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Old 08-24-2017, 01:50 PM   #1
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Genset Load Question

I read about generators needing to run with a load as no load is bad. OK. I have no understanding whatsoever of why but trust it is true as I read it everywhere. When I run my Westerbeke, at least this time of year, it is for the A/C units and battery charging. Pretty much always two MarineAire 12K and a 16K. Sometimes three by adding on another 9K I believe it is. Is this considered a "good" load on the generator? When weather cools down and I don't need A/C I have the water heater, oven/range. Obviously not going to cruise around with an empty oven or hot stove which leaves me with the water heater. Oh, I do have a refrigerator on at all times as well but believe that is running off of 12v.

I'm fairly confused on my electrical system and have an electrician scheduled to come to the boat and help me sort it all out. Battery charger, inverter, combiner, separate 8D's for house and starting and a couple of more batteries that I'm not even sure of. But I digress...
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Old 08-24-2017, 01:52 PM   #2
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I took a generator class at Trawlerfest last year. The instructor (from Northern Lights) said you always want to load it at least 50%. Now the fun part, how much do your loads total up to?????
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Old 08-24-2017, 02:20 PM   #3
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Marine gennies paid for the construction of my boat, so I have a little experience with them.

Don't get too wrapped up in worrying about load. Probably optimal to run 25-75% load, but many units have logged many hours at like 10% and are doing fine. Even mine with around 5000hrs often will run overnight at almost no load, occasionally running an AC unit for 15min.

If you run it at very light load for a long time (several hours) make a point of loading up 50-75% for say an hour to clean things up.

And don't worry about it.

When weather gets cool and you don't need AC, shut gennie off and run loads on inverter.
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Old 08-24-2017, 02:22 PM   #4
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Three air con units is excellent assuming you are not overloading. You didn't say how big your genny is.
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Old 08-24-2017, 02:22 PM   #5
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Quote:
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I read about generators needing to run with a load as no load is bad. OK. I have no understanding whatsoever of why but trust it is true as I read it everywhere. When I run my Westerbeke, at least this time of year, it is for the A/C units and battery charging. Pretty much always two MarineAire 12K and a 16K. Sometimes three by adding on another 9K I believe it is. Is this considered a "good" load on the generator? When weather cools down and I don't need A/C I have the water heater, oven/range. Obviously not going to cruise around with an empty oven or hot stove which leaves me with the water heater. Oh, I do have a refrigerator on at all times as well but believe that is running off of 12v.

I'm fairly confused on my electrical system and have an electrician scheduled to come to the boat and help me sort it all out. Battery charger, inverter, combiner, separate 8D's for house and starting and a couple of more batteries that I'm not even sure of. But I digress...
Most boats have an AC amp meter to monitor load either from shore power or Gen set.
If you know the rating of your Gen, easy to figure out.
Commons size is 7.5kw
7.5kw = 7500 watts
7500 watts/120volts = 62.5 amps
1/2 load would be 31.25 amps

The reason for "loading" the /gen is to obtain proper combustion temps to prevent carbon build up and fouling the injectors (assuming diesel)
This has nothing to do with the engine temp I.E.:temp guage
Same thing on your main(s) engines some people run at low rpm and load for fuel economy. Make sure to run them up for 10-15 min after a run to clean them out
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Old 08-24-2017, 09:03 PM   #6
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My 8kw NL genny only had 150hrs when I bought the boat. When it was installed they went slightly oversized so I try my best to load it. AC units provide good loads. To run my main 16k unit and it's fan and water pump is about 12amps. With all 3 running I'm in the 20-25amp range. So no problem loading when the AC is running. Whenever I run it to charge batteries I also run the water heater and everything else that we might be using at the time to help improve the load. 500 hours so far so good.

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Old 08-24-2017, 09:16 PM   #7
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OK, so my gen is a Westerbeke 8KW.
8kw = 8000 watts
8000/120volts = 66.7 amps
1/2 load would be 33.4 amps

So I need to find out how many amps each piece of 120v equipment draws.
I need to load it, as least reasonably, because the load is to the generator what RPM's are to my engine. 50% load on gen = me cruising at 1600-1800 rpms. Now that I think about it I have an amp gauge by my power panels which showed about 25 amps as we were cruising with the air on the other day.

Now I need to figure out the ammeters on my helm gauge panel. When I am cruising with no generator on they all show 0. Upper helm and lower. With the generator on the port meter reads 5amps or so. Starboard is 0. In a car this gauge shows that my alternator is working and putting out electricity. Haven't a clue what 0 means on a boat. Has to have something to do with the inverter/battery charger/combiner?
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Old 08-24-2017, 09:40 PM   #8
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Two air con units will be in the neighborhood of 25-30 amps total when the compressors are running. I recommend an AC/DC clamp on ammeter for all cruisers. These are not the ones you usually find at Home Depot or Lowes. Read the specs carefully to make sure it can read at least 40 amps DC.
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Old 08-24-2017, 09:59 PM   #9
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Thanks for all the replies. Why would my ammeter gauge, which I think shows the output of the alternators, read 0 when running? Why would only one, port side, read 5-10 amps or so when I am running with the generator on but the starboard read 0?
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Old 08-24-2017, 11:21 PM   #10
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Light loads can cause cylinder glazing. It's most common in high rpm gen sets. Most common in the US, to make 60 cycles, are 3600, 1800, and 1200 rpm. RPM determined by the generator windings. The problem rarely shows in low rpm engines.
MY current boat came with 2 14kw, 1800 rpm sets. Perkins and Onan. Both with glazed cylinders causing high oil consumption and an oil sheen in the exhaust water. Previous owner always ran one 24/7 when away from the dock. I've seen it many times in 3600 rpm engines and not as much in 1800 rpm engines.
Sometimes rings and a honing will solve it. Otherwise an overhaul.
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Old 08-25-2017, 02:23 AM   #11
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Thanks for all the replies. Why would my ammeter gauge, which I think shows the output of the alternators, read 0 when running? Why would only one, port side, read 5-10 amps or so when I am running with the generator on but the starboard read 0?
One might be feeding your nav equipment. Don`t they both show a charge just after starting, to replenish the amps used to start?
Is there an ammeter for each engine, plus an ammeter for house etc usage?
Best I leave this to the electrical guys, but there are some thoughts.
Maybe you need to test your gauges, or test for the actual alt outputs.
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Old 08-25-2017, 06:55 AM   #12
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I don't think I can load mine at 50%. Water heater and battery chargers and fridge is all that's really on it. It's a 4.4 Westerbeke but it has made it 2200+ hours.
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Old 08-25-2017, 08:11 AM   #13
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Apart from keeping a reasonable load on these smaller generators the worst thing is not to run them long enough each time you use it, most of these engines corrode away before you wear them out!!, turn them on and let them run, they will love you for it,

If being raw water cooled remember to change out the rubber impeller every season ! so it wont let you down,
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Old 08-25-2017, 01:28 PM   #14
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Don't get too wrapped up in worrying about load. Probably optimal to run 25-75% load, but many units have logged many hours at like 10% and are doing fine. Even mine with around 5000hrs often will run overnight at almost no load, occasionally running an AC unit for 15min.

If you run it at very light load for a long time (several hours) make a point of loading up 50-75% for say an hour to clean things up.
How do I just % load on my genset? I have a 6kw NL genset. I assume that means that it has a capacity to put out 50amps at 120v?

Are the number of amps being put out a good measure of load?
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Old 08-25-2017, 02:21 PM   #15
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Thanks for all the replies. Why would my ammeter gauge, which I think shows the output of the alternators, read 0 when running? Why would only one, port side, read 5-10 amps or so when I am running with the generator on but the starboard read 0?
If both alternators are connected to the same battery bank(s), one will shut down when the bank gets to nearly fully charge because one will always have a slightly higher voltage setting than another.

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Old 08-25-2017, 02:23 PM   #16
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How do I just % load on my genset? I have a 6kw NL genset. I assume that means that it has a capacity to put out 50amps at 120v?

Are the number of amps being put out a good measure of load?
Yes.
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Old 08-25-2017, 02:27 PM   #17
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Yes.


Thanks.
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Old 08-25-2017, 03:53 PM   #18
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There have been prior threads on running light loads, all of this information is anecdotal or based on one or two data points...

just sayin'...

(I tend to subscribe to the give it some load theory but I think it just makes me feel good)
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Old 08-25-2017, 04:26 PM   #19
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There have been prior threads on running light loads, all of this information is anecdotal or based on one or two data points...

just sayin'...

(I tend to subscribe to the give it some load theory but I think it just makes me feel good)
And I think you're right. It seems reasonable that extended run times at very light loads could cause problems, but 20% or more for extended periods with occasional much heavier loads is probably fine.

Ken
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Old 08-25-2017, 05:03 PM   #20
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If all that's running is the water heater, turn the genset off . Water will stay hot a long time, and with a thermostat the WH will be off a lot in practice. One thing to determine as well is if the WH gets heat from the engines, common on a lot of boats.
Consider getting an inverter to run light loads; talk with your electrician about that.
In the meantime get a copy of Calder's "Boat Owner's Mechanical and Electrical Manual".. essential reading and on-board tool written in plain English.
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