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21-11-2008, 08:48
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#1
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Registered User
Join Date: Oct 2007
Location: On the boat
Boat: Valiant 50
Posts: 509
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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.
__________________
The light at the end of the tunnel are no longer the headlights of the oncoming train......yippee
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21-11-2008, 09:34
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#2
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Moderator Emeritus
Join Date: Sep 2007
Location: Eastern Tennessee
Boat: Research vessel for a university, retired now.
Posts: 10,406
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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.
__________________
David
Life begins where land ends.
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21-11-2008, 10:01
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#3
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Registered User
Join Date: Oct 2007
Location: On the boat
Boat: Valiant 50
Posts: 509
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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.
__________________
The light at the end of the tunnel are no longer the headlights of the oncoming train......yippee
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21-11-2008, 10:31
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#4
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Senior Cruiser
Join Date: Feb 2003
Location: Macatawa Michigan
Boat: Amanda Faye 61' Custom Irwin aftcockpit ketch
Posts: 1,415
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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.
__________________
Gunner
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21-11-2008, 10:42
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#5
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Registered User
Join Date: Oct 2008
Location: Puget Sound
Boat: Irwin 41 CC Ketch
Posts: 2,878
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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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21-11-2008, 11:06
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#6
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Registered User
Join Date: May 2008
Posts: 3,558
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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
Carl
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21-11-2008, 11:57
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#7
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CF Adviser
Join Date: Oct 2007
Boat: Van Helleman Schooner 65ft StarGazer
Posts: 10,280
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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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21-11-2008, 13:15
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#8
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Registered User
Join Date: Sep 2004
Location: Ft. Lauderdale, FL currently CLODs [cruisers living on dirt]
Posts: 423
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Quote:
Originally Posted by neelie
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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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.
__________________
Jon
S/Y Sirius
Moody 47
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21-11-2008, 13:37
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#9
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Registered User
Join Date: Oct 2007
Location: On the boat
Boat: Valiant 50
Posts: 509
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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.
__________________
The light at the end of the tunnel are no longer the headlights of the oncoming train......yippee
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21-11-2008, 13:46
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#10
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Registered User
Join Date: Oct 2008
Location: Puget Sound
Boat: Irwin 41 CC Ketch
Posts: 2,878
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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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23-11-2008, 18:33
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#11
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Registered User
Join Date: Jun 2008
Location: Corpus Christi Texas
Boat: boatless atm
Posts: 762
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Quote:
Originally Posted by Stillraining
Let me know what dumpster you through that Genny into though.
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LOL.... yeah me as well.
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23-11-2008, 23:01
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#12
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Registered User
Join Date: Oct 2008
Location: Pacific Northwest, USA
Boat: FL12 12 ft rowboat, 8 foot sailing dink, 18 foot SeaRay I/O
Posts: 328
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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.
__________________
Ike
"Dont tell me I can't, tell me how I can"
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24-11-2008, 08:39
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#13
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Registered User
Join Date: Oct 2005
Posts: 346
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Quote:
Originally Posted by neelie
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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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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24-11-2008, 09:09
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#14
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Registered User
Join Date: Aug 2007
Location: on the boat. Gulf Coast
Boat: C&C 38'
Posts: 351
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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,
__________________
Jerry and Denver
Happy Old cruisers!
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24-11-2008, 11:18
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#15
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Registered User
Join Date: Mar 2006
Location: Nevada City. CA
Boat: Sceptre 41
Posts: 3,857
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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.
__________________
Fair Winds,
Charlie
Between us there was, as I have already said somewhere, the bond of the sea. Besides holding our hearts together through long periods of separation, it had the effect of making us tolerant of each other's yarns -- and even convictions. Heart of Darkness
Joseph Conrad
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