onan 7KW

[dhenline]

I was wondering if anyone would be able to give me the approximate fuel burn of an Onan Genny running 15K BTU AC for eight hours during the night. I am looking for a general ball park number of how many gallons a generator will use to run the AC on a 44 cat, if you run three AC units during the evening for sleeping

Also, does this generator ramp up and down de;pending on the load?

don

[Jammer]

Ballpark 10 kwh per gallon for any small diesel marine generator running at half to full load. Fuel economy is somewhat worse at light loads.


Nominal power consumption of an air conditioner in watts is BTU divided by EER. Water-cooled marine air conditioners have high EERs, figure 15 ballpark. So you're at 1000 watts. If you're running three of them that's 3000 watts.


Power draw of air conditioners varies depending on inside and outside temperatures. Also, the thermostat will cycle the air conditioners on and off so (if properly sized) all three of them won't run continuously all night. Figure 50% cycle time as a ballpark for a typical hot night.



Now we're at 1500 watts. That's going to be less than ¼ load for most diesel gensets so you won't get 10 kwh/gallon, maybe 7 kwh per gallon. Some arithmetic gives about ¼ gallon per hour or 2 gallons for an 8 hour run on a typical night.


All the Onan diesel generators that I'm aware of run at a constant speed regardless of load.

[Cwens]

Originally Posted by Jammer

All the Onan diesel generators that I'm aware of run at a constant speed regardless of load.

For those not familiar with the innards: AC generators run at a constant speed in order to produce 60 Hz (USA etc) or 50 Hz (Euro etc), but not at a constant fuel consumption: the governor supplies more or less fuel to maintain the target RPM, depending upon load. At nearly zero load, some fuel is burned just to overcome internal friction and keep the motor turning, so fuel burned per kwh generated is nearly infinite... efficiency increases, as Jammer points out, as load increases.

[dhenline]

Thank you very much to both of you of the very nice explanations. Very helpful to a new person such as myself.

[Jammer]

Originally Posted by ChrisOwens

For those not familiar with the innards: AC generators run at a constant speed in order to produce 60 Hz (USA etc) or 50 Hz (Euro etc)

The newer suitcase generators (notably the Honda EU2000i and competing and follow-on products) are variable speed. They have a 3-phase stator and then use a rectifier and inverter to convert the power to single-phase AC at 60/50 Hz. By running at lower speeds at light loads, the fuel consumption, wear, and audible noise are all improved.


As far as I know there are no diesel generators on the market that use this technique.

[dhenline]

interesting that you mentioned the suitcase generators. Honda has come out with a 3000i which weights in 49 lb. My partner in a my upcoming adventures thinks we should forgo the built in Generator and just use a portable Honda to run the AC at night. I love the idea of the weight savings and in general I have been unimpressed with the reliability of the built in generators. Having said that, some would likely call it lunacy to buy a new $900K boat and use a portable generator. We will likely purchase a new 44 CAt for this trip.
Our current game plan is to take delivery of a boat in the MED and over a two year period, sail it to Australia. HE has previously sailed from Florida to Australia in a 35ft Benetau so he is not completely naïve.

Crazy idea of a suitcase generator?

[Jammer]

People do it. A suitcase generator poses a number of practical problems and safety hazards that a permanently installed below-decks diesel generator does not, among them:


1. A relatively serious carbon monoxide risk. People on boats have died. The hazard is particularly acute while sleeping, doubly so if the generator is unattended.
2. Problems related to securing the generator in place. It must be in the open for proper cooling, must be secured in place so that a wave or wake does not dislodge it, and must be reasonably protected from boarding waves and spray. It also must be accessible for starting and operation.
3. The electric shock and fire hazards posed by using high-current portable electric wiring to connect the generator to the boat's electrical system, when at anchor or under way.
4. The fire hazards and inconvenience posed by handling gasoline in the quantities needed.
5. The difficulty of completely defueling the generator for storage below decks for passage.


A fact to consider in your specific situation is that a 3000i is marginal at best for one 15k air conditioner. It cannot run two at once let alone three. The starting load is too high for two air conditioners, and the actual rated output (2800 watts) of the 3000i is insufficient for three air conditioners except under favorable conditions (not too hot).

[dhenline]

its not that I disagree too much with what you say. The partner used a portable honda on his last crossing and loved it. He is a master electrician and is pretty confident it will work. I am a surgeon and dont know much about much other than fixing bones. I think his plan would be to use the generator in the open on the back deck while at anchor. One obvious advantage of a fixed generator is being able to draw off of the fuel tank. We will likely also tanker a large quantity of spare diesel in a fuel bladder. two gallons a night sounds not to bad for the built in generator.
Mostly I hate how unrelieable the built in ones seem.

[Jammer]

Originally Posted by dhenline

The partner used a portable honda on his last crossing and loved it. He is a master electrician and is pretty confident it will work.

Never ascribe to wisdom that which is adequately explained by good luck.

I think his plan would be to use the generator in the open on the back deck while at anchor. One obvious advantage of a fixed generator is being able to draw off of the fuel tank. We will likely also tanker a large quantity of spare diesel in a fuel bladder. two gallons a night sounds not to bad for the built in generator. Mostly I hate how unrelieable the built in ones seem.

Very few sailboats run air conditioning at anchor or on passage. If air conditioning is not used, it is possible to limit their generator runtime to, at most, an hour or two a day. With a catamaran that size it should be possible to fit sufficient solar panels that a generator isn't necessary at all. As a data point I do not have a generator on my 38' mono and don't plan to install one. It does have air conditioning installed by a previous owner which I do not anticipate using more than a few times a year.


The better quality built-in diesel gensets (e.g. Northern Lights) are reasonably reliable. The suitcase generators are reasonably reliable but most reports indicate that the maximum life is in the low thousands of hours (1000-3000) of operation, which is something you'll bump up against fairly quickly if you run it most nights. Of course you can just replace it as they aren't that expensive. Fuel cost ends up being somewhat higher than the diesels. Cost wise I worked it out one time and concluded that the cost is the same over the life of the boat (20+ years).

[dhenline]

[QUOTE=Jammer;3763317]Never ascribe to wisdom that which is adequately explained by good luck.

that is funny



Very few sailboats run air conditioning at anchor or on passage.


in my previous charter experience, I ran my AC most night. Unfortunately, I am pretty addicted to sleeping in a cool, dry environment. Several years ago, I was in the Pacific and very much appreciated the AC on the sailboat. I doubt I would not run my AC much of the time.

thank you

[Tin Tin]

2L (0.5US gal) per hr at 75% load per dealer website. No doubt you will also be running fridges and freezers, ice maker, water maker some times, lights TV etc etc, and they will be powered by a mix of AC and DC (via the battery charger) and also heating water. So running a combo of such devices will get it up to 75%. AT 50% it will use a bit less fuel but not as efficient. Hope you have CO monitors in each cabin.
Typical fuel consumption
No. 2 diesel fuel, L/hr (Gal/hr)No load1/4 load1/2 load3/4 loadFull load
7.0 MDKBK-50 Hz0.7 (0.2)1.1 (0.3)1.5 (0.4)2.0 (0.5)2.8 (0.8)

[dhenline]

thank you Tin Tin

good numbers to know

Don Henline

[skipperpete]

Originally Posted by dhenline

interesting that you mentioned the suitcase generators. Honda has come out with a 3000i which weights in 49 lb. My partner in a my upcoming adventures thinks we should forgo the built in Generator and just use a portable Honda to run the AC at night. I love the idea of the weight savings and in general I have been unimpressed with the reliability of the built in generators. Having said that, some would likely call it lunacy to buy a new $900K boat and use a portable generator. We will likely purchase a new 44 CAt for this trip.

Our current game plan is to take delivery of a boat in the MED and over a two year period, sail it to Australia. HE has previously sailed from Florida to Australia in a 35ft Benetau so he is not completely naïve.



Crazy idea of a suitcase generator?

The type of generator you’re likely to get with the yacht you eventually purchase will probably be the most compact lightweight type on the market (Fischer Panda, Paguro) and as you correctly point out, not the most trouble free types available. If you have the space, can carry the extra weight and are committed to having a reliable ac power supply the choices are between Kohler, Northern lights and Onan which all take up a fair amount of space with a sound shield fitted. Do you really need to run 3 aircons at sea though? Fans and good ventilation seem to work quite well for most of us on passage and its only in marinas that aircons justify their existence.

[BlueH2Obound]

With our Onan 7.5 we figured 1 gal/hr, but in reality it was probably closer to .75 gal/hr running two AC units, 12.5btu and 13.5kbtu.

[dhenline]

skipperPete
I understand that some don't run generators while at anchor. I know that when I charter in the Caribbean ,I really enjoy Air Con in the evening. Maybe I will get used to the heat but I dont want to depend on that. When I visited my friend in the TuaMotos, I LOVED that he had aircon. He said everyone loved his boat because of it. Ironically, he was on a 35 ft benetau Ocenanis and used a window unit in the gangway and a honda portable to run it. Worked like a charm when I was there