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Old 22-09-2017, 07:51   #1
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Quietest 7.5kw generator

I am sure t his have been discussed before, but after 15 minutes search on this forum... with ALL sort of key words ... DID NOT FOUND !!!

I am to buy a Lucia 40ft catamaran, and the generator installed by the FP is a Onan 7.5 KW in the portside engine room ... opposite hull to the owner cabin. OUF !

I want NO NOSE as possible and Onan never mention any DB level, but Panda has no fear to mention for the Fischer Panda AC 8 Mini DP ... Extremely quiet (52dba@7meter).

What are the best choice in this case.
I am on the last days to order and think to only ask for wiring and thru-hull fittings to be installed... or they install the one that I WANT !

Thanks to all specialists.
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Old 22-09-2017, 08:23   #2
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Re: Quietest 7.5kw generator

Perhaps new ones are better but I have read many reports of problems with Fisher-Panda generators and also many reports that they require very frequent maintenance to fix or prevent problems.

In general the quieter generators are ones that run at low rpm. 1800 rpm for USA 60 Hz systems or 1500 rpm for EU 50 Hz systems. In addition to a low rpm generator most can be purchased with an insulated box enclosure that works as a sound shield.

Northern Lights is often considered the best and most reliable and has the options I mentioned.
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Old 22-09-2017, 08:25   #3
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Re: Quietest 7.5kw generator

As a starter, I think it wise to only consider 4 pole generators because they operate at 1/2 the RPMs as a 2 pole.
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Old 22-09-2017, 08:52   #4
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Re: Quietest 7.5kw generator

Wot skip and foggy said.

Be careful with sound level ratings too- note that 52 db@ 7m is going to be a helluva lot more when it's closer than 7m- which it most certainly will be on a 40' boat.

FP gennys have an excellent overland reputation (trucks and emergency vehicles). I've installed a few and they're all still going strong. On boats however, they have a far less distinguished history. The tight packaging and aluminum components all play very poorly with salt water when there is the inevitable leak inside the sound shield. One small leak that you don't notice, and it's only a matter of time (and not much) before it all goes fizzzzzz.
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Old 22-09-2017, 09:02   #5
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Re: Quietest 7.5kw generator

Generator noise measured by a sound meter is almost irrelevant.
Sounds crazy, but most noise on a boat with a generator running is secondary vibrations from the boat itself.
How it's mounted makes more difference than anything, except maybe the exhaust.
You want it mounted to something very, very solid. A plywood board can act like a drum head and send vibrations throughout the boat.
I double isolated mine, meaning the generator in its sound enclosure sits on rubber isolation mounts, but I put a second set of isolation mounts to mount the enclosure on.
It helps, I have a buzzy mid RPM generator and with the AC on in the boat you can't tell it's on. However if you lean your head against the bulkhead in the Salon, you can feel it slightly.
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Old 22-09-2017, 16:46   #6
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Re: Quietest 7.5kw generator

The exhaust is very important. You want one that separates the exhaust gas from the cooling water with the gas exiting above the waterline and the water exiting below the water line. That eliminates the splash you often hear and that can be so annoying.
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Old 22-09-2017, 18:19   #7
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Re: Quietest 7.5kw generator

I have another comment based on my experience with my Kohler 7.5KW machine. It is soo quite running in the bilge without a noise shield I can hardly hear it. A big part of the genny's quite operation is a 4 cylinder engine rotating the generator section. More cylinders, less vibration. But really what I want to discuss is the cast iron exhaust manifold/heat exchanger combo.

My genny is over 25 years old and presents a major problem getting replacement parts. For example, the exhaust manifold is an integral part of the heat exchanger. This year the so called end cap on the heat exchanger rotted away from salt water. NO REPLACEMENT parts were available for this fine but old machine and I looked and looked. I got around this by having the part remanufactured in steel rather than cast iron which of course would be better.

Now I am concerned that I might have a future failure with the remaining section of the cast iron heat exchanger and if that happens I might be screwed unless I can somehow isolate the heat exchanger from the exhaust manifold. Maybe this is a needless worry because the exhaust manifold is cooled with the engine's coolant but all things have a limited life. So my worry remains with getting replacement parts.
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Old 22-09-2017, 18:45   #8
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Re: Quietest 7.5kw generator

All great comments and advice.

Some basic guidelines:

Noise is 80% structure borne, and only 20% airborne.

Mounts are key to reduce the vibration and harmonics as are the heavy mass products used as vibration traps to dissipate the noise energy

Sound shields really inhibit proper maintenance and visual checks, so better to make the whole enclosure a proper sound shield rather than to add their own.
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Old 22-09-2017, 19:06   #9
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Re: Quietest 7.5kw generator

Quote:
Originally Posted by Pelagic View Post

Sound shields really inhibit proper maintenance and visual checks, so better to make the whole enclosure a proper sound shield rather than to add their own.


A lot do, the little Nexgen sound shield is two "clam shell" halves, open the two rubber latches and the two halves come completely apart and once the fan hose is removed, they can both be simply taken off leaving the whole generator exposed with no shield.
Usually though for oil changes, anode replacements etc only the front half has to be removed so you don't even need to remove the air hose.
Some are better accessible for maintenance than others
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Old 23-09-2017, 07:54   #10
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Re: Quietest 7.5kw generator

Yes your have a good point, that even with a sound enclosure w internal rubber mounts, have an extra set underneath would be the best for "hull transmited noise".
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Old 23-09-2017, 08:13   #11
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Re: Quietest 7.5kw generator

Don't forget the OP is buying a 40' catamaran. I would be loathe to put in a massive heavy duty generator (like my 6.5kW Kohler) in a smaller cat on top of the two diesel engines he already has in there. That's a recipe for final destruction of sailing properties.

I would probably put jumbo DC generators on the main engines and use those. LiFePo batteries and a big inverter or ganged inverters.

Plus a massive solar installation.

I guess he wants the generator to run air conditioning. That can be run off shore power, and in the Carib you don't usually need AC that much at anchor. Run it just after sunset for a couple of hours to cool things off, then shut it down - can be done powered by a main engine running a big DC generator, or perhaps even from batteries.

Something like this:

http://www.ecotechalternators.com/ca...e-alternators/

One of these putting out 3 or 4kW would be a perfectly decent load for a 30hp main engine running at moderate RPM.
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Old 23-09-2017, 08:21   #12
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Re: Quietest 7.5kw generator

I visited the Nexgen site and seems they only have 5kw but low speed. The Lucia 40 is offered with a 7.5kw generator, so I have not enough knowledge in this field to think to reduce to 5kw. I plan to add a Bauer that takes 5hp of power, so I think I ma better to stay with 7.5 to 8 generator.
Thank you
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Old 23-09-2017, 08:28   #13
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Re: Quietest 7.5kw generator

Quote:
Originally Posted by Falbala60 View Post
I visited the Nexgen site and seems they only have 5kw but low speed. The Lucia 40 is offered with a 7.5kw generator, so I have not enough knowledge in this field to think to reduce to 5kw. I plan to add a Bauer that takes 5hp of power, so I think I ma better to stay with 7.5 to 8 generator.
Thank you
What do you want to run with a generator?
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Old 23-09-2017, 08:29   #14
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Re: Quietest 7.5kw generator

You want 1800 rpm. The Panda are very quiet as there is always something wrong with them!
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Old 23-09-2017, 08:30   #15
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Re: Quietest 7.5kw generator

I am planning also to add a Bauer diving compressor asking 5hp.
Yes this is quite a small cat for this load, but why FP is proposing an Onan 7.5kw installed in the aft portside engine room... all weight aft, is it better than having it with the water tanks ?

Following the "mind" of FP with their Lucia 40, to "equilibrate portside-starboard" I would put the compressor in the starboard aft engine room... sure lot of weight there, but ...

I will try and idea that I know I could be wrong... all says that "don't put weight at the bow" ...so as the sail is pushing the bow down in the water, the cat COULD be more even under sail.
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