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Old 21-11-2008, 09:48   #1
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Why have an AC Genset? (versus DC)

I have tried a search on this subject and came up with nothing - so here goes.

Can someone explain to me why I should keep my 630 lb, 9kW 110VAC genset?

Would I not be better off installing large (c. 150A) alternators on each engine and then ditching the genset in favor of added battery capacity and solar panels?

Or replace the AC genset with a light weight DC genset with a PTO for a watermaker pump?

I dont see the AC advantage - have I missed something fundamental?

My only big draw AC devices are air conditioners and the water maker. The former will only be used at a marina and the latter can be converted. I have no idea why a massive 9kW genset was originally specified and installed in the first instance.
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Old 21-11-2008, 10:34   #2
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Really it depends on your loads. If you have no large loads then scale down to something smaller especially.

The reason to have a large AC generator is if you have large AC loads that even the largest small yacht inverters (typically 3000 watts) are incapable of powering. So if you don't have a clothes dryer or a large oven or a large air conditioner that you use at sea...then that's your answer.
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Old 21-11-2008, 11:01   #3
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That's exactly how I see it - but surely its not that simple. I must have missed something.

Why then do people install AC gensets so prolifically? Surely they can't all be wanting air conditioning at sea? With the added fuel consumption and need to carry extra fuel it seems like a case of diminishing returns.

I've almost convinced myself to ditch the 9kW Onan.
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Old 21-11-2008, 11:31   #4
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Is the Genset reliable? If it is why take it out?
I do beleive that knowing what I now know I would have installed a large Alternator and more batteries but I would not get rid of a good genset to do it.
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Old 21-11-2008, 11:42   #5
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Two arguments I will thought out..

1) It is better to abuse a gen set for short duration runs every day or two to charge your bats then your prime mover.

2) On a 46' boat 600# of additional weight unless your racing is equivalent to ditching the spare tire in you car to increase gas mileage.
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Old 21-11-2008, 12:06   #6
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Gensets make most sense for those who anchor out (or use a mooring) rather than a dock. Running the main propulsion engine just to charge batteries or support an inverter load at under 2000 rpm is not fuel efficient or particularly good for the engine.

Even then, it's hard to believe that those costs exceed the cost of buying genset and maintenance of a genset. That's why most people run the main engine.

There's a really interesting new option that might be better than adding more batteries or going to a traditional DC generator - the Whispergen - based on a Stirling "external combustion" engine (try to figure that out!) it is very fuel efficient, much quieter than a genset, completely automatic (it turns itself on and off as required for the batteries) - it provides 70 amps battery charging, hot water, and cabin heat (as needed). I saw it at the Annapolis show and was blown away. It has all of the risks of new technology but but it's being distributed in the US by Mack Boring - the primary East Coast Yanmar distributor. It's impressive that they liked it enough to pick it up.

There was also a piece in Professional Boatbuilder last month that was very interesting - it's online.

I'm in no way related to this company and don't even own one - but I'm hoping they turn out to be as good as they look

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Old 21-11-2008, 12:57   #7
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Reasons..?

1/….Basically AC gens duplicate what you use when you are plugged into Shore Power, so even if all your high loads were DC only… then you would still need to recharge using a large AC charger.

2/….There are better selections of more efficient high load AC consumers available than what is available in DC units (Microwaves etc…) …and generally at cheaper prices.

For example, I bought quite economically a portable Air Con unit so as to avoid the excessive sea chest fouling that happens in some tropical marinas. It turned out to be so much more efficient than my Marine Air Cons, that I permanently installed it in the master cabin with the hot air ducting being dispensed through a flange at one of the portholes. It has1/3 the AC consumption of my Marine Air and can be replaced much more cheaply

3/… As someone already said, why put hours on an oversized main engine at anchor, just to charge batteries, when a much smaller, more fuel efficient Gen engine can be used and more easily serviced than the M.Eng and give you other high load services at the same time.
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Old 21-11-2008, 14:15   #8
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Quote:
Originally Posted by neelie View Post
I have tried a search on this subject and came up with nothing - so here goes.

Can someone explain to me why I should keep my 630 lb, 9kW 110VAC genset?

Would I not be better off installing large (c. 150A) alternators on each engine and then ditching the genset in favor of added battery capacity and solar panels?

Or replace the AC genset with a light weight DC genset with a PTO for a watermaker pump?

I dont see the AC advantage - have I missed something fundamental?

My only big draw AC devices are air conditioners and the water maker. The former will only be used at a marina and the latter can be converted. I have no idea why a massive 9kW genset was originally specified and installed in the first instance.
Neelie

1. I would add the large alternators no matter what.
2. As said above running a genset at capacity with a large battery charger i.e. >100amp allows quick charge with a small diesel burn rate. We found it much more efficient and quieter to run genset at .25 gal/hr than our 1+ gal/hr main engine for charging.
3. Big draw items like a/c require genset which is why we have ours and yes we do run it at anchor sometimes.
4. Assuming it is there - I would use it. Based on your comments not sure I would add it if it does not exist.
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Old 21-11-2008, 14:37   #9
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Thanks for the input folks. Perhaps I was remiss in not detailing my plan (which I hasten to add is just still a plan for now, until I research it further)

I was thinking of ditching the gen set and replacing its weight with more batteries - so I have no expectations of weight savings.

In the grand scheme, I had intended adding 6 solar panels and I was hoping that with solar panels, added battery capacity and improved refrigeration insulation, I would only need to run the main engine say once every 4 - 5 sunny days and then only for a couple of hours.

I can't see how running any AC genset can be fuel efficient if battery charging is the sole reason.
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Old 21-11-2008, 14:46   #10
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There is a sect of sailboat owners that wont have an engine either Nellie...as a matter of fact they dont consider you a sailor if you do..IMHO..do what makes the most sense to you and the heck with everyone else's opinion..thats the way me thinks..

Let me know what dumpster you through that Genny into though.
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Old 23-11-2008, 19:33   #11
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Let me know what dumpster you through that Genny into though.
LOL.... yeah me as well.
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Old 24-11-2008, 00:01   #12
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Automotive type alternators are not particularly efficient at charging large battery banks. They do turn out gobs of amps at a constant rate but they are designed to maintain a battery rather than charge it to full capacity like a three or four stage charger would. Gensets charge the batteries through a 3 or 4 stage charger that brings the batteries up to as near 100% as you can get and this is much better for the battery's health and life span.

But a lot of people do charge their batteries off the main engine. I prefer a genset but that's my choice. A genset burns a lot less gas, runs at a more efficient rpm than your main engine (the main engine will essentially be at idle) and puts out less exhaust and Carbon Monoxide. If properly muffled it's quietter.
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Old 24-11-2008, 09:39   #13
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Quote:
Originally Posted by neelie View Post
I have tried a search on this subject and came up with nothing - so here goes.

Can someone explain to me why I should keep my 630 lb, 9kW 110VAC genset?

Would I not be better off installing large (c. 150A) alternators on each engine and then ditching the genset in favor of added battery capacity and solar panels?

Or replace the AC genset with a light weight DC genset with a PTO for a watermaker pump?

I dont see the AC advantage - have I missed something fundamental?

My only big draw AC devices are air conditioners and the water maker. The former will only be used at a marina and the latter can be converted. I have no idea why a massive 9kW genset was originally specified and installed in the first instance.
If you need maybe 5kw (AC + DC) or more almost continuously then an AC genset running full time is probably the way to go. If your needs are less than that but at least 1 or 2kwh a day then a small DC genset, say 5kw or less, with appropriate sized battery bank and inverters is probably optimal. If you use less than 1kwh per day you probably can do without the genset and use the main engine and solar cells to charge the battery bank.

However, even if the DC genset makes sense. Since you already have the AC genset if may be cheaper for you to use it as you would a DC genset, i.e. to charge the battery bank and use an automactic controller to turn on the genset when the bank needs charging or the inverters are pulling a heavy load.
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Old 24-11-2008, 10:09   #14
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Pelagic,

I have seen a portable AC here in South Africa that exhausted the hot air out a flexable hose that looked like a clothes dryer vent hose. The users loved it; it was on rollers and they moved it from their bedroom to their living room as needed. He hade made an adaptor to their built-in braii to exhaust the hot air up the chimmney when it was in that room. It was the standard 250 volts that RSA uses.
What voltage does yours run on? How many BTU's per hour does it remove? Does it have a large start-up current draw? Can you mention a website? Would a little Honda 2kw generator power it?
Thanks,
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Old 24-11-2008, 12:18   #15
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I decided on an AC genset because the cost of adding a bigger alternator and then the wiring and all was equal to about half the cost of a genset. From there you ae just adding alot of hours on the engine just to charge the batteries. I figured that there would be no increase in value to the boat by adding the bigger alternator but a genset would add value and the genset only sips 1/5 of a gallon an hour. Hours on the genset are cheaper b/c I can replace the engine of the genset for alot less than repowering. That was my logic for what it is worth.
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