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Old 05-05-2016, 10:37 PM   #1
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2 Generators

On a boat with 2 gen sets, when do you use each of these?

12 kw Northern Lights generator (2,672 hrs)
6 kw Northern Lights generator (1,163 hrs)
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Old 05-05-2016, 11:07 PM   #2
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Well I would use the 6 KW unless loads exceeded 75% of capacity and then switch to the 12 KW. Realistically, if you run a generator all the time, I would run the 12 KW during the day when loads were higher and the 6 KW at night with the much reduced loads. In areas where air conditioning is required, day time loads would be much higher than night time loads.

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Old 05-05-2016, 11:26 PM   #3
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Well I would use the 6 KW unless loads exceeded 75% of capacity and then switch to the 12 KW.
OK, thanks, Ted.
So using simple math when the 6KW hits 75% and you switch to the 12 KW would it's load then be around 25% and would that be too light for it?

There is reverse air and a water maker. Can the water maker be run under way?
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Old 05-05-2016, 11:45 PM   #4
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Hawg-we have a 20KW and a 12KW. We normally use the 12KW almost exclusively. We only have to use the 20KW if we use the AC, which isn't often up here in the PNW. When on the 12KW, we do have to pay a bit of the attention to the load, since we have a 240V water heater and 240V fridge. We also have to use the 20KW when using the thruster as it's pump runs off the 20.
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Old 05-05-2016, 11:51 PM   #5
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I have a northern lights 16kw and a 6kw. They were put in within a year of each other and now the engine hours are: 16kw 14,000 hours and the 6 kw has 3,000.

The boat is power thirsty, I"ve blown the breaker in the 6 kw more that a couple of times.

So I've put a lot of hours on the big generator at 50% load + or -. So far I've replaced the water pump twice and the engine sensor twice. Change the oil like it is a religion...

I like the little one because it is nearly silent.

But, sure make some water underway, have the crew take a hot shower everyday, bakes some bread. Power is easy to use...
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Old 05-06-2016, 12:16 AM   #6
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Had a huge discussion of this type configuration recently. In areas like South Florida where you run air day and night, not sure how much you're able to use the smaller one. The big factor distinguishing the large one being required vs. the smaller one is air conditioning.

I would say what you're seeing is somewhat typical with 70% of the usage on the larger unit. That's the way you're probably going to be tied to using them. You also have the small one available if an issue with the larger one, just have to limit what you use.

Now, if I was designing a boat, I'd just go with two equal generators 95% of the time. That way you can rotate them and prolong lives.
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Old 05-06-2016, 12:26 AM   #7
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Now, if I was designing a boat, I'd just go with two equal generators 95% of the time. That way you can rotate them and prolong lives.
Now that challenges my logic. How would you determine a common size that is not constantly over or under loaded? Unless, maybe in cases like yours in FL, your loads seldom vary day to day, day to night. Is that it?
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Old 05-06-2016, 01:01 AM   #8
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I've not been doing this long enough to have developed a rhythm. I have two NL gensets - an 8K (with 1423h) and a 20K (with 917h). The boat has always been on the Pacific Coast and the low usage of the AC is probably reflected in the lower hours on the 20K genset.

I try to get the 20K a workout from time to time. In the winter I can run a few zones of cruisair heating (I have 4) - and if it's morning after a day or two at anchor the water heater will be running and the batteries will be needing a good charge. Now throw in a load of laundry with the dryer going and I'm probably up to 14K or so. The 8K meets a lot of my needs most of the time. It's in the lazarette and so it's the one I run if I need one underway. The 20K shares the engine room and I've noticed the temp in the ER rises quite a bit if it's running all day with the engine.

Overall I feel my 20K is under loaded and under utilized. There was one winter night that I tripped my 50A shore power breaker thanks to lots of heat (family visiting) and the dryer running. No problem. I fired up the 20K for a couple of hours. Felt good!

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Old 05-06-2016, 06:44 AM   #9
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"The boat is power thirsty, I"ve blown the breaker in the 6 kw more that a couple of times.

Have you thought about a few load shedding relays?

Frequently some items can be secured ,like a HW heater , or deep freeze for a while to maintain a modest noisemaker load.

A pass thru inverter would also help , using a bit of battery power to help start large motor loads.
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Old 05-06-2016, 07:00 AM   #10
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Originally Posted by Hawgwash View Post
OK, thanks, Ted.
So using simple math when the 6KW hits 75% and you switch to the 12 KW would it's load then be around 25% and would that be too light for it?

There is reverse air and a water maker. Can the water maker be run under way?
FWIW, 75% of 6 Kw is 4.5 Kw. 4.5/12 = 37.5%.
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Old 05-06-2016, 07:55 AM   #11
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Originally Posted by Hawgwash View Post
OK, thanks, Ted.
So using simple math when the 6KW hits 75% and you switch to the 12 KW would it's load then be around 25% and would that be too light for it?

There is reverse air and a water maker. Can the water maker be run under way?
At 75% on 6 KW the load would be 4.5 KW. That would be 37% on the 12 KW unit, which would be an adequate load.

The water maker should be able to be run under way depending on how it's powered. Some have engine driven pumps, but most are electric.

Depending on the size of the boat, I would have a 3 KW sine wave inverter. Many of the under way loads can be run off the inverter reducing generator time. I'm guessing that in the PNW there would be a fair amount of time that heat / AC wouldn't be required which is my biggest need for generator time.

Ted
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Old 05-06-2016, 09:49 AM   #12
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FWIW, 75% of 6 Kw is 4.5 Kw. 4.5/12 = 37.5%.
Yeah, I was using a Taiwanese calculator and it leaks.
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Old 05-06-2016, 09:59 AM   #13
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After reading all this, very happy with a very basic 6.0kw NL now for 15 years...
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Old 05-06-2016, 10:26 AM   #14
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A larger genset combined with a smaller one allows for the installation of a hydraulic get home system driven off the lesser used larger unit. This way the problem of under loading the bigger unit is negated during normal use.

This setup is gaining favor with several builders and companies like Wesmar. Also facilitates use of at rest stabilizers, winches and thrusters. Lots of rationale in dual gensets for some following these paths . Not to forget, as inverters and associated electrics get bigger and more expensive, the utter simplicity of a smaller genset becomes attractive.

Smaller gensets don't have to be made with printed circuit boards or electronic fuel delivery systems. Lots to consider when doing a new build or refit of a vessel with space for alternatives.
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Old 05-06-2016, 10:32 AM   #15
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A larger genset combined with a smaller one allows for the installation of a hydraulic get home system driven off the lesser used larger unit...
My boat didn't come with a get home motor. I've been eyeing up my 20K generator as a power source. I started thinking in terms of hydraulics, but now I'm considering an electric motor.

Rather than adding a PTO for hydraulics why not use the existing PTO for 240V? Motor technology is continuing to improve - and I'm not planning on the upgrade for a few years yet.

What are people's thoughts? Any reason not to go electric for the get home?

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Old 05-06-2016, 10:43 AM   #16
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Originally Posted by FF;
Have you thought about a few load shedding relays?
Besides what appears to be a belly up cock a roach over there in the corner, what am I looking at here?
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Old 05-06-2016, 11:22 AM   #17
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Now that challenges my logic. How would you determine a common size that is not constantly over or under loaded? Unless, maybe in cases like yours in FL, your loads seldom vary day to day, day to night. Is that it?
Well, first with modern generators you toss out the old wive's tails regarding loading and go straight to the manufacturer's recommendations. Northern Lights recommends ideally between 30 and 70% but just not below 25%. Therefore, the only problem you'd have on the 12 kw unit is if your consumption was below 3 kw. I'm guessing you're running the 6 kw unit now between 3 and 4.5 kw load and more toward the 4 to 4.5 range. That would be fine on the 12 kw unit.

Those who argue for the smaller unit quote a rule of not running under 50% load. That is not the rule applicable on current generators. The boat we're on at the moment came with one 21.5 kw generator. We added a second identical as a back up and to reduce wear. Our minimum requirement has been around 6 kw, our average has been around 11 kw and our maximum just over 14 kw. I know someone with a very similar boat and he had a 20 kw and a 9.5 kw. The only use of the 9.5 unit has been just to periodically run it as a preventative measure, but the only unit that gets actually used is the larger unit. The 9.5 doesn't even make them a good back up as to use it, they would need to shut down all air, not use their thrusters, not cook and wash dishes, as their freezers, refrigerators, lighting and a television or stereo, plus instruments and electronics push it up to nearly 70% load.

We did extensive calculations and testing on this boat and the factory installed 21.5 unit before concluding what to do with the second generator. The answer was either skip it, or get a duplicate.

I would encourage anyone considering a second generator to do some math on the first and even some real world testing. By far the majority I've observed with what I call a generator and a half, the half gets very little usage. There are those boats on which the generator and half makes sense but they are few and far between.

The other thing we looked at was size and weight and we were amazed how close they are. An Onan 11.5 weighs 695 lbs and is 41x22x23. An Onan 21.5 weighs 930 lbs and is 44x24x27. For us it was either one 21.5 or two 21.5's.
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Old 05-06-2016, 11:37 AM   #18
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What are people's thoughts? Any reason not to go electric for the get home?

Richard
Richard, it sounds like a good compromise for a retrofit as in your case. VFD proven technology has made electric motors far more manageable and continues to evolve. Having said that a hydraulic system is perhaps more readily available "off the shelf" so to speak.
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Old 05-06-2016, 12:55 PM   #19
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Those who argue for the smaller unit quote a rule of not running under 50% load. That is not the rule applicable on current generators. The boat we're on at the moment came with one 21.5 kw generator. We added a second identical as a back up and to reduce wear. Our minimum requirement has been around 6 kw, our average has been around 11 kw and our maximum just over 14 kw.

We did extensive calculations and testing on this boat and the factory installed 21.5 unit before concluding what to do with the second generator. The answer was either skip it, or get a duplicate.
Certainly this would be a good argument for waiting on the second generator until after developing some solid numbers. Certainly pull wire and have all infrastructure in place for the second one. With solid use numbers, clearly 16 to 18 KW would have been a better all around fit for the boat.

Ted
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Old 05-06-2016, 01:03 PM   #20
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Besides what appears to be a belly up cock a roach over there in the corner, what am I looking at here?
3 Blue Sea MEGA fuses, 2 Blue Sea 6 fuse blocks and 2 circuit breakers.
No relays there.
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