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capta 03-13-2019 01:31 AM

eight kilowatts
 
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?

CVAT 03-13-2019 02:08 AM

Re: eight kilowatts
 
Quote:

Originally Posted by capta (Post 2051588712)
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?

HP and wattage are both measures of power, one electrical and one physical but can be directly converted.

Your electrician friend is correct 1 hp is 745.69 watts so at 10.5 hp the max wattage it can produce is 7829.745 that is assuming 100% conversion of power between mechanical power, hp, and electrical power, kw. Onan is over stating the generators output by rounding to the nearest thousand, at 10.5 hp the maximum it could produce, without loses and assuming 100% conversion is the above stated 7829.745 watts. No diesel generator can convert 100% efficiently from mechanical power to electrical, so in my opinion this is just Onan overstating the engines ability if it is true. When I did a check of Onan generators online the 8 KW models had 22.3 hp engines attached.

By the way for your first example, assuming the generator is capable of multiple voltages it would produce 4.4 KW, half the voltage but twice the amperage, as wattage = voltage X amperage.

Minnewaska 03-13-2019 06:01 AM

Re: eight kilowatts
 
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

outbound 03-13-2019 07:54 AM

Re: eight kilowatts
 
On the current boat first had 4 kw lombardini. It was a total piece of junk. When I replaced it an new 8 kw Northern Lights was available at a extremely good price so installed that.
I never maxed out the output of the 4 kw unit. The current genset goes on rarely (has 17hours since Oct of 2018). When it goes on I charge batteries, vacuum, turn on the AC, watermaker, charge all electronics and tools etc. but obviously still don’t max out the 8kw unit.
I knew I wanted a low rpm unit. Smallest I could find was a 6kw and it was considerably more expensive than the 8kw.
Did I make a mistake in buying the 8kw? How important is it that a genset be run under full load occasionally?

Minnewaska 03-13-2019 08:10 AM

Re: eight kilowatts
 
I'm not sure, but I doubt it's a problem to run a genset below full load. They run at constant rpm anyway. I suspect it may be poor for the diesel to run it at zero load for lengthy periods of time, but no one would do that. Curious too.

CVAT 03-13-2019 08:28 AM

Re: eight kilowatts
 
Quote:

Originally Posted by Minnewaska (Post 2051588736)
I'm not sure, but I doubt it's a problem to run a genset below full load. They run at constant rpm anyway. I suspect it may be poor for the diesel to run it at zero load for lengthy periods of time, but no one would do that. Curious too.

Found this https://boatmags.com/choosing-the-right-generator/ interesting and informative read.

Don L 03-13-2019 08:58 AM

Re: eight kilowatts
 
Quote:

Originally Posted by outbound (Post 2051588734)
Did I make a mistake in buying the 8kw? How important is it that a genset be run under full load occasionally?

you are happy with it so, No you didn't make a mistake

JimsCAL 03-13-2019 09:15 AM

Re: eight kilowatts
 
CVAT's post is spot on.

colemj 03-13-2019 09:49 AM

Re: eight kilowatts
 
Quote:

Originally Posted by Minnewaska (Post 2051588726)
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

Our 8kW Kohler genset has a 14hp engine and is continuously rated for 8kW at 77F. It derates 1% for every 10F above 77F.

Our 5.5kW Nextgen genset has a 11hp engine and is continuously rated for 5.0kW at 77F. I don't know its derate factor for temperature, but know that it doesn't like running higher than ~4.5kW at typical hotter temps.

Mark

Minnewaska 03-13-2019 10:13 AM

Re: eight kilowatts
 
Quote:

Originally Posted by CVAT (Post 2051588740)
Found this https://boatmags.com/choosing-the-right-generator/ interesting and informative read.

That was a good read. This was the answer and it would seem unlikely many genset run with less than 25% load or one probably wouldn't be running them at all. I got a chuckle out of the recommendation that one consider having two different size generators to properly match loads.

Quote:

A generator should never run continually with less than a 25% load. 35% to 70% is optimal.

outbound 03-13-2019 11:54 AM

Re: eight kilowatts
 
So...
We never run the generator for just one load. Doing best guesses we’re at just above the 35% during the usual hour or so the time the thing is on. In retrospect the 6kw would’ve been enough but the 8kw should be alright.
Thanks for the help.

makobuilders 03-15-2019 05:40 AM

Re: eight kilowatts
 
It is very common for generators to have shortened lives because of either low engine loading or just lack of use at all. There have been a lot of heated discussions on the TrawlerForum regarding minimum loading of engines. One of the biggest factors is whether the engine is one of the new Tier-4 electronically controlled common rails, or just an old fashioned mechanical engine.

Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours. Certainly higher loading is better, but unless you're running your aircon, it may be difficult to provide such a high load. Perhaps timing water heater, battery bank bulk charge, electric oven and clothes washer/dryer.

I had two gennies in my power boat and hated both of them. My opinion is that unless you have aircon, then do not mount a genset. Today's energy efficient lighting and appliances, coupled with solar options, are all wonderful, whether for a sailboat or a powerboat.

BTW, outbound commented about installing a larger unit because the price was better, which is understandable. If you find yourself only running 2-3 kWhr over the next year, then consider having the injector pump (or other systems) de-tuned by a mechanic to a lower hp rating. The electrical head end does not care what it is loaded to (as long as the RPMs are correct), it's the diesel unit that needs to run properly.

CVAT 03-15-2019 08:19 AM

Re: eight kilowatts
 
Quote:

Originally Posted by makobuilders (Post 2051589130)
BTW, outbound commented about installing a larger unit because the price was better, which is understandable. If you find yourself only running 2-3 kWhr over the next year, then consider having the injector pump (or other systems) de-tuned by a mechanic to a lower hp rating. The electrical head end does not care what it is loaded to (as long as the RPMs are correct), it's the diesel unit that needs to run properly.

Actually de rating the hp will could the output of the generator as less hp means less mechanical power to convert to electrical power, so de rating the diesel could de rate the output of the generator if the diesel does not generate sufficient power to produce the wattage, i.e if it where a 1hp engine it could produce 745.69 watts maximum assuming 100% conversion efficiency which they are not, if it were de rated to 0.8 hp a 20% reduction then the maximum electrical power that could be produced is 596.55 watts. In the end power is power whether it is Horse Power or electrical and changing one will result in changing the other.

Minnewaska 03-15-2019 08:34 AM

Re: eight kilowatts
 
A great point was made above that all engines need to run or rot. When conserving fuel on a passage, there is nothing wrong with avoiding consumption for days at a time. However, when fuel is available, the engines should be run to full temp and under load routinely. I don't have any science to support it, but I'd say that weekly would be an absolute minimum.

Minnewaska 03-15-2019 08:37 AM

Re: eight kilowatts
 
If one has the place for it, another great multi-tasking use for the genset is a washing machine. When cruising, our Splendide is a luxury beyond words. We do not bother drying (which uses more water than washing), but cleaning up the clothing with real soap and fresh water is amazing. This is never done stand alone, only when using the gen for all other tasks too. I'd say the gen burns a few gallons per week. It's pretty inconsequential.

Don L 03-15-2019 09:00 AM

Re: eight kilowatts
 
Quote:

Originally Posted by makobuilders (Post 2051589130)

Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours.

Several hours?????

Far as I'm concerned all you need to do is run it till it's fully warmed up and fully loaded for 10 minutes or so. I'm not going to listen to the generator for several hours just to exercise the unit. I also only shot for every couple of weeks.

Of course it will be summer soon, running it wouldn't be a problem. :laugh

capta 03-15-2019 12:36 PM

Re: eight kilowatts
 
Quote:

Originally Posted by makobuilders (Post 2051589130)
It is very common for generators to have shortened lives because of either low engine loading or just lack of use at all.
Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours. Certainly higher loading is better, but unless you're running your aircon, it may be difficult to provide such a high load. Perhaps timing water heater, battery bank bulk charge, electric oven and clothes washer/dryer.

I guess I neglected to mention that our last genset was a running take out at 19,000 hours plus. Certainly not without some problems at the end like rusting out freeze plugs and the alloy water pump housing corroding, but none the less a running take out.
As for operating our genset, it gets at least an hour in the morning and evening 365, with a water heater, high amp battery charger, fridge/freezer, and various kitchen appliances during cooking. When not cooking, the 35 gph watermaker goes on with its 17 amp draw, so I'm guessing a light load or little use won't be the death of this one.
In the marine field, I believe electronics are a big mistake in both gensets and main engines. Spare pc boards deteriorate at nearly the same rate as those in use and they are expensive. A good old mechanically regulated genset and main engine are worth their weight in gold if one is beyond the reach of Sea Tow or speedy delivery (1 day overnight usually takes about a week here).

Minnewaska 03-15-2019 03:10 PM

Re: eight kilowatts
 
Cooking has been the time when I'm most likely to run the genset. I refuse to crank it up, the moment I awake. A quiet cup of coffee is life extending. When breakfast or dinner prep starts, on comes the gen, but I like to shut it off to actually eat and relax. Watch the sunset peacefully. Evening becomes the toughest to schedule. Sometimes, sundowner cocktails delay dinner prep and time runs out. :)

capta 03-15-2019 04:58 PM

Re: eight kilowatts
 
1 Attachment(s)
Quote:

Originally Posted by Minnewaska (Post 2051589236)
Cooking has been the time when I'm most likely to run the genset. I refuse to crank it up, the moment I awake. A quiet cup of coffee is life extending. When breakfast or dinner prep starts, on comes the gen, but I like to shut it off to actually eat and relax. Watch the sunset peacefully. Evening becomes the toughest to schedule. Sometimes, sundowner cocktails delay dinner prep and time runs out. :)

With Onan's sound shield, the genset is nearly silent, so it's nothing more than a background hum. Now that we've switched to gas cooking, it's mainly about refrigeration, battery charging, kitchen appliances to facilitate the cook's job and making and heating water.
Now the big question for us is switching from an electric coffee maker to a stovetop method. Any suggestions on which you like best? Please remember we are way south of civilization, so some suggestions won't be of much use to us.
My favorite the last time I had stovetop made coffee is pictured below, but if there is something better these days, I'm all ears.

colemj 03-15-2019 08:52 PM

Re: eight kilowatts
 
No reason to give up your electric coffee maker. If you have decent batteries and a suitable inverter, just run it off the inverter. They draw ~750W for maybe 5-7 minutes, so your batteries won't be hit hard. You will lose maybe 7-9Ah from your batteries.

Mark

capta 03-15-2019 09:05 PM

Re: eight kilowatts
 
Quote:

Originally Posted by colemj (Post 2051589336)
No reason to give up your electric coffee maker. If you have decent batteries and a suitable inverter, just run it off the inverter. They draw ~750W for maybe 5-7 minutes, so your batteries won't be hit hard. You will lose maybe 7-9Ah from your batteries.

Mark

We do that anyway on charter, but sometimes one guest wants regular and the other decaf, so with the gas stove we can now do both at once.

chef2sail 03-15-2019 11:15 PM

Re: eight kilowatts
 
Coffee made in an electric pot or " boiled " is like drinking cheap pink zinfandel.

If you really like coffee....grind your beans before brewing and put it in a French press. Otherwise don't waste your money and buyMaxwell House instant😞

CVAT 03-16-2019 12:21 AM

Re: eight kilowatts
 
What is this heresy Decaf you speak of...kinda like non-alcohol beer...

outbound 03-16-2019 07:53 AM

Re: eight kilowatts
 
We use an Aeropress. It’s a plastic tube with a piston that is pushed down to force the coffee through a fine ss mesh. You get a hockey puck of grinds at the bottom when you’re done. The bottom of the device twists off so you can pop the used grinds out easily. No hassle cleaning it like with a French press. Useable in a seaway. Nothing to buy. No paper filters. Makes excellent coffee.
Current one is 6 years old. Works fine. $35 on internet.
Buy the local coffee. Surprisingly the cheapest brands are often the best.

Minnewaska 03-16-2019 10:53 AM

Re: eight kilowatts
 
Quote:

Originally Posted by capta (Post 2051589272)
With Onan's sound shield, the genset is nearly silent, so it's nothing more than a background hum.

Ours 7.6kw Mase genset is pretty quiet too. It lives inside it's own sound dampening enclosure, all of which is buried in a dedicated soundproofed genset locker under the salon sole and below the water line. You can stand right on top of it and have a conversation, without slightly needing to raise your voice. Nevertheless, on a peaceful morning, I don't even like hearing the dull buzz. If you sit in the cockpit, it's impossible to hear the motor running, but you do hear the exhaust splash.

Truth be told, I'm usually the first one up in the morning and I'm hoping to get some me time, before the boat full of guests arise. The genset is like an alarm clock. One could easily sleep through it, but they'll know I'm up.

Quote:

Now the big question for us is switching from an electric coffee maker to a stovetop method. Any suggestions on which you like best?
I find an old fashioned low tech stove top perculator works just fine. It's far from the best coffee, but with the right beans, it's perfectly acceptable and welcome. I have a high end prosumer espresso machine at home, which is my daily driver. I know good coffee, I just don't feel the need to live every moment in that space. Everything literally tastes better aboard anyway. A French Press makes better coffee than a perculator, IMO, but are more difficult to clean.

My go to onboard, however, is a Boche electric drip. I have a 10 cup and a 4 cup. The later gets decaf, when some want it. Both run off the inverter.

I've recently been introduced to a Nespresso pod based espresso machine. I'm an espresso snob and found some of the pods to be remarkably better than expected. Nothing like the real deal, but totally acceptable for a quick hit. Very expensive pods ($0.75 to $1.00 each), but could easily be run off the inverter as well.

My biggest advancement has been to buy one of what I call the gas station style pump carafes. I can get two full pots in there and have one more brewing, which is necessary when we have a lot of people aboard. Having to wait for someone to finish one pot, before I can start brewing the next, is awkward and leaves some waiting or politely not taking the last half cup.

makobuilders 03-17-2019 06:19 AM

Re: eight kilowatts
 
Quote:

Originally Posted by capta (Post 2051589192)
I guess I neglected to mention that our last genset was a running take out at 19,000 hours plus.

That's a fantastic long life. Did you add an oil purifier or do anything special?

Quote:

Originally Posted by capta (Post 2051589192)
As for operating our genset, it gets at least an hour in the morning and evening 365, with a water heater, high amp battery charger, fridge/freezer, and various kitchen appliances during cooking. When not cooking, the 35 gph watermaker goes on with its 17 amp draw, so I'm guessing a light load or little use won't be the death of this one.

:smile

Quote:

Originally Posted by capta (Post 2051589192)
In the marine field, I believe electronics are a big mistake in both gensets and main engines. Spare pc boards deteriorate at nearly the same rate as those in use and they are expensive. A good old mechanically regulated genset and main engine are worth their weight in gold if one is beyond the reach of Sea Tow or speedy delivery (1 day overnight usually takes about a week here).

Amen brother. That's why with my new construction (a bluewater boat) I'm going with an old fashioned mechanical engine, even though it won't be compliant.

Quote:

Originally Posted by Don0190 (Post 2051589152)
Several hours????? Far as I'm concerned all you need to do is run it till it's fully warmed up and fully loaded for 10 minutes or so.

Generally running for a short time like that is not conducive to genset happiness. Running the engine puts sulfur, acids, carbon and pollutants in the oil, but they're not able to be driven out because the engine/block/oil never get hot enough. Time spent in the filter is minimum. Also when an engine is run infrequently then the startup is "dry" without adequate lube oil on the cylinder walls - a big source of engine wear. But honestly, no one but you knows how you're actually using your systems, so hopefully you're enjoying a good long life in your genset.

makobuilders 03-17-2019 06:28 AM

Re: eight kilowatts
 
Quote:

Originally Posted by CVAT (Post 2051589142)
Actually de rating the hp will could the output of the generator as less hp means less mechanical power to convert to electrical power, so de rating the diesel could de rate the output of the generator if the diesel does not generate sufficient power to produce the wattage... In the end power is power whether it is Horse Power or electrical and changing one will result in changing the other.

Yes that is exactly my point. Let's say you have a 10hp 7.5kW genset but never load it past 2kW. That likely will result in a shorter engine life. So you have the engine de-tuned to (example) 7hp so at max it will produce 5kW. Now you are loading your diesel to 40% which is alot healthier.

Obviously there are limitations but a good mechanic could figure it all out.

I'm looking at gensets and the exact same engine model is available in 12kW or 16kW, so there is a difference in the tuning of the engine.

CVAT 03-17-2019 07:10 AM

Re: eight kilowatts
 
Quote:

Originally Posted by makobuilders (Post 2051589604)

I'm looking at gen sets and the exact same engine model is available in 12kW or 16kW, so there is a difference in the tuning of the engine.

Due to the fact that you want the diesel to run at constant rpm either 3600 or 1800 depending on if it is 2 pole or 4 pole, the difference between the two would most likely be on the generator side not the diesel side, most probably the current regulation circuits. But would have to see detailed breakdown of both units to be sure, but willing to bet a beer that they are probably almost identical except for a few parts.

capta 03-17-2019 01:00 PM

Re: eight kilowatts
 
Quote:

Originally Posted by makobuilders (Post 2051589600)
That's a fantastic long life. Did you add an oil purifier or do anything special?



Amen brother. That's why with my new construction (a bluewater boat) I'm going with an old fashioned mechanical engine, even though it won't be compliant.

Generally running for a short time like that is not conducive to genset happiness. Running the engine puts sulfur, acids, carbon and pollutants in the oil, but they're not able to be driven out because the engine/block/oil never get hot enough. Time spent in the filter is minimum. Also when an engine is run infrequently then the startup is "dry" without adequate lube oil on the cylinder walls - a big source of engine wear. But honestly, no one but you knows how you're actually using your systems, so hopefully you're enjoying a good long life in your genset.

We always changed the oil around 150 hours. We added a couple of ounces of Marvel Mystery oil at each oil change. When I got the genset it had around 6 k hours and of course running it every day of the year (any engine) would extend its lifetime considerably. Nothing is worse on an engine than not running it.
When I stored my truck when away operating a boat for extended periods, the first thing I'd do is take it to get the oil changed, as I believe longterm storage of oil in the engine deteriorates it.
Compliant with what? If you want a suggestion, I'd go with a Gardner, from Joe McCool
(really) in the UK. Absolutely the best, longest lasting, most fuel-efficient diesels I've ever worked with (and heaviest per hp). Had this boat not had an engine w/only 30 hours on it, she would have a Gardner today. My last one pushed a 125-ton steel vessel @ 10 knots in almost any seaway on 5 gph at 375 rpms. At that rpm, no need to change oil frequently!
I agree, 10 minutes is a rather short time to run any diesel, though it is better than nothing.

amandacian 11-02-2019 10:07 AM

Re: eight kilowatts
 
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

colemj 11-02-2019 12:48 PM

Re: eight kilowatts
 
Quote:

Originally Posted by amandacian (Post 2051634244)
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

It depends on the generator. Low speed 4-pole ones like Northern Lights, Kohler, etc are generally rated for continuous load, while high speed 2-pole ones like NextGen, Entec, etc are generally rated at max temporary load.

For example, our previous NextGen was plate-rated 5.5kW, but could only be run at 4.5kW continuous. Our current Kohler is plate-rated 8kW, and that is for 8kW continuous duty.

Mark

Minnewaska 11-03-2019 08:45 AM

Re: eight kilowatts
 
Quote:

Originally Posted by Minnewaska (Post 2051588726)
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

Quote:

Originally Posted by amandacian (Post 2051634244)
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

That's an odd coincidence.

colemj 11-03-2019 09:00 AM

Re: eight kilowatts
 
Huh, and I already gave the same reply. Short memory, I guess.

Mark


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