to install it, whether weight is important, etc.

David
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Old 25-11-2012, 22:39   #5
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Re: Marine Generator. Low revs or high revs?

David has it exactly right.
I have had a genset on my truck for 10 years. The first was a "TruckGen", made by the same folks that make the NexGen, I think. They used a 2cylinder Kubota. Mine was a one cylinder model. They run at a much higher RPM, and thus produce more heat, and heat is the enemy of mechanical stuff. Mine ran at a crisp 2800rpm, and was so loud even the guys with the reefer units would park elsewhere. It lasted about 3 years, then puked coolant out the top and died. Disposable engine. 7hp commercial rototiller engine I later found out.

My current genny is a 8kw direct drive, Marathon 'pancake' generator head married to a 3 cylinder Perkins. Runs at a quiet 1800, but it's bigger, and heavier.
Betwixt the two, I prefer the Perkins unit. So do my truckstop neighbors.

There's a great manufacturer of gensets in Eugene, Oregon. Wrico International. He's very knowledgeable and does custom units for many applications. A unit like mine, only with a Kubota 3 cyl runs about $5200, without an enclosure. A good sheet metal fab shop should be able to make up an aluminum enclosure fairly reasonably.

Wrico International - All the right components in one place.
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Old 25-11-2012, 23:38   #6
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Re: Marine Generator. Low revs or high revs?

You've gotten good advice here. The difference in noise and vibration between low speed and high speed gensets is amazing. However, the downside is also amazing - double the weight, and nearly double the cost, for the same output.
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Old 26-11-2012, 12:43   #7
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Re: Marine Generator. Low revs or high revs?

Rebuild the one you have. Just my $0.02, Mike
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Old 25-05-2013, 22:24   #8
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Well, its not a nautical post, but my genset on my class 8 Volvo semi quit on me yesterday in Phoenix, AZ. I stopped at Southwest Products and had them trouble shoot it. The genset head is toast. They wanted $1450 for a new head, and 6-8 hours off labor. Total of $2200.
NOT!!
I called Temco Products in Clackamas, OR (the manufacturer), they have 3 Marathon 7.5 kw generator heads left. Sold me one for $570!!

I'll have to live w/o the genny for a couple of weeks, until I get home, which sucks, but I'll have a new can on the genny when I get done. Then, I'll take the old head and have it rewound, and keep it as a spare.
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Old 26-05-2013, 08:33   #9
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Re: Marine Generator. Low revs or high revs?

Quote:
Originally Posted by lostsheep View Post
I called Temco Products in Clackamas, OR (the manufacturer), they have 3 Marathon 7.5 kw generator heads left. Sold me one for $570!!

I'll have to live w/o the genny for a couple of weeks, until I get home, which sucks, but I'll have a new can on the genny when I get done. Then, I'll take the old head and have it rewound, and keep it as a spare.
It is unlikely you will get it rewound and refurbed for much less than you paid for your new one. I would just buy another new one.

Mark
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