2 Generators
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O C Diver
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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.
Ted
Ted
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?
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.
Malahide
Member
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...
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...
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.
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.
Britannia
Wannabe
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!
Richard
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!
Richard
FF
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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.
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.
High Wire
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FWIW, 75% of 6 Kw is 4.5 Kw. 4.5/12 = 37.5%.
O C Diver
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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
Yeah, I was using a Taiwanese calculator and it leaks.
Marlinmike
Guru
After reading all this, very happy with a very basic 6.0kw NL now for 15 years...
sunchaser
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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.
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.
Britannia
Wannabe
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?
Richard
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.
CPseudonym
Moderator Emeritus
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.
O C Diver
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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
High Wire
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3 Blue Sea MEGA fuses, 2 Blue Sea 6 fuse blocks and 2 circuit breakers.
No relays there.
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sunchaser
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A few years ago I spent some time on a Northern Marine 57. It had 20 and 6 kw units as well as a decent inverter setup. The 6 kw unit did most of the work as far as keeping the house bank up even while at anchor. The 20s main hours were low in comparison to the 6, relegated to using AC, clothes washing and serving as a hydraulic pump source for the get home.
So many options I can't say one way for size decision is better, especially when taking into account not all of us base survival upon ACs up and running due to location.
So many options I can't say one way for size decision is better, especially when taking into account not all of us base survival upon ACs up and running due to location.
17 was a consideration but then it wouldn't have been interchangeable and would have been overloaded with our maximum load, plus not completely interchangeable parts which we consider that a plus. There's no reason not to go with the 21.5 in our case. No load that is too little for it. Thinking logically, the boat normally only comes with the 21.5. That way too our max load to date of 14.3 would be 84% on the 17 and while that's our max load to date, I'm sure some day we'll top it. We did get all the numbers before deciding, so I certainly agree with you on that to get some real numbers, not just theory, before finalizing second generator.
Capt.Bill11
Guru
12kw - durning the day for hotel service
6kw - at night after dinner when the load is light
Location means a lot. We almost always have A/C on, except the last few days in NY, running heat, but same issue. We do a lot of laundry and cook a lot. Seems most of the females have long hair so lots of shower time and hot water.
If we hadn't added additional freezers then we would have had a different picture. We have switchable units but normally running 3 freezers and one refrigerator.
Our load doesn't lighten as much as others seem to.
In fact with 43-46 degree nighttime temperatures right now, ours would be heavier at night if we were anchoring.
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Gordon J
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I have the exact same generators on my 2003, Ocean Alexander 456. I have owned the boat only for about a month. Typically if I am only charging the batteries and using one air-conditioner, or the stovetop , I will use the 6KW period. All five air-conditioners are 220 V. The 6KW will run the 220 V air conditioner water pump and two of the smaller air-conditioners. All else requires The 12 KW.
For what it's worth, the previous owners ran the 6KW much more than the 12 KW.
The 12 KW has about 1800 hours but was original to the boat. The 6KW was installed in 2010 and it has about 1500 hrs.
Obviously, if I were to run both the flybridge air-conditioners as well as the three below decks, I would need to run the 12 KW.
Gordon
For what it's worth, the previous owners ran the 6KW much more than the 12 KW.
The 12 KW has about 1800 hours but was original to the boat. The 6KW was installed in 2010 and it has about 1500 hrs.
Obviously, if I were to run both the flybridge air-conditioners as well as the three below decks, I would need to run the 12 KW.
Gordon
O C Diver
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Guess I misunderstand what you wrote. When you said 14 KW was your maximum, I assumed it was everything turned on at full load .
BTW, it looks like Cummins / Onan offers two 17.5 KW units. One identical to your 21.5KW but turning 1500 instead of 1800 RPM.
Ted
I was comparing to their regular 17 kw unit. The other 17.5 you mention is 50 Hz. I was saying just over 14 kw was our maximum to date. It's not our theoretical maximum and I do anticipate more sometime. But even with say 14.3 kw usage I wouldn't use a 17 kw generator as that would put it at over 84% load.
Sorry I was a little unclear. The real issue is that the 21.5kw generator was the sizing the builder felt appropriate for that boat and I don't see us ever loading it less than 6 kw which is 28% and generally not under 9 kw which is 42%. So, I'm very comfortable with the 21.5 kw for both the primary and second generator.
Now I see someone else posting that they've almost exclusively used their 6 kw instead of 12 kw. Everyone's usage is different. That's running 2 of the smallest A/C's out of his 5 units. That's not a pattern we would ever fit in. The only A/C we sometimes would not be running is the flybridge.
We could easily get by with just one 21.5 kw gen. The worry of it being down for a while is my main reason for having two and in our case since the second is for backup purposes then makes sense to be the same.
Obviously if you can operate on a smaller unit, that's fine. However, the argument that it was more economical to operate and the argument over load are both weak. At 1/4 load a 21.5 uses 0.8 gph. At 1/2 load an 11.5 uses 0.7 gph. So with the same load, minimal difference in fuel usage.
101TUG
Senior Member
and when on ship you have 2 generators same size don't try to equalize hours, give 1000 hours difference because if you give same hours when problem happen on one you get same problem on other one and you can you in trouble...
so that better to give big difference
Hugues
so that better to give big difference
Hugues
and when on ship you have 2 generators same size don't try to equalize hours, give 1000 hours difference because if you give same hours when problem happen on one you get same problem on other one and you can you in trouble...
so that better to give big difference
Hugues
No two generators are going to give trouble at exactly the same hours. Just not going to happen. You might be years apart.
