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Old 11-10-2012, 06:58   #1
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Generator/Aircon Philosophy Question

I am looking to buy another boat so I am doing a lot of thinking about the systems I want aboard. I kind of want a boat with no systems so I can start from the ground up. It seems to me that 110v generation and power consumption makes little sense to me. 110v gensets have to run at full speed to put out a safe charge, so there is a lot of waste of fuel. Also, 110v motors need three times the amps to start then they need to run, so you have to have a much great capacity just for starting motors. Starting up 110v motors can cause all kinds of problems with voltage spikes blowing up other equipment aboard. When you do need 110v, have a very good converter for those things but you won't need as large of one. I am all for having systems based on 12v. Have a single cylinder diesel genset putting out 250amps. As the batteries approach high charge, cut back the speed of the diesel and save fuel. Use 12v motors for freezer, a/c, watermaker. If you want redundancy, then put engine mounted pumps, compressors on the primary diesel engine. Then have an inverter for running those few 110v things what have no motor which would need a surge of power at start up. It will cost a little more upfront, but if you are living on your boat, the savings on fuel cost will make up for it.
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Old 11-10-2012, 07:39   #2
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Re: Generator/Aircon Philosophy Question

One thing every system will benefit from is "peak shaving" from a good charger/inverter which will power startup loads from the batteries without requiring that much capacity from whatever generator you are using.

12 volt DC generation as you describe has a lot of fans. You can make your own system, or you can buy them from Fischer-Panda or NextGen.

I think it makes perfect good sense, but you should not expect a quantum leap in efficiency gains compared to a good low speed AC generator, which also works pretty well. A low speed AC generator will be almost equal in efficiency to a DC generator as long as you keep about 25% load on it, below that you start to lose marginally, but if you have something like the 4.5kW Northern Lights generator we're only talking about a kilowatt, more or less. An AC generator will be quite a bit more efficient running AC loads directly, compared to running them off an inverter and batteries which had been earlier charged by a DC generator -- the battery charge/discharge cycle has a lot of overhead in it, and becomes even less efficient when the batts are not in perfect condition. But an AC generator will be quite a bit less efficient than a variable speed DC generator running loads a lot smaller than 25% of the AC generator's capacity. So it's all a tradeoff.

No kind of generator really shines at getting lead/acid batteries up to 100% charge from 80% charge. For that, solar is king.

There is no "silver bullet" where power systems are concerned. For most people, the smallest possible low speed AC diesel genset combined with a modern charger/inverter will the the simplest and most reliable -- if not cheapest! -- variant. It will also be a quite a bit quieter than most DC gensets, which usually run at 3000 RPM at full load. A home-made variable speed DC genset combined with modern charger/inverter may be the cheapest variant, and will be more efficient than other approaches under some circumstances -- so has some things going for it. To my mind, the downside is like any self-engineered system -- hell of a lot of work to get it working properly, and probably to keep it working properly -- making it best suited to people with a lot of time on their hands, and love of DYI projects.

Yet another approach, cheapest of all, reasonably quiet, and no work whatsoever -- is just to buy a Honda EU200i portable generator and use that together with a modern inverter/charger. You have to keep gasoline on board, which is a downside, and you will use a bit more fuel per KW/h generated than a diesel genset, but the advantages are huge -- cheapest of all, no hassle whatsoever, quieter than a one-lunger diesel. Around the same power output as a 250 amp DC generator.
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Old 11-10-2012, 07:42   #3
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Re: Generator/Aircon Philosophy Question

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Originally Posted by motion30
Please keep us informed as to what you find in 12 volt air conditioner units

I just did a google "marine dc air conditioning units" and 7 brands popped up on first page.
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Old 11-10-2012, 07:52   #4
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Re: Generator/Aircon Philosophy Question

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Originally Posted by CCBullseye View Post
Quote:
Originally Posted by motion30
Please keep us informed as to what you find in 12 volt air conditioner units

I just did a google "marine dc air conditioning units" and 7 brands popped up on first page.
There is much information about 12v air conditioning in the CF archives -- a search will be worthwhile.

All of the 12v marine air conditioning units I have ever seen are not native 12 volt. They use 110 volt or 230 volt AC motors powered by a small inverter inside the unit.

I am not an electrical engineer, but my hazy and perhaps wrong understanding is that this is so because constant speed AC motors are easier to design and build due to being able to use the AC frequency to time the motor.

If you have a large inverter on board, then there would hardly be anything to gain by using a DC aircon system which simply inverts DC power inside -- you're just duplicating a function which you perform on board anyway with your regular inverter.

Good inverters like Mastervolt and Victron are very efficient.
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Old 11-10-2012, 08:21   #5
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Re: Generator/Aircon Philosophy Question

Quote:
Originally Posted by CCBullseye View Post
Quote:
Originally Posted by motion30
Please keep us informed as to what you find in 12 volt air conditioner units

I just did a google "marine dc air conditioning units" and 7 brands popped up on first page.
Same here. Mr. Google popped up a number of options. However I did a quick look at the first 2-3 units on the list I got and they were 3.5-5 KBTU. Maybe enough to keep one cabin or a small boat on the cool side but would take two or three in a 40' or larger boat, especially with multiple cabins. Will look through the list a little more to see if any larger units are listed.

Dockhead's comment that the DC units may be actually AC motors with a built in inverter is interesting. Would sort of kill most of the benefits to the whole idea of DC A/C except for someone with no AC power source at all. Of course with no source for AC power to run the cooler you would need massive batteries or have to run the engine pretty often.

Will read the specs on these units to see if that issue is covered as well.
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Old 11-10-2012, 08:30   #6
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Re: Generator/Aircon Philosophy Question

OK. Checked out one DC A/C unit. The Marvair.

Marvair - Marine - DC Air Conditioner

They have 12V and 24V models. The specs say they use a 12V DC brushless motor so no inverter.

The 12V is rated at 5000 BTU, pretty small. Power draw at 12V is listed at 30 amps. That's a heap of DC current. You would need over 1000 amp hours of batteries installed to run this overnight and have a little power to spare for lights and other small house loads. Add a 12V refrigerator and make that more like 1500-2000 amp hours worth of batteries. Plus some serious charging capacity to put all those amps back into the batteries.

Seems like there is no simple or practical way around needing a generator to have air conditioning on a boat.
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Old 11-10-2012, 08:32   #7
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Re: Generator/Aircon Philosophy Question

Having spent the majority of my life living without A/C in hot humid summers of the Chesapeake Bay area, having a/c on board is pretty low in my priority list of must haves on the boat. I have always enjoyed sailing up river in July and August to the fresh water where I can swim without stinging nettles. Growing up on a farm with no a/c, I learned many tricks to cool down and handle the heat. One simple way is to stick feet in bucket of cooler water, even 85 degree water will cool one down. In the summer, I keep a wet cloth around my neck, and if it is really hot, keeping a wet cloth on my head or chest too. Also it is critical to drink plenty of water. Just like when it is snowing outside, we slow down and hibernate, so too when it is really hot, like 100+ degrees.

I would rather have a windvane steering, big efficient freezer, water maker, good heating system.
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Old 11-10-2012, 08:49   #8
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Re: Generator/Aircon Philosophy Question

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Originally Posted by CCBullseye View Post
Having spent the majority of my life living without A/C in hot humid summers of the Chesapeake Bay area, having a/c on board is pretty low in my priority list of must haves on the boat. I have always enjoyed sailing up river in July and August to the fresh water where I can swim without stinging nettles. Growing up on a farm with no a/c, I learned many tricks to cool down and handle the heat. One simple way is to stick feet in bucket of cooler water, even 85 degree water will cool one down. In the summer, I keep a wet cloth around my neck, and if it is really hot, keeping a wet cloth on my head or chest too. Also it is critical to drink plenty of water. Just like when it is snowing outside, we slow down and hibernate, so too when it is really hot, like 100+ degrees.

I would rather have a windvane steering, big efficient freezer, water maker, good heating system.
I agree, 99% of the time. I spent two years living aboard a boat in the Bahamas and Caribbean with no air conditioning and was fine with it. Good awnings and ventilation did the trick almost all the time. However I remember a few really unpleasant nights when the bugs were eating me alive but it was too hot to close the hatches. Screens, including spraying the screen with repellent to keep out the noseeums help but on a really still night will also block most of any breeze that might be there.

Since I plan to have the Honda for backup and the boat came with an A/C installed I figure I may as well take advantage of it.
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Old 11-10-2012, 08:50   #9
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Re: Generator/Aircon Philosophy Question

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The 12V is rated at 5000 BTU, pretty small. Power draw at 12V is listed at 30 amps. That's a heap of DC current. You would need over 1000 amp hours of batteries installed to run this overnight and have a little power to spare for lights and other small house loads. Add a 12V refrigerator and make that more like 1500-2000 amp hours worth of batteries. Plus some serious charging capacity to put all those amps back into the batteries.

Seems like there is no simple or practical way around needing a generator to have air conditioning on a boat.
Why not get a 12 volt genset running on a one cylinder diesel with auto start and auto throttle adjustment for needed charging volts? These systems run on less than a pint of fuel an hour, and should only need to run a few times a day and night. 5,000btu is a lot of a/c for a small space. There is no need to cool all of the boat, only the area you are in, and then, it is much more important to get the humidity out of the air then to take it down to 68*. Heck, in my house, I am very comfortable at 78* as long as it is low humidity inside.
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Old 11-10-2012, 09:08   #10
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Re: Generator/Aircon Philosophy Question

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Why not get a 12 volt genset running on a one cylinder diesel with auto start and auto throttle adjustment for needed charging volts? These systems run on less than a pint of fuel an hour, and should only need to run a few times a day and night. 5,000btu is a lot of a/c for a small space. There is no need to cool all of the boat, only the area you are in, and then, it is much more important to get the humidity out of the air then to take it down to 68*. Heck, in my house, I am very comfortable at 78* as long as it is low humidity inside.
Agree again.

Like the the old cliche, it's not the heat, it's the humidity. We too keep our home thermostat around 78. Drives me crazy to go places where the temp is in the low seventies or even below. At a previous job I used to wear a windbreaker to work in August. Some of the staff had to keep little space heaters under their desks. Totally nuts.

I have looked into DC generators and in theory they should be the most efficient option. Can run the diesel at varying rpm to generate only as much power as needed. But on a practical level seems like there are very few units on the market and the ones I looked at were expensive.

If you have found some good options would be very interested. For now based on budget, power rating, quality, size/weight I really like the Honda. Since I would carry gasoline for the dinghy anyway the Honda wouldn't add a new fuel requirement, just need a little more of it.
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Old 11-10-2012, 09:46   #11
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Re: Generator/Aircon Philosophy Question

I have serious issues about carrying a gasoline powered genset. First, where do you stow it, is it stowed in a locker that has a bottom draining air vent? Do you pull it out of locker and put it on deck when running it? How do you run it when it is raining? If you run it in a locker, it should be marinized (did I spell that right) to prevent explosions. How do you carry enough gasoline. It seems like you are talking about a gallon or two of fuel a day, so I would think you would want at least 20 gallons. That is a lot of fuel to carry in jerry jugs.
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Old 11-10-2012, 10:25   #12
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Re: Generator/Aircon Philosophy Question

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I have serious issues about carrying a gasoline powered genset. First, where do you stow it, is it stowed in a locker that has a bottom draining air vent? Do you pull it out of locker and put it on deck when running it? How do you run it when it is raining? If you run it in a locker, it should be marinized (did I spell that right) to prevent explosions. How do you carry enough gasoline. It seems like you are talking about a gallon or two of fuel a day, so I would think you would want at least 20 gallons. That is a lot of fuel to carry in jerry jugs.
Yes to all the concerns.

Unfortunately my boat has very limited deck storage so would have to store the Honda below decks at least long term. Also much less likely to sprout legs and walk away if stored below. But would make sure it was completely drained and fuel free before before bringing it inside.

As far as running, definitely above deck. But even on deck I am concerned about potential for CO poisoning on a very still night with no wind to blow the exhaust away. One of many reasons to install good CO detectors in the boat. Have heard some put the Honda in the dinghy to run it. I have considered that but it would not be an option if there were any other boats in the anchorage.

Have thought about building some sort of combo seat, dock box, locker to put on the stern and store the Honda on deck. That is very far down on the job list at this point. Probably would not run it in the rain except in an emergency and then rig some sort of temporary cover.

Would have to carry more gasoline for the Honda but I'm not planning on frequent use so I hope that will be minimal. I have read tests from users that measured the fuel consumption of the Honda and at full load was less than 0.4/gallon per hour. At +/- 20% load about 0.12 gal/hour. So I'm thinking all night to run a small A/C would take less than 2 gallons. Guess it would depend on how far out in the boonies I am and how much I would use the dinghy and gennie. Hope I could keep the gasoline load to 5-10 gallons max.
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Old 11-10-2012, 10:32   #13
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Re: Generator/Aircon Philosophy Question

From personal experience, a one cylinder diesel engine is NOISEY!
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Old 11-10-2012, 10:47   #14
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Re: Generator/Aircon Philosophy Question

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From personal experience, a one cylinder diesel engine is NOISEY!
One more point for the Honda.

The little light bulb just went off over my head. The boat has a ventilated, deck level locker. Instead of several jerry jugs which fit poorly and waste a lot of space, maybe convert the locker into an on deck gas tank with proper fill, vents and hose to transfer to the outboard and Honda gen.
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Old 11-10-2012, 11:10   #15
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Re: Generator/Aircon Philosophy Question

I guess theres lots of pros and cons about gen sets ! but the one thing I know is comm fish boats all used to have gas engines years ago! and because of fires they all went to diesel! Gas aboard a vessel can be a danger!! I currently have aboard a 3 cyl northren lights 3 1/2 kw genny, in a sound box thats NOT NOISY ! most folks can't hear it run even aboard the boat! As Ive said before I like my comfort, and with a diesel gen set I dont need to carry a supply of gas aboard ! as our dink has oars or sails no out board ! but thats us, and it's our choice ! but if ya want AC, inverters are really not the way to go ! unless ya have HUGE battery banks ! and if ya do your carrying way more weight then a small genny would weigh! just my 2 cents
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