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Old 10-31-2012, 07:00 AM   #21
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Ideally, yes, you should increase the load as much as possible to bring it up to 10kW or higher. Realistically, that is a pain in the keel and costs a lot in fuel.

We sometimes use loadbanks to keep the load up when the "hotel" load is low. I personally despise the things because they waste fuel to heat seawater in order to make up for a poorly considered build specification. But, considering that long term underloading of synchronous generators leads to high emissions, soot, oil sheen, and smoke to the point where some yachts have been asked to find another marina and have had to pay for cleaning their neighbor's hull and waterline, it is a workable solution.

I have been installing particulate filters with great results but that only hides the symptoms, it doesn't cure the problem.

In your case, it might be worth considering a good house battery and an inverter to run the trivial fridge load and give the DG a break. Your neighbors will appreciate it as well.
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Old 10-31-2012, 07:23 AM   #22
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The lower rpm / KW rated generators have longer lives and significantly greater duty cycles between servicings as a result of lower rpm, lighter loads, and less fuel consumed.

Perhaps ,

The slow speed keeps down the piston miles , and the smaller output will consume less fuel.

I believe their longer life may be due to the HIGHER percentage of their permissible loading during normal operation.

I would assume its harder to underload a small unit than a big one.

The "best" setup if noisemaker engine life is desired is probably a DC unit matched to a large sized inverter.

4KW or 6KW inverter is not too expensive and a 300A 24V bus alt could be coupled to a light industrial engine (Kubota?) and set to operate at only the RPM required simply.

The Big inverter would probably have a load sense setup , so would parallel the dock power when loads of air cond require starting.
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Old 10-31-2012, 07:27 AM   #23
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I'll try one more time. A marine alternator operates at a synchronous speed precisely as I stated. It runs at that speed regardless of load. It will run at 900, 1200, 1800, 3600 or whatever rpm it was made to operate to produce (in those cases) 60Hz alternating current. It will turn that rpm at 25kW or 2.5kW output.

When the unit is operated for long periods at 2.5kW the engine suffers serious problems due to underloading. This is a fact of marine generator operation and those of us who live and work with these devices on a daily basis rather than pretend on the internet are fully aware of the costs and problems created by low loading of generators.

When it comes to marine diesel generators, there is no such thing as a "commercial" vs some other type. We have DGs installed in our yachts which range in output from10 to 250kW and everything in between. Some of these yachts operate privately and some operate commercially ... the generators are all the same and work the same and even cost the same and fail the same way for the same reasons.

Low loading of generators is a different issue than the perception of low loading of propulsion engines that dominates the threads on forums such as this one.
Rick, you just like to argue with people for the sake of argument. Nobody here was talking about running an engine at 10% of capacity for a specific RPM / HP. The OP was talking about pushing a 48' motor yacht at hull speed. At say 1,400 rpm, the load on the engines is likely 25% +/- of the capacity of those engines at that rpm.

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Old 10-31-2012, 07:35 AM   #24
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WTF are you talking about?

A 25kW generator is pretty small no matter where it is used and last time I looked there wasn't any difference between a "recreational" generator and one installed on a charter boat or tug or a fishing vessel.

Diesel powered generators, or more correctly - alternators - turn at a synchronous speed related to the number of poles and the desired output frequency. A high speed diesel in the power range we are talking about here that drives a generator to produce 60Hz AC power will commonly run at 1800 rpm or 3600 rpm. Larger units might operate at 900 or 1200 rpm.

Maybe where you come from they only use variable speed generators but the rest of the world relies on conventional synchronous alternators.
That is absolutly 100% correct.

Furthering Rick's discussion of loading vs engine life we have found that a disel fired prime mover in a generator application will get a acceptable mean time between overhauls if it is loaded at least 30% and under 80% of its rated capacity. We measure rated capacity of a generator set as the point where the governor no longer has control of the RPM.
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Old 10-31-2012, 07:36 AM   #25
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Rick, you just like to argue with people for the sake of argument.
I don't generally waste my time arguing with pretenders. I posted to correct your inaccurate statement that "commercial" (whatever that means) generators are suited to operate for "weeks at a time" at very low loads.

Real life operators of those systems know that is false and I responded to advise readers of that fact, not to argue with you.
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Old 10-31-2012, 07:38 AM   #26
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FF's last post pretty much nails it, I think. He is correct.
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Old 10-31-2012, 07:46 AM   #27
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Rick, since you clearly have a limited knowledge of "commercial" generators, let me explain it to you. The generator end for a direct drive generator is wound to match a specific rpm. As an example, if you had a 4-71 DD you could get the generator wound for 1,800 or 1,200 rpm. The 1,200 rpm unit will have a lower KW rating as the 4-71 can't produce as much HP at that RPM. The lower rpm / KW rated generators have longer lives and significantly greater duty cycles between servicings as a result of lower rpm, lighter loads, and less fuel consumed.

If you look at the generators on most trawlers / yachts, the rpm is at the higher end of the continuous duty rating for the engine. This is done to produce the most KWs with the smallest engine, least amount of weight and lowest engine cost.

Ted
Ted, you are correct in theory, but not in practical application.

There are VERY FEW 1200 rpm generator sets in service. This is not a common RPM, although I have used it for continous duty applications at remote microwave sites a few times with great results. But, I had to design the engine/generator set myself as it is not a commercially available item.

Again, looking at commercially abailable generator sets you cannot but "small" generator sets in 1200 RPM. That makes them impractable for any reasonable dissussion.
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Old 10-31-2012, 09:45 AM   #28
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You are correct to be concerned about the 6-71 450 HP versions and longevity. Currently there is a good thread on boatdiesel about 1500 hour J&Ts at 1500 hours and associated issues. If you browse the boatdiesel archives some good info will show up on your question. Hopefully your engines are not the tuner builds but DDs, the DDs generally do better.

Derating is not cheap or easy, it can be done but many bits and pieces are involved. Assuming you are not the original owner, how these engines were operated by the PO will play a role in their remaining life.
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Old 10-31-2012, 10:06 AM   #29
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Sunchaser,
I am on Boat Diesel and I spent the better part of a week solid reading every Detroit post on there and like I said in the original you get everything from good to bad. I guess I'm not going to worry about it and just run them as I see fit. Everyone has an opinion based on there experiances so I am going to do the same.
Yes not the original owner, 3rd actually. The question I posted about the genny was the PO had installed the Phasor 15kw about 2 years before I bought the boat and I am wondering why 15KW, it seems a little overkill unless you want to run everything in the boat at the same time. A 10KW would probably do fine. I know most of the Californians I've seen on the market have had 12.5KW gennys in them so I think that was the size they went with originally. As I've said in the past I'm new to diesels and I am just trying to get a better understanding of them, all my life I've been around and worked on gas, so diesel just seems foreign. I guess after a few more years with the DD's I'll know for sure. I do have to admit, I love the sound, can't even get a big block V-8 to sound as throaty as the 6-71 at idle.
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